A Carbon Bond's Photo Finish

Abstract

Chemistry Photoinduced electron transfer is the initiating step in photosynthesis, so in a broad sense, it's been key to carbon-carbon bond formation since long before chemists (or humans more generally) arrived on the scene. Present-day synthetic organic chemistry rarely relies on that mechanism, though over the past several years its versatility has been coming into renewed focus. More specifically, catalysts such as tris(bipyridyl)ruthenium can easily lend out an electron after absorbing blue light, and substrates in the electron transfer chain often react differently than they would in a more conventional thermal context. DiRocco and Rovis apply this light-accelerated technique to one half of a dual catalytic cycle, oxidizing an amine to a more reactive iminium ion. A second catalyst—an asymmetrically substituted N-heterocyclic carbene—activates an aldehyde toward nucleophilicity in a dark portion of the cycle, ultimately linking the acyl carbon to the α carbon of the amine. The reaction couples a range of aldehydes to substituted tetrahydroisoquinolines, in many cases with enantiomeric excesses above 90%. J. Am. Chem. Soc. 134 , 10.1021/ja3030164 (2012).