Mechanisms and Conformational Control of (4 + 2) and (2 + 2) Cycloadditions of Dienes to Keteniminium Cations.

J Org Chem

Department of Chemistry and Biochemistry , University of California, Los Angeles , California 90095 , United States.

Published: February 2020

Selectivities in (4 + 2) and (2 + 2) cycloadditions of keteniminium cations with 1,3-dienes studied experimentally by Ghosez et al. were explored with ωB97X-D density functional theory. Reactions of keteniminium cations with 1,3-dienes are influenced by the -cis or -trans nature of the diene. -Trans dienes react to give an intermediate enamine that leads to favored formation of (2 + 2) cycloadducts across the keteniminium C-C bond. The first step of the cycloaddition is rate-determining, and reaction occurs by attack on the central carbon of the keteniminium cation and subsequent C-C bond formation. In contrast, -cis constrained dienes lead to preferential formation of (4 + 2) products by both stepwise and concerted mechanisms involving regioselective addition to the keteniminium C-N bond. Diels-Alder reaction occurs via a concerted mechanism if the diene termini are held in close proximity, as in cyclopentadiene.

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Source
http://dx.doi.org/10.1021/acs.joc.9b03340DOI Listing

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