A Comparative Study of the 1-Hexene Hydroformylation Either Under Syngas Conditions or with Paraformaldehyde Catalyzed by Rhodium/Diphosphine Systems

In order to carry out a comparison, the hydroformylation of 1-hexene to their corresponding aldehydes (heptanal and 2-methyl-hexanal) was studied both under syngas conditions and with paraformaldehyde using the catalytic rhodium/diphosphine precatalysts; the catalytic systems were formed in situ by the addition of one or two equivalents of the corresponding diphosphine, Ph₂P(CH₂)_nPPh_2, n = 2 (dppe), 3 (dppp) and 4 (dppb), to the carbonyl complex Rh(acac)(CO)₂. For the hydroformylation reactions under syngas conditions, the more active systems were the ones containing one equivalent of the diphosphine, which produce trigonal bipyramidal species like RhH(CO)₂(diphos). The activity and selectivity of these systems strongly depend on the bite angle of the ligand: when the bite angle increases both parameters are higher (dppb > dppp > dppe). Contrary to these results, for the reaction with paraformaldehyde, the systems containing two equivalents of the diphosphine ligand, which produce the cationic square planar species [Rh (diphos)₂]⁺, were more active than those containing one equivalent; the reaction rate decreases with the enlargement of the carbon chain of the bridge between the two phosphorous atoms of the diphosphine (dppe > dppp > dppb). These results may be explained by a higher steric effect on the metal center, which probably produces a decreasing of the rate of the CH₂O oxidative addition reactions. For both reactions, these effects were explained through DFT calculations of the corresponding resting states.