TEMPO-copper(II) diimine-catalysed oxidation of benzylic alcohols in aqueous media

Article abstract of DOI:10.1002/adsc.200600505

In situ generated copper(II)-diimine complexes combined with TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxyl radical) were studied in the oxidation of benzylic alcohols, the focus being on enviromentally benign reaction conditions. In this respect, reactions were studied in aqueous alkaline solutions and dioxygen was used as an end oxidant. This simple catalytic system turned out to be highly efficient and selective in the oxidation of primary and secondary benzylic alcohols to their corresponding carbonyl compounds. Under optimised reaction conditions [5 mol% of TEMPO, 3 mol% of copper(II) diimine, pH 12.6-13.5, 80°C, 10 bar O₂] benzyl alcohol was quantitatively and selectively oxidised to benzaldehyde. According to ESI-MS studies, coordination of TEMPO, as well as deprotonated benzyl alcohol to the parent copper-diimine complex in aqueous solutions is feasible. Supported by these observations a plausible reaction mechanism is proposed for the oxidation reaction.

Full text of DOI:10.1002/adsc.200600505

FULL PAPERS
DOI: 10.1002/adsc.200600505
TEMPO-Copper(II) Diimine-Catalysed Oxidation of Benzylic
Alcohols in Aqueous Media
Paweł J. Figiel,a Markku Leskelä,a and Timo Repoa,*
a
Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland
Fax : (+358)-9-191-50198; phone: (+358)-9-191-50228; e-mail: timo.repo@helsinki.fi
Received: October 4, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: In situ generated copper(II)-diimine com- diimine, pH 12.6–13.5, 808C, 10 bar O2] benzyl alco-
plexes combined with TEMPO (2,2,6,6-tetramethyl- hol was quantitatively and selectively oxidised to
piperidinyl-1-oxyl radical) were studied in the oxida- benzaldehyde. According to ESI-MS studies, coordi-
tion of benzylic alcohols, the focus being on enviro- nation of TEMPO, as well as deprotonated benzyl al-
mentally benign reaction conditions. In this respect, cohol to the parent copper-diimine complex in aque-
reactions were studied in aqueous alkaline solutions ous solutions is feasible. Supported by these observa-
and dioxygen was used as an end oxidant. This tions a plausible reaction mechanism is proposed for
simple catalytic system turned out to be highly effi- the oxidation reaction.
cient and selective in the oxidation of primary and
secondary benzylic alcohols to their corresponding
carbonyl compounds. Under optimised reaction con- Keywords: alcohols; aqueous-phase catalysis;
ditions [5 mol% of TEMPO, 3 mol% of copper(II) copper; green chemistry; oxidation; TEMPO
Introduction
provement for several catalytic systems with transi-
tion metal complexes[5]. Recently, Sheldon et al.[6] re-
The selective catalytic oxidation of alcohols to their ported an efficient system for the air oxidation of al-
corresponding carbonyl compounds is an important cohols that consists of TEMPO with Cu(II)/bipyridine
but challenging task for modern organic synthesis and complexes. Although this system permits the effective
the chemical industry.[1] Selective oxidation of benzyl- oxidation of primary alcohols under mild conditions,
ic alcohols to aldehydes is a particularly useful chemi- it requires water/acetonitrile (2/1) solutions and addi-
cal transformation, as these aldehydes are important tional base (t-BuOK). To further develop oxidation
intermediates in the synthesis of other organic com- reactions towards environmentally benign processes,
pounds, ranging from pharmaceuticals to plastic addi- consideration must also be given to the reaction
tives as well as in the processing of perfume and fla- media. Non-toxic solvents, including ionic and super-
vouring compounds and in the preparation of certain critical liquids, have recently become popular,[7] but
aniline dyes in the textile industry.[2] Many stoichio- from an economic and environmental viewpoint,
metric oxidants are known for the oxidation of alco- water still remains the undisputed solvent of choice,
hols such as chromates, hypochlorite, permanganates, and has recently been the subject of significant inter-
and others,[3] however their main disadvantage is that est in catalytic reactions.[8] In the light of this, we ex-
most of them are toxic and produce large amounts of tended our studies on the oxidation capability of in-
waste. In this respect, non-waste-producing oxidants situ generated copper(II) complexes[9] towards the
such as air, dioxygen or hydrogen peroxide, provide a TEMPO-mediated oxidation of benzylic alcohols in
base for more enviromentally friendly and relatively aqueous media. The combination of TEMPO and
cheap systems but activation of these oxidants with Cu(II) turned out to be an efficient oxidation system
catalysts is needed.[4]
for both primary and secondary benzylic alcohols
The use of persistent (TEMPO – 2,2,6,6-tetrame- using water as the only solvent, and dioxygen as the
thylpiperidinyl-1-oxyl, and its derivatives) or non-per- oxidant (Scheme 1).
sistent (PINO – phthalimide-N-oxyl) nitroxyl radicals
in oxidation processes seems to offer a significant im-
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Paweł J. Figiel et al.
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gave also high conversions under similar reaction con-
ditions while quantitative conversions of benzyl alco-
hol were achievable when the pH of the reaction solu-
tion is slightly lowered (Table 1, runs 3 and 4). We
can assume that the reasons for the improved activity
at lower pH are related to the complex distribution in
alkaline aqueous solutions as previously described for
Cu
(bipy)2 catalysts.[9e] When the alcohol/catalyst ratio
G
is increased (100:1), the difference among the catalyst
systems becomes more apparent. The catalyst based
on CuSO4 and phenanthroline gives high conversions
and selectivities, while the CuBr2/phen-based system
also provides high selectivity, but with significantly re-
duced conversions (Table 1, runs 7 and 8). Bipyridine
as a ligand also facilitates high conversions with a
Scheme 1.
Results and Discussion
As shown in our previous studies, catalysts for veratr- high alcohol/catalyst ratio, but with a marked reduc-
yl alcohol oxidation can be prepared in situ, simply by tion in selectivity due to benzoic acid formation
mixing a ligand and metal ion precurssor in an alka- (Table 1, run 5 and 6). Interestingly, in this case
line water solution.[9] A similar approach for catalyst CuBr2 as a metal source provided better efficiency
preparation, now accomplished with TEMPO, was ap- than the CuSO4 derivative.
plied here. Reaction conditions, optimised previously
for Cu(II)(phen) catalyst, were chosen for primary Cu(II) salts including Cu
studies and the oxidation of benzyl alcohol was general, they gave fairly similar conversions of benzyl
chosen as a model reaction (Table 1). alcohol as those with CuSO4/phen and CuBr2/bipy
This prompted us to study the influence of other
A
N
ACHTREUNG
In the presence of TEMPO, the oxidation capability under similar reaction conditions. In addition, from a
of in situ generated Cu(II) diimine complexes is sig- series of different bases, including LiOH, NaOH,
nificantly improved, as can be seen for runs 10 and 7 KOH, Ca(OH)2, Ba(OH)2 and t-BuOK, NaOH
in Table 1. The oxidation of benzyl alcohol at 808C in turned out to be the most efficient. Due to the fact
alkaline aqueous solution proceeds nearly quantita- that the CuSO4/phen/TEMPO/NaOH combination
tively when a catalytic amount of TEMPO (5 mol% provides high activity and selectivity, and because it is
vs. substrate) together with in situ generated CuBr2/ a halogen-free system, it was chosen for further stud-
phenanthroline (3 mol% vs. substrate) or CuSO4/phen- ies. The time dependency of the activity of the
anthroline complexes were used (Table 1, runs 1 and CuSO4/phenanthroline system was studied. After
2). The selectivity of the catalysts was also excellent 1.5 h, only 25% of the alcohol was oxidised whereas
(98–99%). The system based on the bipyridine ligand after 3 h, 90% conversion was reached (Table 1, runs
Table 1. Oxidation of benzyl alcohol with in situ made TEMPO/[Cu]/ligand system.[a]
Run
t [h]
pH
Conversion[b] [%]
Selectivity[b] [%]
Catalytic system
TEMPO
Catalyst (substrate:catalyst)
Ligand
1
2
3
4
5
6
7
8
5
5
5
5
5
5
3
3
13.5
13.5
13.5
12.6
12.6
12.6
13.5
13.5
13.5
13.5
13.5
13.5
100
100
92
100
89
77
90
77
25
44
3
98
99
100
100
78
67
99
99
100
100
100
100
+
+
+
+
+
+
+
+
+
-
CuBr2 (33:1)
CuSO4 (33:1)
CuBr2 (33:1)
CuBr2 (33:1)
CuBr2 (100:1)
CuSO4 (100:1)
CuSO4 (100:1)
CuBr2 (100:1)
CuSO4 (100:1)
CuSO4 (100:1)
-
phen
phen
bipy
bipy
bipy
bipy
phen
phen
phen
phen
phen
-
9
1.5
3
3
10
11
12
+
+
3
3
CuSO4 (100:1)
[a]
Reaction conditions: 3 mmol of alcohol, TEMPO (0.15 mmol , 5 mol% vs. substrate), CuSO4 0.09 mmol, 3 mol% and
1,10-phenanthroline (phen) or bypirydine (bipy) (runs 1–4) or 0.03 mmol of CuSO4 and diimine (runs 5–11), 10 bar O2,
808C, NaOH water solution up to 10 mL total volume.
[b]
Conversions and selectivities are based on GC measurements. Accuracy Æ1%. The main side product in these reactions
is benzoic acid.
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TEMPO-Copper(II) Diimine-Catalysed Oxidation of Benzylic Alcohols
FULL PAPERS
Figure 1. Optimisation of the catalytic system. Black dots
Figure 2. The effect of reaction temperature and pressure as
a function of benzyl alcohol conversion. Reaction conditions:
5 mol% of TEMPO and 1 mol% of CuSO4/phen (1:1), 3 h.
represent the experimental points. Reaction conditions:
Benzyl alcohol (3 mmol), TEMPO (5 mol%), CuSO4 (1
mol%) and phen (0–8%) at differrent pH (1–14). Reaction
time 3 h.
As shown in a 3-D plot (Figure 2) at lower pressure
9 and 7), which indicates not only the lengthy induc- (<6 bar) and temperature (<608C) conversions are
tion time but also the stability of the catalyst system rather low (<10%), whereas at temperatures above
under the applied reaction conditions. The control re- 60–808C and pressure of about 8–10 bar the alcohol
action without TEMPO gave a conversion of 44% conversions markedly increased (up to 90%). Al-
after 3 h, while reactions without CuSO4 or phenan- though thermally rather stable, the catalytic activity
throline produced approximately 3% benzaldehyde does not unexpectedly increase linearly with the O2
(Table 1, runs 10–12). These results underline the im- pressure, but reaches its maximum at 8–10 bar. The
portance of each catalyst component for the efficient available results do not provide adequate reasons for
oxidation reaction.
As shown previously for the in situ made nomenon is linked to stability of the catalytic system
Cu(phen) catalyst, pH and ligand concentration are at high O2 concentrations.
crucial factors which determine catalytic activity in Although Cu(diimine) alone can oxidise benzyl al-
the reduced activity but we can assume that this phe-
ACHTREUNG
AHCTREUNG
aqueous solution and these two parameters have a cohol with moderate activity (40%, Table 1), addition
strong interdependency.[9] The three-dimentional plot of a catalytic amount of TEMPO markedly improved
in Figure 1 summarises the optimisation results for the catalytic system, and in the best case leds to quan-
the CuSO4/phen/TEMPO/NaOH system with the titative substrate oxidation. The influence of the
focus being on maximum benzyl alcohol conversion. Cu(II)-diimine catalyst and TEMPO concentrations
Apparently, the most effective system is achieved on the efficiency of the catalytic system were further
with rather high pH (12–14) and with a 1:1 ligand/ studied. As illustrated in Figure 3, an increase in the
metal ratio. Interestingly, for the Cu(phen)n systems catalyst concentration in the absence of TEMPO sig-
G
without TEMPO the optimised ligand to metal ratio nificantly improves the activity when more than 5%
was 2:1, which leads to pentacoordinated Cu(II) com- catalyst loading is applied, however, the conversion
plexes of the type Cu
(phen)(OH)2.[9e] In these Cu(II) remains under 80%. Similar conversions are also at-
G
complexes two coordination sites are available for fur- tainable if a 1% catalyst loading combined with high
ther coordination, e.g., for the substrate. Also, the re- TEMPO concentration (above 8%) is used. For a
sults herein indicate that for optimum activity at least quantitative conversion of benzyl alcohol, catalyst
two coordination sites should be available. Presuma- concentrations between
bly the coordination of both substrate and TEMPO to TEMPO are needed.
3 and 4% and 5–8%
one catalytic centre might be beneficial.
The influence of both reaction temperature and O2 simple catalyst system, the optimised reaction condi-
pressure was studied for the optimised catalyst system.
tions (5 mol% of TEMPO, 3 mol% of Cu2+, ligand to
To demonstrate the applicability and limits of this
Adv. Synth. Catal. 2007, 349, 1173 – 1179
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Paweł J. Figiel et al.
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fine tuning of the structure of the phenanthroline did
not improve the efficiency of the catalyst, for exam-
ple, only with a few substrates, like 3,5-di-MeO-
benzyl alcohol did the use of 4,7-dihydroxyphenan-
throline gave better conversions (Table 2, runs 6 and
7. See also S1 in Suplementary Information).
Unlike the previously reported Cu/TEMPO cata-
lyst,[6] the current catalyst system can also oxidise sec-
ondary benzylic alcohols. Under the applied condi-
tions, 1-phenyl-1-ethanol can be oxidised to acetophe-
none with good yield and 3-MeO-a-Me-benzyl alco-
hol can be converted to the corresponding ketone in
moderate yield (Table 2, runs 10 and 11). Both of
these oxidations are also highly selective. As a limita-
tion, aliphatic alcohols, both primary (1-hexanol), and
secondary (2-pentanol), were not oxidised (Table 2,
runs 12 and 13). These results clearly point out that
benzylic activation of alcohol is needed for the effi-
cient catalytic oxidation.
There are several mechanisms proposed for the
TEMPO-copper assisted alcohol oxidation, mainly
based on UV-Vis or kinetic experiments.[10a] Some of
the most recent publications suggest the simultaneous
coordination of TEMPO and alcohol to the metal
Figure 3. The effect of catalyst and TEMPO concentration
on the conversion of benzyl alcohol. Reaction conditions:
Cu:phen=1:1, 10 bar of O2, 808C, 3 h
metal ratio=1:1, high pH) were chosen for a further centre and reaction in the coordination sphere[6,8,10]
study of the catalyst in the oxidation of different alco- whereas other proposed mechanisms are based on the
hols. Besides benzyl alcohol, also trimethoxy-substi- activity of the nitrozium cation.[11] The role of basic
tuted primary benzylic alcohols, as well as di- and conditions remains unclear, but the presence of
mono-substitued ones can be smoothly and selectively sodium hydroxide likely facilitates the formation of
oxidised to their corresponding aldehydes with mod- Cu(diimine)(OH) species and assists the deprotona-
U
erate to good yields (Table 2).
tion of alcohol.[6,9]
Based on extended oxidation experiments with a
ESI-mass spectrometry is an effective method for
series of substrates, the optimal reaction conditions the characterisation of the reaction intermediates.[12]
evidently depend on the substrate as well. For exam- To adapt this technique to the present system, and to
ple, with the substrates in runs 2, 7 and 8, a lower pH avoid the excessive formation of sodium adducts in
(12.6) gave higher conversions than those measured the ionisation, we applied neutral conditions (pH~7)
at pH 13.5. In addition with some substrates higher for the measurements. In the water solutions consist-
catalyst loadings (5 mol%) were beneficial. Further ing of CuSO4 and bipyridine or phenanthroline, few
Table 2. Oxidation of selected alcohols with TEMPO/CuSO4/phen system.[a]
Run Substrate
Time [h] Temp. [8C] Substrate/catalyst pH
Conversion [%] Selectivity [%]
1
2
3
4
2,4,5-tri-MeO-benzyl alcohol
5
5
5
5
5
5
5
3
3
3
5
3
3
100
100
100
100
100
100
100
80
80
80
100
80
80
20
20
20
20
20
20
20
100
100
100
20
100
100
13,5
12,6
13,5
13,5
13,5
13,5
12,6
12.6
13.5
13.5
13.5
100
77
96
100
99
55
100
68
65
82
62
99
99
99
100
97
94
99
100
100
100
99
-
3,4,5-tri-MeO-benzyl alcohol
2,5-di-MeO-benzyl alcohol
2,4-di-MeO-benzyl alcohol
3,4-di-MeO-benzyl alcohol
3,5-di-MeO-benzyl alcohol
3,5-di-MeO-benzyl alcohol
3,4-di-Cl-benzyl alcohol
3-MeO-benzyl alcohol
1-Ph-1-ethanol
5
6
7[b]
8
9
10
11
12
13
3-MeO-a-Me-benzyl alcohol
1-hexanol
2-pentanol
12.6/13.5 no reaction
12.6/13.5 no reaction
-
[a]
[b]
Conditions: 3 mmol of substrate, 5 mol% of TEMPO, 10 bar O2. Conversions and selectivities based on GC.
4,7-Dihydoxyphenanthroline was used instead of phenanthroline.
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TEMPO-Copper(II) Diimine-Catalysed Oxidation of Benzylic Alcohols
FULL PAPERS
Scheme 2. Proposed mechanism for the catalytic oxidation of benzyl alcohols by O2 and a TEMPO-Cu(II)-diimine system in
alkaline aqueous media.
peaks appeared in ESI-MS (m/z=316 and 375 for optimisation results suggest that the in situ formation
bipy, m/z=339 and 423 for phen) due to the forma- of a Cu
N
N
tion of [Cu(diimine)(SO4)] and [Cu
N
G
ACHTREUNG
plexes. This is in accordance with the results obtained by ESI-MS measurements, this complex can coordi-
from the optimisation of the ligand to metal ratio and nate benzyl alcohol (PhCH2OH), and water is re-
supports the discussion therein. The addition of leased as a side product (Scheme 2a). Obviously, the
TEMPO to these solutions induced new peaks core- abstraction of b-hydrogen from coordinated alcohol
sponding to Cu
A
(phen) or Cu
A
ACHTREUNG
respectively). Similarly, even in these neutral condi- in the current system requires a benzylic activation
tions, benzyl alcohol alone can coordinate to the Cu- and is driven by TEMPO (Scheme 2c). TEMPO-OH
diimine complexes and adequate alkoxide species are is formed simultaneously and is latter reoxidised to
formed. In fact, the peaks corresponding to TEMPO by dioxygen, as described in previously pro-
Cu
(phen) (m/z=351) were also observable when complex formed (Scheme 2d) can be then reoxidised
TEMPO was present. Thus, both Cu(TEMPO)- by O2 to Cu(II), which is a well-known step for cata-
(diimine) and Cu(PhCH2O)
(diimine) complexes can lytic systems containing copper complexes,[13] al-
coexist, but the presence of the Cu(TEMPO)- though, alternatievely TEMPO can be involved in
(PhCH2O)
(bipy) complex could not be confirmed. It that process as well.[6]
A
(bipy) (m/z=327) and Cu
G
N
ACHTREUNG
AHCTREUNG
A
N
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
is likely that, when formed, this species reacts further
rapidly and aldehyde is released.
This mechanism shares similarities with those of
our previous studies on Cu-catalysed oxidations[9] ac-
On the basis of the oxidation results and the ESI- complished with some previously-published TEMPO-
MS measurements for the model system, a mechanism based systems.[6] The, intermediate species observed
for the Cu(II)/TEMPO system is proposed by ESI-MS, Cu
A
G
N
(Scheme 2). Our previous studies on the Cu(II) di-
imine complexes in alkaline media and the present
ACHTREUNG
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Paweł J. Figiel et al.
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utions, with sodium formate as a reference standard. The
peaks observed for the mixture of CuSO4 and phenanthro-
the active species as postulated in former publicati-
ons.[5a,6,10]
line were: m/z=339.9586 corresponding to Cu
(calculated: 339.9573, error À3.9 ppm) and m/z=423.0659,
Cu(phen)2 complex (calculated: 423.0665, error 1.3 ppm).
After the addition of TEMPO the following peaks were ob-
served: m/z=399.1369 due to Cu(phen)(TEMPO) forma-
tion (calculated: 399.1366, error À0.8 ppm), after addition of
benzyl alcohol peak at m/z=351.0574 appears for
Cu(PhCH2O)(phen) (calculated: 351.0553, error 5.9 ppm).
A
G
AHCTREUNG
Conclusions
A
ACHTREUNG
In conclusion, a novel catalytic system for the effi-
cient and selective oxidation of benzylic alcohols, in
alkaline aqueous solutions has been developed. The
effects of different reaction conditions were carefully
studied to achieve the maximum substrate conversion.
These studies have shown that high pH, temperature
and O2 pressure, as well as a 1:1 ratio of ligand:Cu
are needed for effective benzaldehyde production.
The former Cu(II)-diimine system used mainly for the
oxidation of veratryl alcohol in water, is now marked-
ly improved by the use of a catalytic amount of
TEMPO radical. Conversion of primary and secon-
dary alcohols into their corresponding carbonyl prod-
ucts was achieved both efficiently and selectively. The
use of “green” reagents, such as dioxygen as an oxi-
dant and water as the only solvent makes the system
attractive for environmentally sustainable processes.
It is important to note that the ESI-MS experiments
a
A
ACHTREUNG
For the mixture of CuSO4 and bipyridine the following
peaks were observed: m/z=315.9587 corresponding to
Cu
A
ACHTREUNG
m/z=375.0677 corresponding to Cu
AHCTREUNG
375.0665, error À3.0 ppm). The addition of TEMPO results
in the formation of a peak at m/z=375.1339 due to the
Cu
G
G
7.2 ppm), while after the addition of benzyl alcohol a peak
at m/z=327.0579 appears for Cu
N
N
lated: 327.0553, error 7.9 ppm).
References
[1] a) B. M. Choudary, Catal. Today 2000, 57, 17–32;
b) F. A. Carey, R. J. Sundberg, Advanced Organic
Chemistry, 3rd edn., Plenum Press, New York, 1993.
[2] a) M. Musawir, P. N. Davey, G. Kelly, I. V. Kozhevni-
kov, Chem. Commun. 2003, 1414–1415; b) T. Mallat,
A. Baiker, Catal. Today 1994, 19, 247–283.
confirmed the existence of the Cu
N
N
and Cu(diimine)(PhCH2O) species, thus providing an
A
ACHTREUNG
insight into the catalytic system.
[3] a) Ullmanꢀs Encyclopedia of Industrial Chemistry, 6th
edn., Wiley-VCH, Weinheim, 2002; b) Principles of Or-
ganic Synthesis, 3rd edn., Blackie Academic & Proffe-
sional, London, 1993.
ExperimentalSection
Materials
[4] a) I. W. C. E Arends, R. A. Sheldon, Modern Oxidation
Methods, (Ed.: J.-E. Bäckvall), Wiley, Chicester, 2004,
p 83; b) I. P. Skibida, A. M. Sakharow, Catal. Today
1996, 27, 187–193; c) R. Noyori, M. Aoki, K. Sato,
Chem. Commun. 2003, 1977–1986.
Benzyl alcohols and other reagents were used as received
from commercial sources without further purification.
GeneralProcedure
[5] a) For review see: R. A. Sheldon, I. W. C. E. Arends, J.
Mol. Cat. A: Chemical 2006, 251, 200–214; b) R. A.
Sheldon, I. W. C. E. Arends, Adv. Synth. Catal. 2004,
346, 1051–1071; c) Y. Ishii, S. Sakaguchi, T. Iwahama ,
Adv. Synth. Catal. 2001, 343, 393–427; d) W. Adam,
C. R. Saha-Mçller, P. A. Ganeshpure, Chem. Rev. 2001,
101, 3499–3548; e) P. J. Figiel, J. M. Sobczak, Polish J.
Chem. 2001, 75, 869–873; f) R. Liu, X. Liang, C. Dong,
X. Hu, J. Am. Chem. Soc. 2004, 126, 4112–4113; g) S.
Velusamy, A. Srinivasan, T. Punniyamurthy, Tetrahe-
dron Lett. 2006, 47, 923–926; h) P. J. Figiel, J. M. Sobc-
zak, J. J. Ziółkowski, Chem. Commun. 2004, 244–245;
i) F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti,
C. Punta, R. Faletti, R. Paganelli, G. F. Pedulli, Eur. J.
Org. Chem. 2004, 109–119; j) F. Minisci, C. Punta, F.
Recupero, J. Mol. Cat. A: Chemical 2006, 251, 129–
149.
The oxidation reactions were carried out as described previ-
ously in standard glass test tubes (up to 14 test tubes per
one run) placed in a 1-L steel reactor.[9b] The reactor was
placed on a magnetic stirrer and thermostated in an oil
bath. Reaction products were isolated by extraction with
10 mL of ethyl acetate, and analysed quantitatively by GC
(Agilent 6890 chromatograph, Agilent 19091 J-413 capilary
column 0.32 mm”30 m”0.25 mm, FID detector) using inter-
nal standards. To estimate the amount of benzoic acid
formed during the reaction, samples were acidified with hy-
drochloric acid before extraction with ethyl acetate, then the
acid was converted into a methyl ester by reaction with
0.1 mL 0.25M TMSH (trimethylsulfonium hydroxide, Fluka
reagent) prior to GC analysis. Products were identified by
GC-MS analyses (Agilent 6890N equiped with Agilent 5973
mass selective detector, HP 19091 L-102 capilary column,
200 mm”24 m”0.31 mm).
[6] a) P. Gamez, I. W. C. E. Arends, J. Reedijk, R. A. Shel-
don, Chem. Commun. 2003, 2414–2415; b) P. Gamez,
I. W. C. E. Arends, R. A. Sheldon, J. Reedijk, Adv.
Synth. Catal. 2004, 346, 805–811; see also: c) D. Geibl-
meir, W. G. Jary, H. Falk, Monatsh. Chem. 2005, 136,
1591–1599.
Mass Spectrometry
High resolution ESI-MS measurements were performed
using a Bruker Microtof LC mass spectrometer in water sol-
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Adv. Synth. Catal. 2007, 349, 1173 – 1179

TEMPO-Copper(II) Diimine-Catalysed Oxidation of Benzylic Alcohols
FULL PAPERS
[7] a) R. A. Sheldon, Green Chem. 2005, 7, 267; b) Z.
Yang, W. Pan, Enzyme Microb. Technol. 2005, 37, 19–
28; c) F. Shi, Y. Gu, Q. Zhang, Y. Deng, Catal. Surv.
Asia 2004, 8, 179–186; d) E. J. Beckman, J. Supercrit.
Fluids 2004, 28, 121–191; e) A. E. McGowin, 38th Mid-
west Regional Meeting of the American Chemical Soci-
ety, Columbia, MO, USA, November 5–7, 2003, Ab-
stracts, 375.
[10] a) A. Dijksman, I. W. C. E. Arends, R. A. Sheldon,
Org. Biomol. Chem. 2003, 1, 3232–3237; b) P. Ferreira,
E. Philips, D. Rippon, S. C. Tsang , Appl. Catal. B. En-
vironmental 2005, 61, 206–211; c) N. Jiang, A. J. Ra-
gauskas , Org. Lett. 2005, 7, 3689–3692.
[11] a) M. F. Semmelhack, C. R. Schmid, D. A. Cortꢁs, C. S.
Chou, J. Am. Chem. Soc. 1984, 106, 3374–3376; b) R.
Ben-Daniel, P. Alsters, R. Neumann, J. Org. Chem,
2001, 66, 8650–8653; c) M. Contel, P. R. Villuendas, J.
Fernandez-Gallardo, P. J. Alonso, J-M. Vincent, R. H.
Fish, Inorg. Chem. 2005, 44, 9771–9778.
[8] a) U. M. Lindstrçm, Chem. Rev. 2002, 102, 2751–2772;
b) R. Liu, Ch. Dong, X. Liang, X. Wang, X. Hu, J. Org.
Chem. 2005, 70, 729–731.
[12] a) B. C. Gilbert, J. R. Lindsay Smith, A. Mairata i Paye-
ras, J. Oakes, R. Pons i Prats, J. Mol. Cat. A: Chemical
2004, 219, 265–272; b) H. Guo, R. Qian, Y. Liao, S.
Ma, Y. Guo, J. Am. Chem. Soc. 2005, 127, 13060–
13064; c) L. S. Santos, C. H. Pavam, W. P. Almeida, F.
Coelho, M. N. Eberlin, Angew. Chem. Int. Ed, 2004, 43,
4330–4333.
[13] a) D. Samocha-Bonet, D. Lichtenberg , I. Pinchuk, J.
Inorg. Biochem. 2005, 99, 1963–1972; b) S. W. Lam, K.
Chiang, T. M. Lim, R. Amal, G. K-C. Low , Appl.
Catal. B: Environmental 2005, 55, 123–132.
[9] a) H. Korpi, P. Lahtinen, V. Sippola, O. Krause, M. Le-
skelä, T. Repo, Appl. Cat. A: General 2004, 268, 199–
206; b) P. Lahtinen, H. Korpi, E. Haavisto, M. Leskelä,
T. Repo , J. Comb. Chem. 2004, 6, 967–973; c) P. Lahti-
nen, E. Lankinen, M. Leskelä, T. Repo, Appl. Cat. A:
General 2005, 295, 177–184; d) H. Korpi, V. Sippola, I.
Filpponen, J. Sipilä, O. Krause, M. Leskelä, T. Repo,
Appl. Cat. A: General 2006, 302, 250–256; e) H. Korpi,
P. J. Figiel, E. Lankinen, P. Ryan, M. Leskelä, T. Repo,
Eur. J. Inorg. Chem. 2007, in press.
Adv. Synth. Catal. 2007, 349, 1173 – 1179
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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