Triplet Diphenylcarbene Has Staying Power

A triplet diphenylcarbene with unprecedented stability has been reported by chemists at Japan's Mie University (J. Am. Chem. Soc., published online Dec. 31, 2005, dx.doi.org/10.1021/ja056575j).

Katsuyuki Hirai, Hideo Tomioka, and coworkers created an organic diradical with unpaired electrons that can survive almost 24 hours in solution by adding large yet unreactive substituents to the molecule’s ortho positions. Hirai and Tomioka's group characterized the triplet diphenylcarbene and found that the molecule was "at least two orders of magnitude less reactive than the most stable triplet diphenylcarbene thus far known." After nearly a day in solution at room temperature, traces of the triplet carbene were still detectable spectroscopically.

The researchers attribute the molecule's remarkable stability to a perpendicular arrangement of the CF₃ groups. They suspect this orientation shields the carbene center. The Mie group also prepared an unsymmetrical positional isomer of the molecule with like substituents on the same ring. While less stable than its symmetrical isomer, this carbene is nevertheless more stable than previously reported diphenylcarbenes.