Computational studies of [2+2] and [4+2] pericyclic reactions between phosphinoboranes and alkenes. Steric and electronic effects in identifying a reactive phosphinoborane that should avoid dimerization

Computational studies of pericyclic reactions between phosphinoboranes X2BPY2 and alkenes/dienes were undertaken for comparison with organic analogues. Identification of a suitable phosphinoborane required examination of several differently substituted examples. The data show that (F3C)(2)BP(CH3)(2) contains a nearly planar P atom (a requirement for multiple bonding), but it dimerizes without a barrier and so would be unsuitable experimentally. Increasing the size of the P-bound groups lowers the dimerization exothermicity, but an adequate barrier arises only for (F3C)(2)BP(t-Bu)(2), 1. This compound is predicted to undergo facile [2+2] cyclization with C2H2 and C2H4 and [4+2] cyclization with cis-C4H6 and c-C5H6, with barriers of < 10 kcal mol(-1) and exothermicities of > 30 kcal mol(-1). It thus represents a candidate for synthesis and subsequent reactions. The reactivity provides insight into the nature of the B-P bond (whether double, single, or in between) in phosphinoboranes, but does not allow unambiguous classification.