Addition of organozinc reagents to multiple bonds

The addition of organometallics, including functional reagents in ring-forming processes, to multiple bonds is a reaction of importance in synthesis. 

Organometallic reagents of lithium, magnesium, zinc, aluminum, copper, etc. add to alkenes and alkynes.226 Carbozincation was reported to be improved by sonication. Perfluoroalkyl zinc reagents generated in situ add to alkynes in the presence of a copper salt (Fig. 39).227 

Conjugated dienes undergo hydroperfluoroalkylation with perfluoroalkyl iodides and zinc in the presence of titanium catalysts 02 Yields are moderate to good (52-74%) but stereoselectivity is low. From allylic halides functionalized on the a-vinylic position (TMS, carboxylate, phosphonate), additions to terminal alkynes provide functional dienes, used in further synthetic reactions leading to lactones and carboxylic or phosphonic esters.228 

A complex intramolecular reaction occurs with 5-iodoalkynes (Fig. 40).229 Some amounts of iodovinylidenecyclopentane are formed, which formally relates this reaction to the atom transfer cyclization methods developed by Curran.230 The relative amounts of the acyclic reduction product and the cyclic compounds depend on the substituents R and R, the stoichiometry, and the reaction conditions. 

Without sonication, the cyclized iodo compoimd forms in 23% yield, increased to 39% by sonication, then to 53% by using the zinc-copper couple imder sonication. 

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