Transmetalation reaction

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

This reaction is a variation of the hydroformylation reaction. Transmetallation of Rh(I)(acac) with the alkylmercury(I) compound gives ClHg(acac) and an alkylrhodium(I) compound. Oxidative addition of H2 gives a Rh(III) compound, and coordination and insertion of CO gives the acylrhodium(IH) compound. Reductive elimination then gives the product and regenerates Rh(I) — but as a Rh-H, not as Rh(acac). 

Exchange reactions (transmetallation) with other heterocyclopentadienes... 

Step b Negishi reaction transmetallation (A1 to Zn) followed by cross-coupling of the resultant organozinc species with the vinyl halide. The Pd(0) catalyst was generated by treatment of (PhaP PdC with DIBAL-H. 

As usual, to further increase the scope of the reaction, transmetalation of dienyl zirconium complexes, such as 124-127Zr, into the corresponding dienyl organocopper derivatives was performed. Surprisingly, when 126Zr was trans-metalated to copper derivatives by addition of a catalytic amount of CuCl/2LiCl in the presence of allyl chloride for 1 h at +50 °C, a partial isomerization of the dienyl system was found (Scheme 49). 

The formation of organometallic compounds from metals is described in Ch. 5. In this section, a few references are given, dealing with the formation of such compounds by an exchange reaction (transmetallation or ligand exchange) imder solid-liquid conditions and the preparation of compoimds containing non-metal elements. 

The organolithium compounds formed by the reaction shown in eq. (3.1 ) tend to react with hydrocarbon halides by the Wurts coupling reaction and the yields of organolithium compounds are low if the resultant organolithium compounds are very reactive compounds such as allylithium or benzyllithium. Hence, for these reactions, transmetalation reactions, as described below are used. Therefore, this reaction is used for production of organolithium compounds having relatively low reactivity such as alkyllithiums, vinyllithiums and aryllithiums [14—20]. 

Transmetalation Reactions. Transmetalation of tri-methylsilyl-2-trimethylstannylethylene at low temperature with BuLi affords fra 5 -/3-trimethylsilyllithium in high yield, as evidenced by derivatization of 2-methyldimedone isobutyl ether (eq6).3-12-l4... 

Transmetalation Reactions. Transmetalation of trimethy-lsilyl-2-trimethylstannylethylene at low temperature with n-BuLi affords ( )-2-(trimethylsilyl)vinyllithium in high yield (eq 6). ... 

Because of the complexity of the full mechanism, most theoretical studies concentrate on particular steps of the catalytic cycle. This chapter is thus organized by the different steps of the mechanism, rather than by reaction. Oxidative addition and reduction elimination have been studied also in the context of other organometallic reactions. Transmetalation is on the other hand specific of C-C bond formation processes, and because of this, it will be discussed in more detail. Our intention in this contribution is to review the present state of the research in the area. As wiU be seen, a significant part of the articles mentioned have been published after 2003, indicating the current high activity on the topic. Because of this, it is impossible to provide a definitive mechanistic picture. This is rather a progress report on a very active research field. This paper complements a recent review by some of us that focused on cross-coupling [19]. 

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