Early Transition Metal Initiators

It has since been shown that if less coordinating anions are used, then cationic zirconocene alkyls may serve as highly active single-component catalysts. Hence, treatment of Cp2ZrMe2 with 

The block copolymerization of MMA with ethylene was recently described using (202)/ B(C6F5)3.499 The olefin must be polymerized first (as observed with (188) and (190)) and the diblock nature of the product was inferred from solubility behavior. 

Several group 5 complexes have also been examined as MMA polymerization initiators. A series of 1,4-diaza-1,3-diene tantalum complexes have been investigated and a mixture of (206)/ Me3Al was found to polymerize MMA in a living manner at — 30 °C (rr = 78%).500 501 Cp2TaMe3 also polymerizes MMA when activated with two equivalents AlMe3.235 However, initiator efficiencies are low and molecular weight distributions are broad. 

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Radical Chain Mechanism This mechanism also requires a coordinatively unsaturated metal and the presence of a radical initiator Q (trace of 02, hv, etc.). Such a pathway has been proposed for a Ni(II) complex-catalyzed dehalogena-tion of polyhaloarenes [60], and it occurs frequently in the stoichiometric C-X activations with early transition-metal complexes (see [205-207]). 

Uniaxial viscosity, 21 718 Unicomponent metathesis initiators based on early transition metals,... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

As outlined above, the initial reaction of early transition metal organometal-lics with O2 may produce a variety of complexes containing oxygen atoms—in the form of superoxo (02 ), peroxo (02 ), 0x0 (0 ), or alkylperoxo (R02 )... 

The initial success with early transition metals, such as zirconium and titanium, reported by the Buchwald group, included an indirect cycloaddition between an enyne and isocyanides. The first protocol that used [Gp2Ti(PMc3)2] or Ni(GOD) together with triphenylphosphine failed to cyclize the enynes under the pressure of GO, but provided the cyclic imines with trialkylsilyl isocyanides, and bicyclic enones were obtained by hydrolysis of the resultant imine products (Equation (9)). ... 

Mo(r75-C5H5)2H2] and [MoH dppe ]. Our studies of the di- and trihydride complexes of ruthenium and iridium, described above and published previously (27,35), and those of other workers (discussed at the beginning of this chapter), indicate that photoinduced elimination of molecular hydrogen is a common reaction pathway for di- and polyhydride complexes. To demonstrate the photoreaction s generality and its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of polyhydride complexes of the early transition metals. We focused initially... 

Reaction of pentafluoroacetophenone with hexamethyldisilane for 20 h in toluene in a 130°C oil bath in the presence of 10 mol% Rh(cod)2BF4gave a 79-88% yield of 2,3,4,5-tetrafluoro-6-trimethylsilylacetophenone. Not surprisingly, 2,6-difluoroacetophenone affords the mono-Me3Si-F exchange product in somewhat lower yields (33-48%). In the case of a related oxazoline derivative some disilylation accompanied the mono-substituted product. The authors propose a chelate assisted mechanism for the initial C-F activation step [62]. It is interesting to note that these catalytic reactions all involve the later transition metal rhodium with a relatively labile Rh-F bond removed as R3Si-F or HF. However, related catalytic reactions of aromatic C-F bonds have also been discovered for early transition metals and even in the very electropositive lanthanide series. 

The discoveries of several new types of catalysts for (non-aqueous) efhylene and 1-olefin polymerization based on late transition metals over fhe last decade have initiated great interest in fhis field [16, 53-55]. By comparison to early transition metal-based Ziegler or Phillips catalysts used for industrial polyolefin synfhesis [13—... 

The ability of early transition metals to initiate polymerizations has been known and utilized since the mid-20th century. [46] Such Ziegler-Natta catalyst systems with homoleptic and simple Ti and A1 compounds have paved the way to the development... 

Tris(pentafluorophenyl)borane (35) is a powerful Lewis acid with a hard boron center. It interacts strongly with hard Lewis basic centers and abstracts hard anion such as methyl anion and hydride to initiate selective reactions that are otherwise difficult to achieve. The B(CgF5)3-promoted early transition-metal-catalyzed olefin polymerizations are typical reactions [16]. The B(C6F5)3-Ph3SiH system is useful for hydro-silylation of the carbonyl group, in which the boron of 35 activates the Si—H bond rather than the carbonyl group (see 40 in Scheme 3.39) and reduces even the ester carbonyl group effectively [17]. A number... 

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