Triplet diphenylcarbene protected by iodine and bromine groups

Abstract

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‐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.

Generation and reactivities of triplet diphenylcarbenes protected by bulky groups as para substituents

“…The different tendencies observed for zfs and T d can be explained in terms of the steric effect of the ‘wings’. We have reported that the triplet diphenylcarbenes decay in 2‐MTHF at low temperature by abstracting a hydrogen from the solvent to form the corresponding diphenylmethyl radicals . Thus, large ‘wings’ would retard the approach of 2‐MTHF to the carbene center, resulting in the high T d s.…”

“…The strategy, however, encounters a limitation when bulky substituents are employed as protecting groups. Persistent triplet diphenylcarbenes are generated by photolysis of the nitrogen precursors, diphenyldiazomethanes, which are almost always synthesized from the corresponding benzhydrols via the carbamate pathway . However, synthesis of a carbamate derivative having substituents bulkier than a trifluoromethyl group at the ortho position was not successful.…”

“…In benzene, most sterically hindered diphenylcarbenes decay by dimerization at the carbene center to give either tetraphenylethenes or phenanthrenes, which are formed by photocyclization of the ethenes followed by dehalogenation . If dimerization can be prevented by the para substituent, the carbene might become more persistent.…”

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

“…In addition, according to theoretical calculations [17,18], the length of the C-I bond (210 pm) of iodobenzene is greater than that of the C-Br bond (189 pm) in bromobenzene. Recently, we generated and characterized a triplet diphenylcarbene bearing two iodine and two bromine groups at the ortho positions, (2,6-dibromo-4- tert -butylphenyl)(4- tert -butyl-2,6-diiodophenyl)carbene 3 1b [19] (Scheme 1). This species was found to be a persistent triplet carbene, but contrary to our expectations, its half-life ( t 1/2 = 24 s) was only 1.5 times longer than that of 3 1c .…”

Computational methods for contemporary carbene chemistry

“…Although stabilization of singlet carbenes is now well accomplished,9 effective means for the stabilization of triplet carbenes yet remain to be put to test. A common concept is the design of sterically crowded arylcarbenes, e.g., 2,6‐dibromo‐4‐ tert ‐butyl‐2′,6′‐bis(trifluoromethyl)‐4′‐isopropyldiphenylcarbene ( v ) 10–14. This particular approach goes along with considerate spin delocalization across the molecule, thus altering the distinctive reactivity pattern and electronic configuration of a triplet carbene.…”