Organic synthesis multifunctional catalysts

Several architectural paradigms are known for polyketide and fatty acid synthases. While the bacterial enzymes are composed of several monofunctional polypeptides which are used during each cycle of chain elongation, fatty acid and polyketide synthases in higher organisms are multifunctional proteins with an individual set of active sites dedicated to each cycle of condensation and ketoreduction. Peptide synthetases also exhibit a one-to-one correspondence between the enzyme sequence and the structure of the product. Together, these systems represent a unique mechanism for the synthesis of biopolymers in which the template and the catalyst are the same molecule. 

Very few efficient catalytic enantioselective versions of MBH reaction were known up to 1999 despite a considerable amount of efforts devoted to the field. A breakthrough came in 1999 when Hatakeyama and coworkers discovered that p isocupreidine (P ICD) is an efficient catalyst for the MBH reaction [11]. Meanwhile, the use of small organic molecules as catalysts to perform asymmetric transformations has received increasing attention over the past decade. Therefore, chiral multifunctional orga nocatalysts have also been developed rapidly to promote successful enantioselective MBH/aza MBH processes. This chapter mainly summarizes recent advances in the design and synthesis of small organic molecules for the enantioselective aza MBH reactions from 2000. On the basis of these enantioselective aza MBH reactions, a variety of chiral amines can be easily prepared under mild conditions. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

Multifunctional Zeolites Efficient, multifunctional zeolite-based catalysts allowing one-step complex reactions are of great interest in the field of fine chemicals and organic industrial synthesis [58]. Recendy, catalysts based on zeolites are relevant in emerging areas of interest such as the catalytic conversion of biomass to fuels and chemicals (see Section 8.2.1.3 for illustrative examples). This field needs to develop specific multifunctional catalysts having the correct polarity (adsorption properties) and reactant accessibility (porosity), which are efficient in water or biphasic operation with reactants and products of different polarities and sizes. Hence, great opportunities for zeolites and related materials are offered in this new field [59-61]. 

It is well established that MOFs themselves can function as heterogeneous catalysts [10-13,108] due to the presence (either in the pristine MOF or introduced afterwards by post-synthesis modifications) of active sites located at the metallic nodes and/or functional organic linkers. Therefore, combination of two (or more) different framework active sites (such as acid/base), or a framework active site with encapsulated species located inside the pores, affords a means for preparing multifunctional catalysts. In this way, sequential catalytic transformation (usually referred to as tandem or cascade processes [24,109,110]) can... 

Increasing efforts to develop chemical processes with minimized ecological impact and to reduce the emission of potentially hazardous or toxic organic chemicals have stimulated a rapidly growing interest to provide alternatives to this classical approach of synthesis in solution. At the same time, researchers have started to realize that the design and utilization of multifunctional reaction media can add a new dimension to the development of synthetic chemistry. In particular, efficient protocols for phase separations and recovery of reagents and catalysts are urgently required... 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

One of the most fascinating aspects in the history of asymmetric catalysis with its countless successful applications in the stereoselective synthesis of a broad variety of functional groups is the structural variety of the complexes which are able to be used as catalysts [1,2]. Many catalysts have been developed based on different ideas and concepts of mechanistic effect. However, in spite of the abundance of such catalysts which have been successfully applied in asymmetric catalysis, not a handful of them possess multifunctional abilities catalyzing different type of enantioselective reactions. The development of such a type of chiral catalyst, the catalytic effect of which is not limited to one reaction but to different types of asymmetric synthetic organic transformations, remained an attractive challenge for a long time. 

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