Carbon transmetallation reactions

Carbon-carbon bond formation from alkyl and alkenyl zirconium reagents usually involves transmetallation reactions and are discussed in Chapter 8. 

In contrast to the transition metals, where there is often a change in oxidation level at the metal during the reaction, there is usually no change in oxidation level for boron, silicon, and tin compounds. The synthetically important reactions of these three groups of compounds involve transfer of a carbon substituent with one (radical equivalent) or two (carbanion equivalent) electrons to a reactive carbon center. Here we focus on the nonradical reactions and deal with radical reactions in Chapter 10. We have already introduced one important aspect of boron and tin chemistry in the transmetallation reactions involved in Pd-catalyzed cross-coupling reactions, discussed... 

Lithiated aromatics, generated by a transmetalation reaction, have been used for the generation of carbon-phosphorus bonds by displacement of chloride from phosphorus trichloride in the overall... 

For a long time, zirconacyclopentadiene was considered to be inert towards carbon-carbon bond formation. However, through transmetalation to Cu, Ni, Zn, Li, and Al, various kinds of carbon—carbon bond-formation methodologies have been widely developed. One major advantage of this chemistry is that zirconacyclopentadienes can be conveniently prepared in situ, in high yields and with excellent selectivities, from two different alkynes. Combination of the selective formation of zirconacyclopentadienes and the chemistry of Cu, Ni, Zn, Li, or Al provides useful tools in organic synthesis. In the near future, it can be expected that more metals will be used for the transmetalation reactions of zirconacycle derivatives and some catalytic reactions will hopefully be developed. 

Thus ethyl groups may be added to a-carbons as long as benzylic hydrogens are available for replacement. The mechanism which has been proposed by Pines et al. 19) for the reaction consists of the addition of the benzylic carbanion formed by reaction of the aromatic and the catalyst with the olefin followed by a transmetalation reaction with more of the aromatic [Reaction (4)]. 

Stille coupling was also developed in tlie early 1980s and is similar to Suzuki coupling in its sequence. It is used to couple aryl or vinyl halides or triflates with organotin compounds via oxidative addition, transmetallation, and reductive elimination. The oxidative addition reaction has tlie same requirements and preferences as discussed earlier for tlie Heck and Suzuki reactions. The reductive elimination results in formation of tlie new carbon-carbon bond. The main difference is that tlie transmetallation reaction uses an organotin compound and occurs readily without the need for an oxygen base. Aryl, alkenyl, and alkyl stannanes are readily available. Usually only one of tlie groups on tin enters into... 

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... 

Over the past few decades, interest in organozirconium chemistry has been rapidly increasing. This special interest arises from the combination of transition metal behavior, such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, (3-hydride elimination, addition reaction and from the behavior of classical o-carbanion towards electrophiles. The reactivity of the resulting carbanion can be easily modified by a whole gamut of transmetalation reactions. 

The trend shown in Eq. (d) is representative of transmetallation reactions, and consideration of the relative carbon transferabilities based on electronegativity provides predictive power. 

A major part of this chapter will show the synthetic applications of these transmetallation reactions for the perfonnancc of cross-coupling reactions. After a short description of the methods of preparation of organozinc halides (RZnX) and diorganozincs (R2Zn), the utility of these reagents for forming new carbon-carbon bonds will be presented. Emphasis will be... 

Carbon-carbon bond formation via transmetalation reactions... 

CARBON-CARBON BOND FORMATION VIA TRANSMETALATION REACTIONS (CROSS-COUPLING REACTIONS)... 

Carbon-Carbon Bond Formation via Transmetalation Reactions (Cross-Coupling Reactions) 585... 

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