Metallocene reactions

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

Bis-pi-allyl Pd and Pt complexes have been found to catalyze the addition of allyl tributyltin to aldehydes [26]. These catalysts are formed in situ from Pd- and PtCl2-phosphine complexes and the allylstannanes (Fig. 7, step 1). The allylation step is depicted as a metallocene reaction of the aldehyde and an his-allylmetal complex (Fig. 7, step 3). The catalyst is regenerated by attack of the allylic stannane on the alcoholate-palladium complex formed in step 3. Representative additions of allyl and methallyl tributyltin to aldehydes with the pi-allyl platinum catalyst are summarized in Table 14. 

Our strategy to synthesize a compound containing the MesCsSi cation is based on observations from the chemistry of the heavier group 14 element metallocenes. Reaction of the decamethylmetallocenes (Me5Cs)2El with several protic substrates led to the isolation of compounds containing the corresponding MesCsEr cations (Scheme 1) [8]. 

Knobloch DJ, Lobkovsky E, Chirik PJ. Carbon monoxide-induced dinitrogen cleavage with group 4 metallocenes reaction scope and coupling to N-H bond formation and CO deoxygenation. / Am Chem Soc. 2010 132 10553-10564. 

The second aspect, predicting reaction dynamics, including the quantum behaviour of protons, still has some way to go There are really two separate problems the simulation of a slow activated event, and the quantum-dynamical aspects of a reactive transition. Only fast reactions, occurring on the pico- to nanosecond time scale, can be probed by direct simulation an interesting example is the simulation by ab initio MD of metallocene-catalysed ethylene polymerisation by Meier et al. [93]. 

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

As a rule, LLDPE resins do not contain long-chain branches. However, some copolymers produced with metallocene catalysts in solution processes can contain about 0.002 long-chain branches per 100 ethylene units (1). These branches are formed in auto-copolymerisation reactions of ethylene with polymer molecules containing vinyl double bonds on their ends (2). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

Polymerization Reactions. Polymerization addition reactions are commercially the most important class of reactions for the propylene molecule and are covered in detail elsewhere (see Olefin polymers, polypropylene). Many types of gas- or liquid-phase catalysts are used for this purpose. Most recently, metallocene catalysts have been commercially employed. These latter catalysts requite higher levels of propylene purity. 

Step 1 CP2Z1CI2 is converted to the active catalyst by reaction with the promoter methylalumoxane (MAO). A methyl group from MAO displaces one of the chlorine ligands of Cp2ZrCl2. The second chlorine is lost as chloride by ionization, giving a positively charged metallocene. 

Reactions of the metallocene derivatives of molybdenum with pyrazole lead to the mononuclear complexes of the type 22. Structure 22 shows that it cannot be used as a ligand for the preparation of dinuclear complexes owing to geometric constraints [80JOM( 197)291 83JOM(253)53]. In acetone, an unusual complex 23 is formed [83JOM(253)53]. The bidentate ligand is the product of the reaction of pyrazole and acetone. 

T. C. Chung, New utilities of metallocene catalysts and borane reagents in the functionalization and block/graft reactions of polyolefins, MetCon 95 Proceedings, USA, May 1995. 

The successful polymerization of a, >-dienes via ADMET continually produces a small molecule, typically ethylene, and the removal of this small molecule drives the reaction. When Schrock s [W] and [Mo] alkylidenes (14) are used, care has to be taken in maintaining an inert atmosphere devoid of both moisture and air in order to avoid decomposition of the catalyst. For this reason, Schlenk line techniques such as those used to handle Ziegler-Natta or metallocene catalysts and high purity monomers are important. 

Keywords stereoselective DIels-Alder reaction catalysts, DIels-Alder chiral metallocene catalyst review... 

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