Asymmetric Hydroformylation: Synthesis of α-Branched Aryl Aldehydes

Significance

Regioselectivity is a major challenge in the asymmetric hydroformylation of alkenes that are not strongly polarized. The authors report the application of a diazaphospholane ligand (1) they developed previously to the highly selective hydroformylation of simple aryl alkenes (Org. Lett. 2007, 9, 2665; Synfacts 2007, 945). The regiomeric ratio of the aldehydes formed was highly sensitive to the partial CO pressure (for 2a, CO/H₂: 75/75 psi = 6:1; 120/40 psi = 55:1). Also, α-aryl aldehydes are prone to racemization which often limits the scope. Ligand 1 does not racemize the products and, therefore, allows longer reaction times for less reactive substrates such as internal alkenes (4-6; 12-24 h).