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A DFT study on NHC‐catalyzed [4 + 2] annulation of 2H‐azirines with ketones: Mechanism and selectivity
Author(s) -
Li Yan,
Zhang Zhiqiang
Publication year - 2021
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.26557
Subject(s) - chemistry , annulation , regioselectivity , ketone , stereochemistry , catalysis , stereoselectivity , ring (chemistry) , selectivity , medicinal chemistry , organic chemistry
To examine the mechanisms and stereoselectivities in the [4 + 2] annulation between an 2H‐Azirine and a ketone catalyzed by a N‐heterocyclic carbine (NHC), M06‐2X density functional theory (DFT) calculations were performed. The reaction was found to proceed via five steps: (Step 1) the NHC adds nucleophilically to the 2H‐azirine; (Step 2) 1,2‐proton transfer affording the Breslow intermediate; (Step 3) three‐membered ring opening accompanied by another proton transfer; (Step 4) addition to the ketone and (Step 5) regeneration of the active catalyst to give the [4 + 2] cycloadduct. For Step 2, we investigated both direct and mediators (H 2 O and HCO 3 − ) assisted proton transfer, and we found that the HCO 3 − assisted mechanism is preferred. In addition, we found that Step 3 (ring opening) determines the regioselectivity of this reaction and favors the cleavage of the CN bond. Step 4, that is, the CN bond formation step is found to be responsible for the stereoselectivity of the reaction, and the R‐ and S‐configurational products should compete with each other. DFT results agree well with the experimental findings. Moreover, we performed GRI, and NBO analyses to investigate the role the NHC. The mechanistic insights gained herein should be useful for rational designing new NHC‐catalyzed ring opening reactions in the future.