L. Zhang et al. / Journal of Catalysis 354 (2017) 78–83
79
To pursue a more environmentally friendly oxidative method
2.1. Oxidation of other alcohols
and further understand the role of water moleculars in copper-
based and TEMPO-free system, we wish to develop an efficient
dioxygen catalytic system for the selective oxidation of alcohols
under mild conditions. This idea was inspired by our previous
report of copper complexes with hydroxylated bipyridyl-like
ligands [30,31]. Of these complexes, the neutral dinuclear complex
Cu2(ophen)2 (Fig. 1, Hophen = 2-hydroxy-1,10-phenanthroline)
shows interesting and catalyst-related features: (a) planar
butterfly-like dinuclear structures with two Cu(I) centers as the
‘‘body” and two peripheral ophen rings as ‘‘wings”; (b) existence
of coordinately unsaturated Cu(I) atoms; (c) easiness to be one-
electron oxidized into mixed-valent CuICuII(ophen)2 fragment. All
these features suggest that the Cu2(ophen)2 complex might be an
efficient catalyst for the selective aerobic oxidation of alcohols. In
addition, the defined structure of the dinuclear complex is better
than that of systems composed of copper salt/ligands, providing
a good platform for mechanistic investigation.
Herein, we present our preliminary observation and mechanis-
tic investigation of the selective dioxygen oxidation of alcohols cat-
alyzed by the dinuclear complex Cu2(ophen)2 in the absence of
TEMPO and base. To do this, we performed a combination of DFT
and ESI(+)(ꢀ)-MS experimental study. Two distinct reaction
courses of dioxygen and hydroperoxide oxidative dehydrogena-
tions, featuring successive dioxygen activation from mixed-valent
copper(I,II)-superoxide to all-divalent copper(II)-peroxide species
were revealed. Additionally, the role of H2O as a proton mediator
in the Cu2-O2 catalytic cycle was clarified.
To extend the application of this efficient activator of dioxygen,
we then examined the generality of the oxidative reaction. As
shown in Table 1, the oxidation reactions of primary benzyl alco-
hols with electron-rich (1b-e, h-j) or electron-deficient (1f-g) aryl
groups and heteroaryl groups (1k-l) can afford corresponding alde-
hydes in excellent conversions. Surprisingly, the selective oxida-
tion of benzyl alcohol in the presence of oxidizable ANH2 (1h)
was also be achieved. Substrates 1c, 1d and 1e bearing a methoxy
group at para, meta and ortho positions of phenyl ring resulted in
the target products 2c, 2d, and 2e, respectively, in high conver-
sions. The results indicated that the position of substituents in
aromatic rings had no obvious effect on the oxidation of mono-
substituted benzyl alcohols.
Bis-substituted o-benzyl alcohols 1m was selected to examine
potential steric effect, and the corresponding product 2m with
low to medium conversion indicated that increased further steric
hindrance limited the reaction. Aromatic-substituted secondary
alcohols (1n-p) showed good activity with medium to excellent
conversions. Terephthalaldehyde 2q was obtained in high conver-
sion, which indicated that dihydric benzyl alcohol was oxidized
smoothly. Further testing the catalytic activity by the oxidation
of non-activated aliphatic primary alcohols (1r-t) unfortunately
showed low conversions. Importantly, using oxygen bubbling
instead of an oxygen ball, significantly improved the effectiveness
of primary alcohols oxidation (2r and 2s), but conversion still
remained relatively low for secondary alcohol (2t).
Importantly, we found that the amount of water had a large
influence on the reaction rate and the conversion during oxidation.
Using commercially alcohols in which with trace amounts of water
as reactants, the reaction proceeded smoothly under optimized
condition. However, the reaction rate and conversion rate
decreased remarkably when experiments were conducted under
conditions with larger amounts of water (ꢁ10%) or without water
(Fig. S1). All experimental results showed that the trace amount of
water in the raw material promoted the oxidation process of
alcohol.
In order to verify the major oxidation species in this oxidative
process, 2.0 equiv of benzoquinone (BQ, OÅ2ꢀ quencher) was used
to examine the existence of O2Åꢀ. Adding BQ into the reaction sys-
tem of 1a after 2h, we found that the oxidative process was com-
pletely shut down, affording only 59% upon standing for 10 h
(Fig. S2). The great influence of BQ on the oxidation showed that
the reaction was caused by OÅ2ꢀ to a large degree.
2. Results and discussion
In the previous study, we found that coordinately unsaturated
Cu(I) atoms of Cu2(ophen)2 can be easily oxidized into
CuICuII(ophen)2 fragment [30,31]. Therefore, we speculated that
the dinuclear complex would combine with oxygen and the inter-
action will make dioxygen molecule active. The combination of
‘‘L2Cu2-O2” may be an efficient promoter for some aerobic oxida-
tion reactions. To test this idea, a series of probe tests were con-
ducted. The oxidative reaction of benzyl alcohol was investigated
as model reaction. After carefulscreening of different parameters
(Table S1), we found that the oxidation reaction proceeded
smoothly in CH3CN at 60 °C after addition of 5 mol% Cu2(ophen)2
as catalyst with dioxygen as oxidant in the absence of nitroxyl
co-catalyst and any base. The conversions of ethanol and benzyl
alcohol were as high as 89% (O2 bubbling) and 92% (O2 ball),
respectively. Obviously, the extraordinary activity of aerobic oxida-
tive reaction was attributed to the Cu2(ophen)2 active species.
To gain further insight into the reaction mechanism, a com-
bined DFT and ESI(+)(ꢀ)-MS experimental study was performed.
Using the oxidation of ethanol as an example, the ESI-MS spectrum
suggested the presence of key species in the oxidative process
(Figs. S3–7).
2.2. Plausible mechanism of oxidation of alcohol
The DFT study results indicated that two oxidative dehydro-
genation processes occurred (Fig. 2, S1–S22, Tables S3–9), featuring
superoxide/peroxide (I) and hydroperoxide (II) (Scheme 1).
Cu2ðophenÞ2
CH3CH2OH þ O2
CH CH ¼ O þ H O
ð1Þ
!
3
2
2
Cu2ðophenÞ2
CH3CH2OH þ H2O2
CH CH ¼ O þ 2H O
ð2Þ
!
3
2
The DFT calculations showed that dioxygen tends to adsorb the
Cu atom of Cu2(ophen)2 (1, M+ = 515.972, Fig. S3) with superoxide
end-on Cu-OO (2, Figs. S4, 8–11) and end-end Cu-OO-Cu (20,
[M+H]+ = 548.967, [M+H]2+ = 274.988, Figs. S4, 8–11) styles via
Cu/d2z and O2/p⁄ orbitals. Two styles could be mutually
Fig. 1. Planar wings-opened butterfly-like structure with Cu(I) centers as ‘‘body”
and two peripheral ophen rings as ‘‘wings”.