Homogeneous, Early Transition-Metal Catalysts

The molecular design of stereospecific homogeneous catalysts for polymerization and oligomerization has now reached a practical stage, which is the result of the rapid developments in early transition metal organometallic chemistry in this decade. In fact, Exxon and Dow are already producing polyethylene commercially with the help of metallocene catalysts. Compared to the polymerization of a-olefins, the polymerization of polar vinyl, alkynyl and cyclic monomers seems to be less developed. 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

A number of Ziegler-type catalysts based on early transition metals and tri-alkylaluminum or similar organometallic compounds are soluble in hydrocarbon solvents and function as homogeneous hydrogenation catalysts.24,26 Many of the... 

The reaction in Fig. 4.19 is catalyzed by compounds of high-valent, early transition metals such as Mo(VI), W(VI), V(V) and Ti(IV). Molybdenum compounds are particularly effective homogeneous catalysts and are used in the ARCO process in combination with TBHP or EBHP. In the Shell process, on the other hand, a heterogeneous Ti(IV)/Si02 catalyst is used with EBHP in a continuous,... 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

The synthesis of alkylidenes incorporating late transition metals has resulted in alkene metathesis catalysts having unprecedented functional group tolerance. In particular, the discovery that complexes of Group VIII transition metals were efficient ROMP catalysts introduced several advantages. Relative to their early transition metal counterparts, these classical catalysts functioned well in the presence of a variety of polar and protic functional groups (Table 2), and they functioned homogeneously in water. 

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