Nickel-catalyzed reactions 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. 

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. 

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. 

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