Transition metal catalysts configuration

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

The Sn-C bond can be cleaved with hydrogen peroxide44-63 or peroxyacids.64 The reactions take place with retention of configuration, and presumably involve a nucleophilic rearrangement, for example equation 5-50.44 Ozonolysis has been used on a few occasions (e.g. equation 5-51),65 and also oxygen in the presence of a transition metal catalyst.66... 

In 1961, Natta reported one of the first examples of enantioselective catalysis using a transition metal catalyst. In this reaction, an optically active polymer was formed from 1,3-pentadiene using a chiral organoaluminum/VClj catalyst [62]. The optical activity of this polymer results from the main-chain chiraHty of polymer, where the methyl-substituted stereogenic centers are predominantly of one absolute configuration. Since this initial study, significant advances in the enantioselective synthesis of main-chain chiral polymers have been reported using ionic and metal-based techniques. 

The polyolefins produced by transition metal catalysts are characterized by the absence of large amounts of long- or short-chain branching, which causes variability in density, crystallinity, and melting points. Most catalysts used are heterogeneous but some homogeneous systems are known. A two-step mechanism for catalysis is widely accepted (1) adsorption of the monomer, which may be activated by the configuration established in this step, and (2) insertion of the activated monomer into a metal-carbon bond. 

In order for a substrate to be activated by a transition metal in homogeneous catalysis, prior interaction of the substrate with the transition metal catalyst is required. When the complex is coordinafively saturated with an 18 electron configuration [1,7], coordinafive unsaturation is usually created by dissociation of all or part of a bound ligand from the complex thus allowing the resulting coordinafively unsaturated transition metal fragment to enter the catalytic cycle (Scheme 1.4). 

For transition metal catalysts, two-electron reduction was reported for less active metals such as Au and Hg. For the most active catalyst, Pt and Pt-based alloys, four-electron reduction steps are generally believed. However, the detailed mechanism and reduction pathways are not clear and much debate remains. Even for the first electron transfer step, different views still exist [38-40]. Examples of plausible first steps include the following (1) splitting of the 0-0 bond upon oxygen adsorption on two Pt sites (S) in abridge configuration, O2 + 2S —> O + O (2) formation of... 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

The reduction of a carbon-carbon double bond can be carried out using hydrogen in the presence of a transition metal catalyst. Because hydrogen atoms are delivered to either face of the double bond with equal probability, if a new chiral center is created, equal amounts of both the R and S configurations will be produced. 

Olefin Isomerization- a variety of transition metal (RhCl3 H20) catalyst will isomerize doubles bonds to more thermodynamically favorable configurations (i.e. more substituted, trans, conjugated)... 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

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