Halogen atoms, addition-elimination

From the viewpoint of the reaction mechanism, the emphasis in Scheme 5 is focused on products with the general formula R3MCH2CHB1CH2CCI3 (M = Ge, Sn) with a halogen atom in a /3-position to the element M (the so-called normal addition product). These compounds are believed to be unstable and to decompose with the elimination of an R3M radical. The phenomenon is referred to as /S-decomposition or /3-cleavage (Scheme 5). The mechanism presented in this scheme lacks the radical pair stages, while the experimental results47,48 demonstrate CIDNP effects observed for the initial compounds and the main reaction products of the interaction of R3MCH2CH=CH2 with CChBr (Table 6). Thus Scheme 5, which is based on the analysis of the reaction products, needs to be refined. 

Hydrogen abstraction from CHXO may also occur by reaction with another halogen atom (equation 38). The barrier for abstraction of hydrogen from CHFO by fluorine (Y = F) is 1.6 kcalmol"1, whereas that for abstraction of chlorine is 3.2 kcal mol"1. Addition (equation 39) of halogen Y may also occur as a competing process, followed by either a-elimination of HX (equation 40) or dissciation (equation 41) of a weaker CX bond54,55. 

Hydrogen attached to ring carbon atoms of neutral azines, and especially azinium cations, is acidic and can be replaced by a metal formally being removed as a proton. Alkyllithiums can be used as bases for this purpose however, the reaction can be accompanied by addition of the alkyl anion to the ring C=N bond. To avoid this, sterically hindered bases with strong basicity but low nucleophilicity can be utilized. Among these are lithium tetramethylpiperidide (LiTMP) and lithium diisopropylamide (LDA). If the anion contains an ortho halogen atom, then this can be eliminated to form a pyridyne (see Section 3.2.3.10.1). 

As with other aromatic substitutions, the substitution step itself can be considered to involve an approximately sps hybridization at the carbon atom under attack (10). In the idealized substitution process shown in Eq. (16), 10 may constitute either an intermediate or a transition state. If proton loss ensues directly, the process is properly called a substitution. In other situations the intermediate 10 may become allied with a radical or an anion, leading thereby to a covalent adduct 11. The final substituted product 12 may then be formed either by the elimination of H—Z (first H, then Z) or by the reversal to 10, followed by proton loss. The first case is a classical example of an addition-elimination halogenation, where the adduct is an essential species in the process. In the second case, structure 10 is a common intermediate for both the substitution and the addition reactions. Being merely a diversion of 10, the addition product is not essential to the substitution. In consequence of this, the isolation of adduct 11 may not mean that addition-elimination is the principal pathway of substitution reversal to 10 may be faster than the elimination of H—Z ( 2, k3>ki). On the other hand, the mere failure to detect adduct 11 does not rule out an addition-elimination process, for dehydrohalogenation of adduct 11 may be much faster than its formation (ki>klt k2). 

In addition, the hydrated electron acts as a nucleophile, especially with organic molecules that contain halogen atoms (Eq. 6-16). This reaction results in rapid elimination of a halide ion from the initially formed negatively charged organic species. The reaction of Eq. 6-16 is of special interest for the degradation of per-halogenated saturated hydrocarbons that are usually not affected by hydroxyl radicals (Sun et al, 2000). 

Vinylic halogen atoms in perhalogenated alkenes are easily substituted by an addition-elimination mechanism with secondary phosphanes. 

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

In unsaturated aliphatic systems the most important reactions are those of czirbonyl compounds of the type —COX, in which X is a good leaving group such as halogen. In general, most displacement reactions of anionic nucleophiles on the carbonyl carbon atom of acyl halides involve an addition-elimination mechanism " (e.g. equation 16). In such reactions bond-formation is in advance of bond-rupture... 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

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