Table 2 Effect of ligands for the dimerization of butyl acrylate (2) by in situ
form the corresponding head-to-tail products in 86%, 88% and
82% yields, respectively. Methyl vinyl ketone (4) similarly
dimerized under these conditions to give 1,5-diketone (5) in
79% yield [eqn. (2)].§ However, acrylonitrile was difficult to
dimerize due to the strong coordination of cyano group to the
iridium complex.
generated iridium hydride complexa
Yield/%b
Run
Ligand
Conv./%
1c
3
1
2
3
4
5
6
7
8c
—
32
98
71
9
77
71
11
14
10 ( < 1)
86 (10)
60 (7)
2 (trace)
4 ( < 1)
nd
4
1
4
3
4
2
4
2
(MeO)3P
(BuO)3P
(PhO)3P
Bu3P
Cy3P
Ph3P
(2)
nd
2 (trace)
dppe
a After treatment of [IrCl(cod)]2 (0.01 mmol) with 1-butanol (1 mL) in the
presence of Na2CO3 (0.02 mmol) and ligand (0.04 mmol) at 100 °C for 1 h,
1-butanol was removed under reduced pressure, and then toluene (1 mL)
and 2 (1 mmol) were added. The reaction mixture was stirred at 100 °C for
5 h. b Parenthesis shows the yield of dimers other than 1c. c Dppe (0.02
mmol) was used.
Owing to the complexity of the present catalytic system, we
cannot propose a detail reaction path, but the present reaction is
thought to proceed through a similar pathway to that proposed
for the dimerization of acrylates by Ru,3 Rh,4 and Pd.5
In conclusion, we have found that an iridium hydride
complex generated in situ from [IrCl(cod)]2 complex and
1-butanol in the presence of small amounts of Na2CO3 and
(MeO)3P catalyzes the head-to-tail dimerization of acrylates
and methyl vinyl ketone in high selectivity.
be controlled lower than 1% (Run 2).‡ The reaction in the
absence of a ligand under these conditions resulted in the
considerable decrease of the conversion of 2 to 1 (Run 1).
Although the head-to-tail dimerization was accelerated by
PCy3,7 the reaction catalyzed by (MeO)3P alone did not take
place at all under these reaction conditions.
It was found that phosphites serve as good ligands of the
iridium hydride complex generated in situ from [IrCl(cod)]2 and
1-butanol in the presence of Na2CO3 (Runs 2 and 3). When
phosphine ligands like tributylphosphine and tricyclohex-
ylphophine were employed, the polymerization of 2 was
induced rather than the dimerization (Runs 5 and 6). Triphenyl-
phosphine and bidentate ligand, dppe, were inert for the
promotion of the dimerization (Runs 7 and 8).
The effect of alcohols on the generation of an active iridium
hydride complex was examined (Table 3). The reactions by the
[IrCl(cod)]2 upon treatment with methanol and ethanol gave
almost the same results, but no reaction took place when water
was employed in place of alcohols. 2-Butanol was also effective
for the generation of an iridium hydride, while tert-butyl alcohol
did not promote the dimerization of 2 probably because of the
difficulty for the generation of the iridium hydride complex
which catalyzes the dimerization. This shows that the alcohols
possessing a-hydrogen requires to generate the iridium hydride
complex from the [IrCl(cod)]2.
This work was partially supported by a Grant-in-Aid for
Scientific Research (KAKENHI) (S) (No.13853008) from
Japan Society for Promotion of Science.
Notes and references
† A procedure for the dimerization reaction of 2: The solution of a dry
1-butanol (1 mL) containing [IrCl(cod)]2 (0.01 mmol), (MeO)3P (0.04
mmol), and Na2CO3 (0.02 mmol) under Ar were stirred at 100 °C for 1 h.
After 1-butanol was removed under reduced pressure, a solution of a dry
toluene (1 mL) and 2 (1 mmol) was added, then the reaction mixture was
allowed to react at 100 °C for 5 h.
‡ Spectral data for 1c: 1H NMR d 6.86–6.80 (m, 1H), 4.09–4.04 (m, 4H),
3.13 (d, J = 7.1 Hz, 2H), 1.79 (s, 3H), 1.63–1.50 (m, 4H), 1.40–1.24 (m,
4H), 0.86 (t, J = 7.3 Hz, 3H); 13C NMR d170.2, 167.2, 132.5, 130.5, 64.85,
64.52, 34.2, 30.6, 30.5, 19.24, 19.11, 13.74, 13.69, 12.7; MS (70 eV) m/z =
182 (M+), 126, 106, 57, 41.
§ Spectral data for 5: 1H NMR d 6.86–6.80 (m, 1H), 3.13 (d, J = 7.1 Hz,
2H), 2.20 (s, 3H), 2.07 (s, 3H), 1.89 (s, 3H); 13C NMR d 206.0, 196.5, 139.6,
130.2, 36.3, 25.5, 23.1, 11.8; MS (70 eV) m/z = 112 (M+), 81, 59, 53,
43.
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Methyl, ethyl and isobutyl acrylates dimerized in the same
fashion as 2 under the conditions shown in Table 2, Run 2 to
Table 3 Effect of alcohols on the activation of [IrCl(cod)]2 for dimerization
of 2a
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Yield/%
Run
Alcohol
Conv./%
1c
1d
3
1
2
3
4
5
6
MeOH
EtOH
1-BuOH
2-BuOH
t-BuOH
Water
94
91
98
71
72
68
86
61
8
10
9
10
10
2
4
7
1
2
22
nd
no reaction
a After [IrCl(cod)]2 (0.01 mmol) was treated in the presence of Na2CO3
(0.02 mmol) and (MeO)3P (0.04 mmol) in alcohol (1 mL) at refluxing
temperature for 1 h, the alcohol was removed under reduced pressure, and
then toluene (1 mL) and 2 (1 mmol) was added to the reactant. The reaction
was run at 100 °C for 5 h.
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