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

Diphenyldiazomethane with four iodine groups at the ortho positions and two tert-butyl groups at the para positions, i.e., bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N(2)), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N(2) in solution effectively generated the corresponding triplet diphenylcarbene (3)1a, 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)1a was obtained by irradiating 1a-N(2) 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)1a. Transient absorption bands ascribable to (3)1a were observed by laser flash photolysis of 1a-N(2) in a degassed benzene solution and decayed very slowly with a second-order rate constant (2k/εl) of 5.5 × 10(−)(3)·s(−)(1). Steady-state irradiation of 1a-N(2) in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a 31% yield. Triplet carbene (3)1a was also trapped by either oxygen (k(O2) = 6.5 × 10(5) M(−)(1)·s(−)(1)) or 1,4-cyclohexadiene (k(CHD) = 1.5 M(−)(1)·s(−)(1)) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H(2). The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two ortho-iodo and two ortho-bromo groups, (3)1b.