Visible-Light Photoredox-Catalyzed Semipinacol-Type Rearrangement: Trifluoromethylation/Ring Expansion by a Radical-Polar Mechanism

Article abstract of DOI:10.1002/anie.201503210

A visible-light-mediated photoredox-catalyzed semipinacol-type rearrangement proceeding via 1,2 alkyl migration was developed. In this transformation, trifluoromethylation of the C=C bond of α-(1-hydroxycycloalkyl)-substituted styrene derivatives is followed by ring expansion of the 1-hydroxycycloalkyl group to deliver novel cycloalkanones with all-carbon quaternary centers. The reaction proceeds via a radical-polar mechanism, with trifluoromethylation (radical) and ring expansion (ionic) occurring in the same transformation.

Full text of DOI:10.1002/anie.201503210

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International Edition: DOI: 10.1002/anie.201503210
German Edition: DOI: 10.1002/ange.201503210
Photocatalysis
Visible-Light Photoredox-Catalyzed Semipinacol-Type
Rearrangement: Trifluoromethylation/Ring Expansion by a Radical–
Polar Mechanism**
Basudev Sahoo, Jun-Long Li, and Frank Glorius*
Dedicated to Professor F. Ekkehardt Hahn on the occasion of his 60th birthday
Abstract: A visible-light-mediated photoredox-catalyzed semi-
pinacol-type rearrangement proceeding via 1,2 alkyl migration
was developed. In this transformation, trifluoromethylation of
reactions such as in atom transfer radical additions
(ATRAs)[5] or Meerwein-type reactions.[2h] As well as provid-
ing a mild and sustainable route into radical chemistry, a key
feature of visible-light photoredox catalysis concerns its
ability to facilitate radical–polar crossover[6] reactions. In
these processes, a radical intermediate generated during the
reaction undergoes SET with the reduced or oxidized photo-
redox catalyst to afford anions or cations. The combination of
both radical and polar steps in a single mechanism is an
attractive concept for the development of novel reactions, and
impressive visible-light-mediated transformations have been
disclosed over the last few years.[2j]
=
the C C bond of a-(1-hydroxycycloalkyl)-substituted styrene
derivatives is followed by ring expansion of the 1-hydroxycy-
cloalkyl group to deliver novel cycloalkanones with all-carbon
quaternary centers. The reaction proceeds via a radical–polar
mechanism, with trifluoromethylation (radical) and ring
expansion (ionic) occurring in the same transformation.
O
rganic reactions conducted in the presence of visible light
have drawn the attention of synthetic organic chemists
because of their sustainability, cost effectiveness, and attrac-
tive environmental performance.[1,2m] However, this approach
suffers from poor substrate scope since relatively few organic
molecules are capable of absorbing photons in the visible
range of the spectrum. To overcome this limitation, photo-
sensitizer compounds capable of absorbing visible light and
passing the energy on to organic substrates have become
widely employed. Over the last few years in particular,
visible-light photoredox catalysis has enabled the develop-
ment of many novel organic transformations.[2] Visible-light-
mediated photoredox catalysis has also been combined with
other modes of catalysis in dual catalytic systems to achieve
new reactivity and increase efficiency.[3]
We[7] considered whether radical–polar crossover process-
es mediated by visible-light photoredox catalysis could
provide new routes towards valuable trifluoromethylated
compounds.[8,2n] The CF3 group is widely incorporated into
pharmaceuticals and agrochemicals owing to its advantageous
influence on the lipophilicity, metabolic stability, and mem-
brane permeability of many compounds.[9] Visible-light pho-
toredox catalysis has enabled many impressive trifluorome-
thylations of alkenes, (hetero)arenes, and other compounds,
most often through the use of electrophilic trifluoromethyl-
+
ating (CF3 ) reagents (e.g, Umemotoꢀs reagents, Togniꢀs
reagents, CF3I, and CF3SO2Cl) as CF3 radical precursors.[5c,10]
In 2012, Koike, Akita et al. reported oxytrifluoromethyl-
ation of activated alkenes with the use of Umemotoꢀs reagent
as a CF3 source (Scheme 1).[10d] In this process, the cation
generated upon oxidation of the alkyl radical is trapped by an
internal or external oxygen nucleophile.[10d,e,j,o] Similar trans-
formations have since been reported with nitrogen, halogen,
and carbon nucleophiles.[5c,10i,k,l] On the other hand, depro-
tonation[10m,11] or deboronation[10h] of a hydrogen or boron in
the a-position to the cation can also occur to form the
trifluoromethylated alkene derivatives. Alternative reaction
pathways for the alkyl cation have not been widely explored,
however, and much potential exists for further, complexity-
building steps after addition of the photoredox-generated
radical. A major reactivity pathway characteristic of cations
that has not been developed for photoredox radical–polar
crossover reactions involves 1,2 alkyl migration. Such pro-
cesses would give concise access to sp3-trifluoromethylated
compounds featuring a quaternary all-carbon center. Herein,
we report the successful development of a visible-light-
mediated trifluoromethylation reaction to form CF3-substi-
tuted cycloalkanone compounds directly from styrene deriv-
atives. The reaction constitutes the first photoredox-catalyzed
A typical mechanistic cycle for photoredox catalysis
involves the initial excitation of the photocatalyst (often
transition-metal–polypyridyl complexes such as [Ru-
(bpy)3]Cl2·6H2O (bpy = 2,2’-bipyridine)or organic dyes) by
visible light followed by single electron transfer (SET) either
to or from the substrate of interest.[4] This generates radical
anion or cation intermediates, which invariably release
organic radicals capable of engaging in classical radical
[*] B. Sahoo, Dr. J.-L. Li, Prof. Dr. F. Glorius
Westfälische Wilhelms-Universität Münster, International Graduate
School of Chemistry & Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
[**] We are grateful to Dr. Matthew N. Hopkinson for helpful discus-
sions. Generous financial support by the NRW Graduate School of
Chemistry (B.S.), the Alexander von Humboldt Foundation (J.-L. L.)
and the Deutsche Forschungsgemeinschaft (Leibniz Award) are
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503210.
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Table 1: Optimization studies.[a]
Entry
Photocatalyst
(mol%)
CF3+ Source
(equiv)
Yield
[%][b]
1[c]
2
3
4
5
6
7
8
9
[Ru(bpy)3](PF6)2 (2)
[Ru(bpy)3](PF6)2 (2)
[Ru(bpy)3](PF6)2 (2)
[Ru(bpy)3](PF6)2 (2)
[Ru(bpy)3](PF6)2 (2)
fac-[Ir(ppy)3] (2)
[Ir(ppy)2(dtbbpy)](PF6) (2)
Fluorescein (2)
[Ru(bpy)3](PF6)2 (2)
[Ru(bpy)3](PF6)2 (1)
–
2a (1.4)
2a (1.4)
2b (1.4)
2c (1.4)
2d (1.4)
2a (1.4)
2a (1.4)
2a (1.4)
2a (1.2)
2a (1.2)
2a (1.2)
2a (1.2)
60
98
81
9
–
96
97
–
95
94 (74)
–
–
Scheme 1. Visible-light-mediated photoredox-catalyzed trifluoromethy-
lation of alkenes via a radical–polar mechanism.
10
11
12[d]
semipinacol-type rearrangement[12] and it benefits from mild
and convenient conditions (room temperature, no solvent
degassing) and uses light from readily available commercial
sources.
[Ru(bpy)3](PF6)2 (1)
[a] 1a (0.1 mmol), trimethylsilyl trifluoromethanesulfonate (TMSOTf,
0.12 mmol), the CF3 reagent (2), and the photoredox catalyst were
+
mixed in N,N-dimethylformamide (DMF) and stirred at RT for 6 h under
visible-light irradiation. [b] GC yield of 3aa with mesitylene as an internal
reference. Yields of isolated 3aa are given in parentheses. [c] In the
absence of TMSOTf, 3aa was obtained along with 4aa in a 3aa/4aa ratio
of 2.3:1, which was determined by 19F NMR analysis. [d] The reaction was
performed in the dark. bpy=2,2’-bipyridine, ppy=2-phenylpyridine,
dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine, LEDs=light-emitting diodes.
Our reaction design starting from cycloalkanol-substi-
tuted styrene derivatives is shown in Scheme 1. After addition
of the photoredox-generated CF3 radical and subsequent
single-electron oxidation, the cation would then undergo
1,2 alkyl migration of the cycloalkanol group. To validate the
process, alternative reaction pathways involving undesired
intramolecular trapping by the alcohol or deprotonation
would have to be suppressed. In a preliminary test, 1-(1-
phenylvinyl)cyclobutanol (1a) was reacted with 1.4 equiva-
lents of the electrophilic trifluoromethylating reagent 5-
(trifluoromethyl)dibenzothiophenium trifluoromethanesulfo-
nate (2a) in N,N-dimethylformamide (DMF, 0.1m) in the
presence of [Ru(bpy)3](PF6)2 (2 mol%) under visible-light
irradiation from 5 W blue LEDs (lmax = 465 nm). We were
delighted to observe ring-expanded 2-phenyl-2-(2,2,2-trifluor-
oethyl)cyclopentanone (3aa) as the major product in 60%
GC yield, along with the undesired byproduct 2-phenyl-2-
(2,2,2-trifluoroethyl)-1-oxaspiro[2.3]hexane (4aa) in a 3aa/
4aa ratio of 2.3:1 (Table 1, entry 1). To suppress the formation
of byproduct 4aa, the reaction was performed in the presence
of a stoichiometric amount of trimethylsilyl trifluorome-
thanesulfonate (TMSOTf, 1.2 equiv) to protect the hydroxy
group in situ and reduce its nucleophilicity. Gratifyingly, the
desired product 3aa was formed exclusively (98% GC yield)
under these conditions, with no 4aa being detected (Table 1,
entry 2). In a screening of different trifluoromethylating
reagents, Umemotoꢀs reagent with a tetrafluoroborate coun-
teranion (2b, 1.4 equiv) gave the product 3aa in a reduced
yield of 81% (GC yield), while Togniꢀs reagents 2c
(1.4 equiv) and 2d (1.4 equiv) were not suitable (9% GC
yield and no reaction, respectively; Table 1, entries 3–5).
After a short solvent screen, DMF remained as the best
solvent.[13] In a survey of different photoredox catalysts,
[Ir(ppy)3] and [Ir(ppy)2(dtbbpy)](PF6) (dtbbpy = 4,4’-di-tert-
butyl-2,2’-bipyridine) delivered 3aa in high GC yields respec-
tively, while fluorescein dye unfortunately did not catalyze the
formation of 3aa (Table 1, entries 6–8). A 23 W compact
fluorescent lamp (CFL) bulb was also identified as a reliable
source of visible light in this reaction (92% GC yield of
3aa).[13] Upon lowering the equivalents of 2a (1.2 equiv), the
efficiency of the reaction was largely unaffected (95%
GC yield of 3aa, Table 1, entry 9), while reducing the catalyst
loading of [Ru(bpy)3](PF6)2 to 1 mol% also did not hamper
the reaction significantly, with the product 3aa being deliv-
ered in 94% GC yield and 74% isolated yield (Table 1,
entry 10). Control experiments showed that both the photo-
catalyst [Ru(bpy)3](PF6)2 and visible light were crucial for this
transformation (Table 1, entries 11,12).
Under the optimal reaction conditions, the scope and
limitations of this transformation were explored (Scheme 2).
Several electron-withdrawing and electron-donating substitu-
ents on the benzene ring of 1-(1-arylvinyl)cyclobutanol (1a–j)
were well tolerated. Substrates 1k–o, derived from 1-tetra-
lones, 4-chromanone, and 1-indanone, were also well toler-
ated. Interestingly, the reaction of highly electron-rich
substrates 1l and 1n afforded products 3la and 3na in
moderate yields but with good to excellent diastereoselectiv-
ity (d.r. 10:1 and > 25:1, respectively). Incomplete conversion
of substrate 1o under the optimized reaction conditions gave
3oa as a mixture of diastereomers (d.r. 1.5:1) in 53% yield
with 22% recovery of the starting material 1o. Increasing the
amount of 2a (2.0 equiv), however, led to full conversion,
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Scheme 3. Derivatizations of 3aa: a) Oxime formation from 3aa.
b) Reduction of 3aa. c) Baeyer–Villiger oxidation of 3aa. MMPP=mag-
nesium monoperoxyphthalate.
(PF6)2 or visible light shut down the reactivity completely,
thus suggesting a crucial role for both of these elements for
the transformation (Table 1, entries 11,12). The reaction was
inhibited in the presence of the radical scavenger 2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO), with a TEMPO-
trapped CF3 adduct being detected by GC–MS analysis.[13]
During the screening of solvents for optimization, the
formation of 3aa was accompanied by the formation of
a methanol-trapped intermediate (detected by GC-MS anal-
ysis) when methanol was employed as solvent.[13] These
results support the involvement of both radical and ionic
intermediates in this reaction.
Based on the above mechanistic information and liter-
ature reports,[10f,i] we propose a plausible mechanism for the
reaction in Scheme 4. Under visible-light irradiation, the
photoredox catalyst [Ru(bpy)3]2+ is excited to the strongly
reducing excited state *[Ru(bpy)3]2+. SET from this species to
Umemotoꢀs reagent 2a would then generate [Ru(bpy)3]3+ and
an electrophilic CF3 radical. Addition onto the double bond
of the silyl-protected intermediate A, formed in situ from 1a
and TMSOTf, would lead to the stabilized radical intermedi-
ate B. At this point, the key radical–polar crossover step can
Scheme 2. Substrate scope of the trifluoromethylation/ring expansion.
1a–s (0.20 mmol), TMSOTf (0.24 mmol), and DMF (2 mL) were
stirred at RT for 2 h. The CF3 reagent (2a, 0.24 mmol) and [Ru(bpy)3]-
(PF6)2 (0.002 mmol) were then added to the reaction mixture and
stirred at RT for 6 h under visible-light irradiation from blue LEDs. d.r.
values (given in parentheses) were determined by 19F NMR analysis.
[a] Incomplete conversion with 22% recovery of 1o. [b] The reaction
was performed with 2.0 equiv of 2a. [c] Detected by GC–MS analysis.
+
with the formation of 3oa in 65% yield with d.r 1.5:1.
Substrate 1p, which does not have an aryl ring in conjugation
=
with the C C bond, did not afford the product 3pa in an
isolable amount. Substituted oxacyclobutanol substrates (1q–
r) also delivered the corresponding products 3qa and 3ra in
lower yields. Despite low ring strain, 1-(1-phenylvinyl)cyclo-
pentanol (1s) gave the desired product 3sa in a reasonable
yield.
Since the products possess a carbonyl functionality, we
further conducted some follow-up transformations to explore
the versatility of the method (Scheme 3). The product 3aa
was converted to the corresponding oxime derivative 5
(70%), which serves as a potential precursor for Beckmann
rearrangement (Scheme 3a). Reduction of 3aa with NaBH4
in MeOH delivered alcohol 6 in 91% yield with excellent
diastereoselectivity (d.r. 25:1), while Baeyer–Villiger oxida-
tion of the product 3aa with magnesium monoperoxyphtha-
late (MMPP) afforded d-lactone 7 in very good yield
(Scheme 3b,c).
In order to gain mechanistic insight into the reaction,
some preliminary mechanistic experiments were performed.
The absence of either the photoredox catalyst [Ru(bpy)3]-
Scheme 4. Plausible mechanism for the visible-light photoredox-cata-
lyzed trifluoromethylation/ring expansion.
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occur with SET from B to [Ru(bpy)3]3+, thereby regenerating
the photocatalyst and delivering the tertiary cation C.
Alternatively, this step could proceed as part of a radical
chain, with SET occurring directly from B to another
molecule of 2a. The involvement of this pathway is supported
by the reaction quantum yield value (F) of 3.8.[13] A 1,2 alkyl
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=
shift with formation of a C O p bond would then afford the
product 3aa upon loss of the silyl group.
In conclusion, we have successfully developed the first
visible-light-induced photoredox-catalyzed semipinacol-type
rearrangement, which proceeds via 1,2 alkyl migration. This
transformation proceeds via a radical–polar mechanism , in
which sequential SET processes involving the photoredox
catalyst enable both radical and polar steps in the same
mechanism. The reaction provides a novel route to densely-
functionalized CF3-containing compounds under mild con-
ditions and uses visible light from readily available sources.
Keywords: fluorine · photocatalysis · radical reactions ·
ring expansion
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11577–11580
Angew. Chem. 2015, 127, 11740–11744
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Received: April 8, 2015
Published online: June 1, 2015
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