Alkenes, carboalumination hydroalumination

Negishi reported the zirconium-catalyzed enantioselective carboalumination of alkenes, which consisted of a hydroalumination/alkylalumination tandem process.133-135 This permits the asymmetric syntheses of methyl-substituted alkanols and other derivatives, typically with >90% ee, which represents an increase in ee value by 15% from the previously obtained 70-80%.136-138 The hydroalumination/zirconium-catalyzed enantioselective carboalumination of alkenes was carried out using (—)-bis(neomenthylindenyl)zirconium dichloride as the catalyst (Table 15).133... 

Table 15 Hydroalumination zirconium catalyzed enantioselective carboalumination of alkenes...

It has been suggested that alkene or alkyne complexes are mechanistically important intermediates in hydroalumination and carboalumination reactions (124-126). Clear spectroscopic evidence for n interactions stems from investigations of alkenylaluminum compounds having a suitable intramolecular separation between the aluminum center and the double bond (127). IR and NMR data of these compounds show comparably lower alkene stretching frequencies and deshielded vinylic protons. Furthermore, these molecules are monomeric in solution this indicates that the tendency toward 7i-complex formation is stronger than that toward the dimeric bonding usual in aluminum alkyls. 

The Cl2ZrCp2-catalyzed reaction of alkynes with Bu3Al did not undergo car-bometallation under the conditions employed. It instead underwent a hydrogen-transfer hydroalumination [ 17]. As it could be not only a synthetically useful reaction but also intimately related to the Zr-catalyzed carboalumination of alkenes discussed later, a brief discussion of this reaction will be presented in the following section. 

Shortly after the discovery of the Zr-catalyzed carboalumination of alkynes in 1978, attempts were made briefly to observe the corresponding carboalumination of alkenes, but no more than traces of the desired products were formed. This puzzle was soon resolved when the Zr-catalyzed hydrogen-transfer hy-droalumination with BujAl was discovered in 1980 [166]. Since the expected products of carboalumination of alkenes would be isoalkylalanes, they could then undergo hydrogen-transfer hydroalumination in competition with the desired carboalumination. With this rationalization, attempts to develop the Zr-catalyzed asymmetric carboalumination were postponed. 

---