Electrophilic Addition to Acetylene Derivatives

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). 

The most commonly encountered electrophile is the proton, but other species are frequently involved in such additions. 

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. 

The relevant studies of electrophilic addition will be examined according to the nature of the electrophile involved. 

The specific substrates and reaction conditions are indicated in Table 1 and will be referred to by the numbers in the first column. Hydration of adamantylacetylene (6 of Table 1) has not been investigated from a kinetic point of view and the intermediacy of the adamantylvinyl 

---

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. 

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. 

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

Addition reactions of sulphenyl halides (in particular chlorides) to acetylene derivatives have been extensively explored and recently reviewed (Modena and Scorrano, 1968). Although free radical processes may be involved under specific conditions, the addition of both arene-and alkanesulphenyl halides normally occurs by an ionic mechanism, the sulphenyl halide sulphur being the electrophilic centre. 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Subsequent interaction with electrophilic reagents in general yields a mixture of the allenic and the acetylenic derivative trimethylsilylation and alkylation, however, seem to afford the y-functionalized allenic ether [99]. Under strongly polar conditions, i.e. if lithium is replaced by potassium (by addition of t-BuOK) and HMPT is added as co-solvent, the y-metallated allenic ethers rearrange into the a-metallic derivatives [100]. This remarkable isomerization may take place via a head-to-tail arrangement of two metallated molecules ... 

Rates of gas-phase additions of dimethylsilylene, Me2Si, to acetylene and its mono- and dimethyl derivatives showed an approximate ratio of 1 2 4. This behaviour could suggest electrophilic character of the silylene. On the other hand, reactivity of dimethylgermylene, Me2Ge, with acetylenes is high for t-Bu—C=C—CN and thus not favouring electrophilic character of the germylene. ... 

Alkenyl chains can be directly introduced into the thiophene ring by appropriate choice of electrophile. Bis(/-butylsulfonyl)acetylene adds to 3-methyltWophene to give an 82 18 mixture of the 2-alkenyl (83) and 5-alkenyl (84) derivatives <90TL2173>. This Michael-type addition of thiophene does not seem to have a precedent. 

A new synthesis of aldehydes with 2-methyl-2-thiazoline has the advantage of releasing the aldehydes from the thiazolidine intermediate under neutral conditions . Acetylene derivatives can be obtained from aldehydes via dibromomethylene compounds Novel reactions of alkynes with cationoid electrophiles have been published. -Diketones and 2-ketoalkoximes can be obtained by this reaction from acid chlorides and aliphatic nitro compounds respectively Addition of aldehydes to activated carbon-carbon double bonds occurs smoothly in the presence of cyanide ions as catalysts . Poly- -carbonyl compounds have been prepared by condensation of two anions, whereby the enolate salt of a y8-keto ester condenses as an electrophilic anion with strong nucleophiles such as the dianion of benzoylacetone. ... 

These reactions are related to the formation of pyrroles and quinolines from aminocarbonyl compounds and acetylenes (582,583) and may be contrasted with the formation of pyran derivatives by electrophilic attack on an enamine, followed by addition of an oxygen function to the imonium carbon (584-590). 

Reactions of salts of 1,2,3-triazole with electrophiles provide an easy access to 1,2,3-triazol-jV-yl derivatives although, usually mixtures of N-l and N-2 substituted triazoles are obtained that have to be separated (see Section 5.01.5). Another simple method for synthesis of such derivatives is addition of 1,2,3-triazole to carbon-carbon multiple bonds (Section 5.01.5). N-l Substituted 1,2,3-triazoles can be selectively prepared by 1,3-dipolar cycloaddition of acetylene or (trimethylsilyl)acetylene to alkyl or aryl azides (Section 5.01.9). 

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

The electrophilicity of benzyne (90) is 1.95 eV, a value that falls within the range of strong electrophiles in the w scale.102 This value, which is larger than that evaluated for acetylene (32), to = 0.54 eV, allows to explain the reactivity of the benzyne derivatives towards nucleophilic additions. The electrophilicity of the fused four-membered... 

---