Rhodium Porphyrin Catalyzed Regioselective Transfer Hydrogenolysis of C-C σ-Bonds in Cyclopropanes with iPrOH

Article abstract of DOI:10.1021/acs.organomet.9b00290

A new rhodium porphyrin catalyzed regioselective transfer hydrogenolysis of both activated and unactivated cyclopropanes employing ⁱPrOH as the hydrogen source was discovered. The reaction mechanism for the C-C σ-bond activation of cyclopropanes was identified through an initial radical substitution with rhodium(II) metalloporphyrin radical to give a rhodium porphyrin alkyl, followed by hydrogenolysis with ⁱPrOH to give the corresponding acyclic alkanes and regenerate rhodium(II) metalloporphyrin radical.

Full text of DOI:10.1021/acs.organomet.9b00290

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Rhodium Porphyrin Catalyzed Regioselective Transfer
i
Hydrogenolysis of C−C σ‑Bonds in Cyclopropanes with PrOH
Chen Chen, Shiyu Feng, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: A new rhodium porphyrin catalyzed regioselective transfer hydrogenolysis of both activated and unactivated
i
cyclopropanes employing PrOH as the hydrogen source was discovered. The reaction mechanism for the C−C σ-bond
activation of cyclopropanes was identified through an initial radical substitution with rhodium(II) metalloporphyrin radical to
i
give a rhodium porphyrin alkyl, followed by hydrogenolysis with PrOH to give the corresponding acyclic alkanes and
regenerate rhodium(II) metalloporphyrin radical.
by RhII(tmp)· (tmp = 5,10,15,20-tetramesitylporphyrinato
INTRODUCTION
■
dianion) metalloradical to afford Rh(tmp)CH2CH2CH2Ph as
Cyclopropanes are versatile starting materials for the
a stoichiometric ring opening product at room temperature in
construction of 1,3-difunctionalized, carbocyclic or hetero-
neutral conditions.16 On the basis of the earlier success, we
cyclic molecules.1 The reductive ring opening of cyclopropanes
would like to convert the stoichiometric transformation into a
provides a convenient method for the preparation of 1,3-
difunctionalized acyclic alkanes from 1,2-disubstituted cyclo-
catalytic C−C σ-bond hydrogenolysis process with the
i
addition of a weak reducing agent of PrOH (E°(acetone/
propanes,2−4 of which many synthetic methodologies have
isopropanol) = −0.11 V vs SCE).17
been established.1b,5 Various transition metal complexes can
catalyze the hydrogenolysis of both activated and unactivated
RESULTS AND DISCUSSION
■
cyclopropanes to non-cyclic alkanes with H2. A rhodium
We attempted the catalytic transfer hydrogenolysis of cyclo-
catalyzed example is shown in Scheme 1A.6
propylbenzene (1a) with H2O first. Unfortunately, the
isomerization product trans-β-methylstyrene (3a), generated
from the fast β-H elimination of the stoichiometric ring
Transfer hydrogenation with hydrogen source other than H2
gas has raised much attention due to safety issues and ease of
8
handling.7 SmI2 (E°(Sm3+/SmI2) = −0.89 V vs SCE)9 and
opening product, was achieved as the major product (see SI, eq
zinc metal3,10 (E°(Zn2+/Zn) = −0.76 V vs SCE)11 are strong
S1). A stronger reducing agent of isopropanol was then
reducing agents and can promote the transfer hydrogenolysis
examined due to its wide applications in the transfer
of donor−acceptor cyclopropanes via single electron transfer
process (Scheme 1B,C). Cyclopropyl ketones undergo smooth
transfer hydrogenolysis with Bu3SnH through sequential
radical attack and hydrogen atom transfer pathways (Scheme
1D).12 All these systems require stoichiometric amount of
metal reagents as reductant or hydrogen source (Scheme 1B−
D). Organic electron donors (OED) (E°(OED+/OED) =
−1.20 V vs SCE)13 can reduce 1,2-diaryl cyclopropanes via
single electron transfer and further induce C−C σ-bond
hydrogenolysis (Scheme 1E).14 However, the reduction
efficiency and substrate scope still need to be improved.
Rhodium porphyrin complexes have been investigated
extensively in C−C bond activation reactions of various
substrates.15 Most of the transformations generate rhodium
porphyrin alkyls as stoichiometric bond activation products
and the high stability of the Rh−C bond hinders its further
application in catalysis. In 2017, our group reported a
regioselective C−C σ-bond activation of cyclopropylbenzene
hydrogenation reactions, cheap price, and low toxicity.7
To our delight, both cyclopropylbenzene (1a) (eq 1) and
cyclopropyl phenyl ketone (1b) (eq 2) underwent smooth
transfer hydrogenolysis in the presence of 20 mol %
Rh(ttp)Me (ttp = 5,10,15,20-tetrakis(p-tolyl)-porphyrinato
dianion) precatalyst and 50 equiv of PrOH at 200 °C in 5−
12 h to afford the corresponding linear alkanes (2a and 2b) in
i
high yields. Nearly an equal amount of acetone, generated from
i
the dehydrogenation of PrOH, was also observed.
Received: May 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00290
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Scheme 1. Reported Systems for the Hydrogenolysis of C−C σ-Bonds in Cyclopropanes
Table 1. Substrate Scope for Monosubstituted
Cyclopropanes
With this initial success in hand, the reaction conditions
including the loadings of Rh(ttp)Me (0, 10, 20 mol %) and
iPrOH (25, 50 equiv) were optimized using cyclopropylben-
zene (1a) as the model compound (see SI, Table S1). Twenty
mol % of Rh(ttp)Me and 50 equiv of PrOH at 200 °C were
employed as the optimal reaction conditions for further
investigations.
a
a
entry
FG
time (h) 2a−e yield (%) acetone yield (%)
1
2
3
4
Ph (1a)
12
5
100 (2a)
91 (2b)
97 (2c)
89 (2d)
99
66
88
96
C(O)Ph (1b)
COOEt (1c)
CN (1d)
i
32
32
a
NMR yield.
Various commercially available monosubstituted cyclo-
propanes (1a−d) with common representative functional
groups were then examined under the optimal reaction
conditions to investigate the functional group compatibility
of the transfer hydrogenolysis process (Table 1). The more
sterically hindered but weaker bond i (see SI, computed bond
dissociation energies of cyclopropane ring C−C σ-bonds)
underwent hydrogenolysis regioselectively. The chemoselective
reaction of cyclopropyl phenyl ketone (1b) finished in 5 h to
afford 91% yield of butyrophenone (2b) without any reduction
of the carbonyl group (Table 1, entry 2). Ethyl cyclo-
propanecarboxylate (1c) showed lower reactivity and the
reaction took 32 h to give ethyl butyrate (2c) in 97% yield
(Table 1, entry 3). Reaction of cyclopropanecarbonitrile (1d)
was slow to afford butyronitrile (2d) in 89% yield after heating
for 32 h without reduction of the nitrile group (Table 1, entry
4). Therefore, the transfer hydrogenolysis process exhibits
good functional group compatibility with carbonyl, ester, and
nitrile groups remaining intact after reactions.
The transfer hydrogenolysis of 1,1-disubstituted cyclo-
propane diethyl cyclopropane-1,1-dicarboxylate (1e) was also
investigated and afforded 14% yield of diethyl ethylmalonate
(2e) and isopropyl butyrate in 51% yield as the major product
(eq 3). The formation of isopropyl butyrate can be attributed
to the successive thermal decarboxylation and transesterifica-
tion of diethyl ethylmalonate (2e) in the presence of
isopropanol at high temperature (see SI, eq S2).
B
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The catalytic C−C σ-bond hydrogenolysis of 1,2-disub-
stituted cyclopropanes were also explored (Table 2). The most
parallel rhodium porphyrin catalyzed dehydrogenation of
iPrOH,18 which occurs more competitively in the presence of
less reactive cyclopropane substrates.
Table 2. Substrate Scope for 1,2-Disubstituted
Cyclopropanes
To gain more mechanistic understandings, the reaction time
profiles of transfer hydrogenolysis of cyclopropylbenzene (1a)
under the optimal reaction conditions (eq 1) were obtained.
Figures 1 and 2 show the reaction time profiles of rhodium
porphyrin species and organic compounds, respectively.
a
a
time
(h)
2f−i yield
acetone yield
(%)
entry
FG
(%)
b
1
Ph (trans-1f)
C(O)Ph (trans-1g)
COOEt (1h)
6
100 (2f)
88 (2g)
92 (2h)
65
51
65
97
2
3
0.7
24
22
c
4
CN (trans-1i)
82 (2i)
a
b
c
NMR yield. With 5 mol % of Rh(ttp)Me. 1h (trans:cis = 2.8:1.0).
sterically hindered but weakest bond i (see SI, computed bond
dissociation energies of cyclopropane ring C−C σ-bonds)
underwent hydrogenolysis regioselectively. The reactivity of
trans-1,2-diphenylcyclopropane (trans-1f) was enhanced com-
pared with 1a. trans-1f was converted to 1,3-diphenylpropane
(2f) quantitatively in the presence of 5 mol % Rh(ttp)Me
within 6 h at 200 °C (Table 2, entry 1). trans-1-Benzoyl-2-
phenylcyclopropane (trans-1g) was also very reactive and the
catalysis completed within 0.7 h to give 88% yield of 4-
phenylbutyrophenone (2g) (Table 2, entry 2). 2-Phenyl-
cyclopropanecarboxylic acid ethyl ester (1h) (trans:cis =
2.8:1.0) was converted to ethyl 4-phenylbutyrate (2h) in
92% yield after 24 h (Table 2, entry 3). trans-2-Phenyl-
cyclopropanecarbonitrile (trans-1i) underwent reductive ring
opening to afford 82% yield of 4-phenylbutyronitrile (2i) after
heating for 22 h (Table 2, entry 4). Good functional group
compatibility was also achieved for 1,2-disubstituted cyclo-
propanes with enhanced reactivity compared with monosub-
stituted substrates.
Figure 1. Reaction time profiles for rhodium porphyrin species in the
transfer hydrogenolysis of cyclopropylbenzene (eq 1).
1,2,3-Trisubstituted cyclopropanes showed low reactivity
toward reductive ring opening. The stronger C−C σ-bonds ii
(see SI, computed bond dissociation energies of cyclopropane
ring C−C σ-bonds) of trans,cis-2,3-diphenyl-1-cyclopropane-
carboxylic acid ethyl ester (1j) and trans,trans-2,3-diphenyl-1-
benzoylcyclopropane (1k) underwent transfer hydrogenolysis
preferentially (Table 3). For cyclopropane 1j, 20% yield of 2j
and 73% yield of 2j′ were achieved after 48 h, with 5% of 1j
recovered due to its poor reactivity (Table 3, entry 1). The
reaction of 1k was more diversified to afford 40% of 2k′ with
inseparable unknown products after 48 h (Table 3, entry 2).
The high amount of acetone formed can be attributed to the
Figure 2. Reaction time profiles for organic compounds in the transfer
hydrogenolysis of cyclopropylbenzene (eq 1).
At 200 °C in 4 h, the precatalyst Rh(ttp)Me was completely
converted to the stoichiometric ring opening product Rh(ttp)-
CH2CH2CH2Ph exclusively. The structure of Rh(ttp)-
CH2CH2CH2Ph was further confirmed through an independ-
ent synthesis19 (see SI, Figure S16). A similar structure has
been reported in previous work.19b With prolonged heating,
Rh(ttp)CH2CH2CH2Ph was gradually consumed with the
Table 3. Substrate Scope for 1,2,3-Trisubstituted
Cyclopropanes
formation of Rh(ttp)H (δ = −39.4 ppm, JRh−H = 40.0 Hz)20
1
in ∼50% yield at the end of the catalysis (Figure 1).
The amount of starting material cyclopropylbenzene (1a)
decreased once the precatalyst Rh(ttp)Me began to consume
upon heating. trans-β-Methylstyrene (3a) was formed in 1 h,
which can be generated from the β-hydride elimination of
Rh(ttp)CH2CH2CH2Ph followed by rhodium porphyrin
catalyzed olefin isomerization.21 The yield of trans-β-
methylstyrene (3a) gradually increased to ∼25% and remained
the same in around 6 h before complete consumption in the
last 4 h. The desired product propylbenzene (2a) started to
a
a
a
2j−k yield
2j′−k′ yield
acetone yield
(%)
entry
FG
(%)
(%)
b
1
COOEt (1j)
C(O)Ph (1k)
20 (2j)
0 (2k)
73 (2j′)
40 (2k′)
203
158
2
a
b
NMR yield. 5% of 1j recovered.
C
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form in 2 h and increased to 100% yield at the end of the
catalysis. Since trans-β-methylstyrene (3a) was formed prior to
propylbenzene (2a) and completely consumed finally, it may
serve as a consecutive intermediate in the formation of
propylbenzene (2a). The yield of acetone gradually increased
to 99% yield (Figure 2).
On the basis of the above results and established
mechanisms for rhodium porphyrin metalloradical mediated
C−C bond activation of cyclopropanes17 and cyclooctane,23
Scheme 2 depicts a plausible mechanism. Rh(ttp)Me under-
Scheme 2. Proposed Mechanism for the Catalytic Transfer
Hydrogenolysis Process
As Rh(ttp)CH2CH2CH2Ph was identified as a plausible
intermediate in the catalysis, the stoichiometric hydrogenolysis
i
of rhodium porphyrin alkyls with PrOH was therefore
examined as a viable reaction step in the catalysis. Rh(ttp)Me
and Rh(ttp)CH2CH2CH2Ph were subject to the thermal
i
reaction with PrOH under optimal reaction conditions. The
precatalyst Rh(ttp)Me was completely consumed in 15 min
with the quantitative formation of Rh(ttp)H (eq 4). Methane
in 86% yield22 and acetone in 51% yield were formed.
Therefore, the activation of precatalyst Rh(ttp)Me with iPrOH
to afford Rh(ttp)H is a fast step in the catalytic reaction. The
transfer hydrogenolysis of Rh(ttp)CH2CH2CH2Ph is slightly
slower in rate to give propylbenzene (2a) in 100% yield and
i
goes alcoholysis with PrOH to give the reactive Rh(ttp)H as
i
Rh(ttp)H in 66% yield (eq 5). The role of PrOH as the
well as methane and acetone rapidly, analogous to the reported
hydrolysis process with water.24 The rhodium porphyrin
i
mediated dehydrogenation of PrOH to form acetone has
also been reported previously.18 Rh(ttp)H then undergoes
thermal reductive dimerization and equilibrates with H2 and
RhII (ttp)2 upon heating.25 RhII(ttp)·, generated from the
hydrogen source was further supported by the coformation of
acetone in 58% yield. Therefore, the intermediacy of
Rh(ttp)CH2CH2CH2Ph in the formation of the final product
propylbenzene (2a) through transfer hydrogenolysis of the
Rh−C bond is established.
2
homolysis of RhII (ttp)2,26 cleaves the C−C bond of
2
cyclopropylbenzene through a homolytic radical substitution
to form a more stable secondary alkyl radical intermediate
A.17,23 Intermediate A can undergo reversible fast ring closure
to regenerate cyclopropylbenzene27 or abstract a hydrogen
atom from the weak (ttp)Rh−H (∼60 kcal/mol)28 or CH3C−
H(OH)CH3 (∼91 kcal/mol)29 bond to form the stable
Rh(ttp)CH2CH2CH2Ph complex (intermediate B). At high
temperature, intermediate B undergoes facile β-hydride
elimination to give allylbenzene and Rh(ttp)H.21 Allylbenzene
isomerizes to trans-β-methylstyrene (3a) through a Rh(ttp)H
promoted process.21 The fast formation of trans-β-methylstyr-
ene (3a) has been proved by the independent hydrogenolysis
of intermediate B (see SI, Figure S1). Reversible 1,2-addition
between Rh(ttp)H and trans-β-methylstyrene (3a) affords a
Rh(ttp)-2°-alkyl (intermediate C), which reacts with iPrOH to
afford the desired product propylbenzene (2a) and Rh(ttp)H,
thus closing the catalytic cycle (Scheme 2, Pathway I). The
i
The reaction between Rh(ttp)CH2CH2CH2Ph and PrOH
at a lower temperature of 180 °C was then carried out to slow
down the reaction rate and the reduction process for close
reaction monitoring (see SI, eq S3). The reaction time profiles
(see SI, Figure S1) show that trans-β-methylstyrene (3a)
formed instantaneously once the reaction mixture was heated
up. The amount of 3a increased gradually in the first 0.5 h and
then completely consumed in 2.5 h. Propylbenzene (2a)
started to form at 0.5 h, and the yield increased steadily to
reach 99% yield at the end of the catalysis after 4 h. Therefore,
the β-hydride elimination of Rh(ttp)CH2CH2CH2Ph to give
trans-β-methylstyrene (3a) occurs more facile and is in parallel
with the hydrogenolysis of Rh(ttp)CH2CH2CH2Ph to give
propylbenzene (2a). 3a is further converted to 2a, thus serving
as a consecutive and concurrent intermediate.
i
direct reaction between intermediate B and PrOH to afford
propylbenzene (2a) may also operate on the basis of the
hydrogenolysis of Rh(ttp)Me (Scheme 2, Pathway II). In the
presence of a large amount of Rh(ttp)H, intermediates B and
C can also undergo bimolecular reductive elimination with
The transfer hydrogenation of trans-β-methylstyrene (3a) to
propylbenzene (2a) is further ascertained under the optimal
reaction conditions (eq 6). trans-β-Methylstyrene (3a) was
Rh(ttp)H to form RhII (ttp)2 and propylbenzene (2a). The
2
formal “alcoholysis” of intermediates B and C is accelerated
once Rh(ttp)H is formed, like the reported hydrolysis of
Rh(ttp)alkyls with water.24b
For mono- and 1,2-disubstituted cyclopropanes, the weakest
but more substituted C−C σ-bond i is always cleaved
selectively. The regioselectivity can be attributed to the
following reasons. First, the radical stabilization effect induced
by substituents in the formation of intermediate A when bond i
is activated (Scheme 2). Second, the π−π interaction30
hydrogenated with isopropanol catalyzed by Rh(ttp)Me
rapidly in 5 h to afford propylbenzene (2a) quantitatively
together with the generation of acetone in 77% yield.
Therefore, the intermediacy of 3a in the formation of 2a is
also established.
D
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confirmed with GC−MS analysis using naphthalene as internal
standard through comparing with standard sample.
between phenyl substituent in 1,2-disubstituted cyclopropanes
and porphyrin likely direct the bond activation toward bond i
(Table 2) with the formation of a more stable 2° radical
intermediate. The transfer hydrogenolysis of 1,2,3-trisubsti-
tuted substrates (1j−k) presents poor regioselectivity. We do
not understand the origin at this moment.
Transfer Hydrogenolysis of Cyclopropyl Phenyl Ketone.
Cyclopropyl phenyl ketone (1b; 3.4 μL, 0.025 mmol), iPrOH (95 μL,
1.24 mmol), Rh(ttp)Me (3.9 mg, 0.005 mmol), and 400 μL C6D6
were added into a Teflon screw head stoppered NMR tube, degassed
by three freeze−pump−thaw cycles, and then flame-sealed under a
vacuum, heated in an oven in the dark at 200 °C for 5 h. The reaction
1
CONCLUSION
was monitored by H NMR spectroscopy at particular time interval.
■
91% of butyrophenone (2b; 0.023 mmol) and 66% of acetone were
obtained.
In summary, the rhodium porphyrin complex catalyzed transfer
hydrogenolysis of C−C σ-bonds in cyclopropanes with
isopropanol has been realized. Both monosubstituted and
1,2-disubstituted cyclopropanes undergo regioselective bond
cleavage at the weakest ring C−C σ-bond and are thus
converted to the corresponding linear alkanes with high yields.
Transfer Hydrogenolysis of Ethyl Cyclopropanecarboxy-
late. Ethyl cyclopropanecarboxylate (1c; 2.9 μL, 0.025 mmol), iPrOH
(95 μL, 1.24 mmol), Rh(ttp)Me (3.9 mg, 0.005 mmol), and 400 μL
C6D6 were added into a Teflon screw head stoppered NMR tube,
degassed by three freeze−pump−thaw cycles, and then flame-sealed
under a vacuum, heated in an oven in the dark at 200 °C for 32 h. The
1
EXPERIMENTAL SECTION
reaction was monitored by H NMR spectroscopy at particular time
■
interval. 97% of ethyl butyrate (2c; 0.024 mmol) and 88% of acetone
were obtained.
Unless otherwise noted, all reagents were purchased from commercial
suppliers and directly used without further purification. Hexane for
chromatography was distilled from anhydrous calcium chloride.
Benzene was distilled over sodium under nitrogen. Thin-layer
chromatography was performed on precoated silica gel 60 F254 plates.
Silica gel (Merck, 70−230 and 230−400 mesh) was used in column
chromatography to isolate. Neutral alumina (Merck, 90 active neutral,
70−230 mesh)/H2O (∼10:1 v/v) was used in column chromatog-
raphy to isolate. For the reaction conducted in a sealed NMR tube,
the mixture was degassed by three freeze (77 K)−pump (0.005
mmg)−thaw cycles, and then flame-sealed under a vacuum, heated in
an oven in the dark and wrapped with alumina foil to protect from
Transfer Hydrogenolysis of Cyclopropanecarbonitrile. Cy-
clopropanecarbonitrile (1d; 1.9 μL, 0.025 mmol), iPrOH (95 μL, 1.24
mmol), Rh(ttp)Me (3.9 mg, 0.005 mmol), and 400 μL C6D6 were
added into a Teflon screw head stoppered NMR tube, degassed by
three freeze−pump−thaw cycles, and then flame-sealed under a
vacuum, heated in an oven in the dark at 200 °C for 32 h. The
1
reaction was monitored by H NMR spectroscopy at particular time
interval. 89% of butyronitrile (2d; 0.022 mmol) and 96% of acetone
were obtained.
Transfer Hydrogenolysis of Diethyl Cyclopropane-1,1-
dicarboxylate. Diethyl cyclopropane-1,1-dicarboxylate (1e; 4 μL,
0.025 mmol), iPrOH (95 μL, 1.24 mmol), Rh(ttp)Me (3.9 mg, 0.005
mmol), and 400 μL C6D6 were added into a Teflon screw head
stoppered NMR tube, degassed by three freeze−pump−thaw cycles,
and then flame-sealed under a vacuum, heated in an oven in the dark
at 200 °C for 48 h. The reaction was monitored by 1H NMR
spectroscopy at particular time interval. 14% of diethyl ethylmalonate
(2e; 0.0035 mmol) and 70% of acetone were obtained with 16% of 1e
recovered. 51% of isopropyl butyrate (0.013 mmol) was further
confirmed as side product using GC−MS analysis through comparing
with standard sample.
1
exposure to room light before H NMR measurements. The NMR
i
yields were calculated using benzene residue or excess PrOH as
internal standard. Rh(ttp)Me,19a Rh(ttp)CH2CH2CH2Ph,19a trans-
1,2-diphenyl cyclopropane (trans-1f),14 trans-1-benzoyl-2-phenyl-
cyclopropane (trans-1g),31 2-phenylcyclopropanecarboxylic acid
ethyl ester (1h),32 trans-2-phenylcyclopropanecarbonitrile (trans-
1i),33 trans,cis-2,3-diphenyl-1-cyclopropanecarboxylic acid ethyl ester
(1j),34 and trans,trans-2,3-diphenyl-1-benzoylcyclopropane (1k)35
have been characterized and were prepared according to the literature
process.
1H NMR spectra were recorded on a Bruker AV-400 instrument at
400 MHz. Chemical shifts were referenced with the residual solvent
protons in CDCl3 (δ 7.26 ppm), C6D6 (δ 7.15 ppm). Chemical shifts
(δ) are reported as parts per million (ppm) in δ scale downfield from
TMS. Coupling constants (J) are reported in Hertz (Hz). High-
resolution mass spectrometry (HRMS) was performed on a Bruker
Autoflex speed MALDI-TOF instrument using a trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) ma-
trix and CH2Cl2 as the solvent. GC−MS analyses were conducted on
a GCMS-QP2010 Plus system using an Rtx-5MS column (30 m ×
0.25 mm). Details of the GC program are as follows. The column
oven temperature and injection temperature were 100 and 250 °C.
Helium was used as the carrier gas. The flow control mode was
chosen as linear velocity (36.3 cm s−1) with a pressure of 68.8 kPa.
The total flow, column flow, and purge flow were 13.5, 0.95, and 3.0
mL min−1, respectively. Split mode injection with a split ratio of 10.0
was applied. After injection, the column oven temperature was kept at
100 °C for 2 min and was then elevated at a rate of 30 °C min−1 for 5
min until 250 °C. The temperature of 250 °C was kept for 4 min. The
retention time and mass spectrum of organic products obtained were
identical with those of commercially available authentic samples.
Transfer Hydrogenolysis of Cyclopropylbenzene. Cyclo-
Transfer Hydrogenolysis of trans-1,2-Diphenylcyclopro-
pane. trans-1,2-Diphenylcyclopropane (trans-1f; 4.7 μL, 0.025
i
mmol), PrOH (95 μL, 1.24 mmol), Rh(ttp)Me (1.1 mg, 0.0014
mmol), and 400 μL C6D6 were added into a Teflon screw head
stoppered NMR tube, degassed by three freeze−pump−thaw cycles,
and then flame-sealed under a vacuum, heated in an oven in the dark
at 200 °C for 6 h. The reaction was monitored by 1H NMR
spectroscopy at particular time interval. 100% of 1,3-diphenylpropane
(2f; 0.025 mmol) and 65% of acetone were obtained.
Transfer Hydrogenolysis of trans-1-Benzoyl-2-phenylcyclo-
propane. trans-1-Benzoyl-2-phenylcyclopropane (trans-1g; 5.5 mg,
0.025 mmol), iPrOH (95 μL, 1.24 mmol), Rh(ttp)Me (4.0 mg, 0.005
mmol), and 400 μL C6D6 were added into a Teflon screw head
stoppered NMR tube, degassed by three freeze−pump−thaw cycles,
and then flame-sealed under a vacuum, heated in an oven in the dark
1
at 200 °C for 40 min. The reaction was monitored by H NMR
spectroscopy at particular time interval. 88% of 4-phenylbutyrophe-
none (2g; 0.022 mmol) and 51% of acetone were obtained.
Transfer Hydrogenolysis of 2-Phenylcyclopropanecarbox-
ylic Acid Ethyl Ester. 2-Phenylcyclopropanecarboxylic acid ethyl
i
ester (1h; 4.3 μL, 0.025 mmol), PrOH (95 μL, 1.24 mmol),
i
propylbenzene (1a; 3.1 μL, 0.025 mmol), PrOH (95 μL, 1.24
Rh(ttp)Me (4.0 mg, 0.005 mmol), and 400 μL C6D6 were added into
a Teflon screw head stoppered NMR tube, degassed by three freeze−
pump−thaw cycles, and then flame-sealed under a vacuum, heated in
an oven in the dark at 200 °C for 24 h. The reaction was monitored
by 1H NMR spectroscopy at particular time interval. 92% of 4-
phenylbutyrate (2h; 0.023 mmol) and 65% of acetone were obtained.
Transfer Hydrogenolysis of trans-2-Phenylcyclopropane-
carbonitrile. trans-2-Phenylcyclopropanecarbonitrile (trans-1i; 3.6
mmol), Rh(ttp)Me (3.9 mg, 0.005 mmol), and 400 μL C6D6 were
added into a Teflon screw head stoppered NMR tube, degassed by
three freeze−pump−thaw cycles, and then flame-sealed under a
vacuum, heated in an oven in the dark at 200 °C for 12 h. The
1
reaction was monitored by H NMR spectroscopy at particular time
interval. 100% of propylbenzene (2a; 0.025 mmol) and 99% of
acetone were obtained. The yield of propylbenzene (2a) was further
E
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Organometallics
Article
i
mg, 0.025 mmol), PrOH (95 μL, 1.24 mmol), Rh(ttp)Me (3.9 mg,
spectroscopy at particular time interval. 72% of Rh(ttp)H, 99% of
propylbenzene (2a; 0.005 mmol) and 61% of acetone were achieved.
Catalytic Transfer Hydrogenation of trans-β-Methylstyrene
0.005 mmol), and 400 μL C6D6 were added into a Teflon screw head
stoppered NMR tube, degassed by three freeze−pump−thaw cycles,
and then flame-sealed under a vacuum, heated in an oven in the dark
at 200 °C for 22 h. The reaction was monitored by 1H NMR
spectroscopy at particular time interval. 82% of 4-phenylbutyronitrile
(2i; 0.021 mmol) and 97% of acetone were obtained.
Transfer Hydrogenolysis of trans,cis-2,3-Diphenyl-1-cyclo-
propanecarboxylic Acid Ethyl Ester. trans,cis-2,3-Diphenyl-1-
cyclopropanecarboxylic acid ethyl ester (1j; 6.1 μL, 0.025 mmol),
iPrOH (95 μL, 1.24 mmol), Rh(ttp)Me (3.9 mg, 0.005 mmol), and
i
with PrOH at 200 °C. trans-β-Methylstyrene (3a; 3.3 μL, 0.025
i
mmol), PrOH (95 μL, 1.24 mmol), Rh(ttp)Me (3.9 mg, 0.005
mmol), and 400 μL C6D6 were added into a Teflon screw head
stoppered NMR tube, degassed by three freeze−pump−thaw cycles,
and then flame-sealed under a vacuum, heated in an oven in the dark
at 200 °C for 5 h. The reaction was monitored by 1H NMR
spectroscopy at particular time interval. 100% of propylbenzene (2a;
0.025 mmol) and 77% of acetone were obtained.
400 μL C6D6 were added into a Teflon screw head stoppered NMR
tube, degassed by three freeze−pump−thaw cycles, and then flame-
sealed under a vacuum, heated in an oven in the dark at 200 °C for 2
d. The reaction was monitored by 1H NMR spectroscopy at particular
time interval. 20% of 2j (0.005 mmol) and 73% of 2j′ (0.018 mmol)
were achieved with 5% of 1j recovered. 203% of acetone was
obtained.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00290.
■
S
Transfer Hydrogenolysis of trans,trans-2,3-Diphenyl-1-ben-
Reaction for attempted transfer hydrogenolysis with
H2O, conditions optimization, thermal reaction of
diethyl ethylmalonate (2e) with PrOH, computed
bond dissociation energies of cyclopropane ring C−C
σ-bonds, additional experimental procedures, NMR
spectra, GC-MS chromatogram spectra, X-ray crystallog-
raphy data for Rh(ttp)CH2CH2CH2Ph (PDF)
zoylcyclopropane. trans,trans-2,3-Diphenyl-1-benzoylcyclopropane
i
(1k; 7.6 mg, 0.025 mmol), PrOH (95 μL, 1.24 mmol), Rh(ttp)Me
i
(3.9 mg, 0.005 mmol), and 400 μL C6D6 were added into a Teflon
screw head stoppered NMR tube, degassed by three freeze−pump−
thaw cycles, and then flame-sealed under a vacuum, heated in an oven
1
in the dark at 200 °C for 2 d. The reaction was monitored by H
NMR spectroscopy at particular time interval. 40% of 2k′ (0.01
mmol) was achieved. 158% of acetone was obtained.
Accession Codes
Independent Synthesis of Rh(ttp)CH2CH2CH2Ph.19a Rh(ttp)-
Cl (33.4 mg, 0.041 mmol), NaBH4 (27.1 mg, 0.72 mmol), NaOH (1
M) (100 μL) and 1 mL EtOH were added into Teflon screw capped
reaction tube. The mixture was degassed for 3 times and heated to 60
°C for 4 h. After the reaction mixture was cooled down to r.t., 1-
bromo-3-phenylpropane (12.5 μL, 0.082 mmol) was added and the
resultant mixture was further stirred for 10 min at r.t.. EtOH was
removed with reduced pressure. The residue was extracted with water
and DCM for 3 times. The combined organic layers were
concentrated and the target product was recrystallized from
MeOH/DCM to give red solid in 80% yield (29.3 mg, 0.033
CCDC 1908688 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
ORCID
■
1
2
mmol). H NMR (400 MHz, CDCl3) δ −5.00 (dt, JRh−H = 2.9 Hz,
3JH−H = 7.8 Hz, 2 H), −4.09 (m, 2 H), −0.39 (t, 3JH−H, 2 H), 2.7 (s,
3
3
12 H), 5.45 (d, JH−H, 2 H), 6.69−6.78 (m, 3 H), 7.52 (t, JH−H=8, 8
Kin Shing Chan: 0000-0002-1112-6819
H), 7.92−7.94 (m, 4 H), 8.05−8.07 (m, 4 H), 8.71 (s, 8 H). 13C
1
Notes
NMR (100 MHz, CDCl3) δ 13.7 (d, JRh−C = 28.4 Hz), 21.6, 27.7,
The authors declare no competing financial interest.
31.9, 122.4, 124.7, 126.9, 127.4, 127.4, 127.4, 131.5, 133.7, 134.1,
137.2, 139.4, 140.6, 143.3. HRMS (MALDI) calcd for C57H47N4Rh
[M]+ m/z 890.2850, found m/z 890.2838.
ACKNOWLEDGMENTS
We thank Research Grants Council (No. 14300214) of Hong
Kong SAR for financial support.
■
Hydrogenolysis of Rh(ttp)Me with iPrOH. Rh(ttp)Me (4.0 mg,
i
0.005 mmol), PrOH (95 μL, 1.24 mmol), and 400 μL C6D6 were
added into a Teflon screw head stoppered NMR tube, degassed by
three freeze−pump−thaw cycles, and then flame-sealed under a
vacuum, heated in an oven in the dark at 200 °C for 15 min. The
REFERENCES
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reaction was monitored by H NMR spectroscopy at particular time
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DOI: 10.1021/acs.organomet.9b00290
Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.9b00290
Organometallics XXXX, XXX, XXX−XXX