Rearrangement and Fragmentation Reactions

Fragmentation is the reverse of radical addition. Fragmentation of radicals is often observed to be fast when the overall transformation is exothermic. 

Rearrangement of radicals frequently occurs by a series of addition-fragmentation steps. The fragmentation of alkoxyl radicals is especially common because the formation of a carbonyl bond makes such reactions exothermic. The following two reaction sequences are examples of radical rearrangements proceeding through addition-elimination. 

SECTION 10.3. REACTIONS INVOLVING FREE-RADICAL INTERMEDIATES 

A direct comparison of the ease with which unsaturated groups migrate by cycliza-tion-fragmentation has been made for the case of net 1,2-migration  

The two reactions above are examples of a more general reactivity pattern255 256  

In this system, the overall driving force is the conversion of a primary radical to a tertiary one (AH 5 kcal) and the activation barrier incorporates strain associated 

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates 

The starting point for a fragmentation is the molecular ion (El) or the quasimolec-ular ion (Cl). A large number of reactions which follow the primary ionization need to be described here. All the reactions follow the thermodynamic aim of achieving the most favourable energy balance possible. The basic mechanisms which are involved in the generation of spectra of organic compounds will be 

There are two possible mechanisms for the cleavage of carbon chains following ionization, known as a-deavage a.nd formation of carbenium ions. The starting point after ionization is the localization of positive charge on electron-rich structures in the molecule. 

Fragmentation involving formation of carbenium ions takes place at a double bond in the case of aliphatic carbon chains (allylic carbenium ion) or at a branch. 

With alkylaromatics, side chains are cleaved giving a benzylic carbenium ion (benzyl cleavage), which dominates as the tropylium ion m/z 91 in many spectra of aromatics. 

The formation of carbenium ions occurs preferentially at tertiary branches rather than secondary ones. 

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Grota, J., Domke, I., Stoll, I., Schroder, T., Mattay, J., Schmidtmann, M., Bogge, H., and Muller, A. (2005) Synthesis, fragmentation, and rearrangement reactions of annelated cyclobutyl-carbinols. Synthesis, 2321-2326. 

Acylation reactions of l-aryltetrazol-5-ones generally occur on the N(4)-position, but acylation of the exocyclic oxygen atom occurred in up to 50% yield when the acylating agent was 2-methylpropanoyl chloride (73ZC429). Fragmentation and rearrangement reactions of 5-alkoxytetrazoles are discussed in Section 4.13.3.2 and alkylations of these systems in Section 4.13.3.5. 

The reactions to be described in this chapter have in common the formal involvement of even-electron intermediates having unfilled orbitals of low energy. The most familiar of these intermediates are carbonium ions. Sections 8.4 and 8.5 contain examples of fragmentation and rearrangement reactions of carbonium ions that are of synthetic value the discussion of these reactions supplements the mechanistically oriented discussion presented in Part A, Chapter 5. Also important are the neutral divalent carbon and monovalent nitrogen species, carbenes and nitrenes, respectively. 

Sources of Radical Intermediates Introduction of Functionality by Radical Reactions Addition Reactions of Radicals with Substituted Alkenes Cyclization of Free-Radical Intermediates Fragmentation and Rearrangement Reactions... 

Electrochemical fluorination leads to fragmentation, coupling, and rearrangement reactions as well as giving the perfluorinated product. In addition, small amounts of hydrogen can be retained in the cmde product. The products are purified by treatment with base to remove the hydrogen-containing species and subsequently distilled. 

In the presence of various metal ions, 2-(fluoroenone)benzothiazoline has been found to rearrange to A-2-mercaptophenylenimine, while a free radical mechanism involving the homolysis of C-S and C-N bonds has been invoked to explain the formation of 3-phenyl-1,2,4-triazole derivatives from the thermal fragmentation and rearrangement of 2-(arylidenehydrazino)-4-(5//)-thiazolone derivatives. The cycloadducts (36) formed from the reaction of 3-diethylamino-4-(4-methoxyphenyl)-5-vinyl-isothiazole 1,1-dioxide (34) with nitric oxides or miinchnones (35) have been found to undergo pyrolytic transformation into a, jS-unsaturated nitriles (38) by way of pyrrole-isothiazoline 1,1-dioxide intermediates (37). 

R. E. Gawley, The Beckmann reactions Rearrangements, elimination-additions, fragmentations, and rearrangement-cyclizations , in Organic Reactions, Vol. 35 (Ed. A. S. Kende), Wdey-Intersdence, London, 1988, pp. 1-415. 

Fragmentation and recombination reactions predominate in nearneutral, alkaline solutions. Fragments from the cleavage of the carbohydrate, and from its dehydration products, undergo further dehydration, condensation, and intermolecular, Cannizarro-type reactions. The benzilic acid rearrangement, an intramolecular Cannizarro reaction, seems to be inoperative. 

The importance of ring contractions, fragmentations, and rearrangements in these types of reactions was again emphasised, and inferences were drawn from this work as to the mechanism of the reaction, based on steric modelling of the compounds. (See Sect. 4.3). 

R. E. Gawley, The Beckmann Reactions Rearrangements, Elimination-Additions, Fragmentations, and Rearrangement-Cyclizations, Org. React. 1988, 35, 1-420. 

Successive introduction of nitrogen atoms into benzene causes a gradual reduction in aromatic stabilization. The diazines still show typical aromatic behavior in that in most of their reactions they revert to type. However, with the triazines and tetrazines, decreasing aromaticity increases the ease of both thermal and photochemical fragmentations and rearrangements, and of cyclic transition state reactions with other reagents. 

The Beckmann Reactions Rearrangements, Elimination-Additions, Fragmentations, and Rearrangement-Cydizations... 

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