Transmetallation zirconium compounds

As Scheme 16 shows, vinyl(methyl)zinc compounds 17 were obtained from the hydrozirconation of terminal alkynes with Cp2ZrHGl, followed by treatment with dimethylzinc. The initially formed vinyl zirconium complex undergoes rapid transmetallation with ZnMe2 to yield the product.44... 

Organogermanium compounds can be prepared by transmetallation reactions with tin reagents. Examples include Me2PhGeCl (Equation (66)),89 the alkene-functionalized species 26-28, (Equations (67) and (68)),90 and the allenic (Equation (69)) and propargylic (Equation (70)) species 29 and 30.91 A series of aryltrichlorogermanes was prepared from the corresponding tin reagents (Equation (71), Table 9).92 Transmetallation with zirconium species can also be used (Equation (72), Table 10).93... 

Before adding aldehyde 14 a transmetalation from zirconium to zinc is necessary because of low reactivity of the sterically hindered organozirconocene compounds like 18 toward most organic electrophiles.9 Resulting alkenylethylzinc 19 reacts in a 1,2-addition with the cr,y3-unsaturated aldehyde 14 transferring exclusively the alkenyl moiety. The formation of Z -allylic alcohol 20 reveals stereochemical retention of the double bond configuration in the transmetalation and addition steps. 

It is known that the transmetalation reaction from zirconium to copper is a useful tool for the formation of a new carbon-carbon bond. Schwartz reported the first transmetalation from zirconium to copper [27], while Lipschutz [28] and Takahashi [29] used it for synthetic purposes. Thus, the transmetalation reaction of silazirconacydopentene to copper was investigated. To a THF solution of CuCl (2 equiv. to Cp2ZrCl2) and allyl chloride was added a THF solution of silazirconacydopentene 22a, generated from Cp2ZrCl2, alkyne 14a, and Me2PhSiLi 8b, and the solution was stirred at room temperature for 18 h. After the usual workup, the bis-allylated compound 37a was obtained in 76% yield... 

These important compounds with regards to their reactivity are obtained by transmetalation see Transmetalation) between the appropriate metallocene dichloride (see Metallocene Complexes) and an alkyl- or aryl-lithium reagent. Dialkyl and diaryl zirconium complexes are very moisture-sensitive, degrading to form /r-oxo species (see Section 5). 

First, coordinatively unsaturated active palladium catalyst, PdL2, is produced via dissociation of the ligands, which then reacts with acyl halide to give the acylpalladium intermediate. Since deinsertion of CO of the acylpalladium derivatives may occur simul-taneously, the next step, transmetallation (so-called metathesis), is the most crucial for the efficiency of the overall reaction. A variety of organometallic compounds, such as boron, aluminum, copper, zinc, mercury, silicon, tin, lead, zirconium, and bismuth, are used as the partner in this coupling reaction without loss of CO. In this section, the important features of the cross-coupling reactions of a variety of organometallic compounds with acyl halides and related electrophiles are discussed. 

Similarly, copper and zirconium reagents lead to transmetallation reactions with organopalladium compounds. These reagents are particularly useful to introduce a vinyl group into various substrates and has received applications in steroids [30], prostaglandins (Ref. 31) and natural diterpenes [32, 33] synthesis. 

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