Stabilization of Triplet Polynuclear Aromatic carbene

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Triplet di(9-anthryl)carbene has almost completely linear and perpendicular structure, suggesting that unpaired electrons are extensively delocalized and hence the carbene center is protected by four perihydrogens. Thus, the carbene was expected to be persistent. Contrary to this expectation, its lifetime in a degassed benzene at room temperature is only 0.5μs, which is as short as that of triplet diphenylcarbene.This is because it decays by coupling reactions at 10,10'-positions where spin is delocalized. In order to inhibit the coupling reactions, we have generated dianthrylcarbene having two phenyl groups at 10,10'-positions and indicated that it was a dramatically long-lived triplet carbene (t1/2 = 40 min). We found triplet dianthrylcarbene(2a) having 2,6-dimethyl-4-tert-butylphenyl groups at 10,10'-positions was much more persistent with half-life of 50 hours. Since the carbene(2a) decayed by second-order kinetics, it appeared to decay by dimerization at the carbene carbon. In order to inhibit the dimerization, triplet dianthrylcarbene(2b) bearing four bromo groups at 2,7-positions on anthracene rings was generated by photolysis of the corresponding diazo compound and the half-life(t1/2 =6.5 min) of 2b was determined to be shorter than that of dianthrylcarbene(2a).