Organometallic compounds stereoselectivity

A. Olefinic compounds Acetylenic compounds Aromatic compounds Carbonyl compounds F/c-Oxygen compounds Nitrogen compounds Sulfur compounds Halogen compounds Other heteroatom compounds Organometallic compounds Stereoselective and Stereospecific Electrooxidation A. Carboxylic acids Acetoxylation Methoxylation Acetamidation... 

Keywords Azomethine compoimds 1,2-Diamines Organometallic compounds Reduction Stereoselectivity... 

Hard carbon nucleophiles of organometallic compounds react with 7r-allylpalladium complexes. A steroidal side chain was introduced to 24 regio- and stereoselectively by the reaction of the alkenylzirconium compound 26 with the steroidal 7z-allylpalladium complex 25, which was derived from 24 to afford 27 [10]. 

H. Ahlbrecht, Formation of C-C Bonds by Alkylation of ff-Type Organometallic Compounds, in Stereoselective Synthesis (Houben-Weyl) 4th ed. 1996, (G. Helmchen, R. W. Hoffinann, J. Mulzer, E. Schau-mann, Eds.), 1996, Vol. E 21 (Workbench Edition), 2, 645-663, Georg Thieme Verlag, Stuttgart. 

The stereoselective synthesis of alkenes is basically a solved problem. Nowadays, all kinds of alkenes can be synthesized irrespective of whether their double bond is isolated or conjugated with another C=C double bond, a C=C triple bond, or an aromatic ring. This state of affairs is largely due to the discovery and the development of a number of palladium-catalyzed alkenylation and arylation reactions of organometallic compounds. 

Fig. 16.17. Mechanism of the carbocupration of acetylene (R = H) or terminal alkynes (R H) with a saturated Gilman cuprate. The unsaturated Gilman cuprate I is obtained via the cuprolithiation product E and the resulting carbolithiation product F in several steps—and stereoselectively. Iodolysis of I leads to the formation of the iodoalkenes J with complete retention of configuration. Note The last step but one in this figure does not only afford I, but again the initial Gilman cuprate A B, too. The latter reenters the reaction chain "at the top" so that in the end the entire saturated (and more reactive) initial cuprate is incorporated into the unsaturated (and less reactive) cuprate (I). - Caution The organometallic compounds depicted here contain two-electron, multi-center bonds. Other than in "normal" cases, i.e., those with two-electron, two-center bonds, the lines cannot be automatically equated with the number of electron pairs. This is why only three electron shift arrows can be used to illustrate the reaction process. The fourth red arrow—in boldface— is not an electron shift arrow, but only indicates the site where the lithium atom binds next.

A /3-hydroxysilane, like the one shown in Figure 4.38 (top, left), can be prepared stereo-selectively (e.g., via the Cram-selective reduction of an a-silylated ketone according to the reactions in Figure 8.9 or via the Cram-selective addition of organometallic compounds to a-silylated aldehydes similar to what is shown in Table 8.3). These compounds undergo a stereoselective anft -elimination in the presence of add and a stereoselective syn-elimination in the presence of a base (Figure 4.38). Both reactions are referred to as Peterson olefination. The stereochemical flexibility of the Peterson elimination is unmatched by any other HetVHet2 elimination discussed in this section. 

Apparently, the simplest approach would be the carbometalation reaction or, more specifically, the vinylmetalation of alkynes [1-8]. The addition of organo-metallic reagents to functionalized or nonfunctionalized, terminal or nonterminal alkynes, in which the resulting organometallic compound can react with electrophiles, is defined as the carbometalation reaction (Scheme 1). It has been widely explored and applied in the regio- and stereoselective preparation of numerous vinyl metal species. 

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