Regioselectivity in the Addition of Grignard Reagents to Bis(2-benzothiazolyl) Ketone: C- vs. O-Alkylation Using Aryl Grignard Reagents

The reaction between bis(2-benzothiazolyl) ketone (1) and a series of ring-substituted phenyl Grignard reagents gives, in considerable amount, the unexpected O-alkylation product derived from the attack of the Grignard reagent to the carbonyl oxygen atom, thus extending the range of rarely reported cases in which O-alkylation can occur. The expected classic 1,2-addition product and that derived from O-alkylation have been obtained in a relative molar ratio dependent on the substituent on the phenyl ring. Bis(2-benzothiazolyl) aryl carbinols, the classic 1,2-addition products to the carbonyl group of 1, were obtained in high yield through an alternative synthetic route that permitted the limit imposed by O- vs. C-alkylation competition to be overcome.