Carbozincation

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. 

In contrast to the allyltitaniums derived from acrolein cyclic acetals, such as 1,2-dicyclo-hexylethylene acetal shown in Scheme 9.8, those derived from acrolein acyclic acetals react with ketones and imines exclusively at the y-position. As shown in Eq. 9.29, the reaction with chiral imines having an optically active 1-phenylethylamine moiety proceeds with high diastereoselectivity, thus providing a new method for preparing optically active 1-vinyl-2-amino alcohol derivatives with syn stereochemistry [53], The intermediate allyltita-nium species has also found use as a starting material for a carbozincation reaction [54],... 

Allylic zinc reagents are highly reactive reagents which are prone to undergo carbozincation of weakly activated alkenes . Thus, the addition of the mixed methallyl(butyl)zinc 393 with the vinylic boronate 394 provides the intermediate zinc reagent 395. After the addition of an extra equivalent of ZnBri, CuCN 2LiCl and allyl bromide, the reaction mixture was worked up oxidatively providing the alcohol 396 in 83% yield. ... 

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). 

The perfect diastereoselectivity observed in these reactions lead to the assumption that they do not proceed via a simple anionic intramolecular cyclization (i.e. carbozincation) as was proposed in earlier61 publications. Cyclizations of non-propargylic substrates15,62 were shown to proceed with significantly lower diastereoselectivity (cis/trans typically 75/25). Thus, in the case of propargylic compounds, e.g. 114, a simple carbozincation process is unlikely to be operative. It was therefore reasoned that the active species in these reactions is not 115-ZnBr but rather its allenic isomer 120, which undergoes a metallo-ene-allene reaction in a chair-like transition state as depicted in equation 5659a. 

Transition metal-catalyzed/promoted intermolecular carbozincations... 

Transition metal-catalyzed intermolecular carbozincations of alkenes 881... 

Carbozincations are reactions that involve the addition of the carbon—zinc bond of an organozinc reagent 1 across a carbon—carbon multiple bond 2, leading to a new organozinc species 3 (equation 1). 

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