Rhodium-catalyzed hydroformylation of 4-vinylpyridine: 4-ethylpyridine formation via an unusual cleavage of the Rh-C bond by the enolic form of the oxo product

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 (4α), 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 4α in its enolic form.