Steric effects transition metal catalysts

Typical examples of the polymerization of monosubstituted acetylenes are shown in Table 3. Transition metal catalysts that involve Mo, W, and Rh are particularly effective. Whereas Mo and W catalysts are sensitive to polar groups in the monomer, Rh catalysts are tolerant to such groups. Another point is that Mo and W catalysts are effective to various sterically crowded monomers, while Rh catalysts are useful to rather restricted kinds of monomers including... 

The importance of group 5 and 6 transition metal catalysts stems from the fact that they can polymerize sterically crowded acetylenes for which Ziegler catalysts are inactive. On the contrary, the present catalysts 12,13) (e.g, WC16—Ph4Sn) are much less effective toward the unsubstituted acetylene than is Ti(0-n-Bu)4—Et3AL, a Ziegler catalyst this is because the present catalysts are prone to cyclotrimerize acetylene. 

In general, disubstituted acetylenes are sterically more crowded than their monosubstituted counterparts and, consequently, their effective polymerization catalysts are restricted virtually to group 5 and 6 transition-metal catalysts. Among disubstituted acetylenes. 

Our initial idea to use borane monomers, co-alkenylboranes (7-8), in Ziegler-Natta polymerization was based on three considerations, (a) the stability of borane to transition metal catalyst. Because trialkylborane is a Lewis acid, it offers an veiy good chance for them to coexist with the catalyst. In addition, the boron atom is relatively small, steric protection can be effectively applied if needed, (b) the solubility of borane monomers and polymers in the hydrocarbon solvents (hexane and toluene) used in Ziegler-Natta polymen zation. A soluble growing polymer chain is essential to obtain high molecular weight polymer, (c) the versatility of borane groups, which can be transformed to a remarkably fruitful variety of functionalities, as shown by H. C. Brown (9). 

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. 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

On these transition metal-based catalysts, the selechve hydrogenahon of the C=0 group is very difficult because C=C double bond hydrogenahon is both thermodynamically and kinehcally favored, especially in the case of small molecules (e.g., acrolein, crotonaldehyde) where addihonal steric effects are not important [62, 71, 72]. 

Major challenges remain in catalyst development. There will be continuing efforts to increase turnover numbers and rates and to perform these reactions under environmentally friendly conditions. Transition metals, especially Cu, Rh, Ru, Co, Mo, and Zn are effective today will other metals with attendant ligands be found whose electronic and steric properties are superior to those currently optimized ... 

---