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RSC Advances
Notes and references
1 (a) A. S. Hay, H. S. Blanchard, G. F. Endres and J. W. Eustance,
J. Am. Chem. Soc., 1959, 81, 6335; (b) H. Finkbeiner, A. S. Hay, H.
S. Blanchard and G. F. Endres, J. Org. Chem., 1966, 31, 549; (c)
C. C. Price and K. Nakaoka, Macromolecules, 1971, 4, 363; (d)
J. Kresta, A. Tkac, R. Plikryl and L. Malik, Die Macromol. Chem.,
1975, 176, 157; (e) C. Gu, K. Xiong, B. Shentu, W. Zhang and
Z. Weng, Macromolecules, 2010, 43, 1695; (f) P. Gamez, S. Gupta
and J. Reedijk, C. R. Chimie, 2007, 10, 295, and references
therein.
2 (a) G. Pandey, C. Muralikrishna and U. T. Bhalerao, Tetrahedron
Lett., 1990, 26, 3771; (b) R. Gupta and R. Mukherjee,
Tetrahedron Lett., 2000, 41, 7763; (c) B.-S. Liao, Y.-H. Liu, S.-
M. Peng and S.-T. Liu, Dalton Trans., 2012, 41, 1158, and
references therein.
3 P. M. Borsenberger, W. T. Gruenbaum, M. B. Oregan and L.
J. Rossi, J. Polym. Sci., Part B: Polym. Phys., 1995, 33, 2143.
4 (a) A. S. Hay, D. M. White, B. M. Boulette, S. A. Nye and H. S.
I. Herbert, J. Org. Chem., 1988, 53, 5959; (b) L. Barnes, N.
W. Eilerts and J. A. Heppert, Polyhedron, 1994, 13, 743; (c) G.-
C. Chen, C.-J. Huang, A. P. Tu and K. Y. Hwang, US Pat., 7 994
358, 2010.
5 Designing Dendrimers, ed. by S. Campagna, P. Ceroni and F.
Puntoriero, John Wiley & Sons, Hoboken, New Jersey, 2012.
6 (a) J. Kofoed and J.-L. Reymond, Curr. Opin. Chem. Bio., 2005, 9,
Fig. 1 Proposed reaction intermediate within G4-Cu2+
.
12
mononuclear Cu azide complex.18b,c The spectra of the G4-
Cu2+n(N3) series of dendrimer-Cu species exhibited a clear
correlation with the selectivity of the corresponding G4-Cu2+
n
catalysts (Table 1, entries 1–5). G4-Cu2+ (n ¢ 12) indicating the
n
formation of binuclear Cu azide complexes showed high
selectivities for the C–C coupling (Table 1, entries 3–5), while G4-
Cu2+ (n = 2 and 8) having the mononuclear Cu azide complexes
n
gave poor selectivities for the C–C coupling (Table 1, entries 1 and
2). These results demonstrate that the formation of adjacent Cu2+
species within the G4-TEBA nanovoids is essential for high
selectivity for the C–C selective coupling of 1a.1b,1c,2
´
656; (b) B. Helms and J. M. J. Frechet, Adv. Synth. Catal., 2006,
We hypothesize the reaction path for the C–C coupling of 1a to
2a involving the adjacent Cu2+ species within G4-Cu2+12 as follows;
a ligand exchange reaction of a Cu2+-Cl species with 1a occurs to
form a Cu2+-phenolate with concurrent generation of HCl which is
trapped by a tertiary amino group on the dendrimer.1d Within the
G4-Cu2+12, the two Cu2+-phenolates are suitably oriented for the C–
C selective coupling reaction via one-electron oxidation of the
phenolates (Fig. 1), giving 3a,2c and successive oxidation of 3a
yields the final product 2a.20 The regularly arranged tertiary amino
groups which compose the boundaries of the nanovoid thus work
to (1) coordinate Cu ions to generate adjacent active Cu species, (2)
promote the facile ligand exchange of Cu–Cl with 1a through the
trapping of HCl at basic sites21 and (3) accumulate both adjacent
Cu species and basic sites within the confines of the nanovoids.
In conclusion, we found that the PPI dendrimer-encapsulated
348, 1125; (c) A. Ouali and A.-M. Caminade, in Dendrimers:
towards catalytic, material and biomedical uses, ed. A.-M.
Caminade, C.-O. Turrin, R. Laurent, A. Ouali and B. Delavaux-
Nicot, John Wiley & Sons, Chichester, 2011, ch. 4, pp. 183–195;
(d) K. Kirkorian, A. Ellis and L. J. Twyman, Chem. Soc. Rev.,
2012, 41, 6138.
7 (a) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani and K. Kaneda,
J. Am. Chem. Soc., 2004, 126, 1604; (b) Z. Maeno, T. Mitsudome,
T. Mizugaki, K. Jitsukawa and K. Kaneda, Heterocycles, 2012, 86,
947.
8 M. Ooe, M. Murata, T. Mizugaki, K. Ebitani and K. Kaneda,
Nano Lett., 2002, 2, 999.
9 (a) T. Mizugaki, T. Kibata, K. Ota, T. Mitsudome, K. Ebitani,
K. Jitsukawa and K. Kaneda, Chem. Lett., 2009, 38, 1118; (b)
Z. Maeno, T. Kibata, T. Mitsudome, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Chem. Lett., 2011, 40, 180; (c) T. Kibata,
T. Mitsudome, T. Mizugaki, K. Jitsukawa and K. Kaneda, Chem.
Commun., 2013, 49, 167.
Cu2+ complex G4-Cu2+ is capable of catalyzing the regioselective
12
oxidative coupling of 2,6-dimethylphenol to tetramethyldipheno-
quinone. G4-Cu2+ was also applicable to gram-scale synthesis
10 Z. Maeno, T. Mitsudome, T. Mizugaki, K. Jitsukawa and
K. Kaneda, Chem. Lett., 2012, 41, 801.
12
and exhibited much higher catalytic activity than previously
reported catalysts. Moreover, this catalyst could be successfully
reused without significant loss of its activity or selectivity. The
extremely high efficiency of this catalyst may be attributed to the
presence of both adjacent Cu species and basic sites within the
nanovoids of the PPI dendrimer following the encapsulation of the
appropriate quantity of Cu2+ ions.
11 (a) E. M. M. de Brabander-van den Berg and E. W. Meijer,
Angew. Chem., Int. Ed. Engl., 1993, 32, 1308; (b) S. Stevelmans, J.
C. M. van Hest, J. F. G. A. Jansen, D. A. F. J. van Boxtel, E. M.
M. de Brabander-van den Berg and E. W. Meijer, J. Am. Chem.
Soc, 1996, 118, 7398.
12 The UV-vis spectra and the titration curve of Gx-Cu2+n are shown
in Fig. S2–S4 in the ESI. The same phenomena have been
3
previously observed in the UV-vis measurements of dendrimer-
encapsulated Cu complexes. See: Y. Niu and R. M. Crooks,
Chem. Mater., 2003, 15, 3463.
Acknowledgements
This study was supported by JSPS KAKENHI (22360339,
23656514, and 24246129). We thank Dr Tetsuo Honma and
Dr Masafumi Takagaki (JASRI) for XAFS measurements.
13 (a) U. L. Kala, S. Sima, M. R. P. Kurup, S. Krishnan and R.
P. John, Polyhedron, 2007, 26, 1427; (b) S. Pande, M. G. Weir, B.
A. Zaccheo and R. M. Crooks, New J. Chem., 2011, 35, 2054.
9664 | RSC Adv., 2013, 3, 9662–9665
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