Catalysis, continued substitution

Heterocyclic compounds are indispensable in recent far-reaching developments in the material and biological sciences because they can be used as a substructure in functional materials, agrochemicals, and pharmaceuticals. Transition-metal catalysis continues to be a fruitful source of new methods for the synthesis of heterocyclic compounds, since transition-metal catalysts can be used to construct complex structures directly from easily accessible starting materials under neutral and mild reaction conditions. The cycloaddition of unsaturated molecules is one of the most straightforward and atom-economical reactions for constructing substituted heterocyclic compounds [20]. 

One of the most attractive features of the IL/CO2 approach to homogeneous catalysis is the development of continuous processes [7]. Consequently it needs to be demonstrated that the combination of a suitable IL and compressed CO2 can offer more potential for process optimisation than just a simple protocol for batch-wise catalyst recycling. As an example we were able to activate, tune and immobilise Ni catalyst 13 in a continuous-flow system for the hydroviny-lation of styrene (Scheme 3). Styrene is co-dimerised with ethene yielding 3-substituted 1-butenes [26,27]. We could show that this powerful carbon-carbon bond-forming reaction can be achieved with high enantioselectivity in batch-wise operation and in continuous-flow systems. 

Typical phase transfer catalysis in liquid-liquid systems combines processes in which Na+ or K+ salts of inorganic and organic anions derived from strong adds (phenolates, thiolates, carboxylates, etc.) are continuously transferred from aqueous (often alkaline) solutions to the organic phase by the phase transfer catalysts. Applications include nucleophilic substitution, addition, elimination, oxidation, and reduction reactions. 

Commercially available VP is usually over 99% pure but does contain several methyl-substituted homologues and 2-pyrrolidinone. The vinylation of 2-pyrrolidinonc is carried out under alkaline catalysis analogous to the vinylation of alcohols. 2-Pyrrolidinone is treated with ca 5% potassium hydroxide, then water and some pyrrolidinone are distilled at reduced pressure. A ca 1 1 mixture (by vol) of acetylene and nitrogen is heated at 150- L60°C and ca 2 MPa (22 atm). Fresh 2-pyrrulidiuone and catalyst are added continuously while product is withdrawn. Conversion is limited to ca 60% to avoid excessive formation of hy-products. The AI-vinyl-2-pynolidinone is distilled at 70-85°C at 670 Pa (5 mm Hg) and the yield is 70-80%. 

Catalysis of oxidation reactions will continue to be of enormous importance in the future. Areas that continue to be of active interest are the development of efficient methods for the direct epoxidation of olefins, hydroxylation and substitution of aromatics as well as the selective oxidation of alkanes. The application of methods developed for industrial chemicals to the synthesis of more complex molecules is worthy of more attention. A few examples have been discussed in the text. On the whole, however, synthetic chemists have not exploited these methods. 

Information on ligand substitution mechanisms should aid us to understand more profoundly homogeneous catalysis by transition metal complexes, where probably consecutive substitution and transfer reactions of ligands from metal to a substrate and back take place continuously. 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

As stated some years ago [2, 4], shape selective catalysis involving bulky molecules continues to be a thrust area in zeolite catalysis. Consequently, test reactions have been developed which are particularly suited to characterize large and super-large pore molecular sieves [34]. In view of possible commercial applications, recent work focussed on the shape selective synthesis of substituted dinuclear aromatics, i.e., 4,4 -diisopropylbiphenyl and 2,6-dialkylnaphthalenes, due to their potential as components in high-temperature resistent polyesters or as liquid crystals. Recent advances in this field are covered in two excellent review articles [35, 36]. 

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