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Synthesis

Catalyst Promotes Acyl Transfer Three Ways

Palladacycle enhances reaction kinetics by a factor of 100 million

by Stu Borman
December 12, 2011 | APPEARED IN VOLUME 89, ISSUE 50

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Palladacycle catalyst accelerates methanolysis by binding substrate, delivering methoxide to it, and aiding departure of a leaving group.
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Palladacycle catalyst accelerates methanolysis by binding substrate, delivering methoxide to it, and aiding departure of a leaving group.

Computation and experiment have been used to design a catalytic process with a three-part mechanism that makes a metal-promoted acyl transfer reaction occur 108 times faster than the uncatalyzed reaction. Nicholas J. Mosey, R. Stan Brown, and coworkers at Queen’s University, in Kingston, Ontario, developed the unusual catalyst, a palladacycle, to promote the methanolysis of a thioamide (J. Am. Chem. Soc., DOI: 10.1021/ja209605r). They report that the catalyst performs three essential roles: substrate binding; nucleophilic delivery of methoxide to form a stabilized version of a tetrahedral intermediate, which would otherwise be an unobservable high-energy intermediate; and palladium assistance (via transient association with nitrogen) for departure of an amide leaving group, usually the most difficult step in amide or thioamide hydrolysis reactions. “The demonstration of this multifaceted catalytic role suggests how biological catalysts might operate in promoting these sorts of difficult reactions,” Brown says. Brown, Mosey, and coworkers are currently studying the palladacycle-catalyzed reactions of different thioamides, evaluating solvent effects and performing theoretical simulations to better understand the reaction’s large rate acceleration.

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