Oxidative addition, zinc metal alkyl bromides

Early preparations of active zinc utilized the potassium or sodium metal reduction of anhydrous zinc salts in refluxing THF or DME (Protocol l).3,8 These highly divided zinc powders displayed high reactivity towards organic halides in oxidative addition. Alkyl iodides and bromides reacted with the zinc powders at room temperature. Even aryl bromides and iodides would react to form the corresponding arylzinc iodides or bromides at refluxing... 

Monoorganozinc halides (RZnX) can be synthesized by oxidative addition of organic halides to zinc metal. The oxidative addition rate is strongly affected by the reaction conditions (solvent, concentration) [16] and by activation of the zinc [15,17]. Zinc powder or zinc foil, which is activated by treatment with 1,2-dibromoethane and then with trimethylsilyl chloride, will oxidatively add alkyl iodides [18]. The reaction of alkyl bromides, on the other hand, requires more active zinc, which may be prepared by the reduction of zinc chloride with either lithium naphthalenide [19] or lithium metal under ultrasonic.irradiation [20, 21]. 

The first report of the oxidative addition of zinc metal to organic halides dates back to the work of Frankland [1-4] around 1850. He discovered that dialkylz-inc compounds could be prepared from zinc metal and methyl iodide or ethyl iodide. However, the reaction did not proceed with alkyl bromides or chlorides. Also, no aryl halides were found to undergo the oxidative addition reaction. Several approaches have been reported since that time to increase the reactivity of the zinc metal. The majority of these modifications have employed zinc-copper couples [5-8] or zinc-silver couples. However, all of these procedures still only worked with alkyl iodides. Noller used a mixture of alkyl iodides and bromides but found that the mixture must contain a large percent of alkyl iodide [9]. 

Mixed bimetallic reagents possess two carbon-metal bonds of different reactivity, and a selective and sequential reaction with two different electrophiles should be possible. Thus, the treatment of the l,l-bimetailic compound 15 with iodine, at — 78"C, affords an intermediate zinc carbenoid 16 that, after hydrolysis, furnishes an unsaturated alkyl iodide in 61% yield [Eq. (15) 8]. The reverse addition sequence [AcOH (1 equiv), —80 to — 40 C iodine (1 equiv)] leads to the desired product, with equally high yield [8]. A range of electrophile couples can be added, and the stannylation of 15 is an especially efficient process [Eq. (16) 8]. A smooth oxidation of the bimetallic functionality by using methyl disulfide, followed by an acidic hydrolysis, produces the aldehyde 17 (53%), whereas the treatment with methyl disulfide, followed by the addition of allyl bromide, furnishes a dienic thioether in 76% yield [Eq. (17) 8]. The addition of allylzinc bromide to 1-octenyllithium produces the lithium-zinc bimetallic reagent 18, which can be treated with an excess of methyl iodide, leading to only the monomethylated product 19. The carbon zinc bond is unreactive toward methyl iodide and, after hydrolysis, the alkene 19... 

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