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

摘要

As reported by Scheldt 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 a- 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 a- 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.