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

Abstract

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)2BP(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)2BP(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.

Monomeric phosphinoboranes

“…In 2007, a computational study suggested that phosphinoboranes of the type t Bu 2 P B(CF 3 ) 2 should undergo pericyclic reactions with alkenes ([2 + 2] cycloadditions) and dienes ( [4 + 2] cycloadditions) [75]. In 2013, it was shown that the phosphinoborane 25, featuring the dibenzoborole group, undergoes cycloaddition reactions with molecules containing C O, C C or C N bonds (Scheme 18) [26].…”

On the Possibility of Conversion of Alcohols to Ketones and Aldehydes by Phosphinoboranes R2PBR′R′′: A Computational Study

“…To test a simpler system, we have located the analogue of TS(H 2 ) for the simplified complex (CH 3 ) 2 P=BA C H T U N G T R E N N U N G (CF 3 ) 2 (9). [11] The transition state is shown in Figure S6 in the Supporting Information. The barrier for the H 2 uptake by 9 is 17 kcal mol À1 in the gas phase and about 20 kcal mol À1 in toluene.…”

“…The bond between B and P atoms in 1 compares fairly well with those seen experimentally for phosphinoboranes thought to contain a B=P bond. [11] Orbital population analysis carried out for 1 is con- [a] Dr.…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…[54] Stephan and co-workers note that, based on their own calculations, the π-bond is very much polarised towards the phosphorus atom in tBu 2 P-B(C 6 F 5 ) 2 and a similar finding has previously been reported for tBu 2 P-B(CF 3 ) 2 . [52,88] The limited participation of the boron 2p-orbital in this MO suggests that it would dominate the corresponding antibonding orbital, enabling coordination of H 2 to boron. Indeed, the authors calculated the activation energy for a hydrogenation that proceeded in this manner and stated that it was in reasonable agreement with the observed rate.…”