Organic Reducing Agent Sets New Record

Organic electron donors with exceptionally negative redox potentials have emerged as new reagents in organic synthesis to complement traditional metal-based reducing agents. Chemists led by John A. Murphy of the University of Strathclyde, in Scotland, and C. Adam Dyker of the University of New Brunswick, in Canada, have pushed this class of compounds to a new level by designing and synthesizing a substituted bispyridinylidene that is the first reagent strong enough to reduce recalcitrant sulfonamides and malononitriles without additional light-driven photoexcitation (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201505378). The molecule features four strongly π-donating iminophosphorano substituents on the bispyridinylidene skeleton. It has a record redox potential of –1.70 V versus a saturated calomel electrode, surpassing the previous record by about 0.2 V. The team can isolate the compound but found that it can be conveniently generated in situ for multigram-scale reactions. The researchers show that the two-electron donor is capable of reducing aryl halides to arenes. They also found that it’s effective in reductive S–N bond cleavage of arenesulfonamides, an important step in removing tosyl protecting groups during multistep syntheses. The reagent also can remove cyano groups from malononitriles to form mononitriles, a process that usually requires toxic tributyltin hydride and the radical initiator AIBN.