P-Fragmentation Reactions

Carbon-centered radicals may undergo homolytic P-fragmentation reactions, whereby an olefin and a new radical is formed. This reaction is, in fact, the reverse of the polymerization reaction. With neighboring C-C bonds, these P-frag-mentation reactions are usually slow, and only observable, at least on the pulse radiolysis time-scale with negatively-charged polymeric radicals whose lifetime is prolonged by electrostatic repulsion. Then, even the situation of equilibrium polymerization maybe approached (Ulanski et al. 2000 Chap. 9.4). 

The rate of such an (exothermic) reaction is not yet known. In the amino acids, P-fragmentation [reactions (22)-(24)] compete successfully (Bonifacic et al. 2000a for DFT calculations see therein). 

The rate of this reaction (which is the main decay of tertiary alkoxyl radicals) is also strongly enhanced in water as compared to the gas phase and organic solvents. If different substituents can be cleaved off, it is the more highly-substituted one (weaker C-C bond) that is broken preferentially (Riichardt 1987). Thus in the case of secondary alkoxyl radicals, substitution in p-position also decides the ratio of 1,2-H-shift and -fragmentation (Schuchmann and von Sonntag 1982). Because of the fast 1,2-H-shift and p-fragmentation reactions in water, intermolecular H-abstraction reactions of alkoxyl radicals [reaction (61)] are usually inefficient, but intramolecular H-abstraction may occur quite readily if an H atom is in a favorable distance (e.g., six-membered transition state). 

Because of the rapid 1,2-H-shift [reaction (51)] and the ready conversion of the ensuing a-hydroxyalkyl radical into H02702 by 02 [cf. reactions (8) and (10)/(11)], primary and secondary peroxyl are often the precursor of 02 in theses systems. Furthermore, the P-fragmentation reaction (60) creates a new radical and hence a new peroxyl radical, a situation which makes the elucidation of mechanistic details often very difficult if not impossible. The peroxyl radical systems that have been investigated in detail thus far have been discussed by von Sonntag and Schuchmann (1997). 

In the presence of O2, it is converted into the corresponding peroxyl radical [reaction (266)]. The bimolecular decay of this peroxyl radical with the other peroxyl radicals present is this system leads to erythrose [reaction (267)] in 15% yield, i.e., the p-fragmentation reaction of a short-lived oxyl radical intermediate is of minor importance. 

The reactivity of the monomer and the reaction conditions determine the relative importance of P-scission. Fragmentation reactions are generally favored by low monomer concentrations, high temperatures and low pressures. Their significance is greater at high conversion. They may also be influenced by the nature of the reaction medium. 

Certain ketoximes can be converted to nitriles by the action of proton or Lewis acids. Among these are oximes of a-diketones (illustrated above), a-keto acids, a-dialkylamino ketones, a-hydroxy ketones, p-keto ethers, and similar compounds. These are fragmentation reactions, analogous to 17-25 and 17-26. For example, ot-dialkylamino ketoximes also give amines and aldehydes or ketones besides nitriles. 

The p-scission of a phosphoniumyl radical yields a cation and a phosphonyl radical, while its reaction with a nucleophile generates a phosphoranyl radical which can undergo SET reactions and a- or p-fragmentations (Scheme 14). 

During the last two decades, Bentrude et al. [70] has shown that phosphoranyl radicals exhibiting very slow a- and P-fragmentations react with alkyl disulfides via Sh2 homolytic substitution (Scheme 35) [70b]. The reactivity of phosphoranyl radicals in these Sh2 reactions depends strongly on the substituents attached to the phosphorus atom and on the structure of the disulfides [70c]. 

The fragmentation reaction was applied most successfully in the total synthesis of sesquiterpenes 39). For example, the mesylate (110) underwent fragmentation reaction to provide the ketone (111), which could be converted to (112). The compound (112) was eventually transformed to the isolable mixtures of p-himachalene (113), trans-a-himachalene (114) as well as traHs-y-himachalene (115)40). 

Because of the strong electron-withdrawing effect of the MICO), fragment, reactions of nucleophiles, dienes or 1,3-dipoles with vinylcarbene or alkynylcarbene complexes are usually faster than with the corresponding a,P-unsaturated esters [242,253,264]. 

A large number of homolytic fragmentation reactions of carbon radicals with p-leaving groups are known from studies in the gas and condensed phase. 

Another kind of bridge between two phosphonio groups corresponding to the P +—C—C—O—C—C—P+ chain, is favourable to the same type of Ep fragmentation (reaction 189)353. Here the leaving group fragmentation is probably favoured by the interaction nQ->P +. ... 

Quantum yields, free energy changes, — AG°, for the electron transfer reaction from the P fragment to the Q fragment of P-L Q molecules in methylene chloride solution [55]... 

Harayama Y, Yoshida M, Kamimura D, Wada Y, Kita Y (2006) The Efficient Direct Synthesis of N.O-Acetal Compounds as Key Intermediates of Discorhabdin A Oxidative Fragmentation Reaction of a-Amino Acids or P-Amino Alcohols by Using Hypervalent Iodine(III) Reagents. Chem Eur J 12 4893... 

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