The Intricate Assembling of gem-Diphenylpropargylic Units

While optimized procedures for selective propargyljc - versus allenic - attack (in particular by alkynylsilanes) have proven to be compatible with many substitution patterns at the propargylic center, the case of diarylpropargyl electrophiles has remained problematic. The intrinsic reactivity of 1,1-diphenylpropargylic alcohols [R-C C-C(Ph-2)OH (R = TMS, H, Me)] in the presence of various acids (and in the absence of additional nucleophile) has thus been systematically investigated. Whereas the monophenyl analogues [R-C C-CH(Ph)OH] afford the expected bis(phenylpropargyl) ethers, the diphenyl versions undergo complex but quite selective processes to afford various structural types: depending on the acid used, a diatlene, an allerlyne, an indenylatlene, an indanone or a condensed tetra- and pentacycles were obtained. When the reactions were conducted in the presence of an alkynylsilane capable of playing the role of a competing nucleophile, the expected propargylic substitution products - dialkynyldiphenylmethanes or their isomers were never observed, The hitherto unknown simple hydrocarbons diethynyl- and dipropynyl-diphenylinethane could, however, be obtained in low yields through a four-step sequence involving allenylidene- and alkynylruthenium intermediates. ((C) Wiley-VCH Veriag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)