Dynamic stereochemistry of imines and derivatives. Part VIII. Barriers to rotation around the carbon–nitrogen bond in imine N-oxides (nitrones)

Abstract

Semiempirical molecular orbital calculations (CNDO/2 and INDO) are reported for methyleneamine N-oxide, N-(benzylidene)methylamine N-oxide, and diphenylmethyleneamine N-oxide. The calculated dipole moments and the energy difference between E- and Z-isomers are in good agreement with experimental data in solution. The calculated barriers to rotation around the CN bond are about twice the measured barriers to E–Z-interconversion in closely related nitrones. Experimental data for E–Z-isomerization in N-(9-anthrylmethylene)methylamine N-oxide shows the entropy of activation to be close to zero as expected for a rotational process.