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Organocatalytic Asymmetric Friedel-Crafts Alkylation of Indoles with Enones
G. Bartoli, M. Bosco, A. Carlone, F. Pesciaioli, L. Sambri, P. Melchiorre*
Università di Bologna, Italy
24 April 2007 (online)
Melchiorre and co-workers reported an efficient asymmetric organocatalytic alkylation of indoles 1 with simple α,β-unsaturated ketones 2, giving access to a broad range of highly enantioenriched β-indolyl ketones 3. The reaction is catalyzed by amine salt 4, in which both the cation and anion are chiral, through iminium ion catalysis, and tolerates steric and electronic demands of the β-olefin substitutent as well as aromatic ketones. The methodology can also be applied to substituted indoles without affecting the efficiency of the system. However, substitution on the indolic nitrogen leads to a strong decrease of the reactivity and has a detrimental effect on the selectivity.