E. Y. Lee, Y. Kim, J. S. Lee, J. Park
SHORT COMMUNICATION
not detected. Notably, changing the reaction atmosphere benzylic alcohols as well as aliphatic alcohols, whereas the
from argon to oxygen greatly enhanced the reaction effi- yields were somewhat low in the reactions of aliphatic
ciency; methyl cinnamate was formed in 91% and the for- alcohols (Table 2, Entries 1 and 2). A substituent effect was
mation of benzyl cinnamate decreased to less than 2% at not significant in the reactions of benzylic alcohols (Table 2,
80 °C for 20 h (Table 1, Entry 2). The reaction was complete Entries 3–8). The reaction of 1,4-phenylenedimethanol gave
within 12 h at 110 °C with an increased yield of methyl cin- the corresponding diester in 82% yield (Table 2, Entry 7).[11]
namate (Table 1, Entry 3). The reaction in the air was a The vinyl group, which has a capacity to be applied in poly-
little slower than that under an atmosphere of oxygen mer synthesis, survived during the coupling reaction
(Table 1, Entry 4). It is clear that 1 is an excellent catalyst (Table 2, Entry 8).[12] The carbon–carbon double bond in
for the one-pot reaction in comparison with commercially an allyl alcohol and the triple bond in a propargyl alcohol
available catalysts and those reported for related reactions were also compatible with the reaction conditions (Table 2,
(Table 1, Entries 5–8); the reaction with the use of 1 is much Entries 9 and 10). It is notable that the reaction of the pro-
more selective and faster for the formation of methyl cinna- pargyl alcohol is much more efficient than that using [(η6-
mate than those using other catalysts. In addition, in our p-cymene)RuCl2]2 as a homogeneous catalyst, which gave
reaction system, molecular oxygen was the terminal oxidant the corresponding α,β-unsaturated ester only in 28%
and does not require any additives such as base or ligand.
yield.[7] Our catalyst system was also applicable for the reac-
tions of alcohols containing heteroaromatic rings (Table 2,
Entries 11 and 12). The reaction of thiophen-2-ylmethanol
was as efficient as those of benzyl alcohols, but that of pyr-
idin-3-ylmethanol was hard to improve.
Table 1. Catalytic activity comparison.[a]
Entry Catalyst
T
t
Yield [%][b]
[°C]
[h]
2
3
4
Scheme 2. Ru-catalyzed, one-pot oxidation–Wittig reaction with
benzyl alcohol and stabilized Wittig reagents.
1[c]
2
1
110
80
30
20
12
12
12
14
72
24
59
91 Ͻ2
94 Ͻ2
77
48
77
21
8
0
0
0
0
0
0.6
68
78[h]
1
3
1
1
110
110
110
110
150
80
4[d]
5
1
2
8
3
[e]
0.5% Ru/Al2O3
6
7
8
5% Ru/C[e]
Conclusions
[f]
[IrCl(cod)]2
Ru(IMes)(PPh3)(CO)H2
[g]
In summary, we have developed a simple and efficient
method to prepare α,β-unsaturated esters from primary
alcohols and stabilized Wittig reagents through a one-pot
process. The method is applicable to aliphatic alcohols as
well as benzylic ones and does not require any additives
except oxygen, which helps the catalytic dehydrogenation of
alcohols.
[a] A mixture of benzyl alcohol (1.0 mmol), Ph3P=CHCO2Me
(1.1 mmol), and a catalyst (1 mol-% of Ru) in toluene (2.0 mL) was
heated under an oxygen atmosphere (balloon). [b] Determined by
1H NMR spectroscopy. [c] Under an argon atmosphere. [d] In the
air. [e] Commercial Ru catalysts. [f] See ref.[8] [g] See ref.[9] [h] Isola-
tion yield.
The recyclability of 1 was tested in the reaction of benzyl
alcohol with ethyl (triphenylphosphoranylidene)acetate un-
der the conditions outlined in Entry 3 of Table 1. The cata-
lyst could be recovered simply by decantation, but a de-
crease in activity was observed with reuse: first use, 93%;
second use, 85%; third use, 77%; fourth use, 60%.
Experimental Section
Synthesis of Ru/AlO(OH) (1): A mixture of RuCl3·3H2O (Ru con-
tent: 41.25%; 130 mg, 0.53 mmol of Ru) and Al(sec-OBu)3 (9.4 g,
38 mmol) in ethanol (4.6 mL) was heated at 100 °C for 1 h to give
a black suspension. Distilled water (9.0 mL) was added into the
suspension to give a black gel. The gel was filtered at room tem-
perature, washed with acetone, and dried under an argon atmo-
sphere to give 1 as a gray powder (3.2 g, 1.7 wt.-% of Ru).
We tested three more Wittig reagents for the coupling
reaction with benzyl alcohol (Scheme 2). Under the condi-
tions of the Entry 3 of Table 1, ethyl cinnamate and benzyl
cinnamate were obtained in 96 (E/Z = 15:1) and 94% (E/Z
= 14:1) yield, respectively. In the case of the Wittig reagent
containing one additional methyl group at the α carbon
atom, the reaction was relatively slow and required more
catalyst (2.0 mol-% Ru) to give (E)-ethyl 2-methyl-3-phenyl-
acrylate in 81% yield. Then, various primary alcohols were
employed in the coupling reactions with ethyl (triphenyl-
phosphoranylidene)acetate (Table 2). The expected α,β-un-
One-Pot Oxidation–Wittig Reaction: A mixture of benzyl alcohol
(100 µL, 1.0 mmol), methyl (triphenylphosphoranylidene)acetate
(380 mg, 1.1 mmol), and Ru/AlO(OH) (1; 59 mg, 1.0 mol-% of Ru)
in toluene (2.0 mL) was stirred for 12 h at 110 °C under an oxygen
atmosphere (balloon). The reaction mixture was filtered through a
glass filter, and the filtrate was concentrated under reduced pres-
sure. The crude product was purified by column chromatography
saturated esters were formed selectively in the reactions of to give methyl cinnamate (153 mg, 94%).[13]
2944 Eur. J. Org. Chem. 2009, 2943–2946
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim