Mechanism of C-P reductive elimination from trans-[Pd(CH=CHPh)Br(PMePh2)2]

Article abstract of DOI:10.1021/om801207k

The (E)- and (Z)-styryl isomers of trans-[Pd(CH=CHPh)Br(PMePh ₂)₂](1a) and [Pd(n²-PhCH= CHPMePh2) Br(PMePh₂)] (2a) were prepared, and their C-P reductive elimination (1a - 2a) and C-P oxidative addition (2a - 1a

Full text of DOI:10.1021/om801207k

Organometallics 2009, 28, 2527–2534
2527
Mechanism of C-P Reductive Elimination from
trans-[Pd(CHdCHPh)Br(PMePh2)2]
Masayuki Wakioka, Yumiko Nakajima, and Fumiyuki Ozawa*
International Research Center for Elements Science (IRCELS), Institute for Chemical Research,
Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed December 20, 2008
The (E)- and (Z)-styryl isomers of trans-[Pd(CHdCHPh)Br(PMePh2)2] (1a) and [Pd(η2-PhCHd
CHPMePh2)Br(PMePh2)] (2a) were prepared, and their C-P reductive elimination (1a f 2a) and C-P
oxidative addition (2a f 1a) behaviors examined. Kinetics and thermodynamics of the reactions are
strongly affected by E/Z configurations of the styryl group and solvent polarity. Complex (E)-1a readily
undergoes C-P reductive elimination in CD2Cl2 as a polar solvent in high selectivity. On the other hand,
while the (Z)-isomer of 1a is unreactive toward reductive elimination, (Z)-2a undergoes C-P oxidative
addition favorably in nonpolar benzene. X-ray diffraction analysis and DFT calculations for 1a and 2a
provided reasonable accounts for these reaction features. Kinetic examinations revealed two types of
C-P reductive elimination processes, which involve predissociation and association of the PMePh2 ligand,
respectively.
results in Ar/Ar′ exchange between palladium and phosphorus
atoms.8 The C-P reductive elimination has also been postulated
for catalytic formation of aryl-,4 alkenyl-,5 and alkyl-phospho-
niums6 and functionalized phosphines.7
Most of the reductive elimination processes involve the
coupling of two anionic ligands. In contrast, C-P reductive
elimination causes the bond formation between anionic and
Introduction
Reductive elimination is a crucial elementary process, often
serving as the product-forming step in many catalytic organic
transformations.1 Besides classical reactions to afford C-H and
C-C bonds, reductive elimination of a carbon-heteroatom bond
has attracted considerable recent attention in connection with
palladium-catalyzed synthesis of heteroatom compounds.2 While
the C-N and C-O bond formations are of central importance
in such chemistry, there has been a growing interest in C-P
reductive elimination of hydrocarbyl and phosphine ligands from
Pd(II) complexes.3-7 It has been documented that complexes
of the formula [PdAr(X)(PAr′3)2] (Ar, Ar′ ) aryl; X ) halogen)
undergo C-P reductive elimination to give arylphosphonium
halides (PArAr′3 · X) and Pd(0) species.3 This reaction is usually
reversible with C-P oxidative addition, and the overall process
(4) (a) Migita, T.; Shimizu, T.; Asami, Y.; Shiobara, J.; Kato, Y.; Kosugi,
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Chem.-Eur. J. 1999, 5, 3066–3075. (e) Bourgogne, C.; Le, F. Y.; Juen,
P.; Masson, P.; Nicoud, J.-F.; Masse, R. Chem. Mater. 2000, 12, 1025–
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Cheng, C.-H. Organometallics 1998, 17, 676–682. (b) 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.
(c) Kowalski, M. H.; Hinkle, R. J.; Stang, P. J. J. Org. Chem. 1989, 54,
2783–2784. (d) Hinkle, R. J.; Stang, P. J.; Kowalski, M. H. J. Org. Chem.
1990, 55, 5033–5036. (e) Dervisi, A.; Edwards, P. G.; Newman, P. D.;
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2060. (b) Kwong, F. Y.; Chan, K. S. Chem. Commun. 2000, 1069–1070.
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2001, 20, 2570–2578. (e) Lai, C. W.; Kwong, F. Y.; Wang, Y.; Chan, K. S.
Tetrahedron Lett. 2001, 42, 4883–4885. (f) Kwong, F. Y.; Lai, C. W.; Chan,
K. S. Tetrahedron Lett. 2002, 43, 3537–3539. (g) Kwong, F. Y.; Lai, C. W.;
Yu, M.; Tian, Y.; Chan, K. S. Tetrahedron 2003, 59, 10295–10305. (h)
Kwong, F. Y.; Lai, C. W.; Yu, M.; Chan, K. S. Tetrahedron 2004, 60,
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Tetrahedron 2004, 60, 9433–9439.
* To whom correspondence should be addressed. E-mail: ozawa@
scl.kyoto-u.ac.jp.
(1) (a) Ozawa, F. In Current Methods in Inorganic Chemistry Volume
3, Fundamentals of Molecular Catalysis; Yamamoto, A., Kurosawa, H.,
Eds.; Elsevier Science: Amsterdam, The Netherlands, 2003; Chapter 9. (b)
Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
4th ed.; John Wiley & Sons: Hoboken, NJ, 2005; pp 55-60. (c) Yamamoto,
A. Organotransition Metal Chemistry, Fundamental Concepts and Applica-
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J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications
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ReV. 1988, 88, 1031–1046.
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J. F. Nature 2008, 455, 314–322.
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1997, 119, 12441–12453. (c) Grushin, V. V. Organometallics 2000, 19,
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Chenard, B. L. J. Org. Chem. 1995, 60, 12–13. (f) Sakamoto, M.; Shimizu,
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(8) An alternative mechanism involving a phosphido intermediate has
been proposed for the conversion of trans-[PdMe(I)(PPh3)2] to [PdPh(I)-
(PMePh2)(PPh3)]: Morita, D. K.; Stille, J. K.; Norton, J. R. J. Am. Chem.
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10.1021/om801207k CCC: $40.75
2009 American Chemical Society
Publication on Web 03/23/2009

2528 Organometallics, Vol. 28, No. 8, 2009
Wakioka et al.
Table 1. C-P Reductive Elimination of
trans-[Pd{CHdCHPh-(E)}Br(PR3)2] ((E)-1)a
Table 2. C-P Oxidative Addition of
[Pd{η2-(Z)-PhCHdCHPMePh2}Br(PMePh2)] ((Z)-2a)a
b
additive
temp
t1/2
selectivity of
selectivity (%)
(Z)-1a (E)-2a
46 33
PMePh2
(mM)
conversion of
(Z)-2a (%)
run complex solvent (PR3, mM) (°C) (min) 2 (or 3) (%)
run
solvent
1
2
3
4
5
6
7
(E)-1a
(E)-1a
(E)-1a
(E)-1a
(E)-1b
(E)-1b
(E)-1c
CD2Cl2
CD2Cl2
THF-d8
C6D6
CD2Cl2
CD2Cl2
CD2Cl2
0
50
0
0
0
40
40
50
50
40
40
40
ca. 5
168
48
60
<4
82
1
2
3
C6D6
C6D6
CD2Cl2
0
10
0
79
90
27
97c
67
0
12
100
73
56
63
a [(Z)-2a]0 ) 10 mM. All reactions were run at 40 °C for 8 h.
50
0
35
270
96d
43e
selectivity (run 1). The remaining part of (E)-1a was converted
to trans-[PdBr2(PMePh2)2] (5%) and some unidentified pal-
ladium species. We already confirmed that the dibromo complex
is afforded by disproportionation of (E)-1a, and its formation
is inhibited by free PMePh2.11 Actually, in the presence of added
PMePh2 (1 equiv/1a), the C-P reductive elimination proceeded
in 97% selectivity (run 2), where the product styrylphosphonium
complex was obtained as [Pd{η2-(E)-PhCHdCHPMePh2}-
(PMePh2)2]Br ((E)-3a), instead of (E)-2a.12 The conversion of
(E)-1a was strongly affected by solvent polarity; the reaction
decelerated significantly in THF and benzene, and the selectivity
of (E)-2a decreased (runs 3 and 4). The PPh3 complex (E)-1b
behaved similarly, while the reactivity was apparently higher
than that of (E)-1a (runs 5 and 6). On the other hand, the PMe3
complex (E)-1c was much less reactive than (E)-1a and (E)-1b
(run 7).
Unlike the (E)-styryl complexes, the (Z)-isomer trans-
[Pd{CHdCHPh-(Z)}Br(PMePh2)2] ((Z)-1a) was stable toward
C-P reductive elimination; no trace of [Pd{η2-(Z)-PhCHd
CHPMePh2}Br(PMePh2)] ((Z)-2a) was formed at 50 °C for 6 h.
On the contrary, (Z)-2a was found to undergo oxidative addition
of styrylphosphonium ligand (eq 2, Table 2). Heating a C6D6
solution of isolated (Z)-2a at 40 °C for 8 h resulted in the
formation of (Z)-1a in 46% selectivity, together with (E)-2a
(33%) and other palladium complexes including trans-
[PdBr2(PMePh2)2] (4%) (run 1). The formation of (Z)-1a was
entirely suppressed by added PMePh2 (1 equiv/1a), while the
Z-to-E isomerization of the styrylphosphonium ligand giving
(E)-2a continued to proceed at a comparable rate (run 2). The
reaction rate and the selectivity of (Z)-1a decreased significantly
in CD2Cl2 as a polar solvent (run 3).
a [1]0 ) 50 mM. All reactions were examined by 1H NMR spectroscopy
using 1,3,5-trimethoxybenzene as an internal standard. b Half-lives of 1.
c The sum of [Pd{η2-(E)-PhCHdCHPMePh2}(PMePh2)2]Br ((E)-3a) (96%)
d
and (E)-PhCHdCHPMePh2 · Br (1%). The product was [Pd{η2-(E)-
PhCHdCHPPh3}(PPh3)2]Br ((E)-3b). e The selectivity at 58% conversion
of (E)-1c.
neutral ligands. Accordingly, it is expected that the reaction
possesses rather unique mechanistic features, differing signifi-
cantly from common reductive elimination processes. Although
the reductive elimination of arylphosphonium from [PdAr(X)-
(PAr′3)2]-type complexes has been suggested to involve prior
dissociation of one of the phosphine ligands,3a-c intimate
information about the mechanism of C-P reductive elimination
is still limited.9 This is probably due to the concurrent operation
of C-P oxidative addition, which makes the reaction system
complicated.
This paper deals with C-P reductive elimination from trans-
[Pd(CHdCHPh)Br(PMePh2)2] (1a) to afford [Pd(η2-PhCHd
CHPMePh2)Br(PMePh2)] (2a) and [Pd(η2-PhCHdCHPMePh2)-
(PMePh2)2]Br (3a). Unlike the reactions of arylpalladium
complexes,3 the C-P reductive elimination of styryl complexes
forms η2-styrylphosphonium complexes as the products, which
are sufficiently stable for isolation.10 Therefore, we anticipated
that the reaction is capable of mechanistic investigations using
kinetic techniques. We herein describe that C-P reductive
elimination of 1a is strongly dependent on E/Z configurations
of the styryl ligand. It has also been found that two types of
reductive elimination processes are operative, depending on the
amount of free PMePh2 in the system.
Results and Discussion
C-P Reductive Elimination and Oxidative Addition
Reactions. (E)-Styryl complexes having three kinds of tertiary
phosphine ligands ((E)-1a-c) were heated in solution, and the
reaction systems were examined at intervals by 1H NMR
spectroscopy (eq 1). The results are summarized in Table 1.
X-ray Structures of 1a and 2a. It has been found that
reductive elimination and oxidative addition reactions of C-P
bonds are markedly affected by E/Z configurations of the styryl
group and solvent polarity. Since these reactions are essentially
reverse processes, and the conversion of (E)-1a to (E)-2a and
(9) The mechanism of C-P reductive elimination of anionic phosphorus
ligands such as phosphido and phosphonato has been examined using
isolated complexes: (a) Gaumont, A.-C.; Brown, J. M.; Hursthouse, M. B.;
Coles, S. J. Chem. Commun. 1999, 63–64. (b) Adam, M.; Stockland, R. A.,
Jr.; Clark, R.; Guzei, I. Organometallics 2002, 21, 3278–3284. (c) Stockland,
R. A., Jr.; Levine, A. M.; Giovine, M. T.; Guzei, I. A.; Cannistra, J. C.
Organometallics 2004, 23, 647–656. (d) Kohler, M. C.; Stockland, R. A.,
Jr.; Rath, N. P. Organometallics 2006, 25, 5746–5756. (e) Kalek, M.;
Stawinski, J. Organometallics 2008, 27, 5876–5888.
(10) The C-P reductive elimination of a styryl complex has been
reported for trans-[Pd{CHdCHPh-(E)}Br(PPh3)2] (1b).5a
(11) Wakioka, M.; Nagao, M.; Ozawa, F. Organometallics 2008, 27,
602–608.
Complex (E)-1a, bearing PMePh2 ligands, readily underwent
C-P reductive elimination in CD2Cl2 at 40 °C to give the
corresponding styrylphosphonium complex ((E)-2a) in 82%
(12) The PPh3 analogue [Pd{η2-(E)-PhCHdCHPPh3}(PPh3)2]Br ((E)-
3b) has been reported.5a

C-P ReductiVe Elimination
Organometallics, Vol. 28, No. 8, 2009 2529
Figure 1. X-ray structures of (E)-1a and (Z)-1a (one of the crystallographically independent molecules). Thermal ellipsoids are drawn at
the 30% probability level. Hydrogen atoms, expect for the H(1) atom of (Z)-1a, are omitted for clarity. Selected bond distances (Å) and
angles (deg) are as follows. (E)-1a: Pd-Br ) 2.5324(8), Pd-P(1) ) 2.3139(10), Pd-P(2) ) 2.3121(10), Pd-C(1) ) 2.007(2), C(1)-C(2)
) 1.325(3), Br-Pd-P(1) ) 94.15(2), Br-Pd-P(2) ) 90.08(3), C(1)-Pd-P(1) ) 88.32(7), C(1)-Pd-P(2) ) 87.50(7), Pd-C(1)-C(2) )
128.85(18), C(1)-C(2)-C(3) ) 123.4(2). (Z)-1a: Pd-Br ) 2.5233(8), Pd-P(1) ) 2.3187(12), Pd-P(2) ) 2.3128(13), Pd-C(1) ) 2.002(5),
C(1)-C(2) ) 1.341(7), Br-Pd-P(1) ) 91.50(3), Br-Pd-P(2) ) 95.55(3), C(1)-Pd-P(1) ) 87.21(13), C(1)-Pd-P(2) ) 85.75(13),
Pd-C(1)-C(2) ) 129.5(4), C(1)-C(2)-C(3) ) 129.4(4).
Figure 2. X-ray structures of (E)-2a and (Z)-2a. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (deg) are as follows. (E)-2a: Pd-Br ) 2.5669(10), Pd-P(2) ) 2.281(2), Pd-C(1) )
2.135(8), Pd-C(2) ) 2.063(7), C(1)-P(1) ) 1.758(8), C(1)-C(2) ) 1.444(10), C(2)-C(3) ) 1.491(10), Br-Pd-P(2) ) 98.32(6),
C(1)-Pd-C(2) ) 40.2(3), Pd-C(1)-C(2) ) 67.2(4), C(1)-Pd-Br ) 115.1(2), C(2)-Pd-P(2) ) 106.4(2), C(1)-C(2)-C(3) ) 122.0(7),
P(1)-C(1)-C(2) ) 122.2(6). (Z)-2a: Pd-Br ) 2.6201(10), Pd-P(2) ) 2.2872(17), Pd-C(1) ) 2.106(5), Pd-C(2) ) 2.088(5), C(1)-P(1)
) 1.754(5), C(1)-C(2) ) 1.438(7), C(2)-C(3) ) 1.494(7), Br-Pd-P(2) ) 92.78(4), C(1)-Pd-C(2) ) 40.1(2), Pd-C(1)-C(2) ) 69.3(3),
C(1)-Pd-Br ) 111.83(15), C(2)-Pd-P(2) ) 115.24(16), C(1)-C(2)-C(3) ) 124.8(4), P(1)-C(1)-C(2) ) 129.6(4).
that of (Z)-2a to (Z)-1a are operative under similar conditions,
it seems likely that the observed tendencies mainly reflect the
difference in thermodynamic stability between (E)- and (Z)-
isomers of 1a and 2a. We therefore examined their structures
by X-ray diffraction analysis and DFT calculations.
Figure 2 shows the X-ray structures of (E)-2a and (Z)-2a.
The C(1)-C(2) distances (1.444(10), 1.438(7) Å) are compa-
rable to each other. The Pd, C(1), C(2), P(2), and Br atoms are
coplanar in both molecules, as previously observed for related
alkenylphosphonium complexes.5a 5b The Pd-C(1) bonds
,
Figure 1 shows ORTEP drawings of (E)-1a and (Z)-1a. Both
complexes have similar structures with comparable distances
of Pd-C (2.00 Å), Pd-P (2.31 Å), and Pd-Br (2.53 Å) bonds.
A notable difference between the isomers is found for the
orientation of styryl ligands. The (E)-styryl ligand in (E)-1a is
oriented away from the palladium center, successfully evading
the steric congestion within the molecule. On the other hand,
the phenyl group of the (Z)-styryl ligand in (Z)-1a is situated
over the palladium center. Although the interatomic distance
of Pd · · · H(1) (2.494 Å) is in the range of agostic interactions,
a repulsive interaction between them should be considered,
because the C(1)-C(2)-C(3) angle (129.4(4)°) is significantly
enlarged as compared with that of (E)-1a (123.4(2)°).
(2.135(8), 2.106(5) Å) are apparently longer than the Pd-C(2)
bonds (2.063(7), 2.088(5) Å), reflecting the higher trans
influence of PMePh2 than Br.
A remarkable structural feature of (Z)-2a is the axial/
axial/axial arrangement of three phenyl groups of PhCHd
CHP(1)MePh2 and P(2)MePh2 ligands. Since they are situated at
1,3-diaxial positions with one another, a great deal of steric
repulsion should take place between them. This situation is in
sharp contrast with (E)-2a, in which all substituents are
accommodated with enough space. Actually, the P(1)-C(1)-C(2)
angle of (Z)-2a (129.6(4)°) is apparently wider than that of (E)-
2a (122.2(6)°).

2530 Organometallics, Vol. 28, No. 8, 2009
Wakioka et al.
a
Table 3. Relative Energies of 2a to 1a (kcal mol-1
)
solvent
(E)-isomers
(Z)-isomers
(in vacuo)
C6H6
THF
0.0
-2.3
-4.2
-4.6
+4.3
+2.3
+0.4
+1.0
CH2Cl2
a The values of E2a - E1a, estimated by DFT calculations.
Figure 3. NBO charges on the core atoms of 1a and 2a. The values
for (E)- and (Z)-isomers are given with roman and italic typefaces,
respectively.
Figure 4. Time course of conversion of (E)-1a (50 mM) in CD2Cl2
at 40 °C in the absence or presence of added PMePh2.
charged negatively in reality. The negative charge is distributed
on the C(1), C(2), and Br atoms and most remarkably increased
on the C(1) atom as compared with 1a, showing the occurrence
of strong π-back-donation from the palladium center to the η2-
styrylphosphonium ligand in 2a.
Mechanism of C-P Reductive Elimination. As seen from
Table 1 (runs 1, 2), the C-P reductive elimination of (E)-1a is
strongly inhibited by added PMePh2, showing a reaction process
involving predissociation of the PMePh2 ligand (path (a) in
Scheme 1). The three-coordinate intermediate A undergoes C-P
reductive elimination to form B, which combines with PMePh2
to afford (E)-2a. On the other hand, as described below, kinetic
data revealed that the other reaction process involving prior
association of PMePh2 is operative in the presence of added
PMePh2 (path (b)).
DFT Calculations. Table 3 lists the relative energies of 2a
to 1a, estimated by DFT calculations. The geometry optimization
of (E)- and (Z)-isomers of the complexes was carried out using
B3LYP in conjunction with the LANL2DZ basis set and
effective core potential for Pd and the 6-31G(d) basis set for
other atoms, where a diffuse function was added for the Br atom.
Solvent effects were incorporated by PCM single-point calcula-
tions on fully optimized geometries in vacuo. The optimized
structures were in good accordance with the X-ray structures
given in Figures 1 and 2.
The data in Table 3 clearly indicate that C-P reductive
elimination of (E)-isomer [(E)-1a f (E)-2a] is an exothermic
process, whereas that of (Z)-isomer [(Z)-1a f (Z)-2a] is
endothermic (i.e., the reverse process is exothermic). Further-
more, reflecting the presence of the styrylphosphonium ligand
with an ionic character, 2a is more effectively stabilized than
1a in polar solvents for both isomers. These tendencies are
consistent with the experimental observations described above.
Figure 3 compares the charge distribution in 1a and 2a,
evaluated by NBO analysis. The values written in roman and
italic letters correspond to the atomic charges for (E)- and (Z)-
isomers, respectively. The η2-styrylphosphonium palladium
complexes (2) are commonly depicted as zwitterionic species
having positive and negative charges on the phosphorus and
palladium atoms, respectively (see eqs 1 and 2). However, while
the P(1) atom of 2a is charged positively, the Pd atom is not
Figure 4 shows the time course of conversion of (E)-1a (50
1
mM) in CD2Cl2 at 40 °C, followed by H NMR spectroscopy
using 1,3,5-trimethoxybenzene as an internal standard. In the
absence of free PMePh2 (run 1), the PMe signal of (E)-1a at δ
2.07 rapidly decreased (t1/2 ) ca. 5 min), to be replaced by the
PMe signals of (E)-2a at δ 2.83 and 1.50. The reaction progress
was effectively prevented by addition of a small amount of
PMePh2 (5 mM, 0.1 equiv/1a) to the system (run 2). In this
case, however, the reaction dramatically accelerated from the
middle stage, because the added PMePh2 is consumed by
trapping in the reductive elimination product as [Pd{η2-(E)-
1
PhCHdCHPMePh2}(PMePh2)2]Br ((E)-3a). Indeed, in the H
Scheme 1. Proposed Mechanisms for C-P Reductive Elimination of (E)-1a

C-P ReductiVe Elimination
Organometallics, Vol. 28, No. 8, 2009 2531
Table 4. Effects of Added PMePh2 on the First-Order Rate
1
kobsd
1
1
k
Constants for C-P Reductive Elimination of (E)-1aa
)
+
(4)
kK[PMePh2]
[PMePh2] (mM)
104kobsd (s-1
)
data range (%)b
50
64
100
200
0.76(2)
0.87(1)
1.28(2)
2.14(1)
19
29
38
60
Applying the slope and intercept values of Figure 5 to eq 4
results in the following rate and equilibrium constants: k ) 4.8
× 10-4 s-1, K ) 3.7 M-1
.
There are two possible structures for intermediate C. One is
a five-coordinate species, whereas the other is a four-coordinate
ionic complex, [Pd{CHdCHPh-(E)}(PMePh2)3]+Br- ((E)-4a),
generated by ligand displacement of Br in (E)-1a with PMePh2.
The C-P reductive elimination from the latter type intermediate
has been postulated for catalytic conversion of alkenyl triflates
a [(E)-1a]0 ) 50 mM. All reactions were run at 40 °C in CD2Cl2.
b The upper range of the conversion of (E)-1a, taken into the first-order
plot.
and PPh3 to alkenylphosphonium triflates.5c 5d We therefore
,
prepared [Pd{CHdCHPh-(E)}(PMePh2)3]+OTf- ((E)-5a) as a
model of (E)-4a and examined its reactivity toward C-P
reductive elimination (eq 5).
Treatment of (E)-1a (50 mM) with AgOTf (1 equiv) in
CD2Cl2 in the presence of PMePh2 (1 equiv) at -30 °C for
1 h formed (E)-5a in 96% selectivity. The resulting (E)-5a
was highly reactive toward C-P reductive elimination; the
reaction was completed within a few minutes at 40 °C to
give [Pd{η2-(E)-PhCHdCHPMePh2}(PMePh2)2]OTf ((E)-6a)
in 95% selectivity. No notable change in the reaction rate
was observed in the presence of excess PMePh2 (4 equiv).
Since the observed reactivity of (E)-5a (k > 5 × 10-3 s-1 at
40 °C) is at least 10 times higher than that of C (k ) 4.8 ×
10-4 s-1), it is convincing that C is not an ionic species like
(E)-5a, but either a five-coordinate species or a tight ion pair
of [Pd{CHdCHPh-(E)}(PMePh2)3]+ and Br-. Complex C is
then converted to (E)-3a by either a stepwise mechanism
involving the metallophosphorane intermediate [Pd{P(CHd
CHPh)MePh2}Br(PMePh2)2]13 or a concerted mechanism with
concurrent formation of the P-CHdCH and Pd-(η2-CHdCH)
bonds.
Figure 5. Plot of 1/kobsd against 1/[PMePh2] for the kinetic data in
Table 4. The straight line is based on least-squares calculation:
1/kobsd ) (5.7(4) × 102 s M)/[PMePh2] + (2.1(5) × 103 s).
NMR spectra, the PMe signal of (E)-1a was observed to be
coalescent with that of PMePh2 until ca. 20% conversion of
(E)-1a, but thereafter changed to a sharp triplet, showing the
absence of free PMePh2 in the system.
Interestingly, the rate of C-P reductive elimination increased
significantly with increasing amounts of added PMePh2 (runs 3
and 4). Thus, the reaction performed with 50 mM PMePh2 (run
3) was clearly faster than the initial stage of run 2 (5 mM) and
accelerated further with 200 mM PMePh2 (run 4). Although
the concentration of free PMePh2 gradually decreases with the
formation of (E)-3a, the reactions obeyed a good first-order
kinetics (r > 0.999) up to 19% (run 3) and 60% (run 4)
conversion of (E)-1a. Table 4 lists the rate constants (kobsd) thus
estimated. The plot of 1/kobsd against 1/[PMePh2] exhibited a
good linear correlation (r ) 0.995, Figure 5).
These kinetic observations are consistent with the reaction
process of path b in Scheme 1. Since (E)-1a undergoes rapid
ligand exchange with free PMePh2 on an NMR time scale, the
occurrence of a rapid equilibrium between (E)-1a and C is
likely. Intermediate C then undergoes the rate-determining
formation of (E)-3a. In this case, the reaction rate can be
expressed by eq 3, where [Pd-styryl] ) [(E)-1a] + [C], K )
[C]/[(E)-1a][PMePh2], and k stands for the rate constant for the
conversion of C to (E)-3a:
Conclusions
It has been evidenced that C-P reductive elimination of trans-
[Pd{(CHdCHPh)Br(PMePh2)2} (1a) is essentially a reverse
process with C-P oxidative addition of styrylphosphonium
complex [Pd{η2-PhCHdCHPMePh2}Br(PMePh2)] (2a). The
interconversion between 1a and 2a is markedly dependent on
E/Z configurations of the styryl group and solvent polarity. The
(E)-isomer of 1a undergoes C-P reductive elimination easily
in polar CD2Cl2 to afford (E)-2a in high selectivity, whereas
C-P oxidative addition of (Z)-2a giving (Z)-1a takes place
favorably in nonpolar C6D6. These tendencies have been
rationalized by X-ray structural analysis and DFT calculations
kK[(PMePh2]
d[(E)-3a]
dt
d[Pd-styryl]
) -
)
[Pd-styryl]
(3)
dt
1 + k[PMePh2]
(13) A metallophosphorane intermediate has been proposed for F/Ph
exchange of [RhF(PPh3)3] affording [RhPh(PFPh2)(PPh3)2]. Macgregor,
S. A.; Roe, D. C.; Marshall, W. J.; Bloch, K. M.; Bakhmutov, V. I.; Grushin,
V. V. J. Am. Chem. Soc. 2005, 127, 15304–15321.
Consequently, the kobsd values are correlated with the [PMePh2]
values by the following equation:

2532 Organometallics, Vol. 28, No. 8, 2009
Wakioka et al.
vacuum (385 mg, 95% yield). The NMR data were identical to
for 1a and 2a. Complex 2a, bearing a styrylphosphonium ligand,
is a significantly charged molecule, compared with 1a, causing
a strong solvent effect. Moreover, complex (Z)-2a is sterically
unstable due to the occurrence of 1,3-diaxial interactions of
phenyl groups within the molecule and thereby converted to
(Z)-1a by C-P oxidative addition.
those reported.14a 1H NMR (CD2Cl2): δ 7.70-7.63 and 7.43-7.30
3
(m, 30H in total), 6.98-6.87 (m, 3H), 6.40 (dt, JHH ) 15.7 Hz,
3JHP ) 9.7 Hz, 1H), 6.27 (d, 3JHH ) 6.8 Hz, 2H), 5.42 (dt, 3JHH
)
4
15.8 Hz, JHP ) 2.5 Hz, 1H). 31P{1H} NMR (CD2Cl2): δ 24.5 (s).
Synthesis of trans-[Pd{CHdCHPh-(E)}Br(PMe3)2] ((E)-1c).
To a homogeneous solution of (η5-C5H5)(η3-C3H5)Pd15 (106 mg,
0.500 mmol) in toluene (2.0 mL) was added PMe3 (78.9 mg, 1.05
mmol) at 0 °C. The mixture was stirred for 10 min, and (E)-stylyl
bromide16 (366 mg, 2.00 mmol) was added. The solution was stirred
at room temperature for 4 h. Hexane (5 mL) was added with stirring
at 0 °C. A 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
dissolved in a minimum amount of CH2Cl2 (ca. 1 mL) at room
temperature, layered with Et2O (ca. 5 mL), and allowed to stand at
the same temperature to afford pale yellow crystals of the title
compound (165 mg, 74% yield). Mp: 118-120 °C (dec). 1H NMR
(CD2Cl2): δ 7.33-7.21 and 7.12-7.07 (m, 6H in total), 6.43 (d,
3JHH ) 16.5 Hz, 1H), 1.38 (virtual triplet, J ) 3.5 Hz, 18H). 13C{1H}
The reductive elimination from [PdR(R′)L2]-type complexes
has been known to proceed via any of three reaction processes:
(a) dissociative path via a three-coordinate intermediate, (b)
direct path, and (c) associative path with precoordination of an
external L to give a [PdR(R′)L3] intermediate.1a The reaction
processes vary with hydrocarbyl ligands, and alkenyl complexes
generally follow the direct path (b).14 On the other hand, the
present C-P reductive elimination from (E)-1a has been found
to proceed via either the dissociative path (a) or the associative
path (c), depending on the amount of free PMePh2 in the system.
In the absence of free PMePh2, the reaction invokes predisso-
ciation of one of the PMePh2 ligands, giving a three-coordinate
[Pd(CHdCHPh)Br(PMePh2)] intermediate, which undergoes
C-P reductive elimination. This process is effectively sup-
pressed by addition of PMePh2 to the system, and an alternative
process involving prior association of (E)-1a with PMePh2 takes
place.Comparisonofthekineticdatawiththatof[Pd{CHdCHPh-
(E)}(PMePh2)3]+OTf- ((E)-5a) has suggested the intermediacy
of a five-coordinate species or a tight ion pair of [Pd{CHdCHPh-
(E)}(PMePh2)3]+ and Br-, rather than the four-coordinate ionic
species like (E)-5a. Further mechanistic details, especially
focused on the intermediate and transition state structures in
the latter process (i.e., C f (E)-3a in Scheme 1), are now under
investigation.
3
4
NMR (CD2Cl2): δ 145.6 (t, JPC ) 9 Hz), 140.4 (s), 134.3 (t, JPC
1
) 6 Hz), 129.0 (s), 125.9 (s), 125.2 (s), 14.6 (t, JPC ) 15 Hz).
31P{1H} NMR (CD2Cl2):
δ -17.9 (s). Anal. Calcd for
C14H25BrP2Pd: C, 38.08; H, 5.71. Found: C, 37.96; H, 5.68.
Synthesis of [Pd{(Z)-η2-PhCHdCHPMePh2}Br(PMePh2)]
((Z)-2a). (a) Synthesis of (Z)-PhCHdCHPMePh2 · Br. A mixture
of (Z)-P(CHdCHPh)Ph217 (1.22 g, 4.25 mmol) and a 2 M solution
of MeBr in THF (22 mL, 44 mmol) was stirred at room temperature.
After 48 h, a white precipitate generated from the solution was
collected by filtration, washed successively with Et2O, and dried
under vacuum. Recrystallization of the crude product from a mixture
of acetone and Et2O at -20 °C gave pale yellow crystals of the
title compound (1.33 g, 82% yield). Mp: 130 °C. 1H NMR (CDCl3):
Experimental Section
2
3
δ 8.30 (dd, JHP ) 45.4 Hz, JHH ) 13.2 Hz, 1H), 7.86-7.78,
3
7.73-7.67, and 7.63-7.57 (m, 10H in total), 7.22 (t, JHH ) 7.4
General Considerations. All manipulations were carried out
under a nitrogen or argon atmosphere using standard Schlenk
techniques. Nitrogen and argon gases 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
Hz, 1H), 7.13-7.05 (m, 3H), 7.01 (d, 3JHH ) 7.6 Hz, 2H), 2.74 (d,
3JHP ) 13.5 Hz, 3H). 13C{1H} NMR (CDCl3): δ 158.0 (d, JPC
)
4
2
3
1 Hz), 134.6 (d, JPC ) 3 Hz), 133.5 (d, JPC ) 8 Hz), 132.6 (d,
3JPC ) 11 Hz), 130.4 (s), 130.3 (d, 2JPC ) 13 Hz), 128.5 (s), 128.4
(d, 4JPC ) 2 Hz), 120.2 (d, 1JPC ) 89 Hz), 110.0 (d, 1JPC ) 82 Hz),
12.2 (d, JPC ) 58 Hz). 31P{1H} NMR (CDCl3): δ 12.9 (s). Anal.
1
1
shifts are reported in 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. Elemental analysis was
performed by the ICR Analytical Laboratory, Kyoto University.
CD2Cl2, THF-d8, and C6D6 were dried over CaH2, Na/Ph2CO,
and LiAlH4, respectively, distilled, and stored over activated
MS4A. The compounds trans-[Pd{CHdCHPh-(E)}Br(PMePh2)2]
((E)-1a),11 trans-[Pd{CHdCHPh-(Z)}Br(PMePh2)2] ((Z)-1a),14a
and [Pd{η2-(E)-PhCHdCHPMePh2}Br(PMePh2)] ((E)-2a)11 were
prepared according to the literature. Other chemicals were
purchased and used as received.
Synthesis of trans-[Pd{CHdCHPh-(E)}Br(PPh3)2] ((E)-1b).
To a heterogeneous solution of [Pd(η5-C5H5)(η3-C3H5)]15 (106 mg,
0.500 mmol) and PPh3 (275 mg, 1.05 mmol) in toluene (10 mL)
was added (E)-styryl bromide16 (1.83 g, 10.0 mmol) at 0 °C. The
mixture was stirred at room temperature until homogeneous and
then was allowed to stand at the same temperature, causing
precipitation of pale yellow crystals of (E)-1b, which were collected
by filtration, washed successively with Et2O, and dried under
Calcd for C21H20BrP: C, 65.81; H, 5.26. Found: C, 65.67; H, 5.36.
(b) Synthesis of (Z)-2a. The complex Pd(dba)218 (288 mg, 0.500
mmol) and (Z)-PhCHdCHPMePh2 · Br (192 mg, 0.500 mmol) were
dissolved in CH2Cl2 (10 mL), and PMePh2 (100 mg, 0.500 mmol)
was added at room temperature. The mixture was stirred for 1 h
and filtered through a Celite pad, and the filtrate was concentrated
to dryness under reduced pressure. The residue was extracted three
times with a mixed solvent of THF (2 mL) and Et2O, and the
combined extract was concentrated to dryness to give a pale yellow
solid of (Z)-2a. This product was purified three times by repre-
cipitation from CH2Cl2/Et2O (1/15 mL) at -78 °C and then by
recrystallization from CH2Cl2/Et2O at -20 °C (162 mg, 47% yield).
Mp: 117-119 °C (dec). 1H NMR (CD2Cl2): δ 7.87-7.80, 7.71-7.64,
3
7.64-7.45, and 7.31-7.20 (m, 20H in total), 6.81 (d, JHH ) 7.0
Hz, 2H), 6.76 (t, 3JHH ) 7.3 Hz, 1H), 6.57 (t, 3JHH ) 7.7 Hz, 2H),
2
3
3
4.51 (ddd, JHP ) 27.3 Hz, JHH ) 10.7 Hz, JHP ) 5.3 Hz, 1H),
3
3
3
2.87 (ddd, JHP ) 11.9 Hz, JHH ) 10.7 Hz, JHP ) 8.1 Hz, 1H),
2.58 (d, 2JHP ) 13.6 Hz, 3H), 1.71 (d, 2JHP ) 6.1 Hz, 3H). 13C{1H}
3
3
NMR (CD2Cl2): δ 142.7 (dd, JPC ) 7 Hz, JPC ) 1 Hz), 139.0
(d,1JPC ) 29 Hz), 138.8 (d,1JPC ) 29 Hz), 133.9 (d, 3JPC ) 10 Hz),
133.2 (d, 2JPC ) 16 Hz), 133.1 (d, 3JPC ) 14 Hz), 133.0 (s), 132.9
(14) (a) Loar, M. K.; Stille, J. K. J. Am. Chem. Soc. 1981, 103, 4174–
4181. (b) Brown, J. M.; Cooley, N. A. Organoemtallics 1990, 9, 353–359.
(c) Calhorda, M. J.; Brown, J. M.; Cooley, N. A. Organometallics 1991,
10, 1431–1438. (d) Ozawa, F.; Tani, T.; Katayama, H. Organometallics
2005, 24, 2511–2515.
2
2
(d, JPC ) 16 Hz), 131.0 (s), 131.0 (s), 129.6 (d, JPC ) 12 Hz),
(15) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19, 220–
223.
(16) Dolby, L. J.; Wilkins, C.; Frey, T. G. J. Org. Chem. 1966, 31,
1110–1116.
(17) Taillefer, M.; Cristau, H. J.; Fruchier, A.; Vicente, V. J. Organomet.
Chem. 2001, 624, 307–315.
(18) Ukai, T.; Kawamura, H.; Ishii, Y. J. Organomet. Chem. 1974, 65,
253–266.

C-P ReductiVe Elimination
Organometallics, Vol. 28, No. 8, 2009 2533
Table 5. Crystal Data and Details of the Structure Determination for 1a and 2a
(E)-1a (Z)-1a (E)-2a
C34H33BrP2Pd C34H33BrP2Pd C34H33BrP2Pd
(Z)-2a
C34H33BrP2Pd
689.85
formula
fw
689.85
689.85
689.85
cryst size (mm)
cryst syst
a (Å)
b (Å)
c (Å)
0.40 × 0.20 × 0.10
0.28 × 0.20 × 0.08
monoclinic
19.196(6)
16.509(5)
20.194(7)
90
111.543(4)
90
5952(3)
0.40 × 0.19 × 0.18
monoclinic
9.7501(19)
25.206(5)
12.916(2)
90
109.107(3)
90
2999.4(10)
1.523
Cc (#9)
4
2.079
0.5936-0.7060
numerical
3.23-27.48
12 140
5855 (Rint ) 0.0267)
4546
0.08 × 0.05 × 0.01
triclinic
triclinic
7.263(3)
9.097(3)
10.521(4)
20.015(8)
85.512(12)
86.222(12)
79.225(11)
1495.9(10)
1.532
11.925(4)
14.187(5)
77.931(12)
84.226(13)
80.528(12)
1480.9(9)
1.547
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
dcald (g cm-3
space group
Z
)
1.540
P21/c (#14)
8
j
j
P1 (#2)
P1 (#2)
2
2
µ (mm-1
)
2.085
2.095
2.106
transmn factor
0.4894-0.8186
numerical
3.09-27.48
12 140
6577 (Rint ) 0.0340)
5758
0.5915-0.8503
numerical
3.06-27.48
46 439
13 534 (Rint ) 0.0642)
10 727
0.6781-0.6781
empirical
3.08-27.48
12 141
6521 (Rint ) 0.0541)
4103
absorp corr
θ range (deg)
no. of reflns collected
no. of unique reflns
no. of reflns with I > 2σ(I)
no. of variables (restraints)
GOF on F2
345 (0)
1.058
693 (0)
0.930
338 (2)
1.048
345 (0)
1.059
final R indices (I > 2σ(I))
R1 ) 0.0297, wR2 ) 0.0726 R1 ) 0.0498, wR2 ) 0.1305 R1 ) 0.0496, wR2 ) 0.1335 R1 ) 0.0545, wR2 ) 0.0898
R indices (all data)
R1 ) 0.0360,
wR2 ) 0.0761
0.806, -0.823
R1 ) 0.0732,
wR2 ) 0.1496
1.158, -0.515
R1 ) 0.0586,
wR2 ) 0.1374
0.984, -0.900
R1 ) 0.1015,
wR2 ) 0.1121
1.099, -0.865
max. and min. peak (e Å-3
)
5
2
129.3 (d, JPC ) 1 Hz), 129.1 (s), 129.1 (d, JPC ) 12 Hz), 128.4
(E)-5a. 1H NMR (CD2Cl2, -30 °C): δ 7.46-7.15 and 7.02-6.92
(d, 3JPC ) 9 Hz), 128.4 (d, 3JPC ) 9 Hz), 127.3 (s), 126.9 (dd, 1JPC
(m, 33H in total), 6.29 (d, JHH ) 7.3 Hz, 2H), 6.14 (tdd, JHP
)
)
3
3
4
1
4
3
3
3
) 80 Hz, JPC ) 7 Hz), 125.9 (dd, JPC ) 84 Hz, JPC ) 4.3 Hz),
20.6 Hz, JHH ) 16.6 Hz, JHP ) 10.5 Hz, 1H), 5.72 (dd, JHH
4
1
2
125.5 (s), 60.9 (s), 32.3 (dd, JPC ) 78 Hz, JPC ) 38 Hz), 16.6
16.6 Hz, JHP ) 10.2 Hz, 1H), 1.66 (virtual triplet, J ) 3.1 Hz,
(dd, JPC ) 66 Hz, JPC ) 2 Hz), 15.2 (d, JPC ) 15 Hz). 31P{1H}
6H), 1.30 (t, JHP ) 7.2 Hz, 3H). 31P{1H} NMR (CD2Cl2, -30
1
4
1
2
3
3
2
NMR (CD2Cl2): δ 17.5 (d, JPP ) 9 Hz), 6.1 (d, JPP ) 9 Hz).
Anal. Calcd for C34H33BrPPd: C, 59.19; H, 4.82. Found: C, 58.75;
H, 4.92.
°C): δ 7.9 (d, JPP ) 35 Hz), -0.53 (t,2JPP ) 35 Hz).
(E)-6a. 1H NMR (CD2Cl2): δ 7.82-7.76, 7.70-7.56, 7.52-7.23,
7.16-7.10, 6.72-6.66, and 6.66-6.56 (m, 35H in total), 3.72-3.51
2
2
C-P Reductive Elimination and Oxidative Addition. A typical
procedure is as follows. (E)-1a (20.7 mg, 0.030 mmol) and 1,3,5-
trimethoxybenzene (2.5 mg, 0.015 mmol; internal standard) were
placed in an NMR sample tube and dissolved in CD2Cl2 (total 0.6
mL) at room temperature under an argon atmosphere. The sample
tube was placed in an NMR sample probe controlled to 40.0 ( 0.1
(m, 2H), 1.85 (d, JHP ) 13.1 Hz, 3H), 1.57 (d, JHP ) 5.3 Hz,
3H), 1.12 (d, 2JHP ) 6.1 Hz, 3H). 13C{1H} NMR (CD2Cl2): δ 142.1
(dd, JPC ) 12 Hz,3JPC ) 7 Hz), 138.5 (dd, JPC ) 29 Hz, JPC
)
3
1
3
3 Hz), 137.5 (dd, 1JPC ) 31 Hz, 3JPC ) 3 Hz), 136.8 (dd, 1JPC ) 30
3
4
4
Hz, JPC ) 1 Hz), 134.5 (d, JPC ) 3 Hz), 134.4 (d, JPC ) 3 Hz),
1
3
3
134.4 (dd, JPC ) 31 Hz, JPC ) 2 Hz), 133.0 (d, JPC ) 10 Hz),
1
°C. The reaction was examined at intervals by H NMR spectros-
132.6 (d, 3JPC ) 10 Hz), 132.6 (d, 2JPC ) 14 Hz), 132.3 (d, 2JPC
)
2
2
copy using the following marker signals: (E)-1a (δ 2.07, PMe),
(E)-2a (δ 2.83, PMe), (E)-3a (δ 1.18, PMe), trans-
[PdBr2(PMePh2)2] (δ 2.24, PMe).
14 Hz), 131.7 (d, JPC ) 14 Hz), 131.2 (d, JPC ) 13 Hz), 130.7
(d, 4JPC ) 1 Hz), 130.3 (d, 2JPC ) 12 Hz), 130.2 (s), 130.1 (d, 2JPC
4
3
) 12 Hz), 129.6 (d, JPC ) 1 Hz), 129.4 (d, JPC ) 9 Hz), 129.4
(d, 3JPC ) 9 Hz), 129.2 (m), 128.8 (d, 3JPC ) 9 Hz), 128.7 (d, 3JPC
) 9 Hz), 126.2 (m), 125.2 (m), 124.6 (dd, 1JPC ) 89 Hz, 3JPC ) 2
The complex [Pd{η2-(E)-PhCHdCHPMePh2}(PMePh2)2]Br ((E)-
3a) was independently prepared from (E)-2a and PMePh2 (1 equiv)
in CD2Cl2 and characterized by NMR spectroscopy. 1H NMR
(CD2Cl2): δ 7.81-7.21, 7.20-7.05, 6.73-6.67, and 6.67-6.55 (m,
1
3
1
Hz), 124.4 (dd, JPC ) 83 Hz, JPC ) 5 Hz), 121.6 (q, JFC ) 321
2
2
1
Hz), 65.7 (dd, JPC ) 31 Hz, JPC ) 5 Hz), 34.0 (ddd, JPC ) 79
2
2
2
1
35H in total), 3.74-3.58 (m, 2H), 1.98 (d, JHP ) 13.1 Hz, 3H),
Hz, JPC ) 32 Hz, JPC ) 7 Hz), 15.1 (d, JPC ) 19 Hz), 13.0 (d,
1.59 (d, 2JHP ) 4.9 Hz, 3H), 1.12 (d, 2JHP ) 6.1 Hz, 3H). 31P{1H}
NMR (CD2Cl2): δ 21.4 (br), 4.5 (br), 4.2 (br). The formation of
(E)-3a took place instantly at room temperature, as already
documented for (E)-3b.12
1JPC ) 20 Hz), 11.6 (d, 1JPC ) 64 Hz). 31P{1H} NMR (CD2Cl2): δ
2
2
21.3 (d, JPP ) 12 Hz), 4.4 (s), 4.1 (d, JPP ) 12 Hz).
X-ray Structural Analysis. Single crystals of (E)-1a, (Z)-1a,
(E)-2a, and (Z)-2a were grown by slow diffusion of Et2O into
CH2Cl2 solutions at -20 °C ((E)-1a, (E)-2a, (Z)-2a) and -30 °C
((Z)-1a), respectively. The intensity data were collected on a Rigaku
Mercury CCD diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.71070 Å). The intensity data were collected at
173 K and corrected for Lorentz and polarization effects and for
absorption. The structures were solved by heavy atom Patterson
methods (PATTY), expanded using Fourier techniques
(DIRDIF99),19 and refined on F2 for all reflections (SHELXL-97).20
All non-hydrogen atoms were refined anisotropically. The H(1)
Synthesis and C-P Reductive Elimination of [Pd{CHd
CHPh-(E)}(PMePh2)3]OTf ((E)-5a). Complex (E)-1a (27.6 mg,
0.0400 mmol), PMePh2 (8.0 mg, 0.040 mmol), and 1,3,5-tri-
methoxybenzene (3.4 mg, 0.020 mmol; internal standard) were
dissolved in CD2Cl2 (0.75 mL), and AgOTf (10.3 mg, 0.400 mmol)
was added at -30 °C. The solution was stirred at this temperature
for 1 h to precipitate an off-white powder of AgBr. The orange
supernatant was transferred by cannulation to an NMR sample tube
and analyzed by 1H NMR spectroscopy at -30 °C, showing
the formation of (E)-5a (96%) and a small amount of [Pd{η2-(E)-
PhCHdCHPMePh2}(PMePh2)2]OTf ((E)-6a) (3%). At 40 °C, (E)-
5a was converted to (E)-6a within 4 min in 95% selectivity. Since
(E)-5a was thermally too unstable to be isolated, its characterization
was carried out by NMR spectroscopy.
(19) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garc´ıa-Grana, S.;
Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF99 Program System;
University of Nijmegen: The Netherlands, 1999.
(20) Sheldrick, G. M. SHELXL-97; University of Gottingen: Germany,
1997.

2534 Organometallics, Vol. 28, No. 8, 2009
Wakioka et al.
atom of (Z)-1a was located from a differential Fourier map and
refined isotropically. The other hydrogen atoms were placed using
AFIX instructions. Crystal data and details of data collection and
refinement are summarized in Table 5.
Computational Details. All calculations were performed using
the B3LYP level of density functional theory.21 The Pd atom
was described using the LANL2DZ basis set including a double-
basis set with the Hay and Wadt effective core potential (ECP).22
The 6-31G(d) basis set was used for other atoms.23 A diffuse
function was added for Br atoms. Frequency calculations were
carried out to identify all the stationary states as minima (zero
imaginary frequencies). All calculations in this study have been
performed without any symmetry constraints using Gaussian
03.24 The solvent effect was taken into account through single-
point calculations at each optimized geometry in vacuo using
the polarized continuum model (PCM) at 298.15 K.25 The partial
atomic charges were calculated on the basis of natural bond
orbital (NBO) analyses.26
(21) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (c) Becke, A. D.
J. Chem. Phys. 1993, 98, 5648–5652.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (Nos. 18064010 and 20350049)
from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan.
(22) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.
(23) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654–3665.
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03;
Gaussian, Inc.: Pittsburgh, PA, 2004.
Supporting Information Available: Tables with Cartesian
coordinates of the optimized structures; crystallographic data in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM801207K
(25) (a) Miertus, S.; Scrocco, E.; Tomasi, J. J. Chem. Phys. 1981, 55,
117–119. (b) Tomasi, J.; Persico, M. Chem. ReV. 1994, 94, 2027–2094. (c)
Tomasi, J.; Cammi, R. J. Comput. Chem. 1995, 16, 1449–1458.
(26) Glendening, E. D.; Read, A. E.; Carpenter, J. E.; Weinhold, F. NBO
(Version 3.1); Gaussian, Inc.: Pittsburgh, PA, 2004.