Stereocontrolled formation of substituted imidazolidines in the reaction between N-metallated azomethine ylides and isocyanates

Substituted imidazolidines (and not imidazolidin-4-ones) are the unexpected cycloadducts obtained in the reaction between imines and isocyanates. The reaction is shown to take place via stepwise [3+2] cycloaddition between the N-metallated azomethine ylide formed in situ and the starting imine, followed by nucleophilic addition of the resulting imidazolidine on the sp-hybridized carbon atom of the isocyanate. Density-Functional Theory calculations provide a model for the mechanism of this unusual reaction and for the origins of the observed regio- and stereoselectivity.