Hydrozirconation hydrogen transfer

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. 

The nonredox catalytic hydrogen transfer hydrometallation protocol should also be widely applicable to those involving transition metals. Indeed, the Pd-catalyzed hydrogen transfer hydrozirconation discussed in Sect. E appears to represent a prototypical example of this class of reactions. 

One of the shortcomings associated with the hydrogen-transfer hydrozirconation is that it is generally much slower than that with preformed HZrCp2Cl. 

Perhaps the most useful application of the hydrozirconation/protodezirconation sequence is for specific introduction of deuterium label some examples were presented in the previous section. Although most such work indicates that deuterium incorporation is essentially complete, Svenson and coworkers have found that around 20% of the alkane thus obtained from simple alkenes is undeuterated. The amount of alkane-reaction times, so transfer of hydrogen to give alkane prior to hydrolysis is probably responsible. This in situ hydrogenation was found to be even more pronounced for a,p-unsaturated carboxylic acid oxazolines, giving 70% of the undeuterated saturated product. ... 

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