Iridium-catalyzed enantioselective Pauson-Khand-type reaction of 1,6-enynes

Article abstract of DOI:10.1016/j.tet.2005.08.016

Iridium-chiral diphosphine complex catalyzes an enantioselective intramolecular Pauson-Khand-type reaction to give various chiral bicyclic cyclopentenones. The enantioselective reaction proceeds more smoothly and enantioselectively under a lower partial p

Full text of DOI:10.1016/j.tet.2005.08.016

Tetrahedron 61 (2005) 9974–9979
Iridium-catalyzed enantioselective Pauson–Khand-type reaction
of 1,6-enynes
b
b
a
Takanori Shibata,a, Natsuko Toshida, Mitsunori Yamasaki, Shunsuke Maekawa
*
and Kentaro Takagib
aDepartment of Chemistry, School of Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan
bDepartment of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan
Received 7 July 2005; revised 4 August 2005; accepted 4 August 2005
Available online 24 August 2005
Abstract—Iridium–chiral diphosphine complex catalyzes an enantioselective intramolecular Pauson–Khand-type reaction to give various
chiral bicyclic cyclopentenones. The enantioselective reaction proceeds more smoothly and enantioselectively under a lower partial pressure
of carbon monoxide. Moreover, aldehyde can be used as a CO source in the enantioselective carbonylative coupling.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
(1)
The Pauson–Khand reaction is a carbonylative coupling of
alkyne and alkene, which was originally reported using a
stoichiometric amount of a cobalt carbonyl complex in
1973.1 The reaction gives synthetically useful cyclo-
This manuscript discloses further investigation of an
iridium-catalyzed enantioselective Pauson–Khand-type
pentenones; in fact, it has been used as a key reaction in
reaction. Various types of enynes were submitted to the
natural product syntheses.2 In the 1990s, efforts were
reaction under an atmospheric pressure or a lower partial
extended toward developing a catalytic reaction, and
pressure of carbon monoxide.14 Moreover, an iridium-
Jeong reported a practical intramolecular reaction of enynes
catalyzed Pauson–Khand-type reaction using an aldehyde as
using a cobalt–phosphite complex;3 many publications on
a CO source is also presented.
catalytic reaction conditions followed.4 Further progress
was made by reactions using other transition metal
complexes as catalysts, known as a Pauson–Khand-type
reaction.5 Since Buchwald reported Ti-catalyzed intra-
molecular reaction of enynes,6 Ru7 and Rh complexes8
2. Results and discussion
were found to be efficient catalysts. The first catalytic and
enantioselective Pauson–Khand-type reaction was also
realized by Buchwald using a chiral titanium complex,
2.1. Iridium complex-catalyzed enantioselective coupling
with carbon monoxide
where various enynes were transformed into the correspond-
ing chiral bicyclic cyclopentenones in high ee.9 Further
achievements include enantioselective reactions using a
In order to investigate the catalytic activity of an iridium
cobalt complex by Hiroi,10 a rhodium one by Jeong11 and an
complex, we first examined an intramolecular Pauson–
Khand-type reaction of enyne 1a (Eq. 2). The iridium
complex, possessing triphenylphosphine as an achiral
ligand, operated as a more efficient catalyst than that
without phosphine ligands. The results were opposite to
iridium one by us12 in 2000 (Eq. 1), and the development of
an enantioselective Pauson–Khand-type reaction is still an
intriguing topic these days.13
those of a rhodium complex-catalyzed reaction, where the
addition of triphenylphosphine deactivated the catalytic
Keywords: Iridium; Enynes; Carbonylation; Pauson–Khand reaction;
activity,8c and they prompted us to examine chiral ligands
Enantioselective.
for iridium-catalyzed enantioselective intramolecular
Pauson–Khand-type reaction.
*
Corresponding author. Tel./fax: C81 3 5286 8098;
e-mail: tshibata@waseda.jp
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.016

T. Shibata et al. / Tetrahedron 61 (2005) 9974–9979
9975
Also in the case of enyne 1i, possessing a functionalized
substituent on its alkyne terminus, decrease of a partial
pressure of carbon monoxide worked well; moreover, ee
was also significantly improved (entries 11, 12). Enyne,
having 1,1-disubstituted olefin as an alkene moiety, is
known to be rather inactive, and considerable amounts of
enynes 1j,k were recovered, respectively, under an
atmospheric pressure of carbon monoxide (entries 13, 15).
Due to the decrease of the partial pressure of CO, bicyclic
cyclopentenones 2j,k, having a chiral quaternary carbon,
were obtained in acceptable yield and ee (entries 14, 16).
(2)
Next we investigated chiral diphosphines as chiral ligands
(Table 1); with the increase of yield, enantioselectivity was
improved and tolBINAP was found to be best among them.
A good yield of 83% and high ee of 93% were achieved15
(entries 1–5). Decreasing the amounts of catalyst to 5 mol%
gave slightly poorer yield and enantioselectivity; however,
drastic decrease of ee was observed, and a considerable
amount of enyne 1 was recovered by 2 mol% catalyst
(entries 6, 7). In order to increase the catalytic efficiency, the
concentration of the chiral catalyst was found to be
important: the higher yield and the same ee were achieved
using 2 mol% catalyst when the reaction was examined for
longer reaction time under the same concentration as that of
entry 5 (entries 8, 9).
Scheme 1 depicts a possible mechanism: p-complexation of
enyne 1 to chiral catalyst A induces an oxidative coupling to
give metallacyclopentene C, where a chiral carbon is
generated. Carbonyl insertion to C gives acyl complex D,18
and the following reductive elimination provides enone 2
with regeneration of the active iridium species. In the
reaction mixture, the mole amount of CO is much larger
than that of the catalyst, which means that complex A0 and
B0 could also exist by CO coordination. Complex A0 is less
reactive than A, and an oxidative coupling of B0 lowers
enantioselectivity. When the coupling is done under a lower
Various 1,6-enynes were submitted to the reaction using the
chiral iridium catalyst,16 which was prepared in situ from
[IrCl(cod)]2 and tolBINAP (Table 2). Electron-donating and
-withdrawing substituents on the phenyl ring produced
almost no effect, and the corresponding bicyclic enones 2b,c
were obtained in good yield with high ee (entries 1, 2). In
place of aryl groups on the alkyne terminus of enyne, an
isopropenyl group could be possible, yet with moderate
yield (entry 3). Enynes, having alkyl groups on their alkyne
termini, were also good substrates, and methyl-substituted
enyne 1e gave enone 2e in the highest ee of 98% (entries
4–6). Not only oxygen-bridged enynes but also nitrogen-
bridged enyne 1g was enantiomerically transformed into
bicyclic compound 2g (entry 7); however, carbon-bridged
enyne 1h gave carbonylative product 2h in moderate yield
even over longer reaction time, and a considerable amount
of enyne 1h was recovered (entry 8). Decreasing a partial
pressure of carbon monoxide accelerated the carbonylative
coupling also in an iridium-catalyzed system,8c and higher
yield was achieved under a 0.2 partial pressure of carbon
monoxide without any lowering of ee.17 Longer reaction
time realized further better yield of ca. 90% (entries 9, 10).
Scheme 1. A possible explanation for the effect of a partial pressure of CO.
Table 1. Investigation of chiral ligands and amounts of catalyst in iridium-catalyzed enantioselective Pauson–Khand-type reaction
Entry
X
Chiral liganda
[M]/mMb
Time/h
Yield/%
ee/%
1
2
3
4
5
6
7
8
9
10
10
10
10
10
5
2
2
2
CHIRAPHOS
BDPP
DIOP
15
15
15
15
15
7.5
3
12
12
12
12
12
24
48
48
72
13
23
53
64
83
75
33
59
88
!1
22
17
86
93
91
74
93
92
BINAP
tolBINAP
tolBINAP
tolBINAP
tolBINAP
tolBINAP
15
15
a (S,S)-isomers were used for entries 1–3. (S)-isomers were used for entries 4–10.
b Concentration of catalyst.

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T. Shibata et al. / Tetrahedron 61 (2005) 9974–9979
Table 2. Enantioselective Pauson–Khand-type reaction of various enynes under a CO atmosphere
Entrya
1
Enyne
Cyclopentenone
CO/atm
1.0
Time/h
20
Yield/%
80
ee/%
96
2b
2c
1b
2
3
1.0
1.0
20
20
78
54
95
97
1c
1d
2d
4
5
1.0
1.0
20
48
60
75
98
97
1e
2e
2f
6
7
1.0
1.0
20
12
54
85
90
95
1f
1g
2g
2h
8
9
10
1.0
36
36
72
51
71
89
88
85
86
0.2b
0.2b
1h
11
12
1.0
0.2b
72
72
15
50
84
88
1i
1j
2i
2j
13
14
1.0
0.2b
24
72
30
86
88
93
15
16
1.0
0.2b
96
96
22
62
86
94
1k
2k
a [IrCl(cod)]2C2(S)-tolBINAP (10 mol%), toluene, reflux.
b CO (0.2 atm) C Ar (0.8 atm).
partial pressure of CO, the content of A and B increases as
compared with that of A0 and B0, which probably brings
about the acceleration of the coupling and the increase
of ee.19
with failure, and only a trace amount of or no carbonylated
product was detected.
We further examined enyne 3, having no substituent on the
alkyne terminus and having two methyls at the propargylic
position, which deter isomerization of alkyne moiety to
vinylidene complex.20 Enone 4 was obtained yet only with
low ee (Eq. 3). This result implies that the substituent on
the alkyne terminus plays a pivotal role for high
enantioselectivity.
Table 2 shows wide generality of the present iridium-
catalyzed enantioselective Pauson–Khand-type reaction;
however, there is a limitation of enynes (Fig. 1): under the
same reaction conditions as Table 2, enynes, containing
1,2-disubstituted olefin as an alkene moiety, a 1,7-enyne,
and enynes with no substituent on the alkyne terminus met
(3)
Figure 1. Enynes, which did not give carbonylated products.

T. Shibata et al. / Tetrahedron 61 (2005) 9974–9979
9977
Table 3. Examination of enantioselective Pauson–Khand-type reaction using cinnamaldehyde as a CO source
Entry
M
Solvent
X/equiv
Time/h
Yield/%
ee/%
1
2
3
4
5
6
Rh
Rh
Ir
Ir
Ir
None
Xylene
None
Xylene
Xylene
Toluene
20
20
20
20
5
5
36
6
12
9
89
54
27
52
66
25
82
8
88
86
92
95
Ir
5
24
2.2. Iridium complex-catalyzed enantioselective coupling
using an aldehyde as a CO source
complex, which is readily prepared in situ from a
commercially available and stable iridium complex and
chiral diphosphine. Various enynes could be transformed
into chiral bicyclic cyclopentenones in high ee. Especially, a
low partial pressure of carbon monoxide facilitated the
carbonylative coupling and improved the enantioselectivity.
Furthermore, an enantioselective Pauson–Khand-type
reaction using cinnamaldehyde as a CO source could be
also achieved by chiral iridium complex and higher
enantioselectivity was realized than that by the rhodium
complex.
Recently, Morimoto and Kakiuchi21 and we22 indepen-
dently reported a Rh-catalyzed Pauson–Khand-type
reaction using aldehydes as a CO source in place of CO
gas. Enantioselective reaction was also realized, where
solvent-free condition is essential for high yield and ee
(Table 3, entry 1).22b When the coupling was examined in
xylene, it took much longer reaction time to consume enyne
1a and enantioselectivity was extremely low (entry 2). We
next examined an iridium-catalyzed coupling using an
aldehyde as a CO source23 and found that ee was high both
with and without solvent; however, solvent was needed for
high yield (entries 3, 4). Higher yield and ee were achieved
by decreasing the amounts of cinnamaldehyde (entry 5).
These results imply that the chiral rhodium complex would
be stable and less reactive, and it works as a catalyst in harsh
reaction conditions; on the contrary, the chiral iridium
complex would be unstable, and solvent is needed for
operating as an efficient catalyst.
4. Experimental
4.1. General
Optical rotation was measured using Jasco DIP-370
polarimeter. IR spectra were recorded with Horiba FT210
spectrophotometer. NMR spectra were measured with
JEOL AL-400 or Varian VXR-300S spectrometer using
tetramethylsilane as an internal standard and CDCl3 was
used as solvent. Mass spectra were measured with JEOL
JMS-SX102A and elemental analyses with Perkin Elmer
PE2400II. Dehydrated toluene is commercially available
Under the best reaction conditions (Table 3, entry 5), we
examined an enantioselective coupling of several enynes
(Table 4). In each entry, yield was moderate; however, ee
was very high and exceeded that of rhodium-catalyzed
coupling.22b
˚
and it was dried over molecular sieves 4 A and degassed by
carbon monoxide bubbling before use. All reactions were
examined using a CO balloon or a balloon of CO and Ar (2:
8). Spectral data of 2a–2c, 2e–2h, and 2j were already
published by others4b,h,8b,c,9b,11,24,25 and us.12,22
Table 4. Enantioselective Pauson–Khand-type reaction of various enynes
using cinnamaldehyde as a CO source
4.2. Typical experimental procedure for enantioselective
coupling with carbon monoxide (Table 2)
Preparation of a balloon with mixed gas of carbon monoxide
and argon (2:8): CO (2 atm) was introduced into an
autoclave (30 mL) then Ar (8 atm) was introduced into the
autoclave; then the pressurized mixed gas (10 atm) was
released into a balloon at an atmospheric pressure.
Entry
Enyne
Time/h
Yield/%
ee/%
1
2
3
4
5
6
1b
1c
1e
1g
1h
1j
5
9
24
5
24
24
57
56
30
55
51
40
91
91
85
94
87
90
Under an atmosphere of carbon monoxide, tolBINAP
(34.0 mg, 0.050 mmol) and [Ir(cod)Cl]2 (16.8 mg,
0.025 mmol) were stirred in toluene (2.0 mL) at room
temperature. After the addition of a toluene solution
(2.0 mL) of enyne 1 (0.25 mmol), the reaction mixture
was stirred under reflux for an appropriate time (cited in the
table). The solvent was removed under reduced pressure,
and the crude products were purified by thin-layer
3. Conclusion
In summary, we have developed a catalytic and enantiose-
lective Pauson–Khand-type reaction using a chiral iridium

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T. Shibata et al. / Tetrahedron 61 (2005) 9974–9979
chromatography to give chiral cycloadduct 2. Enantiomeric
excess was determined by HPLC analysis using a chiral
column.
time (cited in the table). After the exclusion of excess
cinnamaldehyde and xylene, the crude products were
purified by thin-layer chromatography, and pure bicyclic
enone 2 was obtained. Enantiomeric excess was determined
by HPLC analysis using a chiral column.
4.2.1. 2-Isopropenyl-7-oxabicyclo[3.3.0]oct-1-en-3-one
(2d). Pale yellow oil. IR (neat) 2852, 1712, 1651, 1456,
1
1028, 903 cmK1; H NMR dZ1.80 (s, 3H), 2.22 (dd, JZ
3.0, 17.4 Hz, 1H), 2.72 (dd, JZ3.0, 17.4 Hz, 1H), 3.21–3.25
(m, 2H), 4.33 (dd, JZ5.8, 5.8 Hz, 1H), 4.63 (d, JZ16.6 Hz,
1H), 4.77 (d, JZ16.6 Hz, 1H), 5.21 (s, 1H), 5.61 (s, 1H);
13C NMR dZ22.2, 40.2, 43.3, 66.4, 71.4, 118.0, 134.6,
135.1, 176.5, 206.6; HRMS (EIC) for M found m/e
164.0824, calcd for C10H12O2: 164.0837. [a]3D1 K178.3 (c
1.17, CHCl3, 97% ee). Enantiomeric excess was determined
by HPLC analysis using a chiral column (Daicel Chiralpak
AD-H: 4!250 mm, 254 nm UV detector, room tempera-
ture, eluent: 3% 2-propanol in hexane, flow rate: 1.0 mL/
min, retention time: 15 min for major isomer and 16 min for
minor isomer).
Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Prof. Koichi
Narasaka and Dr. Yuji Koga (University of Tokyo) for
helpful discussion. T. S. thanks the Inamori Foundation for
supporting this work.
References and notes
4.2.2. Diethyl 2-(benzyloxy)methyl-3-oxobicyclo[3.3.0]
oct-1-en-7,7-dicarboxylate (2i). Pale yellow oil. IR (neat)
1. (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.
J. Chem. Soc., Perkin Trans. 1 1973, 977–981. (b) Khand,
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30–32.
1
2982, 1730, 1672, 1267 cmK1; H NMR dZ1.21–1.30 (m,
6H), 1.69 (dd, JZ12.7, 12.7 Hz, 1H), 2.11 (dd, JZ3.3,
17.9 Hz, 1H), 2.64 (dd, JZ6.2, 17.9 Hz, 1H), 2.78 (dd, JZ
7.7, 12.7 Hz, 1H), 2.98–3.06 (m, 1H), 3.34 (d, JZ20.6 Hz,
1H), 3.42 (d, JZ20.6 Hz, 1H), 4.19–4.24 (m, 6H), 4.53 (s,
2H), 7.25–7.34 (m, 5H); 13C NMR dZ14.1, 34.7, 38.8, 41.6,
43.5, 61.1, 61.9, 62.0, 63.1, 73.1, 127.5, 128.2 133.6, 137.8,
170.6, 171.3, 181.2, 207.4; HRMS (FAB) for MC1 found
m/e 387.1811, calcd for C22H27O6: 387.1808. [a]3D1 K48.2
(c 1.11, CHCl3, 88% ee). Enantiomeric excess was
determined by HPLC analysis using a chiral column (Daicel
Chiralpak AS-H: 4!250 mm, 254 nm UV detector, room
temperature, eluent: 10% 2-propanol in hexane, flow rate:
1.0 mL/min, retention time: 13 min for minor isomer and
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en-3-one (2k). IR (neat) 1711, 1021, 919, 764 cmK1; H
NMR dZ2.25 (dd, JZ6.6, 13.5 Hz, 1H), 2.39 (d, JZ
17.4 Hz, 1H), 2.48 (dd, JZ8.1, 13.5 Hz, 1H), 2.69 (d, JZ
17.4 Hz, 1H), 3.40 (d, JZ8.1 Hz, 1H), 4.14 (d, JZ8.1 Hz,
1H), 4.58 (d, JZ16.4 Hz, 1H), 4.93 (d, JZ16.4 Hz, 1H),
5.12 (d, JZ0.9 Hz, 1H), 5.16 (d, JZ4.5 Hz, 1H), 5.59–5.77
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4.3. Typical experimental procedure for enantioselective
coupling using cinnamaldehyde as a CO source (Table 4)
Under an atmosphere of argon, tolBINAP (20.4 mg,
0.030 mmol) and [Ir(cod)Cl]2 (10.1 mg, 0.015 mmol) were
stirred in xylene (1.5 mL) at room temperature. After the
addition of a xylene solution (0.5 mL) of enyne 1
(0.30 mmol) and cinnamaldehyde (198.0 mg, 1.5 mmol),
the reaction mixture was stirred at 120 8C for an appropriate
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would be needed for an efficient carbonyl insertion.
18. The carbonyl insertion between metal and sp2-carbon was
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