title: Umpolung catalysts: comparative assessments on reactivities.
abstract: Umpolung catalysis is studied by a sequence of model reactions (CPCM in THF, B3LYP/6-31G*) with different aldehydes and catalysts. We involved addition of the catalyst to the aldehyde and 1,2-H-migration to form a carbanionic d1-species, which is the crucial intermediate according to the Lapworth- and Breslow-mechanisms. Cyanide, N-methylthiazol-2-ylidene, and a glycol-based phosphite perform as umpolung catalysts, formaldehyde, acetaldehyde, benzaldehyde, and acrolein are substrates in this study. In these aldehyde substrates, alkyl-substitution disfavors but pi-conjugation favors formation of the carbanionic d1-intermediate. The nucleophilic carbene, N-methylthiazol-2-ylidene, is the strongest umpolung catalyst, while the phosphite is about as active as cyanide.
authors.affiliation: Institut für Organische Chemie, Greinstrasse 4, 50939, Köln, Germany. Goldfuss@uni-koeln.de
authors.forename: Bernd
authors.initials: B
authors.lastname: Goldfuss
authors.forename: Maria
authors.initials: M
authors.lastname: Schumacher
mesh_terms: D002244:Carbon / Q000737:chemistry; D002384:Catalysis; D003486:Cyanides / Q000737:chemistry; D004583:Electrons; D005557:Formaldehyde / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D017905:Phosphites / Q000737:chemistry
chemical_list: D003486:Cyanides; D017905:Phosphites; D005557:Formaldehyde; D002244:Carbon