The coupling of arylboronic acids with electron‐rich allylic bromides is accomplished in the absence of any transition‐metal catalyst through conventional heating. The reaction is completely regioselective, affording only the α‐coupled product, and can be carried out under mild aerobic conditions in an organic solvent; the presence of a base is required.

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The preparation of the new ligand 8-(di-tert-butylphosphinooxy) quinoline (1) and the palladium derivatives [PdCl2(1)] (2), [Pd(eta(3)-all)(1)](+) [all = C3H5 (3a), 1-PhC3H4 (3b) and 1,3-Ph2C3H3 (3c)] and [Pd(eta(2)ol)(1)] [ol = dimethyl fumarate (4a) and fumaronitrile (4b)] is reported. The cationic species 3a-3c have been isolated as BF4- salts. The complex 3a(BF4) is obtained either from the reaction of 1 with [Pd(mu-Cl)(eta(3)-C3H5)](2) or from the reaction of ClP(CMe3)(2) with [Pd(eta(3)-C3H5)(8-oxyquinoline)], followed in both cases by chloride abstraction with NaBF4. In the complexes, the ligand 1 is P,N chelated to the central metal, as shown by the X-ray structural analysis of 3a( BF4). At 25 degrees C in solution, 3a(BF4) and 3b(BF4) undergo a fast eta(3) - eta(1) - eta(3) dynamic process which brings about a syn-anti exchange only for the allylic protons cis to phosphorus, while for 4a and 4b a slow rotation of the olefin around its bond axis to palladium takes place. The complexes 2 and 3a(BF4) are efficient catalyst precursors in the coupling of the phenylboronic acid with aryl bromides and chlorides. (C) 2008 Elsevier B. V. All rights reserved

a b s t r a c tThe complex [PdCl 2 (P-N)] containing the basic and sterically demanding 8-(di-tert-butylphosphinooxy)-quinoline ligand (P-N) is a highly efficient catalyst for the coupling of phenylboronic acid with aryl bromides or aryl chlorides. The influence of solvent and base has been investigated, the highest rates being observed at 110 C in toluene with K 2 CO 3 as the base. With aryl bromides the reaction rates are almost independent on the electronic properties of the para aryl substituents, on the contrary, reduced reaction rates are observed when bulky substituents are present on the substrate. Nevertheless the coupling of 2-bromo-1,3,5-trimethylbenzene with phenylboronic acid can be carried out to completion in 2 h using a catalyst loading of 0.02 mol %. Under optimized reaction conditions, turnover frequencies as high as 1900 h À1 can be obtained in the coupling of 4-chloroacetophenone with phenylboronic acid; lower reaction rates are obtained with substrates bearing EDG substituents on the aryl group.

The Heck coupling of aryl bromides with olefins such as styrene or butyl acrylate is efficiently catalysed by the iminophosphine-palladium(0) complex [Pd(dmfu)(P-N)] (dmfu = dimethyl fumarate; P-N = 2-(PPh 2)C 6 H 4-1-CH NC 6 H 4 OMe-4) (1) in polar solvents. With activated aryl bromides such as 4-bromoacetophenone turnover numbers of up to 20,000 can be achieved at 140 • C in 2 h. The presence of electron-donating groups leads to decreased reaction rates, nevertheless, high substrate conversions can be obtained in reasonable reaction times. Kinetic studies indicate that complex 1 is only a precursor of the actual catalytic species. Experiments aimed to demonstrate the intervention of metallic palladium did not lead to conclusive findings.

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