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

Abstract

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.