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

The iodine induced rearrangement of lithium alkynylborates yields substitut ed acetylenes in high yield. Facile control of stereochemistry in these rea ctions gives them considerable synthetic utility. The product-determining s tep 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 c harges of reactants, products, and transition structures were calculated se mi-empirically using the Conductor-like Screening Model for Real Solvents ( COSMO) at the AM1 level. These were compared with gas phase results obtaine d from the same method. We conclude that the concerted iodine induced 1,2 a nionotropic migration is energetically more favorable than the analogous tw o step process by ca. 95 kcal/mol. (C) 2001 Elsevier Science B.V. All right s reserved.