Further Pseudopericyclic Reactions:  An ab Initio Study of the Conformations and Reactions of 5-Oxo-2,4-pentadienal and Related Molecules

It has been proposed that all pseudopericyclic reactions are allowed, regardless of the number of electrons involved. 5-Oxo-2,4-pentadienal (1) is a vinylog of formylketene, and thus its reactions provide a test of this proposal. Ab initio (MP4(full, SDQ)/D95**//MP2/6-31G* + ZPE) calculations were carried out on all the conformations and several reactions of 1, as well as related systems. One of the eight possible conformations of 1 (zZz1) does not exist, but closes without a barrier, via a pseudopericyclic pathway to pyran-2-one (3). IR spectra of 1 in Ar matrices are consistent with three conformations (zZe1, eZz1, and eZe1). Calculated rotational barriers from zZe1 and eZz1 to 3 are consistent with observed kinetics for the of decay of 1. The structures of s-Z- and s-E-3-hydroxy-1,2-propadien-1-one (18 and 19) and the vinylogous cumulene 16 are predicted to have strongly bent geometries. The barrier to internal proton transfer from 16 is only 0.8 kcal/mol, while the barrier from 18 is 23.7 kcal/mol. Barrier heights for these and other pseudopericyclic reactions are shown to correlate with (1) the nucleophilicity and electrophilicity of the reacting centers, (2) the exothermicity of the reaction, and (3) deviations from the ideal geometries for the reaction. Significantly, the barrier heights do not correlate with the number of electrons involved in the reaction.