LETTER
Catalysis of the Heck-type reaction of alkenes with arylboronic acids by
silica-supported rhodium: an efficient phosphine-free reusable catalytic
protocol
Rajiv Trivedi,* Sarabindu Roy, Moumita Roy, B. Sreedhar and
M. Lakshmi Kantam
Received (in Montpellier, France) 8th May 2007, Accepted 27th June 2007
First published as an Advance Article on the web 19th July 2007
DOI: 10.1039/b706904a
A 3-aminopropyl-functionalized silica-supported rhodium(0) cat-
alyst (SiO2–Rh0) was prepared and employed in the Heck-type
coupling of arylboronic acids and alkenes, affording good-to-
excellent yields of substituted alkenes; the catalyst was recov-
ered by filtration and reused for several cycles.
Silica shows many advantageous properties, such as excel-
lent chemical and thermal stability, high surface area, good
accessibility and the fact that organic groups can be robustly
anchored to the surface to provide catalytic centers.5 By
exploiting these advantages, we prepared a 3-aminopro-
pyl-functionalized silica immobilized rhodium(0) catalyst
(SiO2–Rh0) and subsequently tested it in the Heck-type aryla-
tion reaction of alkenes with arylboronic acids.
The rhodium-catalyzed conjugate addition of arylboronic
acids to a,b-unsaturated carbonyl compounds is a well estab-
lished methodology in modern organic synthesis,1 but the
related Heck-type olefination reaction was first reported in
2001 by Lautens et al.2a They have shown that a substrate
incapable of enolization, such as styrene, affords the Heck-
type product, whereas ortho- or para-vinylpyridines provide
the conjugate adducts in the presence of [Rh(COD)Cl]2/4,40-
(phenylphosphinidene)bis(benzenesulfonic acid)dipotassium
salt (TPPDS). Later on, they widened the scope of their
methodology to enolizable alkenes in a toluene–water (1 : 1)
solvent system by using 2-(di-tert-butylphosphino)ethyltri-
methylammonium chloride (t-Bu-amphos chloride) in place
of the TPPDS ligand.2b Just after the report of Lautens et al.,2a
Mori et al. reported a hydroxorhodium complex-catalyzed
carbon–carbon bond forming reaction of silanediols with
a,b-unsaturated carbonyl compounds. Heck-type products
were obtained in anhydrous solvents, whereas in aqueous
solvents, the Michael-type adduct was formed.3 In 2003, Zou
et al. reported the rhodium-catalyzed Heck-type reaction of
arylboronic acids with a,b-unsaturated esters in a water–
toluene biphasic medium.4 All the protocols reported so far
for the Heck-type coupling of alkenes with arylboronic acids
by rhodium are homogeneous in nature and require an
ancillary phosphine ligand. The major drawback of homo-
geneous catalysis is the need to separate the relatively expen-
sive catalyst from the reaction mixture at the end of the
process. One of the most promising solutions to this problem
seems to be the immobilization of the soluble catalysts onto an
insoluble matrix using a simplified protocol. So, to dispense
with the use of ligands, as well as to achieve the reusability of
the precious rhodium, it is highly desirable to design a ligand-
free and recyclable heterogeneous system for this elegant
coupling reaction.
The SiO2–Rh0 catalyst was prepared by stirring 3-amino-
propyl-functionalized silica gel (500 mg) at room temperature
with an aqueous solution of RhCl3 ꢀ xH2O (30 mg in 200 mL)
under a nitrogen atmosphere for about 10 min, followed by
reduction with an excess amount of an aqueous solution of
NaBH4. Just after the addition of NaBH4, the solution
changed its colour from pink to black, confirming the reduc-
tion of rhodium(III) to rhodium(0). Finally, the catalyst was
filtered and washed with water, followed by ether, to afford the
black silica-immobilized rhodium(0) catalyst (SiO2–Rh0). The
loading of rhodium was measured as 0.221 mmol gꢁ1
(ICP-AES). TEM images of the SiO2–Rh0 showed the rho-
dium particles to be in the range 10–20 nm (Fig. 1).
The SiO2–Rh0 catalyst was used in the coupling of phenyl-
boronic acid (1a) and n-butyl acrylate (2a) (Scheme 1) in a
toluene/water (3 : 1) mixture under a nitrogen atmosphere for
24 h to afford n-butyl cinnamate (3a) in good yield (72%),
along with a very small amount of the corresponding 1,4-
Michael addition product (4a) (o1% as measured by 1H
NMR). 3a was obtained in 82% yield when a 1a : 2a (1 : 3)
mixture in toluene–water (5 : 1) was treated with 1.1 mol%
catalyst at 100 1C for 10 h (Table 1, entry 1). A small amount
of water is needed for the reaction to proceed,6 and overall
Inorganic and Physical Chemistry Division, Indian Institute of
Chemical Technology, Hyderabad 500 007, India. E-mail:
rajiv_trivedi_1968@yahoo.com; Fax: +91 40-2716 0921;
Tel: +91 40-2719 3510
Fig. 1 TEM images of SiO2–Rh0 catalyst.
ꢂc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 1575–1578 | 1575