Diphenylcarbene Protected by Four ortho-Iodine Groups: An Unusually Persistent Triplet Carbene.

Diphenyldiazomethane with four iodine groups at the positions and two -butyl groups at the positions, i.e., bis(4--butyl-2,6-diiodophenyl)diazomethane (-N₂), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of -N₂ in solution effectively generated the corresponding triplet diphenylcarbene ³, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at room temperature. The UV-vis spectrum of ³ was obtained by irradiating -N₂ in a 2-methyltetrahydrofuran matrix at 77 K. The ESR spectrum showed no triplet carbene signals, while a radical species was observed at the anticipated temperature of the decomposition of triplet carbene ³. Transient absorption bands ascribable to ³ were observed by laser flash photolysis of -N₂ in a degassed benzene solution and decayed very slowly with a second-order rate constant (2ε) of 5.5 × 10³·s¹. Steady-state irradiation of -N₂ in degassed benzene afforded 9,10-diarylphenanthrene derivative in a 31% yield. Triplet carbene ³ was also trapped by either oxygen ( = 6.5 × 10⁵ M¹·s¹) or 1,4-cyclohexadiene ( = 1.5 M¹·s¹) to afford the corresponding ketone -O or the diarylmethane -H₂. The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two -iodo and two -bromo groups, ³.