A novel addition mechanism for the reaction of "Frustrated Lewis Pairs" with olefins

Density functional theory calculations have been carried out to investigate the addition mechanism for the reactions of "frustrated Lewis pairs" with olefins. Several reactions are studied in this work, which include a three-component reaction between a sterically demanding phosphane [P(tBU)(3)], borane [B(C6F5)(3)], and ethylene, and a two-component reaction between an olefin derivative of phosphane [CH2=CH(CH2)(3)P(tBu)(2)] and B(C6F5)(3). For the two-component reaction, we find a concerted addition mechanism, in which the formation of the B-C and P-C bonds takes place simultaneously. For the three-component reaction, our calculations show that the reaction may be initiated by the weak association of B(C6F5)(3) with ethylene (to form a transient species) and then proceeds in a concerted transition state similar to that in the two-component reaction under study. The natural population analyses for the corresponding transition states indicate that the CH2=CH group (in the two-component reaction) and C2H4 (in the three-component reaction) seem to act as a bridge for electron transfer from the Lewis base center P to the Lewis acid center B. We also investigate the reaction between P(tBU)(3), propylene, and B(C6F5)(3). The results account well for the experimentally observed regioselectivity. In addition, our calculations also indicate that the presence of fluorine atoms in the borane is essential for stabilizing the addition product. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).