Proton Abstraction Reactions

The composition of an enolate mixture may be governed by kinetic or thermodynamic factors. The enolate ratio is governed by kinetic control when the product composition is determined by the relative rates of the two or more competing proton-abstraction reactions. 

Deuterium exchange studies on decaborane and other boranes indicate acidic character of bridge hydrogens. They react with bases undergoing proton abstraction reactions ... 

The second class of TAM RE AC s inventory includes the reactions between the coordinated ligands and external organic reagents. We divide these reactions into nucleophilic and electrophilic attacks and consider them as acid-base interactions. Table III presents their general description. The nucleophilic attacks are either addition reactions to unsaturated coordinated ligands (Reactions 44-46) or abstraction reactions (usually a proton abstraction, Reactions 47-50). The electrophilic attacks are similarly addition reactions (Reactions 51 and 52) and abstraction reactions (usually a hydride abstraction, Reactions 53-59). Reactions 60 to 63 represent some other intermolecular reactions. 

Marcus, 1969) by assuming that the point which represents the transition state on the potential energy profile is the intersection of two parabolas of equal curvature. It was also considered that the proton abstraction reaction can be divided up [eqn (7)] into three discrete steps (0 encounter of the reactants, (ii)... 

Looking back on the history of ketone dianion chemistry, one soon notices that dianion species, derived from / -keto esters, have been in continuous steady use in organic synthesis3,4, as shown in Scheme 2. Thus, ethyl acetoacetate can be converted to the corresponding ketone o a -chainon via consecutive proton abstraction reactions. The resulting dienolate anion reacts with a variety of alkyl halides to give products, resulting from exclusive attack at the terminal enolate anions. 

Once formed, NO can exert indirect effects on biomolecules, including deamination, nitrosylation [25], nitration or proton abstraction reactions, as outlined in Schemes 2,3 and 4 below. 

Fig. 6 (a) Schematics of a polymer-tethered lipid bilayer on a solid support the architecture is composed of a proximal mixed monolayer of lipopolymers and regular low-mass lipids, covered by a monolayer of normal lipids obtained by vesicle fusion. The lipopolymers are covalently attached to the benzophenone-derivative coated substrate by a proton-abstraction reaction as depicted in (b)... 

Although proton abstraction reactions often occur with alkyl halides, displacement reactions are observed when the ylid is treated with organo-metallics, e.g., trimethylbromosilane, trimethylbromogermane, and tri-methylbromostannane 110>. 

Amide ions have been used for many years to induce proton abstraction reactions and therefore might be expected to react with the tetramethylammonium ion in a similar way. In 1935, the decomposition of tetramethylammonium amide in liquid ammonia was initially reported by Franklin 44> to give trimethylamine and methylamine but no experimental details were given. Later, Hazlehurst, Holliday, and Pass 63> repeated Franklin s experiment 44> and noted that a trace of ethylene was evolved. During later attempts to prepare solid tetramethylammonium amide, a... 

The basic decomposition of tetraalkylammonium salts (the Hofmann degradation), has been reviewed extensively so.135) and will not be discussed here in detail. However, it should be noted that both displacement reactions and a-proton abstraction reactions may occur in addition to elimination reaction 30>. Ingold and Patel 75> report that the amount of substitution relative to elimination varies depending upon both the substituent on nitrogen and the base. 

The most common reactions of the ylid (stabilized or not) are proton abstraction and displacement reactions. In general, proton abstraction reactions are favored, since the ylid can be envisaged as a neopentyl carbanion with a high steric requirement and high basicity. 

The glycolytic pathway includes three such reactions glucose 6-phosphate isomer-ase (1,2-proton transfer), triose phosphate isomerase (1,2-proton transfer), and eno-lase (yS-elimination/dehydration). The tricarboxylic acid cycle includes four citrate synthase (Claisen condensation), aconitase (j5-elimination/dehydration followed by yS-addition/hydration), succinate dehydrogenase (hydride transfer initiated by a-proton abstraction), and fumarase (j5-elimination/dehydration). Many more reactions are found in diverse catabolic and anabolic pathways. Some enzyme-catalyzed proton abstraction reactions are facilitated by organic cofactors, e.g., pyridoxal phosphate-dependent enzymes such as amino acid racemases and transaminases and flavin cofactor-dependent enzymes such as acyl-C-A dehydrogenases others. 

Thus, despite the apparent abundance of functional and structural information, important mechanistic issues remain to be resolved for a complete understanding of the strategy by which TIM catalyzes proton abstraction reactions. 

The new primary amine anion (HI), unlike the lactam anion (II), is not stabilized by conjugation with a carbonyl group. It is therefore highly reactive and, once it is formed, it undergoes a rapid proton-abstraction reaction with a caprolactam monomer producing the dimer compound A -caproylcaprolactam (TV), which is a //-acyllactam, and regenerating the lactam anion (II) ... 

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