Hydrocarbonylation of prop-2-en-1-ol to butane-1,4-diol and 2-methylpropan-1-ol catalysed by rhodium triethylphosphine complexes
The hydrocarbonylation of prop-2-ene-1-ol catalysed by [Rh2(O2CMe)4]–PEt3, which gives [RhH(CO)(PEt3)2] as the active species, has been found to produce predominantly butane-1,4-diol and 2-methylpropan-1-ol with small amounts of 2-methylpropane-1,3-diol and propan-1-ol. Neither 2-enal nor 2-methylprop-2-en-1-ol are intermediates in the production of 2-methylpropan-1-ol. By carrying out the reaction under a variety of reaction conditions and by using deuterium-labelling studies it was possible to formulate a mechanism for the production of 2-methylpropan-1-ol which involves formation of the vinyl alcohol, 2-methylprop-1-en-1-ol, as the primary product followed by tautomerism and hydrogenation, provided that at least two PEt3 groups are co-ordinated to the rhodium. A dehydration is proposed to occur during the catalytic cycle from a cationic hydroxycarbene intermediate. Using propenyl ethers as substrates similar products are obtained presumably via loss of alcohol rather than dehydration. If less than two PEt3 groups are co-ordinated to rhodium the major branched-chain product from prop-2-en-1-ol is 2-methylpropane-1,3-diol. This is interpreted as indicating that protonation of the acyl intermediate and dehydration of the hydroxycarbene do not occur because of the lower electron density on the acyl O atom.