Efficient, recoverable, copper-catalyzed aerobic oxidation of alcohols under FBS and thermomorphic mode

Article abstract of DOI:10.1016/j.tetlet.2007.10.074

A series of fluorinated bipyridine derivatives, (4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy) {R_f = n-C₈F₁₇ (1a), n-C₉F₁₉ (1b), n-C₁₀F₂₁ (1c), n-C₁₁F₂₃ (1d)} have been successfully synthesized using 4,4′-bis(bromomethylene)-2,2′-bpy and fluorinated alkoxides. Bpy 1a-d have been characterized by multi-nuclei (¹H, ¹⁹F, and ¹³C) NMR, GC/MS and FTIR. The Cu complexes 2a-d could be generated in situ by stirring ligands 1a-d with CuBr·Me₂S at room temperature, respectively. The 3-component systems 3c-d, CuBr·Me₂S/Bpy (1c-d)/2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO), were successfully used to the aerobic oxidation of alcohols under the fluorous biphasic system (FBS). The resulting products from FBS could be easily recovered by two phase separation with high yields up to 8 runs (>90%). In order to avoid using the expensive fluorous solvents, systems 3a-d, CuBr·Me₂S/Bpy (1a-d)/TEMPO, were also successfully shown to catalyze the aerobic alcohol oxidation under the thermomorphic condition (in C₆H₅Cl), and the yields of oxidation of 4-nitrobenzyl alcohol were close to 100% even after 8 runs. In particular, 3a was most effective under the thermomorphic mode in the chemoselectivity of aerobic oxidation of aliphatic primary alcohols to aldehydes without any overoxidized acids.

Full text of DOI:10.1016/j.tetlet.2007.10.074

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 8823–8828
Efficient, recoverable, copper-catalyzed aerobic oxidation
of alcohols under FBS and thermomorphic mode
Norman Lu* and Yan-Chou Lin
Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan
Received 29 June 2007; revised 10 October 2007; accepted 16 October 2007
Available online 22 October 2007
Abstract—A series of fluorinated bipyridine derivatives, (4,40-bis(RfCH2OCH2)-2,20-bpy) {Rf = n-C8F17 (1a), n-C9F19 (1b), n-C10F21
(1c), n-C11F23 (1d)} have been successfully synthesized using 4,40-bis(bromomethylene)-2,20-bpy and fluorinated alkoxides. Bpy 1a–d
have been characterized by multi-nuclei (1H, 19F, and 13C) NMR, GC/MS and FTIR. The Cu complexes 2a–d could be generated
in situ by stirring ligands 1a–d with CuBrÆMe2S at room temperature, respectively. The 3-component systems 3c–d, CuBrÆMe2S/Bpy
(1c–d)/2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO), were successfully used to the aerobic oxidation of alcohols under the fluor-
ous biphasic system (FBS). The resulting products from FBS could be easily recovered by two phase separation with high yields up
to 8 runs (>90%). In order to avoid using the expensive fluorous solvents, systems 3a–d, CuBrÆMe2S/Bpy (1a–d)/TEMPO, were also
successfully shown to catalyze the aerobic alcohol oxidation under the thermomorphic condition (in C6H5Cl), and the yields of oxi-
dation of 4-nitrobenzyl alcohol were close to 100% even after 8 runs. In particular, 3a was most effective under the thermomorphic
mode in the chemoselectivity of aerobic oxidation of aliphatic primary alcohols to aldehydes without any overoxidized acids.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
environmental persistence. In response to these limita-
tions, several research groups, for example, Gladysz
and co-workers.10,11 and Yamamoto and co-work-
ers,12,13 have introduced some new methodologies. For
example, the solubility based thermomorphic properties
of catalysts in an organic solvent were evaluated as a
new strategy to perform homogeneous catalysis without
the use of fluorous solvents.
The oxidation of alcohols to aldehydes or ketones is one
of the important transformations in organic synthesis.1–4
Although several metal-based oxidizing reagents have
been developed, these reagents usually require stoichi-
ometric amounts of metal oxidants, and thus a large
amount of waste is formed. In this respect, the oxidation
of alcohols using molecular oxygen catalyzed by recover-
able catalysts is ideal from an environmental and atom-
economical viewpoint.5 Knochel and co-workers6 re-
ported the utility of fluorous biphasic catalysis7,8 for oxi-
dation reactions.9 In such a system, perfluoroalkylated
catalysts have been used to achieve a selective solubility
in fluorous solvents, whereas the reagents and starting
materials are soluble in the organic phase. This method
allows a facile separation of the catalyst from the prod-
uct by decantation of the fluorous phase. Although
FBS combines the advantages of homogeneous catalysis
and heterogeneous catalysis, the large-scale use of perflu-
orocarbon solvents has some drawbacks: high cost and
Reported here are the synthesis of novel ligands 1a–d,
and using systems 3a–d, CuBrÆMe2S/Bpy/2,2,6,6-tetra-
methylpiperidine 1-oxyl (TEMPO), as catalysts in the
aerobic oxidation of alcohols under (a) fluorous bipha-
sic system and (b) thermomorphic mode.
As shown in Scheme 1, the preparation of fluorous-
ponytailed 2,20-bpy ligands started from deprotonation
of readily available fluorous alkanols, RfCH2OH, where
Rf = n-C8F17 (a), n-C9F19 (b), n-C10F21 (c), and n-C11F23
(d). Fluorous alkanols, RfCH2OH, were treated with
CH3ONa solution (30% in CH3OH) to give the corre-
sponding alkoxides. The alkoxides were then reacted
with 4,40-bis(BrCH2)-2,20-bpy14–16 to give rise to [4,40-
bis(RfCH2OCH2)-2,20-bpy] (1a–d) with high yields.
The novel ligands of this type were successfully synthe-
sized in our group for the first time.17 The partition
ratios of ligands 1a–d are shown in Table 1. Based on
Keywords: Fluorinated bipyridine; Fluorous biphasic system; Aerobic
oxidation; Alcohols; Thermomorphic.
Corresponding author. Tel.: +886 2 2771 2171x2417; fax: +886 2
2731 7174; e-mail: normanlu@ntut.edu.tw
*
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.074

8824
N. Lu, Y.-C. Lin / Tetrahedron Letters 48 (2007) 8823–8828
tion using four –CH2– units as a spacer and only one
Br
Br
type of fluorous ligand, which was made from
C8F17(CH2)3I. In contrast, in this study both 1c and
1d could be used as bpy-based ligands for the aerobic
oxidation of alcohols under FBS as shown in Table 2.
System 3c, based on ligand 1c, could catalyze the aero-
bic oxidation of 4-nitrobenzyl alcohol with good yields
and excellent recycling abilities up to eight times. System
3d, based on ligand 1d, could also catalyze the same
reaction with the good yields up to five times. During
the separation and recycling,19 Cu complexes 2c–d were
easier to be recovered by FBS, but the effect of the elec-
tron-withdrawing from longer fluorous ponytail in 2d on
the Cu center seemed to slow down catalytic activities.
N
N
N
Rf
ONa
Rf
Rf
O
O
N
a-d
Rf = n-C8F17 (1a), n-C19F19 (1b), n-C10F21 (1c) and n-C11F23 (1d)
Scheme 1. Syntheses of ligands 1a–d.
In order to demonstrate the generic application of Cu
systems 3c,d to the aerobic alcohol oxidation under
FBS, the aerobic oxidation of benzylic, allylic, and ali-
phatic alcohols (1° or 2°) was also tested using 3c–d.
The results showed excellent yields during 3–8 h cataly-
sis for benzylic and allylic alcohols (entries 1–4) listed in
Table 3, but no reactions were observed with aliphatic
alcohols (entries 5 and 6). The possible reason was that
the unactivated aliphatic alcohols were much more diffi-
cult to be oxidized. When we compared these results
with the results obtained by Knochel co-workers,6 li-
gands 1c,d were shown to be almost as good as ligand
A in the aerobic oxidation of activated alcohols. Ligands
1c,d which have a shorter spacer (–CH2OCH2–) may
have made Cu center less reactive than ligand A with a
5-bond spacer, –(CH2)4–. However, ligands 1c–d could
be easily prepared with high yields from cheaper and
widely available fluorinated alcohols and therefore,
could be good replacements.
Table 1. Distribution ratios of ligands 1a–d in different solvent systems
Ligand
CH2Cl2/FC77 DMF/FC77 C6H5Cl/FC77
(1a) (F%:59.8) 1.07:1
(1b) (F%:61.2) 1.04:1
1:4.5
1:6.9
2.6:1
1.09:1
1:29
(1c) (F%:62.3)
1:42
ꢀ0.1:100
(1d) (F%:63.3) 1:70
<0.1:100
1:48
the partition value in C6H5Cl/FC77 system, we selected
the two high fluorine-content ligands 1c,d under FBS
for the aerobic alcohol oxidation. Interestingly, ligands
1a–d were also found to be useful for the aerobic alcohol
oxidation under the thermomorphic mode. Because Cu
complexes 2a–d were found soluble in DMF at 90 °C,
but 2a–d precipitated at 25 °C or below. Because of
the wide availability of fluorous alcohols, a range of
these novel bpy-based ligands 1a–d could then be stud-
ied and gave the insightful information based on the
different lengths of fluorous ponytails. As shown in
Figure 1, the possible structure of Cu complexes 2a–d
is believed to be 4-coordinate geometry18 as proposed
by Tavener and Clark. Subsequently, we focused on
the catalytic ability of the Cu systems 3a–d, which is
based on the 3-component system, CuBrÆMe2S/Bpy/
TEMPO, in this Letter.
A series of fluorinated bpy-based ligands (including
1a–d) were prepared in our group, and their unusual
thermomorphic properties were found to be similar to
what were reported by Gladysz et al. and Yamamoto
et al.10–13 It was interesting to observe ligands 1c–d
and their resulting complexes exhibiting the exponential
increases in solubility when the temperature was raised.
This property has made the homogeneously recoverable
catalyst to precipitate at room temperature possible.
The Cu complexes 2a–d could be generated in situ by
stirring ligands 1a–d with CuBrÆMe2S at room tempera-
ture, respectively. The resulting systems 3a–d,
CuBrÆMe2S/Bpy (1a–d)/TEMPO, could be used for aer-
obic alcohol oxidation as shown in Figure 2. Knochel
and co-workers6 reported the fluorous alkylated bpy
ligand (ligand A) in their studies on the alcohol oxida-
The systems 3a–d were used to homogeneously catalyze
the aerobic oxidation of alcohols (aromatic or aliphatic)
under the thermomorphic mode (in C6H5Cl), and the
catalysts could be recovered and reused eight times with-
out any significant loss of activity. As shown in Table 4,
systems 3a–d could be effectively recycled for the aerobic
oxidation of 4-nitrobenzyl alcohol. Due to the electronic
effect of the longest fluorous ponytail, 3d-catalyzed aer-
obic oxidation of alcohols took longer time to complete,
and the reaction yield decreased significantly after 6
runs. From the Table 4 results, one trend was observed
the shorter the fluorous ponytail, the better the catalyst.
Therefore, the catalytic abilities in the series were:
3a > 3b > 3c > 3d. Furthermore, the ICP-MS results
from four cycles of 3d-catalyzed oxidations showed an
average 1.9% loss of Cu for each recovery. When the
results from FBS and thermomorphic systems were
compared (Tables 2 and 4), one could easily conclude
RfCH2OCH2
CH2OCH2Rf
N
S
N
Cu
Me
Me
Br
Figure 1. The proposed structure of Cu complex 2a–d.18

N. Lu, Y.-C. Lin / Tetrahedron Letters 48 (2007) 8823–8828
8825
HO
O
CuBr.Me2S, 1c-d (2 mol%)
TEMPO (3.5mol%)
C6H5Cl/FC77, 90 ºC, O2
NO2
NO2
R
f
R
f
O
O
N
N
C F
8
17
C F
8
17
4
1a-d
4
N
N
R
C F17(1a),C9F19(1b),
=
f
8
ligand A used by Knochel et al.6
C10F21(1c),C11F23(1d)
Figure 2. Aerobic oxidation of 4-nitrobenzyl alcohol to 4-nitrobenzaldehyde based on ligands 1c,d; ligand A6 is shown for comparison.
that the thermomorphic system was better in catalysis
than FBS.
Table 2. Oxidation of 4-nitrobenzyl alcohol under fluorous biphasic
system
Cycle no.
Yielda (%)
In addition, the pictures of the several stages of reac-
tions are shown in Figure 3. (A) Before reaction, 2a
was insoluble; (B) the reactants were easily soluble in
C6H5Cl, and 2a was still insoluble; (C) during the catal-
ysis, it was a homogenous catalysis when the tempera-
ture was around 80 °C; (D) At the end of reaction, the
catalyst precipitated and was recovered for the next
run when the product mixture was cooled to room
temperature.
A6
1c
1d
93
1
2
3
4
5
6
7
8
93
86
87
88
88
85
81
86
98
82
88
100
100
100
100
>90
82
>99
>99
>99
63
NRb
NR
TEMPO (3.5 mol %), CuBrÆMe2S, 1c,d (2 mol %), 90 °C, O2, 4–8 h.
a The yield was measured by GC/MS (NMP as internal standard).
b NR: no reaction.
In order to test the general application of 3a–d to the
aerobic alcohol oxidations,20,21 a variety of alcohols
were also tested under the thermomorphic mode as
Table 3. Aerobic oxidation of other alcohols under FBS
Entry
Substrate
Product
3c system
Yield (%)
3d system
Time (h)
Time (h)
Yield (%)
F
F
1
2
3
6
100
100
100
7
3
8
>99
OH
O
OH
O
5
7
>99
>99
Br
Br
O
O
C
OCH3
CH
OH
C
C
O
OCH3
OH
O
4
6
100
5
>99
5
6
CH3(CH2)8CH2OH
CH3(CH2)8CHO
NRa
NR
NR
NR
NR
NR
NR
NR
OH
O
CH3(CH2)4CH2
CH3
CH3(CH2)4CH2
CH3
TEMPO (3.5 mol %), CuBrÆMe2S, 1c,d (2 mol %), 90 °C, O2.
a NR: no reaction.

8826
N. Lu, Y.-C. Lin / Tetrahedron Letters 48 (2007) 8823–8828
Table 4. Aerobic oxidation of 4-nitrobenzyl alcohol to 4-nitrobenzaldehyde by using systems 3a–d under thermomorphic mode
Cycle no.
3a
3b
3c
3d
Time (h)
Yield (%)
Time (h)
Yield (%)
Time (h)
Yield (%)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
5
4
4
4
4
5
4
6
93
97
5
4
4
4
4
4
7
7
99
97
99
5
2
2
3
4
4
6
13
>99
96
99
100
93
88
5
5
5
5
5
6
NRa
NR
>99
96
100
100
90
89
NR
NR
>99
>99
>99
92.3
95
95
100
>99
>99
>99
99
80
>99
TEMPO (3.5 mol %), CuBrÆMe2S, 1a–d (2 mol %), 80–90 °C, O2.
a NR: no reaction.
Figure 3. The pictures of the several stages of aerobic oxidation of 4-nitrobenzyl alcohol; (A) before reaction; (B) the reactants were easily soluble in
C6H5Cl, and 2a was insoluble; (C) during the catalysis, it was a homogenous catalysis at 80 °C; (D) at the end of reaction, the catalyst precipitated
and was recovered for the next run.
shown in Table 5. The benzylic and allylic alcohols
(entries a–d) could be easily oxidized by catalysts
3a–d within 7 h. The reason why entry c took 16 h
for the 3d-catalyzed oxidation is mainly because of
the steric hindrance of the substrate. The aliphatic 1°
and 2° alcohols, the unactivated alcohols, could also
be oxidized, but the amount of TEMPO used had to
be raised to 10%. Both unactivated aliphatic alcohols,
e and f, could then be oxidized in the thermomorphic
mode, but not in the FBS (as shown in Table 3). The
possible reason was because the single phase was more
easily achieved in thermomorphic condition than in the
fluorous biphasic condition. The primary alcohol,
1-decanol, (entry e) could be oxidized with excellent
yields. However, the secondary aliphatic alcohol due
to the steric reason was only partially oxidized, and
therefore, even after 17 h the best yield was only
43%. Overall, systems 3a–d, CuBrÆMe2S/Bpy (1a–d)/
TEMPO, were able to oxidize all classes of alcohols
in the neutral condition under the thermomorphic
mode, which suggests that the catalysts could then be
easily recovered19 and reused.
To our surprise, all 3a–d showed good chemoselectivity
and excellent yields (>99%) to aldehydes in the aerobic
oxidation of aliphatic 1° alcohols (Table 5; entry e) with-
out the problem of over-oxidizing.22 In order to test this
type of selective oxidation, other aliphatic 1° alcohols
were also tested and were successfully oxidized by 3a,
with good yields, to aldehydes without trace of over-oxi-
dized acids. As shown in Table 6, 1-octanol (entry i) was
easily and selectively oxidized. The olefinic group (entry
ii) as one functional group present in the molecule was
undisturbed during the oxidation. When the bulky
cyclohexyl group (entry iii) was present, oxidation also
went smoothly. In particular, the heteroatom-containing
(S or N atom) alcohol (entry iv or v) could also be oxi-
dized without being influenced from its chelating effect.
Therefore, 3a can not only oxidize all classes of alcohols
in neutral conditions, but also selectively oxidize pri-
mary alcohols (especially aliphatic) with no trace of
over-oxidized carboxylic acids.
In summary, the novel ligands 1a–d were successfully
prepared with high yields, and the CuBrÆMe2S/Bpy

N. Lu, Y.-C. Lin / Tetrahedron Letters 48 (2007) 8823–8828
Table 5. Aerobic alcohol oxidations by systems 3a–d under thermomorphic mode
8827
Entry
Substrate
3a
3b
3c
3d
Time (h)
Yield (%)
100
Time (h)
Yield (%)
100
Time (h)
Yield (%)
100
Time (h)
Yield (%)
OH
a
6
6
6
6
100
F
OH
b
c
6
7
100
100
7
7
>99
100
6
6
100
100
5
100
100
Br
O
C
OCH3
OH
CH
OH
16
d
6
100
6
93
6
100
5
100
ea
fa
CH3(CH2)8CH2OH
7
>99
43
7
100
23
7
>99
43
7
>99
31
OH
17
17
17
17
CH3(CH2)4CH2
CH3
a For entries e and f, the amount of TEMPO used was raised from 3.5% to 10%. In the 3a case, when 3.5% TEMPO was used, the yield of entry e was
92.6% after 16 h.
Table 6. Aerobic oxidation of primary alcohols by systems 3a under thermomorphic mode
Entry
Substrate
Product
3a
Time (h)
8
Yield (%)
>99
i
CH3(CH2)6CH2OH
CH3(CH2)6CHO
H3C
OH
O
H3C
ii
9
96
CH3
CH3
CH3
CH3
OH
O
iii
iv
8
8
>99
>99
O
OH
OH
S
S
N
O
v
8
>99
N
TEMPO (10%); CuBrÆMe2S, 1c,d (2 mol %), 80 °C, O2; TOF: ca. 6.3 hꢁ1
.
(1a–d)/TEMPO system was useful in the aerobic alcohol
oxidation under the FBS and thermomorphic mode. The
catalytic results from thermomorphic mode were better
than from FBS. In the thermomorphic mode, 3a–d
could catalyze the oxidation of unactivated aliphatic
alcohols as well as benzylic, allylic alcohols. The cata-
lytic abilities within this series are 3a > 3b > 3c > 3d.
To our knowledge, besides the catalysts used by Yamag-
3
uchi and Mizuno and Marko et al.,22 systems 3a–d were
´
the 3rd system capable of oxidizing all classes of alco-
hols. In particular, it was shown that 3a was recoverable
and the most efficient in the series in the chemoselective
oxidation of primary alcohols without adding bases
under the thermomorphic mode.

8828
N. Lu, Y.-C. Lin / Tetrahedron Letters 48 (2007) 8823–8828
3. Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002,
2. Experimental
41, 4538–4542.
4. Schultz, M. J.; Sigman, M. S. Tetrahedron 2006, 62, 8227–
8241.
5. Mallat, T.; Baiker, A. Chem. Rev. 2004, 104, 3037–3058.
6. Betzemeier, B.; Cavazzini, M.; Quici, S.; Knochel, P.
Tetrahedron Lett. 2000, 41, 4343–4346.
Thermomorphic mode: For preparation of 4-nitrobenz-
aldehyde a 25 mL (Schlenk) flask was charged with the
bipyridine 1a (43 mg, 40 lmol; 2 mol %) dissolved in
FC77 (2 mL) and CuBrÆMe2S (8.2 mg, 40 lmol;
2 mol %) dissolved in a small amount of CH3CN leading
to a deep green solution. After evaporation, solid 2a was
formed. Then, a solution of 4-nitrobenzyl alcohol
(306 mg, 2.0 mmol) and TEMPO (10.9 mg, 70 lmol,
3.5 mol %) in chlorobenzene (2 mL) was added. Com-
plex 2a was not soluble in chlorobenzene at rt. However,
when the mixture was stirred at 80 °C, it became one
phase and then further stirred at 80–90 °C for several
hours while a gentle O2 stream was passing. At the
end of the reaction, the mixture was cooled to 0 °C
and the catalyst precipitated, the organic layer was dec-
anted and the precipitated 2a was recovered (and
reused) after the centrifugation.
´
´
7. Horvath, I. T.; Rabai, J. Science 1994, 266, 72–75.
8. (a) Cornils, B. Angew. Chem., Int. Ed. 1997, 36, 2057–
´
2059; (b) Horvath, I. T. Acc. Chem. Res. 1998, 31, 641–
650; (c) de Wolf, E.; van Koten, G.; Deelman, B.-J. Chem.
Soc. Rev. 1999, 28, 37–41; (d) Betzemeier, B.; Knochel, P.
Top. Curr. Chem. 1999, 206, 61–78; (e) Fish, R. H. Chem.
Eur. J. 1999, 5, 1677–1680.
9. (a) DiMagno, S. G.; Dussault, P. H.; Schultz, J. A. J. Am.
Chem. Soc. 1996, 118, 5312–5313; (b) Pozzi, G.; Monta-
nari, F.; Quici, S. Chem. Commun. 1997, 69–70; (c) Pozzi,
G.; Cinato, F.; Montanari, F.; Quici, S. J. Chem. Soc.,
Chem. Commun. 1998, 877–878; (d) Klement, I.; Lutjens,
H.; Knochel, P. Angew. Chem., Int. Ed. 1997, 36, 1454–
1456; (e) Betzemeier, B.; Lhermitte, F.; Knochel, P.
Synlett 1999, 489–491.
10. Wende, M.; Meier, R.; Gladysz, J. A. J. Am. Chem. Soc.
2001, 123, 11490–11491.
11. Wende, M.; Gladysz, J. A. J. Am. Chem. Soc. 2003, 125,
5861–5872.
12. Ishihara, K.; Hasegawa, A.; Yamamoto, H. Synlett 2002,
1299–1301.
13. Ishihara, K.; Kondo, S.; Yamamoto, H. Synlett 2001,
1371–1374.
Acknowledgments
N.L. thanks the National Science Council (NSC
96-2113-M-027-004-MY2 and 95-2113-M-027-002),
Taiwan, for the financial support. Dr. S. J. Wu (Eli Lilly
and Company, USA) was greatly appreciated for discus-
sion and proof-reading.
14. Ciana, L. D.; Dressick, W. J. J. Heterocycl. Chem. 1990,
27, 163–165.
15. Oki, A. R.; Morgan, R. J. Synth. Commun. 1995, 25, 4093–
4097.
16. Will, G.; Boschloo, G.; Rao, S. N.; Fitzmaurice, D. J.
Phys. Chem. B 1999, 103, 8067–8079.
17. Lu, N.; Lin, Y. C.; Chen, J. Y.; Fan, C. W.; Liu, L. K.
Tetrahedron 2007, 63, 2019–2023.
18. Tavener, S. J.; Clark, J. H. J. Fluorine Chem. 2003, 123,
31–36.
Supplementary data
Syntheses of ligands 1a–d and the catalytic reactions
under FBS were described. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2007.10.074.
19. System 3a–d was supposed to be unstable and difficult to
be recycled. The 2c,d and 2a–d could be recycled by FBS
and the thermomorphic mode, respectively.
20. Sheldon, R. A.; Arends, I. W. C. E. J. Mol. Catal. A:
Chem. 2006, 251, 200–214.
References and notes
1. (a) Hill, C. L. In Advances in Oxygenated Processes; JAI
Press: London, 1998; Vol. 1; (b) Hundlucky, M. Oxida-
tions in Organic Chemistry; American Chemical Society:
Washington, DC, 1990.
21. Gamez, P.; Arends, I. W. C. E.; Sheldon, R. A.; Reedijk, J.
Adv. Synth. Catal. 2004, 346, 805–811.
´
22. Marko, I. E.; Gautier, A.; Dumeunier, R.; Doda, K.;
Philippart, F.; Brown, S. M.; Urch, C. J. Angew. Chem.,
Int. Ed. 2004, 43, 1588–1591.
´
2. Marko, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.;
Urch, C. J. Science 1996, 274, 2044–2046.