Electrocyclic ring opening kinetics

Linn and Benson discovered in 1963 the ability of tetracyanoethylene oxide (145) to react with alkenes and alkynes at elevated temperatures in [3 + 2] cycloadditions.139 The kinetics of the reaction of (145) with styrene revealed that the formation of (147) is preceded by a first-order step consisting of the electrocyclic ring opening to the carbonyl ylide (146 Scheme 33). 

This corresponds to the electrocyclic ring-opening of the cyclopropyl cation the preferred reaction pathways should involve + 2 ] or [ jOa + 2a] interactions, (Equation 6.1). In simple unfused ring systems the evidence for disrotatory scission comes from kinetic measurements (see below). The stereochemical test is not available because of the interception of the allyl cation by a counter-ion, (Equation 6.2). However, when the cyclopropyl cation is part of a bicyclic system, for example (1), it is found that electrocyclic cleavage occurs readily even when the number n has the small value of 3 or 4. In these circumstances the conrotatory mode, which would yield the unstable trans-... 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

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