Cyclic mechanisms, Cope rearrangements

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-... 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus cis-1,2-divinylcyclobutane (p. 1131) rearranges smoothly to 1,5-cyclooctadiene, since the geometry is favorable. The trans isomer also gives this product, but the main product is 4-vinylcyclohexene (resulting from 8-33). This reaction can be rationalized as proceeding by... 

In contrast to the preceding mechanisms proposed for [3,3]-sigmatropic shifts, the mechanism of the silver-catalyzed oxy-Cope rearrangement was proposed as a stepwise process (Scheme 3.37). As usual, the reaction would be initiated by silver coordination to the alkyne moiety. Nucleophilic attack of this complex by the double bond would then lead to a cyclic cationic vinylsilver intermediate. Fragmentation would then give the dienone. 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

Elementary uni- and bimolecular reactions will necessarily show first- and second-order kinetic behaviour, but the reverse is not necessarily true a first-order reaction may not be unimolecular and a second-order reaction may not be bimolecular. For example, we considered the decomposition of dibenzylmercury in Chapter 1, in which the mechanism could either be elementary, giving a mercury atom and a 1,2-diphenylethane molecule directly (reaction 2.13a), or the reaction could be complex, with a slow initial homolysis of a carbon-mercury bond, followed by rapid further reactions to give the products (reaction 2.13b). Similarly for the Cope rearrangement of diene 2 to diene 4, the reaction could be elementary, with a concerted cyclic movement of electrons (reaction 2.14a), or might involve a di-radical intermediate 3 which rapidly reacted further to give the observed product 4 (reaction 2.14b). Both these mechanisms would lead to first-order kinetics, so the establishment of first-order kinetic behaviour for both these reaction schemes does not establish the... 

Recently, cyclizations of the isomeric unsaturated carbamates (141) and (142) have been investigated (equation 107). These fundamental reactions are not as regio- and stereo-selective as similar cyclizations starting from cyclic A -acyliminium ions (equations 25 and 26). The loss of stereoselectivity might have to do with the occurrence of cationic aza-Cope rearrangements in combination with chair-chair interconversions (equation 108). Normal cyclization of (142) occurs via conformation (145) and should lead to (144). If sigmatropic rearrangement competes with normal cyclization, (146) arises, but this conformation still leads to (144). Only after chair-chair interconversion to (147) and subsequent cyclization, (143) is formed. Via the same mechanism (141) can produce the abnormal product (144). 

Homoallylic amines containing an allylic hydroxy group rearrange in the presence of an aldehyde and an acid catalyst to yield 3-acylpyrrolidines. If the starting amino alcohol is cyclic, this transformation provides a pyrrolidine-annulated product, in which the initial ring is expanded by one member. The mechanism has been proposed to proceed via a tandem cationic aza-Cope rearrangement/Mannich cyclization1134. 

The 9-membered cyclic ethers 116 have been prepared via oxy-Cope rearrangement of 115, ultimately derived from D-glucose. The 10-membered bis-ether 118 was prepared via Bergman cyclization of the bis-acetylene 117 (R=2,3,4,6-tetra-0-benzyl-p-D-glucosyl). Solution conformational analysis was reported from NMR experiments. Dialkyne 117 was prepared by Pd-catalysed coupling of the P-D-glucosyl-l-alkyne and 1,2-diiodobenzene, and a mechanism was proposed for... 

For that reason these reactions are also alternatively known under the older name - the reactions with cyclic mechanism. The typical representatives of this extensive and synthetically very important class of reactions are, e.g., the ester pyrolysis (1), Diels-Alder reaction (2), Cope (3) and Claisen (4) rearrangements, 1,3 dipolar additions (5) etc. 

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. 

Other Reactions, The allyl Grignard reagent (601) cyclizes on heating by a Cope-like transition state. The product after hydrolysis is predominantly the c/j-cyclopentyl-olefin (602), and a m-cyclopentane (603) is also obtained in the ene rearrangement of the corresponding diene. A non-cyclic mechanism is known to operate in intermolecular addition where intramolecular electrophilic assistance is available in the olefin, e.g, (604). ... 

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. 

When first reported by Cope and Hardy, similarities between this and the Claisen rearrangement were immediately recognized, and it was thus hypothesized that the reaction proceeds through a concerted, intramolecular, cyclic transition state. To confirm this mechanism, a series of experiments were conducted which strongly supported Cope s initial postulate. 

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