A study on the Kabachnik-Fields reaction of benzaldehyde, cyclohexylamine, and dialkyl phosphites

The Kabachnik–Fields reaction of benzaldehyde, cyclohexylamine, and dimethyl phosphite carried out at 80°C in acetonitrile takes place via an imine (PhC[DOUBLE BOND]N[BOND]cHex) intermediate, as the monitoring by in situ Fourier transform IR spectroscopy suggested. The corresponding α-hydroxyphosphonate was also formed in a quantity of 13% that was not converted to α-aminophosphonate under the conditions applied. The outcome was similar to the Kabachnik–Fields reaction with diethyl phosphite as the P-component. Molecular modeling and subsequent DFT calculations carried out under solventless conditions supported the experimental results and indicated the formation of a high number of ideally positioned H bonds as the key determinant for the conformation of the starting, intermediate, and product states. The relative energies of the possible intermediates were in accord with the observation that the formation of the α-hydroxyphosphonate is a “dead-end” route.