Cooperativity Tames Reactive Catalysts

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Science  19 Feb 2010:
Vol. 327, Issue 5968, pp. 965-966
DOI: 10.1126/science.1186764

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When chemists use more than one catalyst to speed up a chemical reaction, it is almost always the case that overall increase in reaction rate is greater than that for either catalyst alone. For example, for acid-catalyzed reactions, combinations of “designer acids”—combinations of organic and inorganic molecules bearing acidic groups—can result in higher reactivity, selectivity, and versatility than the use of individual catalysts (1). However, there can be advantages to adding a second catalyst that actually slows down the rate of an already catalyzed reaction, especially if the goal is suppressing unwanted side reactions. This strategy is similar to that used by a coach of a relay team, who may pair a slower but sure-handed runner with a faster but less agile one. Speed is sacrificed to avoid losing by dropping the baton. On page 986 of this issue, Xu et al. (2) report the use of two catalysts that cooperate to control stereochemistry, that is, to pick between different spatial arrangements of the same product. The initial catalyst is fast but produces an unwanted mixture of stereoisomers; adding the second catalyst suppresses one of these pathways and selects for the desired product.