Letter
Visible Light-Mediated Ullmann-Type C−N Coupling Reactions of
Carbazole Derivatives and Aryl Iodides
Woo-Jin Yoo, Tatsuhiro Tsukamoto, and Shu Kobayashi*
̅
Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: The combined use of an iridium-based photo-
catalyst and a copper salt under blue light emitting diode
irradiation enables the Ullmann-type C−N cross-coupling
reaction between carbazole derivatives and aryl iodides to
proceed under mild conditions.
isible light-driven photoredox catalysis has emerged as a
powerful means to promote efficient organic trans-
Scheme 1. Light-Mediated, Copper-Catalyzed C−N Ullmann-
V
Type Coupling Reactions
formations under mild conditions.1 This is, in part, due to the
realization that photoredox active transition metals can mediate
single electron transfer (SET) processes of various organic
functional groups to generate reactive charged radical
intermediates. Moreover, it has been demonstrated that the
inclusion of photoredox catalysis into other types of catalytic
cycles can lead to new synthetic methodologies and improve the
reaction conditions of established organic transformations.2−4
Owing to the numerous examples of functionally important
nitrogen-containing organic compounds found in the pharma-
ceutical, agrochemical, and material sciences, the development of
efficient methods to construct carbon−nitrogen (C−N) bonds is
of paramount importance. As such, the palladium-catalyzed
Buchwald−Hartwig5 and the copper-catalyzed Ullmann-type6
coupling reaction between aryl halides and nitrogen-based
nucleophiles have been extensively studied. Although the original
report by Ullmann required the use of a stoichiometric amount of
copper under extremely harsh reaction conditions to effectively
form the C−N bond, the use of copper as a catalyst is
advantageous due to its lower cost and toxicity when compared
to palladium. Thus, considerable efforts have been made to
improve the Ullmann-type C−N coupling reaction and catalytic
protocols that permits the cross-coupling reaction to occur under
mild conditions have been developed. Recently, Fu and Peters
disclosed a series of copper-catalyzed protocols for the cross-
coupling reactions of various heteroatom nucleophiles with aryl
and alkyl halides under ultraviolet (UV) light irradiation
(Scheme 1, eq 1).7 Interestingly, they reported that the
copper-catalyzed cross-coupling reaction of indole and iodo-
benzene did not occur under visible light irradiation, even in the
presence of Ru(bpy)32PF6 as a photoredox catalyst.7c However,
the combined use of visible light photoredox catalysis and
transition metal catalysis in cross-coupling reactions has recently
appeared. For instance, Doyle and MacMillan demonstrated a
productive merger between a nickel catalyst and a heteroleptic
iridium photocatalyst to effectively induce the decarboxylative
cross-coupling reaction of aryl and vinyl halides with alkyl
carboxylic acids.4e,k Similarly, Molander reported that alkyltri-
fluoroborates could undergo efficient transmetalation via
photoredox catalyst-mediated SET for the nickel-catalyzed
cross-coupling reaction with aryl halides.4c,d,j In addition, Lu
and Xiao found that a dual catalyst system comprising a nickel
catalyst and a ruthenium-based photoredox catalyst allowed for
an efficient carbon−phosphorus cross-coupling reaction.4a
Related to these redox-neutral cross-coupling reactions, we
recently reported a copper-catalyzed Chan−Lam coupling
reaction of aniline derivatives and aryl boronic acids under
visible light irradiation in air.4b Based on these examples, we
believed that it was possible to achieve the copper-catalyzed
Ullmann-type C−N coupling reaction under visible light
irradiation. Herein, we describe the productive merger between
copper catalysis and photoredox catalysis that enables the cross-
coupling reaction of aryl iodides and carbazole derivatives under
visible light irradiation (Scheme 1, eq 2).
The starting point of our optimization studies was based on Fu
and Peters’s previous conditions for the photoinduced C−N
Ullmann-type coupling reaction7c using carbazole (1a) and
iodobenzene (2a) as model substrates with Ir(ppy)3 as a
photoredox catalyst under white light emitting diode (LED)
irradiation (Table 1).
We initially screened various solvents (entries 1−3) and found
that DMSO provided 9-phenylcarbazole (3a) in a moderate
yield. Since oxygen gas can diminish the performance of a
photoredox catalyst,8 we carefully degassed the reaction mixture
Received: June 5, 2015
© XXXX American Chemical Society
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Org. Lett. XXXX, XXX, XXX−XXX