^{Click the structures and reaction arrows in sequence to view the 3D models and animations respectively}

The reaction goes via an enamine, which attacks the aldehyde Re-face to form the R-product. In the transition state the isopropyl group is pseudoequatorial to minimise the 1,3-diaxial repulsions from the axial methyl group alpha to the nitrogen. This forms the R-aldol product.

Click for more information on Re and Si.

Benjamin List, Richard A. Lerner and Carlos F. Barbas III, Proline-Catalyzed Direct Asymmeric Aldol Reactions, J. Am. Chem. Soc. 2000, 122, 2395-2396.