Palladium-Catalyzed Cross Couplings in Organic Synthesis - Essay Example

Chemical catalyst Catalysis has a vast area of applicability covering various industrial processes, aspects of environment protection as well biological processes. The purpose of this report is to provide a general introduction to the process of chemical catalysis, the involved mechanistic approach in chemical reactions, the classifications of the process, the advantages and disadvantages of different types of catalysis, properties, developmental history and an evaluation in the contexts of different chemical processes. 1. Introduction The 2010 Nobel Prize in chemistry was awarded to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki for their work on “palladium-catalyzed cross couplings in organic synthesis" which in itself is evidence strong enough to validate the significance of catalysis in the fields of modern science, and particularly in industrial processes. Around 9 billion US Dollars defines the volume of the recent market for catalysts in the Global markets. (Dautzenberg, 2002). ‘Catalysis’ is defined as a phenomenon and ‘catalyst’ is the substance which is responsible for that phenomenon. A Catalyst is a substance which can usually rev up the rate of a chemical reaction but remains unaltered itself (physically and chemically) after the reaction. Sometimes particular substances can also slow down the rate of a chemical reaction. Such type of substances are known as inhibitors (Encyclopædia Britannica, 2011). 2. Properties of catalysts i) Catalysts must be regenerated in their prior chemical and physical form after the completion of the reaction. ii) Appreciable effect to the chemical reaction can be produced by an immeasurably small amount of the catalyst. iii) Commencement of a chemical reaction cannot be initiated by a catalyst; a catalyst can only speed up the reaction rate. iv) Final state of a chemical reaction is not affected by catalyst. v) The rate of a forward as well as a backward reaction is affected to the same extent by the Catalyst. 3. Classification of catalysts Catalysts are classified in two different ways - depending on the phase and depending on the nature of the catalytic behaviour. According to the involvement in the phases, i.e., state of aggregation, catalysts are divided into three broad categories- (i) homogeneous catalyst (ii) heterogeneous catalyst and (iii) biocatalyst [Cavani &Feruccio, 1997; Hagen, 2006]. As the name suggests, homogeneous catalyst implies to the situation where the substrate and the catalyst are in the similar phase (gas or liquid). A very common example of a homogeneous catalysis is the conversion of carbon monoxide to carbon- di-oxide catalyzed by nitric oxide where both the substrate and catalyst are in gaseous phase. The fundamental advantages of homogeneous catalysis are that this type of catalysis is atom economic in respect to the other catalysis processes and additionally has higher selectivity in producing the desired product for its milder reaction condition. However, there are disadvantages associated with homogeneous catalysis as well. The problematic regarding homogeneous catalysis is the work up procedure of the reaction. The procedure for catalysis recovery is expensive, there is always a problem of waste management and above all a fair chance of contamination of the product by the catalyst is ever present. Heterogeneous catalysis is defined as the catalytic process where the catalysts and the substrate are in different phases. The catalysts can be in solid phase while the reactant is in gaseous or liquid phase. Hence the reactant molecules get adsorbed in the solid surface of the catalysts. The disadvantages of homogeneous catalysis can be overcome by heterogeneous procedures but these have their own limitations. For instance, controlling the temperature in the exothermic reactions is a crucial problem in heterogeneous catalysis. There is a growing interest of biocatalysis (enzyme catalysis) in the modern era of catalysis development. These enzymes are protein molecules that usually work in the cell or cell membrane. They assume a critical role in carrying out biological processes in physiological systems. Depending on their dimension of catalytic behavior catalysts can be classified in the following way: (i) Redox catalyst, (ii) Acid-base catalyst (iii) Polyfunctional catalysts and (iv) Specific catalyst for the transformation of carbon monoxide. Figure 1: Examples of Catalysts [Rothenberg, 208] Theoretical background In explaining the mechanism of catalyzed reaction, the active participation catalyst in a chemical process should be documented. In the all three types of catalytic process viz., homogeneous, heterogeneous or biocatalysis, the catalyst molecule binds to the reactant molecule to form an intermediate complex which often known as substrate. Subsequent disruptions of this substrate complex yield the product and regenerate the catalyst. Formation of the enzyme-reactant active complex always provides an alteration of the transformation pathway. This alteration leads to lowering in activation energy of the reaction which in turn permits the conversion of large amount of substrate in less time. e.g., the activation energy of unanalyzed ethyl ether decomposition is found to be 53000 cal where as it comes down to 24300 cal when catalyzed with I2 vapor. The rate of a chemical reaction obviously depends on the concentration of the catalyst. Development of catalysts Berzelius (1836) was the person to coin the term ‘catalysis’ (Laidler, 1982). Since then to the modern day, catalysis has gone through a very interesting history of development. The history began with the synthesis of sulphuric acid by using nitric oxide as catalyst. Later Humphrey Davy introduced platinum as a catalyst which could perform oxidation of coal gas efficiently. The thermodynamic explanation of catalysis was proposed by Ostwald in 1880 (Roberts, 2000). In 19th century the only necessity of utilizing catalysis was for higher production. Many transition metals such as zinc, nickel, iron, cobalt, vanadium oxide, titanium, palladium, platinum, chromium, cupper and many more were used for carrying out heterogeneous catalysis. At that time thin and flat films of metals were made by evaporating the metals onto the surface of the substrate. A wide variety of reactions were performed by employing this method. But surface defect and irreproducibility were the common problems associated with the method. That the single crystal (transition metals) heterogeneous catalysis model had definite surface structure and surface properties were well documented. Introduction of ultra high vacuum (UHV) in surface science of catalysts introduced a new era in the development of the process. It facilitated an in depth insight for researchers of the catalyst surface for explaining the atomic-molecular events. In addition to that, this method could be used to prepare ultrapure materials as required for the industrial process (Walczak,1996). Although the activity of the catalyst single crystal model turned out to be a useful tool to enhance perception, industrial processes in the 21st century were more concerned with the selectivity and stability of a catalyst because of the scarcity of raw materials and expensive waste management. Dispersion of the metal crystallites on a supported material (e.g metal oxide) produced new generation of heterogeneous catalysts. The metal atom cluster can vary from a few to large numbers. As the metal atoms interact with the solid support, their reactivity is somewhat different form that when the atoms are in cluster. Apart from the stabilization of the metallic particles, the solid support also influences the chemical reaction occurring in its surface. Platinum adsorbed on aluminum oxide is one of the examples of such type of catalysis which can reform n-hexane from gasoline. The selectivity and stability of a catalyst can be extended in various ways. Selective blocking of the active site of the catalyst’s surface is one of the promising techniques for achieving the required selectivity. For example, in reforming process of the n-hexane if gold is added to the single crystal of platinum then this catalyst displays selectivity towards the desired product. The cyclisation and dehydrocyclysation of n-hexane decreases where as an increase in the product 2- and 3-methylpentane is observed. The utilization of nanoparticles of both metal and the supports also reduces the possibility of byproducts in a chemical reaction since the metals and the support can no longer react with the basic reactant to such byproducts. Evaluation of the catalytic process Homogeneous catalysis Consider an example of a mechanistic view of homogeneous catalysis by nickel hydrides chelating with P-O groups for the formation of new carbon-carbon bond, i.e., to produce the desired alkanes. First, the alky-nickel complex is formed by the reaction of ethylene with metal hydride. Then the insertion of another molecule of ethylene occurs which thereby produces 1-butene. Subsequent ethylene insertion will then result in the comparable α-olefins. Figure 2: schematic diagram of homogeneous catalysis by nickel complex (Hagen, 2006) Heterogeneous catalysis: In homogeneous catalysis first the reactant molecules get adsorbed into the surface of the catalyst. Then the reactant molecules undergo chemical reaction to form new products which are then desorbed from the catalytic surface. An example of homogenous catalysis is the formation of ammonia from H2 and N2 in the surface of iron. First the nitrogen and hydrogen molecules get nearer to the surface of iron. Hydrogen molecules get dispersed on the surface of iron by successive dissociation and chemisorptions. Nitrogen molecules are adsorbed physically on the surface of iron and dissociates. Then the nitrogen atom reacts with hydrogen atom to from NH which subsequently converts to NH3 by another two steps of hydrogen atom addition. The final step is the desorbtion of ammonia molecule from the surface of iron (Swathi and Sebastian, 2008). Figure 3: Schematic diagram of heterogeneous catalysis by iron in the formation of ammonia from hydrogen and nitrogen, ( Swathi & Sebastian, 2008) Discussion According to Ostwald (1895) “a catalyst accelerates a chemical reaction without affecting the position of the equilibrium” (Hagen, 2006). Although it was assumed in early days that catalysts remain unaltered during the chemical transformation, it has been established later that catalysts enter into physical or chemical bonds with the substrates as the catalytic reaction progresses and then get detached from the product and regenerate as the reaction comes to an end. Catalysis is essentially a cyclical process. Theoretically, consumption of catalysts in a catalytic process should not occur. But practically catalysts are found to undergo competing reactions. As a consequence its deactivation occurs and eventually catalysts have to be replaced after a certain number of cycles. Apart form increasing the velocity of a chemical reaction, selectivity is the other dimension of catalyst which make it more significant in an industrial process. Another aspect of catalysts is its usage in green chemistry for prevention of pollution. Catalysts are efficiently used in reaching the goals of green chemistry which include prevention of waste of a chemical process, utilization of the entire reactant to produce the desired product, designing the chemical reaction in such a way so that the byproducts do not interfere with or affect human health, minimization of employment of the auxiliary substance (solvent) and renewal of raw materials whenever and wherever possible and to whatever extent (Anastas P T, 2000). Catalysts cater in the pursuit of attaining this goal successfully. Although the efficiency of modern day catalysts have come far since the days of inception, further research is needed for future catalysts which i) have more selectivity (100%) and produce ‘zero’ waste. ii) operate in moderate temperatures and pressures iii) renew and alternate the feedstock. Conclusion Modern era catalysis is not confined only to industrial chemical synthesis or in biological transformations. It distinctly puts it footprints in managing environmental degradation. A Catalytic converter in automobiles is a familiar example of this. Systematic study of the science of catalysis started two hundred years back and its importance has continued to flourish even in present times. The ever depleting energy resources are continuously changing from biomass to coal and then oil. Hence it has a profound effect in industrial processes as well as in the development of catalysts. Now-a-days since natural gas is evolving as an important energy source, future catalysts should aim to process the natural gases as well as other unconventional energy resources. References Anastas, P T, Bartlett L B, Kirchhoff M M, and Williamson T C (2000) “The role of catalysis in the design, development, and implementation of green chemistry”, Catalysis Today, 2000, 55, p 11–22 Cavani, F and Ferruccio T (1997) “Classification of industrial catalysts and catalysis for the petrochemical industry”, Catalysis Today, 34, 1997, p 269-279 Dautzenberg, F M and Angevine, P J (2002) “Encouraging innovation in catalysis”, Catalysis Today, 75, p 3–15 Encyclopædia Britannica (2011) "catalysis", Encyclopædia Britannica Online, www.britannica.com/EBchecked/topic/99128/catalyst, Accessed 17th Aug, 2011 Hagen J (2006) “Industrial Catalysis- A Practical Approach”, WILEY-VCH Verlag GmbH & Co. KGaA, p 1. Laidler, K J, and Meiser, J. H., (1982) “Physical Chemistry”, Benjamin/Cummings, p-423 Roberts M W(2000) "Birth of the catalytic concept (1800-1900)", Catalysis Letters, 2000, 67 (1), p 1–4 Rothenberg G (2008) “Catalysis”, WILEY-VCH Verlag GmbH & Co. KGaA, p 11 Swathi R S, and Sebastian K L (2008) “Molecular Mechanism of Heterogeneous Catalysis, the 2007 Nobel Prize in Chemistry” Resonance, June, p 548-560. Walczak, M M (1996) “A Laboratory Study of Heterogeneous Catalysis in Ultrahigh Vacuum”, The chemical educator, 1996, 1(2), 1-45. Read More