Silver Spurs Cycloadditions

Nanoparticles have been all the rage in chemistry for years, but synthetic organic chemists have been slow to take up the trend. Now, John A. Porco Jr. and coworkers at Boston University are reporting the first example of a Diels-Alder cycloaddition catalyzed by metal nanoparticles (J. Am. Chem. Soc., DOI: 10.1021/ja102482b). Porco’s group was working on the total synthesis of panduratin A—a natural product with promising anticancer, anti-HIV, and anti-inflammatory activities—when it ran into trouble getting the key Diels-Alder reaction to work. The team screened metal salts in combination with borohydride as catalysts and found that AgBF₄ worked best. Several observations led the researchers to conclude that they were generating silver nanoparticles in solution, so they created a heterogeneous catalyst by fixing the in situ-generated nanoparticles onto silica gel. This material catalyzed the critical Diels-Alder reaction in 85% yield, producing predominately the desired endo isomer. The reaction was conducted in air without the need for excluding water or oxygen. Furthermore, the catalyst can be stored on the benchtop for months and recycled without significant loss of activity.

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