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

Abstract

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-N2), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N2 in solution effectively generated the corresponding triplet diphenylcarbene 31a, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at room temperature. The UV-vis spectrum of 31a was obtained by irradiating 1a-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 31a. Transient absorption bands ascribable to 31a were observed by laser flash photolysis of 1a-N2 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-N2 in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a 31% yield. Triplet carbene 31a was also trapped by either oxygen (kO2 = 6.5 × 105 M−1·s−1) or 1,4-cyclohexadiene (kCHD = 1.5 M−1·s−1) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H2. 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, 31b.

Keywords

• triplet carbene
• diazo compound
• photolysis
• steric hindrance
• substituent effect