Radicals cyclic pathways

Photolysis of the nitrosimines, such as 35a, gives rise to a variety of products that appear to have come from loss of NO and subsequent radical reactions (Scheme 3.22) [196]. Similar products, indicative of radical reactions, are also observed in the thermolysis of sterically hindered nitrosimines (e.g., 40, with Rj = tert-Bu and R2 = 2-MePh) [197]. Steric constraints were proposed to disfavor the cyclic pathway... 

Scheme 6.13 A stepwise pathway via five-membered cyclic intermediate radical 66 in the 1,2 migration of an acyloxy group...

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

Radical cyclization of acyclic sulfinamides 239 provides easy access to cyclic sulfinamides 241 <06AG(E)633>. Conceivably, the reaction pathway involves thiophilic attack by the aryl radical with a concomitant or successive expulsion of the p-tolyl or tert-butyl radical. 

With 4,4-diacyl triafulvenes two principal fragmentation pathways have been observed5 s In 4-aroyl-4-acetyl triafulvenes 241 the molecular ion is followed by a fragment ion of probable structure 242 arising from primary loss of (CvI R), which surprisingly has incorporated a CH2 unit from the acetyl group and the exo-cyclic aryl residue. It is not unlikely that the (C7H6R)-residue corresponds to a substituted tropyl radical due to its well-known formation from electron-impact of benzylic precursors. 

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs]" ion has initially been attributed to a McLafferty rearrangement followed by loss of an alk-oxy radical and final stabilization to a cyclic oxonium ion. [104] However, it has been revealed that four other pathways in total lead to its formation excluding the above one. [105,106] The two most prominent fragmentation pathways are ... 

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