Enantioselective Hydroalumination of Alkenes

In spite of the modest asymmetric induction it was concluded that at least one of the chiral ligands is coordinated to the nickel in the catalyticaUy active species. An alternative interpretation was given by Wilke and coworkers [29]. They could show that (methylsalicyUdene)dimethylaluminum forms a stable adduct with nickel(O) complexes. It was concluded that the asymmetric induction in Pino s experiment might be attributed to a complex in which the chiral Ugand is complexed to the Lewis acidic aluminum. 

This interpretation was supported by further investigations by Giacomelli and coworkers [73]. Racemic 4-phenyl-l-hexene was kineticaUy resolved by isomerization of the double bond using a catalyst system consisting of Al Bus, (R)-N,N-di-methyl-l-phenylethylamine and Ni(mesal)2. Very poor enantioselectivities ee 0.3%) were observed for both the isomerization product and the unreacted aUcene. The authors note that it is essential to first react the alane with the chiral amine. No 

Although some preUminary studies have been carried out [79], uncertainties still exist about the mechanism of this transformation. The hydroalumination step was 

The asymmetric reductive ring opening of oxabenzonorbomene 53 was apphed as a key step in the total synthesis of serotonin re-uptake inhibitor sertraline [77,80], 

Alkynes are much more reactive toward hydroalumination than alkenes. Hence, they readily react with both dialkylaluminum hydrides and LiAlH4 under mild conditions in the absence of a catalyst [1]. However, it is not always possible to avoid side reactions and subsequent transformation of the vinylalanes formed in this transformation [81, 82]. In addition, ds-traws-isomerization of the metallated C=C bond can take place, thereby reducing the stereoselectivity of the overall reaction [83]. 

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. 

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Hydroalummation of Functional Croups 63 2.4.1.2 Enantioselective Hydroalumination of Alkenes... 

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