Spectroscopic evidence of a free-radical mechanism in the reduction of Schiff bases by formic acid

The unprecedented direct detection of the free radical intermediate 7 in the reduction of Schiff bases 2 to the corresponding amines by formic acid using the EPR technique is reported. Monitoring the same reaction by NMR, line broadenings and signal shifts are observed, while the further evolution to products is triggered off by heating. No product formation or line broadening/shifts of the imine derivatives take place upon addition of the isoamyl nitrite spin trapper, which interrupts the chain process capturing the free radical 7. Single electron species have also been detected in the similar reduction of the benzylidene-bis-piperidine 11 by formic acid.