Wacker oxidation in an aqueous phase through the reverse phase-transfer catalysis of a self-assembled nanocage

Article abstract of DOI:10.1246/cl.2000.598

Self-assembled nanocage 1 was found to promote the aerobic, aqueous oxidation of styrene and its derivatives with the aid of (en)Pd(NO₃)₂. Obviously, the reaction was promoted by a double catalysis system: i.e., cage 1 acted as a reverse phase-transfer catalyst, whereas (en)Pd²⁺ as an oxidation catalyst.

Full text of DOI:10.1246/cl.2000.598

5
98
Chemistry Letters 2000
Wacker Oxidation in an Aqueous Phase through the Reverse Phase-Transfer Catalysis
of a Self-Assembled Nanocage
†
†,††
Hirokazu Ito, Takahiro Kusukawa,* and Makoto Fujita*
The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603
†
††
CREST, Japan Science and Technology Corporation (JST), Chikusa-ku, Nagoya 464-8603
(Recevied Febrauary 28, 2000; CL-000193)
Self-assembled nanocage 1 was found to promote the aero-
bic, aqueous oxidation of styrene and its derivatives with the
tion of other metal components will cause unfavorable metal
exchange in the cage framework. Notably, however, cage 1
remains intact upon the addition of the same metal component,
(en)Pd(NO ) . After the solution was heated at 80 °C for 24 h,
aid of (en)Pd(NO ) . Obviously, the reaction was promoted by
3
2
a double catalysis system: i.e., cage 1 acted as a reverse phase-
3
2
2+
1
transfer catalyst, whereas (en)Pd as an oxidation catalyst.
the aqueous phase was again monitored by H NMR. The sig-
nals of styrene disappeared and a new set of signals appeared at
δ 6.6-7.3 and δ 1.7 ppm (Figure 1b). The product, after being
Chemical transformation in aqueous media is a current
trend in synthetic chemistry in view of developing clean tech-
nology.1 One of the most promising approaches to water-
based reactions is to develop reverse phase-transfer catalysts3
which bring organic substrates into aqueous phase, let them
react with aqueous reagents, and take out products from the
aqueous phase. Recently reported coordination nanocage 1 is
expected to be a good and unique candidate for the reverse
phase-transfer catalyst because of its good solubility in water
and high binding ability toward large and neutral organic mole-
cules.4 Although chemical transformations within self-assem-
extracted with CDCl , was identified as acetophenone and the
3
yield was determined by NMR to be 82% (Table 1, run 1).
,2
5
bled cages were reported recently, their phase transfer catalysis
has not been explored. Only a few previous reports dealt with
reverse phase-transfer catalyst, wherein cyclodextrins were
3
employed as catalysts.
In expectation of developing such a functional coordination
system, we studied the catalysis by cage 1 of Wacker oxidation
of olefins which is a typical Pd(II)-promoted catalytic reaction.
Thus, when styrene (0.05 mmol) was suspended in D O (1 mL)
2
solution of 1 (0.005 mmol), styrene was partly transferred into
the aqueous phase forming 1·(styrene) clathrate complex (n =
n
1
6
ca. 3) as monitored by H NMR (Figure 1a). The signals of
styrene were highly upfield shifted due to the enclathration in
the cavity. To this mixture, (en)Pd(NO ) (2, 0.005 mmol, 10
3
2
mol%) was added as an aqueous reagent to promote the reac-
tion efficiently. Since Pd(II)-pyridine bond is reversible, addi-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
599
to acetophenone and its derivatives in good yields (Table 1,
runs 5 and 6). In contrast, the oxidation of p-nitrostyrene,
which is poorly bound by 1, was hardly observed (run 7). The
reaction was also sensitive to the size of the substrate: 2-vinyl-
naphthalene seems less effectively bound by 1 and the yield
was low (run 8).
References and Notes
1
W. A. Herrman and C. W. Kohlpaintner, Angew. Chem.,
Int. Ed. Engl., 32, 1524 (1993); C. -J. Li, Chem. Rev., 93,
2023 (1993).
2
Selected reports or organic transformation in aqueous
media see: D. V. McGrath, R. H. Grubbs, and J. W. Ziller,
J. Am. Chem. Soc., 113, 3611 (1991); K. Sato, M. Aoki, M.
Ogawa, T. Hashimoto, and R. Noyori, J. Org. Chem., 61,
8
310 (1996); K. Sato, M. Aoki, and R. Noyori, Science,
2
81, 1646 (1998); S. Kobayashi, S. Nagayama, and T.
Busujima, J. Am. Chem. Soc., 120, 8287 (1998); S.
Kobayashi, Chem. Lett., 1991, 2087; B. M. Novak and R.
H. Grubbs, J. Am. Chem. Soc., 110, 7542 (1988); D. M.
Lynn, B. Mohr, and R. H. Grubbs, J. Am. Chem. Soc., 120,
1627 (1998).
A. Harada, Y. Hu, and S. Takahashi, Chem. Lett., 1986,
2083; Y. Hu, A. Harada, and S. Takahashi, Synth.
Commun., 18, 2083 (1988). Also see: R. Breslow and D.
D. Steven, Chem. Rev., 98, 1997 (1998); K. Takahashi,
Chem. Rev., 98, 2013 (1998).
M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K.
Yamaguchi, and K. Ogura, Nature, 378, 469 (1995); T.
Kusukawa and M. Fujita, Angew. Chem., Int. Ed. Engl., 37,
3142 (1998); T. Kusukawa and M. Fujita, J. Am. Chem.
Soc., 121, 1397 (1999).
Control experiments revealed that this reaction proceeded
in the aqueous phase. Under the same conditions, no reaction
took place in the absence of cage 1 (Table 1, run 2). Even in
the presence of cage 1, the reaction was suppressed by the addi-
tion of an "inhibitor", 1,3,5-trimethoxybenzene (1 equiv to
styrene), which has much higher affinity to the cage than
styrene (run 3). Small amount of (en)Pd(NO ) (10 mol%) was
essential because the catalytic activity of Pd(II)-linked cage 1
itself for the oxidation of styrene was inferior (conversion: 4%),
unfortunately (run 4).
3
4
5
3
2
Obviously and interestingly, the reaction was promoted by
a unique double catalysis system: i.e., cage 1 acted as a reverse
2+
phase-transfer catalyst, whereas (en) Pd as an oxidation cata-
lyst. Concerning the reverse phase-transfer catalysis of 1, the
catalytic cycle should involve the following steps. First, a sub-
strate (styrene), which itself forms organic phase, is enclathrat-
ed by cage 1 and transferred into the aqueous phase. (ii) Then,
styrene in the cage is oxidized to acetophenone by the action of
Pd(II) reagent. (iii) Finally, acetophenone (less hydrophobic) is
replaced by unreacted styrene (more hydrophobic). The
decreased hydrophobicity of the substance should promote
smooth product-substrate replacement (Figure 2).
J. Kang and J. Rebek, Jr., Nature, 385, 50 (1997); J. Kang,
G. Hilmersson, J. Santameria, and J. Rebek, Jr., J. Am.
Chem. Soc., 120, 3650 (1998); J. Kang, J. Santameria, G.
Hilmersson, and J. Rebek, Jr., J. Am. Chem. Soc., 120,
7389 (1998); C. J. Walter, H. L. Anderson, and J. K. M.
Sanders, J. Chem. Soc., Chem. Commun., 1993, 458.
A titration experiment also showed the 1:3 host-guest stoi-
6
1
chiometry. The H NMR spectrum of 1·(styrene) complex
3
The Pd(II)-promoted Wacker oxidation in the aqueous
phase featured a remarkable aspect: the oxidation smoothly pro-
ceeded without employing any reoxidant of Pd(0) species.
Since the catalytic reaction was completely suppressed with a
degassed solvent under argon atmosphere, the Pd(0) species
involved in the catalytic cycle must be reoxidized to Pd(II) by
air. The aerobic reoxidation might be promoted by a trace
amount of metal contamination in the Pd(II) reagent. The reac-
tion efficiency was unaffected by the addition of Cu(NO ) (10
prepared stoichiometrically (270 MHz, D O, TMS as an
2
external standard): for the host, δ 2.99 (s, 24H; CH ), 8.64
2
(d, J = 6.7 Hz, 24H; PyH ), 9.25 (d, J = 6.7 Hz, 24H;
a
PyH ); for the guest, 3.35 (d, J = 11 Hz, 3H; CH), 3.63 (d,
b
J = 17 Hz, 3H; CH), 4.34 (dd, J = 11, 17 Hz, 3H; CH),
4.97 (t, J = 8 Hz, 3H; ArH(p-)), 5.27 (d, J = 8 Hz, 6H;
ArH(o-)), 5.50 (t, J = 8 Hz, 6H; ArH(m-)). Chemical shifts
shown in Figure 1a are somewhat down-field shifted due to
the complexation with slightly larger amount (> 3.0 equiv)
3
2
1
3
mol%).
of styrene. C NMR (67.5 MHz, D O, TMS as an external
2
Due to the strong binding ability of 1 toward electron-rich
aromatic compounds, the reaction was particularly efficient for
electron-rich substrates. For example, p-methoxystyrene and p-
methylstyrene, which are strongly bound by 1, were converted
standard) δ 47.35 (CH), 112.48 (CH), 124.88 (CH), 125.93
(CH), 126.57 (CH), 127.58 (CH), 135.05 (CH), 135.99
(Cq), 145.71 (Cq), 152.83 (CH), 169.54 (Cq).