Triplet diphenylcarbene protected by iodine and bromine groups

Abstract

AbstractDiphenyldiazomethane 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‐N2), has been synthesized. Triplet diphenylcarbene 31a 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‐N2 in a 2‐methyltetrahydrofuran (2‐MTHF) matrix at 77 K gives the UV–Vis spectrum of 31a, but ESR signals that can be assigned to a triplet species are not detected. The LFP of 1a‐N2 in a degassed benzene solution produces the transient absorption ascribable to 31a at 351 nm, which decays in a second‐order process with a rate constant of 0.17 s−1. The first half‐life of 31a is estimated to be 24 s. Triplet carbene 31a is trapped by oxygen to generate benzophenone oxide 1a‐O, with maximum absorbance at 415 nm, with a rate constant of 7.3 × 106 M−1s−1, and also by 1,4‐cyclohexadiene (CHD) to produce the diphenylmethyl radical (1a‐H2, λmax = 380 nm) with a rate constant of 39 M−1s−1. The carbene is also trapped by methanol with a rate constant of 0.30 M−2s−1. Steady‐state irradiation of 1a‐N2 in degassed benzene affords only a small amount of the phenanthrene 2a as an assignable product. Copyright © 2010 John Wiley & Sons, Ltd.