Tetrahedron Letters
Tribromoisocyanuric acid as a green reagent for benzylic
bromination of alkylarenes
⇑
⇑
Leonardo S. de Almeida, Pierre M. Esteves , Marcio C. S. de Mattos
Departamento de Química Orgânica, Instituto de Química, Universidade Federal do Rio de Janeiro, CP 68545, 21945-970, Rio de Janeiro, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of diverse alkylarenes with tribromoisocyanuric acid (0.34 mol equiv) in reflux EtOAc in the
Received 3 September 2015
Revised 16 October 2015
Accepted 26 October 2015
Available online xxxx
absence of any catalysts or light irradiation produced the corresponding benzyl bromides in 53–88%
yield.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Tribromoisocyanuric acid
Benzylic bromination
Alkylarenes
Radical reaction
Benzylic bromides are important precursors in the synthesis of
1
pharmaceuticals, agrochemicals, bioactive compounds, etc.
Diverse methodologies are described in the literature for their
2
preparation, especially from benzylic alcohols or alkylarenes.
3
Considering green chemistry aspects, however, some methodolo-
Figure 1. Tribromoisocyanuric acid (TBCA).
gies are not attractive as they use harmful reagents, hard condi-
tions, or generate a large amount of waste. The preparation of
benzylic bromides from the corresponding benzylic alcohols is a
drawback due to the utilization of the corrosive HBr or hazardous
reagents such as phosphorus compounds.4 The free-radical ben-
zylic bromination of alkylarenes is easily achieved using molecular
bromine or N-bromo compounds, being the popular NBS the most
used reagent in such reactions (the so-called ‘Wohl-Ziegler
Unfortunately, despite its hazardous effects, molecular bromine
is still very common in industry as well as in academia.11 Diverse
other safer brominating agents have been developed, however,
their preparations involve the utilization of molecular bromine at
some stage. Some years ago, we have introduced the system KBr/
oxone as a green alternative to produce N-Br compounds.12
Within this context, tribromoisocyanuric acid (TBCA, Fig. 1) is a
TM
TM
5
bromination’). However, molecular bromine is toxic, hazardous,
6
1
2a
corrosive, and a strong oxidizing low-boiling liquid; its handling,
stable solid, easily prepared from cyanuric acid, KBr, and oxone
that has been efficiently used for diverse brominating reactions.
13
storage, transport, and manipulation are difficult and problematic,
especially on a large scale. On the other hand, NBS has to be used in
3
,7
Considering green aspects, tribromoisocyanuric acid possesses
advantages compared to other N-bromo analogues as it is capable
of transferring three atoms of bromine to the substrate (corre-
sponding to up to 65% of its mass). Consequently, the stoichiome-
try of reactions involving TBCA is frequently 3:1 (substrate/TBCA).
Continuing our interest on the chemistry of tribromoisocya-
7
a 1:1 molar proportion and presents low atom economy, as it can
transfer only up to 45% of its mass. Furthermore, both reactions are
frequently performed in polychlorinated solvents, which involve
8
serious environmental concerns. Besides, the presence of a radical
9
initiator is always required in such reactions. Therefore, the
1
4
development of alternative methodologies for a green and efficient
nuric acid, herein we communicate its utilization for benzylic
bromination of alkylarenes under free-radical conditions.
benzylic bromination is a subject of intense research.10
Ethylbenzene was chosen as a model substrate to develop the
best conditions for the benzylic bromination using tribromoisocya-
nuric acid (Table 1). Initial studies utilizing equi-molar quantities
of the reagents and a catalytic amount of benzoyl peroxide as a
⇑
(
M.C.S. de Mattos).
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0