Cope rearrangement mechanisms

The Cope rearrangement mechanism can be also strongly affected by other substituents. Thus, the normal electrocyclic process in the thermal isomerization of divinyl aromatics has been suppressed relative to the thermolysis of l,2-bis(trifluorovinyl)naphthalene 438 (in benzene, at 193 °C, 24 h)231. Three major products 440-442 were isolated from the reaction mixture, but none of them was the expected product 439. Also formed in low... 

The mechanism of the PdCh-catalyzed Cope rearrangement has been studied by use of the partially deuterated 1.5-diene 53[46], The coordination of Pd(II) activates the alkene, and cyclization (carbopalladation) takes place to... 

The sequence depicted has been suggested as a plausible reaction mechanism. Diazabasketene primarily reacts via a retro-Diels-Alder reaction to give an azine which, after a Cope rearrangement, undergoes a further retro-Diels -Alder reaction to cleave off hydrogen cyanide. The resulting azabicyclo[4.2.0]octatriene finally isomerizes to the target molecule. 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus c/5-1,2-divinylcyclobutane (p. 1445) rearranges smoothly to 1,5-cycloocta-... 

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 ... 

The mechanism and stereochemistry of the orthoester Claisen rearrangement is analogous to the Cope rearrangement. The reaction is stereospecific with respect to the double bond present in the initial allylic alcohol. In acyclic molecules, the stereochemistry of the product can usually be predicted on the basis of a chairlike TS.233 When steric effects or ring geometry preclude a chairlike structure, the reaction can proceed through a boatlike TS.234... 

Trifluoromethyl imidates show similar reactivity.262 Imidate rearrangements are catalyzed by palladium salts.263 The mechanism is presumably similar to that for the Cope rearrangement (see p. 555). 

This chapter has attempted to demonstrate how secondary deuterium and tritium KIEs can be used to elucidate the mechanisms of reactions and determine the structure of their transition states. In particular, the advantages of using both theoretical calculations and experimental data to solve these problems has been emphasized. Unfortunately, several important topics where the combination of theoretical calculations and experimental work has been very useful in extending our understanding of KIEs could not be discussed. In particular, the extensive studies on the Diels-Alder and the Cope rearrangement by Houk and co-workers (Beno et al., 1996 Houk et al., 1992 Storer et al., 1994) are noteworthy. 

The mechanism involves conversion of V-homoallylamine 116a to imine 117a via a Lewis acid-catalyzed cationic aza-Cope rearrangement (equation 61). Various Lewis acids were tested with yields ranging from 40-99% with high diastereoselectivities. 

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... 

Furthermore, the oxy-Cope rearrangement of allenic cycloheptane alcohol 47 (NaOEt, THF, 20 °C, 12 h, 80%) gave rise only to ring-enlarged product 48 without transannular cyclization (equation 15)29. The above transformations can be rationalized either by the fragmentation-recombination mechanism or by a concerted oxy-Cope mechanism29. 

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 dissociative mechanism of the Cope rearrangement casually mentioned above222 can be illustrated by two examples of Pd-catalyzed reactions. The migration of an allyl group from carbon to carbon in the pyridine system 466 occurs in the presence of a Pd° catalyst236. Refluxing dilute solutions of precursors 466 (R1, R2 = H, Me) in toluene for 7 h or in n -heptane for 24 h gave derivatives 468. The pyridine allyl ether 469 was also... 

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. ... 

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