PHOTOENOLIZATION .10. AB-INITIO SCF-CI STUDY

The processes involved in photoenolisations are theoretically simulated by an ab initio SCF‐CI method, using cis‐2‐butenal as a prototype structure. The prominent role of the hydroxyl group conformation in the resulting transient (2a) is emphasized; its rotation ‘out of the reaction site’ allows the next reaction paths to proceed exothermally. The equilibration of the different types of twisted biradicals in the triplet manifold, which only involves a low energy barrier, is thus possible, populating in quite equal weights the precursors of both E‐ and Z‐dienols. In the singlet state, the formation of the Z‐isomer is expected to be kinetically dominant. An examination of the role of the substituents suggests that, in related systems, the steric crowding induces important structural relaxation of the dienol geometries. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim