Synergistic effect of cyclodextrin-based binuclear complexes in the hydrolysis of amide

Article abstract of DOI:10.1016/S0040-4039(00)00038-1

Novel cyclodextrin-based binuclear metal complexes have been prepared and proved to be far more efficient than their mononuclear complexes in promoting amide hydrolysis. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00038-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1825–1828
Synergistic effect of cyclodextrin-based binuclear complexes in
the hydrolysis of amide
Jia-Ming Yan, Masato Atsumi, De-Qi Yuan and Kahee Fujita ∗
Faculty of Pharmaceutical Sciences, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki 852-8521, Japan
Received 6 December 1999; accepted 24 December 1999
Abstract
Novel cyclodextrin-based binuclear metal complexes have been prepared and proved to be far more efficient than
their mononuclear complexes in promoting amide hydrolysis. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: cyclodextrin; binuclear metal complex; amide hydrolysis.
Amides are resonance-stabilized entities and difficult to hydrolyze. The half-life is estimated to be
hundreds of years for the hydrolysis of a typical peptide bond around neutral pH.1 In nature, various
enzymes use two metal ions in their active center to cooperatively catalyze the hydrolysis of their
substrates.2,3 It is of particular interest to develop artificial models of these binuclear metalloenzymes
because it cannot only promote the understanding of the enzymatic chemistry but also provide potential
applicability in biotechnology. Therefore, a number of binuclear metal complexes have been synthesized
by linking two metal-centers to a molecular scaffold,4,5 mainly focused on phosphate hydrolysis and
sparsely aimed at amide hydrolysis.6,7 These complexes are developed to mimic the active sites of some
binuclear metalloenzymes but most of them lack a hydrophobic substrate-binding site. The first example
of a catalytic binuclear metal complex with hydrophobic binding appeared in 1997. The calixarene-based
binuclear zinc complex showed a 2300-fold rate acceleration in phosphate diester transesterification.8–10
A cyclodextrin (CD)-based bis-tren-Zn(II) complex was also reported, but it appeared that the two metal
centers showed poor cooperation in the phosphate hydrolysis.11 We found that the β-CD-based binuclear
Zn(II) and Cu(II) complexes of 2 have high catalytic activity and the latter has significant cooperation
between the two metal centers in the hydrolysis of amide 4. In this paper, we describe the preliminary
results on the synthesis and catalytic behavior of these complexes.
Ligand 2 was prepared by reductive alkylation of CD diamine 112,13 (Scheme 1). Thus, NaBH3CN
(0.2 g) in MeOH (100 ml) was added dropwise to a MeOH (200 ml) solution containing 1 (0.5 g) and
2-pyridinecarboxaldehyde (0.38 g) at rt overnight, and the solution was stirred at rt for another 2 days.
After evaporation of the solvent, the residue was dissolved in a 10% ammonia solution (100 ml), and
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00038-1
tetl 16318

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Scheme 1.
stirred at rt for 1 day. Chromatography of the reaction mixture afforded 2 in 74% yield. By a similar
procedure, ligand 3 was synthesized for comparison.†
NMR titration of the ligand solution with Zn(NO3)2 showed that ligand 2 can easily form a binuclear
complex with Zn(II). In the 1H NMR spectrum (Fig. 1, top), ligand 2 showed two doublets around 8.37
ppm for 6-H, two doublets around 7.44 ppm for 3-H, and multiplets at 7.65 and 7.17 ppm for 4- and 5-H of
the pyridine residues. When Zn(II) was added and the Zn(II)/2 increased to 1, these signals decreased and
were finally replaced by a new set of signals resonating in the range from 6.9 to 8.7 ppm. The signals at
relatively higher fields gradually diminished when the Zn(II)/2 increased from 1 to 2. Thereafter, further
addition of Zn(II) no longer caused obvious spectral changes. This result implies that the complex 2-
[Zn(II)]2 can be easily obtained by mixing 2 with two equivalents of Zn(II). Examination of the NMR
spectra of 2-[Zn(II)]2 (Fig. 1, bottom) gave important structural information. In complex formation, three
pyridyl groups showed larger downfield shifts (∆δ 0.3–0.5 ppm) for their 4- and 5-H, smaller downfield
shifts (∆δ 0.2–0.3 ppm) for their 6-H but no obvious shift for their 3-H protons. The methylene groups
on these pyridine rings were greatly deshielded and shifted downfield by up to 0.7 ppm. The last one
Fig. 1. 500 MHz 1H NMR spectra of the ligand 2 (top) and its complex 2-[Zn(II)]2 (bottom) in DMSO[d6], assigned based on
2D COSY spectra. The pyridyl protons are numbered according to their attached positions and letters a–d stand for different
pyridine rings. M and C6 represent the methylene protons of the pendant groups and modified sugar units, respectively. The
pyridylmethylene groups are differentiated with numeral numbers. The primed numbers denote the geminal protons resonating
at higher fields
†
Ligands 2 and 3 were characterized by FAB MS, 1H and 13C NMR spectra.

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of the four pyridylmethylene residues, however, demonstrated quite a different shift pattern. Instead of
large downfield shifts, very large upfield shifts were observed for its 4-H and methylene protons (∆δ up
to 0.5 and 1.3 ppm, respectively), indicating that this residue is under a strong shielding of other pyridyl
rings. The remarkable shifts for all the four pyridylmethylene residues suggests that both amino groups
participated in chelating Zn(II), which was ascertained by the moderate downfield shifts (∆δ 0.1–0.4
ppm) for the 6- and 5-H protons of the two functional subunits.
Upon addition of CuCl2 to a solution of 2, a shoulder peak appeared around 290–360 nm. The
absorbance at 320 nm increased with increasing concentration of Cu(II) and reached a plateau when
2 equivalents of CuCl2 were added, which indicated the formation of complex 2-[Cu(II)]2. Titration of
the 2-[Cu(II)]2 with the NaOH solution gave an apparent pKa of 8.8, which was almost the same as that
of 3-Cu(II) (pKa=8.9).
Amide 4 was used as a substrate to probe the catalytic properties of the metal complexes of 2. Initial
rates were detected by following the release of N-methyl-4-nitroaniline at 400 nm, and the kinetic
parameters were listed in Table 1. This amide is inherently stable at pH 8.4 and 25°C and the reaction
half-life is over 1 year. Ligand 2 actually stabilizes the substrate by 10 times. However, when 2-[Zn(II)]2
was used, the reaction was accelerated by at least two orders of magnitude. Greater rate enhancement
was obtained in the case of 2-[Cu(II)]2. When 6.0×10−5 M amide 4 was mixed with 3.0×10−4 M ligand
2 and 6.0×10−4 M Cu(II) at pH 8.4 and 25°C, the pseudo first order rate constant was 8.5×10−6 s−1
,
corresponding to a half-life of 23 h. Under the same reaction conditions, the mononuclear complex 3-
Cu(II) was 50 times less effective than 2-[Cu(II)]2.
Table 1
Pseudo first order rate constantsa of the hydrolysis of 4 (6.00×10−5 M)

1828
The above results reveal that the binuclear complex 2-[Cu(II)]2 is an efficient catalyst for the amide
hydrolysis and the two metal centers can cooperate in catalysis and are essential for obtaining greater rate
acceleration. This synergistic effect was further proved by the pH-rate behavior of the reaction. It was
reported that in the pH range of 5–9, the uncatalyzed hydrolysis of peptide bonds was predominated by
the nucleophilic attack of H2O and was, therefore, independent of pH.1 Nevertheless, the 2-[Cu(II)]2-
mediated hydrolysis of 4 was remarkably influenced by pH variation from 7.5 to 9.3. The reaction
rate increased rapidly when pH was increased from 7.5 to 8.4, and reached an optimum at pH 8.4.
Thereafter, the rate decreased with increasing pH. This pH-rate dependence indicates that a general
acid–base bifunctional catalysis works in the hydrolysis of 4.
By measuring the initial rate at different substrate concentrations, saturation kinetics were observed
for the 2-[Cu(II)]2-mediated hydrolysis of 4. The Michaelis–Menten constant KM was 1.2 mM, the same
magnitude by which a nitrophenyl species binds to β-CD.
In conclusion, the new CD-based binuclear complex 2-[Cu(II)]2 binds and cleaves the amide 4
efficiently. The good synergistic action of the two metal centers makes the binuclear complex much
more efficient than their corresponding mononuclear complex.
Acknowledgements
We thank the Japan Maize Products Co. Ltd for a generous gift of cyclodextrins. D.-Q.Y. thanks the
NSFC for projects 29632004 and 29772023, and the State Education Ministry of China for project 380.
References
1. Smith, R. M.; Hansen, D. E. J. Am. Chem. Soc. 1998, 120, 8910.
2. Wilcox, D. E. Chem. Rev. 1996, 96, 2435.
3. Strater, N.; Lipscomb, W. N.; Klablunde, T.; Krebs, B. Angew. Chem., Int. Ed. Engl. 1996, 35, 2024.
4. Gobel, M. W. Angew. Chem., Int. Ed. Engl. 1994, 33, 1141.
5. van den Benken, E. K.; Feringa, B. L. Tetrahedron 1998, 54, 12958, and references cited therein.
6. Murthy, N. N.; Mahroof-Tahir, M.; Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 10404.
7. Chen, X.; Luo, X.; Song, Y.; Zhou, S.; Zhu, L. Polyhedron 1998, 17, 2271.
8. Molenveld, P.; Kapsabelis, S.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1997, 119, 2948.
9. Molenveld, P.; Engbersen, J. F. J.; Kooijman, H.; Spek, A. L.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 6726.
10. Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. Engl. 1999, 38, 3189.
11. Breslow, R.; Dong, S. D. Chem. Rev. 1998, 98, 1997.
12. Tabushi, I.; Kuroda, Y.; Yokota, K.; Yuan, L. C. J. Am. Chem. Soc. 1981, 103, 711.
13. Hanessian, S.; Benalil, A.; Laferriere, C. J. Org. Chem. 1995, 60, 4786.