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 X 2 BPY 2 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 (F 3 C) 2 BP(CH 3 ) 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 (F 3 C) 2 BP( t -Bu) 2, 1 . This compound is predicted to undergo facile [2+2] cyclization with C 2 H 2 and C 2 H 4 and [4+2] cyclization with cis -C 4 H 6 and c -C 5 H 6, 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.