Angewandte
Chemie
DOI: 10.1002/anie.201400602
Heterocycle Synthesis
Visible-Light-Induced Formal [3+2] Cycloaddition for Pyrrole
Synthesis under Metal-Free Conditions**
Jun Xuan, Xu-Dong Xia, Ting-Ting Zeng, Zhu-Jia Feng, Jia-Rong Chen, Liang-Qiu Lu,* and
Wen-Jing Xiao*
Abstract: A photocatalytic formal [3+2] cycloaddition of 2H-
azirines with alkynes has been achieved under irradiation by
visible light in the presence of organic dye photocatalysts. This
transformation provides efficient access to highly functional-
ized pyrroles in good yields and has been applied to the
synthesis of drug analogues. A primary trial of photocascade
catalysis merging energy transfer and redox neutral reactions
was shown to be successful.
P
hotoredox catalysis driven by visible light has recently
emerged as a flourishing research area owing to its inherent
features of green chemistry and sustainability.[1] In this
context, visible-light-induced photocatalytic cycloadditions
have been successfully achieved for the synthesis of structur-
ally diverse carbo- and heterocyclic molecules.[2] Surprisingly,
however, [3+2] cycloaddition reactions proceeding through
a photocatalytic amine oxidation process has received less
attention. In 2011, our group[3] and the group of Rueping[4]
independently disclosed a photocatalytic oxidation/[3+2]
cycloaddition/oxidative aromatization sequence in which
a metal photosensitizer catalyzed the generation of azome-
thine ylides from tertiary amine moieties (Scheme 1a). In
addition, Zheng and co-workers elegantly developed a [3+2]
cycloaddition of cyclopropylamines with olefins in the
presence of a ruthenium complex and visible light, thus
furnishing various cyclopentane derivatives in good yields
(Scheme 1b).[5] Despite the advances, further exploration of
Scheme 1. Visible-light-induced photocatalytic [3+2] cycloadditions.
bpy=2,2’-bipyridine, bpz=2,2’-bipyrazine, EWG=electron-withdraw-
ing group, DCE=1,2-dichloroethane, NBS=N-bromosuccinimide.
photocatalytic cycloaddition reactions employing non-terti-
ary amine precursors and avoiding the use of precious metal
catalysts is a highly desirable, yet challenging goal.
Polysubstituted pyrroles constitute an important family of
five-membered N-containing heterocycles with diverse bio-
logical properties and synthetic applications.[6] Not surprins-
ingly, great efforts have been devoted to the development of
practical, atom- and step-economic approaches to the pyrrole
architecture.[7] In particular, the exploition of novel reagents
and strategies has led to pyrrole syntheses using mild
conditions and exhibting good functional-group tolerance.
Recently, 2H-azirines (1) were established as highly valuable
synthetic intermediates for constructing different azacyclic
compounds, usually by using transition-metal catalysts or UV
light irradiation.[8] From the viewpoint of green chemistry, it is
significant to update the synthetic transformation of 2H-
azirines under operationally simple and environmentally
friendly conditions. As part of our ongoing research interests
in developing new methods for the synthesis of carbo- and
heterocycles by visible-light photoredox catalysis,[9] we report
herein a novel visible-light-induced [3+2] cycloaddition
reaction of the 2H-azirines 1 with activated alkynes 2
(Scheme 1c). The method provides an efficient route to
various polysubstituted pyrroles under very mild reaction
conditions (visible-light irradiation, metal-free, and room
temperature).
[*] J. Xuan, X.-D. Xia, T.-T. Zeng, Z.-J. Feng, Prof. Dr. J.-R. Chen,
Dr. L.-Q. Lu, Prof. Dr. W.-J. Xiao
CCNU–uOttawa Joint Research Centre, Key Laboratory of Pesticide
& Chemical Biology, Ministry of Education, College of Chemistry
Central China Normal University (CCNU)
152 Luoyu Road, Wuhan, Hubei 430079 (China)
and
Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin (China)
E-mail: luliangqiu@mail.ccnu.edu.cn
Prof. Dr. W.-J. Xiao
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, CAS (China)
[**] We are grateful to the National Science Foundation of China (NO.
21232003, 21202053, and 21272087) and the National Basic
Research Program of China (2011CB808603) for support of this
research. We thank Prof. Li-Zhu Wu and Qin-Yuan Meng in Technical
Institute of Physics and Chemistry (CAS) for the quantum yield
measurements.
Initially, the formal [3+2] cycloaddition reaction was
investigated under irradiation from a 3 W white LED light by
using 2,3-diphenyl-2H-azirine (1a) and dimethyl but-2-yne-
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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