Migratory Aptitudes of Alkyl Groups on Boron: A Computational Study of Halomethyllithium-Induced Migration Reactions

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For reactions of alkyldioxaborolanes with halomethyllithium reagents, migrations of tertiary alkyl groups and benzylic groups often give lower yields of migrated products. The only published computational study calculated the barriers to tert-alkyl migration to be lower than for other types of simple alkyl groups. In contrast to this, the present work shows that, when detailed conformational analysis is carried out, tert-alkyl and benzylic groups have significantly higher gas phase barriers to migration than for other types of alkyl groups, such that for tertiary alkyl groups, oxygen migration of the ethylenedioxy group could compete. The barriers are reduced when a solvent model is included, which reduces the importance of oxygen migration.