Nonlinear effects in proline-catalysed aldol reactions under solvent-free conditions based on the ternary phase behaviour of scalemic proline

Ball milling was employed in order to study the phase behaviour of the proline-catalysed aldol reaction between solely solid substrates under solvent-free conditions. A significant nonlinear relationship between the enantiomeric excess of the catalyst (proline) and that of the aldol product was observed. The nature of this nonlinear effect originated from the ternary phase behaviour of scalemic proline. Significant differences were detected between reactions in which a mixture of either (S)- proline and (R)- proline or rac-proline and (S)- proline was employed.