Gauging the Donor Ability of the C-Si Bond. Results from Low-Temperature Structural Studies of gauche and Antiperiplanar β-Trimethylsilyl-cyclohexyl Esters and Ethers by Use of the Variable Oxygen Probe.

Abstract

A plot of C–OR bond distance against pK_a(ROH) for esters and ether derivatives of the antiperiplanar β-silyl alcohol (4) has a slope of –5.310^–3. This represents a stronger response of the C–OR distance than was observed for the corresponding gauche β-silyl alcohol (5) and its derivatives. This result is consistent with the greater donor ability of a C–Si bond compared with a C–C bond. Comparison of these plots with those previously reported for derivatives (3) of tetrahydropyran-2-ol reveals that a C–Si bond is a weaker donor than an oxygen non-bonded pair of electrons. An estimate of the pK_a value for 2,4-dinitrobenzenesulfenic acid (14) was derived from these plots.