Angewandte
Chemie
Photoredox Catalysis
Trifluoromethylchlorosulfonylation of Alkenes: Evidence for an Inner-
Sphere Mechanism by a Copper Phenanthroline Photoredox Catalyst**
Dattatraya B. Bagal, Georgiy Kachkovskyi, Matthias Knorn, Thomas Rawner,
Bhalchandra M. Bhanage, and Oliver Reiser*
Dedicated to Henri Brunner on the occasion of his 80th birthday
Abstract: A visible-light-mediated procedure for the unprece-
dented trifluoromethylchlorosulfonylation of unactivated
alkenes is presented. It uses [Cu(dap)2]Cl as catalyst, and
contrasts with [Ru(bpy)3]Cl2, [Ir(ppy)2(dtbbpy)]PF6, or
eosin Y that exclusively give rise to trifluoromethylchlorination
of the same alkenes. It is assumed that [Cu(dap)2]Cl plays
a dual role, that is, acting both as an electron transfer reagent as
well as coordinating the reactants in the bond forming
processes.
unactivated alkenes, suggesting an inner-sphere mechanism
that determines the outcome of the reaction beyond a photo-
initiated electron transfer. In contrast, commonly used
ruthenium or iridium complexes or eosin Y give rise to
trifluoromethylchlorination with concurrent loss of sulfurdi-
oxide, being in agreement with an outer-sphere electron
transfer mechanism commonly assumed in photoredox cata-
lyzed ATRA (atom-transfer radical addition) reactions.[11]
Thermal cleavage of triflyl chloride catalyzed by [Ru-
(Ph3P)2Cl2] was demonstrated by Kamigata et al.[12] for the
trifluoromethylchlorination of alkenes. Han et al. showed that
the same process can be achieved by photoredoxcatalysis
using [Ru(bpy)3]Cl2 (Scheme 1).[9b] In both cases the mech-
T
he introduction of the trifluoromethyl group into organic
molecules usually leads to a significant improvement of the
chemical and metabolitic stability of drug candidates.[1–3]
Numerous procedures, including nucleophilic, electrophilic,
or radical approaches were developed for the installation of
the CF3 group,[4] including recent examples in which visible-
light-mediated redox photocatalysis is used as key step.[5]
Ruthenium- and iridium-based complexes can effectively
catalyze the cleavage of CF3I,[6] Togniꢀs reagent,[7] Umemotoꢀs
reagent,[8] or triflyl chloride.[9] The role of these photoredox
catalysts in such transformations is assumed in the transfer of
an electron to the trifluoromethyl source by an outer-sphere
mechanism, providing CF3 radicals that undergo further
transformations.
Exploring the potential of [Cu(dap)2]Cl[10] (À1.43 V vs.
SCE; dap = 2,9-bis(para-anisyl)-1,10-phenanthroline) as
a visible-light-driven photoredox catalyst, we report herein
the unprecedented trifluoromethylchlorosulfonylation of
Scheme 1. Trifluoromethylations of alkenes by visible-light photocatal-
ysis.
anism is assumed to involve the oxidative Ru2+/Ru3+ cycle,
and electron transfer to triflyl chloride is accompanied with
SO2 extrusion.
[*] D. B. Bagal, Dr. G. Kachkovskyi,[+] M. Knorn, T. Rawner,
Prof. Dr. O. Reiser
Institut fꢀr Organische Chemie, Universitꢁt Regensburg
Universitꢁtsstrasse 31, 93053 Regensburg (Germany)
E-mail: oliver.reiser@chemie.uni-regensburg.de
Following our development of ATRA reactions by visible
light photocatalysis using [Cu(dap)2]Cl,[13] we examined the
feasibility of this catalyst towards trifluoromethylations of
alkenes. Surprisingly, we observed that green light (LED
530 nm) irradiation of an acetonitrile solution of allylbenzene
(1a) and triflyl chloride (2 equiv) in the presence of 1 mol%
[Cu(dap)2]Cl resulted in an ATRA process without SO2
extrusion, giving rise to 2a in moderate yield (Table 1,
entry 1). The expected product 3a was only observed in
trace amounts. The presence of a base significantly improves
the yield of 2a, which was isolated in 86% (1 mol%
[Cu(dap)2]Cl) or 63% (0.5 mol% [Cu(dap)2]Cl) yield, respec-
tively, upon addition of K2HPO4 (entries 2,3). Variation of
solvents (entries 4,5) results in a slight decrease in yield;
D. B. Bagal, Prof. Dr. B. M. Bhanage
Department of Chemistry, Institute of Chemical Technology
Matunga, Mumbai-400 019 (India)
[+] Current address: Institute of Bioorganic Chemistry and Petro-
chemistry of National Academy of Science of Ukraine
Murmanska str 1, 02660, Kyiv (Ukraine)
[**] The authors thank the DFG (GRK 1626, Photocatalysis), the DAAD
and the Humboldt foundation for financial support. We thank
Sabine Stempfhuber, Universitꢁt Regensburg, for carrying out the X-
ray crystal structure analysis of 5d, and Prof. R. D. Little, UC St.
Barbara, for helpful discussions.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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