Ill-defined catalysts

The formation of relatively ill-defined catalysts for epoxide/C02 copolymerization catalysts, arising from the treatment of ZnO with acid anhydrides or monoesters of dicarboxylic acids, has been described in a patent disclosure.968 Employing the perfluoroalkyl ester acid (342) renders the catalyst soluble in supercritical C02.969 At 110°C and 2,000 psi this catalyst mixture performs similarly to the zinc bisphenolates, producing a 96 4 ratio of polycarbonate polyether linkages, with a turnover of 440 g polymer/g [Zn] and a broad polydispersity (Mw/Mn>4). Related aluminum complexes have also been studied and (343) was found to be particularly active. However, selectivity is poor, with a ratio of 1 3.6 polycarbonate polyether.970... 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

For a more in depth coverage of the use of ill-defined catalysts in cross-metathesis, see Ivin KJ, Mol JC (1997) Olefin metathesis and metathesis polymerisation. Academic Press, San Diego, Chap 9... 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

In general, the termination reactions of these polymerizations are not well understood but, depending upon the metal and the monomer, reductive coupling of the metal carbene fragments to give alkene and reduced metal complexes is one possibility. Another termination reaction appears to be initiated by -Hydride Elimination from the carbene complex. These mechanisms have been observed in well-defined catalyst systems, and are possible in the ill-defined systems also. The fact that most catalysts are sensitive to oxygen and moisture (or other proton sources) means that termination of the polymer chain by added or adventitious sources of water is a common problem, especially for the ill-defined catalysts. 

Olefin metathesis is a reaction that is over fifty years old and has been developed over this time period from a process nm at high temperatures with ill-defined catalysts by unknown mechanisms to a process that can be conducted imder nuld conditions with designed catalysts by mechanisms that occur by established steps. Olefin metathesis, and the related alk3me metathesis, fully cleaves carbon-carbon double and triple bonds and reforms these bonds to generate new alkenes and alkynes. The reaction is often under equilibrium control, but certain classes of reactions can be conducted in a selective fashion that is controlled by relative rates or thermod)mamic preferences. This reaction can open strained rings to form polymers or small dienes. It can close small rings and macrocycles by a reaction that is driven by the expulsion of ethylene that makes the reaction favored entropically or by running in an open system under non-equilibrium conditions. It can also be run as a "cross metathesis" to form imsymmetrical alkenes when the steric or electronic properties of the two alkenes properly match. 

Thus, the key observations and hypotheses are as follows. First, the formation of higher order polynuclear copper acetylides is detrimental to the rate and the outcome of the reaction. Therefore, solvents that promote ligand exchange (e.g., water and alcohols) are preferred over apolar, organic solvents which promote aggregation of copper species. Ill-defined catalysts perform better in the CuAAC precisely for this reason. 

During the past two decades, the alkene metathesis reaction has developed from its early appUcations in large-scale processes with heterogeneous and ill-defined catalyst systems to a standard technique in synthetic chemistry and polymer laboratories. The development of well-defined and often bench-stable precatalysts " has been key to the widespread use of alkene metathesis in modem target synthesis projects. The impact of this useful reaction was recognized in 2005 by the award of the Nobel Prize in Chemistry to Yves Chauvin, Robert Gmbbs, and Richard Schrock. Astmc has published an excellent article on the early history of the alkene metathesis reaction, which covers the determination of the mechanism and the rejection of alternative hypotheses, so this early history will not be discussed here. 

Rinehart et al. [1, 2] and Michelotti and Keaveney [3] reported the first successful emulsion ROMP using water-soluble ruthenium, iridium, and osmium chlorides activated by a reducing agent. These ill-defined catalyst systems were... 

Although metathesis polymerization was well known from previous work using ill-defined catalysts, it was only with the introduction of single component catalysts that well-defined polymerizations using ring-opening metathesis polymerization (ROMP) became possible. 

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