Regioselectivity in the Rhodium-Catalyzed Hydroformylation of Styrene as a Function of Reaction Temperature and Gas Pressure

The regioselectivity of styrene hydroformylation in the presence of Rh4(CO)12, as catalytic precursor is strongly affected by reaction parameters: the branched isomer, which prevails in all cases, decreases with increasing reaction temperature and decreasing gas pressures. These results have been rationalized on the basis of the isomerization of the branched alkylrhodium intermediate to the linear one, favoured by high temperature and low CO and Hz pressures. The isomeric ratio found at room temperature, strongly in favour of the branched isomer (98%), reflects the regioselectivity in the formation of alkylrhodium intermediates, no isomerization being observed under these conditions. The isomerization process has been investigated by deuteroformylation experiments at partial substrate conversion.