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DOI: 10.1055/s-0028-1087324
Asymmetric Hydroformylation: Synthesis of α-Branched Aryl Aldehydes
Contributor(s):Mark Lautens, Frédéric MénardUniversity of Wisconsin-Madison, USA
Highly Enantioselective Hydroformylation of Aryl Alkenes with Diazaphospholane Ligands
Org. Lett. 2008, 10: 4553-4556
Publication History
Publication Date:
20 November 2008 (online)
Key words
hydroformylation - rhodium - diazaphospholane
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).
Comment
Whereas b:I ratios with terminal alkenes were sensitive to CO partial pressure (2, 3), internal alkenes showed no dependence to the gas mixture (4-6). Examination of electronic effects showed that the desired branched product was favored with EWG on the aromatic moiety (2a-c). A Hammett plot suggests that a negative charge buildup in the TS is stabilized by delocalization on the aromatic moiety bearing EWG, thus formation of α-aryl aldehydes is favored over β-aryl aldehydes.
Review
B. Breit, W. Seiche Synthesis 2001, 1-36.