A Stable Route to Selectivity

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Science  23 Nov 2012:
Vol. 338, Issue 6110, pp. 1010
DOI: 10.1126/science.338.6110.1010-d

In general, the purpose of catalysis is to speed up reactions that otherwise proceed too slowly. Asymmetric catalysis is something of a special case though: When the catalyst's role is to bias a reaction toward one particular product isomer, problems arise if the background reaction is too fast on its own. This circumstance limited a prospectively promising route to chiral allylic amines. Specifically, decades-old chemistry established that adducts of terminal olefins and imido-sulfur compounds quickly rearrange to form carbon-nitrogen bonds. Bao and Tambar realized that stabilization of such adducts could slow down the bond-forming event sufficiently for a catalyst to dictate stereoselectivity. Benzenesulfonyl sulfurdiimide proved the optimal reagent, affording an adduct so slow to rearrange that it persisted for days below 0°C. The authors could then introduce a palladium catalyst coordinated by a chiral ligand to coax the adducts toward allylic amine products with enantioselectivities exceeding 90%. The reaction tolerated electrophilic substituents on the olefin framework, such as aldehydes, nitriles, and halides.

J. Am. Chem. Soc. 134, 10.1021/ja307851b (2012).

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