Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1143–1145
Cobalt-catalyzed photolytic methoxycarbonylation of
bromoalkanes in the presence of a Lewis acid
Daniel Cash,a Angela Combsa and Veljko Dragojlovicb,*
aDepartment of Math, Science and Technology, Farquhar College of Arts and Sciences, Nova Southeastern University,
3301 College Avenue, Fort Lauderdale, FL 33314, USA
bOceanographic Center, Nova Southeastern University, 8000 North Ocean Drive, Dania, FL 33004, USA
Received 14 November 2003; revised 2 December 2003; accepted 3 December 2003
Abstract—Addition of a water-stable Lewis acid, indium trichloride, improved the rate of photochemical methoxycarbonylation of
bromoalkanes. Primary and secondary bromoalkanes were carbonylated in good yields. Carbonylation of tertiary substrates was
somewhat more difficult and some of them reacted too slowly for the reaction to be of a preparative value.
Ó 2003 Elsevier Ltd. All rights reserved.
It has been reported that photocarbonylation of
iodoalkanes proceeds with ease while more readily
available and less expensive bromoalkanes are unreac-
tive.1;2 Photochemical carbonylation reactions are done
in the presence of an inexpensive cobalt catalyst at room
temperature and at atmospheric pressure of carbon
monoxide. However, photochemical carbonylations
sometimes suffer from low reaction rates and limited
reaction scale. Lewis acids have been shown to increase
the rate of thermal carbonylation reactions.3 Recently,
aluminum-based Lewis acids were successfully employed
in cobalt-catalyzed thermal carbonylations of epoxides
and aziridines.4;5 We investigated the use of Lewis acids
as additives in order to improve the reaction rate and
make photocarbonylation of bromoalkanes feasible.
ped with a 450 W medium pressure mercury lamp and
Vycor filter and adjusting the concentrations of the
cobalt catalyst (5 mol %), Lewis acid, and bromoalkane.
Photocarbonylation reactions of bromoalkanes were
done in a mixture of methanol and acetone as solvents.
In photocarbonylation of alkenes, it was established
that acetone was necessary for the reaction to proceed.6–8
The authors assumed that acetone was a sensitizer.7
Due to different induction periods as well as a change in
the reaction rate due to acid (HBr) buildup, it is difficult
to compare the relative rates or the catalyst turnover
numbers. The best way to compare efficiency of different
catalytic systems appears to be comparison of reaction
times and yields. Photolytic methoxycarbonylation of
bromocyclohexane in the presence of Co(acac)2 as a
catalyst source yielded only 3% of methyl cyclohexane-
carboxylate after 24 h (Table 1, entry 1). It was proposed
that, in thermal carbonylation reactions, Lewis acids
increase the reaction rate by coordinating to the carbon
monoxide ligand.3 Addition of commonly used Lewis
acids (BF3ÆMeOH, AlCl3, GaCl3) to the reaction mix-
ture failed to improve the yields or increase reaction
rates (Table 1, entries 2–4). In fact, the reaction rate of
methoxycarbonylation was retarded and the main
product was the corresponding aldehyde. Lanthanide
chlorides also retarded the reaction rate with formation
of the aldehyde as the major reaction products (Table 1,
entries 5 and 6). Since protic acids have been shown to
suppress photochemical cobalt-catalyzed methoxycarb-
onylation and to give a mixture of aldehydes and acetals
in low yields,8 we speculated that the Lewis acids reacted
A modified quartz Ace micro-scale photochemical setup
equipped with a PenRey 5.5 W low pressure mercury
lamp was employed. A relatively weak UV source
allowed for the reactions to be done on a small scale and
with high reproducibility. For carbonylation to proceed
at a reasonable rate, a relatively high catalyst loading
(25–50 mol %) had to be employed. Once the reaction
conditions were optimized on a micro-scale setup, the
procedure was scaled up to a gram-scale reaction by
employing a quartz Ace macro-scale apparatus equip-
Keywords: Bromoalkanes; Cobalt; Photocarbonylation; Lewis acid;
Indium trichloride.
* Corresponding author. Tel.: +954-262-8332; fax: +954-262-3931;
0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.009