Kinetic Stabilization of Triplet Carbenes by Steric Protectors

这项研究是为了通过空间保护来稳定三重态卡宾。由于最近合成的稳定的单线态卡宾颠覆了长期以来的观点，即卡宾在室温下不够稳定，不足以在宏观上分离出来，因此稳定三重态卡宾成为一个具有挑战性的目标。立体保护是稳定三重态卡宾的理想方法，因为当保护基团引入到卡宾中心附近时，不仅可以阻止外部试剂对卡宾中心的作用，而且可以通过增加卡宾角度来实现热力学稳定。然而，由于卡宾对电子的贪婪胃口，即使在C-H键等弱电子源的情况下，它们也可以与之反应，当烷基被用作保护基团时，这一策略遇到了限制。例如，叔丁基已被成功地用作最有效的保护基团之一，但在保护三重卡宾中心方面甚至不如甲基有效。因此，开发一种立体拥挤但对三重卡宾不起反应的保护基团是至关重要的。我们制备了三(芳基)重氮甲烷，并用激光光解生成了相应的卡宾，发现三苯基卡宾对三联卡宾有很好的保护作用，某些芳基卡宾的寿命延长了约10^5倍。我们还合成了双[1，2，3，4，5，6，7，8-辛氢-1，4，5，8-二(乙基)蒽基]-重氮甲烷，并用激光光解生成了相应的卡宾。这是第一个在室温下在溶液中存活超过秒的全烃三重卡宾。

This research was undertaken to stabilize a triplet carbene kinetically by steric protection. The stabilization of a triplet carbene emerges as a challenging target since the recent syntheses of stable singlet carbenes have upset the long-standing view that carbenes are not stable enough to be isolated in macroscopic scale at room temperature.Steric protection is an ideal method of stabilizing the triplet since a protecting group when introduced near carbene center not only blocks the carbene center from external reagents but also results in thermodynamic stabilization by increasing the carbene angle. However, due to a voracious appetite of carbenes for electrons, with which they can react even with poor sources of electrons, such as C-H bonds, this strategy encounters limitation when alkyl groups are employed as protecting groups. For instance, the tert-butyl group, which has been successfully employed as one of the most effective protecting groups, was found to be less effective even than methyl in protecting triplet carbene center.In this light, it is crucial to develope a protecting group which is sterically congesting but unreactive toward triplet carbenes. Bicycloalkyl groups are very attractive since a bridgehead C-H bond is less succeptible to be abstracted and the bridging chains must act as protectors.We have prepared triptycyl (aryl) diazomethanes and generated the corresponding carbenes by laser flash photolysis to find that the triptycyl group is exceptionally effective in protecting triplet carbenes, increasing the lifetime of some arylcarbenes by a factor of ca.10^5.We have also prepared bis [1,2,3,4,5,6,7,8-octahydro-1,4,5,8-di (ethano) anthryl] -diazomethane and generated the corresponding carbene by laser flash photolysis to find that the diphenylcarbene protected by [2.2.2] bicyclooctyl group has halflife over 1 sencond. This is the first all-hydrocarbon triplet carbene surviving more than seconds in solution at room temperature.