TRANSITION STRUCTURES OF THERMALLY ALLOWED DISROTATORY ELECTROCYCLIZATIONS - THE PREDICTION OF STEREOSELECTIVE SUBSTITUENT EFFECTS IN 6-ELECTRON PERICYCLIC-REACTIONS

The transition structures for the disrotatory electrocyclizations of a variety of 1-substituted 1,3,5-hexatrienes (F, CH3, CN, CHO, NO, BH2) have been located using ab initio molecular orbital calculations and the 3-21G basis set. Energy corrections for electron correlation were performed using second-order Moller-Plesset theory and the 6-31G basi s set. A comparison of the two disrotatory processes, involving either inward or outward rotation of the substituent, supports the theoretic al model of torquoselectivity developed for the conrotatory processes of 3-substituted cyclobutenes. Electronic effects have a smaller influ ence on the preference for inward or outward rotation of the substitue nt in the hexatrienes, and steric effects have a larger influence than in the cyclobutene electrocyclizations.