Chemoselectivity in the rhodium-catalyzed hydroformylation of 4-vinylpyridine: crucial role of phosphine ligand in promoting carbonylation instead of hydrogenation

Hydroformylation of 4-vinylpyridine (4VP) in benzene with Rh4(CO)12/PMe2Ph or Rh4CO12 as catalytic precursor shows completely different chemoselectivity, carbonylation product (branched aldehyde) largely prevailing with the first catalyst, hydrogenation product 4-ethylpyridine (4EP) with the second one. Different phosphines and P/Rh ratios were also used, and a comparison with 3-vinylpyridine (3VP) under the same experimental conditions was made too. In all the experiments 3VP exclusively gives aldehidic products. In the case of 4VP, hydrogenation prevails on carbonylation at low P/Rh ratio (<0.5), while for higher values more than 80% of carbonylation product is obtained. The strong electron-donor phosphine ligand changes the polarization of the carbonrhodium bond making this carbon suitable for the migratory insertion process and hence determining the acyl–metal intermediate formation precursor of the aldehyde.