Heterolytic bond scission

The converse of polymerization is heterolytic bond scission leading either to R3Sn+ or R3Sn species. Tricoordinate organotin(IV) cations can readily be synthesized at room temperature by hydride or halide abstraction reactions in benzene or other solvents.For example, with R = Me, Bu or Ph ... 

In the absence of solvation mechanisms, the process of homolytic bond scission in organic compounds requires much less energy than heterolytic bond scission... 

Marks and co-workers employed organolanthanide complexes of the type Cp2Ln-R, where metal-centered redox processes cannot be involved. They studied PhSiH3 polymerization using kinetics and thermochemical measurements and proposed a four-center, heterolytic bond-scission/bond-forming sequence analogous to the Tilley mechanism.62b... 

The electron population changes are shown in Fig. 11 [9]. Apparently, the H atomic population decreases very much in the reaction, which clearly indicates that the H atom becomes protonic. On the other hand, the electron population of the phenyl group somewhat increases. In the transition state, the H atom is more positively charged than that in the reactant, while the phenyl group is more negatively charged than that in the reactant. From these electron populations, it should be concluded that the C-H o-bond activation via the metathesis occurs in a heterolytic bond scission. 

Shiraki T, Diesendruck CE, Moore JS (2014) The mechanochemical production of phenyl cations through heterolytic bond scission. Faraday Discuss. doi 10.1039/C4FEXKK)27G... 

The most important cracking reaction, however, is the carbon-carbon beta bond scission. A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation. This can be represented by the following example ... 

Because most heteroatom-carbon single bonds are less stable than carbon-carbon bonds, traceless linkers can be synthesized on the basis of nearly all heteroatoms. The enthalpies of C-X bonds are, however, only relevant for homolytic bond scission. Many linkers are cleaved heterolytically, and kinetic stability toward hetero-lytic bond cleavage is decisive in these linkers. 

W. A. Lee, T. C. Bruice, Homolytic and heterolytic oxygen-oxygen bond scissions accompanying oxygen transfer to iron(III) porphyrins by percarboxylic adds and hydroperoxides. A mechanistic criterion for peroxidase and cytochrome P-450, ]. Am. Chem. Soc. 107 (1985) 513. 

The similar results were presented for the C-H o-bond activation of benzene by Pd(tT -02CH)2. These results suggest that the MP4(SDQ) and CCSD(T) methods with proper basis sets provide reliable results in this reaction but we had better apply carefully the DFT(B3LYP) method to this kind of reaction. Because this reaction takes place in a heterolytic C-H o-bond scission, as will be described below, it is considered likely that the weak point of the DFT(B3LYP) method here results from the polarized electron distribution of this reaction system. 

Kinetics of the reactions were followed by the use of the sodium salt of 2,2 -azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as an easily oxidizable trap for the reactive higher valent iron-porphyrin intermediates and any radical products derived from YOOH (Y = alkyl or H). The one electron oxidation of ABTS provides ABTS-+ (Xmax = 660 nm). Regardless of whether YO-OH bond scission is homolytic (eqla) or heterolytic (eq2a), a... 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

Investigations conducted to elucidate the dechlorination mechanism of 4-chlorophenol have revealed that elimination of HCl via heterolytic C-Cl bond scission from aqueous 4-chlorophenol in the presence of oxygen produces an initial transient carbene, 4-oxocyclohexa-2,5-dienyhdene that has a triplet character and is stable in aqueous solutions. The addition of Oj to this intermediate affords a benzo-quinone-O-oxide, from which loss of an oxygen atom yields the major reaction product p-benzoquinone. The involvement of a carbene intermediate has also been indicated for 2-chlorophenoL ... 

Presumably, the heterolytic scission of the carbon-sulfur bond in the oxide is assisted by the hydrogen bonding, in addition to the inherent strain of the three-membered ring. 

It has been shown that the transformation 254 - 251 -t- 253 proceeds with first-order kinetics and that increase in solvent polarity is associated with an increase in the rate. It is proposed that the reaction involves a novel unimolecular heterolytic scission of the sulfur—sulfur bond in the symmetrical disulfide (254). The kinetics of the alkaline hydrolysis of anhydro-2-mercapto-4,5-diphenyl-l,3,4-thiadiazolium... 

Heterolytic scission of the -S-S- bond in which only electrophilic assistance is involved is the exception rather than the rule in reactions involving bond fission of this type. Kice et a/.165,166 have demonstrated that a variety of S-S bond cleavages involve concomitant electrophilic and nucleophilic catalysis including (a) the formation of aryl thiolsulfones from aryl thiolsulfinates and aryl sulfinic acids and (b) the hydrolysis (acetic acid—1% water and 60%... 

Although the general mode of photolysis is homoly tic (X — Y— X. + Y.) the extraordinary resonance stabilization of the cation in Eq. (8) would effect heterolytic scission of the carbon-carbon bond. 

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