Organic reaction mechanism carbanions

As noted in Section 4.2.1, the gas phase has proven to be a useful medium for probing the physical properties of carbanions, specifically, their basicity. In addition, the gas phase allows chemists to study organic reaction mechanisms in the absence of solvation and ion-pairing effects. This environment provides valuable data on the intrinsic, or baseline, reactivity of these systems and gives useful clues as to the roles that solvent and counterions play in the mechanisms. Although a variety of carbanion reactions have been explored in the gas phase, two will be considered here (1) Sn2 substitutions and (2) nucleophilic acyl substitutions. Both of these reactions highlight some of the characteristic features of gas-phase carbanion chemistry. 

Manchester) in 1904, showed for the first time how kinetics could be used to reveal the mechanism of an organic reaction. The carbanion mechanism has since been confirmed not only by the iodination work, but also by studies of stereochemistry and isotopic exchange. 

Biocatalytk decarboxylation is a imique reaction, in the sense that it can be considered to be a protonation reaction to a carbanion equivalent intermediate in aqueous medimn. Thus, if optically active compoimds can be prepared via this type of reaction, it would be a very characteristic biotransformation, as compared to ordinary organic reactions. An enzyme isolated from a specific strain of Alcaligenes bronchisepticus catalyzes the asymmetric decarboxylation of a-aryl-a-methyhnalonic acid to give optically active a-arylpropionic acids. The effect of additives revealed that this enzyme requires no biotin, no co-enzyme A, and no ATP, as ordinary decarboxylases and transcarboxylases do. Studies on inhibitors of this enzyme and spectroscopic analysis made it clear that the Cys residue plays an essential role in the present reaction. The imique reaction mechanism based on these results and kinetic data in its support are presented. 

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. 

The fate of the onium carbanion Q+R incorporated into the organic phase depends on the electrophilic reaction partner. The most studied area in the asymmetric phase-transfer catalysis is that of asymmetric alkylation of active methylene or methine compounds with alkyl halides, in an irreversible manner. The reaction mechanism illustrated above is exemplified by the asymmetric alkylation of glycine Schiff base (Scheme 1.5) [8]. 

The author believes that students are well aware of the basic reaction pathways such as substitutions, additions, eliminations, aromatic substitutions, aliphatic nucleophilic substitutions and electrophilic substitutions. Students may follow undergraduate books on reaction mechanisms for basic knowledge of reactive intermediates and oxidation and reduction processes. Reaction Mechanisms in Organic Synthesis provides extensive coverage of various carbon-carbon bond forming reactions such as transition metal catalyzed reactions use of stabilized carbanions, ylides and enamines for the carbon-carbon bond forming reactions and advance level use of oxidation and reduction reagents in synthesis. 

The mechanisms of the usual organic reactions are now clearly established, and the reactions are classified as ionic, radical, and molecular. More detailed classifications have also been made. The mechanisms of many reactions involving non-transition metal compounds are clear enough for example, in the Grig-nard or Reformatsky reaction, the first step is the irreversible oxidative addition of alkyl halides to form Mg-carbon or Zn-carbon bonds, in which the carbon is considered to be a nucleophilic center or carbanion which reacts with various electrophiles. 

The interfacial mechanism is confirmed by stirring rates around 700-800, necessary to obtain reproducible results [29]. Moreover, a number of interfacial deprotonations and further reactions of carbanions are described in the absence of catalysts. For example, phenylacetonitrile is alkylated by 1-iodobutane and 50% aqueous NaOH at 80°C. Under these conditions the concentration of phenylacetonitrile in the aqueous phase and its carbanion sodium salt in the organic phase were less than 2 and 5 ppm, respectively [30]. 

Scheme 9.2 Reaction mechanism of CO2 fixation by RUBISCO. Adapted from http //chemwiki. ucdavis.edu/Organic Chemistry/Organic Chemistry With a BioIogicai Emphasis/Chapter 13%3A Reactions with stabiIized carbanion intermediates I/Section 13.5%3A Carboxyla tion and decarboxyiation reactions...

Maercker, A. The Wittig Reaction Organic Reactions 1965, 14, 270-490. Maryanoff, B. E. Reitz, A. B. The Wittig Olefmation Reaction and Modifications Involving Phosphoryl-Stabilized Carbanions. Stereochemistry, Mechanism, and Selected Synthetic Aspects Chemical Reviews 1989, 89, 863-927. Blanchette, M. A. Choy, W. Davis, J. T. Essenfeld, A. P. Masamune, S. Roush, W. R. Sakai, T. Horner-Wadsworth-Emmons Reaction Use of Lithium Chloride and an Amine for Base-Sensitive Compounds Tetrahedron Lett. 1984, 25, 2183-2186. 

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