1.2- Elimination reactions, characteristics rearrangements

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

A variety of new reaction pathways are made possible by the ability of the aromatic ring to reduce bond-dissociation energies (Equations 4.41 and 8.81), stabilize the ion and radical sites, and provide an unsaturated site for rearrangement, displacement, and elimination reactions (Equations 4.36, 8.110, 8.122, 8.123). As an example of Equation 4.36, (M — CHjCO)" " is a peak characteristic of aryl acetate spectra (CgHgO" is the base peak in the spectrum of CgHsOCOCHa). 

The reactions of copper salts with diacyl peroxides have been investigated quite thoroughly, and the mechanistic studies indicate that both radicals and carbocations are involved as intermediates. The radicals are oxidized to carbocations by Cu(II), and the final products can be recognized as having arisen from carbocations because characteristic patterns of substitution, elimination, and rearrangement can be discerned " ... 

Characteristic features of this mechanism are that (i) the rate of the reaction does not depend on the concentration of the base and the kinetics are first order (in substrate) (ii) the reaction may not be stereospecific (iii) the elimination/substitution ratio is mostly independent of the leaving group (but in solvents of low ionization energy ion pairs are formed and then the ratio depends upon the leaving group) (iv) by-products are formed via rearrangements (v) the reaction is reversible (vi) generally the most stable alkene is formed (Zaitsev orientation see Section 5.1.2.5)... 

The characteristics of this mechanism are that (i) the attacking base and the substrate both take part in the rate-determining step, which has second-order kinetics overall (first-order in base and first-order in substrate) (ii) a large primary isotope effect is usually observed (iii) since the mechanisms of 5n2 and E2 differ much more than those of 5n1 and 1 reactions, the substitution/elimination ratio can be controlled in most cases by choosing appropriate conditions (iv) no rearrangement reactions are observed (v) the rate of elimination depends upon the strength of the base (vi) the stereospecificity of an E2 reaction depends on the conditions (see Section 5.1.2.4). 

In general, allenes are prepared by the same types of reactions which lead to alkenes elimination of hydrogen halides, halogens and water from adjacent carbon atoms. However, some methods are characteristic of allene synthesis, e.g. the dehalogenation of gem-dihalocyclopropanes, rearrangement of alkynes and the 1,4-addition to vinylalkynes. 

An X-ray diffraction analysis of ( )-flavipucine (30) has confirmed its structure and has revealed that the oxiran ring is Z in configuration. Rearrangement of the compound may be effected in several ways, best by simply refluxing in xylene (80% yield). The structure (31) of the rearrangement product has been confirmed by spectral measurements and by its reduction with zinc-acetic acid to (32), the formation of which can be rationalized, and which has u.v. spectral characteristics of a 4-hydroxy-2-pyridone rather than a 3-hydroxy-2-pyridone. These and other observations tend to confirm (31) as the rearrangement product to the exclusion of (33). " An elimination-addition pathway has been established for the epoxidation reaction employed in the synthesis of flavipucine. ... 

Pong Bi Bi reaction mechanism. As they contain PLP, the mechanism is almost certainly similar to that known for the animal aminotransferases (Fig. 1). Details of this mechanism are discussed by Braunstein (1973) and by Metzler (1977). The apoenzyme moiety determines substrate specificity and confers high catalytic efficiency, as well as suppressing side reactions and eliminating the metal requirement characteristic of nonenzymatic transamination. Initially the amino acid binds to an anchoring site on the enzyme. Condensation then takes place between the amino acid and the enzyme pyridoxal-lysine imine to form an aldimine. Following further rearrangements, a ketimine is produced. Ketimine formation is then followed by a hydrolysis to... 

The methods of obtaining a particular diemical bond are subdivided according to types of formation. Four types are distinguished addition (O), rearrangement (iH), exchange (jt), and elimination (f ). The last part of the symbol refers to the bonds which are destroyed in the reaction or to a characteristic element which is eliminated. 

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