ESR study on the .sigma.- and .pi.-radical cations formed by one-electron oxidation of phenyldiazomethanes

Diarylcarbene Cation Radicals: Generation and Chemical Reactivity in Solution

“…Under these reaction conditions, the reported spectra of diphenyl diazo cation radical, 2g,h λ max = 650 nm, can be obtained. Methyl aromatic substitution results in a red shift of λ max for 1 • + and 2 • + , as has been observed for diarylmethyl cation, radical, and carbene.…”

“…Oxidation of carbenes should significantly enhance their electrophilicity and eliminate the chemical ambiguities observed for carbene singlet and triplet spin states. Although recent work has demonstrated that carbene cation radicals can be generated in solution by the unimolecular loss of nitrogen from the corresponding oxidized diazo precursor, their chemical reactivity has not been well studied. In this context, we recently reported the stereospecific cyclopropanation of diarylcarbene cation radicals with alkenes …”

“…Unfortunately, only relative reactivities could be obtained because of the relatively slow loss of nitrogen from the oxidized diazo precursors. However, recent experiments demonstrated that the diphenylcarbene cation radical could be generated in a matrix by the photolysis of the diphenyldiazomethane cation radical. 2g,h We wish to now report the generation of diarylcarbene cation radicals in solution by double laser flash photolysis and the measurement of their absolute reactivity with nucleophiles and radicalophiles.…”

“…Photoinduced electron transfer from 1 to NTBBQ following the initial 355-nm pulse yields the bis( p -methylphenyl)diazomethane cation radical, 1 • + (λ max = 680 nm) and NTBBQ • (λ max = 520 nm). ,,, Irradiation of 1 • + with the delayed second pulse results in rapid, <10 ns, photoinduced unimolecular loss of nitrogen to form the bis( p -methylphenyl)carbene cation radical, 2 • + (λ max = 410 nm) 2g,h, Under similar experimental conditions, photolyses of bis( p -chlorophenyl)- and bis( p -methoxyphenyl)diazomethane cation radicals, λ max = 675 and 725 nm, respectively, yield bis( p -chlorophenyl)- and bis( p -methoxyphenyl)carbene cation radicals, λ max = 415 and 435 nm, respectively.

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“…The high radical reactivity of 2 • + , which is unusual for many cation radicals, may be rationalized by its structure. Experiments and calculations on diarylcarbene cation radicals suggest that the ground state electronic structure is σ • /π + , in which the odd electron is in a σ orbital with an empty π orbital. 2g, This spatial separation of spin and charge for 2 • + apparently results in high radical reactivity, whereas for many cation radicals, in particular π-cation radicals, the lack of separation renders them inert toward O 2 and hydrogen atom donors.…”