4 years ago

Vinyl (Thio)Ethers versus Enamines: A DFT Insight into Their Divergent Reactivities toward Nitroalkenes

Vinyl (Thio)Ethers versus Enamines: A DFT Insight into Their Divergent Reactivities toward Nitroalkenes
Isabelle Chataigner, Hélène Gérard
Despite their apparent similarities, vinyl (thio)ethers and enamines display divergent reactivities toward nitroalkenes. Whereas [4+2] cycloadduct derivatives are generally obtained in the first case, a variety of adducts are observed in the second, and depend on the substrates and reaction conditions. Herein, a rationalization of this versatility is proposed through a theoretical study, in which the interactions between these electron-rich alkenes and nitroethylene are compared in conjugate addition, [4+2] and [2+2] cycloadditions, and ene reaction processes. The conjugate zwitterionic adduct is shown to play a key role. With vinyl (thio)ethers, its formation is disfavored, whereas, for enamines, it appears as a stabilized intermediate, leading to the thermodynamically favored formal ene adduct. This adduct is, however, kinetically disfavored, which opens up the way to control the selectivity of the whole process. Behavioral insights: The divergent behavior of vinyl (thio)ethers and enamines toward nitroalkenes is explained by the formation of a key zwitterionic adduct (see scheme). Whereas the stability of the C=N+ bond favors its formation with enamines, the lower propensity of the oxygen and sulfur atoms to form C=O+ or C=S+ bonds leads to easier generation of the [4+2] cycloadduct with vinyl (thio)ethers. In both cases, the formal ene adduct is thermodynamically favored.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701961

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