Visible light-mediated intermolecular [2 + 2] photocycloaddition of 1-aryl-2-nitroethenes and olefins

Article abstract of DOI:10.1039/c9ob01146c

Despite the importance of cyclobutanes there are not many direct [2 + 2] photocycloaddition reactions which can be performed with visible light in the absence of a catalyst. A notable exception is the reaction of 1-aryl-2-nitroethenes and olefins which can be performed at a wavelength of λ = 419 nm or λ = 424 nm in CH₂Cl₂ as the solvent. In the present study, a total of 15 1-aryl-2-nitroethenes were found to undergo a [2 + 2] photocycloaddition with 2,3-dimethyl-2-butene (28-86% yield) and a set of 12 olefins was studied in their photocycloaddition to 1-phenyl-2-nitroethene (37-88% yield). All mechanistic results are in agreement with a triplet reaction pathway and with the intermediacy of a 1,4-diradical.

Full text of DOI:10.1039/c9ob01146c

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Visible light-mediated intermolecular [2 + 2]
photocycloaddition of 1-aryl-2-nitroethenes
Cite this: DOI: 10.1039/c9ob01146c
and olefins†
Lisa-Marie Mohr,
Andreas Bauer,
Christian Jandl and Thorsten Bach
*
Despite the importance of cyclobutanes there are not many direct [2 + 2] photocycloaddition reactions
which can be performed with visible light in the absence of a catalyst. A notable exception is the reaction
of 1-aryl-2-nitroethenes and olefins which can be performed at a wavelength of λ = 419 nm or λ =
424 nm in CH2Cl2 as the solvent. In the present study, a total of 15 1-aryl-2-nitroethenes were found to
undergo a [2 + 2] photocycloaddition with 2,3-dimethyl-2-butene (28–86% yield) and a set of 12 olefins
was studied in their photocycloaddition to 1-phenyl-2-nitroethene (37–88% yield). All mechanistic results
are in agreement with a triplet reaction pathway and with the intermediacy of a 1,4-diradical.
Received 16th May 2019,
Accepted 2nd July 2019
DOI: 10.1039/c9ob01146c
rsc.li/obc
any experimental details. In 1980, the Sakurai group described
the [2 + 2] photocycloaddition of β-nitrostyrene with indene
Introduction
In 1887, when studying the properties of phenylnitroethylene (Scheme 1) which was performed with a high-pressure mercury
(β-nitrostyrene, 1-phenyl-2-nitroethene), Priebs observed that lamp in a pyrex vessel.7 Further photocycloaddition studies of
the yellow coloured substrate was converted upon exposure to 1-aryl-2-nitroethenes were reported by Ramkumar and
light to a colourless product which he suspected to be a Sankararaman (Michael type addition of silyl enol ethers to
polymer.1 Meisenheimer and Heim reinvestigated the reaction β-nitrostyrene),8 by Chapman and co-workers ([2 + 2] photo-
in 1907 proposing a dimeric product in analogy to the photodi- cycloaddition of β-nitrostyrene and 2,3-dimethylbutadiene),9
mer of cinnamic acid.2 They were not able to prove this and most recently by Ferreira and co-workers (Cr-catalysed
hypothesis, however, and it took another 65 years until the [4 + 2] cycloaddition of trans-β-nitro-para-methoxystyrene and
cyclobutane structure of the photodimer was established by 1,3-dienes).10
Shechter and co-workers.3 The [2 + 2] photodimerization was
We became interested in the intermolecular [2 + 2] photo-
performed by exposure of solid trans-β-nitrostyrene to sunlight cycloaddition11 of 1-aryl-2-nitroethenes in the context of our
and the conversion was ca. 70% after 4–6 weeks of irradiation. work on visible light-mediated reactions.12 In preliminary
The relative configuration of the two diastereomeric products studies (Scheme 1),13 we found that a smooth reaction
was later established by Desiraju and Pedireddi in an X-ray occurred when the title compounds (c = 20 mM) were irra-
crystallographic study.4
diated in a solution of the olefin (10 equiv.) in dichloro-
Despite the fact that this precedence suggested that methane at λ = 419 nm. The reaction scope was limited,
β-nitrostyrene can be involved in a [2 + 2] photocycloaddition
reaction when exposed to visible light, the very few attempts to
obtain [2 + 2] photocycloaddition products of β-nitrostyrene
were performed with mercury lamps as UV irradiation sources.
An initial report by Chapman and co-workers5 referred to work
performed in the context of a Ph.D. thesis6 but did not provide
Department Chemie and Catalysis Research Center (CRC), Technische Universität
München, 85747 Garching, Germany. E-mail: thorsten.bach@ch.tum.de;
Fax: +49 89 28913315; Tel: +49 89 28913330
†Electronic supplementary information (ESI) available: Synthetic procedures
and full characterization for all starting materials (1) and products (2, 3, 4, 6, 7),
emission spectrum of 1a, quantum yield for 2a. CCDC 1915359. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
C9OB01146C
Scheme 1 Previous studies on the title reaction. Visible light-mediated
reactions were performed with fluorescent lamps (emission maximum:
λ = 419 nm).
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however, and the irradiation conditions were not fully opti-
mized. We have now performed a more comprehensive array of
experiments with a total number of 15 different 1-aryl-2-
nitroethenes and with an additional set of 12 olefins.
Moreover, further mechanistic studies were performed to shed
light on the course of the [2 + 2] photocycloaddition. In this
context, an unprecedented ring opening reaction of 1,1-
dicyclopropylethylene was observed. Full details of our experi-
mental work are presented in this account.
Reaction scope
The 1-aryl-2-nitroethenes 1 employed in our study (Fig. 1) were
prepared from nitromethane and the respective aromatic alde-
hydes in a Henry reaction.14 The condensation was performed
with ammonium acetate in nitromethane or in a mixture of
nitromethane and acetic acid. A colour change of the refluxing
solution indicated a successful elimination of the intermediate
alcohol to the nitroethene which was isolated exclusively as the
trans-isomer. Only the 2-thiophenyl product 1e was not accessi-
ble by this method and required the use of a stronger base
(NaOH in MeOH)15 to induce the Henry reaction.
Fig. 2 UV/Vis spectra of selected 1-aryl-2-nitroethenes (c = 1 mM,
CH2Cl2).
Table 1 Intermolecular [2
+ 2] photocycloaddition of 1-aryl-2-
nitroethenes (1) and 2,3-dimethyl-2-butene: optimal reaction conditions
for the individual substrates
UV/Vis-spectra16 of all 1-aryl-2-nitroethenes were recorded
in dichloromethane solution and selected spectra are depicted
in Fig. 2 (see the ESI† for all spectra). An electron withdrawing
group at the phenyl group led to a hypsochromic shift relative
to the parent compound 1a as shown for the 4-cyanophenyl
derivative 1d. An electron donating group showed the opposite
effect and the 4-methylphenyl (1b) and the 4-methoxyphenyl
(1c) derivatives absorb at longer wavelength relative to 1a. The
absorption coefficient is typically in the range of 10 000–20 000
M−1 cm−1 indicating that the absorption is due to an allowed
transition (vide infra). All compounds are coloured which is in
line with an – at least minimal – absorption in the visible
range (λ > 380 nm).
Entry Substrate Cond.a t [h] Conv.b [%] Product Yieldc [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
A
C
C
C
C
C
C
B
C
B
A
C
C
C
12
3
4
6
2
4
4
6
5
7
7
7
24
3
100
100
100
100
100
100
100
100
100
73
100
57
100
100
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
59
67
77
44
86
50
54
58
33
38
35d
28
66
79
9
10
11
12
13
14
The previous preliminary irradiation experiments13 were
conducted exclusively at room temperature with fluorescent
lamps which display a relatively broad emission spectrum and
an emission maximum at λ = 419 nm (Table 1, conditions A).
a The reactions were performed under conditions A, B, and C (see ESI†
for further details). For each reaction the best conditions are listed in
the table. Irradiation was discontinued after the indicated time period
t. b The conversion is based on the amount of re-isolated starting
material. c Yield of isolated product. d Olefinic by-product (11%), see
narrative.
In search for optimal conditions, we also performed the reac-
tion with a light-emitting diode (LED) at λ = 424 nm at
ambient temperature (conditions B) and at −78 °C (conditions
C). Every substrate was tested under all conditions in its reac-
tion with 2,3-dimethyl-2-butene and the best conditions for
the individual substrate are recorded in Table 1 (for the com-
plete set of results see the ESI†).
Unfunctionalized and heteroaromatic substrates (entries 1,
2, 5, 13 and 14) reacted consistently well and in good yields
(59–86%). Methoxy and halogen substitution in para-position
of the 1-phenyl-2-nitroethenes was compatible with the reac-
tion (entries 3 and 6–8) and the respective products 2c, 2f–2h
Fig. 1 Structures of the 1-aryl-2-nitroethenes 1 employed in this study.
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were obtained in moderate to good yields (50–77%). An elec- substrate 1a with olefins that had not been studied in previous
tron withdrawing group (entries 4, 9 and 11) retarded the reac- work or that gave better yields under conditions B and
tion slightly which reflects a smaller absorption cross section C. Products 3a–3d were obtained as single isomers while cyclo-
of the substrates in the visible range (cf. compound 1d in butanes 3e–3h were formed as diastereomeric mixtures. It was
Fig. 2). In addition, side reactions were observed which were possible in all cases to isolate the major isomer and to assign
particularly significant for compound 1k (entry 11) and which its relative configuration (see ESI† for further details). The
will be discussed in the mechanistic section. The reactions of given yield refers to the total yield of all diastereoisomers (dr =
the meta- and ortho-chloro substituted 1-phenyl-2-nitroethenes diastereomeric ratio).
(entries 10 and 12) proceeded sluggishly and were stopped
Electron deficient olefins (e.g. 1,1-dichloroethene, methyl
after seven hours. Starting material was recovered as a mixture acrylate, allylic alcohol) showed no reaction in attempted inter-
of the respective cis- and trans-compound. Likewise, whenever molecular [2 + 2] photocycloaddition reactions with trans-
a reaction was stopped before completion, the recovered 1-aryl- β-nitrostyrene (1a). In the reaction to product 3f there was no
2-nitroethenes were isolated as cis-/trans-mixtures. The compo- indication for a ring opening of the cyclopropyl ring and
sition in the photostationary state reflects the different absorp- seven-membered carbocyclic by-products were not detected.
tion properties of the individual geometric isomers at the The fact that silyl enol ethers gave cyclobutanes 3g and 3h as
chosen irradiation wavelength.13,17 The only substrate which the only isolable products was surprising. In previous photo-
did not show any [2 + 2] photocycloaddition reaction was 1-(4′- chemical studies,8 Michael addition products were observed
N,N-dimethylamino)phenyl-2-nitroethene despite the fact that suggesting an addition reaction of the silyl enol ether with
it displays a particularly extensive absorption in the visible opposite regioselectivity. For comparison, we prepared the
region. There was no decomposition of starting material and it Michael addition product of 1-(trimethylsilyloxy)cyclopentene
is likely that intramolecular relaxation pathways18 occur more and trans-β-nitrostyrene by a thermal reaction.19 However, this
rapidly than the intermolecular addition to the olefin. very same product was not detectable in the crude product
Products 2 were isolated as single diastereoisomers with the mixture of the [2 + 2] photocycloaddition reaction neither by
aryl group (Ar) and the nitro group in trans-position at the TLC nor by GLC analysis. It should be noted that different
cyclobutane ring. This assignment was corroborated by NOE irradiation conditions (λ > 250 nm) and a different substrate
experiments which revealed a contact between the ortho stoichiometry (1a : silyl enol ether = 1 : 1) were used by
protons at the C1 phenyl group and the proton at C4. It is also Ramkumar and Sankararaman in their experiments.8 Still, it
in line with the relative configuration found in previously remains open why the regioselectivity should be completely
reported [2
+
2] photocycloaddition products of trans- reverted (vide infra). In all [2 + 2] photocycloaddition products
3 the better donor substituent of the former olefin is posi-
β-nitrostyrene (1a).7,9
In our preliminary communication, the reaction of trans- tioned at carbon atom C2 relative to carbon atom C4 which
β-nitrostyrene (1a) with indene, vinyl ethyl ether, 2,3-dimethyl- carries the nitro group.
butadiene, and cyclopentene under visible light irradiation
Intramolecular [2 + 2] photocycloaddition reactions were
(conditions A) was reported.13 Scheme 2 displays reactions of attempted with 1-phenyl-2-nitroethenes that had an alkenyl
group linked to the ortho position of the phenyl ring, such as
substrate 1o 20 (Scheme 3). Irrespective of the length of the
tether there was no indication for an intramolecular reaction
which could be due to the intrinsically low reactivity of a term-
inal olefin. An alternative explanation would be that initial C–C
bond formation has to occur in the intramolecular reaction
at the β-position of the nitrostyrene which might be electroni-
cally disfavored (vide infra). The chromophore of 1o is still
reactive upon excitation as demonstrated by the intermolecular
[2 + 2] photocycloaddition of 2,3-dimethyl-2-butene to product 2o.
Although synthetic applications of the nitrocyclobutanes
were not in the focus of our current study, it was probed
whether aminocyclobutanes would be accessible by a straight-
Scheme 2 Intermolecular [2 + 2] photocycloaddition of 1-phenyl-2-
nitroethene (1a) and various olefins: optimal reaction conditions for the
individual olefins (t = reaction time, dr = diastereomeric ratio).
Scheme 3 Inter- vs. intramolecular [2
1-aryl-2-nitroethene 1o: exclusive formation of product 2o.
+ 2] photocycloaddition of
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Scheme 4 Reduction of nitrocyclobutane 1a to aminocyclobutane 4.
forward reduction.21 Gratifyingly, the reduction of nitrocyclo-
butane 1a, as a representative example, with zinc22 proceeded
smoothly and without any loss of the stereochemical infor-
mation. Product 4 was obtained in 77% yield (Scheme 4).
Scheme 5 Suggested reaction course of the [2 + 2] photocycloaddition
between 1-aryl-2-nitroethenes and olefins via triplet 1,4-diradical 3D
(top); formation of by-product 6 (Table 1, entry 11) via 1,4-diradical 5
(bottom).
Mechanistic studies
There is consensus in the literature that the longest wave-
length absorption of 1-aryl-2-nitroethenes correlates to a ππ*
transition into the respective first excited singlet state (S1).16,18
For trans-β-nitrostyrene (1a), calculations have been performed
that allow to visualize the electron density in the highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO).23 There is no detectable fluo-
rescence for typical 1-aryl-2-nitroethenes if the energy of S1 is
above 2.95 eV (285 kJ mol−1).16b Among other arguments, the
fact, that fluorescence is not even observed in the solid state,
suggests a rapid intersystem crossing (ISC) from the singlet to
the triplet hypersurface leading to a population of the lowest
lying triplet state (T1). Only 1-phenyl-2-nitroethenes with a
strong electron donating group (e.g. NMe2) exhibit fluo-
rescence with a fluorescence quantum yield of ca. 0.1 in
benzene.18 The photophysical behaviour of 1-(4′-N,N-dimethyl-
amino)phenyl-2-nitroethene has been studied by transient
absorption spectroscopy. The ISC is extremely rapid [τ(S1) ≅
6 ps] in a non-polar solvent (cyclohexane) and remains fast
[τ(S1) ≅ 70 ps] in a solvent of moderate polarity.18
The triplet energies of compounds 1a, 1c, and 1g have been
determined from their phosphorescence emission at 77 K in
an EtOH matrix to be E(T1) = 228 kJ mol−1, 226 kJ mol−1, and
219 kJ mol−1, respectively.16b We recorded the phosphor-
escence spectrum of compound 1a in an EtOH matrix at 77 K
and obtained a value of E(T1) = 229 kJ mol−1 (see ESI†). The
nature of the triplet state for compounds 1 has not been exten-
sively explored. Cowley assigned to it an nπ* character which
would be in accord with the high ISC rate and with the
absence of fluorescence from S1.16b
studies, there were again products of a photo-ene reaction24
isolated as by-products. In the present study, the reaction of
substrate 1k (Table 1, entry 11) turned out to be particularly
prone to undergo the reaction that likely involves an intra-
molecular hydrogen abstraction in 1,4-diradical 5 thus generat-
ing product 6.
Another way to substantiate the existence of a 1,4-diradical
3D is based on the ring opening of a cyclopropyl-substituted
alkyl radical.25 In the reaction to product 3f, there was no indi-
cation for such a process, but when employing 1,1-dicyclo-
propylethylene26 as substrate a new product was isolated apart
from the regular [2 + 2] photocycloaddition product 3i. Proof
for its tricyclic structure 7 rests – apart from extensive NMR
analysis – on the isolation of the related product 8 from the
reaction between 1-(4′-methoxycarbonyl)phenyl-2-nitroethene
(1i) and 1,1-dicyclopropylethylene (Scheme 6).
X-Ray crystallographic analysis of product 8 (Fig. 3) revealed
the fact that both cyclopropyl rings had opened in the reaction
sequence and that the exocyclic double bond was formally
Our mechanistic suggestion (Scheme 5) for the reaction
course involves the nπ* triplet state 1 (T1) as the key intermedi-
ate which is accessed from 1 (S1) by ISC. Electron loss at the
oxygen n orbitals and population of the π* orbital with an elec-
tron leads to electron deficiency at the α-carbon atom (photo-
chemical Umpolung) which is the preferred position of olefin
attack to generate triplet diradical 3D. ISC and subsequent ring
closure lead to cyclobutane products but side reactions are
possible from 3D prior or after ISC.
Scheme 6 Intermolecular [2 + 2] photocycloaddition of 1-phenyl-2-
nitroethene (1a) and 1,1-dicyclopropylethylene to product 3i and by-
product 7 (top); by-products 8 obtained from 1-(4’-methoxycarbonyl)
phenyl-2-nitroethene and 7-d5 obtained from deuterated substrate 1a-
3
Any detectable side reactions which occur from D support
a pathway on the triplet hypersurface. As in our preliminary
d5 (bottom).
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Scheme 8 Intermolecular [2 + 2] photocycloaddition of 1-phenyl-2-
nitroethene (1a) and β-methylstyrene (14): non-stereospecific reaction
course, but incomplete stereoconvergence.
Fig. 3 Structure of by-product 8 as obtained by single-crystal X-ray
diffraction.
further details). Due to the rapid isomerisation it is experi-
(E)-configured. In order to explore the fate of the hydrogen mentally difficult to determine whether both isomers are
atom in the ortho position of the phenyl ring, which is involved involved in the [2 + 2] photocycloaddition but it is likely.
in a C–C bond formation step, we submitted aldehyde 1a-d5 to Regarding the olefin, the stereochemical integrity during the
the [2 + 2] photocycloaddition with 1,1-dicyclopropylethylene [2 + 2] photocycloaddition is high. Reactions performed with
(Scheme 6). Product 7-d5 was isolated as by-product (11%) either trans- or cis-β-methyl styrene (14, Scheme 8) delivered
together with the major product 3i-d5 (61%). The deuterium the cyclobutanes 3j and 3j′. The recovered olefin was still dia-
atom was found at the terminal carbon atom of the ethyl stereomerically pure in either case which indicates that the
group which is attached to the exocyclic (E)-double bond.
olefin is not photochemically excited under the irradiation
Invoking a 1,4-diradical 9 for the reaction of 1a and 1,1- conditions.
dicyclopropylethylene the formation of 7 can be tentatively
The photocycloaddition of olefins 14 was not stereospecific
explained by the reaction cascade depicted in Scheme 7. Ring regarding the olefinic double bond. Starting from trans-14 sig-
opening of the cyclopropane leads to 1,7-diradical 10 which nificant amounts of product 3j′ were obtained with the two
seems unsuited for seven-membered ring formation. Instead, substituents at C2 and C3 in the cis-position. Vice versa, cis-
the radical in α-position to the phenyl group attacks the β-methyl styrene (cis-14) gave also notable quantities of the 2,3-
double bond to produce 1,4-diradical 11 which opens to 1,7- trans-product 3j. In the absence of any detectable cis/trans iso-
diradical 12. The proximity of the primary radical center to the merisation of β-methyl styrene during the reaction the non-
phenyl ring in this intermediate may initiate a stereoselective stereospecifity is further evidence for the intermediacy of a
3
C–C bond formation with the former ortho hydrogen atom now triplet 1,4-diradical D in which rotation around single bonds
being perfectly exposed for an intramolecular hydrogen in possible.27
abstraction. Indeed, molecular models suggest that this
process is feasible in intermediate 13 leading to product 7.
A final comment is warranted on a possible involvement of
single electron transfer (SET) processes. The redox potential of
The efficiency of the intermolecular [2 + 2] photocycloaddi- trans-β-nitrostyrene (1a) in its triplet state can be estimated by
tion is limited not only by the low absorption coefficient of the its triplet state energy E(T1) = 229 kJ mol−1 and by its ground
1-aryl-2-nitroethenes but also by their rapid cis/trans isomerisa- state redox potential.28 Based on the known redox potential
tion in the excited state.18 The quantum yield for the reaction E1/2(1a/1a•−) = −0.44 V (ref. 29) a calculated value E1/2(1a*/1a•−
)
1a → 2a at λ = 382 nm was determined as 0.04 (see ESI† for in the order of +1.90 V is obtained for the triplet state.
Thermodynamically, the oxidation of several electron rich
olefins with Eox(olefin•+/olefin) < +1.90 V would thus seem
feasible, e.g. of 2,3-dimethyl-2-butene (Eox = +1.50 V),30 2,3-
dihydrofuran (Eox = +1.40 V),31 and 1-tert-butyl-1-(trimethyl-
silyloxy)ethene (Eox = +1.34 V).32 However, several other reactive
olefins, e.g. methylenecyclohexane (Eox = +2.62 V),33 exhibit a
redox potential far too high for an electron transfer to be poss-
ible. In addition, SET reactions34 are typically performed in
polar solvents to assist charge separation which is more
difficult in a nonpolar solvent. The fact that the observed
photocycloaddition works also in benzene and that it is accel-
erated by a triplet sensitizer13 makes the involvement of SET
processes unlikely. Further circumstantial evidence is based
on the absence of by-products which would be expected in the
reaction of dienes ([4 + 2] cycloaddition) and of 2,3-dimethyl-2-
butene. The side products mentioned earlier (e.g. product 6,
Scheme 5) should exhibit
a different regioselectivity of
Scheme 7 Suggested formation of by-product 7 from 1,4-diradical
intermediate 9.
addition35 had they been formed in an SET process.
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spectra were measured on a PerkinElmer Lambda 35 UV/Vis
spectrometer.
Conclusions
To summarize, we have established a straightforward access to
various 1-aryl-4-nitrocyclobutanes by a visible light-mediated
intermolecular [2 + 2] photocycloaddition. The substituents
Experimental Procedures
(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-benzene (2a).
are trans-positioned within the cyclobutane but it is known Representative procedure (conditions A): A solution of
that the relative configuration can be inverted to cis by a depro- nitroethene 1a (29.8 mg, 200 μmol, 1.00 equiv.) and
tonation/protonation sequence.3 In addition, the nitro group 2,3-dimethyl-2-butene (234 μL, 166 mg, 2.00 mmol, 10.0 equiv.)
can be interconverted to an amine if desired as has been in dichloromethane (10 mL, c = 20 mM) was irradiated
shown in the present study by reduction to product 4. at λmax = 419 nm for twelve hours at room temperature. The
Accordingly, the method makes also trans- and cis-1-aryl-4-ami- crude product was purified by column chromatography
nocyclobutanes accessible if desired.
(P/Et2O = 20/1) to yield 2a (26.9 mg, 1.16 mmol, 59%) as a
Although the nitro chromophore bears electronically some yellow coloured oil. The analytical data obtained matched
analogy to a carbonyl group, the photochemical behaviour of those reported in the literature.13
1-phenyl-2-nitroethene is different from cinnamic aldehyde.
1-Methyl-4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene
While the latter compound does not form cyclobutanes upon (2b). Representative procedure (conditions C): A solution
direct irradiation36 the former compound and its analogues of nitroethene 1b (16.3 mg, 100 μmol, 1.00 equiv.) and
are suitable substrates for [2 + 2] photocycloaddition reactions, 2,3-dimethyl-2-butene (119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.)
as shown in this study. For 1-aryl-2-nitroethenes, an intrinsic in dichloromethane (5 mL, c = 20 mM) was irradiated at λmax
=
feature of their excited state seems to be the propensity to 424 nm for three hours at −78 °C. The crude product was puri-
react only with electron rich olefins.
fied by column chromatography (P/Et2O = 20/1) to yield 2b
(16.5 mg, 66.7 μmol, 67%) as a pale-yellow coloured oil. The
analytical data obtained matched those reported in the
literature.13
1-Methoxy-4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene
(2c). The reaction was performed in analogy to the representa-
Experimental
General information
All preparations and manipulations of air and moisture sensi- tive procedure for conditions C (see above) with nitroethene 1c
tive compounds were carried out in flame dried glassware (17.9 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
under an argon atmosphere using standard Schlenk tech- (119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
niques. Dry solvents were either obtained water and oxygen (5 mL, c = 20 mM) (t = 4 h). Purification by column chromato-
free by a Braun MB SPS purification system or from commer- graphy (P/Et2O = 9/1) yielded 2c (20.3 mg, 77.1 μmol, 77%) as a
cial sources (see ESI†). Irradiation experiments were either yellow coloured oil. The analytical data obtained matched
performed in a Rayonet RPR-100 photochemical reactor those reported in the literature.13
(Southern New England Ultra Violet Company, Branford, CT,
USA) at λmax = 419 nm (16 lamps, cool white, 8 W, Osram)37 The reaction was performed in analogy to the representative
or with a high power light emitting diode (LED) at λmax procedure for conditions C (see above) with nitroethene 1d
4-(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-benzonitrile (2d).
=
424 nm (Roithner Lasertechnik, 350 mA, UF ∼ 3.4 V). Flash (17.4 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
column chromatography was performed with silica 60 (119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
(Merck, 230–400 mesh) as the stationary phase. Infrared (IR) (5 mL, c = 20 mM) (t = 6 h). Purification by column chromato-
spectra were recorded on a PerkinElmer Frontier IR FTR graphy (P/Et2O = 9/1) yielded 2d (11.4 mg, 43.9 μmol, 44%) as
spectrometer by ATR technique. Nuclear magnetic resonance colourless solid. The by-product was only observed in traces.
(NMR) spectra were recorded at room temperature either on a The analytical data obtained matched those reported in the
Bruker AVHD 300, AVHD 400, AVHD 500 or an AV 500 cryo. literature.13
1H NMR spectra were referenced to the residual proton signal
2-(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-thiophene (2e).
of the respective solvent. 13C NMR spectra were referenced to The reaction was performed in analogy to the representative
the 13C-D multiplet of the solvent employed. Assignment and procedure for conditions C (see above) with nitroethene 1e
multiplicity of the 13C NMR signals were determined by two- (15.5 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
dimensional NMR experiments (COSY, HSQC, HMBC). The (119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
relative configuration of diastereoisomers was corroborated (5 mL, c = 20 mM) (t = 2 h). Purification by column chromato-
by NOESY. Melting points were determined using a Kofler graphy (P/Et2O = 19/1) yielded 2e (20.5 mg, 85.7 μmol, 86%) as
melting point apparatus (“Thermopan”, Reichert), with a a yellow coloured oil. The analytical data obtained matched
range quoted to the nearest whole number. Mass spectro- those reported in the literature.13
metry (MS) was performed on a GC-coupled Agilent system
1-Fluoro-4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene
(EI, 70 eV). High resolution mass spectrometry (HRMS) was (2f). The reaction was performed in analogy to the representa-
performed on a Thermo Scientific LTQ FT Ultra (ESI) or a tive procedure for conditions C (see above) with nitroethene 1f
Thermo Scientific DFS HRMS spectrometer (EI). UV/Vis (16.7 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
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(119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane 312.0594; found: 312.0593, calcd for C14H1981BrNO2 [M + H]+:
(5 mL, c = 20 mM) (t = 4 h). Purification by column chromato- 314.0573; found: 314.0573.
+
graphy (P/Et2O = 15/1) yielded 2f (12.6 mg, 50.1 μmol, 50%) as
Methyl 4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzoate
a colourless solid. Rf = 0.53 (P/Et2O = 9/1); IR: ˜ν (cm−1) = 3079, (2i). The reaction was performed in analogy to the representa-
2967, 2871, 1538, 1510, 1460, 1371, 1226, 1151, 1135, 844, 761; tive procedure for conditions C (see above) with nitroethene 1i
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.09–7.00 (m, 4H, Har), (20.7 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
3
3
4.85 (d, J = 10.1 Hz, 1H, H-4′), 3.92 (d, J = 10.1 Hz, 1H, H-1′), (119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
1.24 (s, 3H, CH3-2′), 1.16 (s, 3H, CH3-3′), 1.14 (s, 3H, CH3-2′), (5 mL, c = 20 mM) (t = 5 h). Purification by column chromato-
0.70 (s, 3H, CH3-3′); 13C NMR (101 MHz, CDCl3): δ (ppm) = graphy (P/Et2O = 9/1) yielded 2i (9.47 mg, 32.5 μmol, 33%) and
1
3
162.1 (d, JCF = 254.4 Hz, C-1), 132.2 (s, C-4), 128.5 (d, JCF
=
a by-product (see ESI,† 1.23 mg, 4.22 μmol, 4%) as a mixture
2
7.9 Hz, C-3, C-5), 115.6 (d, JCF = 21.4 Hz, C-2, C–6), 85.2 (colourless solid). Rf = 0.64 (P/Et2O = 1/1); mp: 112 °C; IR:
(d, C-4′), 48.9 (d, C-1′), 45.0 (s, C-3′), 39.3 (s, C-2′), 24.2 [q, (C-3′) ˜ν (cm−1) = 2954, 1717, 1541, 1433, 1371, 1277, 1181, 1151,
CH3], 22.8 [q, (C-2′)CH3], 21.4 [q, (C-3′)CH3], 19.4 [q, (C-2′) 1138, 1110, 1017, 860, 756; 1H NMR (400 MHz, CDCl3):
CH3]; MS (EI): m/z (%) = 205 (45) [M − NO2]+, 163 (100) δ (ppm) = 8.00 (d, J = 7.3 Hz, 2H, H-2, H-6), 7.18 (d, J = 7.3
3
3
[M − NO2 − C3H6]+, 106 (54) [C7H6F]+; HRMS (EI, 70 eV): calcd Hz, 2H, H-3, H-5), 4.92 (d, J = 9.9 Hz, 1H, H-4′), 4.01 (d, J =
3
3
for C14H18FNO2+ [M]+: 251.1316; found: 251.1316.
9.9 Hz, 1H, H-1′), 1.24 (s, 3H, CH3-3′), 1.20 (s, 3H, CH3-2′), 1.16
1-Chloro-4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene (s, 3H, CH3-3′), 0.69 (s, 3H, CH3-2′); 13C NMR (101 MHz,
(2g). The reaction was performed in analogy to the representa- CDCl3): δ (ppm) = 167.0 (s, CH3CO2Ar), 141.8 (s, C-1), 130.0
tive procedure for conditions C (see above) with nitroethene 1g (d, 2C, C-2, C-6), 129.1 (s, C-4), 127.0 (d, 2C, C-3, C-5), 84.6
(18.3 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene (d, C-4′), 53.2 (q, CH3CO2Ar) 49.5 (d, C-1′), 45.1 (s, C-2′), 39.7
(119 μL, 84.1 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane (s, C-3′), 24.3 [q, (C-3′)CH3], 22.7 [q, (C-2′)CH3], 21.5 [q, (C-2′)
(5 mL, c = 20 mM) (t = 4 h). Purification by column chromato- CH3], 19.5 [q, (C-3′)CH3]; MS (EI): m/z (%) = 245 (92)
graphy (P/Et2O = 19/1) yielded 2g (14.4 mg, 53.8 μmol, 54%) as [M − NO2]+, 203 (39) [M − NO2 − C3H6]+, 171 (51), 159 (34),
a pale yellow coloured solid. Rf = 0.54 (P/Et2O = 9/1); IR: 84 (100) [C6H12]+, 69 (35); HRMS (ESI): calcd for C16H22NO4
+
˜ν (cm−1) = 3073, 2972, 2958, 1539, 1494, 1370, 1153, 1135, [M + H]+ 292.1543; found: 292.1544.
1089, 877, 839, 767; 1H NMR (400 MHz, CDCl3): δ (ppm) =
1-Chloro-3-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene
7.33–7.29 (m, 2H, H-2, H-6), 7.06–7.02 (m, 2H, H-3, H-5), 4.85 (2j). The reaction was performed in analogy to the representa-
3
3
(d, J = 10.0 Hz, 1H, H-4′), 3.92 (d, J = 10.0 Hz, 1H, H-1′), 1.24 tive procedure for conditions B (see above) with nitroethene 1j
(s, 3H, CH3-2′), 1.17 (s, 3H, CH3-3′), 1.15 (s, 3H, CH3-2′), 0.70 (18.4 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
(s, 3H, CH3-3′); 13C NMR (101 MHz, CDCl3): δ (ppm) = 134.9 (s, (119 μL, 84.2 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
C-4), 133.0 (s, C-1), 128.9 (d, 2C, C-2, C-6), 128.4 (d, 2C, C-3, (5 mL, c = 20 mM) (t = 7 h). Purification by column chromato-
C-5), 84.9 (d, C-4′), 49.0 (d, C-1′), 45.1 (s, C-3′), 39.4 (s, C-2′), graphy (P/Et2O = 19/1) yielded 2j (10.3 mg, 38.5 μmol, 38%) as
24.3 [q, (C-3′)CH3], 22.8 [q, (C-2′)CH3], 21.5 [q, (C-3′)CH3], a colourless oil. Starting material was recovered as trans-
19.5 [q, (C-2′)CH3]; MS (EI): m/z (%) = 221 (29) [M − NO2]+, 179 isomer trans-1j (5.00 mg, 27.2 μmol, 27%). Rf = 0.53 (P/Et2O =
(100) [M − NO2 − C3H6]+, 125 (64) [C7H6Cl]+; HRMS (EI, 70 eV): 9/1); IR: ˜ν (cm−1) = 3066, 2967, 1598, 1538, 1371, 1151, 1138,
calcd for C14H18ClNO2+ [M]+: 267.1021; found: 267.1021.
1-Bromo-4-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene 7.29–7.22 (m, 2H, Har), 7.09–7.08 (m, 1H, Har), 6.99 (dtd, J =
(2h). Representative procedure (conditions B): A solution of 7.1 Hz, J = 1.8 Hz, 0.8 Hz, 1H, Har), 4.86 (d, J = 10.0 Hz, 1H,
nitroethene 1h (22.8 mg, 100 μmol, 1.00 equiv.) and 2,3- H-4′), 3.93 (d, J = 10.0 Hz, 1H, H-1′), 1.24 (s, 3H, CH3-3′), 1.18
1081, 877, 814, 772; 1H NMR (400 MHz, CDCl3): δ (ppm) =
3
4
3
3
dimethyl-2-butene (119 μL, 84.2 mg, 1.00 mmol, 10.0 equiv.) in (s, 3H, CH3-2′), 1.14 (s, 3H, CH3-3′), 0.72 (s, 3H, CH3-2′);
dichloromethane (5 mL, c = 20 mM) was irradiated at λmax
=
13C NMR (101 MHz, CDCl3): δ (ppm) = 138.6 (s, C-3), 134.7 (s,
424 nm for six hours at room temperature. The crude product C-1), 129.9 (d, CarH), 127.3 (d, CarH), 127.2 (d, CarH), 125.2
was purified by column chromatography (P/Et2O = 20/1) to (d, CarH), 84.7 (d, C-4′), 49.2 (d, C-1′), 45.0 (s, C-3′), 39.5
yield 2h (18.0 mg, 56.0 μmol, 58%) as a yellow coloured oil. (s, C-2′), 24.3 [q, (C-2′)CH3], 22.7 [q, (C-3′)CH3], 21.5 [q, (C-2′)
Rf = 0.43 (P/Et2O = 9/1); IR: ˜ν (cm−1) = 2962, 2925, 1552, 1464, CH3], 19.4 [q, (C-3′)CH3]; MS (EI): m/z (%) = 221 (32) [M −
1
1376, 1259, 1072, 1010, 861, 797; H NMR (500 MHz, CDCl3): NO2]+, 179 (100) [M − NO2 − C3H6]+, 125 (32) [C7H6Cl]+; HRMS
δ [ppm] = 7.48–7.44 (m, 2H, H-2, H-6), 7.00–6.96 (m, 2H, H-3, (ESI): calcd for C14H19ClNO2 [M + H]+: 268.1099; found:
+
3
3
H-5), 4.85 (d, J = 10.1 Hz, 1H, H-4′), 3.90 (d, J = 10.1 Hz, 1H, 268.1098.
H-1′), 1.23 (s, 3H, CH3-3′), 1.17 (s, 3H, CH3-2′), 1.14 (s, 3H,
3-(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-benzonitrile (2k).
CH3-3′), 0.70 (s, 3H, CH3-2′); 13C NMR (101 MHz, CDCl3): The reaction was performed in analogy to the representative
δ (ppm) = 135.5 (s, C-1), 131.8 (d, 2C, C-2, C-6), 128.7 (d, 2C, procedure for conditions A (see above) with nitroethene 1k
C-3, C-5), 121.1 (s, C-4), 84.8 (d, C-4′), 49.1 (d, C-1′), 45.1 (s, (34.8 mg, 200 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
C-3′), 39.4 (s, C-2′), 24.3 [q, (C-3′)CH3], 22.8 [q, (C-2′)CH3], 21.5 (237 μL, 168 mg, 2.00 mmol, 10.0 equiv.) in dichloromethane
[q, (C-3′)CH3], 19.5 [q, (C-2′)CH3]; MS (EI): m/z (%) = 265 (20) (10 mL, c = 20 mM) (t = 7 h). Purification by column chromato-
[M − NO2]+, 168 (100) [M − NO2 − Br]+, 143 (56) [M − NO2 − graphy (P/Et2O = 19/1) yielded 2k (18.2 mg, 70.5 μmol, 35%)
Br − C3H6]+; HRMS (ESI): calcd for C14H1979BrNO2 [M + H]+: and the by-product 6 (2.32 mg, 8.99 μmol, 11%) as a mixture
+
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(colourless liquid). 2k: Rf = 0.53 (P/Et2O = 9/1); IR: ˜ν (cm−1) = procedure for conditions C (see above) with nitroethene 1m
3079, 2960, 2231, 1533, 1372, 1151, 1136, 793; 1H NMR (15.0 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
(400 MHz, CDCl3): δ (ppm) = 7.59–7.55 (m, 1H, Har), 7.46 (td, (119 μL, 84.2 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
3J = 7.7 Hz, 4J = 0.6 Hz, 1H, Har), 7.41–7.39 (m, 1H, Har), (5 mL, c = 20 mM) (t = 24 h). Purification by column chromato-
3
7.36–7.33 (m, 1H, Har), 4.87 (d, J = 10.0 Hz, 1H, H-4′), 3.97 (d, graphy (CH2Cl2/MeOH = 40/1) yielded 2m (15.5 mg, 66.2 μmol,
3J = 10.0 Hz, 1H, H-1′), 1.25 (s, 3H, CH3-3′), 1.20 (s, 3H, 66%) as a yellow coloured oil. Rf = 0.25 (CH2Cl2/MeOH = 19/1);
CH3-2′), 1.16 (s, 3H, CH3-3′), 0.71 (s, 3H, CH3-2′); 13C NMR IR: ˜ν (cm−1) = 3445, 3008, 2930, 1597, 1448, 1386, 1225, 1052,
1
(101 MHz, CDCl3): δ (ppm) = 138.2 (s, C-3), 131.5 (d, CarH), 1025, 992, 760; H NMR (500 MHz, CDCl3): δ (ppm) 8.59–8.37
3
4
130.9 (d, CarH), 130.6 (d, CarH), 129.6 (d, CarH), 118.7 (s, (m, 2H, H-5, H-6), 7.43 (dt, J = 7.9 Hz, J = 2.0 Hz, 1H, H-3),
CarCN), 113.0 (s, C-1), 84.4 (d, C-4′), 49.1 (d, C-1′), 45.2 (s, C-3′), 7.31–7.27 (m, 1H, H-4), 4.90 (d, 3J = 10.0 Hz, 1H, H-4′), 3.96
39.6 (s, C-2′), 24.3 [q, (C-2′)CH3], 22.7 [q, (C-3′)CH3], 21.5 (d, 3J = 10.0 Hz, 1H, H-1′), 1.25 (s, 3H, CH3-2′), 1.19 (s, 3H,
[q, (C-2′)CH3], 19.4 [q, (C-3′)CH3]; MS (EI): m/z (%) = 212 (32) CH3-3′), 1.16 (s, 3H, CH3-2′), 0.73 (s, 3H, CH3-3′); 13C NMR
[M − NO2]+, 170 (100) [M − NO2 − C3H6]+, 116 (20) [C8H6N]+; (126 MHz, CDCl3): δ (ppm) = 148.7 (d, C-5/C-6)*, 148.6 (d, C-5/
HRMS (ESI): calcd for C15H19N2O2 [M + H]+: 259.1440; found: C-6)*, 134.5 (d, C-3), 131.9 (s, C-2), 123.4 (d, C-4), 84.2 (d, C-4′),
+
259.1442. 6: Rf = 0.53 (P/Et2O = 9/1); IR: ˜ν (cm−1) = 2923, 2231, 47.4 (d, C-1′), 45.3 (s, C-3′), 39.4 (s, C-2′), 24.2 [q, (C-2′)CH3],
1549, 1365, 1148, 905, 798, 691; 1H NMR (500 MHz, CDCl3): 22.7 [q, (C-3′)CH3], 21.6 [q, (C-2′)CH3], 19.3 [q, (C-3′)CH3] (* the
δ (ppm) = 7.54 (dd, 3J = 7.6 Hz, 4J = 1.6 Hz, 1H, H-4)*, assignments are interconvertible); MS (EI): m/z (%) = 188 (100)
7.41–7.38 (m, 2H, H-2, H-5), 7.35 (d, 3J = 8.0 Hz, 1H, H-6)*, [M − NO2]+, 146 (72) [M − NO2 − C3H6]+, 132 (40) [C9H10N]+;
5.04 (s, 1H, CHH-5′), 5.00 (s, 1H, CHH-5′), 4.76 (dd, 3J = HRMS (ESI): calcd for C13H19N2O2 [M + H]+: 235.1441; found:
+
2
3
12.0 Hz, 2.2 Hz, 1H, H-2′), 3.32 (dd, J = 15.0, J = 11.9 Hz, 1H, 235.1441.
2
3
CHH-1′), 2.94 (dd, J = 15.0, J = 2.2 Hz, 1H, CHH-1′), 1.87 (d,
2-(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-naphthalene (2n).
4J = 1.4 Hz, 3H, CH3-4′), 1.29 (s, 3H, CH3-3′), 1.23 (s, 3H, The reaction was performed in analogy to the representative
CH3-3′) (* the assignments are interconvertible); 13C NMR procedure for conditions C (see above) with nitroethene 1n
(126 MHz, CDCl3): δ (ppm) = 147.3 (s, C-4′), 138.3 (s, C-3), (19.9 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene
133.3 (d, C-6)*, 132.4 (d, C-2), 131.2 (d, C-4)*, 129.9 (d, C-5), (119 μL, 84.2 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
118.6 (s, CN), 114.5 (t, C-5′), 113.2 (s, C-1), 96.0 (d, C-2′), 43.0 (5 mL, c = 20 mM) (t = 3 h). Purification by column chromato-
(s, C-3′), 34.4 (t, C-1′), 24.7 [q, (C-3′)CH3], 21.6 [q, (C-3′)CH3], graphy (P/Et2O = 19/1) yielded 2n (22.4 mg, 79.1 μmol, 79%) as
19.6 [q, (C-4′)CH3] (* the assignments are interconvertible); MS a yellow coloured oil. Rf = 0.56 (P/Et2O = 9/1); IR: ˜ν (cm−1) =
(EI): m/z (%) = 196 (15) [C14H14N]+, 170 (58) [C12H12N]+, 116 2954, 1537, 1459, 1367, 1139, 858, 810, 755; 1H NMR
(100) [C8H6N]+; HRMS (ESI): calcd for C15H19N2O2 [M + H]+: (500 MHz, CDCl3): δ (ppm) = 7.85–7.79 (m, 3H, Har), 7.54–7.52
+
259.1441; found: 259.1439.
(br. s., 1H, Har), 7.51–7.43 (m, 2H, Har), 7.26–7.23 (m, 1H, Har),
3
3
1-Chloro-2-(2′,2′,3′,3′-tetramethyl-4′-nitrocyclobutyl)-benzene 5.06 (d, J = 10.1 Hz, 1H, H-4′), 4.14 (d, J = 10.1 Hz, 1H, H-1′),
(2l). The reaction was performed in analogy to the representa- 1.28 (s, 3H, CH3-2′), 1.26 (s, 3H, CH3-3′), 1.20 (s, 3H, CH3-2′),
tive procedure for conditions C (see above) with nitroethene 1l 0.73 (s, 3H, CH3-3′); 13C NMR (101 MHz, CDCl3): δ (ppm) =
(18.4 mg, 100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene 134.2 (s, C-2), 133.5 (s, C-5/C-10)*, 132.6 (s, C-5/C-10)*, 128.4
(119 μL, 84.2 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane (d, CarH), 127.8 (d, CarH), 126.5 (d, CarH), 125.9 (d, CarH),
(5 mL, c = 20 mM) (t = 7 h). Purification by column chromato- 125.7 (d, CarH), 125.1 (d, CarH), 85.0 (d, C-4′), 49.6 (d, C-1′),
graphy (P/Et2O = 19/1) yielded 2l (7.46 mg, 27.9 μmol, 28%) as 45.1 (s, C-3′), 39.5 (s, C-2′), 24.4 [q, (C-2′)CH3], 22.8 [q, (C-3′)
a colourless oil. Starting material was recovered as cis-isomer CH3], 21.6 [q, (C-2′)CH3], 19.5 [q, (C-3′)CH3] (* the assignments
cis-1l (7.94 mg, 43.2 μmol, 43%). Rf = 0.54 (P/Et2O = 9/1); IR: are interconvertible); MS (EI): m/z (%) = 237 (28) [M − NO2]+,
˜ν (cm−1) = 3061, 2973, 2960, 1541, 1372, 1153, 1135, 1033, 876, 181 (100) [M − NO2 − C4H8]+, 127 (10) [C10H7]+; HRMS (ESI):
807, 754; H NMR (400 MHz, CDCl3): δ (ppm) = 7.41 (dd, J = calcd for C18H22NO2+ [M + H]+: 284.1645; found: 284.1646.
1
3
4
7.6 Hz, J = 1.6 Hz, 1H, H-6), 7.28–7.18 (m, 2H, H-5, H-4), 7.16
2-Nitro-1-phenylspiro[3.5]nonane (3a). The reaction was per-
(dd, 3J = 7.5 Hz, 4J = 1.9 Hz, 1H, H-3), 4.99 (d, 3J = 10.2 Hz, 1H, formed in analogy to the representative procedure for con-
3
H-4′), 4.43 (d, J = 10.2 Hz, 1H, H-1′), 1.26 (s, 3H, CH3-2′), 1.24 ditions C (see above) with nitroethene 1a (14.9 mg, 100 μmol,
(s, 3H, CH3-3′), 1.19 (s, 3H, CH3-3′), 0.73 (s, 3H, CH3-2′); 1.00 equiv.) and methylenecyclohexane (136 μL, 96.1 mg,
13C NMR (101 MHz, CDCl3): δ (ppm) = 134.7 (s, C-2), 134.1 1.00 mmol, 10.0 equiv.) in dichloromethane (5 mL, c = 20 mM)
(s, C-1), 130.3 (d, C-6), 128.4 (d, C-5/C-4)*, 128.1 (d, C-3), 126.8 (t = 14 h). The crude product was purified by column chrom-
(d, C-5/C-4)*, 84.4 (d, C-4′), 46.8 (d, C-1′), 44.5 (s, C-2′), 40.5 atography (P/Et2O = 20/1) to yield 3a (15.0 mg, 61.9 μmol, 61%)
(s, C-3′), 24.9 [q, (C-3′)CH3], 22.7 [q, (C-2′)CH3], 21.8 [q, (C-2′) as a colourless oil. Starting material 1a was recovered as a
CH3], 19.4 [q, (C-3′)CH3] (* the assignments are interconverti- mixture of isomers (6.64 mg, 42.9 μmol, 43%, cis/trans =
ble); MS (EI): m/z (%) = 221 (32) [M − NO2]+, 179 (100) 12 : 88). The analytical data obtained matched those reported
[M − NO2 − C3H6]+, 125 (28) [C7H6Cl]+; HRMS (ESI): calcd for in the literature.13
C14H19ClNO2+ [M + H]+: 268.1099; found: 268.1094.
6-Nitro-7-phenyl-2-oxabicyclo[3.2.0]heptane (3b). The reac-
2-(2′,2′,3′,3′-Tetramethyl-4′-nitrocyclobutyl)-pyridine
(2m). tion was performed in analogy to the representative procedure
The reaction was performed in analogy to the representative for conditions B (see above) with nitroethene 1a (14.9 mg,
Org. Biomol. Chem.
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100 μmol, 1.00 equiv.) and 2,3-dihydrofuran (75.5 μL, 70.0 mg, [M − NO2]+, 118 (100) [C8H6O]+, 90 (8); HRMS (ESI): calcd for
1.00 mmol, 10.0 equiv.) in dichloromethane (5 mL, c = 20 mM) C16H14NO3+ [M + H]+: 268.0968; found: 268.0970.
(t = 6 h). The crude product was purified by column chromato-
7-Nitro-8-phenyl-2,5-dioxabicyclo[4.2.0]octane (3e). The reac-
graphy (P/Et2O = 9/1 → 4/1) to yield 3b (8.10 mg, 36.9 μmol, tion was performed in analogy to the representative procedure
37%) as a yellow coloured oil. Starting material was recovered for conditions A (see above) with nitroethene 1a (29.8 mg,
as cis-isomer cis-1a (2.00 mg, 13.4 μmol, 13%). The analytical 200 μmol, 1.00 equiv.) and 2,3-dihydro-1,4-dioxin (159 μL,
data obtained matched those reported in the literature.13
172 mg, 2.00 mmol, 10.0 equiv.) in dichloromethane (10 mL,
(2′,2′-Diethyl-4′-nitrocyclobutyl)-benzene (3c). The reaction c = 20 mM) (t = 14 h). Purification by column chromatography
was performed in analogy to the representative procedure for (P/Et2O = 9/1 → 4/1) yielded 3e (34.0 mg, 145 μmol, 72%, dr =
conditions A (see above) with nitroethene 1a (29.8 mg, 52 : 19 : 29) as an orange coloured oil. Starting material 1a was
200 μmol, 1.00 equiv.) and 1,1-diethylethylene (244 μL, recovered as a mixture of isomers (4.50 mg, 30.2 μmol, 15%,
168 mg, 2.00 mmol, 10.0 equiv.) in dichloromethane (10 mL, cis/trans = 55 : 45). NMR data are given for the major diastereo-
c = 20 mM) (t = 24 h). The crude product was purified by isomer depicted in Scheme 2. Rf = 0.06 (P/Et2O = 4/1); IR:
column chromatography (P/Et2O = 30/1) to yield 3c (20.7 mg, ˜ν (cm−1) = 3031, 2923, 1545, 1375, 1132, 1043, 874, 751; 1H NMR
88.7 mmol, 44%) as a yellow coloured oil. Starting material 1a (400 MHz, C6D6): δ (ppm) = 7.02–6.93 (m, 5H, Har), 4.26 (virt. t,
was recovered as a mixture of isomers (4.70 mg, 31.5 μmol, 3J ≅ 3J = 7.8 Hz, 1H, H-7), 3.81 (virt. t, 3J ≅ 3J = 8.6 Hz, 1H, H-6),
16%, cis/trans = 44 : 56). Rf = 0.45 (P/Et2O = 19/1); IR: ˜ν (cm−1) = 3.56 (dd, 3J = 9.8 Hz, 7.3 Hz, 1H, H-8), 3.40–3.28 (m, 2H,
3063, 3030, 2965, 1542, 1455, 1366, 784; 1H NMR (500 MHz, CHH-3, CHH-4), 3.19–3.15 (m, 2H, CHH-3, CHH-4), 2.99 (d, 3J =
3
CDCl3): δ (ppm) = 7.34 (t, J = 7.5 Hz, 2H, meta-Har), 7.29–7.24 9.8 Hz, 1H, H-1); 13C NMR (101 MHz, C6D6): δ (ppm) = 136.5
(m, 1H, para-Har), 7.23–7.20 (m, 2H, ortho-Har), 5.27 (virt. q, (s, Car), 129.0 (d, 2C, ortho-CarH)*, 127.9 (d, 2C, meta-CarH)*,
3
3
3J ≅ J = 8.7 Hz, 1H, H-4′), 3.98 (d, J = 9.1 Hz, 1H, H-1′), 2.40 126.9 (d, para-CarH), 82.3 (d, C-7), 77.9 (d, C-6), 75.2 (d, C-1),
(dd, 2J = 11.9 Hz, 3J = 8.5 Hz, 1H, CHH-3), 2.30 (dd, 2J = 68.3 (t, C-3), 68.1 (t, C-4), 51.2 (d, C-8) (* the assignments are
11.9 Hz, J = 8.6 Hz, 1H, CHH-3), 1.76 (dq, J = 14.8 Hz, J = interconvertible); MS (EI): m/z (%) = 235 (16) [M]+, 189 (52) [M
3
2
3
7.5 Hz, 1H, CHHCH3), 1.64 (dq, J = 14.8 Hz, J = 7.5 Hz, 1H, − NO2]+, 117 (72) [C9H9]+, 91 (100) [C7H7]+; HRMS (ESI): calcd
2
3
CHHCH3), 1.30–1.19 (m, 2H, CH2CH3), 0.96 (t, J = 7.5 Hz, 3H, for C12H14NO4+ [M + H]+: 236.0917; found: 236.0918.
3
CH2CH3), 0.60 (t, 3J = 7.4 Hz, 3H, CH2CH3); 13C NMR
(1′-Cyclopropyl-3′-nitrocyclobutane-1′,2′-diyl)-dibenzene (3f).
(126 MHz, CDCl3): δ (ppm) = 136.6 (s, Car), 128.6 (d, 2C, meta- The reaction was performed in analogy to the representative
CarH), 127.5 (d, 2C, ortho-CarH), 127.2 (d, para-CarH), 76.6 procedure for conditions A (see above) with nitroethene 1a
(d, C-4′), 53.8 (d, C-1′), 41.8 (s, C-2′), 33.7 (t, C-3′), 31.7 (14.9 mg, 100 μmol, 1.00 equiv.) and (1-cyclopropylvinyl)-
(t, CH2CH3), 26.4 (t, CH2CH3), 8.62 (q, CH2CH3), 7.98 benzene (144 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane
(q, CH2CH3); MS (EI): m/z (%) = 187 (4) [M − NO2]+, 157 (12) (5 mL, c = 20 mM) (t = 16 h). Purification by column chromato-
[M − NO2 − C2H6]+, 117 (100) [C9H9]+; HRMS (ESI): calcd for graphy (P/Et2O = 20/1) yielded 3f (25.6 mg, 86.6 μmol, 88%,
C14H20NO2+ [M + H]+: 234.1488; found: 234.1489.
dr = 67 : 33) as a pale-yellow coloured oil. NMR data are given
1-Nitro-2-phenyl-1,2,2a,7b-tetrahydrocyclobuta[b]benzofuran for the major diastereoisomer depicted in Scheme 2. Rf = 0.69
(3d). The reaction was performed in analogy to the representa- (P/Et2O = 9/1); IR: ˜ν (cm−1) = 3028, 1542, 1496, 1368, 1028, 824,
1
tive procedure for conditions B (see above) with nitroethene 1a 770; H NMR (500 MHz, CDCl3): δ (ppm) = 7.17–7.13 (m, 6H,
(14.9 mg, 100 μmol, 1.00 equiv.) and benzofuran (108 μL, Har), 7.00–6.98 (m, 2H, Har), 6.83–6.77 (m, 2H, Har), 5.13 (virt.
118 mg, 1.00 mmol, 10.0 equiv.) in dichloromethane (5 mL, c = q, 3J ≅ 3J = 9.0 Hz, 1H, H-3′), 4.07 (d, 3J = 9.5 Hz, 1H, H-4′), 3.01
2
3
2
20 mM) (t = 36 h). Purification by column chromatography (P/ (dd, J = 12.3 Hz, J = 8.1 Hz, 1H, CHH-2′), 2.60 (dd, J = 12.4
3
3
Et2O = 19/1) yielded 3d (12.8 mg, 47.9 μmol, 48%) as a yellow Hz, J = 9.2 Hz, 1H, CHH-2′), 1.43 [tt, J = 8.3 Hz, 5.6 Hz, 1H,
2
3
coloured oil. Starting material was recovered as cis-isomer cis- CH(CH2)2], 0.77–0.66 [m, 2H, CH(CH2)2], 0.57 [virt. tt, J ≅ J ≅
1a (5.40 mg, 36.2 μmol, 36%). Rf = 0.53 (P/Et2O = 9/1); IR: 3J = 8.6 Hz, J = 5.5 Hz, 1H, CH(CH2)2], 0.44 [virt. dq, J = 9.0
3
2
˜ν (cm−1) = 3062, 3032, 2923, 1543, 1474, 1368, 1218, 1095, 1051, Hz, 3J ≅ 3J = 5.5 Hz, 1H, CH(CH2)2]; 13C NMR (101 MHz,
1
1019, 814; H NMR (500 MHz, C6D6): δ (ppm) = 6.98–6.96 (m CDCl3): δ (ppm) = 141.1 (s, C-1a), 136.2 (s, C-4a), 128.4 (d, 2C,
3
3H, meta-Har, para-Har), 6.84 (t, J = 7.5 Hz, 1H, H-5), 6.79 (d, CarH), 128.3 (d, 2C, CarH), 128.0 (d, 2C, CarH), 127.8 (d, CarH),
3J = 7.5 Hz, 1H, H-4), 6.61–6.56 (m, 2H, ortho-Har), 6.45 (t, J = 127.5 (d, CarH), 126.8 (d, 2C, CarH), 76.9 (d, C–3′), 55.1 (d,
3
3
3
7.5 Hz, 1H, H-6), 6.29 (d, J = 7.5 Hz, 1H, H-7), 5.13 (dd, J = C-4′), 47.3 (s, C-1′), 32.1 (t, C-2′), 22.6 [d, CH(CH2)2], 3.21 [t, CH
7.4 Hz, 4.2 Hz, 1H, H-2a), 4.97 (ddd, 3J = 9.4 Hz, 4.2 Hz, 4J = 1.5 (CH2)2], 2.11 [t, CH(CH2)2]; MS (EI): m/z (%) = 247 (4) [M −
Hz, 1H, H-1), 3.72 (virt. t, 3J ≅ 3J = 9.3 Hz, 1H, H-2), 3.50 (virt. t, NO2]+, 205 (32) [M − NO2 − C3H6]+, 117 (100) [C9H9]+; HRMS
3J ≅ J = 8.4 Hz, 1H, H-7b); 13C NMR (101 MHz, C6D6): δ (ppm) (ESI): calcd for C19H20NO2 [M + H]+: 294.1488; found:
3
+
= 160.9 (s, C-3a), 135.4 (s, C-2′), 129.6 (d, C-5), 128.6 (d, meta- 294.1488.
CarH/para-CarH)*, 128.3 (d, C-7), 127.9 (d, meta-CarH/para-CarH)
[1-(tert-Butyl)-3-nitro-2-phenylcyclobutoxy]trimethylsilane
*, 127.6 (d, 2C, ortho-CarH)*, 124.9 (s, C-7a), 121.6 (d, C-6), 111.5 (3g). The reaction was performed in analogy to the representa-
(d, C-4), 85.6 (d, C-1), 80.6 (d, C-2a), 45.6 (d, C-2), 44.6 (d, C-7b) tive procedure for conditions B (see above) with nitroethene 1a
(* the exact assignment was not possible due to significant (14.9 mg, 100 μmol, 1.00 equiv.) and [(3,3-dimethylbut-1-en-2-
overlap with the solvent C6D6); MS (EI): m/z (%) = 221 (12) yl)oxy]trimethylsilane (216 μL, 172 mg, 1.00 mmol, 10.0 equiv.)
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in dichloromethane (5 mL, c = 20 mM) (t = 24 h). Purification CHH-1′), 2.46 (ddd, 2J = 14.0 Hz, 3J = 10.6 Hz, 6.0 Hz, 1H, CHH-
by column chromatography (P/Et2O 50/1) yielded 3g 1′), 2.19–2.03 (m, 2H, H-3′), 1.82–1.72 (m, 1H, CHH-2′),
=
(12.3 mg, 38.3 μmol, 38%, dr = 77 : 23) as a yellow coloured oil. 1.70–1.58 (m, 1H, CHH-2′), 1.18 (s, 3H, CH3-3″), 1.10 (s, 3H,
NMR data are given for the major diastereoisomer depicted in CH3-3″), 1.09 (s, 3H, CH3-2″), 0.67 (s, 3H, CH3-2″); 13C NMR
Scheme 2. Rf = 0.70 (P/Et2O = 19/1); IR: ˜ν (cm−1) = 3063, 3031, (126 MHz, CDCl3): δ (ppm) = 141.7 (s, C-1), 138.6 (d, C-4′),
2958, 1546, 1480, 1395, 1368, 1252, 1146, 1029, 870, 833; 1H 133.2 (s, C-2), 129.8 (d, C-6), 127.1 (d, C-5)*, 127.0 (d, C-3),
NMR (500 MHz, CDCl3): δ (ppm) = 7.33–7.31 (m, 5H, Har), 5.13 125.9 (d, C-4)*, 115.3 (t, C-5′), 85.3 (d, C-4″), 45.6 (d, C-1″), 44.4
(virt. q, 3J ≅ 3J = 8.5 Hz, 1H, H-3), 4.24 (d, 3J = 8.6 Hz, 1H, H-2), (s, C-2″), 40.1 (s, C-3″), 33.8 (t, C-3′), 32.4 (t, C-1′), 30.6 (t, C-2′),
2
3
2
2.93 (dd, J = 13.2 Hz, J = 8.2 Hz, 1H, CHH-4), 2.50 (dd, J = 24.6 [q, (C-2″)CH3], 22.8 [q, (C-3″)CH3], 21.8 [q, (C-2″)CH3],
3
13.2 Hz, J = 8.6 Hz, 1H, CHH-4), 0.97 [s, 9H, C(CH3)3], 0.12 [s, 19.4 [q, (C-3″)CH3] (* the assignments are interconvertible);
9H, OSi(CH3)3]; 13C NMR (126 MHz, CDCl3): δ (ppm) = 136.4 MS (EI): m/z (%) = 301 (2) [M]+, 255 (20) [M − NO2]+, 199 (49)
(s, Car), 129.0 (d, 2C, ortho-CarH), 128.1 (d, 2C, meta-CarH), [M − NO2 − C3H6]+, 143 (100) [C11H11]+; HRMS (ESI): calcd for
127.3 (d, para-CarH), 83.3 (s, C-1), 78.2 (d, C-3), 52.2 (d, C-2), C19H27NO2+ [M + H]+: 302.2115; found: 302.2115.
37.9 [s, C(CH3)3], 34.3 (t, C-4), 25.8 [q, 3C, C(CH3)3], 2.6 [q, 3C,
2,2,3,3-Tetramethyl-4-phenylcyclobutan-1-amine
(4).
OSi(CH3)3]; MS (EI): m/z (%) = 275 (17) [M − NO2]+, 219 (18) According to a literature known procedure:22 Zn powder
[M − NO2 − C(CH3)3]+, 117 (100) [C9H9]+, 73 (36) [Si(CH3)3]+; (350 mg, 5.36 mmol, 25.0 equiv.) was added in small portions
HRMS (ESI): calcd for C17H28NO3Si+ [M + H]+: 322.1833; to
found: 322.1833.
a
stirred solution of nitrocyclobutane 2a (50.0 mg,
214 μmol, 1.00 equiv.) in a mixture of water/acetic acid (2 mL;
Trimethyl[(6-nitro-7-phenylbicyclo[3.2.0]heptan-1-yl)oxy] 1/1 v/v). The suspension was stirred for four hours at room
silane (3h). The reaction was performed in analogy to the temperature. Aqueous NaOH solution (c = 5 M) was added
representative procedure for conditions C (see above) with until pH = 7 was reached. The solution was extracted with di-
nitroethene 1a (14.9 mg, 100 μmol, 1.00 equiv.) and (cyclo- chloromethane (2 × 50 mL). The combined organic layers were
pent-1-en-1-yloxy) trimethylsilane (156 mg, 1.00 mmol, 10.0 washed with saturated aqueous NaCl solution (100 mL), dried
equiv.) in dichloromethane (5 mL, c = 20 mM) (t = 24 h). over Na2SO4, filtered and concentrated in vacuo to yield 4
Purification by column chromatography (P/Et2O
=
40/1) (33.6 mg, 165 μmol, 77%) as a colourless oil. IR: ˜ν (cm−1) =
yielded 3h (11.7 mg, 38.3 μmol, 38%, dr = 61 : 39) as a colour- 3060, 2959, 2866, 2604, 1566, 1458, 1449, 1358, 1337, 1270,
less oil. NMR data are given for the major diastereoisomer 1132, 885, 810; 1H NMR (400 MHz, CDCl3): δ (ppm) =
depicted in Scheme 2. Rf = 0.70 (P/Et2O = 9/1); IR: ˜ν (cm−1) = 7.29–7.21 (m, 2H, meta-HPh), 7.19–7.08 (m, 3H, ortho-HPh
,
3
3
3004, 2926, 1542, 1497, 1364, 1264, 1045, 1018, 891, 822, 758; para-HPh), 3.37 (d, J = 9.9 Hz, 1H, H-1), 2.86 (d, J = 9.9 Hz,
1H NMR (500 MHz, CDCl3): δ (ppm) = 7.38–7.32 (m, 2H, meta- 1H, H-4), 1.51 (br. s, 2H, NH2), 1.03 (s, 3H, CH3-2), 1.01 (s, 3H,
3
Har), 7.29–7.25 (m, 3H, ortho-Har, para-Har), 5.35 (dd, J = 10.1 CH3-3), 0.93 (s, 3H, CH3-2), 0.59 (s, 3H, CH3-3); 13C NMR
Hz, 8.8 Hz, 1H, H-6), 4.06 (d, 3J = 8.8 Hz, 1H, H-7), 3.09 (virt. t, (101 MHz, CDCl3): δ (ppm) = 139.6 (s, CPh), 128.3 (d, 2C, meta-
3J ≅ 3J = 9.2 Hz, 1H, H-5), 2.00–1.91 (m, 4H, CH2-3, CHH-2,
CPhH), 127.7 (d, 2C, ortho-CPhH), 126.1 (d, para-CPhH), 56.8 (d,
CHH-4), 1.89–1.77 (m, 1H, CHH-2), 1.62–1.44 (m, 1H, CHH-4), C-4), 55.5 (d, C-1), 41.7 (s, C-2), 39.6 (s, C-3), 24.2 [q, (C-3)CH3],
−0.14 [s, 9H, OSi(CH3)3]; 13C NMR (126 MHz, CDCl3): δ (ppm) 22.6 [q, (C-2)CH3], 21.1 [q, (C-3)CH3], 18.7 [q, (C-2)CH3]; MS
= 136.5 (s, Car), 129.5 (d, 2C, ortho-CarH), 128.8/128.5 (d, 2C (EI, 70 eV): m/z (%) = 132 (5) [M − C4H9N]+, 119 (100) [M −
meta-CarH), 127.5/127.3 (d, para-CarH), 83.5 (s, C-1), 81.0 (d, C4H9N − CH3]+, 91 (13) [C7H7]+, 71 (31) [C4H9]+, 56 (11); HRMS
C-6), 51.9 (d, C-7), 49.9 (d, C-5), 40.1 (t, CH2-2), 26.1 (t, CH2-4), (ESI): calcd for C14H22N+ [M + H]+: 204.1741; found: 204.1748.
25.9 (t, CH2-3), 1.64 [q, 3C, OSi(CH3)3]; MS (EI): m/z (%) = 259
(2′,2′-Dicyclopropyl-4′-nitrocyclobutyl)-benzene
(3i)
and
(88) [M − NO2]+, 169 (60) [M − NO2 − OSi(CH3)3]+, 91 (28) 1-nitro-3-propylidene-2,3,3a,4,5,9b-hexahydro-1H-cyclopenta[a]
[C7H7]+, 73 (100) [Si(CH3)3]+; HRMS (ESI): calcd for naphthalene (7). A solution of nitroethene 1a (14.9 mg,
C16H24NO3Si+ [M + H]+: 306.1522; found: 306.1522.
100 μmol, 1.00 equiv.) and 1,1-dicyclopropyl-ethylene (109 mg,
1-(Pent-4′-en-1′-yl)-2-(2″,2″,3″,3″-tetramethyl-4″-nitrocyclo- 1.00 mmol, 10.0 equiv.) in dichloromethane (5 mL, c = 20 mM)
butyl)-benzene (2o). A solution of nitroethene 1o (21.7 mg, was irradiated at λmax = 419 nm for twelve hours at room temp-
100 μmol, 1.00 equiv.) and 2,3-dimethyl-2-butene (356 μL, erature. Purification by column chromatography (P/Et2O = 40/
252 mg, 3.00 mmol, 30.0 equiv.) in dichloromethane (5 mL, c = 1) yielded 3i (18.0 mg, 69.9 μmol, 70%) and 7 (2.20 mg,
20 mM) was irradiated at λmax = 419 nm for 18 hours at room 8.55 μmol, 9%) both as a yellow coloured oil. 3i: Rf = 0.58 (P/
temperature. Purification by column chromatography (P/Et2O Et2O = 19/1); IR: ˜ν (cm−1) = 3079, 3004, 1542, 1449, 1369, 1017,
1
= 4/1) yielded 2o (13.8 mg, 45.8 μmol, 46%) as a yellow 822, 758; H NMR (400 MHz, CDCl3): δ (ppm) = 7.44–7.20 (m,
3
3
3
coloured oil. Rf = 0.68 (P/Et2O = 9/1); IR: ˜ν (cm−1) = 3066, 2931, 5H, Har), 5.20 (virt. q, J ≅ J = 8.7 Hz, 1H, H-4′), 3.97 (d, J =
2870, 1542, 1460, 1371, 993, 912, 751; 1H NMR (500 MHz, 9.2 Hz, 1H, H-1′), 2.06 (dd, 2J = 12.2 Hz, 3J = 8.7 Hz, 1H,
3
3
CDCl3): δ (ppm) = 7.15–7.09 (m, 3H, H-3, H-4, H-5), 7.07–7.03 CHH-3′), 1.71 (dd, J = 12.2 Hz, J = 8.3 Hz, 1H, CHH-3′), 1.09
(m, 1H, H-6), 5.83 (ddt, 3J = 17.0 Hz, 10.2 Hz, 6.7 Hz, 1H, H-4′), [tt, 3J = 8.4 Hz, 5.5 Hz, 1H, CH(CH2)2], 0.60–0.25 [m, 6H,
5.02 (virt. dq, 3J = 17.2 Hz, 2J ≅ 4J = 1.7 Hz, 1H, CHH-5′), CH(CH2)2, CH(CH2)2], 0.20–0.13 [m, 1H, CH(CH2)2]; 13C NMR
4.98–4.92 (m, 2H, CHH-5′, H-4″), 4.15 (d, 3J = 10.2 Hz, 1H, (101 MHz, CDCl3): δ (ppm) = 136.4 (s, Car), 128.5 (d, 2C, meta-
H-1″), 2.76 (ddd, 2J = 14.0 Hz, 3J = 10.5 Hz, 5.3 Hz, 1H, CarH), 127.7 (d, 2C, ortho-CarH), 127.2 (d, para-CarH), 76.6 (d,
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C-4′), 55.5 (d, C-1′), 41.7 (s, C-2′), 27.8 (t, C-3′), 20.2 [d, CH CDCl3): δ (ppm) = 138.8 (s, C-3), 137.0 (s, C-5a), 134.4 (s, C-9a),
(CH2)2], 14.6 [d, CH(CH2)2], 1.93 [t, CH(CH2)2], 1.74 [t, CH 126.0 (d, CvCHCH2CH2D), 92.2 (d, C-1), 47.6 (d, C-9b), 42.1
(CH2)2], 0.90 [t, CH(CH2)2], 0.72 [t, CH(CH2)2]; MS (EI): m/z (%) (d, C-3a), 34.6 (t, C-2), 27.7 (t. C-5), 27.3 (t, C-4), 22.8 (t,
1
= 211 (4) [M − NO2]+, 169 (16) [M − NO2 − C3H6]+, 117 (100) CvCHCH2CH2D), 14.1 (t, JCD = 19.4 Hz, CvCHCH2CH2D)
[C9H9]+; HRMS (ESI): calcd for C16H20NO2+ [M + H]+: 258.1448; (the aromatic signals of carbon atoms linked to deuterium
found: 258.1449. 7: Rf = 0.69 (P/Et2O = 19/1); IR: ˜ν (cm−1) = atoms were not visible in the 13C-NMR spectrum); MS (EI): m/z
3419, 2928, 1722, 1547, 1367, 1023, 856, 791; 1H NMR (%) = 215 (65) [M − NO2]+, 185 (100) [M − NO2 − C2H4]+, 171
(500 MHz, CDCl3): δ (ppm) = 7.17–7.05 (m, 4H, Har), 5.45 (ttd, (36), 132 (20) [C10H4D4]+, 95 (6) [C7H3D4]+; HRMS (ESI): calcd
4
3J = 7.1 Hz, J = 2.5 Hz, 1.6 Hz, 1H, CvCHCH2CH3), 4.90 (virt. for C16H15D5NO2+ [M + H]+: 263.1802; found: 263.1804.
3
3
3
3
q, J ≅ J = 7.3 Hz, 1H, H-1), 3.90 (virt. t, J ≅ J = 7.5 Hz, 1H,
(3′-Methyl-4′-nitrocyclobutane)-1,2-diyl-dibenzene
(3j/3j′).
H-9b), 3.07–2.94 (m, 2H, H-3a, CHH-2), 2.90 (dddd, 2J = 17.3 General procedure for the [2 + 2] photocycloaddition of 1a to
Hz, J = 7.9 Hz, J = 2.7 Hz, 1.4 Hz, 1H, CHH-2), 2.80–2.75 (m, trans-β-methylstyrene: a solution of nitroethene 1a (29.8 mg,
1H, CHH-5), 2.73–2.68 (m, 1H, CHH-5), 2.07–1.98 (m, 2H, 200 μmol, 1.00 equiv.) and trans-β-methylstyrene (259 μL,
CvCHCH2CH3), 1.84 (ddt, 2J = 13.8 Hz, 3J = 6.2 Hz, 4.7 Hz, 1H, 236 mg, 2.00 mmol, 10.0 equiv.) in dichloromethane (5 mL, c =
3
4
2
3
CHH-4), 1.67 (dtd, J = 13.8 Hz, J = 9.5 Hz, 4.8 Hz, 1H, CHH- 20 mM) was irradiated at λmax = 419 nm for twelve hours at
4), 1.00 (t, J = 7.5 Hz, 3H, CH3); 13C NMR (126 MHz, CDCl3): δ room temperature. Purification by column chromatography (P/
3
(ppm) = 138.7 (s, C-3), 137.1 (s, C-5a), 134.4 (s, C-9a), 129.3 (d, Et2O = 50/1) yielded 3j/3j′ (28.1 mg, 1.05 mmol, 53%, dr =
C-6), 128.6 (d, C-9), 127.2 (d, C-8), 126.6 (d, C-7), 125.9 (d, 46 : 54) as a colourless oil. Starting material was recovered as
CvCHCH2CH3), 92.2 (d, C-1), 47.7 (d, C-9b), 42.0 (d, C-3a), cis-isomer cis-1a (6.60 mg, 42.3 μmol, 22%). General procedure
34.6 (t, C-2), 27.8 (t. C-5), 27.3 (t, C-4), 22.8 (t, CvCHCH2CH3), for the [2 + 2] photocycloaddition of 1a to cis-β-methylstyrene:
14.1 (q, CH3); MS (EI): m/z (%) = 181 (100) [M − NO2 − C2H4]+, a solution of nitroethene (29.8 mg, 200 μmol, 1.00 equiv.) and
167 (40) [C13H11]+, 128 (16) [C10H8]+; HRMS (ESI): calcd for cis-β-methylstyrene (260 μL, 236 mg, 2.00 mmol, 10.0 equiv.) in
C16H20NO2+ [M + H]+: 258.1448; found: 258.1449.
dichloromethane (5 mL, c = 20 mM) was irradiated at λmax
=
Methyl (E)-1-nitro-3-propylidene-2,3,3a,4,5,9b-hexahydro-1H- 419 nm for twelve hours at room temperature. Purification by
cyclopenta[a]naphthalene-7-carboxylate (8). Colourless solid. column chromatography (P/Et2O
= 50/1) yielded 3j/3j′
Rf = 0.53 (P/Et2O = 19/1); IR: ˜ν (cm−1) = 3423, 2955, 1720, 1550, (45.8 mg, 1.12 mmol, 56%, dr = 77 : 23) as a colourless oil.
1437, 1368, 1284, 1105, 762; 1H NMR (500 MHz, CDCl3): δ Starting material was recovered as cis-isomer cis-1a (6.80 mg,
3
(ppm) = 7.80–7.77 (m, 2H, H-6, H-8), 7.14 (d, J = 8.6 Hz, 1H, 45.6 μmol, 23%). The analytical data obtained matched those
H-9), 5.46 (virt. tq, 3J = 6.9 Hz, 4J ≅ 4J = 2.4 Hz, 1H, reported in the literature.13
CvCHCH2CH3), 4.88 (virt. q, 3J ≅ 3J = 7.3 Hz, 1H, H-1),
3.96–3.91 (m, 1H, H-9b), 3.90 (s, 3H, CO2CH3), 3.11–2.95 (m,
2H, H-3a, CHH-2), 2.95–2.80 (m, 2H, CHH-2, CHH-5),
Conflicts of interest
2.78–2.66 (m, 1H, CHH-5), 2.08–1.96 (m, 2H, CvCHCH2CH3),
2
3
1.91–1.81 (m, 1H, CHH-4), 1.68 (dtd, J = 13.9 Hz, J = 9.3 Hz, There are no conflicts to declare.
3
4.8 Hz, 1H, CHH-4), 1.00 (t, J = 7.5 Hz, 3H, CvCHCH2CH3);
13C NMR (126 MHz, CDCl3): δ (ppm) = 138.7 (s, C-3), 137.1 (s,
C-5a), 134.4 (s, C-9a), 129.3 (d, C-6), 128.6 (d, C–9), 127.2 (d,
C-8), 126.6 (d, C-7), 125.9 (d, CvCHCH2CH3), 92.2 (d, C-1),
Acknowledgements
52.3 (q, CO2CH3) 47.7 (d, C-9b), 42.0 (d, C-3a), 34.6 (t, C-2), Financial support by the European Research Council under
27.8 (t. C-5), 27.3 (t, C-4), 22.8 (t, CvCHCH2CH3), 14.1 (q, the European Union’s Horizon 2020 research and innovation
CvCHCH2CH3); MS (EI): m/z (%) = 284 (19) [M − OCH3]+, 253 programme (grant agreement No. 665951 – ELICOS) is grate-
(54), 149 (100) [C9H9O2]+, 115 (49), 91 (63) [C7H7]+; HRMS fully acknowledged.
(ESI): calcd for C18H22NO2 [M + H]+: 316.1543; found:
+
316.1545.
(E)-1-Nitro-3-(propylidene-3-d)-2,3,3a,4,5,9b-hexahydro-1H-
Notes and references
cyclopenta[a]naphthalene-6,7,8,9-d4 (7-d5). Yellow coloured oil.
Rf = 0.70 (P/Et2O = 19/1); IR: ˜ν (cm−1) = 3418, 2928, 1711, 1547,
1368, 1261, 1024, 858, 803, 752; 1H NMR (400 MHz, CDCl3):
δ (ppm) = 5.45 (tq, 3J = 7.1 Hz, 4J = 2.4 Hz, 1H, CvCHCH2CH3),
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