Sterically hindered silyl perchlorates: synthetic and mechanistic studies

Abstract

Sterically hindered silyl perchlorates were initially examined in hopes of finding evidence of a trivalent cationic silicon species or silylenium ion. Although our studies failed to uncover solid evidence of ionization, it was dis covered that these compounds had unique synthetic potential as silylating agents. All silyl perchlorates examined to date have been found to be highly reactive, relative to their halogenide counterparts, toward nucleophilic agents. t^Butyldimethylsilyl perchlorate offers a significant improvement over the widely used t-butyldimethylsilyl chloride in that it reacts readily with tertiary alcohols. The di-tbutylmethylsilyl moity, a heretofore unknown alcohol blocking group, was also examined and found to be dramatically more resistant to acidic and basic solvolysis than the t^butyldimethylsilyl group. The route to prepare the tri-t^butylsilylperchlorate, although successful, was found to be too arduous for the group to be of any synthetic utility in the pro tection of alcohols. Silyl perchlorates were found to react readily with carbanions. Di-t-butylfluorenyl silane prepared in this manner was used as a precursor to a sterically and electronically stabilized silene; however, attempts to effect the conversion failed. The reaction of silyl perchlorates with carbanions is of limited utility in the synthesis of sterically hindered organosilanes since it was found that reaction of di-^-butylsilyl perchlorate with a carbanion containing hydrogens beta to the anionic center, such as ^t-butyllithium resulted in reduction of the perchlorate group rather than alkylation at silicon. All silyl perchlorates examined, save the tri-^-butyl, were found to be readily available through a hydride exchange reaction between the silyl hydride and trityl per chlorate. The kinetics of this and other similar hydride exchange reactions involving trityl fluoroborate and hexafluoro antimonate were examined and the reaction was found to be second order overall, first order in silane and first order in trityl cation. The reaction was demonstrated to be anion independent with these ionic trityl salts, thus suggesting that the reaction may proceed through a silylenium ion. Rather anomalous results were obtained with trityl bromide suggesting that perhaps a free radical chain reaction mechanism may be involved.