Zinc Chloride organozinc reagents

An unusual syn addition to epoxides occurs when 1,3-diene monoepoxides are treated with organozinc reagents. Thus, the cyclic vinyl epoxide 72 was converted to the cis-ethyl-cyclohexenol 75 with diethyl zinc in methylene chloride and trifluoroacetic acid. The syn addition is believed to derive from an initial coordination of the oxiranyl oxygen to the organozinc compound, which then delivers the alkyl group to the same face. This transfer is facilitated by a relaxation of the sp3 hybridization brought about by the Lewis acidic zinc center and the allylic character of the incipient carbocation <020L905>. 

The acylation of organozinc reagents derived from dialkyl 1-bromo-1,1-difluoromethylphosphonates with acyl chlorides proceeds smoothly at room temperature to afford the desired products in moderate to good yields (60-77%, Scheme 3.75). Esters, lactones, and triflates did not react at all, even at Although these zinc and cadmium reagents appear to have similar reactivities,... 

Metallation of l-methoxy-2-butyne with n-butyllithium at -78 C, followed by the addition of one equivalent of zinc chloride, generates the very reactive and unisolable organozinc intermediate. Reaction of this zinc reagent with cyclohexenone affords the alkynic alcohol in 95% yield as a 65 35 mixture of diasteieomers. Subsequent reduction of the triple bond provides either the trans alkene or the cis alkene, depending on the reaction conditions. A modified oxy-Cope rearrangement thus provides the dia-steieomeric ketones, which have been used for the synthesis of juvabione (Scheme 29). ... 

Yields are appreciably better if the Grignard reagent is first converted in an organozinc compound zinc chloride in ether is dropped into the Grignard solution at a rate sufficing to keep the solution at the boiling point ... 

The most common method for the formation of an organozinc reagent involves the insertion of zinc metal into the carbon-iodine bond of an alkyl iodide. For allylic substrates the corresponding bromide or even chloride can be used. The zinc metal normally needs to be activated by washing first with 1,2-dibromoethane and... 

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