Hydroalumination transition metal catalysts

Early attempts by Asinger to enlarge the scope of hydroalumination by the use of transition metal catalysts included the conversion of mixtures of isomeric linear alkenes into linear alcohols by hydroalumination with BU3AI or BU2AIH at temperatures as high as 110°C and subsequent oxidation of the formed organoaluminum compounds [12]. Simple transition metal salts were used as catalysts, including tita-nium(IV) and zirconium(IV) chlorides and oxochlorides. The role of the transition metal in these reactions is likely limited to the isomerization of internal alkenes to terminal ones since no catalyst is required for the hydroalumination of a terminal alkene under these reaction conditions. 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

ASYMMETRIC HYDROALUMINATION WITH TRANSITION-METAL CATALYSTS... 

Hydroalumination of 1-alkynes generally proceeds without the aid of transition metal catalysts. However, the reaction sometimes suffers from undesirable side-processes, including the formation of alk-l-ynylalanes or protonolysis of the intermediate alk-l-enylalanes. In particular, these side-products increase significantly in the reaction of... 

Compared with alkynes, alkenes are rather inert to hydroalumination. Most required an external catalyst that aided hydride transfer from aluminum to alkenes [169]. A variety of transition metal catalysts was extensively studied for this among these the Group IV metals Ti and Zr were found generally apphcable. 

In view of the weak affinity of the ionic A1—H bond for olefins [15-21], the hydroalumination of olefins has been studied extensively in the presence of transition metal catalysts. Among these, only Ti and Zr catalysts have proved to be effective for obtaining hydroalumination products and their subsequent functionalization with various... 

It is noteworthy that the transition metals that serve as hydroaluminating catalysts are also active in establishing the equilibrium between aluminum alkyls and their decomposition products, aluminum, hydrogen and alkene (equation 12). Accordingly, these metals, in addition to hafnium, niobium, vanadium, scandium and lanthanum, have found use as activators for the direct synthesis of aluminum alkyls (equation 12, to the left). Probably most of these metal salts will also be capable of accelerating the hydroalumination reaction. 

In Section 3.11.1.4 it was pointed out that salts of certain transition metals, lanthanides and actinides promote the hydroalumination reaction. Since such metal salts are introduced into the reaction in their high oxidation states it can be assumed that the metal ions are rapidly reduced to a lower oxidation state and that this state is the active catalyst. For nickel(II) salts, Wilke has shown conclusively that the active agent is a nickel(0)-alkene complex. Analogously, for titanium(IV) salts, such as TiCU, Ti(OR>4 and Cp2TiCl2, it is most likely that a titanium(III) state is involved. The possible role of such metal centers in accelerating hydroalumination will be considered in the next section. 

Careful and extensive kinetic studies have been carried out only for hydroaluminations with dialkyl-aluminum hydrides. Adequate kinetic information is still lacking on transition metal catalyzed hydroalumination and on the hydroaluminating action of complex metal hydrides, MAlRnH4 n. Preliminary studies on the nickel-catalyzed process have revealed an unstable rate behavior brought about by the deactivation of the catalyst with time. ... 

In 1954, Ziegler and coworkers [11,12] reported that traces of nickel salts dramatically alter the course of the growth reaction of ethylene with trialkylalanes, the Aufbau process. Instead of the low molecular weight polyethylene which was expected, the only product was butene. This observation culminated in Ziegler s discovery of transition metals that were highly effective in polymerizing ethylene, an accomplishment for which he later shared the Nobel Prize. It also opened the door to transition metal catalyzed hydroalumination reactions. In 1968, Eisch and Foxton showed that addition of nickel(II) salts increased the rate of the hydroalumination of alkynes by approximately 100-fold [13]. The active catalyst was believed to be a nickel(O) species. 

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