2,2′,4,4′,6,6′-Hexabromodiphenylcarbene. The First Stable Triplet Carbene in Fluid Solution at Low Temperature and in the Crystal State at Room Temperature

摘要

Since the isolation of carbenes in the singlet state has been realized very recently, the stabilization of a triplet carbene emerges as a challenging target. The stabilization of a triplet carbene must be better accomplished by steric protection than thermodynamic effects since the latter strategy seems to result in the stabilization of the singlet state. Attempts have been made along this line whereby triplet diphenylcarbenes (DPC) were stabilized by introducing a series of substituents at the ortho positions. Among the protecting groups thus far examined, the methyl group is most effective in protecting the carbenic center; didurylcarbene is shown to be some 5 orders of magnitude longer-lived than the parent DPC. When two more methyl groups are introduced at the para positions, the carbenic center is more effectively blocked from external reagents due to buttressing effects; however, it is more easily trapped by the ortho methyl groups which are brought closer to the carbene center. The carbene therefore becomes shorter-lived. Bromine atoms appear to be effective as protecting groups because, while the van der Waals radius is similar to that of methyl, bromine