SILANOL REACTIVITY - EVALUATION OF SILANOLATE AS A METALATION-DIRECTING GROUP

Alkali metal salts of dimethylarylsilanol and di-tert-butylphenylsilanol were treated with organometallic reagents to determine the viability of silanolates as ortho-metalation-directing groups. When dimethylphenylsilanol was treated with an excess of n-butyllithium, metalation was not observed. Heating these reactions led to substitution of the methyl and phenyl groups on silicon by n-butyl groups. When a m-methoxy group was present, metalation of the aromatic ring was observed between the two substituents. The more hindered di-tert-butylphenylsilanol was inert to excess n-butyllithium, but treatment of the potassium salt of this silanol with n-butyllithium results in metalation largely at the meta and para positions, with only traces of ortho products. Calculated deprotonation enthalpies of arylsilanes using AM1 can explain some of the observed acidities from this study as well as observations made in a related study of arylsilane metalation by Schlosser et al.