Carbozincation Reactions

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],... 

Nickel or palladium catalysed conjugate addition and other carbozincation reactions... 

The more difficult cross-coupling reaction between Csp -Csp can be accomplished by mixed zincate compounds, prepared by the reaction of diorganozinc derivatives with Me2CuMgCl-MgCl(CN) (equation 59). Substitution reaction at Csp centers are also efficiently catalyzed by Ni salts when the alkyl halide contains a double bond in remote position (Scheme 22). Changing the double bond for a triple bond leads to an intramolecular carbozincation reaction (equation 60). 

Carboaluminationf and carbozincation reactions of alkynes with Cp2ZrCl2 as a catalyst have been reported (Eqs 33 and 34). 

Progress from a synthetic point of view has been achieved in the selective oxidation of alkyl zinc reagents with oxygen in perfluoro-hexane solutions to give good yields of either hydroperoxides or alcohols. [28] As many zinc organometallics are easily obtained from olefins via a hydroboration/boron-zinc exchange sequence or by a nickel catalyzed hydro- or carbozincation reaction, this method may proof its synthetic potential in the future development. 

The cw-directing ability of a 3-carboalkoxy group in cyclopropenes can be used to great advantage in net carbozincation reactions catalyzed by either Cul or Toluene is the solvent in which best diastereoselectivities are realized both THF and Et20 led to poorer results. 

Hydro- and carbozincation reactions provide a means to access organozinc reagents from alkenes or alkynes. The approach can be highly useful in some specific situations. 

The carbozincation reaction of alkenes [97] has so far been mainly used in the case of ethyl group incorporation or in the case of zinc enolates or aza-enolates [98, 99]. By contrast, the Ni-mediated carbozincation of di-substituted alkynes such as 91 is more general, but salt-free diorganozincs are necessary and the regioselectivity of the carbozincation reaction is excellent only for arylacetylenes and TMS-substituted acetylenes (Scheme 4.22) [100]. It is also of interest that the carbozincation of unactivated alkynes can be achieved without a metal catalyst by zinc-atom radical transfer processes [101, 102]. 

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