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
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- 作者：Veera Reddy Yatham ; Wacharee Harnying ; Darius Kootz ; Jörg-M. Neudörfl ; Nils E. Schlörer ; Albrecht Berkessel
- 刊名：Journal of the American Chemical Society
- 出版年：2016
- 出版时间：March 2, 2016
- 年：2016
- 卷：138
- 期：8
- 页码：2670-2677
- 全文大小：627K
- ISSN：1520-5126
文摘
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.