Influence of the Organocatalyst in the Aldol/Mannich-Type Product Selectivities in C-C Bond Forming Reactions

Several organocatalysts were tested in the cross condensation of isobutyraldehyde and acetone. Formation of aldol-type and Mannich-type ("aldol condensation") products was assessed, and Aldol/Mannich proportion studied under several reaction conditions and at different conversions. Organocatalysts able to form Seebach's oxazolidinones, proline and prolinol, led to high Aldol/Mannich relationships (60-10, depending on organocatalyst, reaction conditions, and conversions), whereas organocatalysts unable to form such oxazolidinones (pyrrolidine, O-methylprolinol and proline 'butyl ester) yielded much lower Aldol/Mannich relationships in all conditions and conversions studied (<2.8). These results suggest that Seebach's oxazolidinones might act as "controllers" of the reaction, thus partly avoiding the formation of the Mannich-type adducts, by removing activated forms of aldehyde from the catalytic cycle.