Nucleophilic subsitution at silicon: Synthesis of sterically-hindered bis(2,6-di-t-butylaryloxy)silanes

Abstract

The reaction of the monomethylsilane ( 8a) with two equivalents of the 4-(carboalkoxy)-2,6-di-t-butyl-substituted phenol ( 7b) in toulene using triethylamine as an acid acceptor gave the bis(aryloxy) adduct ( 9a). The analogus reaction of the dimethylsilane ( 8b) with sodium 2,6-di-t-butyl-4-(methoxycarboxyl)-phenolate ( 7a) gave only the monosubstitution product ( 10a). The reaction of the corresponding phenolate ( 7e) with 8b gave a mixture of 7a, 10a, and bis-adduct ( 9b), whereas, in the presence of 15-crown-5, the bis-adduct 9b was obtained. The bis-adducts 9c–e were prepared in an analogous manner. The reaction of n-hexyl 3,5-di-t-butyl-4-hydoxylbenzoate ( 7h) with the diphenylsilane ( 8c) gave only the monosubstitution product 12, while forcing conditions gave, unexpectedly, the methyl ether 13. The reaction of 4-(carboalkoxyethyl)-2,6-di-t-butylphenol ( 16a) with 8a gave the bis adduct. The reaction of 16a with 8b in THF, without a crown ether, gave a low yield of the monosubstitution product. The bis-adducts 17b–c were obtained by the reaction of 8b with the corresponding phenolates ( 16a–b) in tetraglyme. Compound 17b was also obtained by the reaction of 8b with 16a in THF with a crown ether. These results are discussed in terms of charge dispersal in the phenolate ion and the corresponding effect upon both ion-pairing and aggregation in solution.