Ruthenium-catalyzed hydrative dimerization of allenes

Article abstract of DOI:10.1246/cl.2005.504

Hydrative dimerization and hydration of allenes proceeded in the presence of a ruthenium catalyst and a strong acid such as trifluoroacetic acid. γ,δ-Unsaturated ketones and methyl ketones were isolated in moderate combined yields. No isomeric compound (isomeric enone) was isolated. Copyright

Full text of DOI:10.1246/cl.2005.504

504
Chemistry Letters Vol.34, No.4 (2005)
Ruthenium-catalyzed Hydrative Dimerization of Allenes
Shinichi Saito,ꢀ Naotomo Dobashi, and Yasuo Wakatsukiꢀy;#
Department of Chemistry, Faculty of Science, Tokyo University of Science,
Kagurazaka, Shinjuku, Tokyo, 162-8601
yOrganometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research),
Hirosawa 2-1, Wako 351-0198
(Received January 25, 2005; CL-050117)
Hydrative dimerization and hydration of allenes proceeded
CF3COOH (Entry 1). It was important to add both the ruthenium
in the presence of a ruthenium catalyst and a strong acid such
as trifluoroacetic acid. ꢀ,ꢁ-Unsaturated ketones and methyl ke-
tones were isolated in moderate combined yields. No isomeric
compound (isomeric enone) was isolated.
complex and CF3COOH, otherwise the reaction did not proceed
(Entries 2, 3). A bidentate Ru catalyst, CpRuCl(dppm),7b was
not effective (Entry 4). Among the Ru catalysts8 we examined,
Ru3(CO)12 was most effective, and the dimeric ꢀ,ꢁ-unsaturated
ketone 2b (41%) and the monomeric ketone 3b (18%) were iso-
lated (Entry 5).9 It is noteworthy that no isomeric enone other
than 2b was isolated. The best yield of 2 was observed when
10 mol % of CF3COOH was used (Entries 5–7). Though the
yields of the products did not change significantly when the re-
action was carried out under CO instead of Ar (Entry 8), the for-
mation of some by-products was suppressed and the isolation of
the products was easier.10 The reaction proceeded rapidly at an
elevated temperature (120 ꢁC) but the yield of 2b decreased (En-
try 9). The effect of the acids on this reaction was also investi-
gated, and CF3COOH turned out to be the best additive: the
yields of the products were much lower when the reaction was
carried out in the presence of other acids such as CH3COOH,
p-TsOH, or HCl, or in the absence of acid (Entries 8–13).11
We also examined the effects of the solvent on this reaction
and carried out the reaction in tBuOH–H2O, EtOH–H2O,
THF–H2O, CH3CN–H2O. However, the yields of the products
decreased significantly.
Ruthenium complexes are useful catalysts for the hydration
of various unsaturated hydrocarbons.1 Recent studies revealed
that the highly selective hydration of alkynes,2,3 dienes,4 and oth-
er unsaturated hydrocarbons5 occurred in the presence of an ap-
propriate ruthenium catalyst. On the other hand, the ruthenium-
catalyzed hydration of allenes (1,2-dienes) has not been well ex-
plored.
Our interest in the hydration reactions of unsaturated hydro-
carbons led us to study the hydration of allenes in the presence of
various ruthenium catalysts. In this paper we report the rutheni-
um-catalyzed hydrative dimerization and hydration of allenes.
The results of the reaction of 1,2-undecadiene (1a)6 and phe-
nethylallene (1b)6 in the presence of ruthenium catalysts are
summarized in Table 1. The initial screening of the ruthenium
complexes revealed that the hydrative dimerization of 1a
7a
proceeded slowly in the presence of CpRuCl(PPh3)2 and
Table 1. Ruthenium-catalyzed hydrative dimerization and hydration of 1a and 1b
Ru cat. (2 mol%, Ru)
O
O
additive (10 mol%), 100 °C
R
R
R
•
+
R
iPrOH - H2O
1a (R = n-C8H17)
1b (R = PhCH2CH2)
(2.5 − 0.75 mL)
2a,b
3a,b
under Ar or CO (1 atm)
Entry
Substrate
Catalyst
Additive
Gas
Time/h
Yield of 2/%a
Yield of 3/%a
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
CpRuCl(PPh3)2
CpRuCl(PPh3)2
—
CF3COOH
—
Ar
Ar
Ar
Ar
Ar
Ar
Ar
CO
CO
Ar
Ar
CO
CO
18
82
63
18
18
18
18
18
3f
18
17
24
24
26
0
0
0
0
0
CF3COOH
CF3COOH
CF3COOH
CF3COOHc
CF3COOHd
CF3COOH
CF3COOH
CH3COOH
p-TsOH
CpRuCl(dppm)b
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
0
0
41
24
18e
40
33
0
15
24
—
18
15
8e
22
21
10
18
HClg
—
13
ꢂ11h
aIsolated yields. bdppm = Ph2PCH2PPh2. cThe reaction was carried out in the presence of 5 mol % CF3COOH. dThe reaction was
carried out in the presence of 20 mol % CF3COOH. Yields were determined by NMR. The reaction was carried out at 120 ꢁC.
e
f
gConc. HCl aq was added. The product was contaminated with a small amount of other by-products.
h
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
505
Table 2. Ru3(CO)12–CF3COOH-catalyzed hydrative dimeriza-
tion and hydration of allenes
a cationic Ru-allene complex 5. The addition of water to 5 would
yield a ketone derivative 6. A dimeric complex 7 would be
formed when allene 1 further reacted with 6. Protonolysis of 6
and 7 would lead to the formation of 3 and 2, respectively.12 Al-
ternatively, the formation of 2 might proceed via the formation
and acid-catalyzed hydrolysis of a ruthenacyclopentane.13 We
favor the mechanism proposed in Scheme 1, since the formation
of 3 as well as 2 could be reasonably explained.
In summary, we found a new ruthenium-catalyzed hydrative
dimerization of allenes. The combination of the ruthenium cata-
lyst and a strong acid turned out to be very important for the
progress of the reaction. Further elaboration of this reaction is
underway.
Ru3(CO)12 (2 mol%, Ru)
CF3COOH (10 mol%)
R
•
iPrOH - H2O
(2.5 – 0.75 mL)
120 °C, CO (1 atm)
1a-f
O
O
R
+
2a-f
R
R
3a-f
Time Yield of 2 Yield of 3
/h
Entry
R
/%
/%
1
2
3
4
5
6
n-C8H17 (1a)
PhCH2CH2 (1b)
n-C6H13 (1c)
n-C10H21 (1d)
NCCH2CH2 (1e)
Cyclohexyl (1f)
18a
3
3
6
7
12
40
38
39
31
27
17
22
19
10
18
25
8
References and Notes
#
Present address: Department of Chemistry, College of
Humanities and Sciences, Nihon University, Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan. E-mail: waky@chs.
nihon-u.ac.jp
1
For reviews of the Ru-catalyzed reactions, see: a) T. Naota, H.
Takaya, and S.-i. Murahashi, Chem. Rev., 98, 2599 (1998). b)
B. M. Trost, F. D. Toste, and A. B. Pinkerton, Chem. Rev.,
101, 2067 (2001). See also: c) M. Beller, J. Seayad, A. Tillack,
and H. Jiao, Angew. Chem., Int. Ed., 43, 3368 (2004).
a) M. Tokunaga and Y. Wakatsuki, Angew. Chem., Int. Ed.,
37, 2867 (1998). b) T. Suzuki, M. Tokunaga, and Y.
Wakatsuki, Org. Lett., 3, 735 (2001). c) M. Tokunaga, T.
Suzuki, N. Koga, T. Fukushima, A. Horiuchi, and Y.
Wakatsuki, J. Am. Chem. Soc., 123, 11917 (2001). d) D. B.
Grotjahn and D. A. Lev, J. Am. Chem. Soc., 126, 12232
(2004). For reviews, see: e) M. Tokunaga and Y. Wakatsuki,
J. Synth. Org. Chem. Jpn., 58, 587 (2000). f) Y. Wakatsuki,
Z. Hou, and M. Tokunaga, Chem. Rec., 3, 144 (2003).
For a closely related Rh-catalyzed hydrative dimerization,
see: Y. J. Park, B.-I. Kwon, J.-A. Ahn, H. Lee, and C.-H.
Jun, J. Am. Chem. Soc., 126, 13892 (2004).
aThe reaction was carried out at 100 ꢁC.
The generality of the reaction was examined and the results
of the reaction of various allenes were summarized in Table 2.
While the reactions of primary alkylallenes, including a cyanoal-
lene, proceeded smoothly and the dimerized products were iso-
lated in 27–40% yields (Entries 1–5), the yield of 2 significantly
dropped when cyclohexylallene (1f) was used as the substrate
(Entry 6). Though we carried out the reaction of other allenes
such as phenylallene, phenoxyallene, and ethoxycarbonylallene
as well as disubstituted allenes, the corresponding dimer was not
isolated.
Since it was not clear whether 2 was formed by the reaction
of 2 molecules of 1 with a molecule of water, or by the reaction
of 1 with 3, we examined the reaction of 1b in the presence of 2-
octanone (4). The analysis of the products revealed that 2b was
the only dimeric compound which was isolated from the reaction
mixture, and 4 was not incorporated. Therefore, it is clear that 2
was not formed by the reaction of 3 with 1.
2
3
4
F. Stunnenberg, F. G. M. Niele, and E. Drent, Inorg. Chim.
Acta, 222, 225 (1994).
5
6
B. M. Trost and M. T. Rudd, Org. Lett., 5, 4599 (2003).
For the preparation of allenes, see: L. Brandsma and J. F.
Arens, Recl. Trav. Chim., 86, 734 (1967).
The mechanism of this reaction was tentatively assumed as
shown in Scheme 1. Thus, the allene would react with a cationic
Ru species, formed by the protonation of the Ru3(CO)12, to give
7
8
9
For the preparation of the ruthenium complexes, see: a) M. I.
Bruce and N. J. Windsor, Aust. J. Chem., 30, 1601 (1977). b)
G. S. Ashby, M. I. Bruce, I. B. Tomkins, and R. C. Wallis,
Aust. J. Chem., 32, 1003 (1979).
Though we carried out this reaction in the presence of
Mo(CO)6, RuCl3, Ru2(CO)4(C5H5)2, RuCl(CO)2(C5H5),
[RuCl2(C6H6))]2, or Ru(cod)(cot), compound 2 was not iso-
lated and a small amount of 3 (0–21%) was obtained.
The formation of non-polar compounds such as 1,3-undeca-
diene was observed as the by-products. The analysis of the
GC-MS spectra indicated the formation of dimeric hydrocar-
bons.
Ru(0)
•
R
H
1
H
2
Ru H
R
Ru H
H
Ru H
7
3
•
10 CO may act as a ligand and prevent the decomposition of the
catalyst.
R
O
R
5
H2O
•
R
Ru H
11 The formation of 3 in the absence of acid (Entry 13) may be
explained in terms of the formation of a small amount of
formic acid by the hydration of CO in the reaction mixture.
12 The incorporation of deuterium to the olefinic moiety (52% d)
(and ketone ꢂ-hydrogens, 41% d) of 3 was observed when the
reaction was carried out in 2-propanol-d8-D2O.
H
Ru H
Ru H
R
O
1
R
O
R
O H
6
13 Five-membered ruthenacycles are postulated as intermediates
for various ruthenium-catalyzed reactions. See, Ref 1.
Scheme 1. Proposed mechanism for the hydrative dimerization
and hydration of 1 catalyzed by Ru(0)/acid.
Published on the web (Advance View) March 5, 2005; DOI 10.1246/cl.2005.504