Triplet diphenylcarbene protected by iodine and bromine groups

Diphenyldiazomethane having two iodine and two bromine groups at the ortho positions, (2,6‐dibromo‐4‐ tert ‐butylphenyl)(2,6‐diiodo‐4‐ tert ‐butylphenyl)diazomethane ( 1a ‐N 2 ), has been synthesized. Triplet diphenylcarbene 3 1a is generated from the precursor and is characterized by UV–Vis spectroscopy at low temperature and laser flash photolysis (LFP) techniques at room temperature. Irradiation of 1a ‐N 2 in a 2‐methyltetrahydrofuran (2‐MTHF) matrix at 77 K gives the UV–Vis spectrum of 3 1a , but ESR signals that can be assigned to a triplet species are not detected. The LFP of 1a ‐N 2 in a degassed benzene solution produces the transient absorption ascribable to 3 1a at 351 nm, which decays in a second‐order process with a rate constant of 0.17 s −1 . The first half‐life of 3 1a is estimated to be 24 s. Triplet carbene 3 1a is trapped by oxygen to generate benzophenone oxide 1a ‐O, with maximum absorbance at 415 nm, with a rate constant of 7.3 × 10 6  M −1 s −1 , and also by 1,4‐cyclohexadiene (CHD) to produce the diphenylmethyl radical ( 1a ‐H 2 , λ max  = 380 nm) with a rate constant of 39 M −1 s −1 . The carbene is also trapped by methanol with a rate constant of 0.30 M −2 s −1 . Steady‐state irradiation of 1a ‐N 2 in degassed benzene affords only a small amount of the phenanthrene 2a as an assignable product. Copyright © 2010 John Wiley & Sons, Ltd.