Bond-stretch isomerism in [2,2,2]propellanes

Abstract

The possible existence of bond-stretch isomers for the system 1-aza-4- boratricyclo[2,2,2,0^1,4]octane (abto) ↔ 1-aza-4-borabicyclo[2,2,2]octane (abbo) has been investigated by using molecular orbital theory at the PMO, EHT, CNDO, INDO and ab initio levels. As a consequence of the interplay between through-space and through-bond interactions, the stretching of the central bond is accompanied by an intended but avoided crossing between an occupied and a virtual molecular orbital. The closed (abto) and open (abbo) forms are therefore expected on the basis of conventional orbital symmetry arguments, to be separated by a potential energy barrier. Our ab initio calculations suggest that the barrier separating abto and abbo is small, with the latter being the more stable isomer. These results may be rationalized in terms of the disparity in the energies of the 2p atomic orbitals of boron and nitrogen which weakens their direct stabilizing interaction in abto. Calculation shows abto to be more polar than abbo and the dipole moment reverses direction during the bond-stretching process. The effect of solvent and substituents on the bond-stretch equilibrium is discussed.