Organocopper reagents allylic halides

Allylic halides usually give both SN2 products and products of substitution with an allylic shift (SN2 products) although the mixed organocopper reagent RCu— BF3 is reported to give mainly the SN2 product.22 Allylic acetates undergo displacement with an allylic shift (SN2 mechanism).23 The allylic substitution process may involve initial... 

The conjugate addition reactions probably occur by a mechanism similar to that for substitution on allylic halides. There is probably an initial complex between the cuprate and enone.44 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.45 Solvent... 

Scheme 57 illustrates an example of this process coupling of the lithio reagent derived from vinyl stannane 237 with epoxide 242 leads to the C-disaccharide derivative 243 in good yield. A double transmetallation to give an organocopper reagent 244 can facilitate the C-glycosylation reaction with allyl halides [80] (Scheme 58). 

The reaction of allyl halides with organocopper species usually gives regiochemical mixture of direct and S 2 attack. Exceptions to this trend are 3,3-disubstituted 1-bro-moalkenes, which give reliable direct substitution with lower-order cuprates or RCu organocopper reagents. This fact has been put to use in the synthesis of several prenelated... 

Hydrolysis of the reaction mixture afforded 63 in 80% yield as a single cis diastereomer, and the formation of cyclic organozinc bromide was checked by iodinolysis, by coupling reactions with vinyl iodides in the presence of a catalytic amount of Pd(PPh3)4, or by reaction with allyl halides after transmetallation of the organozinc bromide into an organocopper reagent [60] (Scheme 7-53). 

Reaction of alkynylcoppers with 5 cquiv. of iodomethylzinc iodide results in insertion of four methylene groups to provide organocopper/zinc reagents of type 2, which can be trapped by allylic halides (equation I). 

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