ChemInform Abstract: Generation and Characterization of Persistent Triplet Diphenylcarbene

Abstract

In order to stabilize triplet carbenes kinetically to extent that they are able to survive under normal conditions, attemps were made to protect the carbenic center of triplet diphenylcarbenes by introducing a series of substituents at the ortho position. Thus, diphenylcarbenes bearing chloro, methyl, t-butyl and bromo groups at the ortho positions were generated by photolysis of the corresponding diazo compounds and were characterized not only by product analysis but also by direct observation using spectroscopic means. Thus, decachloro- and decamethyldiphenylcarbenes were shown to be four and five orders of magnitude longer-lived than the parent diphenylcarbene, respectively. The replacement of all the o-CH3 groups of decamethyldiphenylcarbene with CD3 groups greatly decreased the intramolecular H abstraction rate from the o-methyl groups and resulted in the formation of triplet carbene having half-life over second in solution at room temperature for the first time. On the other hand, t-butyl group was shown to be disappointingly less effective protector of the triplet carbene due to its ability of H transfer to the carbenic center.2, 2', 4, 4', 6, 6'-Hexabromodiphenylcarbene was found to be very persistent in the triplet states. Thus, EPR signals of the carbene generated in single crystals of the diazomethane survived up to room temperature without loss of intensity for months. In 2-methyltetrahydrofuran matrix the carbene showed no decomposition even at 120 K where the samples were completely fluid. Replacement of bromine groups at 6 and 6' positions of the hexabromodiphenylcarbene with t-butyl groups resulted in increase in the lifetime of the triplet carbene under normal conditions to ca 20 seconds which is 108 times longer lived than the parent diphenylcarbene.