Concerted rearrangement

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

The reactivity of diazo carbonyl compounds appears to be related to the conformational equilibria between s-cis and s-trans conformations. A concerted rearrangement is favored by the s-cis conformation.237 The /-butyl compound 19, which exists in the s-trans conformation, gives very little di-/-butylketene on photolysis.238 A similarly... 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

To check this possibility and to exclude any possible concerted rearrangement we used a NaY zeolite impregnated with NaBr, to see if we could observe substitution of the bromide in the organic moiety. After the reaction, besides the chloride products, we also observed the alkylbromides (scheme 5). An interesting point is the relative distribution of the bromides, favoring the allylcarbinyl bromide instead of the cyclic bromides, as was found in solution (//). 

These include a series of pericyclic reactions initiated from Si(7t,7t ). Pericyclic reactions are concerted reactions with a cyclic transition state. While in this transition state, a concerted rearrangement of electrons takes place, which causes o- and Jt-bonds to simultaneously break and form. 

Using coupled cluster CCSD(T)//B3P86 methods, the Ea values for HERON rearrangement of model hydrazines 215b-d have also been computed to be between 24 and 34 kcalmoU The authors ruled out alternative concerted rearrangements, which were computed to be energetically unfavourable. 

The carbonyl ylide 1 can undergo an internal cyclization reaction to generate the corresponding epoxide 2, which is in fact an equilibrium process, and epoxides themselves have frequently served as precursors to carbonyl ylides. Other pathways such as concerted rearrangements and internal proton transfers have also been observed to neutralize the charged ylide intermediate and give substituted ethers as represented by 3. Perhaps the best known studies and most synthetically useful... 

The synthesis of benzo[h ]quinolines (124) and (125) from the propargylaminonaph-thalenes involves a remarkable number of concerted rearrangements and a cyclization... 

Figure 13.8. Nitrenium ion formation versus concerted rearrangement.

The behavior of various alkyloxaairanee at elevated temperatures under pyrolysis conditions has also been, studied. Under these conditions amides generally are obtained in good yield. These reactions are carried out in the gas phase at 200-300° and are a direct consequence of the relatively weak oxygen-nitrogen bond. Whether unpairing of the electrons of this bond occurs is uncertain but the reaction may best be described as a concerted rearrangement as indicated below ... 

The anows can be written in either directional sense since these reactions are concerted rearrangements with all bond making-bond breaking taking place at die same time. This example emphasizes the fact that curved-arrow notation is merely an electron bookkeeping method. 

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