Cope rearrangement diradical mechanism

It has been suggested that another type of diradical two-step mechanism may be preferred by some substrates. Indeed, a nonconcerted Cope rearrangement has been reported. In this pathway, the 1,6 bond is formed before the 3,4 bond breaks ... 

This finding confirms an opinion that, at least in some cases, diradicals such as 18 can be the actual intermediates in the non-concerted Cope rearrangement, so-called stepwise cyclization-then-cleavage mechanism. Berson and coworkers who previously excluded diyl intermediate in the acetylenic Cope rearrangement20 designed in their next work24... 

Another type of diradical intermediate species (27) in Cope rearrangement is formed during thermolysis of optically active frans-4,9-dimethyl-1,2,6,7-cyclodecatetraene 2425 which was studied in order to distinguish between concerted and stepwise mechanisms of Cope rearrangement. The transformation of optically active trans-24 via a concerted mechanism would lead to optically active tetraenes 25 and 26, while the participation... 

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). 

The Cope rearrangement of the highly strained diene (32) (Scheme 13) is shown to proceed by a non-concerted mechanism involving the diradical (33), which may be trapped by oxygen to give the peroxide (34). A full kinetic study confirms the intermediacy of the diradical. ... 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

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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