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. 2013 Nov 1;3(11):2599-2605.
doi: 10.1021/cs400689a.

Copper/TEMPO-Catalyzed Aerobic Alcohol Oxidation: Mechanistic Assessment of Different Catalyst Systems

Jessica M Hoover  1 , Bradford L Ryland  1 , Shannon S Stahl  1
Affiliations

Copper/TEMPO-Catalyzed Aerobic Alcohol Oxidation: Mechanistic Assessment of Different Catalyst Systems

Jessica M Hoover et al. ACS Catal. .

Abstract

Combinations of homogeneous Cu salts and TEMPO have emerged as practical and efficient catalysts for the aerobic oxidation of alcohols. Several closely related catalyst systems have been reported, which differ in the identity of the solvent, the presence of 2,2'-bipyridine as a ligand, the identity of basic additives, and the oxidation state of the Cu source. These changes have a significant influence on the reaction rates, yields, and substrate scope. In this report, we probe the mechanistic basis for differences among four different Cu/TEMPO catalyst systems and elucidate the features that contribute to efficient oxidation of aliphatic alcohols.

Keywords: TEMPO; aerobic; alcohol oxidation; copper; kinetics; mechanism.

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Figures

Figure 1
Figure 1
Cyclic voltammograms of CuCl (± bpy) and TEMPO in DMF (dashed blue line) and MeCN (solid red line). See Supporting Information for details.
Figure 2
Figure 2
The oxidation of CyCH2OH by CuCl with (formula image) and without (formula image) bpy in DMF. Standard conditions: 0.4 M CyCH2OH, 0.04 M TEMPO, 0.04 M CuCl, 0.04 M bpy, 600 torr O2, 27 °C.
Figure 3
Figure 3
Rates of oxidation of PhCH2OH by CuIOTf/bpy/NMI/TEMPO with the addition of H2O. Standard conditions: 0.2 M PhCH2OH (0.5 mmol), 10 mM CuI(OTf), 10 mM bpy, 10 mM TEMPO, 20 mM NMI, 600 torr O2, 27 °C. Oxidation of PhCH2OH by CuIIBr2/bpy/KOtBu/TEMPO in MeCN/H2O (formula image) included for comparison.
Figure 4
Figure 4
(A) X-ray structure of [(bpy)Cu(OH)]2(OTf)2, which crystallizes as a dimer of dimers (see Supporting Information for details). The triflate counterions are omitted for clarity. (B) The oxidation of CyCH2OH by CuIOTf (formula image) and [(bpy)Cu(OH)]2(OTf)2. (formula image). Data were obtained by monitoring pressure changes during catalytic turnover. Reaction conditions: 10 mM (bpy)Cu, 10 mM TEMPO, 20 mM NMI: 0.2 M CyCH2 OH, 1 atm O2, 2.5 mL MeCN, 27 °C.
Figure 5
Figure 5
Dependence of (I) CuI(OTf)/bpy/TEMPO/NMI, (data adapted from Ref. 8). (II) CuII(OTf)2/bpy/TEMPO/DBU, and (III) CuII(OTf)2/bpy/TEMPO/NMI catalyzed alcohol oxidation on initial (A) dioxygen pressure, (B) [Cu/bpy], (C) [PhCH2OH], and (D) [TEMPO]. The curves in B are derived from a fit of the data to rate = c1[Cu]2/(c2 + c3[Cu]). The curves in C are derived from a nonlinear least-squares fit of the data to rate = c1[alcohol]/(c2 + c3[alcohol]). Standard reaction conditions: 0.2 M PhCH2OH, 10 mM [Cu], 10 mM bpy 20 mM NMI or DBU, 10 mM TEMPO, 600 torr O2, 2.5 mL MeCN, 27 °C.
Figure 6
Figure 6
Dependence of CuIOTf/bpy/TEMPO/NMI catalyzed benzyl alcohol oxidation on added (A) NMIH+ OTf− and (B) DBUH+ OTf−. The reaction catalyzed by CuII(OTf)2/bpy/-TEMPO/NMI (formula image) is included in A for comparison. Rates were obtained by monitoring pressure changes during catalytic turnover. Standard reaction conditions: 10 mM CuI(OTf), 10 mM bpy, 20 mM NMI, 10 mM TEMPO, 0.2 M alcohol.
Scheme 1
Scheme 1. Simplified Catalytic Cycle for CuIOTf/bpy/NMI/TEMPO Catalyzed Aerobic Alcohol Oxidation
Scheme 2
Scheme 2. Formation of Bis-μ-Hydroxo CuII Dimer
Scheme 3
Scheme 3. Proposed Mechanism for the (bpy)CuII/TEMPO Catalyzed Aerobic Oxidation of Alcohols
Scheme 4
Scheme 4. Catalyst Oxidation Sequence Accounting for a Mixed 1st /2nd Order Cu Dependence
See this image and copyright information in PMC

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