Concerted rearrangement Cope rearrangements

Scheme 1.6 Illustration of a concerted reaction (Cope rearrangement).

Originally very few types of such rearrangements were known e.g., Copes rearrangement, Claisen rearrangement and some 1, 5 hydrogen shift in some dienes, but now many others have been discovered. The common feature of such reactions is that they are concerted, uncatalysed and involve a bond migration through a cyclic transition state. 

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). 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

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 Cope, oxy-Cope, and anionic oxy-Cope rearrangements belong to the category of [3,3J-sigmatropic rearrangements. Since it is a concerted process, the arrow pushing here is only illustrative. Cf. Claisen rearrangement. 

A formal synthesis of ( )-Eseroline (69) via a 3-aza-4-oxa-Cope rearrangement was reported. An iV-aryl A-hydroxycarbamate was reacted with 2-phenylsulfanylpropanoic acid to yield the O-acylhydroxamic acid derivative 65, R = H that rearranged in the presence of potassium bis(trimethylsilyl)amide. The [3,3] and [3,5] rearrangement products, respectively 66 and 67, were formed (equation 22). If the para-substituted hydroxamic acid 65, R = OCOBu-t is used, no [3,5] rearrangement product is observed and the [3,3] rearrangement product 68 is the only product formed (equation 23). The authors proposed two parallel mechanisms, a concerted pathway and an ionic mechanism by an ion-pair recombination. 

Saczewski and Debowski reported the l,4-diaza-3-oxa-Cope rearrangement of N-cyanate anilides (equation 52). Prototropic rearomatization of 176 and internal nucleophilic addition afford the corresponding benzimidazolinone 177, usually in moderate yields (32-78%). A concerted [3,3]-sigmatropic rearrangement is proposed based on the absence of para rearrangement product that usually results from homolysis or heterolysis of the N—O bond followed by recombination of the two radicals or ions. 

In a typical concerted thermal [3,3] sigmatropic (Cope) rearrangement (equation 44), 4,5-dihydrooxepin (70) was formed from cis- 1,2-divinylethylene oxide which was generated in situ from sym-divinylethylene carbonate (63JOC1383). 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

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... 

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