Catalysts, role in environmentally benign

Antibody catalysts, role in environmentally benign synthesis of chemicals, 125-126 Aqueous-supercritical carbon dioxide medium, phase-transfer catalytic oxidation, 144-145 Arene cw-dihydrodiols, biocatalytic conversion of aromatics to optically pure synthons for pharmaceutical industry, 180-195... 

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

This review primarily focuses on the numerous reactions catalysed by water soluble transition metal complexes with emphasis on their implications for development of new environmentally benign processes in aqueous media based on the easy and quantitative separation of products from the catalyst as well as the avoidance of organic solvents. Moreover it contains, to our knowledge, the first comprehensive overview of water soluble ligands which play a key role for the development of efficient organometallic catalysis in aqueous media. 

The use of a class of pentafluorophenyl Pt(ll) complexes as catalysts allows the efficient epoxidation of simple terminal alkenes with environmentally benign hydrogen peroxide as the oxidant. Key features of this system are very high substrate selectivity, regioselectivity, and enantioselectivity, at least for this class of substrates. These properties are related to the soft Lewis acid character of the metal center that makes it relatively insensitive to water but, at the same time, capable of increasing the electrophilicity of the substrate by coordination. The reversal of the traditional electrophile/nucleophile roles in epoxidation helps explain the unprecedented reactivity observed. 

Catalysis will play a vital role in the 21 century in the environmentally benign synthesis of new and existing chemicals. These processes will be designed with an emphasize on both environmental and economic factors. This chapter describes the types of catalysts currently used in the syntheses of bulk and fine chemicals and identifies those most likely to have a beneficial impact, economically as well as environmentally, for chemical manufacturing in the future. 

In a second example, we examine reactions that relate to the Ostwald oxidation process, where NH3 is converted to NO with high selectivity. The reaction is typically run at high temperatures of around 1100 K over Pt/Rh alloy catalysts. The NO that forms is subsequently converted into nitric acid via a series of consecutive reaction steps. At lower temperatures, ammonia reacts to form Ng and NgO instead. The low-temperature conversion of ammonia to N2 would be much more desirable in that it would lower energy costs and, in addition, replace NO, an atmospheric pollutant, with N2, which is environmentally benign. We will describe here the low-temperature catalytic conversion of ammonia to form N2. For a review of high-temperature oxidation, in which coupling with gas-phase radical chemistry plays an important role, we refer to Ref. [50]. 

In 2007, Milstein reported an approach for the transition metal catalysed intermolecular formation of amides from alcohols and amines in the absence of a hydrogen acceptor (Scheme 12.19). In contrast with conventional amide synthesis from activated carboxylic acid derivatives which produces chemical waste, this environmentally benign approach produces hydrogen gas as the only byproduct. The catalyst used for this reaction is a dearomatised Ru(PNN)pincer complex which serves as a bifunctional catalyst. The ligands, as well as the metal centre, play a role in bond making or bond breaking steps of the catalytic cycle. 

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