Reaction of trans-Pd(styryl)Br(PMePh2)2 with styryl bromide affording 1,4-diphenyIbutadiene. An unexpected homocoupling process induced by P-C reductive elimination

Article abstract of DOI:10.1021/om701128e

Reaction of (Z)-styryl bromide (1) with PhB(OH)₂ in toluene at 80 °C for 1 h in the presence of Pd(PPh₃)4 (1.5 mol %) and an aqueous solution of K₂CO₃ (3 equiv) affords (Z)-stilbene as the cross-coupling product in 99% yield. On the other hand, the same reaction of the (E)-isomer of 1 forms considerable amounts of homocoupling products (1,4-diphenylbutadiene (2, 22%) and biphenyl (27%)) in addition to the cross-coupling product ((E)-stilbene, 73%). The formation of 2 was examined by kinetic experiments using trans-Pd(CH=CHPh)Br(PMePh₂)₂ (3) as a model of the presumed intermediate. Complex 3 reacts with 1 at 50 °C for 5 h, giving 2 (91%) and irans-PdBr₂(PMePh₂) ₂ (4, 92%), together with a small amount of Pd(η²- PhCH=CHPMePh₂)Br(PMePh₂) (5, 8%). The reaction rate shows firstorder dependence on the concentration of 3 and 5, respectively, but is independent of the concentration of 1. A novel homocoupling process induced by P-C reductive elimination from 3 giving 5 is proposed.

Full text of DOI:10.1021/om701128e

602
Organometallics 2008, 27, 602–608
Reaction of trans-Pd(styryl)Br(PMePh2)2 with Styryl Bromide
Affording 1,4-Diphenylbutadiene. An Unexpected Homocoupling
Process Induced by P-C Reductive Elimination
Masayuki Wakioka, Masato Nagao, and Fumiyuki Ozawa*
International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto
UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed NoVember 9, 2007
Reaction of (Z)-styryl bromide (1) with PhB(OH)2 in toluene at 80 °C for 1 h in the presence of
Pd(PPh3)4 (1.5 mol %) and an aqueous solution of K2CO3 (3 equiv) affords (Z)-stilbene as the cross-
coupling product in 99% yield. On the other hand, the same reaction of the (E)-isomer of 1 forms
considerable amounts of homocoupling products (1,4-diphenylbutadiene (2, 22%) and biphenyl (27%))
in addition to the cross-coupling product ((E)-stilbene, 73%). The formation of 2 was examined by kinetic
experiments using trans-Pd(CHdCHPh)Br(PMePh2)2 (3) as a model of the presumed intermediate.
Complex 3 reacts with 1 at 50 °C for 5 h, giving 2 (91%) and trans-PdBr2(PMePh2)2 (4, 92%), together
with a small amount of Pd(η2-PhCHdCHPMePh2)Br(PMePh2) (5, 8%). The reaction rate shows first-
order dependence on the concentration of 3 and 5, respectively, but is independent of the concentration
of 1. A novel homocoupling process induced by P-C reductive elimination from 3 giving 5 is proposed.
Scheme 1
Introduction
Dehalogenative homocoupling of aryl and alkenyl halides
promoted by nickel1 and palladium2 complexes is a useful means
of C-C bond formation,3 especially for the synthesis of
π-conjugated polymers.4 This reaction also takes place quite
often as a side reaction of the catalytic cross-coupling of organic
halides with organometallic reagents.5,6
Scheme 1 illustrates a schematic view of homocoupling
processes previously reported. The first step is oxidative addition
of organic halides (RX) to low-valent transition metal species
(MLn) (step a). The resulting MR(X)Ln may undergo two
reaction processes (A and B) affording homocoupling products
(RR). Process A involves metathesis of MR(X)Ln to give MR2Ln
and MX2Ln (step b).7 The former forms RR on reductive
elimination (step c), whereas the latter is reduced to MLn when
the reaction is conducted in catalytic systems containing
reducing agents such as zinc and organometallic reagents (step
d). This type of process was first documented by Tsou and Kochi
for a nickel system using aryl bromides (ArBr) as substrates,
where intermolecular exchange of aryl and bromo ligands
between diamagnetic NiIIBr(Ar)(PEt3)n and paramagnetic
NiIIIBr2(Ar)(PEt3)n was postulated to afford NiIII(Br)Ar2(PEt3)n
and NiIIBr2(PEt3)n.8 More recently, Osakada and Yamamoto
examined metathesis of diamagnetic MAr(X)(bipy) complexes
(M ) Ni, Pd) in detail and demonstrated high reactivity of
cationic [MAr(bipy)]+ species.9
* To whom correspondence should be addressed. E-mail: ozawa@
scl.kyoto-u.ac.jp.
(1) (a) Semmelhack, M. F.; Helquist, P. M.; Jones, L. D. J. Am. Chem.
Soc. 1971, 93, 5908–5910. (b) Semmelhack, M. F.; Helquist, P. M.;
Gorzynski, J. D. J. Am. Chem. Soc. 1972, 94, 9234–9236. (c) Kende, A. S.;
Liebeskind, L.; Braitsch, D. M. Tetrahedron Lett. 1975, 3375–3378. (d)
Zembayashi, M.; Tamao, K.; Yoshida, J.; Kumada, M. Tetrahedron Lett.
1977, 4089–4092. (e) Takagi, K.; Hayama, N.; Inokawa, S. Bull. Chem.
Soc. Jpn. 1980, 53, 3691–3695. (f) Colon, I.; Kelsey, R. D. J. Org. Chem.
1986, 51, 2627–2637. (g) Iyoda, M.; Otsuka, H.; Sato, K.; Nisato, N.; Oda,
M. Bull. Chem. Soc. Jpn. 1990, 63, 80–87.
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On the other hand, process B involves oxidative addition of RX
to paramagnetic MIRLn or anionic [MRLn]- species generated by
(6) For diene formation as a side reaction of cross-coupling, see: (a)
Negishi, E.; Takahashi, T.; Akiyoshi, K. J. Organomet. Chem. 1987, 334,
181–194. (b) Stille, J. K.; Groh, B. L. J. Am. Chem. Soc. 1987, 109, 813–
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10.1021/om701128e CCC: $40.75
2008 American Chemical Society
Publication on Web 02/01/2008

Homocoupling Process of Styryl Bromide
Organometallics, Vol. 27, No. 4, 2008 603
chemical or electrochemical reduction of MR(X)Ln (step e f step
f).10 The reaction forms MIIIR2(X)Ln or MR2Ln, which reductively
eliminates RR (step g). This type of process was originally proposed
for nickel-catalyzed reactions by Colon and Kelsey10 and expanded
to palladium-catalyzed systems by Amatore and Jutand.11
(E)/(Z) isomerization of styryl group takes place uniquely in
the homocoupling process.
This paper deals with the homocoupling process of styryl
bromide promoted by palladium(0) phosphine complexes. In
the course of our study on stereocontrolled synthesis of
poly(phenylene vinylene) (PPV) using the Suzuki-Miyaura
cross-coupling,12 we found that the (Z,Z)-isomer of bis(2-
bromoethenyl)benzene cleanly forms PPV, whereas the corre-
sponding (E,E)-isomer provides a polymer containing a notable
amount of butadiene unit in the main chain resulting from
homocoupling.13 In this context, we attempt in this study to
clarify the following points using a model reaction of isolated
trans-Pd(CHdCHPh)Br(PMePh2)2 (3)14 with styryl bromide to
give 1,4-diphenylbutadiene: (i) the effect of (E)/(Z) configuration
of the styryl group; (ii) the mechanism of the homocoupling
process. It has been found that the butadiene formation proceeds
via a novel homocoupling process induced by P-C reductive
(1)
(2)
elimination from
3
to give Pd(η2-PhCHdCHPMePh2)-
Next, we examined the homocoupling reaction giving 2 in
stoichiometric systems using trans-Pd(CHdCHPh)Br(PMePh2)2
(3) bearing (Z)- and (E)-styryl groups. While the PMePh2
complexes were employed instead of PPh3 analogues for
solubility reason, their reactions with styryl bromides faithfully
reproduced the characteristic points observed in the catalytic
systems. Thus, (Z)-3 was almost unreactive to (Z)-styryl bromide
((Z)-1).16 On the other hand, (E)-3 smoothly reacted with (E)-
styryl bromide ((E)-1, 20 equiv) in C6D6/CD2Cl2 (5:1) at 50 °C
for 5 h to afford a geometrical mixture of 1,4-diphenylbutadiene
(2, (E,E)/(E,Z) ) 48/52, totally 91%), together with trans-
PdBr2(PMePh2)2 (4, 92%) (eq 3). Accordingly, (i) much higher
reactivity of (E)-styryl isomer than (Z)-styryl isomer toward
homocoupling and (ii) the occurrence of (E)/(Z) isomerization
of styryl group during the homocoupling process were evidenced
in the stoichiometric systems as well. As described below, the
remaining part of (E)-3 (8%) is converted to styrylphosphonium
complex 5, which is formed by P-C reductive elimination of
styryl and PMePh2 ligands.15,17
Br(PMePh2) (5).15
Results
Effect of (E)/(Z) Configuration of the Styryl Group. Cross-
coupling reactions of (Z)- and (E)-styryl bromides (1) with
PhB(OH)2 were examined under the catalytic conditions for PPV
synthesis. Thus a 1:1 mixture of 1 and PhB(OH)2 was heated
at 80 °C for 1 h in toluene in the presence of Pd(PPh3)4 (1.5
mol %) and an aqueous solution of K2CO3 (3 equiv). In
agreement with the polymerization results, (Z)-1 selectively
underwent cross-coupling, giving (Z)-stilbene in quantitative
yield (eq 1). On the other hand, the reaction of (E)-1 afforded
considerable amounts of homocoupling products (i.e., 1,4-
diphenylbutadiene (2, 22%) and biphenyl (27%)), along with
(E)-stilbene as the cross-coupling product (73%) (eq 2). It should
be noted that butadiene 2 contains a significant amount of (E,Z)-
isomer in addition to the (E,E)-isomer that is expected from
the geometry of starting (E)-1. Since the cross-coupling product
retains totally the original (E) configuration, it is likely that the
(9) (a) Yamamoto, T.; Wakabayashi, S.; Osakada, K. J. Organomet.
Chem. 1992, 428, 223–237. (b) Yagyu, T.; Hamada, M.; Osakada, K.;
Yamamoto, T. Organometallics 2001, 20, 1087–1101. (c) Suzaki, Y.;
Osakada, K. Organometallics 2003, 22, 2193–2195. (d) Suzaki, Y.; Osakada,
K. Bull. Chem. Soc. Jpn. 2004, 77, 139–145. (e) Suzaki, Y.; Yagyu, T.;
Osakada, K. J. Organomet. Chem. 2007, 692, 326–342.
(10) Colon, I.; Kelsey, D. R. J. Org. Chem. 1986, 51, 2627–2637.
(11) (a) Amatore, C.; Jutand, A. Organometallics 1988, 7, 2203–2214.
(b) Jutand, A.; Mosleh, A. J. Org. Chem. 1997, 62, 261–274. (c) Amatore,
C.; Jutand, A. J. Orgenomet. Chem. 1999, 576, 254–278. (d) Jutand, A.
Eur. J. Inorg. Chem. 2003, 2017–2040.
(3)
(12) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483. (b)
Ref 5a, Chapter 2.
Kinetic Examination of the Homocoupling Mechanism.
Figure 1 shows the time-course of eq 3. Palladium dibromide 4
forms at the expense of (E)-3. At the same time, complex 5
increases, while this complex is converted at the final stage to
(13) (a) Katayama, H.; Nagao, M.; Nishimura, T.; Matsui, Y.; Umeda,
K.; Akamatsu, K.; Tsuruoka, T.; Nawafune, H.; Ozawa, F. J. Am. Chem.
Soc. 2005, 127, 4350–4353. (b) Katayama, H.; Nagao, M.; Nishimura, T.;
Matsui, Y.; Fukuse, Y.; Wakioka, M.; Ozawa, F. Macromolecules 2006,
39, 2039–2048.
(14) (a) Loar, M. K.; Stille, J. K. J. Am. Chem. Soc. 1981, 103, 4174–
4181. (b) Son, T.; Yanagihara, H.; Ozawa, F.; Yamamoto, A. Bull. Chem.
Soc. Jpn. 1988, 61, 1251–1258.
(16) (a) Treatment of (Z)-3 with (Z)-1 (20 equiv) in C6D6/CD2Cl2 (5:1)
at 50 °C for 5 h gave a small amount of trans-PdBr2(PMePh2)2 (4, ca. 2%),
while most of (Z)-3 remained unreacted. GLC analysis revealed no formation
of 1,4-diphenylbutadiene (2). (b) Complex (Z)-3 was stable at 50 °C for
5 h in C6D6 in the presence of (E)-1 (20 equiv).
(17) (a) Heating (E)-3 in C6D6 at 50 °C for 6 h resulted in the formation
of 5 in 52% yield as confirmed by NMR spectroscopy. Also formed in the
solution are butadiene 2 ((E,E)/(E,Z) ) 48/52, 23%) and dibromopalladium
4 (8%). (b) Unlike (E)-3, (Z)-3 was stable in C6D6 at 50 °C for 6 h.
(15) P-C reductive elimination giving alkenylphosphonium complexes
has been reported: (a) Rybin, L. V.; Petrovskaya, E. A.; Rubinskaya, M. I.;
Kuz’mina, L. G.; Struchkov, Y. T.; Kaverin, V. V.; Koneva, N. Y. J.
Organomet. Chem. 1985, 288, 119–129. (b) Duan, J.-P.; Liao, F.-L.; Wang,
S.-L.; Cheng, C.-H. Organometallics 1997, 16, 3934–3940. (c) Huang, C.-
C.; Duan, J.-P.; Wu, M.-Y.; Liao, F.-L.; Wang, S.-L.; Cheng, C.-H.
Organometallics 1998, 17, 676–682.

604 Organometallics, Vol. 27, No. 4, 2008
Wakioka et al.
Table 1. Kinetic Data for Formation of 4 in the Reactions of (E)-3
and (E)-1a
run
[5]0 (mM)
104k1 (s-1 b
)
102k2 (s-1 M-1 c
)
0d
1
2
0
5
10
15
2.5(1)e
2.37(2)
2.36(2)
2.54(4)
1.80(2)
2.72(2)
3.86(6)
3
a Reaction conditions: [(E)-3]0 ) 50 mM, [(E)-1]0 ) 1.0 M, in C6D6/
CD2Cl2 (5:1), at 50 °C. b Estimated by regression analysis of the kinetic
data (up to 63% conversion of (E)-3) using the following equation:
ln{[(E)-3]0/([(E)-3]0 - [4]t)} ) k1t. c Estimated by regression analysis of
the kinetic data (up to 83% conversion of (E)-3) using the following
equation: ln{[(E)-3]0/([(E)-3]0 - [4]t)} ) k2([5]t × t). d Data for Figure
1. e Data for less than 20% conversion of (E)-3 were omitted from the
calculation because the concentration of 5 was too low to be accurately
analyzed (<3%).
Figure 1. Time course of the reaction of (E)-3 (50 mM) with (E)-1
(1.0 M) in C6D6/CD2Cl2 (5:1 v/v) at 50 °C. Palladium complexes
1
3–6 and butadiene (2) were followed by H NMR spectroscopy.
Figure 3. Plot of ln{[(E)-3]0/([(E)-3]0 - [4]t)} vs ([5]t × t) for the
reactions of Figure 2.
the amount of added 5 increases.19 It should be noted that the
time-yield curves for 4 have a simple form (not S-shaped).
Actually, the reactions obeyed first-order kinetics up to 63%
conversion of (E)-3.
Figure 2. Time course of the reaction of (E)-3 (50 mM) with (E)-1
(1.0 M) in C6D6/CD2Cl2 (5:1 v/v) at 50 °C in the presence of
isolated 5.
Table 1 lists the first-order rate constants (k1) thus obtained.
The k2 values in the fourth column are second-order rate
constants estimated by applying kinetic data to the following
equation: ln{[(E)-3]0/([(E)-3]0 - [4]t)} ) k2([5]t × t). Thus, since
the gradual formation of 5 from (E)-3 could not be ignored even
in the presence of added 5, the formation rate of 4 was
compensated for the concentration of 5 at time t. This treatment
resulted in good linear correlations for all runs up to 83%
conversion of (E)-3, giving almost identical k2 values (2.4(1)
× 10-2 s-1 M-1) (Figure 3). Hence the first-order dependence
of the reaction rate on the concentration of (E)-3 and 5,
respectively, was confirmed: d[4]/dt ) k2[(E)-3][5].
Table 2 compares half-lives of (E)-3 under various conditions.
The reaction is highly sensitive to solvent polarity and clearly
faster in a polar solvent (runs 1–3; runs 4 and 5). On the other
hand, the reaction rate is less sensitive to the concentration of
(E)-styryl bromide ((E)-1) (runs 1 and 4; runs 2 and 5). Although
small variations of half-lives are observed depending on the
concentration of (E)-1, they are attributable to the change in
polarity of reaction media. Thus, in C6D6 as a nonpolar solvent,
the other palladium species 618 with liberation of [PhCHd
CHPMePh2]+Br- (7). It was confirmed that the amount of
butadiene 2 is consistent with that of 4 through the reaction.
Clearly, the reaction curves for the conversion of (E)-3 and the
formation of 4 are S-shaped, showing the occurrence of an
autocatalytic process. Since the reaction is apparently accelerated
as the amount of 5 increases, we next investigated the reaction
of (E)-3 with (E)-1 in the presence of added 5. Complex 5 was
prepared separately according to the procedure reported for PPh3
analogues.15c
Figure 2 shows the time course in the presence of added 5,
where the amounts of 4 and 5 are based on the amount of (E)-3
initially employed ([(E)-3]0 ) 50 mM). Both the yield and the
formation rate of 4 are enhanced by addition of 5 to the system
([5]0 ) 5–15 mM, runs 1–3). Furthermore, the amount of 5
generated from (E)-3 during the reaction clearly decreases as
(18) Although 6 could not be isolated, it was assigned as [Pd(CHd
CHPh)(µ-Br)(PMePh2)]2 based on the similarity of NMR data [1H NMR:
(19) (a) The yield of 4 in each run of Figure 2(%): 94 (run 1), 96 (run
2), 98 (run 3). (b) The maximum amount of 5 derived from (E)-3 (%): 6
(run 1), 4 (run 2), 2 (run 3). Complex 5 was converted to 6 and 7 at the
final stage as described for Figure 1.
2
δ 2.14 (d, JHP ) 11.4 Hz, PCH3); 31P{1H} NMR: δ 15.2 (s)] to related
[PdR(µ-Cl)(PMePh2)]2 (R ) Ph, CH2Ph). Anderson, G. K. Organometallics
1983, 2, 665–668.

Homocoupling Process of Styryl Bromide
Organometallics, Vol. 27, No. 4, 2008 605
Table 2. Half-Lives of (E)-3 in Reactions with (E)-1a
run [(E)-1]0 (M)
solvent
additive
t1/2 (min)
1
2
3
4
5
6
7
1.0
1.0
1.0
0.50
0.50
1.0
C6D6
200
100
48
240
76
C6D6/CD2Cl2 (5/1)
C6D6/CD2Cl2 (1/1)
C6D6
C6D6/CD2Cl2 (5/1)
(5)
C6D6/CD2Cl2 (5/1) PMePh2 (5 mM)
280
130
1.0
C6D6/CD2Cl2 (5/1) 7 (1.5 mM)b
Discussion
a Reaction conditions: [(E)-3]0
)
50 mM, at 50 °C. b 7:
[PhCHdCHPMePh2]+Br-.
The following points have emerged from the experimental
results. (1) (Z)-styryl bromide ((Z)-1) is much less reactive than
the corresponding (E)-isomer ((E)-1) toward dehalogenative
homocoupling, giving 1,4-diphenylbutadiene (2), because the
(Z)-styrylpalladium intermediate ((Z)-3) is sufficiently stable
toward P-C reductive elimination. (2) In contrast, (E)-styrylpal-
ladium ((E)-3) readily undergoes P-C reductive elimination to
give styrylphosphonium complex (5), which induces the reaction
of (E)-3 with (E)-1 to afford 2 and palladium dibromide 4. (3)
The homocoupling reaction involves (E)/(Z) isomerization of
the styryl group to give a mixture of (E,E)- and (E,Z)-isomers
of 2. (4) The rate of formation of 2 and 4 shows first-order
dependence on the concentration of (E)-3 and 5, respectively,
but is independent of the concentration of (E)-1. (5) A polar
solvent accelerates the reaction, whereas free PMePh2 and
[PhCHdCHPMePh2]+Br- (7) retard the reaction. (6) The styryl
group of styrylphosphonium complex 5 is not incorporated into
the homocoupling product (2). (7) Palladium dibromide (4 or
4a) readily reacts with PhB(OH)2 in the presence of aqueous
K2CO3 to afford biphenyl and a Pd(0) species.
the reaction is faster at higher concentration of (E)-1, because
(E)-1 possesses higher polarity than benzene (run 1 > run 4).
To the contrary, the higher concentration of (E)-1 makes the
reaction slower when the reaction solution contains CD2Cl2 as
a highly polar solvent (run 2 < run 5). Accordingly, it is
concluded that the rate of homocoupling (i.e., the formation of
butadiene 2) is essentially independent of the concentration of
(E)-styryl bromide ((E)-1). As seen from runs 6 and 7, the reaction
is retarded by free PMePh2 and [PhCHdCHPMePh2]+Br- (7),
respectively, while the effect of 7 is small.
Labeling Experiment. The kinetic experiments revealed a
crucial role of styrylphosphonium complex 5 in the homocou-
pling process. In this connection, the reaction of (E)-3 with (E)-1
was examined in the presence of Pd(η2-m-MeC6H4CHd
CHPMePh2)Br(PMePh2) (5a, 0.2 equiv, 10 mM) in place of 5.
As shown in eq 4, the reaction gave 1,4-diphenylbutadiene 2
and palladium dibromide 4 in almost quantitative yields. Thus,
the m-MeC6H4CHdCH group of 5a was not incorporated into
the product.
Taking these observations into account, we propose the
mechanism in Scheme 2 for the conversion of (E)-1 and (E)-3
into 2 and 4, which consists of initiation (steps a, b) and
production (steps c, d) processes. First, styryl complex (E)-3
undergoes P-C reductive elimination to give styrylphosphonium
complex 5 (step a). Unlike the arylpalladium analogues
(PdAr1(X)(PAr2 ) ; Ar1, Ar2 ) Ph, p-tolyl, etc.),20 the P-C
3 2
coupling proceeds irreversibly, as confirmed by the labeling
experiment in eq 4. Complex 5 then reacts with styryl bromide
((E)-1) to afford a three-coordinated complex A with liberation
of [PhCHdCHPMePh2]+Br- (7) (step b).21
(4)
Complex A serves as a key intermediate in the production
process. Intermolecular exchange of the styryl ligand in A and
the bromo ligand in (E)-3 forms bis(styryl) complex B and
dibromo complex 4 (step c). Reductive elimination of 2 from
B, followed by oxidative addition of (E)-1 to a resulting Pd(0)
species, regenerates A (step d). Accordingly, A catalyzes the
conversion of (E)-1 and (E)-3 into 2 and 4. In this case, since
the reaction rate is independent of the concentration of (E)-1,
step d must be much faster than step c. This situation is very
convincing because the C-C reductive elimination from related
cis-Pd(CHdCHPh)(Me)(PMePh2)2 and the oxidative addition
of (E)-styryl bromide to a Pd(0) phosphine complex proceed
readily even at room temperature.14,22
Relation between Stoichiometric and Catalytic
Reactions. The reaction of styryl complex (E)-3 with (E)-styryl
bromide ((E)-1) afforded butadiene 2 and palladium dibromide
4 in over 90% yields (eq 3). When this reaction proceeds
catalytically, 4 should be reduced to a Pd(0) species, which
subsequently undergoes oxidative addition of (E)-1 to reproduce
(E)-3. Since the catalytic reaction in eq 2 formed a comparable
amount of biphenyl along with 2, it is likely that PhB(OH)2
serves as a reducing agent in conjunction with a base. In fact,
complex 4 smoothly reacted with PhB(OH)2 (3 equiv) in toluene
in the presence of an aqueous solution of K2CO3 (9 equiv) (eq
5). The reaction was completed in 30 min at 50 °C to give
biphenyl in 90% yield. Similarly, the PPh3 complex 4a formed
a quantitative yield of biphenyl in 10 min. The observed
reactivity of 4a was high enough to be operative as an
elementary process in the catalytic conversion of (E)-1.
The reaction rate showed first-order dependence on the
concentration of (E)-3 and 5, respectively. Apart from (E)-3 as
the substrate of step c, the kinetic relation of 5 to the production
(21) T-shaped, three-coordinated complexes have been isolated for
arylpalladium halides: (a) Stambuli, J. P.; Buhl, M.; Hartwig, J. F. J. Am.
Chem. Soc. 2002, 124, 9346–9347. (b) Stambuli, J. P.; Incarvito, C. D.;
Buhl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 1184–1194.
(22) (a) Brown, J. M.; Cooley, N. A. Organometallics 1990, 9, 353–
359. (b) Calhorda, M. J.; Brown, J. M.; Cooley, N. A. Organometallics
1991, 10, 1431–1438. (c) Ozawa, F. In Current Methods in Inorganic
Chemistry; Yamamoto, A., Kurosawa, H., Eds.; Elsevier: Amsterdam, 2003;
Vol. 3,Chapter 9.
(20) Aryl complexes undergo reversible P-C reductive elimination
leading to intramolecular exchange of the PdAr1 and PAr2 groups: (a) Kong,
K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113, 6313–6315. (b) Goodson,
F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997, 119, 12441–
12453. (c) Grushin, V. V. Organometallics 2000, 19, 1888–1900. (d)
Grushin, V. V. Chem.-Eur. J. 2002, 8, 1006–1014. (e) Grushin, V. V.
Organometallics 2007, 26, 4997–5002.

606 Organometallics, Vol. 27, No. 4, 2008
Wakioka et al.
Scheme 2
Scheme 3
process deserves discussion. It is assumed that 5 is converted
to A and 7 by reaction with (E)-1. In fact, 7 is generated in the
system at the final reaction stage, along with 6 as a dimer of A
(Figure 1). However, since 6 and 7 are not observed in the early
to middle stage of the reaction, it must be considered that 5
and A are rapidly interconverted, and the equilibrium lies far
on the side of 5.23 In this situation, complex 5 remains
apparently unchanged in the reaction system, because A is
recycled as catalyst in the production process and therefore its
concentration is maintained. On the other hand, when (E)-
3is almost completely consumed at the final stage, A is
dimerized to a stable form, to be converted entirely to 6 and 7.
The 1,4-diphenylbutadiene (2) was obtained as a mixture of
(E,E)- and (E,Z)-isomers. Since (Z)-styryl complex ((Z)-3) was
not observed in the system during the reaction, the (E)/(Z)
isomerization of the styryl group should be operative in step c
in Scheme 2, causing intermolecular exchange of the styryl
ligand in (E)-3 and the bromo ligand in A. Scheme 3 illustrates
a plausible mechanism responsible for this phenomenon. The
first step is very probably η2-coordination of the styryl ligand
of (E)-3 to the vacant coordination site of A. The styryl ligand
then changes its coordination to an η1-fashion, in conjunction
with the transfer of bromo ligand in least motion. This change
in coordination mode of the styryl ligand provides intermediate
D involving a contribution of the canonical structure given in
Scheme 3, in which free rotation of the µ-styryl ligand is
possible. Accordingly, the two geometrical isomers of 2 are
formed.
reductive elimination of the styryl and PMePh2 ligands from
(E)-3, followed by oxidative addition of (E)-1 to resulting
styrylphosphonium complex 5. We have also shown that (E)/
(Z) isomerization of the styryl group, which takes place during
the formation of 1,4-diphenylbutadiene (2), is reasonably
accounted for by a mechanism involving intermolecular ex-
change of the styryl ligand between A and (E)-3.
Dehalogenative homocoupling catalyzed by palladium com-
plexes has been examined mainly for the reactions with aryl
halides as substrates.2 Process B in Scheme 1 is currently most
accepted for such reactions, where oxidative addition of aryl
halides to anionic [MRLn]- intermediates formed by two-
electron reduction of MR(X)Ln is considered.11 In contrast, we
found a considerably different type of process for the reaction
of (E)-styryl bromide ((E)-1) to give 1,4-diphenylbutadiene (2).
The C-C coupling process proposed in Scheme 2 resembles
our previous observations for the reactions of trans-PdAr2L2
with RX (R ) Me, aryl; L ) PEt2Ph), where trans-PdR(X)L2
formed by oxidative addition of RX to a palladium(0) species
catalyzes the conversion of trans-PdAr2L2 and RX into ArR.24
Experimental Section
General Considerations. All manipulations were carried out
under a nitrogen or argon atmosphere using standard Schlenk
techniques. Nitrogen and argon gas were dried by passing through
P2O5 (Merck, SICAPENT). NMR spectra were recorded on a Bruker
Avance 400 spectrometer (1H NMR 400.13 MHz, 13C NMR 100.62
MHz, and 31P NMR 161.97 MHz). Chemical shifts are reported in
Conclusion
We have demonstrated a novel homocoupling process of (E)-
styryl bromide promoted by palladium(0) phosphine complexes.
Thus the reaction of (E)-styryl complex ((E)-3) with (E)-styryl
bromide ((E)-1) is promoted by a catalytic amount of coordi-
natively unsaturated complex A, which is generated by P-C
1
δ (ppm), referenced to the H (residual protons), and 13C signals
of deuterated solvents or to the 31P signal of an external 85% H3PO4
standard. Mass spectra were measured on a Shimadzu GC-MS
QP2010 spectrometer (EI, 70 eV). GLC analysis was performed
on a Shimadzu GC-14B instrument equipped with a FID detector
and a CBP-1 capillary column (25 m × 0.25 mm). Melting points
were measured with a Yanaco MP-S3 instrument. Elemental
analysis was performed by the ICR Analytical Laboratory, Kyoto
University.
(23) A polar solvent increases the equilibrium concentration of A and
7, causing enhancement in the reaction rate. On the other hand, addition of
7 to the system decreases the concentration of A and reduces the reaction
rate. The retardation effect of free PMePh2 may be attributed to several
reasons. For example, intermediate A is very probably captured as (E)-3.
Furthermore, since complex
5
is converted to [Pd(η2-PhCHd
CHPMePh2)(PMePh2)2]+Br- by the coordination of PMePh2, the formation
of A is possibly hindered by increasing coordination stability of the
styrylphosphonium ligand.
(24) (a) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto, A.
Organometallics 1986, 5, 2144–2149. (b) Ozawa, F.; Hidaka, T.; Yamamoto,
T.; Yamamoto, A. J. Organomet. Chem. 1987, 330, 253–263.

Homocoupling Process of Styryl Bromide
Organometallics, Vol. 27, No. 4, 2008 607
Toluene was dried over sodium benzophenone ketyl, distilled,
and stored over activated molecular sieves (MS4A). C6D6 and
CD2Cl2 were dried over lithium aluminum hydride (for C6D6) or
calcium hydride (for CD2Cl2), transferred under vacuum, and stored
over activated molecular sieves (MS4A). Et2O, THF, and CH2Cl2
(Wako, dehydrated) were used as received. (E)-Styryl bromide
((E)-1) was purified in an alkaline condition to remove con-
tamination of the (Z)-isomer.25The following compounds were
synthesized according to the literature: (Z)-styryl bromide ((Z)-1),26
Pd(PPh3)4,27(Z)-trans-Pd(CHdCHPh)Br(PMePh2)2 ((Z)-3),14a trans-
PdBr2(PMePh2)2 (4),28 trans-PdBr2(PPh3)2 (4a).29
(CDCl3): δ 19.6 (s). Anal. Calcd for C22H22BrP: C, 66.51; H, 5.58.
Found: C, 66.62; H, 5.67.
Synthesis of Pd(η2-PhCHdCHPMePh2)Br(PMePh2) (5). To
a solution of PMePh2 (128 mg, 0.639 mmol) in CH2Cl2 (6.4 mL)
were added Pd(dba)232 (368 mg, 0.639 mmol) and 7 (245 mg, 0.639
mmol). The solution was stirred at room temperature for 3 h and
filtered through a Celite pad. The solvent was removed under
reduced pressure. The residue was dissolved in CH2Cl2 (3 mL) and
diluted with Et2O (10 mL), and a green solid formed in the solution
was removed by filtration. This operation was repeated three times
to give a crude product (255 mg, 58% yield). Recrystallization from
CH2Cl2/Et2O at -30 °C gave yellow crystals of the title compound
Synthesis of (E)-trans-Pd(CHdCHPh)Br(PMePh2)2 ((E)-3).
PMePh2 (411 mg, 2.05 mmol) was added to a solution of
(η5-C5H5)(η3-C3H5)Pd30 (213 mg, 1.00 mmol) in toluene (1.0 mL)
at 0 °C. The mixture was stirred for 10 min, and (E)-1 (732 mg,
4.00 mmol) was added. The homogeneous solution was stirred at
room temperature, causing gradual precipitation of a white solid.
After 3 h, hexane (5 mL) was added, and the mixture was cooled
to -20 °C. The white precipitate formed in the system was collected
by filtration, washed successively with hexane (2 × 2 mL) and
Et2O (2 × 2 mL), and dried under vacuum. The crude product was
purified by recrystallization from CH2Cl2/Et2O at room temperature,
washed with Et2O, and dried under vacuum at 0 °C to afford pale
brown crystals of the title compound (603 mg, 87% yield). The
NMR data were identical with those reported.14b 1H NMR (CD2Cl2):
δ 7.65–7.58 and 7.45–7.35 (m, 20H in total), 7.05 (m, 2H), 6.93
1
(224 mg, 51% yield). Mp: 126–128 °C (dec). H NMR (CD2Cl2):
δ 8.04–7.97, 7.75–7.42, 7.24–6.86, and 6.81–6.78 (m, 25H in total),
2
3
3
3.34 (ddd, JHP ) 19.8 Hz, JHH ) 10.7 Hz, JHP ) 4.1 Hz, 1H),
3
3
3
3.12 (ddd, JHP ) 14.0 Hz, JHH ) 10.7 Hz, JHP ) 8.2 Hz, 1H),
2.83 (d, 2JHP ) 13.7 Hz, 3H), 1.50 (d, 2JHP ) 6.2 Hz, 3H). 13C{1H}
3
1
NMR (CD2Cl2): δ 144.8 (d, JPC ) 11.4 Hz), 139.3 (d, JPC
)
1
3
28.1 Hz), 137.0 (d, JPC ) 29.4 Hz), 134.9 (d, JPC ) 10.1 Hz),
2
3
133.5 (s), 134.8 (s), 132.8 (d, JPC ) 14.4 Hz), 132.6 (d, JPC
)
2
2
9.9 Hz), 132.5 (d, JPC ) 14.0 Hz), 129.9 (d, JPC ) 11.3 Hz),
2
4
129.2 (s), 129.1 (d, JPC ) 12.4 Hz), 128.9 (d, JPC ) 1.2 Hz),
128.7 (s), 128.4 (d, 3JPC ) 9.0 Hz), 128.2 (d, 3JPC ) 9.1 Hz), 127.4
(dd, 1JPC ) 72.7 Hz, 4JPC ) 9.5 Hz), 125.7 (s), 124.9 (s), 124.9 (d,
1JPC ) 93.3 Hz), 59.5 (d, JPC ) 5.0 Hz), 28.3 (dd, JPC ) 81.4
2
1
2
1
1
Hz, JPC ) 35.9 Hz), 14.6 (d, JPC ) 18.8 Hz), 13.7 (d, JPC
)
66.5 Hz). 31P{1H} NMR (CD2Cl2): δ 18.9 (d, 3JPP ) 7.0 Hz), 6.7
3
3
(m, 1H), 6.54 (tt, JHH ) 16.0 Hz, JHP ) 9.6 Hz, 1H), 6.47 (d,
3JHH ) 7.3 Hz, 2H), 5.64 (dt, 3JHH ) 16.0 Hz, 4JHP ) 1.9 Hz, 1H),
2.07 (virtual triplet, J ) 3.3 Hz, 6H). 31P{1H} NMR (CD2Cl2): δ
7.8 (s).
3
(d, JPP ) 6.3 Hz). Anal. Calcd for C34H33BrPPd: C, 59.19; H,
4.82. Found: C, 59.05; H, 4.94.
Pd(η2-m-MeC6H4CHdCHPMePh2)Br(PMePh2) (5a) was simi-
larly obtained as yellow crystals from Pd(dba)2 (130 mg, 0.225
mmol), 7a (89.5 mg, 0.225 mmol), and PMePh2 (45.1 mg, 0.225
mmol) (16%). Mp: 124–126 °C (dec). 1H NMR (CD2Cl2): δ
8.04–7.96, 7.78–7.45, 7.26–7.08 and 7.00–6.90 (m, 20H in total),
6.88 (t, 3JHH ) 7.5 Hz, 1H), 6.81 (d, 3JHH ) 7.5 Hz, 1H), 6.66 (d,
3JHH ) 7.6 Hz, 1H), 6.56 (s, 1H), 3.37 (ddd, 2JHP ) 19.9 Hz, 3JHH
Synthesis of (E)-[PhCHdCHPMePh2]Br (7). This compound
was prepared by the procedure reported for (E)-[PhCHd
CHPPh3]Br.15c A solution of (E)-1 (802 mg, 4.38 mmol) and
PMePh2 (878 mg, 4.38 mmol) in toluene (22 mL) was stirred at
31
100 °C in the presence of Pd(PMePh2)4 (199 mg, 0.219 mmol)
for 24 h. The white precipitate formed in the system was collected
by filtration, washed with hexane and Et2O, and dried under vacuum
3
3
3
) 10.7 Hz, JHP ) 4.0 Hz, 1H), 3.11 (ddd, JHP ) 14.3 Hz, JHH
) 10.7 Hz, 3JHP ) 8.2 Hz, 1H), 2.82 (d, 2JHP ) 13.7 Hz, 3H), 2.11
1
(1.57 g, 94%). Mp: 181–183 °C. H NMR (CDCl3): δ 7.89–7.74,
(s, 3H), 1.51 (d, JHP ) 6.2 Hz, 3H). 13C{1H} NMR (CD2Cl2): δ
2
2
7.70–7.65, and 7.51–7.37 (m, 17H in total), 3.06 (d, JHP ) 13.7
3
1
Hz, 3H). 13C{1H} NMR (CDCl3): δ 154.9 (d, 3JPC ) 4.7 Hz), 134.7
144.6 (d, JPC ) 12.3 Hz), 139.5 (d, JPC ) 27.8 Hz), 138.2 (s),
1
3
4
2
3
137.0 (d, JPC ) 29.4 Hz), 134.9 (d, JPC ) 10.1 Hz), 133.5 (br),
(d, JPC ) 3.0 Hz), 133.6 (d, JPC ) 20.3 Hz), 132.9 (d, JPC
)
2
2
10.5 Hz), 131.9 (s), 130.2 (d, 2JPC ) 12.7 Hz), 129.2 (s), 129.1 (s),
119.7 (d, 1JPC ) 89.7 Hz), 106.4 (d, 1JPC ) 89.0 Hz), 10.7 (d, 1JPC
) 58.5 Hz). 31P{1H} NMR (CDCl3): δ 19.6 (s). Anal. Calcd for
C21H20BrP: C, 65.81; H, 5.26. Found: C, 65.58; H, 5.32.
132.8 (d, JPC ) 14.2 Hz), 132.5 (d, JPC ) 13.9 Hz), 129.9 (d,
3JPC ) 11.2 Hz), 129.2 (s), 129.1 (d, JPC ) 12.5 Hz), 128.9 (s),
3
128.6 (s), 128.5 (d, 3JPC ) 9.1 Hz), 128.2 (d, 3JPC ) 9.0 Hz), 127.4
(dd, 1JPC ) 72.7 Hz, 4JPC ) 9.5 Hz), 126.5 (s), 125.7 (s), 124.8 (d,
1JPC ) 93.3 Hz), 122.6 (s), 59.8 (d, 2JPC ) 4.9 Hz), 28.2 (dd, 1JPC
(E)-[m-MeC6H4CHdCHPMePh2]Br (7a) was similarly prepared
as a white powder by the reaction of (E)-m-methylstyryl bromide
(312 mg, 1.58 mmol) and PMePh2 (317 mg, 294 mmol) in toluene
(8 mL) in the presence of Pd(PMePh2)4 (71.8 mg, 0.0791 mmol)
(585 mg, 93%). Mp: 138–140 °C. 1H NMR (CDCl3): δ 7.88–7.76
2
1
) 81.5 Hz, JPC ) 35.5 Hz), 21.5 (s), 14.6 (d, JPC ) 18.7 Hz),
13.7 (d, 1JPC ) 66.3 Hz). 31P{1H} NMR (CD2Cl2): δ 18.8 (d, 3JPP
) 7.5 Hz), 6.7 (d, 3JPP ) 6.2 Hz). Anal. Calcd for C35H35BrP2Pd:
C, 59.72; H, 5.01. Found: C, 59.44; H, 5.00.
Catalytic Reactions. To a 10 mL Schlenk tube containing
PhB(OH)2 (48.8 mg, 0.400 mmol), Pd(PPh3)4 (6.9 mg, 6.0 µmol),
and hexamethylbenzene (32.5 mg, 0.200 mmol; internal standard)
were added successively toluene (2.0 mL), an aqueous solution of
potassium carbonate (3.0 M, 0.40 mL, 1.2 mmol), and 1 (73.2 mg,
0.400 mmol). The mixture was stirred at 80 °C for 1 h. The coupling
products were identified by GC-MS using authentic samples and
analyzed quantitatively by GLC.
and 7.72–7.55 (m, 13H in total), 7.40 (dd, 3JHP ) 23.1 Hz, 3JHH
)
2
17.4 Hz, 1H), 7.34–7.27 (m, 2H), 3.10 (d, JHP ) 13.7 Hz, 3H),
2.38 (s, 3H). 13C{1H} NMR (CDCl3): δ 155.2 (d, 3JPC ) 4.4 Hz),
4
2
138.9 (s), 134.7 (d, JPC ) 2.9 Hz), 133.5 (d, JPC ) 20.1 Hz),
132.9 (d, 3JPC ) 10.8 Hz), 132.8 (s), 130.2 (d, J ) 12.9 Hz), 129.7
(s), 129.0, (s), 126.4 (s), 119.8 (d, 1JPC ) 89.5 Hz), 105.9 (d, 1JPC
1
) 89.3 Hz), 21.2 (s), 10.7 (d, JPC ) 58.5 Hz). 31P{1H} NMR
Kinetic Experiments. A typical procedure is as follows. The
compounds (E)-3 (20.7 mg, 0.030 mmol), (E)-1 (110 mg, 0.60
mmol), and 1,3,5-trimethoxybenzene (2.5 mg, 0.015 mmol; internal
standard) were placed in an NMR sample tube and dissolved in
C6D6/CD2Cl2 (5:1 v/v) (total 0.6 mL) at room temperature under a
nitrogen atmosphere. The sample tube was capped with a rubber
(25) Dolby, L. J.; Wilkins, C.; Frey, T. G. J. Org. Chem. 1966, 31,
1110–1116.
(26) Kim, S. H.; Wei, H.-X.; Willis, S.; Li, G. Synth. Commun. 1999,
29, 4179–4185.
(27) Coulson, D. R. Inorg. Synth. 1971, 13, 121–124.
(28) Louch, W. J.; Eaton, D. R. Inorg. Chim. Acta 1978, 30, 243–250.
(29) Hartley, F. R. Organomet. Chem. ReV., Sect. A 1970, 6, 119–137.
(30) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19, 220–
223.
(32) Ukai, T.; Kawamura, H.; Ishii, Y. J. Organomet. Chem. 1974, 65,
253–266.
(31) Kuran, W.; Musco, A. Inorg. Chim. Acta 1975, 12, 187–193.

608 Organometallics, Vol. 27, No. 4, 2008
Wakioka et al.
septum and placed in an NMR probe controlled to 50.0 ( 0.1 °C.
and an aqueous solution of potassium carbonate (3.0 M, 0.30 mL,
0.90 mmol). The mixture was stirred at 50 °C for 10 min and
analyzed by GC-MS and GLC, showing the formation of biphenyl
in quantitative yield. The reaction of 4 (trans/cis ) 93/7) was
similarly conducted.
1
The reaction was followed at intervals by H NMR spectroscopy
using the following marker signals: (E)-3 (δ 1.98, PMe), 4 (δ 2.08,
PMe), 5 (δ 2.55, 1.64, PMe), 6 (δ 2.14, PMe), 7 (δ 2.73, PMe),
(E,E)-2 (δ 6.51, vinylic H), (E,Z)-2 (δ 6.31, vinylic H), 1,3,5-
trimethoxybenzene (δ 6.14, Ar). 31P{1H} NMR spectrum measured
after completion of the reaction exhibited three singlets assignable
to 4 (δ 5.4), 6 (δ 15.2), and 7 (δ 19.3).
Reaction of trans-PdBr2(PPh3)2 (4a) with PhB(OH)2. To a 10
mL Schlenk tube containing 4a (79.1 mg, 0.100 mmol), PhB(OH)2
(36.6 mg, 0.300 mmol), and hexamethylbenzene (16.2 mg, 0.100
mmol; internal standard) were added successively toluene (1.0 mL)
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research on Priority Areas (No.
18064010, “Synergy of Elements”) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
OM701128E