title: 1,4-Bis-Dipp/Mes-1,2,4-Triazolylidenes: Carbene Catalysts That Efficiently Overcome Steric Hindrance in the Redox Esterification of α- and β-Substituted α,β-Enals.
abstract: As reported by Scheidt and Bode in 2005, sterically nonencumbered α,β-enals are readily converted to saturated esters in the presence of alcohols and N-heterocyclic carbene catalysts, e.g., benzimidazolylidenes or triazolylidenes. However, substituents at the α- or β-position of the α,β-enal substrate are typically not tolerated, thus severely limiting the substrate spectrum. On the basis of our earlier mechanistic studies, a set of N-Mes- or N-Dipp-substituted 1,2,4-triazolium salts were synthesized and evaluated as (pre)catalysts in the redox esterification of various α- or β-substituted enals. In particular the 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes overcome the above limitations and efficiently catalyze the redox esterification of a whole series of α/β-substituted enals hitherto not amenable to NHC-catalyzed transformations. The synthetic value of 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes is further demonstrated by the one-step bicyclization of 10-oxocitral to (racemic) nepetalactone in diastereomerically pure form.
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Veera Reddy
authors.initials: VR
authors.lastname: Yatham
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Wacharee
authors.initials: W
authors.lastname: Harnying
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Darius
authors.initials: D
authors.lastname: Kootz
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Jörg-M
authors.initials: JM
authors.lastname: Neudörfl
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Nils E
authors.initials: NE
authors.lastname: Schlörer
authors.affiliation: Department of Chemistry (Organic Chemistry), University of Cologne , Greinstrasse 4, 50939 Cologne, Germany.
authors.forename: Albrecht
authors.initials: A
authors.lastname: Berkessel