Reactivity
study of novel metal carbene complexes can offer new
opportunities in catalytic carbene transfer reactions as well as in
other synthetic protocols. Metal complexes with quinoid carbene (QC)
ligands are assumed to be key intermediates in a variety of metal-catalyzed
QC transfer reactions using diazo quinones, which demands development
of the chemistry of QC transfer of well characterized metal–QC
complexes. Herein we report the isolation and QC transfer of ruthenium
porphyrins [Ru(Por)(QC)] which contribute the first examples of (i)
structurally characterized metal–QC complex (by X-ray crystallography)
and (ii) isolated metal–QC complex that undergoes QC transfer
reaction. The complexes [Ru(Por)(QC)] were prepared from reaction
of [Ru(Por)(CO)] with diazo quinones and exhibited dual reactivity,
i.e., hydrogen atom transfer (HAT) as well as QC transfer. The stoichiometric
QC transfer reactions from these Ru–QC complexes to nitrosoarenes
(ArNO) afforded nitrones in up to 90% yield, and the corresponding
catalytic reactions were also developed. Both the stoichiometric and
catalytic reactions for a series of QC ligands bearing electron-donating
and -withdrawing substituents showed a reverse substituent effect
on the QC transfer reactivity. Complexes [Ru(Por)(QC)] are also reactive
toward C–H and X–H (X = N, S) bonds and can catalyze
aerobic oxidation of 1,4-cyclohexadiene; their stoichiometric HAT
reactions with unsaturated hydrocarbons gave product yields of up
to 88%. The unique dual reactivity and electronic feature of [Ru(Por)(QC)]
were studied by spectroscopic means and density functional theory
(DFT) calculations.