Highly selective Negishi cross-coupling reaction of a zinc-metallated ferrocenyl p-tolyl sulfoxide:: New chiral ferrocene-based quinone ligands

A highly selective Negishi coupling of zinc-metallated ferrocenyl p-tolyl sulfoxide with aryl bromides was developed. With Pd(PPh3)(4) as catalyst, a well-matched system in terms of reactivity of organometallic compound, aryl bromide, and catalyst, was obtained. The scope of the reaction was studied by the use of aryl bromides 7a-g, which afforded high yields of coupling products. The reaction conditions for the preparation of ferrocenyl p-tolyl sulfoxide, according to the Andersen method, were optimised to give a yield of 80% and an enantiomeric excess of 98%. Target compound 1b was obtained by deprotection of acetate-protected hydroquinone 8f, employing sodium borohydride in dimethoxyethane, in a yield of 87%. Target compounds 2b and 3b decomposed during workup. The possibility of internal electron transfer, between the ferrocene moiety and the hydroquinone moiety, may account for this instability. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.