Ab initio molecular orbital studies of sigmatropic rearrangements

Ab initio molecular‐orbital theory with the STO‐3G and 4‐31 G basis sets has been used to study the 1,3‐sigmatropic hydrogen rearrangements: propene→propene, formic acid→formic acid, and vinyl alcohol→acetaldehyde, and the1,5‐shifts:1,3‐pentadiene→ 1,3‐pentadiene and β‐hydroxyacrolein→ β‐hydroxyacrolein. Transition states have been determined using gradient procedures. Improved descriptions of the energies of the reactions have been obtained using 3 × 3 configuration interaction. In accord with expectations based on orbital‐symmetry considerations, the calculated barriers are considerably greater for 1,3‐ than for 1,5‐shifts. The forbidden pathway for the degenerate 1,3‐shift in propene is predicted to require less activation energy than the allowed pathway, a result that can be rationalized in terms of interactions with subjacent and superjacent orbitals.