Hydrogen transfer hydroalumination

Negishi reported the hydrogen transfer hydroalumination of alkenes with (/-Bu AKTIBA) and catalytic amounts of palladium and other late transition metal complexes.125 Although uncatalyzed hydroaluminations of alkenes with di-and trialkylalanes at elevated temperatures have long been known, their scope and limitations as well as their synthetic utility have not been extensively explored. 

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. 

Since the hydrogen-transfer hydroalumination with Bu3Al is catalyzed with Cl2ZrCp2 presumably through bimetallic activation, hydrogen-transfer hy-drozirconation might, in turn, be catalyzed by Al compounds. This indeed was the case. Moreover, a variety of metal compounds including Al, Zn, Ag, Pd, and even Si were shown to catalyze the reaction, as indicated by the results summarized in Scheme 19 [172]. Further simplification and improvements are desirable and appear to be feasible. 

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. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

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