Aromatic Hydroxylation Reactivity of a Mononuclear Cu(II)−Alkylperoxo Complex

Abstract

The reactions of copper(II) complexes and hydrogen peroxide (H 2 O 2 ) have been studied extensively in order to gain insight into reactive intermediates involved in copper monooxygenases and copper oxidases as well as copper-catalyzed oxidation reactions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Several types of mononuclear and dinuclear copper/active-oxygen complexes have been reported, and their structures and physicochemical properties have been explored in detail. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, less is known about the intrinsic reactivity of the generated copper/ active-oxygen complexes.We herein report a new copper(II)-alkylperoxo species 2 X [2-hydroxy-2-hydroperoxypropane (HHPP) adduct], which is generated by the reaction of H 2 O 2 and copper(II) complex 1 X supported by the bis(pyridylmethyl)amine tridentate ligand containing msubstituted phenyl groups at the 6-positions of the pyridine rings (L X ) in acetone in the presence of triethylamine (NEt 3 ) (Scheme 1). The alkylperoxo intermediate 2 X undergoes an efficient aromatic ligand hydroxylation reaction, producing phenolate complex 4 X via another intermediate 3 X . Kinetic studies on the aromatic hydroxylation process are reported here together with spectral characterization of 2 X .Starting mononuclear copper(II) complexes 1 X supported by ligand L X (X ) NO 2 , Cl, H, Me, or OMe; Y ) ClO 4 -or H 2 O; S ) CH 3 CN or H 2 O) were prepared by mixing the ligands and Cu-(ClO 4 ) 2 ‚6H 2 O in acetone or acetonitrile (Figures S1-S4). 20 The reaction of 1 X and H 2 O 2 (1 equiv) was then examined in acetone at -70°C in the presence of triethylamine (1 equiv). Figure 1A shows a spectral change for the reaction of 1 NO 2 as a typical example, where intermediate 2 NO 2 exhibiting a characteristic absorption band at 420 nm ( ) 1350 M -1 cm -1 ) together with a weak d-d band at 630 nm ( ) 200 M -1 cm -1 ) becomes apparent. Spectroscopic titration for the generation of 2 NO 2 established that the stoichiometry of 1 NO 2 to H 2 O 2 was 1:1 ( Figure S5). Similar spectral changes were obtained with other ligand systems ( Figures S6-S9). A more detailed characterization of intermediate 2 X was carried out for 2 NO 2 since it showed higher stability than any other (as discussed further below).Intermediate 2 NO 2 generated with H 2 16 O 2 showed isotope sensitive Raman bands at 855, 823, 792, and 545 cm -1 when an acetone-d 6 (CD 3 COCD 3 ) solution of 2 NO 2 was excited with a 441.6 nm laser light ( Figure S10). These Raman bands shifted to 825, 803, 785, and 525 cm -1 , respectively, when H 2 18 O 2 was used ( Figure S10). The appearance of multiple Raman bands in the 800 cm -1 region and their associated isotope shifts (∆ν ) 30, 20, and 7 cm -1 ) as well as their intensity patterns are similar to those reported from resonance Raman studies of copper(II)-alkylperoxo (Cu II -OOR) and iron(III)-alkylperoxo (Fe III -OOR) complexes (R ) tert-butyl and cumyl), where such bands have been assigned as...

Generation, Characterization, and Reactivity of a CuII–Alkylperoxide/Anilino Radical Complex: Insight into the O–O Bond Cleavage Mechanism

“…Based on the analogy of the resonance Raman features with those of reported Cu II -alkylperoxide complexes, [8][9][10][11] a similar type of alkylperoxide complex may be generated in the present reaction. Thus, the band at 831 cm -1 is assigned to the O-O stretching vibrations, whereas the band at 604 cm -1 is due to the Cu-O stretching vibration.…”

Quantum mechanical calculations suggest that lytic polysaccharide monooxygenases use a copper-oxyl, oxygen-rebound mechanism

“…Thus, we consider C-H oxidation by a Cu(II)-oxyl ROS, dubbed the "oxyl mechanism" (Fig. 3) as this species has been previously predicted to exhibit a more strongly oxidative character than Cu(II)-superoxo (31,32 (20,33,34). From this system, we test two different geometries for oxidative attack at the C1 and C4 carbons to evaluate different enzyme-substrate poses (SI Appendix, Fig.…”

Copper–Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

“…180,184 An acetone molecule is functionalized to yield the novel species 13 , the characterization of which is described below (section 2.2.2). The 2-hydroxy-2-peroxypropane ligand was formed in an analogous way upon reaction of an iron(II) complex with H 2 O 2 in acetone.…”