Kinetic Stabilization of Triplet Carbenes by Steric Protectors

本研究旨在通过空间位阻保护来稳定三重态卡宾。三重态卡宾的稳定化成为一个具有挑战性的目标，因为最近稳定的单重态卡宾的合成已经打乱了长-重态卡宾的稳定化。空间保护是稳定三重态的理想方法，因为当在卡宾中心附近引入保护基时，不仅阻断卡宾中心与外部试剂的接触，而且导致热力学稳定通过增加卡宾角。然而，由于卡宾对电子的贪婪胃口，它们甚至可以与差的电子源（例如C-H键）反应，当烷基用作保护基时，这种策略遇到限制。例如，叔丁基是一种非常有效的保护基，但在保护三重态卡宾中心时，其效果甚至不如甲基，因此，开发一种空间上拥挤但对三重态卡宾不反应的保护基是至关重要的。双环烷基是一个非常有吸引力的基团，因为桥头C-H键不易被夺取，桥链必须起保护作用.我们制备了三蝶基（芳基）重氮甲烷并通过激光闪光光解生成相应的卡宾，发现三蝶基在保护三重态卡宾方面特别有效，使某些芳基卡宾的寿命增加了约10^5倍。我们还制备了双[1，2，3，4，5，6，7，8-八氢-1，4，5，8-二（乙蒽基）-重氮甲烷，并通过激光闪光光解生成相应的卡宾，发现[2.2.2]双环辛基保护的二苯基卡宾的半衰期超过1秒。这是第一个在室温下在溶液中存活超过秒的全烃三重态卡宾。

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.