Dalton Transactions PAPER Cite this: Dalton Trans., 2014, 43, 13578 Received 5th June 2014, Accepted 27th July 2014 DOI: 10.1039/c4dt01562b www.rsc.org/dalton 2-Quinoxalinol diamine Cu(II) complex: facilitating catalytic oxidation through dual mechanisms† Yuancheng Li, TaeBum Lee, Kushan Weerasiri, Tanyu Wang, Emily E. Buss, Michael L. McKee and Anne E. V. Gorden* The Cu(II) complex 1, Cu(II)-6-N-3,5-di-tert-butylsalicylidene-6,7-quinoxalinol-diamine, has been developed to address problems with current methods of catalytic oxidation using tert-butyl hydro- peroxide (TBHP). Complex 1 demonstrated an increased capability to utilize TBHP while limiting interfe- rence from free radical reactions and was demonstrated to be highly effective in the oxidations of a variety of olefins. Introduction Direct transformation of an allylic CH 2 group to a carbonyl is of great importance given the versatility of the resultant α,β-un- saturated enones or 1,4-enediones as synthetic starting materials or drug precursors. 1,2 Although oxidations using organic catalysts have been reported, 3 to perform these oxi- dations efficiently, most often the oxidant tert-butyl hydroper- oxide (TBHP) has been used in combination with a metal catalyst. 4 Limitations remain, including harsh reaction con- ditions, difficult purification procedures, toxic wastes, low functional group tolerance, and high costs. A key drawback of utilizing TBHP is the poor regioselectivity of the tert-butyl peroxy radical (t-BuOO • ) that can be produced during the course of the oxidation (eqn (1)). 4–6 Here, we report a new 6-N-3,5-di-tert-butylsalicylidene-6,7-quinoxalinol-diamine Cu(II) complex catalyst 1 that can be used with TBHP in a method that diminishes the potential for free radicals in allylic oxidation. ð1Þ Experimental Materials and methods All reagents were obtained commercially without further puri- fication. Yields reported are for isolated mass yields after chromatography. The leucine methyl ester hydrochloride, 1,5- di-fluoro-2,4-di-nitrobenzene (DFDNB), ammonium formate, 3,5-di-tert-butyl-2-hydroxybenzaldehyde, ammonium hydroxide (5.0 N), palladium on carbon (wet, 5%), tert-butyl hydropero- xide (TBHP, 5.0–6.0 M solution in decane) were purchased from Aldrich. The 18 O 2 was purchased from Icon in a 100 mL break- seal (gas at atmospheric pressure, 98 atom %). The 1 H NMR and 13 C NMR spectra were recorded on 250 or 400 MHz instru- ments as solutions in CDCl 3 as indicated; chemical shifts (δ) are reported in ppm relative to Me 4 Si. Reaction progress was monitored by thin-layer chromatography (TLC) using 0.25 mm silica gel precoated plates; spots were detected with UV light and revealed with I 2 . Chromatographic purifications were per- formed using Fisher (60 Å, 70–230 mesh) silica gel. HRMS data were collected with electron spray ionization. All UV-Vis data was collected using a Cary 50 UV-Vis spectrophotometer with a xenon lamp with an equipment range from 200 to 1000 nm. Atomic absorption spectroscope (Varian AA240), its software (AA240FS) and hollow cathode lamp (HLC; Ni 232.0 nm, optimum working range: 0.1–20 mg L -1 ; Mn 279.5 nm, optimum working range: 0.02–5 mg L -1 ; Cu 324.8 nm, optimum working range: 0.03–10 mg L -1 ) are from Varian Inc. † Electronic supplementary information (ESI) available: Experimental details, cyclic voltammetry, GC-MS spectra, and calculation details. See DOI: 10.1039/ c4dt01562b Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849- 5319, USA. E-mail: anne.gorden@auburn.edu 13578 | Dalton Trans. , 2014, 43, 13578–13583 This journal is © The Royal Society of Chemistry 2014