Hydrogenation, zirconocene-catalyzed

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

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. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Tetrasubstituted alkenes are among the most challenging substrates for catalytic hydrogenation reactions. Towards this end, Buchwald and co-workers recently reported efficient and highly enantioselective Zr-catalyzed hydrogenations of a range of styrenyl tetrasubstituted alkenes (Scheme 6.41) [123]. Precedents based on efficient polymerization reactions promoted by cationic zirconocenes led these workers to consider similar catalyst species, derived from dimethylzirconocene 107, for this purpose. 

A chiral zw a-metallocene triflate complex catalyzes the Diels-Alder cycloaddition reaction between an oxazolidinone-based dienophile and cyclopentadiene [206]. Triflate in titanocene and zirconocene complexes is labile [207,208] and thus the polarity of solvent influences the reactivity and enantioselectivity. Asymmetric hydrogenation of imines and enamines catalyzed by chiral aw a-titanocene catalyst provides amines with high enantioselectivity [209,210]. 

Cationic zirconocene complexes can catalyze the hydrogenation of alkenes, a property which has been used to terminate olefin polymerization [31]. As shown in Scheme 3, dimethyzirconocene complex 26 can be activated by methyalumox-... 

Carvill, A. Tritto, I. Locatelli, R Sacchi, M. C. Polymer microstructure as a probe into hydrogen activation effect in aw a-zirconocene/methylaluminoxane catalyzed propene polymerizations. Macrvmolecules 1997, 30, 7056-7062. 

In one of the earliest examples of a catalytic direct C—H activation process, Jordan and Taylor described the zirconium-catalyzed coupling of a-picoline (71) and propene (78) under an atmosphere of hydrogen (Scheme 10.26). It is presumed that the catalyticaUy active zirconocene species is formed upon hydrogenolysis. Coordination of the active catalyst by the 2-picoline nitrogen promotes C—H insertion at the a-position. ... 

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