Insertion and carbonylation reactions of styrene promoted by [HRh(CO)(x)- (PMe3)(3-x)] (x=1, 2) compounds: A theoretical investigation

摘要

In this work we have applied Quantum Mechanical calculations to investigate the first two elementary steps (olefin insertion mid carbonylation) in the hydroformylation of styrene, using the model catalysts of the type [HRh(CO)(x)(PMe3)(3-x)](x = 1, 2), which are supposed to be the catalytic species that will be present depending on the CO pressure. The migratory styrene insertion reaction and CO insertion reaction into the metal-alkyl bond were investigated at the MP4(SDQ) level using the BP86 optimized geometries. Additionally the Spin Component Scale procedure on the MP2 and MP3 energies (SCS-MP2 and SCS-MP3) was also applied. It is shown that, at normal hydroformylation conditions and normal CO and H-2 pressure, the active catalytic species is preferentially formed in trans arrangement, trans-[HRh-(CO)(PMe3)(2)](,) 2b. Because of the greater steric hindrance around the rhodium atom, the electronic effects of the phosphine do not contribute significantly to the stability of the catalyst. The MPn calculations overestimate the stability of the pi-complexes, in comparison with the BP86 value, as much as 25 kcal/mol, with large fluctuations along the perturbation series. The activation energies predicted by the BP86 method, in comparison with the MPn results, are under-estimated about 5 kcal/mol. The reaction and the coordination energies are very sensitive to the theoretical level employed. Our results indicate that the competitive trapping of the styrene by different catalytic species, one leading to the branched and the other leading to the linear species, as was experimentally proposed to explain the selectivity, only seems to hold if we consider the subsequent CO insertion step. This assumption based only on the olefin insertion reaction, as the selectivity determining step, does not apply since the branched product will always preferentially be formed. All methods employed here predict the selectivity in good agreement with the experimental selectivity of 70-95% in favor of the branched product. (C) 2008 Wiley Periodicals, Inc.