P-Elimination pathway

Rajagopal and Burris (1999) studied the degradation reaction of ethylene dibromide in water with zero-valent iron. Ethylene dibromide degraded rapidly following pseudo-first order kinetics. The observed end products were ethylene and bromide ions and were probably formed via a P-elimination pathway. Bromoethane and vinyl bromide were not observed as possible intermediate products. 

The coupling reaction by which the aromatic group from one residue of mono- or diiodotyrosine is joined in ether linkage with a second residue is also catalyzed readily by peroxidases. One dehydroalanine residue is formed for each molecule of hormone released.108 A possible mechanism involves formation of an electron-deficient radical, which can undergo (3 elimination to produce a dehydroalanine residue and an aromatic radical. The latter could couple with a second radical to form triiodothyronine or thyroxine. However, as depicted in Eq. 25-6, the radical coupling may occur prior to chain cleavage. While P elimination (pathway... 

Ureidomethylphosphonium salts, which contain a hydrogen substituent on N-1, follow a p-elimination pathway similar to the one indicated in equation (IS) giving the tertiary phosphine, but if the N-1 position is blocked by a methyl group the reaction follows the 5np pathway giving the tertiary phosphine oxide (Scheme 41),229... 

SAMPLE SOLUTION (a) First analyze the structure of 2-bromo-2,3-dimethylbu-tane with respect to the number of possible p elimination pathways. 

Coupling of dihydrofuran with an alkene-zirconocene complex and subsequent addition of an electrophile, provided the cw-disubstituted homoallylic alcohol, as shown in the example below. An insertion/p-elimination pathway involving the formation of an oxazirconacyclooctene intermediate was proposed <04AG(E)3932>. 

Evidence of an undesired P-elimination pathway can be demonstrated when enantioenriched a-phenyl pyrrolidine is subjected to the standard reaction conditions. Although this amine couples in 60% yield with bromobenzene, the resulting product is racemic. This indicates that the palladium-amido intermediate is in equilibrium with an imine. 

N-Ary piperazines can be synthesized using this catalyst and unprotected piperazines.IS Key to this protocol is the use of excess piperazine to favor mono-ary lation versus the di-arylated product. Moderate yields of the mono-arylated product can be obtained with a variety of aryl bromides. The major side product is the de-halogenated arene caused by the p-elimination pathway. 

Significantly, although the cyanide in 48 is drawn with specific stereochemistry, either stereoisomer would be acceptable in that the drawn structure could lead to 50 through a concerted E2-type P-elimination pathway, while its undrawn epimer could afford access to the same compound through dehydration of the neighboring tertiary alcohol. 

To have a stable alkyl, we must block the p-elimination pathway for decomposition. This can happen for... 

The p-elimination pathway is the fastest process and the most usual, leading, after the reductive elimination of the metal, to the formation of 1,3- and/or 1,4-dienes. The p-elimi-nation requires a vacant coordination site on the metal and a cis relationship between carbon—metal and carbon—hydrogen... 

Recent studies show that zero-valent metals (e.g., iron, zinc, and palladium), sulfide minerals (like FeS and FeS2), and green rust can act as abiotic agents and increase the reductive dechlorination rates of chlorinated ethanes and ethenes. For example, in the presence of zero-valent iron, P-elimination is the main degradation pathway of chlorinated ethenes. In the P-elimination pathway, chlorines in vicinal carbon atoms are eliminated and a third C-C bond will be formed (Tobiszewski and Namiesnik, 2012 Brown et al., 2009 and Ma and Wu, 2008). Kara et al. (2005) reported fliat with P-elimination, TCE, c-DCE, and t-DCE reduction occmred 40, 10, 100 times faster, respectively, in comparison to hydrogenolysis (Kara et al. 2005). 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

Neutral borabenzene-PMe3, generated through the route described in Scheme 2, reacts with a variety of anionic nucleophiles to furnish 5-substituted borataben-zenes (Scheme 7).15 This approach provides efficient access to boratabenzenes that bear a range of boron substituents (H, C, N, O, P) with diverse electronic and steric properties.16 Mechanistic studies establish that this novel aromatic substitution process follows an addition-elimination pathway. 

Attack on aromatic species can occur with radicals, as well as with the electrophiles (p. 131) and nucleophiles (p. 167) that we have already considered as with these polar species, homolytic aromatic substitution proceeds by an addition/elimination pathway ... 

The P-alkoxy elimination pathway is important during the incorporation of oxygen-containing monomers. Therefore, it is often necessary to provide distance between the olefin and the polar group, or to prevent chain walking close to the group that can be eliminated by the placement of a quaternary carbon spacer [87], The incorporation of acrolein dimethyl acetal is accompanied by reduced activity and full catalyst... 

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

Trichloroethylene (TCE) and perchloroethylene (PCE) require cleavage of the carbon-halogen bonds. Two methods of cleavage are P-elimination by dehy-drohalogenation, as shown in Equation (13.14), and nucleophilic substitution by either water or hydrogenolysis in Equation (13.15). The proposed pathways for reduction of chloroethylenes by zero-valent iron are as follows ... 

The influence of reaction conditions upon activity was explored, revealing that 13 is 16 times more active than is 14, a fact that can perhaps be attributed to the enhanced steric bulk of the former. This property has previously been identified as fundamentally important in avoiding termination by the p-hydride elimination pathway. 71,172 The activity of the 13/MAO system was further probed, revealing a significant increase in turn-over number (TON) upon varying the pressure in the range 1.1 >30... 

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