Cycloreversion, benzene dimer

The results of a study of the catalytic cycloreversion of the benzene dimers (195) and (196) have been published. 

Experimental and theoretical (B3LYP/6-31G calculations) results for 2 4-2-cycloreversion of anti-o, > -benzene dimer (38) and -o,o -naphthalene dimer (39) reveal that the process, which is symmetry forbidden in the ground state, is accelerated through electron transfer by at least 10 -fold. The observed inverse secondary isotope effect of hs/ d8 = 0.91 0.02 for the PET-induced cycloreversion of (39) is due to the isotope effect on the pseudo-equilibrium between (39) and (39" "), rather than on the subsequent chemical step. 

Cycloreversion of TPCB" and dimerization of St are summarized in Scheme 3, in which TT-(t-St)2, cr-St2, and the t-St /t-St pair are involved as the key intermediates. ir-(t-St)2 takes various overlapping arrangements of the rr-electrons between two benzene rings, and exhibits a large charge resonance (CR) band at shorter X of 680-800 nm than those of other dimer reported [84], On the other hand, a-St 2 has an acyclic linear structure. 

A (2 + 2)-cycloreversion was observed when the dimers (174a and 174b) of benzo[f>]furan were heated at 250°.200 Benzo[f>]furan was isolated as the only product. Similarly, the dimer of 2,6-diphenyl-4//-thiopyran-4-one (129) underwent (2 + 2)-cycloreversion upon refluxing in benzene.136 The cycloadducts of flavone and diphenylacetylene (262)191 and 3,4-benzo-2,5-dioxabicyclo[4.2.0]octa-3,7-diene (261a)29 eliminated diphenylacetylene and acetylene, respectively, when heated at 250° to 300°. 

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