The hydroformylation of 4-vinylpyridine (4VP) with rhodium catalyst Rh4(CO)12 modified with phosphine ligands (PMe2Ph) is not completely chemoselective to the branched aldehyde 2-(4-pyridyl)propanal (4 α), the corresponding hydrogenation product 4-ethylpyridine (4EP) always being formed. A series of experiments carried out at different concentrations of rhodium catalyst and 4VP as well as at various degrees of conversion indicate that the amount of hydrogenation product increases with increasing concentration and conversion of the substrate but is only slightly affected by the catalyst concentration. Deuterioformylation experiments carried out in the presence of the phosphine-modified catalyst have established the origin of the hydrogenation product. The recovered 4EP is monodeuterated, the deuterium atom appearing exclusively in the methyl group of 4EP. All the observations are consistent with cleavage of the rhodium−carbon bond in the branched alkyl-metal intermediate by the acidic proton of aldehyde 4 α in its enolic form.

In the Rh 4 (CO) 12 -catalyzed hydroformylation of the vinylpyridine isomers, a very different chemoselectivity between 3-and 4-vinylpyridine is observed: whereas the former is fully transformed into the corresponding branched aldehyde, the latter only undergoes hydrogenation to 4-ethylpyridine. A different behavior of the corresponding branched alkylmetal intermediates related to the different polarization of the Rh-C bond has been suggested to explain the above results.