Heck reaction in ionic liquids catalyzed by a Pd-benzothiazole carbene complex

Article abstract of DOI:10.1016/S0040-4039(00)01592-6

A Pd-catalyst with benzothiazole carbene ligands allows, in ionic liquids as solvent, very fast and efficient reactions of haloaromatics with alkenes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01592-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8973–8976
Heck reaction in ionic liquids catalyzed by a
Pd–benzothiazole carbene complex
Vincenzo Calo`,* Angelo Nacci, Luigi Lopez and Nicola Mannarini
Centro CNR ‘MISO’, Dipartimento di Chimica, Universita` di Bari, via Amendola 173, 70126 Bari, Italy
Received 23 May 2000; revised 30 August 2000; accepted 14 September 2000
Abstract
A Pd-catalyst with benzothiazole carbene ligands allows, in ionic liquids as solvent, very fast and
efficient reactions of haloaromatics with alkenes. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Heck reaction; ionic liquids; carbenes.
There has been much interest recently in the use of ionic liquids as solvents for a variety of
organic and organometallic reactions. These ionic liquids, which are easily recycled and with no
effective vapour pressure, are good solvents for metal catalysts. Thus, they offer an attractive
alternative to conventional organic solvents for the clean synthesis of reaction products that are
not catalyst-contaminated. As a consequence, it is likely that many chemical reactions can be
conducted in these novel solvent systems with potential enhancement in both yield and
selectivity. A few types of catalytic reactions have been carried out in these solvents. Examples
are Friedel–Crafts reactions,1 Diels–Alder reactions,2 alkylations,3 Pd-catalyzed allylations,4
hydrogenation5 and hydroformylation.6 It has been observed by Jeffery,7 Hermann,8 Reetz9 and
Buchwald10 that in the Heck reaction, the addition of tetraalkylammonium or tetraphenyl
phosphonium salts to conventional solvents, leads to higher catalytic activities. A few papers
dealing with the Heck reaction in ionic liquids with Pd-complexes and phosphane ligands have
been reported.11–13
Recently we reported14 the synthesis of a Pd catalyst 1 with benzothiazole carbenes as
ligands,15 which proved to be an efficient catalyst for Heck reaction of aryl iodides and activated
aryl bromides.16
* Corresponding author. E-mail: calo@chimica.uniba.it
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01592-6

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We now report that complex 1 is an excellent catalyst in the phosphine-free Heck reaction of
aryl bromides in ionic liquids.
In a model reaction, the Heck coupling of bromobenzene with butyl acrylate was carefully
monitored17 with respect to conversion using 1 mol% of catalyst at 130°C and 2% of sodium
formate as the reducing agent for palladium in tetrabutylammonium bromide (TBAB) as
solvent. With this catalyst, we did not observe an induction period as found for analogous
reactions performed with (imidazole) Pd carbene complexes8 or palladium acetate.9b By using
sodium acetate as the base, 94% conversion was obtained after 4 h. An astonishing increase of
the reaction rate was observed by replacing the acetate with sodium carbonate with bromoben-
zene being converted into butyl cinnamate within 10 min (Table 1, runs 1 and 2, respectively).
Although the carbonate was the base of choice, we replaced it with sodium bicarbonate with the
aim of limiting concurrent formation of tributylamine, derived from Hofmann elimination (by
Table 1
Arylation of butyl acrylate catalysed by complex 1a

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sodium carbonate) of the tetrabutylammonium salt. This base replacement, although it slightly
decreases the reaction rate, avoids solvent decomposition. Also, less activated bromoaromatics
react easily with high conversion and p-nitrochlorobenzene gave the nitrocinnamic ester in 95%
yield in 1 h (Table 1, run 9). Besides the acrylate, styrene reacts under the same reaction
conditions with 4-bromotoluene in less than 2 h to give a 90% isolated yield of a 9:1 mixture of
trans-4-methylstilbene and 1-(4-methylphenyl)-1-phenylethene, respectively.
Actually we do not have an explanation for such high reaction rates. Probably, the presence
of carbene ligands increases the reduction rate of the Pd(II) complex by formate and the
formation of very small palladium nanoparticles stabilized by TBAB18 would catalyze the
reaction.
Acknowledgements
This work was in part financially supported by Ministero dell’Universita` e della Ricerca
Scientifica e tecnologica, Rome, and the University of Bari (National Project: ‘Stereoselezione in
Sintesi Organica: Metodologie ed Applicazioni’).
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17. Typical procedure: A pyrex reaction flask was charged with tetrabutylammonium bromide (3 g) and heated at
130°C. To the stirred molten salt were added in strict order the catalyst (125 mg, 0.19 mmol), sodium formate
(26 mg, 0.38 mmol), haloarene (19 mmol), sodium bicarbonate (3.4 g, 40 mmol) and butyl acrylate (2.4 g, 19
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.