Ring-chain tautomerism as a factor in the reaction between Grignard reagents and substituted phthalides

Article abstract of DOI:10.1016/S0040-4020(01)97419-8

With phthalide two equivalents of Grignard reagent react rapidly in a reaction which could not be controlled to give stepwise addition. In contrast, 3,3- and 4,7-disubstituted phthalides react with only one equivalent of organometallic reagent. The primary addition product formed from one equivalent each of phthalide and Grignard reagent exists in a ring-chain tautomerism whose position is controlled by the interaction between the substituents present in the phthalide and the alkyl group introduced by the Grignard reagent. With substituted phthalides, ring opening of the primary addition product is prevented by these interactions and thus a second equivalent of Grignard reagent does not react. In the absence of substituents, ring opening and the reaction of a second equivalent occurs. Somewhat related effects appear to exist in the reaction between Grignard reagents and phthalic anhydride and in the dehydration of substituted phthalyl alcohols to their corresponding phthalans.