Elimination reactions forming acetylenes

In a number of elimination reactions carried out in liquid ammonia, an alkali alkynylide is formed. In these cases the ammonia has to be removed completely or partly by evaporation before the acetylene is liberated by hydrolysing the reaction mixture. The beat produced in the remaining solid mass by this hydrolysis must be "neutralized efficiently and this is realized by adding a large amount of finely crushed ice over a very short period. For this reason, the reaction must be earned out in a wide-necked round-bottomed flask (fig. 8). Examples are the preparation of HOCOR and HC=CCH(OR>2 (Chap. IX, exps. 10 and 11). 

Elimination reactions are considered when simple, volatile haloacetylenes are to be prepared. Dichlaroacetylene. far example, has been prepared as an ethereal solution [124] (the undiluted compound is highly explosive) by treating CljC CHCl with KOH at 200 C. Dibromo-acetylene is formed under milder conditions, namely from Br2C=CHBr and ethanolic... 

Alkynes (acetylenes, RCsCR) may be prepared by the elimination of a hydrogen halide from alkenyl halides under vigorous conditions. This is exemplified by the preparation of phenylacetylene from cinnamic acid via the dibromide and (o-bromostyrene (Scheme 3.26). The contrast between the conditions required for the bromodecarboxylation and for the second elimination to form the alkyne reveals the difference in reactivity between an alkyl and an alkenyl halide. Alternative modes of elimination, such as allene formation or rearrangement reactions, restrict the use of this procedure. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

To understand the carbanion mechanism in flavocytochrome 62 it is useful to first consider work carried out on related flavoenzymes. An investigation into o-amino acid oxidase by Walsh et al. 107), revealed that pyruvate was produced as a by-product of the oxidation of )8-chloroalanine to chloropyruvate. This observation was interpreted as evidence for a mechanism in which the initial step was C -H abstraction to form a carbanion intermediate. This intermediate would then be oxidized to form chloropyruvate or would undergo halogen elimination to form an enamine with subsequent ketonization to yield pyruvate. The analogous reaction of lactate oxidase with jS-chlorolactate gave similar results 108) and it was proposed that these flavoenzymes worked by a common mechanism. Further evidence consistent with these proposals was obtained by inactivation studies of flavin oxidases with acetylenic substrates, wherein the carbanion intermediate can lead to an allenic carbanion, which can then form a stable covalent adduct with the flavin group 109). Finally, it was noted that preformed nitroalkane carbanions, such as ethane nitronate, acted as substrates of D-amino acid oxidase 110). Thus three lines of experimental evidence were consistent with a carbanion mechanism in flavoenzymes such as D-amino acid oxidase. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

The procedures presented in this chapter represent basic reactions of alkynes. That involving hydration of 2-methyl-3-butyn-2-ol through electrophilic addition of water to the tr-system is a reaction analogous to the conversion of acetylene to acetaldehyde, a precursor to acetic acid and acetone. In addition, the formation of an alkyne via an elimination reaction illustrates an alternate approach to forming a carbon-carbon triple bond, although one that is not nearly so easy experimentally as adding water to calcium carbide to make acetylene ... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

The mechanism of the Fiesselmann reaction between methylthioglycolate and a,P-acetylenic esters proceeds via consecutive base-catalyzed 1,4-conjugate addition reactions to form thioacetal Enolate formation, as a result of treatment with a stronger base, causes a Dieckmann condensation to occur providing ketone 8. Elimination of methylthioglycolate and tautomerization driven by aromaticity provides the 3-hydroxy thiophene dicarboxylate 9. 

A striking result of this reinvestigation (128, 129) is the observation that the ratio of the product ketone to the acetylene formed from a-bromo-p-aminostyrene is a function of the pH (Table Vll) but that the rate at which they are formed is not. As the pH increases from 3.9 to 13.1, the relative yield of acetylene increases from 16% to 85%. Therefore, the acetylene formation by elimination of a proton from the vinyl cation (path b in route D in Scheme XI) is more susceptible to an increase in base strength than is ketone formation via the enol (path a). This observation is a rare case of pH control over product composition in a 1-El reaction. 

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