Alkyne Carbozincation

More recently, the scope of the Ni catalysed alkyne carbozincation has been extended to the cases of co-alkynyl aldehydes and even mixtures of aldehydes and alkynes.65 These reactions are generally accompanied by alkylation with organozincs (Scheme 11.20). 

As is clear from the mechanisms, these reactions cannot occur with methylmetals. Their extensions beyond ethylmetallation are possible, but are prone to various side reactions [201,202], In contrast to the widely observable Zr-catalyzed carboalumination of al-kynes discussed earlier, the alkyne version of the Zr-catalyzed ethylmagnesation has not been widely observable, the only successful examples being those of conjugated diynes [203], In this context, further investigation of the Zr-catalyzed carbozincation of alkynes reported as early as 1983 [204,205] appears to be very desirable. 

Also promising are both intermolecular and intramolecular carbozincation of alkynes catalyzed by №( ) in THF-NMP. The intramolecular cyclic carbozincation has also been observed with -iodo-l-alkenes (Scheme 20). 

Arylzinc reagents are completely inert towards alkenes and alkynes in the absence of any added catalyst, whereas the reported examples of uncatalyzed intermolecular carbozincations involving alkylzincs appear to be restricted to the more nucleophilic di(tert-butyl)zinc. 

Intramolecular carbozincations of alkynes appeared relatively restricted in scope, at least under the conditions that were used to generate the open-chain organozinc species. 

Thus, whereas intermolecular carbozincations of alkenes and alkynes appear restricted in scope and limited to the more reactive di(terf-butyl)zinc, the zinc-induced cyclization of... 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

Due to the low reactivity of alkyl and arylorganozinc reagents towards alkenes and alkynes, it appears clear that the carbozincation chemistry for this class of reagents is intimately associated with transition metal catalysts. Some of the metal-catalyzed/promoted reactions do indeed produce organozinc reagents as the final organometallic species that can further react with an appropriate electrophile, whereas other processes lead to highly functionalized products by an entirely different pathway. 

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