Acetylene, electrophilic additions

Summary of Electrophilic Additions to Acetylenes and Allenes Involving Vinyl Cations... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Fahey, R. C., The Stereochemistry of Electrophilic Additions to Olefins and Acetylenes, 3, 237. Farina, M., The Stereochemistry of Linear Macromolecules, 17, 1. 

Electrophilic Additions to Olefins and Acetylenes, Stereochemistry of (Fahey) 3 237... 

ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS A. Chlorinating agents Sodium hypochlorite solution 7V-Chloro succi n i m i de Antimony pentachloride Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Controlled chlorination of acetylenes... 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

Acetylene undergoes electrophilic addition reaction with bromine in the dark. Bromine adds successively to each of the two n bonds of the aUcyne. In the first stage of the reaction, acetylene is converted to an alkene, 1,2-dibromoethene. In the final stage, another molecule of bromine is added to the n bond of this alkene, and produces 1,1,2,2-tetrabromoethane. 

Succinimide and DMAD give a Michael 1 1 adduct,113 but iV-bromo-succinimide with acetylenic esters in acetic acid led to electrophilic addition of bromine to the triple bond.114... 

Electrophilic additions of the halogens to alkynes have not been much studied. In acetic acid a given olefin reacts with bromine 103 to 105 times more rapidly than the corresponding acetylene. However, the relative rates are enormously solvent-dependent and decrease with solvent polarity. Thus bromina-tion of styrene is 2590 times faster than bromination of phenylacetylene in acetic acid, but only 0.67 times as fast in HzO. Solvation of the transition state must be... 

Electrophilic addition of iodine(III) reagents to the triple bond of alkynes leading to alkenyl(phenyl)iodonium salts is also possible. It has been effected in some alkynes, including acetylene, with Phl+F TfO" [95] or PhI+OH TfO [129] or Phi and XeF2 in MeS03H [130] to afford 2- -trifyloxy-alkenyl(phenyl)iodo-nium salts. Terminal alkynes reacted with p-TolylIF2 in Et3N-5HF to afford similarly jS-E-fluoroalkenyl(phenyl)iodonium fluorides (Scheme 43) [131]. 

We have expanded our collection of stereoselective reactions even more in the making of alkenes by the Wittig reaction (chapter 15), from acetylenes (chapter 16), by thermodynamic control in enone synthesis (chapters 18 and 19) and in sigmatropic rearrangements (chapter 35). We have seen that such E- or Z-alkenes can be transformed into three-dimensional stereochemistry by the Diels-Alder reaction (chapter 17), by electrophilic addition (chapters 23 and 30), by carbene insertion (chapter 30) and by cycloadditions to make four-membered rings (chapters 32 and 33). 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Relevant mechanistic studies of electrophilic additions to acetylene derivatives and the first demonstration of a unimolecular solvolysis of vinyl halides (Grob and Cseh, 1964) stimulated a still growing interest in the field of vinyl cations. 

A. Electrophilic Addition to Acetylene Derivatives Vinyl cations can be generated by addition to a carbon-carbon triple bond of a variety of positively charged electrophiles, (equation 1). 

Alternative mechanistic routes for electrophilic additions to acetylene derivatives are also possible. According to the principle of microscopic reversibility, the same spectrum of different mechanisms is expected for addition to the triple bond as for elimination from a double bond. 

Relative rates of electrophilic additions to acetylene and ethylene derivatives... 

In solution, propargyl cations may in principle be generated by electrophilic addition to vinylacetylenes (equation 34). However, by all the evidence, the electrophilic attack by protons occurs more readily at the terminal acetylenic carbon than at the terminal vinyl carbon, and cation 176 is generated instead (de la Mare and Bolton, 1966). 

Selenenyl chlorides add to alkenes, often via an AdE2 mechanism involving a bridged seleniranium ion intermediate (19) (equation 14). These reactions are therefore highly stereospecitic, resulting in anti addition. The regiochemistry of the process can be under either kinetic or thermodynamic control. In some cases, initial anti-Markovnikov products were observed at low temperature and Markovnikov adducts dominated after further equilibration. Analogous electrophilic additions to acetylenes and aUenes (Scheme 9) have also been reported. When selenenyl hahdes react with alkenes in the presence of other nucleophiles such... 

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