Cp*Li as a base: application to palladium-catalyzed cross-coupling reaction of aryl-X or alkenyl-X (X=I, Br, OTf, ONf) with terminal acetylenes

Article abstract of DOI:10.1016/j.tet.2007.11.095

The reaction of aryl-X or alkenyl-X (X=I, Br, OTf, ONf) with terminal acetylenes in the presence of a catalytic amount of Pd(OAc)₂ provided the alkynylated products in good yields by using Cp^*Li (Cp^*=1,2,3,4,5-pentamethylcyclopentadienyl) as a base.

Full text of DOI:10.1016/j.tet.2007.11.095

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1829e1833
www.elsevier.com/locate/tet
Cp*Li as a base: application to palladium-catalyzed cross-coupling
reaction of aryl-X or alkenyl-X (X¼I, Br, OTf, ONf)
with terminal acetylenes
*
Minoru Uemura, Hideki Yorimitsu , Koichiro Oshima
*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received 7 November 2007; received in revised form 28 November 2007; accepted 28 November 2007
Available online 4 December 2007
Abstract
The reaction of aryl-X or alkenyl-X (X¼I, Br, OTf, ONf) with terminal acetylenes in the presence of a catalytic amount of Pd(OAc)2
provided the alkynylated products in good yields by using Cp*Li (Cp*¼1,2,3,4,5-pentamethylcyclopentadienyl) as a base.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Pentamethylcyclopentadiene; Cross-coupling reaction; Lithium acetylides; Alkynylation
1. Introduction
at reflux for 1 h gave 1-octynylbenzene (3aa) in 82% yield
(Table 1, entry 7). Other lithium bases, such as n-BuLi, lithium
diisopropylamide (LDA), lithium hexamethyldisilazide
(LHMDS), and lithium cyclopentadienide (CpLi), did not
assist the coupling reaction efficiently (entries 1e4). However,
lithium indenide resulted in the formation of 3aa in 24%
yield (entry 5). Interestingly, the reaction with lithium
Pentamethylcyclopentadienide (Me5Cꢀ5 , Cp*ꢀ) has been
used as a ligand of transition metal complexes for 40 years.1
Although Cp*ꢀ has unique properties, such as six-electron
donation and steric bulkiness by the five methyl groups on
the five-membered ring, it has not been used for other pur-
poses. Then we have been exploring applications of Cp*ꢀ as
a reagent in organic synthesis.2 Now, we have been interested
in Cp*ꢀ as a base. We expected that its strong basicity and ste-
ric bulkiness would exhibit interesting reactivities. We have
found that the reaction of aryl-X or alkenyl-X (X¼I, Br,
OTf, ONf) with terminal acetylenes in the presence of a cata-
lytic amount of palladium provided the alkynylated products
in good yields by using Cp*Li as a base.
Table 1
Effect of lithium bases on palladium-catalyzed cross-coupling reaction of
iodobenzene (1a) with 1-octyne (2a)
lithium base (1.5 equiv)
Pd(OAc)2 (7.5 mol%)
THF, reflux, 1 h
n-C6H13
C
n-C6H13
I
C
C
+
C
H
(1.5 equiv)
2a
1a
3aa
Entry
Lithium base
NMR yield/%
2. Results and discussions
1
2
3
4
5
6
7
a
n-BuLi
3
3
Lithium diisopropylamide (LDA)
Lithium hexamethyldisilazide (LHMDS)
Lithium cyclopentadienide (CpLi)
Lithium indenidea
3
Treatment of a suspension of Cp*Li in THF with iodobenz-
ene (1a), 1-octyne (2a), and a catalytic amount of Pd(OAc)2
3
24
55
82b
Lithium tetramethylcyclopentadienide
Cp*Li
* Corresponding authors. Tel.: þ81 75 383 2441; fax: þ81 75 383 2438.
E-mail addresses: yori@orgrxn.mbox.media.kyoto-u.ac.jp (H. Yorimitsu),
oshima@orgrxn.mbox.media.kyoto-u.ac.jp (K. Oshima).
Derived from indene and n-BuLi.
Isolated yield.
b
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.095

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M. Uemura et al. / Tetrahedron 64 (2008) 1829e1833
Table 2
Reactions of various aryl iodides 1 with terminal acetylenes 2
tetramethylcyclopentadienide, which lacks one methyl group
compared with Cp*Li, gave 3aa in lower yield than that
with Cp*Li yet in higher yield than that with CpLi (entry 6).
Although many kinds of alkynylmetals have been used for
palladium-catalyzed cross-coupling reaction of aryl-X or al-
kenyl-X, there are no reports on the cross-coupling reaction
with lithium acetylides.3 The coupling reaction with lithium
acetylides is difficult because the reaction of a palladium cata-
lyst with an excess amount of lithium acetylides provides
lithium palladate 4, which does not exhibit any catalytic
activity (Eq. 1).4 We thought that gradual formation of
lithium acetylides by means of Cp*Li would avoid the forma-
tion of 4.
Cp*Li (1.5 equiv)
R2
R2
C
Pd(OAc)2 (7.5 mol%)
THF, reflux, 1 h
I
C
C
+
C
H
R1
R1
1
2 (1.5 equiv)
3
Entry
R1
R2
Isolated yield/%
1
H (1a)
1a
n-C6H13 (2a)
c-C6H11 (2b)
82 (3aa)
85 (3ab)
93 (3ac)a
69 (3ad)b
72 (3ae)a,b
81 (3ba)
82 (3ca)
70 (3da)
71 (3ea)
83 (3fa)
74 (3ga)
83 (3ha)
83 (3ia)c
67 (3ja)
2
3
1a
1a
Ph (2c)
TES (2d)
TMS (2e)
4
5
1a
6
p-Me (1b)
p-OMe (1c)
p-CF3 (1d)
p-Cl (1e)
p-COOEt (1f)
p-CN (1g)
m-Me (1h)
o-Me (1i)
1-Naphthyl iodide (1j)
2a
2a
2a
2a
2a
2a
2a
2a
2a
7
8
+
Cl2Pd(PPh3)2
(
C
4
Li
C
C
R
Li2Pd
C R)4
ð1Þ
9
(excess)
10
11
12
13
14
a
To confirm the gradual generation of lithium acetylides,
we carried out the following experiments. Treatment of lith-
ium acetylide 5 with Cp*H for 1 h at reflux followed by an
addition of benzaldehyde at 0 ꢁC gave a mixture of octynyl-
ated alcohol 6 and pentamethylcyclopentadienylated alcohol
7 (Eq. 2). On the other hand, 7 was obtained selectively by
using the reaction mixture of 5 and Cp*H, which was treated
for 11 h at reflux. These facts indicate that the pKa value of
Cp*H is smaller than that of 2a and that a high energy barrier
exists between 2aþCp*Li and 5þCp*H. A similar reaction
with cyclopentadiene (CpH), instead of Cp*H, only for 1 h
followed by an addition of benzaldehyde did not give 6 but
gave Cp adduct-derived compounds such as 6-phenylfulvene.
This result means that the energy barrier between 2aþCpLi
and 5þCpH is lower than that between 2aþCp*Li and
5þCp*H. Not only the pKa values of lithium bases but also
the high energy barrier is important to promote the cross-coup-
ling reaction. The reaction of a mixture of 2a and Cp*Li
with benzaldehyde at reflux or at 0 ꢁC provided 6 or 7, re-
spectively (Eq. 3). The results mean that 5 is generated
only at reflux. The reason why 7 is not provided at reflux
conditions is that nucleophilic addition reaction of Cp*Li to
benzaldehyde is thermally reversible and that of 5 to benz-
aldehyde is irreversible. We are sure that the equilibrium
between 2aþCp*Li and 5þCp*H is mostly biased toward
2aþCp*Li and that a modest amount of 5þCp*H would be
generated at reflux.
PPh3 of 15 mol % was added.
The reaction was carried out at 50 ꢁC for 2 h.
The reaction was carried out for 1.5 h.
b
c
Table 2 summarizes the results obtained by the Cp*Li-me-
diated reaction of aryl iodides 1 with terminal acetylenes 2.
Aromatic iodide 1 bearing an electron-donating group (Table
2, entry 7) or an electron-withdrawing group (entry 8) afforded
3 in high yields. Surprisingly, ester (entry 10) or cyano moiety
(entry 11) on the aromatic ring did not undergo nucleophilic
addition reaction of Cp*Li or lithium acetylides during ther-
mal conditions. Reaction of meta- or ortho-substituted 1 also
proceeded smoothly (entries 12 and 13). 1-Naphthyl iodide
could be used in this reaction (entry 14). The reaction with
phenylacetylene (2c) or trimethylsilylacetylene (2e) afforded
3 in good yields by using 15 mol % of triphenylphosphine
(entries 3 and 5).
Not only aryl iodides but also aryl bromides, triflate, or
nonaflate could be also used as substrates in the Cp*-mediated
reaction (Table 3). In these cases, triphenylphosphine was
essential as a ligand.
By utilizing the advantages of Cp*Li as a base, we find two
applications. The reaction of 2-iodobenzoic acid with 2 pro-
vided isocoumarin 8 and (Z)-3-alkylidenephthalide 9 in good
PhCHO
n-C6H13
n-C6H13
C
HO
Cp*
(1.0 equiv)
+
C
HO
C
+
Cp*H
C
THF
0 °C, 30 min
5
(1.1 equiv)
Li
Ph
ð2Þ
reflux, time
(1.1 equiv)
Ph
6
77
0
7
1 h
11 h
:
:
23
100
n-C6H13
yields (Eq. 4).5 We also found that Cp*Li can be used as
a bulky base to prepare lithium enolates. With this reactivity,
we accomplished one-pot synthesis of enynes 12 from ketone
10 via nonaflate 11 in good yields (Eq. 5).
C
+
Cp*Li
+
PhCHO
6
+
7
C
THF, 30 min
reflux
(1.1 equiv)
(1.0 equiv)
2a
H
92 :
8
(1.1 equiv)
0
: 100
0 °C
ð3Þ

M. Uemura et al. / Tetrahedron 64 (2008) 1829e1833
1831
Table 3
Reactions of various aryl bromides, triflate, or nonaflate with 1-octyne (2a)
3.1.1. General procedure for palladium-catalyzed cross-
coupling reaction of aryl iodides 1 with terminal acetylenes
2 using Cp*Li
Cp*Li (1.5 equiv)
Pd(OAc)2 (7.5 mol%)
PPh3 (15 mol%)
n-C6H13
C
A solution of n-BuLi in hexane (1.60 M, 0.47 mL,
0.75 mmol) was added to a solution of 1,2,3,4,5-penta-
methyl-1,3-cyclopentadiene (0.13 mL, 0.80 mmol) in THF
(5 mL) at 0 ꢁC. The mixture was stirred for 10 min at the
same temperature. Pd(OAc)2 (8.4 mg, 0.038 mmol), 2a
(82.7 mg, 0.75 mmol) in THF (0.5 mL), and 1a (102 mg,
0.50 mmol) in THF (0.5 mL) were added to the reaction mix-
ture. The resulting mixture was stirred for 1 h at reflux. The
reaction mixture was quenched with saturated aqueous
NH4Cl, and the mixture was extracted with hexane. The com-
bined organic parts were washed with brine, dried over anhy-
drous Na2SO4, and concentrated in vacuo to give a crude oil.
The oil was purified by chromatography on silica gel (Wako-
gel C-200, hexane) to afford 3aa (75.5 mg, 0.41 mmol, 82%).
X
C
+
2a
THF, reflux, 1 h
(1.5 equiv)
3
R
R
Entry
X
R
Isolated yield/%
1
2
3
4
5
a
Br
Br
H
79 (3aa)
74 (3ca)c
79 (3da)
58 (3aa)
88 (3aa)c
p-OMe
p-CF3
H
Br
OTfa
ONfb
H
Tf¼trifluoromethanesulfonyl.
b
c
Nf¼1,1,2,2,3,3,4,4,4-nonafluoro-1-butanesulfonyl.
The reaction was carried out for 1.5 h.
Cp*Li (2.5 equiv)
R2
O
R2
2a or 2b
Pd(OAc)2
(1.5 equiv)
(7.5 mol%)
+
I
+
3.1.2. General procedure for palladium-catalyzed cross-
coupling reaction of aryl bromides, triflate, or
nonaflate with 1-octyne (2a) using Cp*Li
A solution of n-BuLi in hexane (1.60 M, 0.47 mL,
0.75 mmol) was added to a solution of 1,2,3,4,5-penta-
methyl-1,3-cyclopentadiene (0.13 mL, 0.80 mmol) in THF
O
THF, reflux, 1 h
ð4Þ
O
8
O
COOH
9
8a+9a: R2 = n-C6H13, 70% (68:32)
8b+9b: R2 = c-C6H11, 78% (63:37)
H
C C
R
(1.5 equiv)
Pd(OAc)2 (7.5 mol%)
PPh3 (15 mol%)
R
1) Cp*Li (2.5 equiv)
O
ONf
C
2) NfF* (1.2 equiv)
C
reflux, 1 h
THF, 0 °C
ð5Þ
10
11
12
12a: R = n-C6H13, 64%
12b: R = Ph, 70%
*NfF = 1,1,2,2,3,3,4,4,4-nonafluoro
-1-butanesulfonyl fluoride
12c: R = TMS, 69% (50 °C, 2 h)
3. Experimental
3.1. Instrumentation and materials
(5 mL) at 0 ꢁC. The white suspension was stirred for 10 min
at the same temperature. Pd(OAc)2 (8.4 mg, 0.038 mmol),
PPh3 (19.7 mg, 0.075 mmol), 1-octyne (82.7 mg, 0.75 mmol)
in THF (0.5 mL), and bromobenzene (78.5 mg, 0.50 mmol) in
THF (0.5 mL) were added to the reaction mixture, and the
mixture was stirred for 1 h at reflux. The reaction was termi-
nated with saturated aqueous NH4Cl, and the mixture was
extracted with hexane. The combined organic layers were
washed with brine, dried over anhydrous Na2SO4, and concen-
trated in vacuo to give a crude oil. Chromatographic purifica-
tion on silica gel (Wakogel C-200, hexane) afforded 3aa
(73.4 mg, 0.39 mmol, 79%).
1H NMR (500 MHz) and 13C NMR (125.7 MHz) spectra
were taken on a Varian UNITY INOVA 500 spectrometer un-
less otherwise noted. Chemical shifts (d) are in parts per mil-
lion relative to chloroform at 7.26 ppm for 1H and at 77.0 ppm
for 13C. IR spectra were determined on a SHIMADZU FTIR-
8200PC spectrometer. TLC analyses were performed on com-
mercial glass plates bearing 0.25-mm layer of Merck silica gel
60F254. Silica gel (Wakogel 200 mesh) was used for column
chromatography. Gel permeation chromatography (GPC) was
performed by LC-908 (Japan Analytical Industry Ltd., two
in-line JAIGEL-2H, toluene, 7.8 mL/min, UV and RI detec-
tors). Elemental analyses were carried out at the Elemental
Analysis Center of Kyoto University.
Unless otherwise noted, materials obtained from com-
mercial suppliers were used without further purification.
1,2,3,4,5-Pentamethyl-1,3-cyclopentadiene was purchased from
Kanto Chemical Co. THF (dehydrated, stabilizer free) was
purchased from Kanto Chemical Co., stored under nitrogen,
and used without distillation. All reactions were carried out
under argon atmosphere.
3.1.3. General procedure for palladium-catalyzed
annulation of 2-iodobenzoic acid with terminal acetylenes
2 using Cp*Li
A solution of n-BuLi in hexane (1.60 M, 0.78 mL,
1.25 mmol) was added to a solution of 1,2,3,4,5-penta-
methyl-1,3-cyclopentadiene (0.20 mL, 1.3 mmol) in THF
(5 mL) at 0 ꢁC. The mixture was stirred for 10 min at 0 ꢁC.
Pd(OAc)2 (8.4 mg, 0.038 mmol), 2a (82.7 mg, 0.75 mmol) in
THF (0.5 mL), and 2-iodobenzoic acid (124 mg, 0.50 mmol)
in THF (0.5 mL) were added to the reaction mixture, and
the mixture was stirred for 1 h at reflux. After being quenched

1832
M. Uemura et al. / Tetrahedron 64 (2008) 1829e1833
with water, the mixture was extracted with ethyl acetate. The
combined organic parts were washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo to give a
crude oil. Purification on silica gel (Wakogel C-200, hexanee
ethyl acetate¼20:1) afforded a mixture of 8a and 9a
(8ae9a¼68:32, 80.8 mg, 0.35 mmol, 70%).
(t, J¼7.0 Hz, 3H), 2.42 (t, J¼7.0 Hz, 2H), 4.36 (t, J¼7.0 Hz,
2H), 7.41e7.45 (m, 2H), 7.93e7.97 (m, 2H); 13C NMR
(CDCl3) d 14.04, 14.30, 19.50, 22.53, 28.54, 28.60, 31.33,
61.01, 80.12, 93.90, 128.81, 129.13, 129.33 (ꢂ2), 131.39
(ꢂ2), 166.20. Found: C, 78.92; H, 8.78%. Calcd for
C17H22O2: C, 79.03; H, 8.58%.
3.1.4. General procedure for palladium-catalyzed one-pot
synthesis of enynes 12 from 10 via 11
3.2.2. Phenyl 1,1,2,2,3,3,4,4,4-nonafluoro-1-
butanesulfonate
IR (neat) 1488, 1427, 1354, 1230, 1204, 1145, 1133, 1036,
A solution of n-BuLi in hexane (1.60 M, 0.78 mL,
1.25 mmol) was added to a solution of 1,2,3,4,5-pentamethyl-
1,3-cyclopentadiene (0.20 mL, 1.3 mmol) in THF (5 mL) at
0 ꢁC. The mixture was stirred for 20 min at the same tempera-
ture. Cyclohexanone (49.1 mg, 0.5 mmol) in THF (0.5 mL)
was added to the resulting white suspension, and the mixture
was stirred for 30 min at 0 ꢁC. 1,1,2,2,3,3,4,4,4-Nonafluoro-1-
butanesulfonyl fluoride (181 mg, 0.60 mmol) in THF (0.5 mL)
was added to the reaction mixture, and the mixture was stirred
for 30 min at 0 ꢁC. 1-Octyne (2a, 82.7 mg, 0.75 mmol) in
THF (0.5 mL), PPh3 (19.7 mg, 0.075 mmol), and Pd(OAc)2
(8.4 mg, 0.038 mmol) were added to the reaction mixture,
and the mixture was stirred for 1 h at reflux. Water was
added to quench the reaction, and the mixture was extracted
with hexane. The combined organic parts were washed with
brine, dried over anhydrous Na2SO4, and concentrated
in vacuo to give a crude oil. The oil was purified by chroma-
tography on silica gel (Wakogel C-200, hexane) to afford 12a
(60.3 mg, 0.32 mmol, 64%).
1
891, 772, 686 cmꢀ1; H NMR (CDCl3) d 7.28e7.31 (m, 2H),
7.37e7.42 (m, 1H), 7.44e7.48 (m, 2H); 13C NMR (CDCl3)
d 107.44e118.48 (m, ꢂ4), 121.35 (ꢂ2), 128.35, 130.25
(ꢂ2), 149.89. Found: C, 31.87; H, 1.50%. Calcd for
C10H5F9O3S: C, 31.93; H, 1.34%.
3.2.3. 3-Hexylisocoumarin (8a) and (Z)-3-
heptylidenephthalide (9a) (68:32)
IR (neat) 2929, 2857, 1782, 1733, 1658, 1484, 1161, 1024,
1
983, 759, 691 cmꢀ1; H NMR (CDCl3) d 0.89 (t, J¼7.5 Hz,
3H), 1.24e1.74 (m, 8H), 2.47 (dt, J¼7.5, 7.5 Hz, 2ꢂ0.32H),
2.52 (t, J¼7.5 Hz, 2ꢂ0.68H), 5.64 (t, J¼7.5 Hz, 1ꢂ0.32H),
6.25 (s, 1ꢂ0.68H), 7.32e8.27 (m, 4H); 13C NMR (CDCl3)
d 14.01, 14.05, 22.49, 22.56, 25.81, 26.85, 28.66, 28.96,
29.22, 31.48, 31.60, 33.51, 102.82, 109.76, 119.59, 120.10,
124.43, 124.98, 125.22, 127.49, 129.27, 129.48, 134.17,
134.67, 137.63, 139.58, 145.58, 158.33, 163.11, 167.21.
Found: C, 78.13; H, 7.83%. Calcd for C15H18O2: C, 78.23;
H, 7.88%.
3.1.5. Preparation of phenyl 1,1,2,2,3,3,4,4,4-nonafluoro-1-
butanesulfonate
3.2.4. 3-Cyclohexylisocoumarin (8b) and (Z)-3-
cyclohexylmethylenephthalide (9b) (63:37)
A solution of n-BuLi in hexane (1.60 M, 3.44 mL,
5.5 mmol) was added to a solution of phenol (471 mg,
5.0 mmol) in THF (17 mL) at 0 ꢁC. The mixture was stirred
for 10 min at the same temperature. 1,1,2,2,3,3,4,4,4-Nona-
fluoro-1-butanesulfonyl fluoride (1.81 g, 6.0 mmol) in THF
(1 mL) was added to the reaction mixture, and the mixture
was stirred for 30 min at room temperature. The reaction
was quenched with water, and the mixture was extracted
with ethyl acetate. The combined organic parts were washed
with brine, dried over anhydrous Na2SO4, and concentrated
in vacuo to give a crude oil. The oil was purified by chroma-
tography on silica gel (Wakogel C-200, hexaneeethyl
acetate¼40:1) to afford phenyl 1,1,2,2,3,3,4,4,4-nonafluoro-
1-butanesulfonate (1.52 g, 4.0 mmol, 81%).
IR (Nujol) 1776, 1727, 1718, 1649 cmꢀ1
;
1H NMR
(CDCl3) d 1.28e2.08 (m, 10H), 2.41e2.48 (m, 1ꢂ0.63H),
2.79e2.88 (m, 1ꢂ0.37H), 5.50 (d, J¼9.5 Hz, 1ꢂ0.37H),
6.23 (s, 1ꢂ0.63H), 7.34e8.27 (m, 4H); 13C NMR (CDCl3)
d 25.63, 25.86 (ꢂ3), 25.92 (ꢂ2), 30.56 (ꢂ2), 32.85 (ꢂ2),
35.28, 41.84, 100.85, 115.04, 119.64, 120.26, 124.37,
125.20, 125.22, 127.45, 129.27, 129.42, 134.15, 134.60,
137.74, 139.78, 144.16, 162.35, 163.12, 167.25. Found: C,
79.13; H, 7.20%. Calcd for C15H16O2: C, 78.92; H, 7.06%.
Mp 78.0e79.0 ꢁC.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Government of Japan. M.U.
acknowledges JSPS for financial support.
3.2. Characterization data
The compounds 3aa, 3ac, and 3ae are commercially avail-
able. The products 3ab,6 3ad,7 3ba,8 3ca,8 3da,8 3ea,9 3ga,8
3ha,8 3ia,8 3ja,10 12a,11 12b,12 and 12c12 can be found in
the literature.
References and notes
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King, R. B.; Bisnette, M. B. J. Organomet. Chem. 1967, 8, 287; (b)
3.2.1. Ethyl 4-(1-octynyl)benzoate (3fa)
IR (neat) 2932, 2859, 1720, 1608, 1466, 1367, 1306, 1272,
Rohl, H.; Lange, E.; Gossl, T.; Roth, G. Angew. Chem. 1962, 74, 155.
¨
¨
1175, 1106, 1097, 1021, 857, 770, 697 cmꢀ1 1H NMR
;
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(CDCl3) d 0.91 (t, J¼7.0 Hz, 3H), 1.28e1.64 (m, 8H), 1.39

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´
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