a
Table 2 Ullmann coupling of various aryl iodides using Au@PMO
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Yield (%)
Entry
1
Substrate
Product
95
4
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Reaction conditions: iodobenzene (1 mmol), Au@PMO (1 mol%), K PO
b
(3 equiv.), 100 1C, 16 h, NMP, under air (5 ml). GC-MS yield, n-decane
c
used as an internal standard. Au@PMO (2 mol%), 130 1C, 36 h.
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in good to excellent yields (Table 2, entries 1–5). The Ullmann
reaction of aryl halides is a well-documented process and it has
been shown that it is normally accompanied by the formation of
ArH via hydro-dehalogenation of ArX over widely used Pd-based
9
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20
catalysts. However, it is worthy of note that our new gold-
catalyzed Ullmann protocol is highly selective for coupling of aryl
iodides to their biaryls and no de-iodination to the corresponding
arene (ArH) was observed. Our studies showed that this method in
the present form is not applicable for the Ullmann coupling of aryl
bromides even upon increasing the reaction temperature to 130 1C,
and catalyst loading to 2 mol% after a prolonged reaction time
1
1
1
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(
2
(Table 2, entry 6). The recyclability of Au@PMO was also
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18
five successive reaction runs (Fig. S9, ESIw).
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In each run the leaching of Au species into solution was negli-
gible as determined by atomic absorption spectroscopy (AAS) (less
than the detection limit of 1 ppm). Furthermore, no appreciable
catalytic activity was observed from the residue of NMP extraction
of Au@PMO even after prolonged reaction times.
1
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1
1
In conclusion, we have demonstrated for the first time that a
phenylene-containing PMO supported AuNP (Au@PMO) is an
effective and reusable heterogeneous catalyst for the Ullmann
coupling of aryl iodides. Further studies to clarify the role of
gold nanoparticles, the support, and the actual mechanism of
the process are currently under investigation in our laboratories.
The authors acknowledge IASBS Research Councils for
support of this work. We also appreciate Dr H. Vali from
McGill University for TEM analysis.
1
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18 See ESIw for experimental details.
1
2
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Notes and references
1
(a) M. Haruta, Chem. Rec., 2003, 3, 75; (b) M. C. Daniel and
D. Astruc, Chem. Rev., 2004, 104, 293; (c) A. Corma and
H. Garcıa, Chem. Soc. Rev., 2008, 37, 2096.
`
1
0454 Chem. Commun., 2011, 47, 10452–10454
This journal is c The Royal Society of Chemistry 2011