Asymmetric [2+2] cycloaddition of (E)-2-(diphenylphosphanyl)styrene promoted by a chiral metal template

Abstract

An organoplatinum complex containing the (S)-form of orthometalated [1-(dimethylamino)ethyl]naphthalene as the chiral auxiliary promotes the asymmetric [2+2] dimerization of (E)-2-(diphenylphosphanyl)styrene to generate two isomeric chelating diphosphanyl-substituted cyclobutane platinum template complexes in the ratio of 6:1. The four substituents on the cyclobutane rings adopt the pseudo-equatorial positions with an all-trans arrangement. The naphthylamine auxiliary can be removed chemoselectively from the template products by treatment with concentrated hydrochloric acid to form the corresponding dichloroplatinum(II) complexes, which, upon subsequent ligand displacement with aqueous cyanide, liberate the enantiomerically enriched free diphosphane ligand in high yields. Recomplexation of the liberated diphosphane to the (R)-form of the platinum template, followed by fractional crystallization, allows separation of the optically pure platinum template complex containing the diphosphane ligand (1R,2R,3R,4R)-1,2- bis(diphenylphosphanyl)-3,4-diphenylcyclobutane, from which the free diphosphane can be subsequently liberated efficiently. © Wiley-VCH Verlag GmbH & Co. KGaA, 2005.