One-pot Reactions on Bifunctional Catalysts

The environmental and economical benefits of one-pot catalytic fine chemical syntheses, in which various successive chemical steps are accomplished in the same reaction vessel, generally over a bifunctional (or multifunctional) catalyst, are obvious. The reduction in the number of synthetic and separation steps has various positive consequences environmentally more sustainable processes (higher atom economy and lower environmental factors), lower operating costs, lower production of wastes and in general an improvement in the safety conditions.[1 31 The environmental advantages are still more remarkable when the transformation of renewable raw materials, such as mixtures of natural terpenes or carbohydrates are concerned. 

The simplest way is to carry out the successive steps, not only in the same pot, but also under the same conditions. However, this is not always possible. Indeed, high yields in the desired product can be obtained only when the last step is quasi irreversibly shifted towards the formation of this product, when, sometimes, the operating conditions satisfying this condition favour the formation of large amounts of undesired products. In this case, the one-pot reaction is carried out in two stages 

Catalysts for Fine Chemical Synthesis, Vol. 4, Microporous and Mesoporous Solid Catalysts Edited by E. Derouane 2006 John Wiley Sons, Ltd 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC 2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

In the following section, some relevant examples of bifunctional catalytic systems hosted in or supported on either microporous or mesoporous materials are reported. In Tables 8.1 and 8.2 there is a list of the catalysts tested in vapour-phase fixed bed reactor and in liquid-phase batch reactor, respectively. 

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There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

One-Pot Multistep Synthesis of Ketones on Bifunctional Zeolite Catalysts. One-pot multistep reactions constitute an elegant and efficient way to decrease the number of chemical and separation steps, hence, to develop greener synthesis processes. Bifunctional metal-acidic or metal-basic zeolite catalysts, which can be prepared easily with the desired properties (e.g., distribution of the... 

Selective carbon-carbon bond formation is one of the most important reactions in both industrial and academic fields. Clean carbon-carbon bond formation depends on the development of heterogeneous acid and base catalysts, as described in this part. Mechanistic considerations can also identify new carbon-carbon bond formations. Section 6.2.2.1 focuses on catalysis by montmorilonite clays because various types of such solid acid catalysts have been simply prepared. The following section reviews recent developments in solid base catalysts for such chemistry and the final section of this chapter describes solid acid-base bifunctional catalysts, which are important concepts for one-pot sequential reactions. 

An interesting combination is that of a Heck coupling with an enantioselective dihydroxylation reaction. To achieve this, a bifunctional catalyst consisting of active palladium and osmium species anchored on silica gel through a mercaptopropyl spacer, and a cinchona alkaloid, respectively, was prepared and applied using N-methylmorpholine N-oxide as a cooxidant This one-pot process of alkene formation and subsequent dihydroxylation afforded diols of type 282 in excellent yields and with high enantiomeric excesses (Scheme 8.59) [385]. 

In 2010, Monge et al. reported a one-pot tandem reaction by combining bifunctional thiourea and Au complex [77], affording dihydropynole derivatives in moderate yields and high enantioselectivities. The reaction was based on a bifunctional thiourea-catalyzed Mannich-type reaction and a subsequent Au-catalyzed alkyne hydroamination and isomerization of propargylated malononitrile and N-Boc-protected imines (Scheme 9.72). Notably, acidic additive proved cracial to prevent deactivation of the gold catalyst and enhance the reactivity and selectivity. 

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