Organocuprates, addition with alkyl halides

There are two major reactions of organocuprates, and both give products reminiscent of a carbon nucleophile (1) reaction with alkyl halides and (2) conjugate addition with a,p-unsaturated ketones. Conjugate addition to a, 3-unsaturated ketones is promoted when ether is used as a solvent.381 The substitution reaction is promoted by the use of THF or ether-HMPA as a solvent. l As mentioned earlier, the mechanism of these reactions probably involves a one-electron transfer, although other mechanistic proposals are in the literature,but the synthetic result is that expected of a carbon nucleophile. 85 The general reactivity of organocuprates with electrophiles follows the order ... 

Solvent effects on chemoselectivity. Use of donor solvents (THF or HMPT) enhances the rate of displacement reactions of lithium organocuprates with alkyl halides, but retards conjugate addition to enones. House and Lee have examined the effect of solvents on the reaction of (CH3)2CuLi with the bromo enone 1. Reaction of 1 with the cuprate in ether-(CH3)2S gives the product of... 

The problem of 1,2 versus 1,4 addition persists with most nucleophiles in their reactions with a,P-unsaturated carbonyl compounds. However, one class of carbanion reagent gives almost exclusively 1,4 addition the organocuprates. These reagents were introduced in Chapter 15 (Section 15.6), where they reacted with alkyl halides to give coupling products. 

In addition to organic halides, alkyl tosylates (R OTs, Section 9.13) also react with organocuprates (RgCuU) to form coupling... 

All of the above reactions illustrate the powerful nucleophilicity that organocuprates exhibit toward carbon. Secondary tosylates react with inversion of stereochemistry, as in the classical Sn2 substitution reaction. The overall mechanism probably consists of two steps. First, an oxidative addition to the metal occurs. The formal oxidation state of copper in the intermediate from this addition step is -1-3. This step is followed by migration of one of the alkyl groups from copper. The addition of halides and tosylates to transition metal species with low oxidation states is a common reaction type in transition metal chemistry. 

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