Kinetic Stabilization of Triplet Carbenes by Steric Protectors

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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.