Cope rearrangement of 1,5-dienes

Among the best known examples, involving a carbon moiety, is the shift from one carbon atom to another observed in the Cope rearrangement of 1,5-dienes (49— 50 not to be confused with the... 

TCNE takes place (Scheme 57). Tomioka reported the (3 + 2) photocycloaddition between 1,1,2-triarylcyclopropanes and vinyl ethers in the presence of p-DCB [162]. Mizuno and Otsuji reported the (4 -l- 2) photocycloaddition between 1,2-diarylcyclopropanes and DCA [23]. The 1,4-radical cation produced as an intermediate of the Cope rearrangement of 1,5-dienes via photoinduced electron transfer can be trapped by molecular dioxygen, giving bicyclic dioxanes (Scheme 58) [163]. This photooxygenation takes place in a stereospecific manner. 

The Cope rearrangement of 1,5-dienes is the all-carbon analog of the Claisen... 

A careful study of the mechanism of the oxy-Cope rearrangement of 1,5-diene alkoxides has provided a neat synthesis of ery//jro-juvabione (68) (Scheme 6). This work has shown that the [3,3] sigmatropic process proceeds in a concerted fashion predominantly via a chair transition state. 

A palladium(II)-catalysed [3,3]-aza-Claisen formal sigmatropic rearrangement of 3-allyloxy-5-aryl-l,2,4-oxadiazoles indicates the intervention of a cationic palladacycle similar to the one postulated for the Cope rearrangement of 1,5-dienes (Scheme 141). " °... 

These two dienes (33 and 34) are in equilibrium, and the equilibrium favors the more thermodynamically stable compound. In this case, the more stable diene is 34, where each C=C unit is disubstituted (see Chapter 12, Section 12.1). This rearrangement is called the Cope rearrangement, after Arthur C. Cope (United States 1909-1966). The temperatures required for a Cope rearrangement are sometimes quite high, and the fact that an equilibrium is established between the two dienes is a problem. If the substitution pattern of the dienes is identical, the mixture of products may be difficult to separate. An example is the Cope rearrangement of 1,5-diene 35, which, upon heating, gives 36. There is one disubstituted C=C unit in 35 and one disubstituted C=C unit in 36, so the equilibrium will not favor one over the other and close to a 1 1 mixture of these two dienes is expected. In other words, this reaction will produce a mixture and it is probably difficult to separate one from the other. 

The Cope rearrangement of 1,5-dienes also takes place via a cyclic six-electron transition state. In this example, the product is an equilibrium mixture of isomeric dienes. The favored product is the diene on the right, which contains the more highly substituted double bonds. 

Following are three examples of Cope rearrangements of 1,5-dienes. Show that each product can be formed in a single step by a mechanism involving redistribution of six electrons in a cyclic transition state. 

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