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Experimental Section
Table 6. Oxidative dimerization of miscellaneous phenol-related com-
pounds with AuCNT.[a]
General procedure for the oxidation of 1,4 and 1,2-dihydroxyben-
zenes (1a–e and 3b,c) into the corresponding quinones (2a–e and
4b,c). Dihydroxybenzene (0.23 mmol), K2CO3 (0.23 mmol) and
AuCNT (100 mL of a 3 mm suspension in H2O—0.13 mol%) were
stirred in a biphasic mixture of CHCl3 (1.5 mL) and H2O (0.5 mL) at
room temperature. The reaction mixture was stirred until complete
consumption of the starting material (confirmed by TLC). After
completion of the reaction, the catalyst was removed by filtration
and the aqueous phase was extracted with CH2Cl2. The combined
organic layers were dried on Na2SO4, filtered, and evaporated
under vacuum. In most cases, the product was obtained pure with-
out any further purification.
Entry Substrate
7
t [h] Product
8
Yield [%][b]
1
7a 24
8a 95
General procedure for the oxidation of 2-aminophenols (5a,b) into
the corresponding aminophenoxazinone (6a,b). Aminophenol
(0.23 mmol), K2CO3 (0.23 mmol) and AuCNT (100 mL of a 3 mm sus-
pension in H2O—0.13 mol%) were stirred in a biphasic mixture of
CHCl3 (1.5 mL) and H2O (0.5 mL) at room temperature. The reaction
mixture was stirred until complete consumption of the starting
material (confirmed by TLC). After completion of the reaction, the
catalyst was removed by filtration and the aqueous phase was ex-
tracted with CH2Cl2. The combined organic layers were dried on
Na2SO4, filtered, and evaporated under vacuum. The dimerized
product was obtained pure without any further purification.
2[c]
7b 35
8b 99
3
7c 24
8c 99
Recycling experiment. Under air, to a stirred solution of hydroqui-
none 1a (0.23 mmol) in a CHCl3/H2O 3:1 mixture (2 mL), were
added K2CO3 (0.23 mmol) and the AuCNT catalyst (100 mL of
a 3 mm suspension in H2O—0.13 mol%). After 6 h of reaction at
room temperature, the catalyst was recovered by centrifugation
and reused without further purification.
[a] Conditions: 7 (0.23 mmol), AuCNT (0.13 mol%), K2CO3 (1 equiv), CHCl3/
H2O 3:1 (2 mL), room temp., under air. [b] Isolated yields. [c] Methanol
was used as solvent.
In furtherance of this study, we investigated the behavior of
phenol-related compounds upon treatment with the AuCNT
catalyst. Thiophenol (7a) was converted into the correspond-
ing dimer 8a, which incorporated a disulfide bond, within 24 h
of reaction in 95% yield (Table 6, Entry 1). The reaction of
2,6-di-tert-butylphenol (7b) led to the oxidative coupling of 7b
followed by further oxidation into dimeric 8b (Entry 2). It is
worth noting that the conversion took place at room tempera-
ture without any additional oxidant, as opposed to recent re-
ports on the same transformation catalyzed by TiO2-supported
gold nanoparticles,[18] molybdate surfactants,[19] or dicopper(II)
complexes,[20] which all required H2O2 and heating. 4-Methoxy-
phenol (7c) was fully converted to ether 8c within 24 h
through an oxidative OꢀC ortho-coupling (Entry 3) in lieu of
the CꢀC para-coupling observed in the above case.
TON and TOF experiment. Hydroquinone 1b (38 mg, 0.23 mmol),
K2CO3 (32 mg, 2.3 mmol) and AuCNT (10 mL of a 3 mm suspension
in H2O—0.013 mol%) were mixed in a CHCl3/H2O 3:1 mixture
(2 mL), and stirred for 16 h. The catalyst was then removed
through filtration and the filtrate was concentrated under vacuum
1
and 55% yield of 2b was measured by H NMR.
TON for 2 b¼ product ðmmolÞ=catalyst ðmmolÞ
¼ 0:1265=0:00003 ¼ 4 217
TOF for 2 b ¼ TON=time ¼ 4 217=16 ¼ 263 hꢀ1
Spectral data for compounds 2a-e; 4b,c; 6a,b; 8a-c. 2a: 1H NMR
(CDCl3): d=6.79 (s, 4H) ppm. 13C NMR (CDCl3): d=136.4,
187.1 ppm. 2b: 1H NMR (CDCl3): d=1.28 (s, 9H), 6.59 (s, 1H),
6.67 ppm (d, 2H). 13C NMR (CDCl3): d=29.0, 35.2, 131.4, 134.8,
138.6, 155.9, 187.3, 188.3 ppm. 2c: 1H NMR (CDCl3): d=2.03–2.04
(m, 9H), 6.55 ppm (s, 1H). 13C NMR (CDCl3): d=12.0, 12.3, 15.8,
133.0, 140.7, 140.8, 145.3, 187.4, 187.8 ppm. 2d: 1H NMR (CDCl3):
d=6.80–6.83 (m, 1H), 6.91–6.94 (d, 1H), 7.02 ppm (d, 1H). 13C NMR
(CDCl3): d=133.7, 136.0, 136.8, 144.1, 179.0, 184.9 ppm. 2e:
1H NMR (CDCl3): d=6.97 (s, 2H), 7.74–7.77 (m, 2H), 8.07–8.09 ppm
(m, 2H). 13C NMR (CDCl3): d=126.3, 131.8, 133.9, 138.6, 185.0 ppm.
In summary, the AuCNT system reported herein compares fa-
vorably with other supported noble metal catalysts in terms of
overall efficacy as it is suitable for the room temperature oxida-
tion of various substrates (phenols, hydroquinones, catechols,
aminophenols, and thiols), operates under air atmosphere (no
external oxidant needed), can readily be recycled, and induces
efficient conversion with as little as 0.013 mol% of gold con-
tent. Further investigations are currently underway to extend
the scope of our AuCNT nanohybrid.
1
4b: H NMR (CDCl3): d=1.21 (s, 9H), 6.25 (s, 1H), 6.35–6.38 (d, 1H),
7.16–7.19 ppm (m, 1H). 13C NMR (CDCl3): d=27.7, 35.6, 123.7,
129.3, 140.0, 162.1, 180.2 ppm. 4c: 1H NMR (CDCl3): d=1.21 (s, 9H),
1.26 (s, 9H), 6.21 (d, 1H), 6.92 ppm (d, 1H). 13C NMR (CDCl3): d=
27.8, 29.2, 35.4, 36.0, 122.0, 133.4, 149.9, 163.2, 179.9, 181.1 ppm.
1
6a: H NMR ([D6]DMSO): d=6.34 (s, 2H), 6.79 (2H, br s), 7.35–7.40
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ChemCatChem 2014, 6, 719 – 723 722