Electrophilic additions to alkynes

FIGURE 11.36 Alkenes are more reactive than alkynes in addition reactions. 

FIGURE 11.38 Meciaanism of mercury-cataiyzed addition ofwatertoan aikyne. 

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For further discussion of this topic see the article The Electrophilic Addition to Alkynes in the November 1993 edition of theVourna/ of Chemical Education (p 873) Additional common tary appeared in the Novem ber 1996 issue... 

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The... 

The exact behavior and mechanism of electrophilic additions to alkynes is clearly strongly dependent upon the reaction conditions. In a highly polar and strongly acidic but weakly nucleophilic solvent such as trifluoroacetic acid, addition via a vinyl cation intermediate is favored whereas in less polar, more nucleophilic solvents such as acetic acid, a different mechanism prevails. 

Predicting the outcome of electrophilic additions to alkynes from an extension of alkene reactivity usually works well, and can be applied to halogenations and hydrations. Hydration of an alkyne has a subtle twist, however the product is a ketone This can still be rationalized quite readily, though. 

For reviews of electrophilic addition to alkynes, including much evidence, see Rappoport React. Interned. (Plenum) 1983, 3, 427-615, pp. 428-440 Stang Rappoport Hanack Subramanian Vinyl Cations, Academic Press New York. 1979. pp. 24-151 Stang Prog. Phys. Org. Chem. 1973, 10. 205-325 Modena Toncllato Adv. Phys. Org. Chem. 1971, 9, 185-280. pp. 187-231 Richey Richey, in Olah Schleycr Carbonium Ions, vol. 2 Wiley New York, 1970. pp. 906-922. 

Sections Table 9.4 summarizes electrophilic addition to alkynes. 

Overall electrophilic addition to alkynes is rare, although bromination of alkynylsilanes has been described and takes place in the trans sense (equation 60)132. 

The relative stability of vinyl and saturated cations in solution can in principle be evaluated by following three approaches (a) from the competitive formation of vinyl and saturated cations in electrophilic addition to allenes (b) from the relative rates of electrophilic addition to alkynes and alkenes (c) from the relative rates of solvolysis of vinyl and saturated derivatives. 

The mechanism of electrophilic addition to alkynes is the same as with alkenes. Orientation of addition of unsymmetrical reagents to unsymmetrical alkynes is determined by the stability of the intermediate carbocation. 

The basic mechanisms that are considered to be involved in electrophilic additions to alkynes are outlined below. The first involves a discrete vinyl cation. In general, this reaction will lead to a mixture of the two stereoisomeric addition products. Mechanisms B and C depict bridged intermediates formed without (B) or with (C) participation of a second electrophilic molecule. Mechanisms B and C should lead to anti addition. Mechanism D is a termolecular process that would be expected to be a stereospecific anti addition. Mechanisms A and B are of the Adg2 type, whereas C and D are classified as Ad S. Each of these mechanisms may involve a prior complex formation between the alkyne and an electrophile. 

A two-sulphur electrophilic addition to alkynes was reported by Bock and coworkers . The reagent is a mixture of the sulphur chloride S2CI2 and aluminium trichloride dissolved in dichloromethane. Initially, 1,2-dithiete 38 is formed, but in the preparative procedure the isolated product is a mixture of 2,6- and 2,5-di-fcr -butyl-1,4-dithiin (39 and 40). When the reaction is performed under nitrogen in a closed tube, the substituted dithiete is oxidized to its radical cation (equation 23). 

Back and Muralidharan described the formation of benzeneselenenyl / -toluene-sulphonate (48) and its electrophilic addition to alkynes. The benzeneselenenyl p ... 

Table 9.3 summarizes reactions that reduce alkynes to alkenes and alkanes. Table 9.4 summarizes electrophilic addition to alkynes. 

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