Triplet Diphenylcarbenes Protected by Trifluoromethyl and Bromine Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature

Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the
ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1−N2) and bis(2-bromo-4-phenyl-6-trifluoromethylphenyl)diazomethane (S-1−N2), are prepared. Triplet diphenylcarbenes (U-31 or S-31) are generated from
those precursors and are characterized by ESR, UV/vis spectroscopy at low temperature, as well as time-resolved UV/vis spectroscopy at room temperature. Those carbenes are shown to be at least 2 orders of
magnitude less reactive than the most stable triplet diphenylcarbene thus far known, that is, bis(2,6-dibromo-4-phenylphenyl)carbene. It has been also shown that S-31 is significantly more stable than U-31 even though
both have the same two kinds of substituents. It is suspected that the perpendicular alignment of the two
most bulky groups is a more effective way to shield the carbenic center than the planar one. By this way,
triplet substituted diphenylcarbene surviving nearly a day in solution at room temperature is realized for
the first time.