Multicenter rearrangement

An old reaction patented by Lange [19] involves the coupling of naphthyl-diazonium salts with sodium sulfite to yield azonaphthalene. A more recent study of this reaction indicates that the first step of the reaction is the formation of an aryl-iyn-diazosulfonate which couples with another molecule of a diazonium salt and by a multicentered rearrangement ultimately affords an azo compound [20, 21]. The reaction is represented in Eq. (7). The validity of... 

Alternatively, covalent propagation may proceed by a pericyclic reaction involving a multicenter rearrangement such as a group transfer polymerization no examples of these type of reactions have been reported so far. 

Nevertheless, some research groups challenge the concept of a dynamic equilibrium between dormant covalent species and carbenium ions, and instead insist that covalent species can react directly with monomer. They propose that this occurs by a pseudocationic mechanism involving a multicenter rearrangements [259], such as those shown in Eq. (78), for the pseudocationic polymerization of styrene initiated by perchloric acid [260]. 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

Huang, J. Kertesz, M. Stepwise Cope rearrangement of cyclo-biphenalenyl via an unusual multicenter covalent x-bonded intermediate, J. Am. Chem. Soc. 2006, 128, 7277-7286. 

Multicenter fragmentations encompass eliminations, retro Diels-Alder, rearrangement, and other reactions involving the cleavage of more than one bond and the concurrent formation of one or more new bonds. 

The unimolecular dissociation of Mc3Si+ (78) has been studied experimentally . As indicated by CAD studies and isotopic exchange reactions, ion 78 readily interconverts upon excitation to isomer 79 (equation 34). The reaction mechanism for this interesting interconversion has been proposed to proceed via a concerted 1,2-hydrogen/l, 2-methyl migration. This dyotropic rearrangement corresponds to a thermally allowed process. In a similar fashion the ethene elimination from 78 was proposed to proceed via a dyotropic multicenter reaction mechanism (equation 35). 

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