Is the iminium ion mechanism viable in the piperidine-catalyzed 1,4-conjugate addition reaction of nitroalkanes to α,β-unsaturated ketones?

Reactions leading to carbon-carbon bond formation, such as the addition of nitroalkanes to alpha,beta-unsaturated ketones, are very important for organic transformations. In the past 15 years, an asymmetric version using amine-thiourea catalysts has been developed. While tertiary amines work via base catalysis, primary and secondary amines are believed to work through iminium catalysis. In this work, we have investigated theoretically the iminium ion catalysis of a model system: the addition of nitromethane to 4-phenylbut-3-en-2-one catalyzed by piperidine in methanol solution. Our results point out that for this system, the iminium mechanism has a very unfavorable transition state with a free energy barrier of 39 kcal mol(-1), involving elimination of the hydroxide ion from the carbinolamine intermediate to form the iminium ion.