Metal-Diazo Radicals of α-Carbonyl Diazomethanes

Abstract

Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [Rh Radicals are highly reactive intermediates, long sought because of their importance in understanding reaction mechanisms and finding new reactions 1 . However, their extremely short lifetimes make their direct detection difficult and impede the development of radical chemistry. Carbenes, from nitrogen-releasing diazo compounds 2 , are the versatile active species in a broad range of reactions, including C-H functionalization 3,4 , carbon-carbon bond formation 5,6 , cyclopropanation 7,8 and polycarbene construction 9-11 . Usually, these reactions are carried out with metal catalysts, and the intermediates, generally considered to be metal-carbenes, have attracted much attention over the past decade [12][13][14][15][16] . Recent studies have shown that some metal-carbenes within paramagnetic metal catalysts (such as Co II , Rh II , and Ir II ) are carbon-centered metal-carbene radicals, not closed-shell metal-carbenes [17][18][19] . As such, their metal-diazo-adduct precursors are reasonably postulated to also possess radical character.Because of the vigorous N 2 evolution and high efficiencies of these reactions, few studies have reported such radicals [20][21][22][23] . The EPR signals of some diazo radical cations of diphenyldiazomethanes and their derivatives have been recorded after electrochemical oxidation or irradiation in Freon matrices [24][25][26] . In frozen toluene at 40 K, a carbon-centered Co-carbene radical was detected by EPR spectroscopy, along with several other paramagnetic cobalt species 17 , which complicated the spectral analysis. Diazo radicals may coexist with carbene radicals within reaction systems, and distinguishing them from one another is likewise difficult. Thus, the detection of metal-diazo radicals and their transformations into metal-carbene radicals remain unsolved challenges in diazo chemistry.To overcome these challenges, we employed room-temperature electron paramagnetic resonance (RT-EPR) spectroscopy with a spin-trapping technique 27 , which offers a convenient and powerful way to identify both diazo radicals and carbene radicals and to simultaneously investigate their behaviors. Because of the persistent radical effects of the redox-active metal and nitroxide 1 , the transient radical intermediates can be either directly detected by EPR or captured by radical traps (Fig. 1). In this paper, we report that the use of [RhCl(cod)] 2 (cod = 1,5-cyclooctadiene) allowed the RT-EPR detection of a rather stable dinitrogen radical of α -carbonyl diazomethane as a unique quintet signal. Density function theory (DFT) calculations support the EPR analyses, assigning a significant amount of spin density (97.2%) to the diazo moiety. To our knowledge, this work represents the first unequivocal spectroscopic confi...

Computation Sheds Insight into Iron Porphyrin Carbenes’ Electronic Structure, Formation, and N–H Insertion Reactivity

“…The SNOs of 1OS RC-EDA close reveal that iron donates electron density to EDA (Figure a). Recent reports similarly found spin density on the N 2 moiety of rhodium and copper alkyl diazoacetate systems. , This interaction leads to a significant Fe–C bond, of 2.23 Å. This electron donated from iron to EDA enters a C–N antibonding orbital, resulting in a significantly longer C–N distance in 1OS RC-EDA close than in 1CS RC-EDA far (Table S11).…”

Iron-Catalyzed Tunable and Site-Selective Olefin Transposition

“…13 Analysis of the reaction mixture in both Fe-catalyzed systems by time-and temperature-dependent electron paramagnetic resonance (EPR) studies showed no signals that would be expected to arise from radical species generated in a MHAT process (see the Supporting Information for details). Control experiments with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) or 5,5-dimethyl-1-pyrroline N-oxide (DMPO) 14 also did not support the possibility of related radical intermediates (see the Supporting Information for details).…”

Ultrasonic role to activate persulfate/chlorite with foamed zero-valent-iron: Sonochemical applications and induced mechanisms

“…The DMPO-X 1 (♥) signal with a single nitrogen (a N = 16.50 G (16.30–16.60), a triplet 1:1:1) [44] may contribute from the oxidation of DMPO itself, but according to study of Kondo et al [45] , peaks labeled “♥” may be due to DMPO•-H adducts showing a primary triplet (a N = 16.50 G) further split by two secondary hydrogens (⋄, a H β = 22.40 G). From Fig.…”