Shufang Zhang et al. / Chinese Journal of Catalysis 35 (2014) 1864–1873
1865
molecular oxygen alone, so a cocatalyst is required for activa‐
tion of molecular oxygen [17].
and 13C nuclear magnetic resonance (NMR) spectra were rec‐
orded using a Bruker Avance III 400 MHz spectrometer. The
chemical shifts (δ) are reported in parts per million (ppm) and
coupling constants (J) in Hz. GC analysis was performed using
an Agilent Technologies 6820 instrument with a flame‐ioniza‐
tion detector. High‐resolution (HR) MS was performed using a
Bruker Daltonics micrOTOF‐QII mass spectrometer. Ultraviolet
(UV)‐visible spectra were recorded using an Agilent Cary 60
spectrometer. Electrochemical measurements were conducted
with a CHI 660E potentiostat using a platinum‐button working
electrode, non‐aqueous Ag/Ag+ reference, and a platinum wire
counter electrode at a scan rate of 100 mV/s. Elemental analy‐
sis was performed using a Vario EL cube elemental analyzer.
Copper complexes are more attractive as they are biomi‐
metic functional models of the mononuclear copper enzyme
galactose [18]. In 1984, Semmelhack et al. [19] reported the
aerobic oxidation of allyl and benzyl alcohols to the corre‐
sponding aldehydes and ketones catalyzed by 10 mol% CuI/
TEMPO in N,N‐dimethylformamide (DMF) at room tempera‐
ture. Subsequent research mainly focused on the choice and
design of nitrogen‐containing ligands such as 1,10‐phenanthro‐
line [20], 2,2'‐bipyridine (bpy) [21–25], 1,4‐diazabicyclo[2.2.2]
octane [26], 4‐pyrrolidinopyridine [27], pyrazole‐pyridine [28],
salen‐H4 [29], and 1,4,7‐triazacyclononane [30] to improve the
catalytic activity and extend the substrate scope. Recently, a
breakthrough was achieved by Stahl and coworkers [21–25].
They used N‐methylimidazole as an additive in a CuI‐bpy/
TEMPO catalyst system for the highly selective and efficient
transformation of a broad range of alcohols, including allylic,
benzyl, and aliphatic derivatives, with heterocycles and other
heteroatom‐containing groups. It should be noted that this
catalyst system used non‐commercial CuI(OTf) as the copper
source.
Although significant progress has been made in developing
TEMPO/Cu catalyst systems, readily available systems for the
efficient oxidation of alcohols under mild conditions are still
needed. In our continuing efforts to develop new systems for
oxidation of alcohols catalyzed by TEMPO derivatives [31–34]
and new applications of N4 ligands in different reactions
[8–10], we considered that a tetradentate nitrogen ligand
(Scheme 1) combined with copper(I) ions and TEMPO deriva‐
tives might be active in oxidation of alcohols. We found that an
easily obtainable catalyst system comprising CuI, N,N'‐dime‐
thyl‐N,N'‐bis(2‐pyridylmethyl)ethane‐1,2‐diamine (BPMEN),
and 4‐OH‐TEMPO exhibited high efficiency and wide substrate
scope, including allylic, benzyl, and aliphatic primary alcohols,
at room temperature using air as the oxidant without any addi‐
tives such as a base.
2.2. Preparation and characterization of ligands and
Cu(BPMEN)I complex
The ligands, i.e., BPMEN, BPMPN, and BPMCN, were synthe‐
sized as previously reported [35,36].
1
BPMEN: H NMR (CDCl3) δ = 8.54 (ddd, J = 4.9, 1.7, 0.8 Hz,
2H), 7.63 (td, J = 7.7, 1.8 Hz, 2H), 7.42 (d, J = 7.8 Hz, 2H), 7.14
(ddd, J = 7.4, 4.9, 1.0 Hz, 2H), 3.69 (s, 4H), 2.65 (s, 4H), 2.28 (s,
6H); 13C NMR (CDCl3) δ = 159.4, 149.1, 136.3, 123.0, 121.9, 64.2,
55.5, 42.9.
BPMPN: 1H NMR (CDCl3) δ = 8.53 (d, J = 4.5 Hz, 2H), 7.63 (t, J
= 8.4 Hz, 2H), 7.39 (d, J = 7.8 Hz, 2H), 7.14 (dd, J = 6.8, 5.5 Hz,
2H), 3.65 (s, 4H), 2.54–2.44 (m, 4H), 2.25 (s, 6H), 1.83–1.73 (m,
2H); 13C NMR (CDCl3) δ = 159.5, 149.0, 136.3, 123.0, 121.9, 63.9,
55.8, 42.5, 25.2.
1
BPMCN: H NMR (CDCl3) δ = 8.42 (dt, J = 4.9, 1.3 Hz, 2H),
7.53–7.49 (m, 4H), 7.08–7.01 (m, 2H), 3.79 (dd, J = 48.3, 14.6
Hz, 4H), 2.65–2.55 (m, 2H), 2.22 (s, 6H), 1.92 (dd, J = 10.4, 2.4
Hz, 2H), 1.74–1.65 (m, 2H), 1.22 (td, J = 12.3, 6.4 Hz, 2H),
1.13–1.05 (m, 2H); 13C NMR (CDCl3) δ = 161.2, 148.6, 136.3,
123.0, 121.6, 64.5, 60.4, 36.7, 25.8, 25.8.
CuI (0.25 mmol) and BPMEN (0.25 mmol) were added to
CH3CN (1 mL) in an Ar atmosphere, and the mixture was
stirred at room temperature for 2 h. After the reaction, the sol‐
vent was removed under vacuum to yield a Cu(BPMEN)I com‐
plex, which was then washed with CH3CN and diethyl ether and
dried under vacuum. HRMS (ESI‐MS) calcd for C16H22CuN4 [M −
I]+: 333.1121; found: 333.1135. Anal. calcd for C16H22CuN4I·
0.7MeCN: C 42.84%, H 5.12%, N 13.38%; found C 42.69%, H
4.96%, N 13.45%.
2. Experimental
2.1. General information
Gas chromatography‐mass spectrometry (GC‐MS) was per‐
formed using an Agilent Technologies 7890A/5975C system. 1H
2.3. Typical procedure for oxidation of alcohols
The copper salt and the ligand (each 0.025 mmol) were
added to CH3CN (1 mL) in an Ar atmosphere and stirred for 30
min. Then 4‐OH‐TEMPO (0.025 mmol) and substrate (0.5
mmol) were added successively, and the mixture was stirred at
room temperature. The reaction progress was checked using
thin‐layer chromatography. The reaction conversion and yield
were obtained from GC measurements using nitrobenzene or
nonane as an internal standard, or by column chromatography.
Scheme 1. Oxidation of alcohols catalyzed by copper salt/TEMPO de‐
rivative/N4 ligand. BPMEN = N,N'‐dimethyl‐N,N'‐bis(2‐pyridylmethyl)
ethane‐1,2‐diamine, BPMPN = N,N'‐dimethyl‐N,N'‐bis(2‐pyridinylme‐
thyl)propane‐1,3‐diamine, BPMCN = trans‐N,N'‐dimethyl‐N,N'‐bis(2‐
pyridylmethyl)cyclohexane‐1,2‐diamine.
2.4. NMR data of some products