Computational Study of the Isomerization Reactions of Borirane

Borirane is isoelectronic to the cyclopropyl cation, but is stable toward electrocyclic ring opening to 2-bora-propa-1,3-diyl A, the boron analogue of the allyl cation. A computational investigation using density functional theory (B3LYP) in combination with highly correlated electronic structure theory methods of the coupled-cluster [CCSD(T)] and multireference configuration interaction (MRCI) type in conjunction with basis sets of up to quadruple-ζ quality reveal that formation of A is endothermic by roughly 15 kcal mol^-1 and that A collapses almost without barrier (0.2 kcal mol^-1) to borirane. The vinylborane isomer B is more stable than borirane and its formation is associated with a barrier of 36-38 kcal mol^-1. Methyl methylideneborane E, only slightly less stable than B, can only be accessed by pathways involving barriers of at least 60 kcal mol^-1.