The palladium-iminophosphine catalyst for the reactions of organostannanes

Article abstract of DOI:10.1016/S0022-328X(98)01056-0

A palladium complex coordinated by an iminophosphine ligand was found to be a remarkably active catalyst for the coupling of organostannanes with aryl halides. The mechanistic studies show that the reaction of an alkynylstannane proceeds through an unprecedented catalytic cycle which involves an oxidative addition of the organostannane to the Pd(0)-iminophosphine complex. The catalyst was demonstrated to be also useful for the carbostannylation of alkynes and the homocoupling reaction of organostannanes.

Full text of DOI:10.1016/S0022-328X(98)01056-0

Journal of Organometallic Chemistry 576 (1999) 169–178
Review
The palladium–iminophosphine catalyst for the reactions of
organostannanesꢀ
Eiji Shirakawa a,*, Tamejiro Hiyama b
a Graduate School of Materials Science, Japan Ad6anced Institute of Science and Technology, Hokuriku, Asahidai, Tatsunokuchi,
Ishikawa 923-1292, Japan
b Department of Material Chemistry, Graduate School of Engineering, Kyoto Uni6ersity, Sakyo-ku, Kyoto 606-8501, Japan
Abstract
A palladium complex coordinated by an iminophosphine ligand was found to be a remarkably active catalyst for the coupling
of organostannanes with aryl halides. The mechanistic studies show that the reaction of an alkynylstannane proceeds through an
unprecedented catalytic cycle which involves an oxidative addition of the organostannane to the Pd(0)–iminophosphine complex.
The catalyst was demonstrated to be also useful for the carbostannylation of alkynes and the homocoupling reaction of
organostannanes. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Cross-coupling; Carbostannylation; Homocoupling; Catalytic cycle; Oxidative addition
1. Introduction
to a latent electrophilicity of the Pd(0) complex: the
reaction with an organostannane nucleophile gives rise
to a novel nucleophilic Pd(II) complex. This character
contrasts sharply to that of a palladium complex with a
conventional phosphine ligand and will be presented in
the following chapters (Scheme 1).
The palladium-catalyzed cross-coupling of organos-
tannanes with aryl halides is one of the most versatile
carbon–carbon bond forming reactions featured by
high chemoselectivity [1–3]. This means, however, that
sometimes the reaction is not reactive enough and
requires rather drastic conditions. The inertness of a
C–Sn bond is evidenced also by the fact that the
transition metal-catalyzed reaction of organostannanes
other than the cross-coupling is extremely rare.
2. Cross-coupling reaction of organostannanes with aryl
halides
In this paper, we demonstrate that the cross-coupling
reaction of organostannanes with aryl halides is effi-
ciently catalyzed by a palladium complex coordinated
by a bidentate iminophosphine ligand [4a]. We also
discuss its catalytic cycles [4b], the carbostannylation of
alkynes [4c] and the homocoupling reaction [4d,e].
The unique characters of the palladium complex
coordinated by an iminophosphine may be attributed
2.1. Introduction
The currently accepted catalytic cycle of the cross-
coupling reaction of organostannanes with aryl halides
ꢀ Dedicated to Professor Jiro Tsuji and Richard F. Heck.
* Corresponding author. Fax: +1-81-761-51-1635.
E-mail address: shira@jaist.ac.jp (E. Shirakawa)
Scheme 1.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01056-0

170
E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
Scheme 2.
involves three distinct steps: (1) oxidative addition of an
aryl halide to an electron-rich Pd(0) complex, (2)
transmetalation of the resulting electrophilic Pd(II)
complex with a nucleophilic organostannane, and (3)
reductive elimination to give a cross-coupled product
and regenerate the Pd(0) complex [5].
Farina and his co-workers surveyed the ligand for
this coupling reaction and revealed that the reaction
was accelerated in the magnitude of three orders by
such a modest donor ligand as tri(2-furyl)phosphine or
triphenylarsine as compared with triphenylphosphine
[5]. The ligand is assumed to dissociate from a Pd(II)
intermediate to make a vacant coordination site needed
for the rate-determining transmetalation. This working
hypothesis is rationalized by an inhibitory effect of an
additional ligand and by the slow rates observed with a
stronger donor like triphenylphosphine.
phine complexes to give in 24 h 1-phenyl-2-(4-trifl-
uoromethyl)phenylethyne (3) in 89% yield. A palladium
complex coordinated by aminophosphine AP, where
the imino moiety in IP is replaced by a dimethyl-
aminomethyl group, was slightly less active, giving 79%
of 3. Under the same conditions, a palladium complex
with tri(2-furyl)phosphine (TFP) complex (TFP/Pd =
2) or 1,3-bis(diphenylphosphino)propane (DPPP) gave
3 in 57 or 34% yield, respectively. The use of
triphenylphosphine, one of the most conventional phos-
phine ligands, was not effective, giving only 1% yield of
3 [7].
It is noteworthy that P–N ligands IP and AP are
more effective than TFP, which has been reported by
Farina and his co-workers to be the best phosphine
ligand for the palladium-catalyzed coupling of iodoben-
zene with tributyl(vinyl)tin [5]. They showed by kinetic
study that the dissociation of a ligand from the Pd(II)
intermediate to make a vacant coordination site for a
vinyltin should be responsible for the high catalytic
activity. In the reaction with IP, the expected intermedi-
ate in the catalytic cycle is complex 4 (cf. Scheme 4,
vide infra) where both phosphino and imino groups
2.2. Palladium–iminophosphine as an acti6e catalyst
Assuming that the reaction would proceed according
to the catalytic cycle mentioned above, we expected
that
a
palladium complex coordinated by an
iminophosphine should be a good catalyst for the cross-
coupling reaction for the following reason. An imino
ligand is said to have a p-acceptor character [6] which
would make a Pd(II) complex electrophilic to accelerate
the reaction of Pd(II) with a nucleophilic organometal-
lic reagent. In contrast, a phosphino group, which
generally has a strong affinity to Pd(0), will assist the
metal to react with an electrophile. Thus, the two
different coordinating groups may play respective roles
cooperatively in a catalytic cycle.
First, the catalytic activities of a palladium complex
coordinated by N-(2-diphenylphosphinobenzylidene)-2-
phenylethylamine (IP) and some other phosphine-based
ligands were compared in the coupling reaction of
4-(trifluoromethyl)iodobenzene (1) with tributyl-
(phenylethynyl)tin (2) (Scheme 2). IP was found to give
higher reaction rate than any other palladium–phos-
coordinate to palladium through a six-membered
chelate. Thus, the dissociation of either phosphorus or
nitrogen atom in 4 is unlikely [8,9]. The fact that IP is
more effective than TFP suggests that a vacant coordi-
nation site proposed by Farina is not always required
for the present coupling reaction, or alternatively that
the reaction proceeds through a different pathway by
use of IP ligand. Contrary to the high yields observed
with P–N ligands IP, AP and DPPP gave 3 in low
yield. Thus, the coordination of the nitrogen atom
appears to be essential for the high catalytic activity
and formation of a six-membered chelate appears to be
rather trivial.
In order to clarify the role of the nitrogen ligand, we
compared the total rate of the cross-coupling reaction
under the catalytic conditions and the rate of
transmetalation step in a stoichiometric reaction using

E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
171
Scheme 3.
one of bidentate ligands IP, AP and DPPP (Schemes 3
and 4). The rate of the cross-coupling reaction in an
early stage was estimated by the time (T1/4) required for
25% conversion; the rate of transmetalation was esti-
mated by the conversion of the reaction of Pd(II)
complex 4, 5 or 6 with 2 in 15 h. Although IP gave the
highest rate for both reactions, AP gave a rate faster for
the cross-coupling and slower for the transmetalation
than DPPP. These results contrast sharply to the gener-
ally accepted catalytic cycle, where the rate of the
cross-coupling reaction is proportional to the rate of
the rate-determining transmetalation step. This dis-
crepancy led us to propose a different catalytic cycle.
lyzed coupling of 2 with 1, using IP or DPPP as a
ligand (Scheme 6). According to Cycle A, complex 4
and 5, prepared by the reaction of 1 with the corre-
sponding Pd(0) complexes, are the expected intermedi-
ates. Thus, if the reaction would take place according
to Cycle A, no induction period should be observed
with either 4 or 5, and the rates in the early stage of the
reaction should be similar to or higher than those with
the corresponding catalysts prepared in situ. If this is
not the case, another catalytic cycle, Cycle B, must be
working. To estimate the rate in the early stage of the
reaction, we measured the time (T1/4) required for 25%
conversion. Conversion was monitored on the basis of
consumed 1 by 19F-NMR of the reaction mixture.
When complex 4 or 5 was used as a catalyst, the
2.3. Alternati6e catalytic cycle
The currently accepted catalytic cycle for the cou-
pling reaction is represented by Cycle A in Scheme 5,
which starts with oxidative addition of an aryl halide to
a Pd(0) complex. An alternative cycle must involve the
reaction of a Pd(0) complex with an organostannane,
the resulting Pd(II) complex then reacting with an aryl
halide as illustrated by Cycle B in Scheme 5. However,
this sort of catalytic cycle [10], or an oxidative addition
of an organostannane to a Pd(0) complex [11], has
never been presented before.
To gain an insight into the new catalytic cycle, we
first examined the reaction rate of the palladium-cata-
Scheme 5.
Scheme 4.

172
E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
Scheme 6.
product directly derived from the complex also was
counted in conversion.
We found that T1/4 with preformed catalyst 4 was
440 min, much larger than T1/4 (=72 min) obtained
with the catalyst prepared in situ from IP and [PdCl(p-
C3H5)]2. This means that the catalyst of [PdCl(p-
C3H5)]2–IP did not mediate the coupling reaction
according to Cycle A. In contrast, the catalysts both
preformed and in situ prepared with DPPP, promoted
the reaction comparably, T1/4 being 1270 and 1260 min,
respectively. These results suggest that Cycle A is in-
volved in these cases.
Fig. 1. 31P{1H}-NMR (109 MHz) spectrum of the reaction mixture
(at ca. 38% conversion) of the coupling of 2 with 1 in THF in the
presence of [PdCl(p-C3H5)]–DPPP.
We next monitored by 31P{1H}-NMR the behavior
of the palladium complex during the reaction (Figs. 1
and 2). Since intermolecular exchange of an Ar group
as shown Scheme 7 was not observed at 25°C in 1 d,
oxidative addition of 1 to a palladium(0) complex
coordinated by IP or DPPP must be irreversible. Thus,
complex 4 or 5 [12], once produced, cannot but get into
Cycle A. Furthermore, examinations under the stoi-
chiometric conditions shown in Scheme 8 revealed that
transmetalation was the rate-determining step in Cycle
A.
The peaks assigned to complex 5 predominated in the
reaction catalyzed by [PdCl(p-C3H5)]2–DPPP (Fig. 1),
showing that the reaction proceeded according to Cycle
A and that transmetalation was the rate-determining
step. By contrast, the reaction with [PdCl(p-C3H5)]2–IP
showed only a small amount of complex 4 (Fig. 2). If
the reaction proceeded according to Cycle A, complex 4
must have been observed dominantly as discussed
above, because the rate-determining step should be
transmetalation. Instead, two major species were ob-
served and identified as 7 and 8 (Figs. 2 and 3, [13,14]).
Fig. 2. 31P{1H}-NMR (109 MHz) spectrum of the reaction mixture
(at 2–33% conversion) of [PdCl(p-C3H5)]2–IP catalyzed coupling of 2
with 1 in THF.
Scheme 7.
Scheme 8.

E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
173
Fig. 4. 31P{1H}-, 119Sn{1H}- and H-NMR (THF) parameters (chem-
ical shift, JSn–P and JP–P) of Pd(0)/DPPP and its reaction products
with 2 or Bu3SnI.
1
Fig. 3. 31P{1H}-, 119Sn{1H}- and 1H-NMR (THF-d8) parameters
(chemical shift and JSn–P) of Pd(0)/IP and its reaction products with
2 or Bu3SnI.
Scheme 9.
These were characterized in a following way: (1) forma-
tion of a Pd–Sn bond was confirmed by a large shift in
119Sn-NMR, (2) coordination of the nitrogen was evi-
denced by a downfield shift (ca. 0.8 ppm) of the
methylene protons adjacent to the nitrogen atom, and
(3) the cis-configuration of the phosphino and stannyl
groups was assigned on the basis of small coupling
constants between phosphorous and tin ([11]a,b,d). Al-
though the stoichiometric reaction of Pd(0)–DPPP with
2 or tributyltin iodide also generated the corresponding
species 9 and 10 (Fig. 4), they were not detected under
the conditions of the cross-coupling reaction (Fig. 1).
All these observations suggest that the coupling reac-
tion of 2 with 1 using [PdCl(p-C3H5)]2–IP as a catalyst
should proceed through an oxidative addition of a
C–Sn bond of an organostannane to a Pd(0) complex.
Namely, Cycle B in Scheme 5 accounts for all of the
above observations.
2.4. Application to 6arious organostannanes and aryl
halides
High catalytic activity of the palladium complex co-
ordinated by IP was also demonstrated in the cross-
coupling of
a
variety of aryl halides with
organostannanes (Scheme 9 and Table 1). The reaction
of aryl iodides with 2 in the presence of 5 mol% of the
catalyst gave over 86% yields of the corresponding
ethynylation products, irrespective of an electron-with-
drawing or -donating substituent on the phenyl group.
Table 1
Coupling of organostannanes with aryl halides catalyzed by a Pd–IP complexa
R–SnBu3
Ar–X
Solvent
Temp (°C)
Time (h)
Yield (%) of R–Arb
2
2
2
2
2
2
2
C6H5–I
THF
THF
THF
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
50
50
50
80
90
90
80
90
90
24
18
29
32
72
48
48
43
140
93
86
91
92
90
84
93
90
84
4-EtOCO–C6H4–I
4-MeO–C6H4–I
2-CF3–C6H4–I
4-Ac–C6H4–Br
4-OHC–C6H4–Br
C6H5–OTfc
Tributyl(vinyl)tind
Tributyl(phenyl)tine
4-EtOCO–C6H4–I
4-EtOCO–C6H4–I
a The reaction was carried out in a solvent (5 ml) using an aryl halide (0.44 mmol), an organostannane (0.48 mmol), [PdCl(p-C3H5)]2 (0.011
mmol) and IP (0.022 mmol).
b Isolated yield based on aryl halide.
c The reaction was carried out in the presence of tetrabutylammonium bromide (1.3 mmol) using 2 (0.66 mmol).
d Tributyl(vinyl)tin (0.66 mmol) was used.
e Tributyl(phenyl)tin (0.87 mmol) was used.

174
E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
nylation products via carbopalladation or stannylpalla-
dation. This turned out to be the case.
3.2. Carbostannylation
Treatment of tributyl(phenylethynyl)tin (2) with a 1:2
mixture of [PdCl(p-C3H5)]2–IP (5 mol% of Pd) under
an acetylenic atmosphere (1 atm) in THF at 50°C for 2
h gave tributyl[(Z)-2-(phenylethynyl)ethenyl]tin [23] in
81% yield [24] as a single isomer through an exclusive
syn-addition (Scheme 10). The use of triphenylphos-
phine (two equivalents to palladium) in place of IP gave
only 48% yield in a prolonged period (43 h) [25]. A
Pd(0)–DPPP complex, which also was shown to be
added oxidatively by 2, was much less effective to give
the carbostannylation product in 28% yield even after
22 h.
Scheme 10.
Aryl bromides and triflate also underwent the cross-
coupling, although a longer reaction time was required.
Vinylation and phenylation of aryl iodide were also
successful by use of tributyl(vinyl)tin and tri-
butyl(phenyl)tin, respectively, in the presence of the
palladium–iminophosphine catalyst.
The carbostannylation of various alkynes catalyzed
by the Pd–IP catalyst was next examined (Scheme 10,
Table 2). The reaction of 2 with ethyl propiolate gave
carbostannylation products consisting of regioisomers
in a 20/80 ratio. The carbostannylation of ethyl 2-bu-
tynoate resulted in higher regioselectivity, though
higher temperature and longer reaction time were re-
quired. A ketonic acetylene, 1-butyn-3-one, reacted
with 2 smoothly with a regioselectivity similar to ethyl
propiolate. The reaction of arylacetylenes with 2 was
relatively slow to give the carbostannylation products
in high yields but with a reversed regioselectivity. The
reaction of ethoxyacetylene proceeded with high re-
gioselectivity. Tributyl(1-hexyn-1-yl)tin also reacted
with alkynes, giving the corresponding alkenylstan-
nanes with the regioselectivity similar to 2 [26].
3. Carbostannylation of alkynes
3.1. Introduction
Carbometalation of alkynes produces cis-substituted
alkenylmetals and is an extremely useful method for a
stereoselective olefin synthesis, since the resulting
alkenylmetals can be transformed further to variously
substituted ethylenes [15]. In particular, carbocupration
[16], zirconium-catalyzed carboalumination [17] and
nickel-catalyzed carbozincation [18] have a high syn-
thetic potential due to wide applicability. Although
alkenylstannanes are useful synthetic precursors for
various olefinic targets [19], no report has been pub-
lished on the transition metal-catalyzed carbostannyla-
tion of alkynes [20–22]. We envisaged that palladium
complex 7 should react with alkynes to give carbostan-
The catalytic cycle should first involve the oxidative
addition of an alkynylstannane to the Pd(0) complex as
discussed before. Successive insertion of an alkyne to
Table 2
Carbostannylation of alkynes with a Pd–IP catalysta
R1
Ph
R2
R3
Solvent
Temp (°C)
Time (h)
Yieldb (%)
A : Bc
H
Hd
H
Me
H
THF
THF
Dioxane
THF
THF
THF
THF
50
50
90
50
50
50
50
2
3
90
4
21
44
5
81
78
57
76
81
82
52
–
CO2Et
CO2Et
Ac
20 : 80
1 : \99
15 : 85
92 : 8
91 : 9
\99 : 1
Ph
H
4-CH3C6H4
EtO
H
He
Bu
H
CO2Et
Ph
Hd
H
H
THF
THF
THF
50
50
50
4
16
29
66
72
80
–
12 : 88
92 : 8
a The reaction was carried out in a solvent (5 ml) using an alkynylstannane (0.46 mmol) and an alkyne (1.38 mmol) in the presence of IP (0.022
mmol) and [PdCl(p-C3H5)]2 (0.011 mmol).
b Isolated yield based on alkynylstannane.
c Determined by 1H- or 119Sn-NMR.
d The reaction was carried out under an acetylenic atmosphere (1 atm).
e Ethoxyacetylene (0.46 mmol) was used.

E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
175
Scheme 11.
the C–Pd bond (carbopalladation) [27] of 7 followed by
reductive elimination is likely to afford the carbostan-
nylation product and regenerate the Pd(0) complex
(Scheme 11). The opposite regioselectivity observed in
use of arylacetylenes and alkynes having a carbonyl
group can be explained by the following facts. In the
cases of arylacetylenes, carbopalladation giving alkenyl-
palladiums would prefer A% to B% by a steric reason.
Accordingly, alkenylstannanes A are produced as main
products. In contrast, electron-deficient alkynes having
a carbonyl group is likely to suffer the Michael addition
of the alkynyl group giving B predominantly through
B¦.
with 1-bromo-2-phenylethyne and 4-nitroiodobenzene
[28] gave the respective coupled products in good
yields. Iodolysis or hydrolysis afforded the correspond-
ing alkenyl iodide or enyne in a good yield, respectively.
4. Homocoupling reaction of organostannanes
4.1. Introduction
The palladium-catalyzed homocoupling of organo-
stannanes is much less familiar than the cross-coupling
reaction. This may be ascribed to the fact that an
appropriate re-oxidant of an intervening Pd(0) com-
plex, making the homocoupling reaction catalytic, is
additionally needed. Actually, all the examples of the
palladium-catalyzed homocoupling of organostannanes
[29] useful for organic synthesis [30] employs such an
oxidant as t-BuOOH [31], benzoquinone [32], 1,2-
dichloroethane [33], 2,3-dibromoacrylate [34] or 1,2-di-
iodoethene [35] except for the homocoupling of
alkenylstannanes catalyzed by bis(acetonitrile)dichloro-
palladium(II) [36]. We have found that a palladium–
iminophosphine complex efficiently catalyzes the homo-
coupling of organostannanes in the presence of such a
mild and easily available oxidant as air [37] or allyl
acetate.
Finally, the utility of the carbostannylation products
is demonstrated by the transformation of the resulting
(Z)-alkenyltin compounds (Scheme 12). For example,
the Pd–tri(2-furyl)phosphine-catalyzed coupling [5]
4.2. Homocoupling reaction
Scheme 12. (a) PhCꢀCBr (0.92 equivalents), Pd2(dba)3, (2-furyl)3P (5
mol% of Pd, Pd/(2-furyl)3P=1/4), NMP, 50°C, 18 h, 58% (based on
PhCꢀCBr). (b) 4-O2NC6H4I (0.92 equivalents), Pd2(dba)3, (2-furyl)3P
(5 mol% of Pd, Pd/(2-furyl)3P=1/4, toluene, 90°C, 13 h, 85% (based
on 4-O2NC6H4I). (c) I2 (1.4 equivalents), THF, 0°C, 40 min, 92%. (d)
Conc. HCl, THF, r.t., 1 h, 81%.
The catalytic activity of a palladium complex coordi-
nated by a phosphine-based ligand was examined in the
homocoupling reaction of tributyl[4-(trifluoromethyl)-
phenyl]tin (11) (Scheme 13). Yields of 4,4%-bis(tri-

176
E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
Table 4
Homocoupling of organostannanes with a Pd–IP(Ph) catalysta
R in R–SnBu3
Temp (°C) Time (h)
Yield (%)b of R–R
Ph
70
50
50
70
50
4
5
36
102
44
10
66
84
76
81
80
75
60
4-O2N–C6H4
4-OHC–C6H4
4-MeO–C6H4
2-NC–C6H4
4-HOCH2–C6H4 70
3-MeOCO2
Scheme 13.
fluoromethyl)biphenyl (12) obtained after 4 h are sum-
marized in Table 3. A palladium complex coordinated
by N-(2-diphenylphosphinobenzylidene)aniline (IP(Ph))
showed higher reaction rate than any other palladium
complexes examined. Thus, the reaction of 11 in N,N-
dimethylformamide (DMF) in the presence of 5 mol%
of Pd–IP(Ph) at 40°C for 4 h in open air gave over 95%
yield of homocoupled product 12. A palladium complex
coordinated by IP was slightly less potent, giving 12 in
83%. Under the same reaction conditions, a palladium–
triphenylphosphine complex (ligand/Pd =2) gave 12 in
64% yield [38]. A palladium catalyst without any phos-
phine ligand was much less active, giving only 15%
yield of 12. The reaction under an argon atmosphere
strictly set up gave 12 in B5% yield, indicating that
oxygen is essential for the homocoupling reaction [30a].
Kinetic experiments in various solvents show that DMF
is the best solvent [39].
50
2.5
–C6H4
2-Thienyl
2-Pyridyl
(E)-PhCHꢁCHc
PhCꢀC
50
70
50
50
50
53
72
4
6
11
60
79
68
31
34
BuCꢀC
a The reaction was carried out in DMF (2 ml) in open air using an
organostannane (0.8 mmol) in the presence of [PdCl(p-C3H5)]2 (8.0
mmol) and IP(Ph) (0.016 mmol).
b Isolated yield based on organostannane.
c (E,E)-1,4-Diphenylbutadiene was obtained.
mol% of the catalyst to give the corresponding biaryls
in moderate to good yields, irrespective of the electron-
withdrawing or -donating substituent on the phenyl
ring. Heteroarylstannanes also gave homocoupled
products in moderate to good yields. (E)-Tributyl(2-
phenylethenyl)tin also underwent the oxidative dimer-
ization in the presence of the Pd–IP(Ph) catalyst to give
(E,E)-1,4-diphenyl-1,3-butadiene in 68% yield. Al-
though alkynylstannanes reacted very fast, the expected
products, 1,3-diynes, were isolated in yields less than
34%, probably due to the instability of the products
under the reaction conditions.
The Pd–IP(Ph) catalyst was applied to the homocou-
pling of a variety of aryl-, alkenyl- and alkynylstan-
nanes (Scheme 14 and Table 4). In particular,
arylstannanes reacted smoothly in the presence of 2
Table 3
Palladium-catalyzed homocoupling of 11a
During our investigation of the Pd–IP-catalyzed car-
bostannylation of alkynes, we found that the reaction
of an alkynylstannane with allyl propiolate gave the
homocoupling product of the alkynylstannane in addi-
tion to the carbostannylation product. Therefore, we
expected that allyl acetate should work effectively as an
oxidant in the homocoupling of alkynylstannanes. This
turned out to be the case.
The reaction of tributyl[4-(trifluoromethyl)phenyl-
ethynyl]tin with allyl acetate (1 equivalence) in the
presence of 5 mol% of Pd–IP(Ph) in DMF for 1 h at
40°C gave 1,4-bis[4-(trifluoromethyl)phenyl]-1,3-bu-
tadiyne in \95% yield (Scheme 15). The effect of
Ligand
Ligand/Pd
Solvent
Yield (%)b of 12
IP(Ph)
IP
1
1
2
–
1
1
1
1
DMF
DMF
DMF
DMF
DMF
THF
\95 (87)c
83
64
15
B5
30
Ph3Pd
None
IP(Ph)e
IP(Ph)
IP(Ph)
IP(Ph)
Toluene
CHCl3
20
16
a The reaction was carried out at 40°C for 4 h in a solvent (2 ml)
in open air using 11 (0.32 mmol) in the presence of [PdCl(p-C3H5)]2
(8.0 mmol) and a ligand (0.016 mmol).
b Determined by 19F-NMR.
c Isolated yield is given in the parenthesis.
d Triphenylphosphine (0.032 mmol) was used.
e Under an argon atmosphere set up through three freeze-thaw
cycles.
Scheme 14.
Scheme 15.

E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
177
ligand and solvent on the catalytic activity was similar
to that observed in the use of oxygen as an oxidant. It
is worthy to note that allyl acetate does not react as a
coupling partner of an alkynylstannane in spite of some
reports on this topic [40].
Various alkynylstannanes were applied to the palla-
dium-catalyzed homocoupling reaction using allyl ac-
etate as an oxidant. Arylethynylstannanes generally
gave the corresponding 1,3-diynes in good yields, irre-
spective of the kind of the substituent on the phenyl
ring. 1-Hexyn-1-ylstannane was also reacted smoothly
to give 5,7-dodecadiyne in a good yield.
washing the combined organic layer with water (10 ml)
and brine (10 ml), drying the organic layer over anhy-
drous magnesium sulfate, and evaporation, followed by
gel permeation chromatography, gave the correspond-
ing coupling product. The results are summarized in
Table 4.
5.3.2. With allyl acetate as an oxidant
An alkynylstannane (0.32 mmol) was added to a
solution of allyl acetate (0.32 mmol), IP(Ph) (6.0 mg,
0.016 mmol) and [PdCl(p-C3H5)]2 (3.0 mg, 8 mmol) in
DMF (2 ml), and the resulting mixture was stirred at
40°C for the time depicted in Scheme 15. Quenching
with water (10 ml), extraction with diethyl ether (30
ml), washing the combined organic layer with water (10
ml×3) and brine (10 ml), drying the organic layer over
anhydrous magnesium sulfate, and evaporation, fol-
lowed by gel permeation chromatography, gave the
corresponding coupling product. The results are sum-
marized in Scheme 15.
5. Typical procedures
5.1. Cross-coupling reaction
A solution (5 ml) of IP (8.6 mg, 0.022 mmol), an aryl
halide (0.44 mmol), and [PdCl(p-C3H5)]2 (4.0 mg, 0.011
mmol) was degassed by three freeze-thaw cycles. To
this solution was added an organostannane (0.48
mmol), and the mixture was stirred at the temperature
indicated in Table 1. After the time specified in Table 1,
a 1 M KF aqueous solution (2 ml) was added, and the
reaction mixture was stirred at room temperature for 30
min. Filtration through a Celite pad was followed by
extraction with ethyl acetate (50 ml). The organic layer
was washed successively with water and brine, and
dried over anhydrous magnesium sulfate. Evaporation
of the solvent followed by gel permeation chromatogra-
phy gave the corresponding coupling product. The re-
sults are summarized in Table 1.
6. Conclusions and prospect
The findings discussed herein will provide us with
clues to further investigation concerning the coupling
reaction of not only organostannanes but also other
organometallic reagents. The novel catalytic cycle
demonstrated here should work also in the cross-cou-
pling reaction of other organometallic reagents.
The newly disclosed nucleophilic Pd(II) complex, (L–
L%)Pd(R)(SnR%), will find wide applications leading to
3
novel methodology for C–C bond forming reactions, as
we disclosed the carbostannylation of alkynes.
Furthermore, the electrophilic Pd(0) complex dis-
cussed herein will also open its own future by reacting
with various nucleophilic reagents other than
organostannanes.
5.2. Carbostannylation of alkynes
A solution of IP (8.6 mg, 0.022 mmol), [PdCl(p-
C3H5)]2 (4.0 mg, 0.011 mmol), and an alkyne (1.38
mmol) in a solvent (5 ml) was degassed by four freeze–
thaw cycles. To this solution was added an organostan-
nane (0.46 mmol), and the mixture was stirred at the
temperature indicated in Table 2. After the time spe-
cified in Table 2, the solvent was evaporated. Gel
permeation chromatography of the residue gave the
corresponding carbostannylation product. The results
are summarized in Table 2.
Acknowledgements
The authors thank Professor Tamio Hayashi (Kyoto
University) for helpful discussions.
5.3. Homocoupling reaction
References
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E. Shirakawa, T. Hiyama / Journal of Organometallic Chemistry 576 (1999) 169–178
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.