Claisen rearrangement radical mechanism

The temperature required for aza-Claisen rearrangement is about 100-150°C higher than those for the corresponding ethers [3]. The trend is the same in the rearrangment of fluorinated compounds as shown below. The totally unique products formation observed in aza-Claisen rearrangement of fluorinated enamines as compared with those of ethers suggests a different mechanism via, not a concerted six-membered transition state, but a stepwise radical pathway [4]. 

The Cope rearrangement, like the Claisen rearrangement, is a no mechanism reaction and thus does not involve ionic or radical intermediates. For practical purposes the result is that Cope rearrangements are independent of catalysts and of the nature of the solvent, and that substituent effects are slight. Steric influences, however, are considerable cw-1,2-divinylcyclobutane rearranges to 1,5-cyclooctadiene within a few minutes at 120° ... 

Because both radical and ionic mechanisms have been proposed for this reaction and no mechanistic detail has been given in the literature, a new ionic mechanism similar to the Abnormal Claisen Rearrangement is proposed and outlined here. 

The conversion of (298) into (300) upon gas-phase thermolysis occurs by a Cope rearrangement and not by a [1,3] carbon shift or a bisallyl radical mechanism. Thus, similar treatment of (299) affords only (301). Rearrangement of 3-oxobicyclo-[3,2,l]oct-6-ene to 3-oxobicyclo[3,3,0]oct-6-ene occurs only at much higher temperature and presumably by way of a non-concerted mechanism. The nickel-catalysed reaction of norbornene with butadiene affords the 1 2 cxo-adduct (302) which upon thermolysis is converted into the valence isomer (303). Concerted and non-concerted Claisen-type rearrangements have been observed in the A, -phosphorin series. Tricyclo[7,3,0,0 ]dodeca-2,5,7,10-tetraene (304), which is available in six steps starting from bicyclo[3,3,0]octa-3,7-dien-2,6-dione, appears to be an essentially static structure (as assessed by n.m.r. spectroscopy) up to 141 The slow... 

The Claisen rearrangement also proceeds via the radical mechanism, which requires either a high temperature (T > 700 K or the introduction of an initiator at a moderate temperature... 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

No mechanism is the designation given, half in jest, half in desperation, to "thermo-reorganization" reactions like the Diels-Alder and the Claisen and Cope rearrangements in which modem, mechanistic scrutiny discloses insensitivity to catalysis, little response to changes in medium and no involvement of common intermediates, such as carbanions, free radicals, carbonium ions and carbenes. (Doering, 1962) [7]... 

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