Inter- and Intra-Molecular Protium-Deuterium Exchange in the Rhodium-Catalyzed Deuterioformylation of Styrene

Rhodium-catalyzed deuterioformylation of styrene has been investigated at various temperatures (25-130°C) and for different degrees of substrate conversion. The isomeric aldehydes 2-phenylpropanal and 3-phenylpropanal have been analyzed by MS and 1H NMR spectroscopy, and the distribution of deuterated species and deuterium content at each carbon atom have been determined. At room temperature, only the two aldehydes expected on a stoichiometric basis are formed and neither HD nor labeled styrenes are found in the reaction mixture. At high temperature, variable deuterium incorporation occurs in the reactants and in the products. In particular, an increasing of deuterium content in the aldehydes with increasing of reaction temperature and substrate conversion and a larger amount of deuterium incorporated in the branched aldehyde have been observed. These results have been explained taking into account the different behaviors of the two alkyl metal intermediatest oward the @-hydridee limination process: at room temperature this process is not reversible for both isomers, whereas at high temperature it is still irreversible for the linear alkyl but becomes reversible for the branched one.