NMR Studies on Formation and Stability of Prolinol and Prolinol Ether Enamines

Significance

The solution equilibria of reactions between various prolinol-derived secondary amines and aliphatic aldehydes were studied by NMR spectroscopy. For all enamines studied, the E-configuration is adopted predominantly. For prolinol ether derived enamines with small ­aldehyde alkyl chains the presence of small amounts of the Z-isomer was observed in the solution. The enamine intermediates for prolinol-­derived catalysts were detected in situ for the first time. With diarylprolinols, significant initial formation of enamines was observed; however, their amount is quickly decreased by the formation of the oxazolidine, which is reflected by loss of catalytic activity. This deactivation can be overcome by destabilizing the endo-oxazolidine and by stabilizing the prolinol enamine with bulkier and electron-withdrawing aryl substituents.