title: New hydrogen-bonding organocatalysts: Chiral cyclophosphazanes and phosphorus amides as catalysts for asymmetric Michael additions.
abstract: Ten novel hydrogen-bonding catalysts based on open-chain P(V)-amides of BINOL and chinchona alkaloids as well as three catalysts based on rigid cis-P(V)-cyclodiphosphazane amides of N (1),N (1)-dimethylcyclohexane-1,2-diamine have been developed. Employed in the asymmetric Michael addition of 2-hydroxynaphthoquinone to β-nitrostyrene, the open-chain 9-epi-aminochinchona-based phosphorus amides show a high catalytic activity with almost quantitative yields of up to 98% and enantiomeric excesses of up to 51%. The cyclodiphosphazane catalysts show the same high activity and give improved enantiomeric excesses of up to 75%, thus representing the first successful application of a cyclodiphosphazane in enantioselective organocatalysis. DFT computations reveal high hydrogen-bonding strengths of cyclodiphosphazane P(V)-amides compared to urea-based catalysts. Experimental results and computations on the enantiodetermining step with cis-cyclodiphosphazane 14a suggest a strong bidentate H-bond activation of the nitrostyrene substrate by the catalyst.
authors.affiliation: Department of Chemistry, Universität zu Köln, Greinstrasse 4, D-50939 Köln, Germany.
authors.forename: Helge
authors.initials: H
authors.lastname: Klare
authors.affiliation: Department of Chemistry, Universität zu Köln, Greinstrasse 4, D-50939 Köln, Germany.
authors.forename: Jörg M
authors.initials: JM
authors.lastname: Neudörfl
authors.affiliation: Department of Chemistry, Universität zu Köln, Greinstrasse 4, D-50939 Köln, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Goldfuss
keywords: DFT computations; Michael addition; hydrogen bonding; organocatalysis; phosphazanes