Ionic cleavage

In contrast to the abundant examples of radical cleavage, only a few proposals of ionic cleavage of carbon-carbon a bonds have been put forward in the long saga of mechanistic studies on heterolytic cleavage reactions. 

Aldehydes and diethyl(triethylgermyl)phosphine give (38) while unsaturated aldehydes and ketones form 1,4-dipolar addition products (39), presumably via ionic cleavage of the germanium-phosphorus bond. Hydrolysis of these compounds produces the y-substituted phosphorus aldehydes or ketones. -... 

Rearrangements characteristic of carbonium ions have been cited as evidence for ionic cleavages 193h... 

It can be expected that p, is larger for the ionic cleavage of a CH bond in solution than for the ionic cleavage of an OH bond in solution, for the following two reasons. 

It is assumed that the p1 value referring to the ionic cleavage in water of an XH bond remains unchanged in transition states with different groups Y, and correspondingly, the value of p2 referring to a YH bond does not depend on the particular X group in the transition state. For reactions of the type... 

The ring-expansion of thiete intermediate 238 is obviously ionic. Cleavage of a carbon-sulfur bond in benzothietes, for example, 199, may occur on alkylation assisted by a 3-amino group. " A 4-methylspirothiete related to 214 is in equilibrium with the exo-methylene isomer. ... 

The observed N->Si dissociation satisfies the tendencies of both opposing effects the bulk of the cyclohexyl forces cleavage of one of the bonds, but the electron-withdrawal prevents ionic cleavage. 

Vacuum ultraviolet irradiation of matrix-isolated complexes may result in ionic cleavage of M-L bonds. Ni(CO)3, Cr(C0)5, and Fe(CO) have been produced in this manner from Ni(C0)4, Cr(CO)g, and Fe(C0)3, respectively (143). 

In addition to the substituted metal-metal-bonded complex, they observed formation of a disproportionated complex MofCOElPPlbk 1 Mo(CO)3Cp, 45 A more quantitative study was reported by Burkett et al.46 who studied ionic cleavage of the metal-metal bond as shown in reaction 10.28 (X = SCN, Cl, Br, I). 

It must be remembered, therefore, that the silicon atom is the most electropositive partner of not only the Si-C bond but also the Si-H bond. Thus, ionic cleavages of such bonds as Si-0, Si-C, and Si-H proceed by electrophilic attack (E+) on silicon s bonding mate and nucleophilic attack (Nu ) on silicon itself (Scheme I). 

One of the first recognized preparations of an oxazole was that of 2,4,5-triphenyloxazole by the action of ammonia on benzil.141 The reaction is very complex and leads to a variety of products (including 2,4,5-tri-phenylimidazole) depending on the temperature, time, and solvent. A number of mechanisms to explain the mode of formation of these products have already been critically discussed in previous reviews.1-4 In the latest study by Wenkert and Mekler,142 the reaction between benzil and ammonia is interpreted as involving the ionic cleavage of a benzil-ammonia complex containing two benzil units. 

Many organic peroxides can be employed, one of the more widely used ones being dicumyl peroxide. Dicumyl peroxide decomposes either thermally to yield free radicals or in acid media by an ionic cleavage mechanism without the production of free radicals. Since the free-radical mechanism is required for the polymer vulcanization reaction, the ionic cleavage decomposition has to be suppressed by the use of a non-acidic medium. The factor determines what type of filler can be used. 

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