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DOI: 10.1055/s-2007-968749
Enantioselective α-Enolation of Aldehydes by SOMO Activation
Contributor(s):Benjamin List, Subhas Chandra PanPrinceton University and California Institute of Technology, Pasadena, USA
Enantioselective Organocatalytic Singly Occupied Molecular Orbital Activation: The Enantioselective α-Enolation of Aldehydes
J. Am. Chem. Soc. 2007, 129: 7004-7005
Publication History
Publication Date:
24 July 2007 (online)
Key words
SOMO activation - α-enolation - aldehydes
Significance
An organocatalytic enantioselective α-enolation of aldehydes using SOMO activation has been developed. The combination of imidazolidinone catalyst 1, ceric ammonium nitrate (CAN; 2 equiv), water (2 equiv) and 2,6 di-tert-butyl pyridine (DTBP; 2 equiv) has been identified as efficient conditions for the reaction. The SOMO-activated radical cation 6 reacts with the silyl enol ether 3 to generate α-siloxy radical 7, which is further oxidized with CAN to cation 8, and finally product 4 is formed by removal of the silyl group. With 20 mol% of catalyst 1 moderate to high yields (55-92%) and excellent enantioselectivities (er = 93:7 to 98:2) have been obtained for different aldehydes and enolsilanes.
Comment
The present authors and the Sibi group have independently introduced the concept of SOMO activation in asymmetric catalysis (for highlights, see: Synfacts 2007, 649 and 654). Murakami and co-workers have earlier developed an achiral non-catalytic α-enolation of preformed enamines by SOMO activation (Chem. Lett. 1992, 2099). The asymmetric approach as demonstrated by the present authors provides an easy access to chiral 1,4-dicarbonyl compounds. The key radical cation intermediate 6 either reacts by resonance structure 6b or via an unusual Umpolung (6c).