Reactions with concerted rearrangements

Bimolecular reactions with concerted rearrangement of bonds... 

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. 

The Schmidt reaction of ketones with hydrazoic acid is believed to be a similar rearrangement, again with concerted trans migration and elimination of nitrogen. 

Micellar rate enhancements of bimolecular, non-solvolytic reactions are due largely to increased reactant concentrations at the micellar surface, and micelles should favor third- over second-order reactions. The benzidine rearrangement typically proceeds through a two-proton transition state (Shine, 1967 Banthorpe, 1979). The first step is a reversible pre-equilibrium and in the second step proton transfer may be concerted with N—N bond breaking (17) (Bunton and Rubin, 1976 Shine et al., 1982). Electron-donating substituents permit incursion of a one-proton mechanism, probably involving a pre-equilibrium step. 

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. 

The mechanism of this reaction is obscure. One suggested mechanism, analogous to the vapor phase reaction, involves concerted decarboxylation of the pyruvic acid to yield a triplet hydroxy carbene which can either dimerize or attack another molecule of pyruvic acid to yield the observed product.91 Dimerization seems to be the less likely process since the carbene can rearrange to acetaldehyde or react with water. Further, this mechanism predicts that acetoin will be formed when pyruvic acid is irradiated in any solvent that does not possess readily abstractable hydrogen atoms, such as benzene, a solvent in which no reaction is observed. One possible explanation of this discrepancy is that the solvation of the pyruvic acid is extremely different in benzene and in water. However, the specific role that the water plays in the reaction has not been determined. 

In the conversion of the pyrazoline 33, formed from the / -nitrostyrene 32 and DPD, to the l//-pyrazole 34 with HCI, migration of the phenyl group was believed to be concerted with loss of N02. 87 It is, however, possible that the reaction proceeds via rearrangement of an intermediate 3//-pyrazoIe 35 (Scheme 11). 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

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