Cope rearrangements bond making/breaking

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

On the basis of MlNDO/3 calculations, Dewar argued for a non-concerted mechanism for the parent Cope rearrangement, involving formation of cyclohexane-1,4-diyl (A) as an intermediate [12]. Several years later, Dewar published another paper in which he boldly claimed that multi-bond reactions, such as the Cope rearrangement, cannot, in general, involve synchronous bond making and bond breaking [13]. 

The simulated (6/6)CASSCF calculations were performed at several partially optimized Ciu geometries. The calculations found that the Cope TS has an interallylic separation of / = 2.062 A, which is close to that of / = 2.023 A for the fully optimized RHF/3-21G TS. Thus, the simulated (6/6)CASSCF/3-21G calculations found that bond making and bond breaking occur synchronously in the Cope rearrangement via an aromatic TS (B). 

Even when the cyanide group is not directly involved in the reaction, it can have a considerable influence on the reaction mechanism. A study of kinetic deuterium isotope effects on a series of Cope rearrangementsshowed that rearrangement of the 3,3-dicyanohexa-1,5-diene (132) had the /rn/te ratio at C(4) (i.e. bond breaking) greater than the A d/ h ratio at C(6) bond making) rather than the reverse, which was observed with other substituents. A theoretical studyof the reaction indicated that this reaction is... 

Subsequent calculations have confirmed the allowed, aromatic nature of the transition state using various DFT methods.The breaking bond in the eneyne is on an average 1.83 A and the making bond is 1.88 A. Unfortunately, like the Cope rearrangement CASSCF calculations on this system indicate favorable formation of a diyl intermediate (bond distances of 1.87 and 1.67 A, respectively which reveals the need for inclusion of Dynamic Correlation. This and the related reactions have been reviewed by Hopf. ... 

Cope rearrangements of 2,6-disubstituted bicyclo[5,l,0]octa-2,5-dienes have been studied to determine the extent of bond making and bond breaking at the transition state. The interpretation of the results was complicated by the enforced boat-transition state, but comparison with polycyclic systems supports the hypothesis that, unlike Cope rearrangements in acyclic dienes, bond cleavage is at least synchronous with bond formation. ... 

The Cope rearrangement has a nonpolar transition state (Fig. 20.51), but this time it is not a simple hydrogen atom that migrates but a whole allyl radical. An analysis of the symmetry of the orbitals involved shows why this reaction is a relatively facile thermal process but is not commonly observed on photochemical activation. As we break the C(l)—C(l) bond, the phases of the overlapping lobes must be the same. The HOMO of the allyl radical is 4>2, that information allows us to fill in the symmetries of the two allyl groups making up the transition state. 

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