Fluorous biphasic catalytic oxidation of alkenes and aldehydes with air and 2-methylpropanal in the presence of (β-perfluoroalkylated tetraphenylporphyrin)cobalt complexes

Article abstract of DOI:10.1002/ejoc.200600027

A simple and facile fluorous biphasic catalytic oxidation of alkenes and aldehydes with 1 atm of air and 2-methylpropanal in the presence of the (β-perfluoroalkylated tetraphenylporphyrin)cobalt complexes [Co{TPP(C ₈F₁₇)₄}] (Co2) has been demonstrated for the first time. This kind of β-perfluoroalkylated tetraphenylporphyrin may be used in other catalytic applications because of its ready availability and unique properties. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600027

FULL PAPER
DOI: 10.1002/ejoc.200600027
Fluorous Biphasic Catalytic Oxidation of Alkenes and Aldehydes with Air and
2-Methylpropanal in the Presence of (β-Perfluoroalkylated
tetraphenylporphyrin)cobalt Complexes
Chao Liu,[a] Dong-Mei Shen,[a] and Qing-Yun Chen*[a]
Keywords: Biphasic catalysis / Cobalt / Fluorinated ligands / Oxidation / Porphyrinoids
A simple and facile fluorous biphasic catalytic oxidation of
alkenes and aldehydes with 1 atm of air and 2-methylpro-
alkylated tetraphenylporphyrin may be used in other cata-
lytic applications because of its ready availability and unique
panal in the presence of the (β-perfluoroalkylated tetraphen- properties.
ylporphyrin)cobalt complexes [Co{TPP(C8F17)4}] (Co2) has
been demonstrated for the first time. This kind of β-perfluoro-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
fluids[7] would be particularly helpful in oxidation reactions.
For example, 5,10,15,20-tetrakis(heptafluoropropyl)por-
phyrin has been successfully used as a fluorocarbon-soluble
sensitizer for the photooxidation of allylic alcohols to hy-
droperoxide in a fluorous biphasic system.[8]
Introduction
Transition metal complexes based on porphyrins have
been used extensively to catalyze a range of fundamentally
and practically important chemical transformations,[1] in-
cluding an array of atom/group-transfer reactions such as
epoxidation and hydroxylation and nitrene (aziridination
and amination) and carbene (cyclopropanation and carbene
insertion) transfers.[1–3] Metalloporphyrin oxidation cata-
lysts not only constitute unique biomimetic models for cy-
tochrome P-450 enzymes but have also practical application
in oxidation reactions. Electronically and sterically modi-
fied porphyrins incorporating strongly electron-with-
drawing substituents at the pyrrolic β- and/or meso-position
have been found to be more active and stable catalysts for
oxygenation reactions than related electron-donating com-
plexes,[3] and this has been rationalized by means of compu-
tational tools and photoelectron spectroscopy.[4] Among the
numerous electron-withdrawing groups, perfluoroalkyl
groups are advantageous because they are inert and
strongly σ-electron-withdrawing. Moreover, theoretical and
electrochemical studies show that these substituents should
effectively stabilize the HOMO of the porphyrin macrocy-
cle,[4] thus increasing the porphyrin’s stability toward oxi-
Nevertheless, there have been only a few reports of cata-
lytic oxidation by meso-perfluroalkylated porphyrins within
a fluorous biphasic system.[8,9] To the best of our knowl-
edge, hydrocarbon oxygenation reactions catalyzed by β-
perfluoralkylated metalloporphyrins have never been re-
ported, perhaps because of synthetic difficulties. In connec-
tion with our recent report on the convenient and efficient
synthesis of various β- and meso-perfluoroalkylated por-
phyrins, as well as their intramolecular radical cycliza-
tions,[10] we report here a facile fluorous biphasic catalytic
oxidation of alkenes and aldehydes with 1 atm of air and 2-
methylpropanal in the presence of the (β-perfluoralkylated
tetraphenylporphyrin)cobalt complexes [Co{TPP(C8F17)4}]
(Co2).
Results and Discussion
The best way to prepare the required β-perfluoralkylated
dation. More importantly, introduction of perfluoroalkyl tetraphenylporphyrin ligand H2TPP(C8F17)4 (2) is by de-
groups onto the porphyrin macrocycle can improve solubil- metalation of its corresponding copper complex
ity[5] and makes metalloporphyrin complexes potentially [Cu{TPP(C8F17)4}] (Cu2) with CF3COOH (TFA)/concd.
useful catalysts in fluorous media.[6] Perfluoroalkylated por- H2SO4. The copper complex was conveniently prepared by
phyrins have been used for fluorous biphasic catalysis, a Pd-catalyzed cross-coupling of the readily available
which has appealing features such as the easy separation (β-octabromoporphyrin)copper complex [CuTPP(Br)8]
and recycling of the perfluorinated organometallic com- (Cu1)[11] with C8F17I/Cu in good yield, as described pre-
plexes. In addition, the high solubility of oxygen in these viously.[10a] We did not try to determine the substitution
pattern of H2TPP(C8F17)4 (2) and the fluorinated chains
[a] Key Laboratory of Organofluorine Chemistry, Shanghai Insti-
are therefore shown at random positions. Insertion of the
tute of Organic Chemistry, Chinese Academy of Sciences,
metal atom (Co, Ni, Fe, or Mn) into the free-base porphy-
354 Fenglin Road, Shanghai 200032, China
E-mail: Chenqy@mail.sioc.ac.cn
rin H2TPP(C8F17)4 (2), according to a literature pro-
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C. Liu, D.-M. Shen, Q.-Y. Chen
FULL PAPER
cedure,[12] afforded the corresponding metalloporphyrins.
Although the fluorine load of these metalloporphyrins (F
Ϸ 55%) is lower than that required to make these fluorous
biphasic oxidation catalysts soluble in perfluorocarbons (F
Ͼ 60%)[6] (and Pozzi[13] has reported that the introduction
of four C8 perfluoroalkyl tails at the periphery of a tetraar-
ylporphyrin is not sufficient to ensure its solubility in per-
fluorocarbons), they readily dissolve in perfluorodecalin, al-
though not in CH3CN, CH2Cl2, acetone, or alcohols, as
determined by visual tests. The remarkably high fluorous
affinity of M2 (M = Cu, Ni, Co, Fe, Mn) for perfluorocar-
bons is probably related to their increasing electroneutrality
upon introduction of a perfluoroalkyl chain at the β-posi-
tions because the very low dielectric constants of perfluoro-
carbons diminishes the solubilization of charged complexes,
even though they are highly fluorinated.[14] Indeed, we
found that the solubility of M2 (M = Cu, Ni, Co) in perflu-
orocarbons is higher than that of M2 (M = Fe, Mn) because
of the cationic nature of the latter. The present results also
confirm that the fluorine load alone is not sufficient to be
able to predict the solubility of a molecule, and other fea-
tures such as the number, length, and position of the RF
substituents must be taken into account.[14] In contrast,
these (β-perfluoralkylated tetraphenylporphyrin)metal com-
plexes [M{TPP(C8F17)4}] (M2; M = Cu, Ni, Co, Fe, Mn)
are very soluble in diethyl ether, making the workup conve-
nient, and this can be ascribed to their favorable interaction
Scheme 1. Synthesis of various (β-perfluoralkylated tetraphenyl-
porphyrin)metal complexes [M{TPP(C8F17)4}] (M2).
tions is generally required in order to maintain the activ-
ity of the metal catalysts. Our catalytic system is more con-
venient and efficient and uses air instead of pure oxygen.
Table 1. Epoxidation of stryene under fluorous biphasic conditions
with the RF tails.[13] The partition coefficient of Co2 in per- catalyzed by various (β-perfluoralkylated tetraphenylporphyrin)-
metal complexes [M{TPP(C8F17)4}] (M2).[a]
fluorodecalin/CH3CN (1:1, v/v) is very high at 20 °C; it can-
M{TPP(C8F17)4} (M2) mol-% Time [h] Yield [%][b]
not be detected by UV/Vis spectroscopy in the CH3CN
layer, despite its characteristic absorption at 435 nm, after
stirring a solution of Co2 in perfluorodecalin with CH3CN
(1:1, v/v) for 2 h. We therefore took advantage of the prefer-
ential solubility of M2 (M = Cu, Ni, Co, Fe, Mn) in per-
fluorodecalin for their isolation from the reaction mixture
and for the fluorous biphasic catalytic oxidation of alkenes
and aldehydes (Scheme 1).
Entry
1
2
3
4
5
6
7
8
–
–
24
24
20
24
24
12
12
12
0
0
Cu2
Ni2
Fe2
Mn2
Co2
Co2
Co2
0.4
0.4
0.4
0.4
0.4
0.1
1.0
55
42
40
65
60
67
Using styrene as a model substrate, we first evaluated the
catalytic oxidation activity of various (β-perfluoralkylated
tetraphenylporphyrin)metal complexes [M{TPP(C8F17)4}]
(M2; M = Cu, Ni, Co, Fe, Mn). The results are summarized
in Table 1. The reactions were carried out at room tempera-
ture in flasks carefully shielded from direct sunlight. A solu-
[a] Reaction conditions: styrene (1.0 mmol), 2-methylpropanal
(2.0 mmol) in perfluorodecalin (5 mL) and CH3CN (5 mL) were
stirred vigorously under 1 atm of air in the dark. [b] Determined
by GC using C12H26 as internal standard.
The optimized reaction conditions described above are
tion of the complex in perfluorodecalin was added to a suitable for other alkenes as well (Table 2). In general, vari-
solution of styrene in CH3CN containing a twofold excess ous alkenes can be epoxidized smoothly in acceptable yield.
of 2-methylpropanal, as a sacrificial reducing agent,[15] with For typical fluorous catalysis the most important aspect is
respect to the styrene. The resulting biphasic mixture was the simple separation, recycling, and re-use of the catalyst.
vigorously stirred under 1 atm of air. After completion of Experiments aimed at determining the extent of catalyst re-
the reaction, the two layers were easily separated and the cyclability were carried out with stilbene. The fluorous
CH3CN layer was analyzed by GC. In the absence of M2 phase was reused up to five times without apparent loss of
the styrene did not react. Although the fluorous biphasic activity (Entries 7–12).
oxidation can go smoothly in the presence of various M2
To further investigate the generality and scope of the sys-
complexes, [Co{TPP(C8F17)4}] (Co2) was found to be the tem, the protocol was extended to simple aromatic alde-
most efficient catalyst. The suitable loading of catalyst was hydes (Table 3). In most cases, the oxidation reaction pro-
0.4 mol-%. It is worth mentioning that air is a safe, clean, ceeded smoothly in good yields. As expected, the reactivity
and freely available oxidant, although the use of 1 atm of of aldehydes with electron-donating substituents was higher
air in most aerobic oxidation systems is still limited as pure than those with electron-withdrawing ones (Entries 2 and 3
oxygen or a high pressure of air under severe reaction condi- vs. 7 and 8).
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Eur. J. Org. Chem. 2006, 2703–2706

Fluorous Biphasic Catalytic Oxidation of Alkenes and Aldehydes
FULL PAPER
Table 2. Epoxidation of different alkenes with 1 atm of air cata-
lyzed by [Co{TPP(C8F17)4}] (Co2) under fluorous biphasic condi-
tions.[a]
Experimental Section
General: 1H (300 MHz) and 19F (282 MHz) NMR spectra were re-
corded with a Bruker AM-300 or Varian-360L spectrometer.
Chemical shifts are reported in ppm relative to TMS as an internal
standard (δ = 0 ppm) for 1H NMR spectra and CFCl3 as an exter-
nal standard (negative for upfield) for 19F NMR spectra. Deuter-
ated solvents for NMR were purchased from Cambridge Isotope
Laboratories, Aldrich, or Acros. MS and HRMS data were re-
corded with a Hewlett–Packard HP-5989A spectrometer and a Fin-
nigan MAT-8483 mass spectrometer, respectively. UV/Vis spectra
were measured with a Varian Cary 100 spectrophotometer. Elemen-
tary analyses were obtained with a Perkin–Elmer 2400 Series II
Elemental Analyzer. TLC analysis was performed on silica gel
plates and column chromatography on silica gel (mesh 300–400),
which were both obtained from Qingdao Ocean Chemicals. DMSO
was distilled from CaH2. All the other solvents and chemicals were
of reagent grade, purchased commercially, and used without further
purification unless otherwise noted.
Synthesis of Copper Powder: CuSO4·5H2O (32 g, 128 mmol) was
dissolved in H2O (160 mL) and zinc powder (8 g, 123 mmol) was
added in several small portions. The resulting solution was stirred
for 30 min. A HCl solution (2 , 40 mL) was then added and the
mixture stirred for another 30 min. The precipitated red copper was
collected by filtration under reduced pressure, washed with acetone,
dried under vacuum at room temperature and kept under N2.
[a] Reaction conditions: alkene (1.0 mmol), 2-methylpropanal
(2.0 mmol), and Co2 (0.4 mol-%) in perfluorodecalin (5 mL) and
CH3CN (5 mL) were stirred vigorously under 1 atm of air in the
dark. [b] Isolated yields. [c] GC yields using C12H26 as the internal
standard. [d] Reaction run with the fluorous layer recovered from
the previous Entry.
General Procedure for Preparing [Cu{TPP(C8F17)4}] (Cu2): An
oven-dried 50-mL Schlenk flask was charged with [Cu{TPP(Br)8}]
(Cu1; 100 mg, 1.0 equiv.), Pd2(dba)3·CHCl3/AsPh3 (10 mol-%/
80 mol-%), and Cu (80 equiv. with respect to Cu1). The flask was
then evacuated and refilled with argon (three cycles). DMSO
(20 mL) and C8F17I (40 equiv. with respect to Cu1) were then
added at room temperature. The resulting mixture was stirred at
100 °C for 3 h and then allowed to reach room temperature. The
reaction mixture was poured on the top of a short silica column
and washed with CH3CN under pressure; the eluate was discarded.
The column was then eluted with Et2O. The filtrate was collected
and washed with water three times. The organic layer was dried
with Na2SO4 and the solvents were evaporated to dryness. The re-
sulting solid was purified by flash column chromatography using
petroleum ether as eluent to provide the desired product Cu2.
Yield: 146 mg (82%). MS (MALDI): m/z = 2347.0 [M+]. UV/Vis
Table 3. Oxidation of aromatic aldehydes with 1 atm of air cata-
lyzed by [Co{TPP(C8F17)4}] (Co2) under fluorous biphasic condi-
tions.[a]
(PE): λmax (%)
= 436 (18.9), 578 (1.2), 623 (1.0) nm.
C76H24CuF68N4·5H2O (2437.1): calcd. C 37.43, H 1.41, N 2.30;
found C 37.00, H 1.22, N 2.00.
Demetalation of Cu2: [Cu{TPP(C8F17)4}] (100 mg, 1.0 equiv.) was
dissolved in TFA/concd. H2SO4 (4:1, v/v; 4 mL) and Et2O (4 mL)
and the mixture was stirred at room temperature for 2 h, then
poured carefully into ice/water and extracted with Et2O. The or-
ganic layer was washed with saturated NaHCO3 solution and
water. It was then dried with Na2SO4 and the solvents were evapo-
rated to dryness. The resulting solid could be used directly for the
next reaction.
[a] Reaction conditions: aldehyde (1.0 mmol), 2-methylpropanal
(2.0 mmol), and Co2 (0.4 mol-%) in perfluorodecalin (5 mL) and
CH3CN (5 mL) were stirred vigorously under 1 atm of air in the
dark. [b] Isolated yields.
Conclusion
In summary, we have demonstrated for the first time a
simple and facile fluorous biphasic catalytic oxidation of
alkenes and aldehydes with 1 atm of air and 2-methylpro-
panal in the presence of the (β-perfluoralkylated tetraphen-
H2TPP(C8F17)4 (2): Yield: 94 mg (96%). MS (MALDI): m/z =
2286.1 [M+]. UV/Vis (PE): λmax (%) = 443 (28.3), 552 (1.1), 606
(1.7), 716 (1.0) nm. HRMS (MALDI): calcd. for [C76H27N4F68]+
2287.1088; found 2287.11443. C76H26F68N4·5H2O (2376.2): calcd.
ylporphyrin)cobalt complex [Co{TPP(C8F17)4}] (Co2). Be- C 38.40, H 1.53, N 2.36; found C 37.91, H 1.06, N 1.95.
cause of the ready availability and unique properties of β-
General Procedure for Cobalt and Nickel Insertion into
H2TPP(C8F17)4 (2):[12a] H2TPP(C8F17)4 (2; 100 mg, 1.0 equiv.),
M(OAc)2·4H2O (5.0 equiv.), and DMF (10 mL) were heated at
100 °C for 1 h. The resulting mixture was cooled to room tempera-
perfluoralkylated porphyrins, they may be applied for other
catalysis reactions. Further work on this catalytic system is
now in progress.
Eur. J. Org. Chem. 2006, 2703–2706
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C. Liu, D.-M. Shen, Q.-Y. Chen
FULL PAPER
ture, taken up in Et2O, and transferred to a separatory funnel. The
mixture was washed with water three times and the organic layer
was filtered through a short silica column. The filtrate was collected
and the solvents were evaporated to dryness to yield the desired
products. This compound was sufficiently pure for further reactions
and an analytical sample was obtained by flash column chromatog-
raphy using petroleum ether as eluent.
[1]
[2]
The Porphyrin Handbook (Eds.: K. M. Kadish, K. M. Smith,
R. Guilard), Academic Press, San Diego, 2000–2003, vol. 1–
20.
a) F. Montanari, L. Casella, Metalloporphyrin-Catalyzed Oxi-
dations, Kluwer Academic Publishers, Dordrecht, The Nether-
lands, 1994; b) R. A. Sheldon, Metalloporphyrins in Catalytic
Oxidations, Marcel Dekker, New York, 1994.
[Co{TPP(C8F17)4}] (Co2): Yield: 97 mg (95%). MS (MALDI): m/z
= 2343.0 [M+]. UV/Vis (PE): λmax (%) = 435 (17.5), 574 (1.3), 623
(1.0) nm. HRMS (MALDI): calcd. for [C76H24N4F68Co]+
2343.0232; found 2343.02415. C76H24CoF68N4·H2O (2361.0):
calcd. C 38.65, H 1.11, N 2.37; found C 38.47, H 1.45, N 2.17.
[3]
a) D. Dolphin, T. G. Traylor, L. Y. Xie, Acc. Chem. Res. 1997,
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Handbook of Fluorous Chemistry (Eds.: J. A. Gladysz, D. P.
Curran, I. T. Horváth), Wiley-VCH, Weinheim, 2004; b) Mod-
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Dobbs, M. R. Kimberley, J. Fluorine Chem. 2002, 118, 3–17
and references cited therein.
[4]
[5]
[Ni{TPP(C8F17)4}] (Ni2): Yield: 96 mg (94%). MS (MALDI): m/z
= 2343.0 [M+]. UV/Vis (PE): λmax (%) = 441 (28.4), 560 (1.9), 600
(1.0) nm. HRMS (MALDI): calcd. for [C76H24N4F68Ni]+
2343.0294; found 2343.02630. C76H24F68N4Ni·1.5H2O (2369.0):
calcd. C38.50, H 1.15, N 2.36; found C 38.12, H 0.85, N 2.29.
[6]
General Procedure for Iron and Manganese Insertion into
H2TPP(C8F17)4 (2):[12b] H2TPP(C8F17)4 (2; 100 mg, 1.0 equiv.),
MCl2·4H2O (5.0 equiv.), 2,6-lutidine (two drops), and Et2O/MeOH
(4:1, v/v; 50 mL) were heated at 40 °C overnight. The resulting mix-
ture was cooled to room temperature and washed with water three
times. The organic layer was dried with Na2SO4 and the solvents
were evaporated to dryness. The resulting solid was purified by
flash column chromatography with petroleum ether as eluent.
[7]
[8]
J. G. Riess, M. L. Blanc, Pure Appl. Chem. 1982, 54, 2383–
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S. G. Dimagno, P. H. Kussault, J. A. Schultz, J. Am. Chem. Soc.
1996, 118, 5312–5313.
[Fe{TPP(C8F17)4}Cl] (Fe2): Yield: 94 mg (91%). MS (MALDI): m/z
= 2375.0 [M+]. UV/Vis (PE): λmax (%) = 364 (8.3), 442 (23.2), 512
(1.0), 560 (1.3) nm. HRMS (MALDI): calcd. for
[C76H24N4F68ClFe]+ 2374.9925; found 2374.99475. C76H24F68FeN4
(2340.0): calcd. C 38.41, H 1.02, N 2.36; found C 38.36, H 1.16, N
2.24.
[9] G. Pozzi, F. Montanari, S. Quici, Chem. Commun. 1997, 69–
70.
[10] a) C. Liu, Q. Y. Chen, Synlett 2005, 8, 1306–1310; b) C. Liu,
Q. Y. Chen, Eur. J. Org. Chem. 2005, 3680–3686; c) Z. Zeng,
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mun. 2005, 4982–4984; e) C. Liu, D. M. Shen, Q. Y. Chen,
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Guo, Q. Y. Chen, Acta Chim. Sin. 2004, 62, 288–294; g) L.
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[11] P. Bhyrappa, V. Krishnan, Inorg. Chem. 1991, 30, 239–245.
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Tetrahedron 1997, 53, 6145–6162.
[Mn{TPP(C8F17)4}Cl] (Mn2): Yield: 97 mg (93%). MS (MALDI):
m/z = 2374.0 [M+]. UV/Vis (PE): λmax (%) = 446 (10.5), 496 (3.6),
610 (1.0), 680 (1.0) nm. HRMS (MALDI): calcd. for
[C76H24N4F68ClMn]+
2373.9891;
found
2374.99785.
C76H24ClF68MnN4 (2340.0): calcd. C 38.43, H 1.02, N 2.36; found
C 38.43, H 1.22, N 2.44.
General Procedure for the Fluorous Biphasic Catalytic Oxidation of
Alkenes and Aldehydes: A 50-mL Schlenk flask was charged with
an alkene or aldehyde (1.0 mmol), 2-methylpropanal (2.0 mmol),
Co2 (0.4 mol-%), perfluorodecalin (5 mL), and CH3CN (5 mL).
The resulting mixture was stirred vigorously under 1 atm of air in
the dark. After completion of the reaction, the fluorous layer was
recovered, washed with CH3CN, and reused in further runs. The
combined CH3CN layers were subjected to GC analysis or flash
column chromatography to yield the desired products.
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Acknowledgments
We thank the Natural Science Foundation of China (nos. 20272026,
D20032010 and 20532040) for support of this work.
Received: January 16, 2006
Published Online: April 21, 2006
2706
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2703–2706