The Unusually Slow Hydrolysis Rate of Silyl Methyl Ketals in Benzoquinone Systems. The Question of Siloxy Stabilization of an Adjacent Positive Charge and Stereoelectronic Effects on Ketal Hydrolysis

Article abstract of DOI:10.1021/jo00273a033

The kinetics of the acid-catalyzed monohydrolysis of six quinone bisketals were studied under pseudo-first-order conditions.The tert-butyldimethylsilyl methyl and the tert-butyldiphenylsilyl methyl ketals respectively hydrolyse about 400 times slower than does the corresponding dimethyl ketal and react nearly at the rate of the ethylene glycol ketal.An important consideration in understanding these results was the carbonium ion stabilizing ability of the siloxy substituent.Thus, ? and ?⁺ values for the tert-butyldimethylsiloxy and the tert-butyldiphenylsiloxy groups were measured.These values indicated that siloxy groups are nearly as effective as a methoxy group in stabilizing a positive center provided a low-energy conformer is available for interaction of the oxygen nonbonding electron pair with the ?-center.If delocalization of the oxygen lone-pair electrons by the silicon is important, it does not manifest itself in the ? or ?⁺ values.The data collected also illustrate the strong conformational dependence of siloxy groups interacting with ?-systems, and it is this effect that is thought to be the major contributor to the lower rate of hydrolysis observed for the silyl methyl ketal moieties in these quinone systems.