Organometallic Catalysis: Some Contributions to Organic Synthesis

Synthetic chemicals are extremely important in all fields of our life and their cheap production in large amounts is necessary for the successful economic growth of modern civilization. Catalysis offers the unique possibility of fast and selective synthesis of desired chemical molecules without consumption of extra amounts of energy and theoretically without the consumption of catalyst itself. Presently, most of industrial chemical processes are based on catalysis. The steady replacement of classical stoichiometric methodologies with cleaner catalytic alternatives is also promoted by a growing environmental awareness and an increasing pressure on chemical industry to reduce a waste production. The amounts of waste (undesired products) produced per kilogram of product, designated as E factor, are especially high in the manufacture of fine chemicals (up to 100 kg), while oil refining and bulk chemicals industry usually produce ca. 0.1 and 1-5 kg of by-products, respectively, per kilogram of product. This situation is partly due to the use of multi-step syntheses in fine chemistry but also due to the predominant utilization of stoichiometric rather than catalytic methods. On the other hand, most of modern petrochemical technologies are one-step catalytic processes with high atom utilization. Transition metals have become an important instrument in organic synthesis due to their ability to activate organic substrates and promote various interactions between them. A rapid progress in the study of organometallic and coordination compounds has led to the development and successful industrial application of a number of catalytic processes based on use of these compounds as catalysts. The major advantage of organometallic catalysis that has led to its widespread adoption by industry is selectivity, the ability to produce pure products in high yield. The main reasons why transition metals contribute so essentially in catalysis are the follows: a) bonding ability: the ability to form both πand σ-bonds with other moieties; b) wide choice of ligands: transition elements readily form chemical bonds with almost every other element and almost any organic molecule; c) ligand effects: a ligand can influence the behavior of a transition metal catalyst by varying the steric or electronic environment at the active site; d) variability of oxidation state and co-ordination number; e) ability to readily interchanges between oxidation states during a catalytic reaction: transition metals can be readily involved in redox processes. The mechanism of catalytic reactions promoted by transition metal complexes can be rationalized as a sequence of stoichiometric steps linked in a cyclic way thus forming a catalytic cycle. First, the formation of an organometallic compound occurs via the coordination or addition of substrate(s) to a metal center. That is followed by a series of intramolecular transformations of the activated substrate(s), which can involve also external groups and substrate(s), and the decomposition of the organometallic compound to give reaction products. Generally, metal complexes are changed as a result of such transformations and their regeneration is necessary to complete a catalytic cycle. Stoichiometric transformations involved in a catalytic process are the fundamental reactions of coordination and organometallic chemistry: ligand coordination, oxidative addition, insertion, isomerization, reductive elimination, etc. No doubt much of the interest arises from the potential commercial importance of these reactions and for this reason, most of the earlier works were concerned with the preparative aspects of organometallic catalysis. However, during the last 30 years a considerable number of mechanistic works, including studies of the organometallic chemistry involved in catalytic reactions, appeared in the literature.