Theoretical Investigations toward the [4 + 2] Cycloaddition of Ketenes with N‑Benzoyldiazenes Catalyzed by N‑Heterocyclic Carbenes: Mechanism and Enantioselectivity

• Published in: Journal of organic chemistry Vol. 77; no. 23; pp. 10729 - 10737
• Main Authors: Zhang, Wenjing, Zhu, Yanyan, Wei, Donghui, Li, Yunxia, Tang, Mingsheng
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 07-12-2012
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; N-heterocyclic carbene; Catalytic reaction; Configuration; Enantioselectivity; Theoretical study; Energy barrier; Ketenes; Azo compound; Cycloaddition; Reaction mechanism; Density functional method; Enantiomeric excess; Catalyst
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo302044n

Density functional theory (DFT) calculations have been performed to provide the first detailed computational study on the mechanism and enantioselectivity for the [4 + 2] cycloaddition reaction of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes (NHCs). Two possible mechanisms have been studied: first is the “ketene-first” mechanism (mechanism A), and second is the novel “diazene-first” mechanism (mechanism B). The calculated results reveal that mechanism B is more favorable than mechanism A because it is not only of lower energy barrier but also more consistent with the provided general experimental procedure (Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed. 2009, 48, 192–195). The enantioselectivity-determining step is demonstrated to present during the first process of cycloaddition, and the main product configuration is verified to agree with the experimental ee values very well. This study should be of some worth on forecasting how different substituent groups of catalysts and/or reactants affect the enantioselectivity of products. The obtained novel mechanistic insights should be valuable for not only rational design of more efficient NHC catalysts but also understanding the general reaction mechanism of [4 + 2] cycloaddition of ketenes.