Concerted rearrangement aromatic Cope

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). 

There appears to be much interest in the mechanism of various pericyclic transfer-mations, particularly of the Cope rearrangement. A pair of interacting allyl radicals, an aromatic species, or a 1,4-cyclohexanediyl diradical are the possible intermediates and transition states for the rearrangement represented here as resonance hybrids in the transformation of 1,5-hexadiene (Scheme 4.16). Two high-order theoretical studies indicate that the Cope rearrangement is concerted and proceeds via an aromatic chair transition state (33).362,364... 

A similar reaction was observed when 2-methylene-l,l-diphenylcyclopropane (8, = H) was heated to 135°C. The isolated 2-methyl-3-phenylindene (9, R = R = H) was probably formed via a 2,7a-dihydro-2-methylene-3-phenyl-liy-indene and subsequent 1,5-hydrogen shift.An analogous reaction occurred when 2-ethylidene-l,l-diphenylcyclopropane (8, R = H R = Me) and 2-(l-methylethylidene)-l,l-diphenylcyclopropane (8, R = R = Me) were submitted to thermolysis. These reactions can be rationalized by both a diradical mechanism and a concerted Cope-type rearrangement, followed by a hydrogen shift to restore the aromatic system. 

The mechanism of Cope rearrangement is considered as concerted and pericyclic, via a six-membered cyclic transition-state. The transition-state of Cope rearrangement is considered as 6n electrons aromatic transition-state. The transition-state of the Cope rearrangement can be either chair conformation or boat conformation. Alternatively, the Cope rearrangement can also be considered to occur via a diradical transition-state. 

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