Deprotonation-triggered heavy-halogen migrations as a key to the structural elaboration of 2,2-difluoro-1,3-benzodioxole

Bromine migration in lithiated 4-bromo-2,2-difluoro-1,3-benzodioxoles was achieved by protecting the 7-position of the bicycle by trialkylsilyl group, thus giving access to 5-bromo-2,2-difluoro-1,3-benzodioxole 4-iodo- and 4-carboxy-derivs. after protodesilylation. Although proton abstraction from the 4-position of 2,2-difluoro-1,3-benzodioxole occurs with exceptional ease, lithiation of the more remote 5-position can only be achieved if no oxygen-adjacent site remains unoccupied. Thus, unlike 4-bromo-2,2-difluoro-1,3-benzodioxole (1), 4-bromo-2,2-difluoro-7-SiR3-1,3-benzodioxoles (5a,b; R = Me, Et) were lithiated by LDA to generate the 5-lithio-intermediate that isomerizes instantaneously by bromine migration to give 5-bromo-2,2-difluoro-4-lithio-7-SiR3-1,3-benzodioxoles, which were iodinated or carboxylated in 4-position. Upon neutralization and carboxylation, 5-bromo-2,2-difluoro-1,3-benzodioxole (8) and 5-bromo-2,2-difluoro-1,3-benzodioxole-4-carboxylic acid (3) are formed nearly quant. A similar basicity gradient-driven heavy-halogen migration can be accomplished starting from 2,2-difluoro-4,7-diiodo-1,3-benzodioxole (11). [on SciFinder (R)]