Disrotatory versus conrotatory electrocyclic ring opening of Dewar benzene:: the conrotatory pathway is preferred and does not involve trans-benzene

For the electrocyclic ring opening of Dewar benzene (2) into benzene (1), both a disrotatory and conrotatory pathway with distinct transition states, TS1 and TS2, respectively, were found at the CASSCF(10,10)/6-311G** level of theory. The importance of the CASSCF(10,10) active space for the proper description of TS1 and TS2 was illustrated by similar calculations using a smaller active space, viz. CASSCF(2,2), CASSCF(4,4) and CASSCF(6,6). Although TS2 represents a true saddle point, TS1 appears to be a higher order saddle point at these levels of theory. Single-point multi-reference SDCI (MRCI) calculations at the CASSCF(10,10)/6-311G** geometries and natural orbitals were performed at all stationary points to obtain more reliable total energies. In contrast to common belief, TS2 lies below TS1 (by 6.62 kcal/mol), i.e. the conrotatory process is favored. Moreover, CASSCF(10,10)/6-311G** Intrinsic Reaction Coordinate (IRC) calculations show that upon conrotatory electrocyclic ring opening 2 does not give the extraordinarily strained traits-benzene (3), i.e. cis, cis, trans-cyclohexa-1,3,5-triene. The conversion of 3 into 1 and vice versa is a distinct process on the C6H6 potential energy surface. (C) 1999 Elsevier Science B.V. All rights reserved.