Communications
Allylic Compounds
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Direct Cross-Coupling of Allylic C(sp ) H Bonds with Aryl- and
Vinylbromides by Combined Nickel and Visible-Light Catalysis
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Abstract: An efficient protocol for the direct allylic C(sp ) H
bond activation of unactivated tri- and tetrasubstituted alkenes
and their functionalization with aryl- and vinylbromides by
nickel and visible-light photocatalysis has been developed. The
arylation of a-heterosubstituted C(sp ) H bonds using the
combination of a nickel catalyst and an iridium photo-
catalyst.[11] Mechanistic studies revealed that the excited NiII-
ArBr species, which is generated through triplet–triplet
energy transfer with the excited photocatalyst, effects the
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method allows C(sp ) C(sp ) formation under mild reaction
conditions with good functional-group tolerance and excellent
regioselectivity.
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C H activation by homolysis of the Ni Br bond. We there-
fore envisioned that this concept of photocatalytically gen-
erated bromine radical for hydrogen atom abstraction could
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A
llylarene units are important structural features in natural
be further extended to allylic C(sp ) H bonds of simple
products and bioactive molecules.[1] Furthermore, allylarenes
serve as ubiquitous synthetic intermediates in organic
chemistry for numerous transformations.[2] Thus, considerable
attention has been devoted to the construction of this
olefins (Scheme 1). Herein, we present the development of
a general and regioselective method for allylic arylation that
exhibits broad scope across a wide range of (tri- and
tetrasubstituted) olefins and aryl- and vinylbromide electro-
philic coupling partners. To our knowledge such a trans-
formation has not been accomplished to date.
moiety.[3–6] The construction of C(sp ) C(sp ) bonds by
direct functionalization of C H bonds is a powerful tool for
the synthesis of highly functionalized and complex alkenes.
However, compared with the significant progress in direct
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oxidation and amination of allylic C H bonds, routes toward
direct arylation are narrowly explored. Recently, Nakamura
disclosed an iron-catalyzed direct arylation of olefins with aryl
Grignard reagents, but only limited examples and low yields
were achieved.[7] In light of the pioneering work of Arnold
and co-workers on photoinduced cross-coupling of dicyano-
benzene with olefins,[8] MacMillan and co-workers recently
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Scheme 1. Our strategy for direct arylation of allylic sp C H bonds of
olefins.
reported the direct arylation of allylic C(sp ) H bonds using
thiyl radicals as hydrogen-atom abstractors, and less available
cyanoarenes as coupling partners.[9] Hence, the development
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of direct, general, and regioselective C(sp ) C(sp ) bond-
forming reactions with readily available substrates would be
of great importance for both synthetic organic chemistry as
well as materials and pharmaceuticals development.
Initially, we investigated the coupling of readily available
methyl 4-bromobenzoate (1a) and tetramethylethylene
(TME; 2a) as a model reaction and the results are summar-
ized in Table 1. A preliminary evaluation of photocatalysts
showed that the reaction proceeds smoothly using [Ir(dF-
(CF3)ppy)2(bpy)]PF6 (4) in the presence of NiCl2·glyme
(glyme = ethylene glycol dimethyl ether), 4,4’-di-tert-butyl-
2,2’-bipyridine (dtbbpy), and 2,6-lutidine at room temper-
ature under irradiation with blue LEDs for 48 hours, afford-
ing the desired product 3a in 84% yield (entry 1). To our
surprise, the organic dye [Acr-Mes]ClO4 (5) gave the desired
product 3a in a comparable yield while [Ru(bpy)3]PF6 (6)
proved to be ineffective (entries 2 and 3). From a cost-
effective and sustainable perspective, the commercially
available acridinium 5 is the preferred photocatalyst.[12] To
be noted, this is the first report on the combination of an
acridinium photocatalyst and a nickel metal catalyst.[13]
Replacing NiCl2·glyme with the air sensitive Ni(cod)2 failed
to improve the yield (entry 4). The reaction performed with
lower efficiency when other solvents were used (entries 5 and
6). Furthermore, lowering the concentration hampered the
formation of 3a severely (entry 7). Evaluation of different
Visible-light photoredox and metal dual catalysis has
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emerged as an effective strategy for diverse C C and C X
(X = N, O, S, P) bond formations in a redox-, atom-, and step-
economical fashion.[10] Key to this success is the capacity of
photocatalysts to act as both strong oxidants and reductants
by single-electron transfer (SET) upon irradiation with visible
light. Recently, Molander and co-workers reported the direct
[*] Dr. L. Huang, Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: magnus.rueping@rwth-aachen.de
Prof. Dr. M. Rueping
King Abdullah University of Science and Technology (KAUST), KAUST
Catalysis Center (KCC), Thuwal, 23955-6900 (Saudi Arabia)
E-mail: Magnus.Rueping@Kaust.edu.sa
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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