Preference of electron transfer to nucleophilic addition in reactions of ketene silyl acetals with quinones and catalysis of magnesium ion

beta,beta-Dimethyl-substituted ketene silyl acetal (1a) reduces p-chloranil and other activated quinones with electron-withdrawing substituents to produce the carbon-oxygen adduct, the hydrolysis of which yields the corresponding hydroquinone ether. The structure of the hydroquinone ether has been determined by the X-ray crystal analysis. The reactions are significantly slowed down in benzene where the charge-transfer spectra of electron donor-acceptor complexes formed between la and the activated quinones are observed. The comparison of the observed rate constant with that predicted for the electron transfer process from la to p-chloranil indicates that the addition of 1a to p-chloranil proceeds via the electron transfer from 1a to p-chloranil. Although no reaction takes place between 1a and p-benzoquinone, the electron affinity of which is significantly smaller than that of p-chloranil, the reduction of p-benzoquinone by 1a occurs efficiently in the presence of magnesium ion. The kinetic expression of the Mg2+ catalysis changes from the first-order to second-order in [Mg2+] under the conditions that Mg2+ forms the 2 : 1 complexes with the corresponding radical anions. The catalytic effects of Mg2+ are approximately the same as those observed for the electron transfer reduction of these oxidants, demonstrating the important contribution of the Mg2+-catalyzed electron transfer process in the addition of 1a to p-benzoquinone. On the other hand, the reaction of a nonsubstituted ketene silyl acetal (1d) with p-fluoranil yields the carbon-carbon adduct rather than the carbon-oxygen adduct. The much larger rate constants of 1d than those of 1a despite the higher oxidation potential of 1d suggest that the 1,2-addition to p-fluoranil occurs via the nucleophilic attack of 1d, which is much less sterically hindered than 1a, to the positively charged carbonyl carbon of p-fluoranil rather than an alternative electron transfer pathway.