C−H Activation from Iron(II)‐Nitroxido Complexes

Abstract

The reaction of nitroxyl radicals TEMPO (2,2′,6,6′-tetramethylpiperidinyloxyl) and AZADO (2-azaadamantane-N-oxyl) with an iron(I) synthon affords iron(II)-nitroxido complexes (ArL)Fe(κ1‐TEMPO) and (ArL)Fe(κ2-N,O-AZADO) (ArL = 1,9-(2,4,6-Ph3C6H2)2-5-mesityldipyrromethene). Both high-spin iron(II)-nitroxido species are stable in the absence of weak C–H bonds, but decay via N–O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4-cyclohexadiene. Whereas (ArL)Fe(κ1-TEMPO) reacts to give a diferrous hydroxide [(ArL)Fe]2(μ-OH)2, the reaction of four-coordinate (ArL)Fe(κ2-N,O-AZADO) with hydrogen atom donors yields ferric hydroxide (ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C–H substrate and are consistent with rate-determining hydrogen atom transfer.

C−H Activation from Iron(II)‐Nitroxido Complexes

“…[8] Reaction of the iron(I) precursor ( Ar L)Fe [9] (1)w ith TEMPO or AZADO radical (TEMPO = 2,2',6,6'-tetramethylpiperidinyloxyl radical;A ZADO = 2-azaadamantane-Noxyl radical) in benzene at room temperature leads to an immediate color change from dark purple to bright pink and Single-crystal X-ray diffraction data and 57 Fe Mçssbauer spectroscopy support the formation of ( Ar L)Fe(k 1 -TEMPO) (2)f eaturing at hree-coordinate iron center (Figure 2a,c). [10] Bond metrics and angles (NÀO1 .441 (3) ,sum of angles on nitrogen = 3328 8)a re similar to those previously reported for an oxygen bound k 1 -TEMPO ligand on iron and are consistent with the reduction of TEMPO (N À O1 .283(9) ) to [TEMPO] À . [7,11] The 57 Fe Mçssbauer spectrum for ( Ar L)Fe-(AZADO) (3)f eatures as ingle quadrupole doublet with an isomer shift as well as aq uadrupole splitting that are significantly increased as compared to the values observed for 2 (d = 0.74 mm s À1 , j DE Q j= 0.59 mm s À1 for 2, d = 0.95 mm s À1 , j DE Q j= 1.51 mm s À1 for 3) ( Figure 2c).…”

Chemoselective Catalytic α-Oxidation of Carboxylic Acids: Iron/Alkali Metal Cooperative Redox Active Catalysis

“…On the basis of a series of mechanistic studies, we present a plausible catalytic cycle in Figure . Iron­(II)/ L5 complex I would first be oxidized by TEMPO, generating iron­(III) species II . Then, the carboxylate exchange would proceed to afford the iron carboxylate III with the concomitant formation of TEMPOH, which would act as an oxidant .…”

“…Iron(II)/L5 complex I would first be oxidized by TEMPO, generating iron(III) species II. 40 Then, the carboxylate exchange would proceed to afford the iron carboxylate III with the concomitant formation of TEMPOH, which would act as an oxidant. 31 The formed iron carboxylate III would be in equilibrium with di-or multinuclear iron species IV.…”

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N–O Reagent as the “Amino” Source and “Oxidant”

“…Betley and co-workers have reported high-spin Fe(II)-nitroxido species which are stable in the absence of weak C–H bonds, but decay via N–O bond homolysis undergoing C–H activation. 159 So far from our experimental results, spectroscopic analyses and theoretical calculations, it is evident that Int I converts to Int II , an Fe(III)-NH • radical species [high-spin Fe(III)-iminyl radical species]. Int II subsequently participates in the N -transfer reaction; the detailed mechanism of aminomethoxylation of styrenyl olefin has been delineated in the subsequent section via DFT calculations ( vide infra ).…”