Substituent Effects in the Silylation of Secondary Alcohols: A Mechanistic Study

Relative rates for the silylation of C4-substituted 1-(naphthalen-1-yl)ethanol substrates with tert-butyldimethylsilyl chloride (TBSCl) in CDCl₃ catalyzed by 9-azajulolidine (TCAP) have been measured. Hammett plot analysis of the resulting selectivity data yields two intersecting linear correlations. A small positive slope of ρ=+0.09 is observed for donor-substituted alcohols, while silylation rates for acceptor-substituted alcohols correlate best with a slightly larger negative slope (ρ=-0.48). The reaction of methanol with TBSCl catalyzed by 4-(N,N-dimethylamino)pyridine (DMAP) has been studied at several different theoretical levels in chloroform solution. Silyl-group transfer between silylated DMAP and methanol occur over an exceedingly flat surface with barely defined minima and transition states. Reaction pathway calculations for the Lewis base and general base catalyzed mechanisms for reaction of TBSCl with C4-substituted 1-(naphthalen-1-yl)ethanol compounds predict a close competition of both pathways.