Influence of an additional gas on the hydroformylation of cyclohexene withCo-2(CO)(6)(PBu3)(2)

The influence of an additional gas on the hydroformylation of cyclohexene i n the presence of Co-2(CO)(6)(PBu3)(2) has been tested. The rate of the hydroformylation is reduced by the presence of an appropria te amount of dinitrogen, argon or xenon as additional gas. The conversion d ecreases as the pressure of the additional gas increases. Helium, on the ot her hand, does nor show any influence. These results are in agreement with the previous data reported for the hydr oformylation of the same olefin in the presence of Co-2(CO)(8) even if the entity of the reduction of the reaction rate is now less evident. The reduc ed influence of the additional gas may be attributed to the more severe con ditions necessary to perform the reaction or to the higher stability of the catalytic system. The analogy between these two catalytic systems is confirmed by the compara ble influence displaced by the additional gas. This influence on the reacti on rate may be attributed, on a molecular basis, to a competition among the additional gas and dihydrogen and/or olefin to a coordinatively unsaturate d place on the catalytically active complex. The formation of an additional gas containing complex reduces the concentration of the active cobalt inte rmediate available for the catalysis and, as a consequence, the hydroformyl ation rate. These data are not. sufficient to identify the step of the catalytic proces s influenced by the presence of the additional gas, however, indicate the i nvolvement of a dihydrogen or an olefin containing complex in the rate dete rmining step of the hydroformylation. The formation of a cobalt complex containing an additional gas as ligand (d initrogen, argon or xenon) in the conditions required to perform the hydrof ormylation is supported by these experiments. (C) 2001 Elsevier Science B.V . All rights reserved.