Stabilization of Triplet Polynuclear Aromatic carbene

三重态二（9-蒽基）卡宾具有几乎完全线性和垂直的结构，这表明未配对电子广泛离域，因此卡宾中心受到四个周围氢原子的保护。因此，碳被认为是持久的。与预期相反，在室温下，它在脱气苯中的寿命仅为0.5μs，与三态二苯卡宾的寿命一样短。这是因为它通过在10,10'位的偶联反应而衰减自旋离域。为了抑制偶联反应，我们合成了在10,10'位置上有两个苯基的二甲基卡宾，并表明它是一个寿命极长的三重卡宾（t1/2 = 40 min）。我们发现在10,10'位上有2,6-二甲基-4-叔丁基苯基基团的三联体二乙烯基（2a）更持久，半衰期为50小时。由于卡宾（2a）是通过二级动力学衰变的，它似乎是通过在卡宾碳上的二聚化衰变的。为了抑制二聚化作用，通过重氮化合物的光解反应生成了蒽环上2,7位上有4个溴基团的三联体二烷基羰基（2b），并测定了2b的半衰期（t1/2 =6.5 min）短于二烷基羰基（2a）。

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).