Heterolytic and Homolytic Pathways

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

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Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

The diatomic elements act as oxidants and as electrophiles. Halogen molecules cleave readily via both heterolytic and homolytic pathways ... 

Early contributions by Cristol and Bindel [1] set the stage for benzyl chloride and benzyl bromide photochemistry. More recently, Bartl, Steenken, Mayr, and McClelland [34] have implicated both heterolytic and homolytic cleavage from nanosecond LFP of diphenylmethyl halides. Transient absorption spectroscopy in the 70-ns time frame allowed for observation of both cationic and radical intermediates. The ratio of the two pathways (heterolytic/ homolytic) was essentially the same for the chloride and the bromide. 

Detailed mechanistic studies on the photolysis of triaryl-, diarylalkyl- and aryldialkylsulfonium salts have been carried out by Saeva and co-workers [63c, 78]. These built upon early work [3c, 79], as reviewed by Coyle [80], which recognized the importance of both heterolytic and homolytic cleavage pathways. 

Figure 10 The proposed reaction pathways for the tyrosyl radical generated upon phenolic hydrogen atom abstraction by the 5 -deoxyadenosyl radical in ThiH. Heterolytic and homolytic Ca-C/3 bond cleavage events are shown. Both pathways converge in the ultimate formation of glyoxylate and cresol.

The intrinsic difference between covalent and coordinate bonds can also be recognized from the fact that a given bond will generally exhibit an inherent preference for either homolytic (covalent) or heterolytic (coordinative, zwitterionic ) dissociation. Such disparate modes could be distinguished, for example, by the disparate responses to bond-dissociative distortions in the presence of an external electric field. Consistent with the principle of microscopic reversibility, we naturally choose to envision bond formation as occurring by the reverse heterolytic or homolytic pathway that leads to dissociation. The intrinsic coordinative or covalent character of the bond could therefore be defined in terms of its preferred dissociation (rather than formation) pathway, an experimentally measurable property. 

The products of reductive cyclization incorporate two non-exchangeable hydrogen atoms. Homolytic and heterolytic hydrogen activation pathways may now be discriminated on the basis of hydrogen-deuterium crossover experiments. Reductive cyclization of the indicated nitrogen-tethered enyne under a mixed atmosphere... 

Similar relative reactivities for homolysis versus heterolysis have been observed in the fragmentation of l-NMOPh versus 1-NpOBz and 2-NMOPh versus l-NpOBz. In these cases, both the heterolytic and the homolytic pathways were observed. However, the heterolytic reactions were reported to be about 10 times faster than the homolytic reactions. 

Because of these clear differences of chemical pathway in the enzymatic and model reactions, it is difficult to assess the actual factor by which the apoenzyme enhances the reactivity of reduced flavin with O2. It is fairly clear, however, that the environment of the apoenzyme suppresses homolytic pathways and, presumably by general-acid-base and steric mechanisms, allows only heterolytic bond cleavages. 

In many cases both homolytic and heterolytic pathways afford the same products, e.g. alcohols and ketones from hydrocarbons, which means that results have to be interpreted with care. Certain elementary tests for homolytic pathways need to be performed, e.g. inhibition by a radical scavenger such as lonol indicates a free radical chain mechanism and loss of yield on flushing with an inert gas suggests the intermediacy of dioxygen in reactions with H2O2 or RO2H. More sophisticated reality tests can also be performed to demonstrate the intermediacy of alkoxy radicals in oxidations with RO2H [17]. 

However, H2 can bind in stable fashion to very electron-deficient metals, which are weak backbonders, nearly as well as to more electron-rich metals. Calculations show that for highly electrophilic metals the reduction in BD is almost completely offset by increased electron donation from H2 to the electron-poor metals. It is important to note there are two completely different pathways for cleavage of H-H and X-H bonds homolytic cleavage (OA) and heterolytic cleavage. Both pathways... 

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