NITROSOACETALDEHYDE AND ITS ENOL AND OXIME ISOMERS - THEORETICAL INVESTIGATION OF AN ASYMMETRIC 1,5-SIGMATROPIC HYDROGEN SHIFT

Ab initio molecular orbital calculations with minimal (STO-3G) and split-valence (4-31G) basis sets have been carried out for nitrosoacetaldehyde (O=NCH2CH=O, 1), nitrosovinyl alcohol (O=NCH=CHOH, 2), and glyoxal monoxime (HON=CHCH=O, 3). Optimized structures and relative energies of several conformations of 1, 2, and 3 are reported. The enol 2 is predicted to be slightly lower in energy than the keto isomer 1, i.e., the nitroso substituent appears to reverse the ordering of stabilities from that observed in the parent keto/enol pair, acetaldehyde and vinyl alcohol. The oxime 3 is, however, still lower in energy. Two paths connecting the enol 2 and the oxime 3 are considered: firstly, that involving successive 1,3 shifts (i.e., proceeding via the aldehyde 1): secondly, a direct 1,5-sigmatropic shift. The transition state 4 for the 1,5-hydrogen shift has been determined and this pathway provides a low-energy route for the isomerization of 2 to 3. The structure of 4 resembles more closely the higher energy isomer 2 than the lower energy isomer 3, which is consistent with the Hammond postulate. Copyright © 1979, American Chemical Society. All rights reserved.