Electrophilic Additions to Acetylenes

Electrophilic Additions to Acetylenes.— Addition of aluminium-hydrogen across unsymmetrical acetylenes (165) can be directed to yield cis or trans addition products. cw-Addition is promoted by the presence of one equivalent of tertiary amine. The aluminium-carbon bond in the products is 

Treatment of t-butyl(phenyl)acetylene with di-isobutylaluminium hydride under various conditions has been studied. The products are (166) and (167) 

Solvolysis of the cyclodecyne derivative (172) in aqueous ethanol involves participation of the triple bond, and the only products of the reaction are the 

Acetylenes (176) and (177) are both converted into ( )-l-chloro-3,3-dimethyl-l-phenylbutene (178) on treatment with t-butyl chloride and 

A study of acetylenic bond participation in the biogenetic-like olefin cyclization of (179) has been extended to media which are deficient in good nucleophiles for trapping the vinyl cation (180). In methylene chloride-trifluoracetic acid at —78 C the product obtained was the chlorodiene (181), apparently formed by reaction of the vinyl cation (182) with methylene chloride. Vinyl cations (180) and (182) apparently do not differ appreciably in stability.  

---

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. 

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... 

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... 

Calculations on electrophilic additions to acetylene were reported for a number of hydrocarbon cations, including CH" [129], C3H3" [130, 131], and phenylvinylium ion [132]. It was predicted that the cyclopropenylium cation, the most stable form of C3H, forms an ion-molecule complex with acetylene but does not undergo further addition [131]. The linear propargyl cation, however, reacts with acetylene without an apparent barrier, to form many different C5H5 isomers [130,131]. Wang et al. computed the AMI potential energy surfaces and carried out RRKM and microcanonical variational transition-state analysis for the rate of reaction of phenylvinylium ion with acetylene [132]. 

The kinetic data on electrophilic addition to acetylenes are summarised in Table 7. As usual, the table is subdivided into sections depending upon the nature of the electrophile initiating the addition. Undesignated rate coefficients are moles, 1 and sec activation enthalpies (A// ) are expressed in kcal. mole , and entropies (A5 ) in cal. deg . mole (eu), generally at 25°C. 

Other studies of electrophilic additions to acetylenes include the reaction of tribenzylaluminium with acetylene the reaction of phthalic anhydride with ynamines to give indanediones or benzoxepinones the reaction of adamantan-l-ol with acetylene in 98 % sulphuric acid leading to adamantane-aldehyde and adamantyl methyl ketone and the effects of organosilicon substituents on the properties of a triple bond. "... 

---