A, p Elimination reactions

If a hydrogen atom is present in the p position of the halogen, a p-elimination reaction becomes possible. If the unsaturated compound obtained is particularly unstable, the reaction will be dangerous. 

FIGURE 7.5 Preparation of a protected dipeptide by the mixed-anhydride method, employing a chloroformate that generates a cleavable urethane.13 The urethane impurity is destroyed by a P-elimination reaction. NMM = A-methylmorpholine, Msc = methane-sulfonylethoxycarbonyl. 

In addition to decomposition by loss of free radicals, Ga(C2H5)3 and Ga compounds with larger alkyl groups can undergo decomposition through a P-elimination reaction (118, 130). 

Bayer, Pan, and Kazmaier (7) proposed that labile sulfur originates from cysteinyl residues by a p-elimination reaction in the presence of... 

The dehydrohalogenation of alkyl halides, discussed in Chapter 8, is one way to introduce a n bond into a molecule. Another way is to eliminate water from an alcohol in a dehydration reaction. Dehydration, like dehydrohalogenation, is a P elimination reaction in which the elements of OH and H are removed from the a and P carbon atoms, respectively. 

Amines, like alcohols, contain a poor leaving group. To undergo a P elimination reaction, for example, a 1 ° amine would need to lose the elements of NH3 across two adjacent atoms. The leaving group, NH2, is such a strong base, however, that this reaction does not occur. 

Zaitsev rule (Section 8.5) In a P elimination reaction, a rule that states that the major product is the alkene with the most substituted double bond. 

Hofmann rule The principal alkene formed in the decomposition of quaternary ammonium hydroxides that contain different primary alkyl groups is always ethylene, if an ethyl group is present. Originally given in this limited form by A.W. Hofmann, the rule has since been extended and modified as follows When two or more alkenes can be produced in a P-elimination reaction, the alkene having the smallest number of alkyl groups attached to the double bond carbon atoms will be the predominant product. This orientation described by the Hofmann rule is observed in elimination reactions of quaternary ammonium salts and tertiary sulfonium salts, and in certain other cases. 

Complex 20 reacts with Li"Bu in hexane at room temperature to give the dihydride OsH2(CO)(T 2-CH2=CHEt)(P Pr3)2 (191), which is isolated as a colorless oil in quantitative yield. The reaction most probably involves the replacement of the chlorine by a butyl group to give initially OsH"Bu(CO)(P Pr3)2 (192), which evolves into the dihydride derivative by a P-elimination reaction [76]. 

In many fungi a proportion of the wall mannan is labile to dilute alkali and oligosaccharides of mannose are released by a p-elimination reaction in its presence. This mannan is linked to serine or threonine residues in polypeptide by way of a mannosyl-hydroxyamino acid (>-glycosidic bond a linkage which has no known counterpart in higher animals. The synthesis of this structure has been studied in some detail in Saccharomyces cerevisiae, and there is evidence for similar mechanisms in other fungi. 

Davies L. Functional and stereochemical specificity at the carbon atom of substrates in threonine dehydratase-catalyzed a.P elimination reactions. J Biol Chem 1979 254 4126-4131. 

The acronym Er(-I has been defined as an Elimination process occurring by a Radical Chain mechanism involving a unimolecular decomposition of the radical anion derived from the substrate followed by electron transfer with a reductant to yield a carbanionic species capable of undergoing a P-elimination reaction. 

PLP dependent enzymes belong to five enzyme classes transferases, hydrolases, lyases, isomerases and oxidoreductases. Nearly half of PLP-dependent enzymes are involved in important steps of amino acid metabolism (decarboxylation, a,p-elimination reactions, racemization and transamination). During the reaction of amino acids (and other amino compounds), the... 

Photolysis of [Ir(u5- neomenthyl)C5H4)(PMe3)(H)2l in cyclohexane or benzene leads to intermolecular C-H activation of solvoit molecules. Reaction of enantiopure [Ir(i) - neomenthyl -C5H4)(PMe3)(Me)(I)] with silver hexafluoroantimonate proceeds via a planar at iridium intermediate and a C-H oxidative addition. A p- elimination reaction completes the preparation of a hydrido-alkene cation which retains very high optical purity. ... 

The 0-linked oligosaccharides of L-serine and L-threonine are alkali labile. When treated with alkali, these 0-linked oligosaccharides are released by a P-elimination reaction to form unsaturated amino acids and the free sugar chain (see Fig. 9.2B,C and section 12.11 in Chapter 12). 5-Linked oligosaccharides to L-cysteine are also released by an identical reaction (see Fig. 9.2F). 0-linked oligosaccharides to hydroxy-L-lysine, hydroxy-L-proline, or phenolic hydroxyl of L-tyrosine are not readily released by alkali. 

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