Communications
[20]
A mixture of solid Au/HT, trans-stilbene oxide (1), and
-propanol in toluene was stirred at 1108C under an Ar
2
atmosphere for 4 hours (Table 1, entry 1). Selective deoxyge-
[
a]
Table 1: Deoxygenation of trans-stilbene oxide (1) using Au catalysts.
[
b]
[b]
Entry
1
Catalyst
Alcohol
Yield [%]
Sel. [%]
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
2-propanol
2-propanol
1-phenylethanol
benzyl alcohol
1-octanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
99
99
99
91
37
60
43
19
5
>99
>99
>99
>99
>99
>99
>99
>99
>99
–
[
c]
2
3
4
5
6
7
8
9
1
1
1
Au/Al O3
2
Au/MgO
Au/TiO2
Au/SiO2
HAuCl4
0
1
2
<1
<1
0
Au O3
–
–
2
Scheme 2. Deoxygenation of trans-stilbene oxide (1) using hydrotalcite-
supported metal particles. M=metal.
bulk Au metal
[a] Reaction conditions: catalyst (Au: 0.45 mol%), toluene (5 mL),
alcohol (10 mmol). [b] Determined by LC methods using an internal
standard technique. [c] At 608C, 12 h, Au/HT (Au: 1.4 mol%).
(Table 2, entries 10 and 11), and trans-3,4-epoxy-2-octanone
(Table 2, entry 32) also proceeded with retention of their
configurations. However, cis-stilbene oxide and cis-2-octene
oxide gave a mixture of the Z/E-alkene stereoisomers
(Table 2, entries 8, 9, and 28). Interestingly, Ag/HT showed
unique chemoselectivity for the deoxygenation; only styrene
oxide derivatives proved to be reactive (Table 2, entries 5, 9,
11, 14, 16, and 18). Such a considerable difference in activity
between the gold and silver NPs may be useful as a means for
selective de-epoxidation of a compound bearing several
epoxide moieties. Furthermore, these solid catalysts were
recoverable by simple filtration after the deoxygenation
without any loss in their activity or selectivity after several
uses (Table 2, entries 2, 3, 6, and 7).
nation of 1 occurred to afford trans-stilbene (2) in 99% yield
without any by-products such as 1,2-diphenylethane, 1,2-
diphenylethanol, or benzyl phenyl ketone, which could arise
from the hydrogenation or isomerization of 1. Au/HT
successfully promoted the deoxygenation reaction even at
6
08C (Table 1, entry 2). Other alcohols such as 1-phenyl-
ethanol and benzyl alcohol also worked well, whereas an
aliphatic primary alcohol, 1-octanol, was not reactive
[
23]
(
Table 1, entries 3–5). The choice in the solid support used
was found to significantly influence the catalytic activity;
among the tested supports, HT was the best. Al O and
2
3
[21]
MgO were also effective (Table 1, entries 1, 6, and 7),
whereas Au/TiO and Au/SiO resulted in low yields of 2
Tunneling electron microscopy (TEM) images of Au/HT
after the reuse revealed that the average diameter and size
distribution of the gold NPs were similar to those of the fresh
Au/HT, and that no aggregation of the used gold NPs was
apparent. By using Au l-edge X-ray absorption fine structure
(XAFS) methods, atomic-scale analysis of Au/HT showed
that the intensity of the Fourier-transform (FT) peak derived
from the Au–Au shell at 2.8 ꢀ was unchanged, supporting the
observation that the gold NPs after reuse were the same size
2
2
(
Table 1, entries 8 and 9). Other gold compounds such as
HAuCl , Au O , and the bulk metal did not result in
4
2
3
formation of 2 (Table 1, entries 10–12). Other metal NPs
supported on HT, which have catalytic potential for aerobic
alcohol oxidation such as Pd, Rh, Pt, Ru, Ag, and Cu, were
also examined under similar reaction conditions in the
deoxygenation of 1 (Scheme 2). Notably, HT-supported
silver NPs also functioned as an efficient catalyst, giving 2 in
high yield, and the other metal NPs did not display any
catalytic activity.
[20]
after recycling compared to that in the original sample.
Similar results were also obtained in the analyses of the Ag/
HT catalyst. These results are consistent with the retention of
the catalytic activities of the NPs during recycling experi-
ments.
The reaction scope for Au/HT and Ag/HT in the
[
22]
deoxygenation of epoxides is summarized in Table 2. Au/
HT could deoxygenate a wide range of epoxides, affording the
corresponding alkenes in excellent yields. For example,
epoxides having phenyl (Table 2, entries 1, 8, 10, 12, 15, and
The deoxygenation of expoxides using Au/HT was also
applicable to a preparative-scale reaction. For example, 1
(3.9 g; 20 mmol) successfully gave 2 [(3.4 g; 95% (isolated)],
ꢀ1
1
7), alkyl (Table 2, entries 23, 25, and 28), ether (Table 2,
entry 19), carbonyl (Table 2, entries 30 and 32), hydroxy
Table 2, entry 34), and olefinic (Table 2, entry 36) groups
wherein the TOF and TON were up to 270 h and 20000,
respectively (Table 2, entry 4). These values are three orders
of magnitude greater than those in previously reported
catalyst systems, such as Tp’ReO with PPh (TOF, TON:
(
were successfully employed. Deoxygenation of trans-stilbene
oxide (Table 2, entries 1–7), trans-1-phenylpropylene oxide
3
3
ꢀ
1
[16]
ꢀ1
[17]
0.2 h , 20),
[Re(catecholato)] with PPh (13 h , 19),
3
5
546
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 5545 –5548