DOI: 10.1002/chem.201601819
Communication
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Organic Synthesis
Hantzsch Ester as a Photosensitizer for the Visible-Light-Induced
Debromination of Vicinal Dibromo Compounds
Wenxin Chen,[a] Huachen Tao,[a] Wenhao Huang,[a] Guoqiang Wang,[b] Shuhua Li,*[b]
Xu Cheng,*[a] and Guigen Li[a, c]
Abstract: The debromination of vicinal dibromo com-
pounds to generate alkenes usually requires harsh reac-
tion conditions and the addition of catalysts. Just recently
the visible-light-induced debromination of vicinal dibromo
compounds emerged as a possible alternative to com-
monly used methods, but the substrate scope of this reac-
tion is limited and a photocatalyst is necessary for the suc-
cessful conversion of the starting compounds. A catalyst-
free visible-light-induced debromination of vicinal dibro-
mo compounds with a base-activated Hantzsch ester as
photosensitizer is reported. The method has a wide sub-
strate scope and a broad functional-group compatibility.
Vicinal dibromination has been established as an efficient
method for the protection of CÀC double bonds in organic
synthesis, and the corresponding deprotection can be carried
Scheme 1. Visible-light-induced debromination to form alkenes.
out with metal reductants, however, harsh reaction conditions
are required; for example, the zinc-mediated reduction typical-
ly requires an acidic medium at elevated temperatures.[1] Fur-
demonstrated the Ru(bpy)3+-catalyzed debromination of vici-
thermore, the use of strongly reductive metal species limits
the functional-group compatibility.[2] Recently, the visible-light-
nal dibromocarbonyl compounds to yield a,b-unsaturated mol-
ecules.[5] Both of these methods require a costly transition-
induced mesolysis emerged as a powerful strategy for cleaving
CÀBr bonds.[3] Because the functional groups commonly found
metal catalyst. In 2014, the photoreductive debromination
in organic molecules do not absorb visible light, this reaction
requires a photocatalyst that acts as an electron shuttle
(Scheme 1).
using sexithiophene as photocatalyst was demonstrated by
Scaiano and co-workers, who focused on stilbene and a-car-
bonyl dibromo compounds.[6] Despite these reports, a general
strategy for unmasking alkenes, especially unfunctionalized
ones, from dibromo precursors has not been reported. In this
study, we investigated the catalyst-free visible-light-induced
deprotection of a wide range of substrates by a method that
uses the Hantzsch ester as light harvester, electron donor, and
hydrogen donor. Beyond its passive role as a reductant in
closed-shell[7] and open-shell chemistry,[8] the Hantzsch ester
can be self-activating with the help of an inorganic base to di-
rectly achieve an electron transfer.
In 1986, Willner et al reported the Ru(bpy)3+-catalyzed de-
protection of 1,2-dibromostilbene under visible-light irradiation
with NADH as the terminal reductant.[4] In 2011, Reiser et al
[a] W. Chen, H. Tao, W. Huang, Prof. Dr. X. Cheng, Prof. Dr. G. Li
Institute of Chemistry and Biomedical Sciences
School of Chemistry and Chemical Engineering
Nanjing University, Xianlin Road, 163, Nanjing, (China)
[b] G. Wang, Prof. Dr. S. Li
We chose p-bromophenyl-1,2-dibromoethane (1a), a styrene
precursor, as a typical substrate for the optimization of the de-
bromination reaction to form the alkene 3a (Table 1). The de-
protection of 0.2 mmol 1a was carried out with 1.1 equiv of
the Hantzsch ester 2 and 1.1 equiv of a base under irradiation
with a 12 W white LED at room temperature. DMSO was found
to be the optimal solvent (Table 1, entry 6), but DMF also gave
good yields (Table 1, entry 1). In acetonitrile, DCM, THF, or tolu-
ene, the reaction did not run to completion (Table 1, entries 2–
Key Laboratory of Mesoscopic Chemistry of Ministry of Education
Institute of Theoretical and Computational Chemistry
School of Chemistry and Chemical Engineering
Nanjing University, Hankou Road, 22, Nanjing (China)
[c] Prof. Dr. G. Li
Department of Chemistry and Biochemistry, Texas Tech University
Memorial Circle & Boston, TX 79409–1061 Lubbock(United States)
Supporting information for this article can be found under:
Chem. Eur. J. 2016, 22, 1 – 6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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