Carbon Heterolytic fission

The carbanions are formed during heterolytic fission of a C—X bond in which the carbon atom is more electronegative than X. 

The heterolytic fission of a bond between a carbon atom and a leaving group (L) can in principle occur in either of two ways. 

C. Heterolytic Fission of Bonds Attached to a Vinyl Carbon Atom. . 231... 

A rationale for the regiochemistries observed for the polychloroarene radical anions may be developed by considering the transition states for the two competing processes (Scheme 8). The loss of chloride ion in route (a) generates phenyl radical. The transition state for this process would, therefore, be expected to exhibit some radical localization at C-l. The shape of the transition state might be expected to be bent rather than planar, since heterolytic fission of the carbon-chlorine bond in a coplanar transition state would lead to an excited state (a phenyl cation with an extra electron in the n molecular orbital), while heterolytic fission of a bent system (such as C) could lead directly to a phenyl radical. Thus, the transition state for route (a) might very well possess some of the character of a delocalized anion with a bent localized radical center (C), while the transition state for chlorine atom loss, by a similar argument, would resemble a delocalized radical with a bent localized carbanionic center (D). 

The heterolytic fission of a C—X bond in an organic molecule, in which X is more electronegative than carbon, generates the negatively charged anion (X ) and positive charged species known as carbocations (called carbonium ions in the older literature). 

Carbanions are considered to be derived by the heterolytic fission of the C—X bond in an organic molecule in which carbon is more electronegative than X. 

Initially, the nitrogen carried the positive charge and then, after the heterolytic fission of the double bond, the carbon bears the positive charge. 

In this mechanism, it is proposed that the first step consists of the heterolytic fission of the bond that joins the carbon atom with the group that will be substituted. In the molecule (CH3)3CBr, identify the... 

This analysis of the simple addition of an electrophilic bromine molecule to a symmetrical alkene or alkyne has highlighted many points. First, there is the induction of a temporary dipole of the soft electrophile by the n electrons of the carbon/carbon double bond. Second, there is the heterolytic fission of the bromine molecule, and the subsequent formation of the cyclic bromonium ion. Third, this cyclic intermediate places certain restrictions on the potential line of attack for the second reagent, and so controls the structural and stereochemical consequences for the product. 

The pressure dependences of formation, homolytic and heterolytic fission of chromium-carbon bonds in aqueous solutions were investigated for the [(H20)5CrR]2+ complexes, where R = different alkyl groups [32-34], The reactions (Eqs. 5 and 6) were followed by spectrophotometry [35], the alkyl radicals being generated in situ by pulse radiolysis [36]. 

The recognition (Ben Ishai and Berger 1951) that the benzyloxycarbonyl group is cleaved by acidolysis, particularly by HBr in acetic acid, had a major impact on the development of protecting groups. In the heterolytic fission of the carbon-oxygen bond the benzyl cation plays an important role by facilitating the decomposition of the protonated itermediate ... 

Heterolytic fission of the Br-Br bond eventually takes place to form a bromide ion, Br". The Jt electrons move to form a o bond between one of the carbon atoms and the nearest bromine atom. The movement of a pair of electrons away from the other carbon atom results in the production of an electron-deficient carbocation. Finally, the bromide ion acts as a nucleophile and forms a (dative) bond to the carbocation intermediate (Figure 20.26b). The product of this reaction is 1,2-dibromoethane (CH2BrCH2Br). 

However, heterolytic fission of the Si—C bond usually occurs more readily than that of a C—C bond because of the greater ionic character of the former (compare differences in electronegativity). The split can be achieved by a nucleophilic attack on silicon, an electrophilic attack on carbon, or a more or less concerted action of both types of attack. 

Ozonides react with catalytic quantities of chlorosulfonic acid (0.3 equivalents) in dichloromethane at 20 °C to yield 3,6-dialkyl-1,2,4,5-tetraoxans and/or 1,4-dialkyl-2,3,5,6,ll-pentaoxabicyclo[5.3.1]undecanes. The reaction of ozonides with chlorosulfonic acid has been extensively investigated by Miura and co-work-ers. The reaction pathways appear to vary with different substituents the proposed mechanism involves heterolytic fission of the carbon-oxygen bond of the peroxide bridge. For example, methylcyclopentene ozonide 472 reacted stereo-selectively with the reagent in dichloromethane to give initially compound 473 which subsequently rearranged to form the trans tetraoxan 474 (Equation 151). 

The heterolytic fission of a bond can involve a C—X bond, where X is an atom more electronegative than carbon. 

The OH ion donates a pair of electrons to the 6+ carbon atom, forming a new covalent bond. At the same time, the C—Br bond is breaking. The Br atom takes both the electrons in the bond (an example of heterolytic fission of a covalent bond, see page 196) and leaves as a Br ion. 

Silicon, in its compounds, forms bonds of a covalent nature and is, with limitations, similar to saturated carbon. However, because silicon is decidedly more electropositive than carbon, its bonding to other elements has more ionic character and, therefore, is subject to a greater ease of heterolytic fission by polar reagents. Also, the available d orbitals of silicon probably play an important role in many reactions at silicon. ... 

Decarboxylation reactions may be induced (depending on the acid) in a variety of ways thermally, bacteriologically, photochemically, or even by electrolysis as in the anodic reaction of the Kolbe synthesis. Thermally induced decarboxylation of many carboxylic acids in solution proceeds by a bimolecular mechanism involving addition of a nucleophilic solvent molecule to an electrophilic carbon atom on the root molecule - preferably at a carbon adjacent to (a) or one removed from (P) the carboxyl carbon (Fraenkel et al. 1954 Clark 1958, 1969). An electrophile-nucleophile pair is formed in the transition state, which subsequently undergoes heterolytic fission (i.e., decomposition of a molecule into two ions of opposite charge) to yield CO2, a proton, and a carbanion the latter two species are reactive intermediates, which then combine rapidly (Brown 1951). The solvent molecule departs unaffected and in this sense the solvent may be considered... 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

Another factor to be investigated in the metathesis process is the effect of bases in the reaction media. Bases such as triethylamine are added in the experimental conditions to stabilize the formic acid product because otherwise the product is thermodynamically less stable than the separate carbon dioxide and dihydrogen reactants. As discussed above, the o-bond methathesis involves the heterolytic H-H bond fission, which would be accelerated by the presence of the base. This effect was theoretically investigated in the four-center o-bond metathesis between RhOn1-... 

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