1,2 Anionotropic rearrangement of lithium-(1-alkynyl) triorganoborates: semi-empirical analysis of the transition state

Abstract

The iodine induced rearrangement of lithium alkynylborates yields substituted acetylenes in high yield. Facile control of stereochemistry in these reactions gives them considerable synthetic utility. The product-determining step is a 1,2 migration of an alkyl group from boron to an adjacent carbon. The transition states for both the two step rearrangement and the concerted bimolecular reactions were compared. Free energy differences and partial charges of reactants, products, and transition structures were calculated semi-empirically using the Conductor-like Screening Model for Real Solvents (COSMO) at the AM1 level. These were compared with gas phase results obtained from the same method. We conclude that the concerted iodine induced 1,2 anionotropic migration is energetically more favorable than the analogous two step process by ca. 95 kcal/mol.