Bimetallic reagents

Another important reaction via transmetallation is carbon-metal bond formation by reaction with bimetallic reagents. This is a useful synthetic method for various main group organometallic reagents. 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

A useful application of the silver-mediated additions is 1,3 -diene synthesis by three-carbon elongation of aldehydes [48,51,53]. The bimetallic reagent 3-trimethylsilyl-l-propenylzirco-nocene chloride (A Scheme 8.23) reacts with aldehydes under the influence of a catalytic amount of Ag+ to give the intermediate zirconocene-alkoxide B, which then undergoes a Peterson-type 1,4-elimination of TMS alkoxide to stereoselectively afford ( )-dienes (fc/Z > 96 4) (Scheme 8.23). A Wittig reaction yields the same products without stereoselectivity (ca. 1 1 mixtures of E- and Z-isomers). 

The importance of chelating for the configurational stability of organozinc reagents has been recently demonstrated by Normant and Marek. The reaction of the bimetallic reagent... 

Metalametallations of alkenes and alkynes are useful methods for the construction of 1,2-dimetala-alkanes and 1,2-dimetala-l-alkenes, which react subsequently with suitable electrophiles to form substituted alkanes and alkenes. Metalametallation is carried out usually with bimetallic reagents of the type R Si-M R, or R Sn-M R in which M = B, Al, Mg, Cu, Zn, Si or Sn. Some metalametallations proceed without catalysts Cu, Ag and Pd compounds are good catalysts. The metalametallation with bimetallic compounds, such as Si-B, Si-Mg, Si-Al, Si-Zn, Si-Sn, Si-Si, Sn-Al or Sn—Sn bonds, catalysed by transition metal complexes, is explained by the oxidative addition of the bimetallic compounds to form 478, and insertion of alkene generates 479. Finally 1,2-dimetallic compounds 480 are formed by reductive elimination. Dimetallation of alkynes proceeds similarly to give 481. Dimetallation is syn addition. 

Add the solution of the bimetallic reagent to the CuCN 2LiCI solution at -60°C. Allow to warm to 0°C, stir for 5 min, and cool the resulting light green solution to -30°C. 

It is convenient, if not strictly systematic, to note here the intriguing and potentially useful bimetallic reagents obtained by the addition of allylzinc... 

Reaction of the bimetallic reagents AuCl 2(p-dppm) with... 

Zinc and zirconium 1,1-bimetallic reagents 4.104, prepared by the hydrozirconation of alkenylzinc halides 4.103, react with carbonyl compounds to produce alkenes with high R-stereoselectivity (Scheme 4.51). Ketones give an E/Z mixture of stereoisomers . 

The reaction can be used to convert 1,1-bimetallic reagents of zinc and magnesium, such as 11, to unsaturated nitriles, such as 12 (Scheme 9-13) [42]. The regioselectivity of the... 

Bimetallic reagents such as 18 and 19 can be smoothly allylated under mild conditions leading to polyfunctional products (Scheme 9-20) [48,49. ... 

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... 

The chemistry of magnesium-zinc 1,1-bimetallic reagents led to the discovery of several new reaction of interest for synthetic organic chemistry. These organometallics should also find applications as building blocks for the construction of new materials and the preparation of new bimetallic catalysts. 

Geminal bimetallic reagents 497-499 see also Bimetallic compounds)... 

Bimetallic reagents are available from reaction of allylmagnesium halides and ZnBr2 to alkenyllithiums. ... 

The unique selectivity of reagents 1 and 2 can be explained by the higher selectivity of the C—Cu bond compared to the C—Zn bond toward electrophiles. These mixed bimetallic reagents arc useful for preparation of polyfunctional products. 

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