Reaction mechanisms rearrangement intermediates

Similar reaction was described also for 463, the reaction mixture contained 95% of 464 and 5% of 467, the product of cyclization of the rearranged intermediate 466. The proposed mechanism of this rearrangement is shown in Scheme 74 (78ZOR105). 

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

On the basis of all these experiments various mechanisms have at some stage been advanced for the Fries rearrangement involving the free acylium ion or as a tightly bound ion pair, Ji-complexes and cyclic intermediates. It is clearly impossible to reconcile all the experimental data by one reaction mechanism. It is probable that many such mechanisms are possible, each one operative under a certain set of conditions. 

Nagayama et al. [36] studied a-sulfonation using nuclear magnetic resonance (NMR). They reported the presence of two intermediates. The first intermediate is the adduct of S03 to the carbonyl oxygen formed at low temperatures. In contrast to the mechanism of Stein et al., they did not propose a rearrangement of this intermediate but a second addition of S03 to the activated a-hydrogen to give the second intermediate. The reaction of the intermediate with sodium hydroxide can lead to the disodium salt if the neutralization is immediate or to the sodium a-sulfo fatty acid ester if the neutralization is delayed. 

The reaction mechanism is based on protonation of the hydroxyl moiety, rearrangement of the phenyl group and simultaneous cleavage of water, creating a carbocation as intermediate [135]. This cation is hydroxylated by water. Thereby, an unstable hemiacetal is formed that splits into two molecules, phenol and water. 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

However, the idea, that 96 may rearrange to the ortho isomer 93 via substituent migration or valence bond tautomerization, which would enable the CH3 loss to proceed as described in (20), could not be substantiated by experimental facts. For example, the secondary decompositions of the [M—CH3]+ ions formed from 93 and 96 are different with regard to the reaction channels and both the kinetic energy release and peak shapes associated with the reactions of interest. Moreover, the CA spectra of the [M—CH3]+ ions exhibit distinct differences. Thus, the [M—CH3]+ ions posses different ion structures and, consequently, a common intermediate and/or reaction mechanism for the process of methyl elimination from ionized 93 and 96 are very unlikely (22). 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

The photolytic decomposition of orf/zo-azidobiphenyls 507 to carbazoles 508 is not as high yielding as the thermolytic procedure, and the reaction times are longer. However, the reaction proceeds under milder conditions (488,489). The reaction mechanism has been studied, and it was concluded that two intermediates were present in the reaction, both of which were capable of rearranging to carbazole (490) (Scheme 5.4). 

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

Rearrangements, when they do occur, are taken as evidence for carbocation intermediates and point to the SN1 mechanism as the reaction pathway. Rearrangements are never observed in SN2 reactions. 

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