Homogeneous acid-base catalysis transition metals

Supramolecular catalysis can involve passive effects such as the confining of two reactive molecules within a cavity and active effects where the catalyst interacts with the substrate via an active site. The active site may be metal-based as in other kinds of homogeneous catalyst based on transition metals or Lewis acids, or my involve interactions such as hydrogen bonding to bring about both polarisation of the reactants and their mutual spatial organisation. 

Prior to 1970 the term homogeneous catalysis usually connoted acid-base catalysis, and this is the best understood type of catalysis . Since then, however, homogeneous catalysis has come to mean primarily transition-metal-complex catalysis, which has been the object of intensive research motivated in part by a series of technological accomplishments . ... 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

Until recently, access to optically active 3-hydroxyalkanoates by enantioselective transition metal catalysis was based primarily on the heterogeneous hydrogenation of 3-oxo esters by Raney nickel modified with tartaric acid and sodium bromide (see Section 2.3.1.1.). As far as homogeneous catalysis is concerned, the best optical induction (71 % ee) in the hydrogena-... 

In case of homogeneous catalysis a catalyst is in the same phase as the reactants. Three types of homogeneous catalysis are usually considered gas phase catalysis, acid-base and by transition metals. 

In homc meous catalysis the catalyst (e.g., an acid, a base, or a transition metal complex) is dissolved in a liquid phase. An example from the chemical industry is hydro-formylation, in which olefins react with synthesis gas (CO/Hj mixture) on cobalt or rhodium catalysts to give aldehydes [Weissermel 2003], The practical applications of homogeneous catalysis date back to the eighth century. At this time mineral acids were used to prepare ether by dehydration of ethanol. [Thomas 1994]. 

It is difficult to summarize all the progress of the homogeneous catalytic aldol reaction due to the space limitations. With representative or selected examples, this article focuses on those efforts in homogeneous catalysis and will be discussed with selected examples according to the different types of catalysts il) transition metals based Lewis acids, such as palladium, copper, iron, etc (2)... 

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