Nickel-catalyzed carbozincation

Formation of the cyclopentane ring and concomitant arrangement of the two side chains in the cis fashion is another efficient approach to methyl < />/-jasmon ate as delineated in Scheme 9. In the upper sequence, nickel-catalyzed carbozincation of 17 with Et2Zn produced organozinc 18, which was converted to a copper reagent for coupling with 1-bromoacetylene to produce 19 efficiently.849 The ene reaction of 20 and 23 has afforded 21 and 24, respectively.858 859 In the latter case, 23 was generated in situ by retro Diels-Alder reaction of 22. 

Among the most prominent examples of this reaction are the carbocupra-tion [5], the zirconium-catalyzed carboalumination [9-12], the nickel-catalyzed carbozincation [13-14], the allylmetalation of metalated alkynes [15], the allyl-zirconation [16], the allylgallation [17], the allylmanganation [18-19], and the alkyllithiation [20] reactions. 

Addition to unactivated triple bonds nickel-catalyzed carbozincation... 

The nickel-catalyzed carbozincation has been developed into a broadly useful method for the cyclization of aUcenes tethered to alkyl iodides (Scheme The fimctionahzed... 

A nickel-catalyzed carbozincation in which an allylzinc adds across the double bond of an unsaturated acetal has also been reported (Scheme 79). ] This procedme effectively provides a reverse-polarity approach to the functionalization of unsaturated carbonyl derivatives. Non-allylic Grignard reagents, however, add to cychc unsaturated acetals with the opposite regiochemistry. This latter procedure provides the basis for an enantioselective conjugate addition to cyclic enones (in 53% ee for the cyclopentenyl substrate and 85% ee for the cyclohexenyl homologue). ... 

Scheme 4.22 Preparation of alkenylzinc reagents by nickel-catalyzed carbozincation of alkynes [100].

Nickel-catalyzed carbozincation can also proceed smoothly vith alkynes for the synthesis of substituted alkenes regio- and stereoselectively. The reaction may proceed via an intramolecular version leading to tri- or tetrasubstituted alkenes. Thus, treatment of ry-iodoalkyne with RjZn in the presence of 7.5 mol% Ni(acac)2 in THF and NMP yield the cyclopentane derivative in 62% yield [102,112]. Presumably, a reductive elimination step from the corresponding organonickel intermediate may occur. It is noteworthy that no cydization is observed when internal alkyne is used. 

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 nickel-catalyzed hydrozincation or carbozincation of double or triple bonds are worthy of note. Thus, treatment of an alkene with diethylzinc in the presence of a catalytic amount of Ni(acac)2 and COD affords the corresponding organozinc reagent. Since diethylzinc will react with aliphatic bromide or iodide, such zinc-... 

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