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Presidential Green Chemistry Challenge: 1998 Academic Award (Trost)

Professor Barry M. Trost of Stanford University


The Development of the Concept of Atom Economy


Innovation and Benefits: Professor Trost developed the concept of atom economy: chemical reactions that do not waste atoms. Professor Trost's concept of atom economy includes reducing the use of nonrenewable resources, minimizing the amount of waste, and reducing the number of steps used to synthesize chemicals. Atom economy is one of the fundamental cornerstones of green chemistry. This concept is widely used by those who are working to improve the efficiency of chemical reactions.

Summary of Technology: The general area of chemical synthesis covers virtually all segments of the chemical industry—oil refining, bulk or commodity chemicals, and fine chemicals, including agrochemicals, flavors, fragrances, pharmaceuticals, etc. Economics generally dictates the feasibility of processes that are "practical". A criterion that traditionally has not been explicitly recognized relates to the total quantity of raw materials required for the process compared to the quantity of product produced or, simply put, "how much of what you put into your pot ends up in your product." In considering the question of what constitutes synthetic efficiency, Professor Barry M. Trost has explicitly enunciated a new set of criteria by which chemical processes should be evaluated. They fall under two categories—selectivity and atom economy.

Selectivity and atom economy evolve from two basic considerations. First, the vast majority of the synthetic organic chemicals in production derive from nonrenewable resources. It is self-evident that such resources should be used as sparingly as possible. Second, all waste streams should be minimized. This requires employment of reactions that produce minimal byproducts, either through the intrinsic stoichiometry of a reaction or as a result of minimizing competing undesirable reactions (i.e., making reactions more selective).

The issues of selectivity can be categorized under four headings—chemoselectivity (differentiation among various functional groups in a molecule), regioselectivity (locational), diastereoselectivity (relative stereochemistry), and enantioselectivity (absolute stereochemistry). The chemical community at large has readily accepted these considerations. In too many cases, however, efforts to achieve the goal of selectivity led to reactions requiring multiple components in stoichiometric quantities that are not incorporated into the product, thus creating significant amounts of waste. How much of the reactants end up in the product (i.e., atom economy) traditionally has been ignored. When Professor Trost's first paper on atom economy appeared in the literature, the idea generally was not adopted by either academia or industry. Many in industry, however, were practicing this concept without explicitly enunciating it. Others in industry did not consider the concept because it did not appear to have any economic consequence. Today, all of the chemical industry explicitly acknowledges the importance of atom economy.

Achieving the objectives of selectivity and atom economy encompasses the entire spectrum of chemical activities—from basic research to commercial processes. In enunciating these principles, Professor Trost has set a challenge for those involved in basic research to create new chemical processes that meet the objectives. Professor Trost's efforts to meet this challenge involve the rational invention of new chemical reactions that are either simple additions or, at most, produce low-molecular-weight innocuous byproducts. A major application of these reactions is in the synthesis of fine chemicals and pharmaceuticals, which, in general, utilize very atom-uneconomical reactions. Professor Trost's research involves catalysis, largely focused on transition metal catalysis but also main group catalysis. The major purpose of his research is to increase the toolbox of available reactions to serve these industries for problems they encounter in the future. However, even today, there are applications for which such methodology may offer more efficient syntheses.

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