Hydroformylation of myrcene: Metal and ligand effects in the hydroformylation of conjugated dienes

The hydroformylation of myrcene catalyzed by Rh and Pt/Sn catalysts containing different P-donor ligands leads to the formation of a number of mono- and dialdehydes. Nine major products of the reaction have been characterized, showing that they arise from the n-alkyl and η3-allyl intermediates, formed through the reaction of the metal catalysts with the less substituted C=C bond of the substrate. Thus, 4-methylene-8-methyl-7-nonenal is the major aldehyde formed with Pt/Sn catalysts, regardless of the P-donor ligand used. This aldehyde is also the main product of the reaction catalyzed by the Rh/xantphos system (xantphos = 9,9-dimethyl-4,6-bis(diphenylphosphino)xantene). However, with ligands such as bisbi (bisbi = 2,2′-bis((diphenylphosphino)methyl)-1,1′-biphenyl), also with bite angles around 120°, but with more flexible backbones than xantphos, rhodium catalysts yield mainly cis- and trans-3-ethylidene-7-methyl-6-octenal. These two aldehydes are also formed in the reactions catalyzed by Rh and P-donor monodentate ligands or the bidentante ones with bite angles around 90° (dppe, dppp). For the last type of ligands, an increase in the flexibility of the backbone reduces the selectivity for the β,γ-unsaturated aldehydes.