Green Chemistry Principle #9
Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
Contributed by Roger A. Sheldon, PhD, Emeritus Professor of Biocatalysis and Organic Chemistry, Delft University of Technology and CEO of CLEA Technologies B.V.
A primary goal of green chemistry is the minimization or preferably the elimination of waste in the manufacture of chemicals and allied products: “prevention is better than cure”. This necessitates a paradigm shift in the concept of efficiency in organic synthesis, from one that is focused on chemical yield to one that assigns value to minimization of waste. What is the cause of waste? The key lies in the concept of atom economy: “synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product”. In the reaction scheme we compare, for example, the reduction of a ketone to the corresponding secondary alcohol using sodium borohydride or molecular hydrogen as the reductant. Reduction with the former has an atom economy of 81% while reduction with the latter are 100% atom economic, that is everything ends up in the product and, in principle, there is no waste.
Unfortunately, hydrogen does not react with ketones to any extent under normal conditions. For this we need a catalyst such as palladium-on-charcoal. A catalyst is defined as “a substance that changes the velocity of a reaction without itself being changed in the process”. It lowers the activation energy of the reaction but in so doing it is not consumed. This means that, in principle at least, it can be used in small amounts and be recycled indefinitely, that is it doesn’t generate any waste. Moreover, molecular hydrogen is also the least expensive reductant and, for this reason, catalytic hydrogenations are widely applied in the petrochemical industry, where the use of other reductants is generally not economically viable. It is only in the last two decades, however, following the emergence of green chemistry, that catalysis has been widely applied in the pharmaceutical and fine chemical industries, with the goal of minimizing the enormous amounts of waste generated by the use of stoichiometric inorganic reagents. This involves the use of the full breadth of catalysis: heterogeneous, homogeneous, organocatalysts and, more recently, Nature’s own exquisite catalysts: enzymes. The latter are particularly effective at catalyzing highly selective processes with complex substrates under mild conditions and, hence, are finding broad applications in the pharmaceutical and allied industries. Moreover, they are expected to play an important role in the transition from a chemical industry based on non-renewable fossil resources to a more sustainable bio-based economy utilizing renewable biomass as the raw material, yet another noble goal of green chemistry.
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