CHIMIA (Feb 2009)
Aminophosphine Palladium Pincer Complexes for Suzuki and Heck Reactions
Abstract
The aminophosphine-based pincer complexes [C6H3-2,6-{NHP(piperidinyl)2}2Pd(Cl)] (2) and [C6H3-2,6-{OP(piperidinyl)2}2Pd(Cl)] (3) are readily prepared from cheap starting materials by sequential addition of 1,1?,1?-phosphinetriyltripiperidine and 1,3-diaminobenzene or resorcinol to solutions of [Pd(cod)(Cl)2] (cod = cyclooctadiene) in toluene under N2 in 'one pot'. Compounds 2 and 3 proved to be not only excellent catalysts for the Suzuki and the Heck cross-coupling reactions, but they are also very convenient to use: The toluene solutions of the 'one-pot' syntheses can be used directly for the catalytic reactions, thereby saving the time-consuming isolation of the catalysts. The Suzuki cross-coupling reaction catalyzed by 2 and 3 can be performed in air at 100 °C in toluene of technical quality: in the presence of only 0.001mol% of catalyst, several electronically deactivated and sterically hindered aryl bromides are quantitatively coupled with phenylboronic acid within a few minutes of reaction time. Furthermore, complex 2 enables the use of activated and non-activated aryl chlorides as coupling partners in the Suzuki reaction. Compounds 2 and 3 have also been shown to be highly active and reliable Heck catalysts: Very low catalyst loadings and short reaction times are required for the quantitative coupling of several electronically deactivated and sterically hindered aryl bromides with various olefins at 140 °C. At increased temperatures, even electronically deactivated and sterically hindered aryl chlorides can be efficiently coupled with olefins in the presence of only 0.01 mol% of catalyst.
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