Stereoretentive elimination and trans-olefination of the dicationic dipalladium moiety [Pd2Ln]2+ bound on 1,3,5-trienes

Article abstract of DOI:10.1021/ja0576740

The reaction of [Pd₂(CH₃CN)₆][BF ₄]₂ (1) with 1,3,5-hexatriene, 1,6-diphenyl-1,3,5- hexatriene (DPHT), or 2,2,9,9-tetramethyl-3,5,7-decatriene (DBHT) afforded bi-η³-allyldipalladium complexes 3, 4, or 5. The reaction of 1 and DBHT proceeded in a stereospecific (syn) manner when the reaction was carried out in CD₂Cl₂ under aerobic conditions, while a mixture of two diastereomers was formed under N₂ atmosphere. The two diastereomers (5-E,Z,E-antifacial and 5-E,E,E-antifacial) formed from DBHT were isolated, and the structure of 5-E,Z,E-antifacial, which was kinetically formed from the reaction of 1 and (E,E,E)-DBHT, was determined by X-ray diffraction analysis. Addition of phosphine ligands (PPh₃ or dppm) to the dinuclear adduct 5-E,Z,E-antifacial or 5-E,E,E-antifacial in acetonitrile resulted in the stereospecific (syn) elimination of [Pd₂(PPh ₃)₂(CH₃CN)₄][BF₄] ₂ (2) or [Pd₂(dppm)₂(CH₃CN) ₂][BF₄]₂ (6). During the PPh ₃-induced dinuclear elimination, the phosphine adducts 7 that retain bi-η³-allyldipalladium structure were observed initially. The phosphine adduct generated from 5-E,E,E-antifacial was isolated and structurally characterized by X-ray diffraction analysis. The reaction of 1 and DPHT in CH₂Cl₂ afforded unique dipalladium sandwich compounds [Pd₂(μ-η³:η³-DPHT) ₂][BF₄]₂ (8). Interconversion between the sandwich complexes and half-sandwich complexes occurred in a stereoretentive manner. The structure of the sandwich complex 8-E,Z,E formed from 4-E,E,E-antifacial and (E,Z,E)-DPHT was determined by X-ray diffraction analysis. Transfer of the dipalladium moiety [Pd₂(CH ₃CN)₄]²⁺ from DPHT ligand of 4-E,E,E-antifacial onto DBHT ligand proceeded in a Stereoretentive manner. The observed stereoretentive dinuclear process is featured by the painwise behavior of two palladium atoms sitting on the triene π-plane. In the dinuclear elimination, the two Pd atoms that are initially in the divalent state and bound on the opposite faces (antifacial) come to the synfacial positions to form a Pd-Pd bond prior to dissociation. These results represent the unique property of conjugated olefin as the multidentate ligands for metal-metal moieties.

Full text of DOI:10.1021/ja0576740

Published on Web 03/10/2006
Stereoretentive Elimination and Trans-olefination of the
Dicationic Dipalladium Moiety [Pd2Ln]2+ Bound on
1,3,5-Trienes
Tetsuro Murahashi,* Hiromitsu Nakashima, Tomoki Nagai, Yukari Mino,
Taketoshi Okuno, M. Abdul Jalil, and Hideo Kurosawa*
Contribution from the Department of Applied Chemistry, Graduate School of Engineering,
Osaka UniVersity, and PRESTO, Japan Science and Technology Agency (JST),
Suita, Osaka, 565-0871, Japan
Received November 10, 2005; E-mail: tetsu@chem.eng.osaka-u.ac.jp; kurosawa@chem.eng.osaka-u.ac.jp
Abstract: The reaction of [Pd2(CH3CN)6][BF4]2 (1) with 1,3,5-hexatriene, 1,6-diphenyl-1,3,5-hexatriene
(DPHT), or 2,2,9,9-tetramethyl-3,5,7-decatriene (DBHT) afforded bi-η3-allyldipalladium complexes 3, 4, or
5. The reaction of 1 and DBHT proceeded in a stereospecific (syn) manner when the reaction was carried
out in CD2Cl2 under aerobic conditions, while a mixture of two diastereomers was formed under N2
atmosphere. The two diastereomers (5-E,Z,E-antifacial and 5-E,E,E-antifacial) formed from DBHT were
isolated, and the structure of 5-E,Z,E-antifacial, which was kinetically formed from the reaction of 1 and
(E,E,E)-DBHT, was determined by X-ray diffraction analysis. Addition of phosphine ligands (PPh3 or dppm)
to the dinuclear adduct 5-E,Z,E-antifacial or 5-E,E,E-antifacial in acetonitrile resulted in the stereospecific
(syn) elimination of [Pd2(PPh3)2(CH3CN)4][BF4]2 (2) or [Pd2(dppm)2(CH3CN)2][BF4]2 (6). During the PPh3-
induced dinuclear elimination, the phosphine adducts 7 that retain bi-η3-allyldipalladium structure were
observed initially. The phosphine adduct generated from 5-E,E,E-antifacial was isolated and structurally
characterized by X-ray diffraction analysis. The reaction of 1 and DPHT in CH2Cl2 afforded unique dipalladium
sandwich compounds [Pd2(µ-η3:η3-DPHT)2][BF4]2 (8). Interconversion between the sandwich complexes
and half-sandwich complexes occurred in a stereoretentive manner. The structure of the sandwich complex
8-E,Z,E formed from 4-E,E,E-antifacial and (E,Z,E)-DPHT was determined by X-ray diffraction analysis.
Transfer of the dipalladium moiety [Pd2(CH3CN)4]2+ from DPHT ligand of 4-E,E,E-antifacial onto DBHT
ligand proceeded in a stereoretentive manner. The observed stereoretentive dinuclear process is featured
by the pairwise behavior of two palladium atoms sitting on the triene π-plane. In the dinuclear elimination,
the two Pd atoms that are initially in the divalent state and bound on the opposite faces (antifacial) come
to the synfacial positions to form a Pd-Pd bond prior to dissociation. These results represent the unique
property of conjugated olefin as the multidentate ligands for metal-metal moieties.
Introduction
conjugated olefins. Historically, the interaction between metal-
metal bonded species and olefin has received considerable
Conjugated sp2-carbon frameworks act as unique π-coordi-
nating polydentate ligands for multinuclear metal moieties.1-7
We recently found that pπ-conjugated polyenes act as excellent
template ligands for a linear palladium chain.3-7 These findings
prompted us to examine the mechanism of addition, elimination,
and trans-olefination of a Pd-Pd bonded moiety to/from pπ-
attention as models for adsorption/desorption on metal sur-
faces.8,9 It is known that some dinuclear metal-metal species
add to conjugated olefin to afford “di-σ-bonded species” where
a metal-metal bond is cleaved.10-14 Typically, Mn2(CO)10 reacts
with 1,3-butadiene or 1,3,5-hexatriene under the irradiation
condition to form Mn2(µ-η3:η1-butadiene)(CO)9 or Mn2(µ-η3:
(1) Markezich, R. L.; Whitlock, H. W. J. Am. Chem. Soc. 1971, 93, 5291.
(2) (a) Wadepohl, H.; Bu¨chner, K.; Pritzkow, H. Organometallics 1989, 8,
2745. (b) Adams, R. D.; Wu, W. Organometallics 1993, 12, 1243. (c) Weng,
W.; Arif, A. M.; Ernst, R. D. Organometallics 1998, 17, 4240. (d) Mashima,
K.; Fukumoto, H.; Tani, K.; Haga, M.; Nakamura, A. Organometallics 1998,
17, 410. (e) Fukumoto, H.; Mashima, K. Organometallics 2005, 24, 3932.
(3) Murahashi, T.; Mochizuki, E.; Kai, Y.; Kurosawa, H. J. Am. Chem. Soc.
1999, 121, 10660.
(8) (a) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzner,
W. R. Chem. ReV. 1979, 79, 91. (b) Muetterties, E. L. Chem. Soc. ReV.
1982, 11, 283.
(9) For adsorption of olefin on Pd or Pt surface: (a) Cremer, P. S.; Su, X.;
Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942 and
references therein. (b) Szulczewski, G.; Levis, R. J. J. Am. Chem. Soc.
1996, 118, 3521. In this paper, it was proposed that desorption of di-σ-
bonded ethylene from Pt(111) proceeds via a radical intermediate.
(10) (a) Beck, W.; Niemer, B.; Wieser, M. Angew. Chem., Int. Ed. Engl. 1993,
32, 923 and references therein. (b) Casey, C. P.; Audett, J. D. Chem. ReV.
1986, 86, 339 and references therein.
(11) (a) Kreiter, C. G.; Lipps, W. Angew. Chem., Int. Ed. Engl. 1981, 20, 201.
(b) Kreiter, C. G.; Lipps, W. Chem. Ber. 1982, 115, 973. (c) Kreiter, C.
G.; Leyendecker, M. J. Organomet. Chem. 1985, 292, C18.
(12) Guggolz, E.; Oberdorfer, F.; Ziegler, M. L. Z. Naturforsch., B: Chem. Sci.
1981, 36, 1060.
(4) Murahashi, T.; Nagai, T.; Mino, Y.; Mochizuki, E.; Kai, Y.; Kurosawa, H.
J. Am. Chem. Soc. 2001, 123, 6927.
(5) Murahashi, T.; Higuchi, Y.; Katoh, T.; Kurosawa, H. J. Am. Chem. Soc.
2002, 124, 14288.
(6) Tatsumi, Y.; Nagai, T.; Nakashima, H.; Murahashi, T.; Kurosawa, H. Chem.
Commun. 2004, 1430.
(7) It was also discovered that perylene binds a tetrapalladium chain through
its tetraene edge: Murahashi, T.; Uemura, T.; Kurosawa, H. J. Am. Chem.
Soc. 2003, 125, 8437.
9
10.1021/ja0576740 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 4377-4388
4377

A R T I C L E S
Murahashi et al.
Scheme 1. Typical Examples of Dinuclear Addition, Elimination,
and Trans-olefination Involving Conjugated Olefins
reported that the Rh-Rh bonded species having rigid porphyrin
ligands adds to styrene via a radical chain process.18
However, no systematic study has been undertaken of the
dinuclear processes involving a pπ-conjugated olefin in which
the stereochemistry is well documented, probably due to the
lack of appropriate model systems. Involvement of concerted
processes such as the dimetalla-Diels-Alder pathway and the
migratory insertion pathway may result in highly stereospecific
dinuclear reactions, while stepwise processes such as radical
pathways should give rise to nonstereospecific reactions. It is
also known that several olefin-bridged dimetal complexes
without metal-metal bonds were synthesized by the reaction
of cationic mononuclear olefin complexes with metal nucleo-
philes.10 Typically, [Re(CO)5(η2-C4H6)]+ reacted with [Re(CO)5]-
to afford Re(CO)5(µ-η1:η1-C4H6)Re(CO)5.19 Although the ster-
eochemistry of such an ionic process has not been reported, a
net stereospecific dinuclear addition may be involved if anti-
attack of a metal nucleophile to the coordinated olefin takes
place.
Scheme 2. Stereochemistry of Some Dinuclear Processes
We recently developed the chemistry of Pd-Pd bonded
complexes [Pd2(CH3CN)6][BF4]2 (1)20 and [Pd2(CH3CN)4(PPh3)2]-
[BF4]2 (2),21 which are isolable but highly reactive with
unsaturated hydrocarbons under mild conditions,21-23 due to the
substitutionally labile property of acetonitrile ligands on di-
palladium centers. Furthermore, as the bridging ability of the
nitrile or PPh3 ligands is poor,22 the dinuclear processes
involving such Pd-Pd species might be free from strong
geometrical constraints imposed by bridging auxiliary ligands
often required to support M-M bonds in other complexes. In
hopes of better understanding the dinuclear processes of the
η3-hexatriene)(CO)8 (Scheme 1a).11 The mechanism of dinuclear
elimination of o-xylylene from the benzodicobaltacyclohexene
(Scheme 1b) was studied by Hersh and Bergman, and they
proposed the involvement of a retro-Diels-Alder-type dinuclear
elimination process.14 The dinuclear trans-olefination process
has also been studied.15 Typically, diosmacyclobutane reacts
with 1,3-dienes to afford formal [2+2] cycloaddition products
from OsdOs and the 1,3-diene with elimination of ethylene
(Scheme 1c).15b
The mechanistic studies have been conducted concerning the
dinuclear processes involving mono-olefin. Norton and co-
workers revealed that the dinuclear trans-olefination process
involving diosmacyclobutane and mono-olefin proceeds in a
stereospecific (syn) manner (Scheme 2a), and they proposed a
stepwise mechanism involving preequilibrium between
µ-η1:η1-olefin and η2-olefin complexes.16 The dinuclear pro-
cesses are not always stereoretentive. Johnson and Gladfelter
reported that a Ru2 center in Ru2(dmpm)2(CO)5 (dmpm ) Me2-
PCH2PMe2) adds to mono-olefin in a nonstereospecific manner
(Scheme 2b).17 It should be mentioned that Halpern et al.
2+
Pd2 moiety embedded on conjugated olefin, we started
examining the addition, elimination, and trans-olefination of the
[Pd2Ln]2+ moiety.
The syn and anti dinuclear additions to terminally di-
substituted conjugated olefin can lead to different diastereomers.
We elected 1,6-disubstituted 1,3,5-trienes as the conjugated
olefin, which would give the addition products as stable bi-η3-
allyl dimetal complexes as found in Scheme 1a.11 In Scheme
3, the two series of stereoisomers of bi-allyl complexes resulting
from hypothetical suprafacial dinuclear addition to 1,6-disub-
stituted 1,3,5-hexatriene are summarized, where fast equilibria
via π-σ-π isomerization at the η3-allylpalladium moiety24 are
assumed to be attained. For example, the trienes having odd
numbers of olefinic E double bonds give one series of the
products different from another derived from the trienes having
even numbers of E double bonds (including zero E). Intercross-
(17) Johnson, K. A.; Gladfelter, W. L. Organometallics 1991, 10, 376.
(18) (a) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985,
107, 4333. (b) Wayland, B. B.; Feng, Y.; Ba, S. Organometallics 1989, 8,
1438.
(13) (a) Chisholm, M. H.; Huffman, J. C.; Lucas, E. A.; Lubkovsky, E. B.
Organometallics 1991, 10, 3424. (b) Barry, J. T.; Bollinger, J. C.; Chisholm,
M. H.; Glasgow, K. C.; Huffman, J. C.; Lucas, E. A.; Lubkovsky, E. B.;
Streib, W. E. Orgaometallics 1999, 18, 2300.
(14) (a) Hersh, W. H.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 6992. (b)
Hersh, W. H.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1983,
105, 5834. (c) Hersh, W. H.; Bergman, R. G. J. Am. Chem. Soc. 1983,
105, 5846.
(19) Beck, W.; Raab, K.; Nagel, U.; Sacher, W. Angew. Chem., Int. Ed. Engl.
1985, 24, 505.
(20) Murahashi, T.; Nagai, T.; Okuno, T.; Matsutani, T.; Kurosawa, H. Chem.
Commun. 2000, 1689.
(21) Murahashi, T.; Otani, T.; Mochizuki, E.; Kai, Y.; Kurosawa, H. J. Am.
Chem. Soc. 1998, 120, 4536.
(22) Murahashi, T.; Otani, T.; Okuno, T.; Kurosawa, H. Angew. Chem., Int.
Ed. 2000, 39, 537.
(15) (a) Takats, J. Polyhedron 1988, 931. (b) Spetseris, N.; Norton, J. R.; Rithner,
C. D. Organometallics 1995, 14, 603. A related trans-dimetalation involving
1,3-diene was reported in ref 14c.
(23) Murahashi, T.; Okuno, T.; Nagai, T.; Kurosawa, H. Organometallics 2002,
21, 3679.
(16) (a) Hembre, R. T.; Scott, C. P.; Norton, J. R. J. Am. Chem. Soc. 1987,
109, 3468. (b) Hembre, R. T.; Ramage, D. L.; Scott, C. P.; Norton, J. R.
Organometallics 1994, 13, 2995. (c) Ramage, D. L.; Wiser, D. C.; Norton,
J. R. J. Am. Chem. Soc. 1997, 119, 5618. (d) Bender, B. R.; Ramage, D.
L.; Norton, J. R.; Wiser, D. C.; Rappe´, A. K. J. Am. Chem. Soc. 1997,
119, 5628.
(24) π-σ-π-isomerism of allylpalladium complexes: (a) Vrieze, K.; MacLean,
C.; Cosse, P.; Hilbers, C. W. Recl. TraV. Chim. Pays-Bas 1966, 85, 1077.
(b) Vrieze, K.; Volger, H. C. J. Organomet. Chem. 1967, 9, 537. (c) van
Leeuwen, P. W. N. M.; Vrieze, K.; Praat, A. P. J. Organomet. Chem. 1969,
20, 219. (d) van Leeuwen, P. W. N. M.; Praat, A. P. J. Organomet. Chem.
1970, 21, 501. (e) Powell, J.; Shaw, B. L. J. Chem. Soc. A 1967, 1839.
9
4378 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

+
Trans-olefination of the Dipalladium Moiety [Pd2Ln]2
A R T I C L E S
Scheme 3. Two Series from Suprafacial Dinuclear Addition to
1,6-Disubstituted 1,3,5-Hexatrienes
Scheme 5. Intra-series Products from s-cis or s-trans Conformers
of (E,E,E)-Triene through Suprafacial Dinuclear Addition
Scheme 6
Scheme 4
ing across the two series cannot be attained by conventional
π-σ-π allylic isomerization.24,25 Thus, analysis of the product
distribution of each series from an “odd E triene” or an “even
E triene” would provide stereochemical information on the
dinuclear processes involving 1,3,5-triene (Scheme 4).26
It is noticeable that the type of conformation, s-cis or s-trans,
which a given 1,3,5-triene takes at any C-C single bond, does
not affect the nature of the product series. For example, three
conformers arising from s-cis/s-trans conformations of 1,6-
disubstituted (E,E,E)-triene, that is, s-trans,s-trans triene,
s-cis,s-trans triene, and s-cis,s-cis triene, give one series of
products upon suprafacial dinuclear addition (Scheme 5).
In a previous communication,4 we reported that dinuclear
addition of the complex 1 to 1,6-diphenyl-1,3,5-hexatriene
(DPHT) afforded a mixture of isomers, each included in the
different series. However, it was difficult to estimate the
diastereoselectivity of the dinuclear addition step, as will be
mentioned in section 1 in this Article, because the inter-series
isomerization between two diastereomers proceeded competi-
tively with the dinuclear addition reaction. As mentioned above,
the intercrossing between two series cannot be explained by
π-σ-π allylic isomerization. The configurational lability of
the η3-allyl moiety25 during the dinuclear addition reaction using
DPHT hampers the stereochemical investigation of the dinuclear
processes as well as isolation of the thermally unstable isomer.
The stereochemical study of the reverse reaction, dinuclear
elimination, may also provide valuable information of the
dinuclear processes (Scheme 6),26 if the dinuclear elimination
process is much faster than the isomerization causing inter-
crossing between the series. The stereochemical information on
the dinuclear trans-olefination process is also complementary
to the dinuclear addition and elimination processes (Scheme 6).26
For such studies, it is necessary to isolate the two series of
products, separately.
In this contribution, we report successful isolation of two
series of products by employing 2,2,9,9-tetramethyl-3,5,7-
decatriene (DBHT).27 It is proven that the dinuclear elimination
and transfer of dipalladium moieties bound on trienes is highly
stereospecific (syn). Furthermore, details of unique sandwich
coordination of DPHT to a dipalladium center [Pd2]2+ are
reported.
Results and Discussion
1.1. Dinuclear Addition Reactions Involving [Pd2(CH3CN)6]-
[BF4]2 (1) or [Pd2(µ-PPh3)2(CH3CN)2][PF6]2 (2′′) and 1,3,5-
Hexatriene (HT). The homoleptic acetonitrile dipalladium(I)
complex [Pd2(CH3CN)6][BF4]2 (1) was prepared according to
the reported method.20 The structure of 1 was determined here
by X-ray structure analysis (Figure 1). The Pd-Pd bond (2.486-
(1) Å) is the shortest among ever known PdI-PdI bonds.28 The
Pd-N distances (2.165(9), 2.147(9) Å) trans to the Pd-Pd bond
are longer than those (1.976(8), 2.001(8), 1.988(8), 1.987(8)
(25) (a) Kurosawa, H.; Ogoshi, S.; Chatani, N.; Kawasaki, Y.; Murai, S. Chem.
Lett. 1990, 1745. (b) Granberg, K. L.; Ba¨ckvall, J. J. Am. Chem. Soc. 1992,
114, 6858.
(26) The half-sandwich type, bi-η3-allyl dipalladium complexes can take two
conformations where two Pd atoms are located either on the same face or
opposite faces of the triene plane, with the latter being normally more stable.
(27) In this Article, 2,2,9,9-tetramethyl-3,5,7-decatriene is abbreviated as DBHT,
which is derived from a conventional name “1,6-di-t-Bu-1,3,5-hexatriene”.
(28) Murahashi, T.; Kurosawa, H. Coord. Chem. ReV. 2002, 231, 207.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4379

A R T I C L E S
Murahashi et al.
Figure 3. ORTEP drawings of 4-E,E,E-antifacial (left) and 5-E,Z,E-
antifacial (right) (50% probability ellipsoids, BF4 anions were omitted for
clarity). Selected bond lengths (Å) for 4-E,E,E-antifacial, Pd1-C1
2.132(4), Pd1-C2 2.129(4), Pd1-C3 2.128(4), C1-C2 1.393(5), C2-C3
1.411(5), C3-C3* 1.474(7); for 5-E,Z,E-antifacial, Pd1-C1 2.159(8),
Pd1-C2 2.111(9), Pd1-C3 2.121(8), Pd2-C4 2.113(9), Pd2-C5
2.111(9), Pd2-C6 2.179(9), C1-C2 1.40(1), C2-C3 1.39(1), C3-C4
1.49(1), C4-C5 1.39(1), C5-C6 1.42(1).
Figure 1. ORTEP drawing of 1 (50% probability ellipsoids, BF4 anions
were omitted for clarity). Selected bond distances (Å) and angles (deg):
Pd1-Pd2 2.486(1), Pd1-N1 2.165(9), Pd2-N2 2.147(9), Pd1-N3
1.976(8), Pd1-N4 2.001(8), Pd2-N5 1.988(8), Pd2-N6 1.987(8), Pd1-
Pd2-N6 86.4(2), Pd1-Pd2-N5 87.7(2), Pd1-Pd2-N2 174.7(2), Pd2-
Pd1-N3 84.4(2), Pd2-Pd1-N4 86.6(2), Pd2-Pd1-N1 178.1(2).
mixture of E-1,3,5-hexatriene and Z-1,3,5-hexatriene is under-
stood by the involvement of rapid allylic π-σ-π isomerization.
When the isolated 3b was dissolved in CD3CN, partial dinuclear
elimination involving dissociation of free 1,3,5-hexatriene from
3b was observed,30 where [Pd2(PPh3)2(CD3CN)4][PF6]2 (2′)21
was formed (37% after 4 h).
Figure 2. ORTEP drawing of 3b (50% probablity ellipsoids, PF6 anions
were omitted for clarity). Selected bond distances (Å): Pd1-N1 2.067(9),
Pd1-P1 2.305(2).
Å) cis to the Pd-Pd bond, probably due to a strong trans-
influence of the Pd-Pd bond relative to that of acetonitrile
ligand. The Pd-N lengths at the cis positions to the Pd-Pd
bond are similar to those in the corresponding homoleptic
mononuclear Pd(II) complex [Pd(CH3CN)4][BF4]2 (1.956(8)
Å).29
1.2. Dinuclear Addition Reactions Involving [Pd2(CH3CN)6]-
[BF4]2 (1) and 1,6-Diphenyl-1,3,5-hexatriene (DPHT). As was
previously communicated,4 the homoleptic acetonitrile di-
palladium(I) complex [Pd2(CH3CN)6][BF4]2 (1) reacted with all-
trans-1,6-diphenyl-1,3,5-hexatriene ((E,E,E)-DPHT) in CH3CN
at 23 °C to afford a half-sandwich complex 4 as a mixture of
two diastereomers (4-E,Z,E-antifacial/4-E,E,E-antifacial ) 22/
78 after crystallization) (eq 3).4 The 1H NMR monitoring
experiments showed that the isomer ratio changed gradually
(54/46 after 10 min, 46/54 after 1 h, 20/80 after 1 day). The
major isomer of 4 was isolated by recrystallization, and its
structure was determined by X-ray crystallographic analysis
(Figure 3, left).4 Two Pd atoms coordinated on opposite faces
of the (E,E,E)-DPHT ligand, and the major isomer is abbreviated
as 4-E,E,E-antifacial. The η3-coordination mode of each Pd
atom as well as the single bond character of the central C3-
C3* bond length (1.474(7) Å) indicated that the oxidation state
of the dipalladium moiety in 4-E,E,E-antifacial is 2 × PdII.
1H and 13C{1H} NMR spectra of 4-E,E,E-antifacial (Scheme
Complex 1 reacted with 1,3,5-hexatriene (HT, a mixture of
E- and Z-isomers) in acetonitrile at 25 °C for 1 h to afford a
half-sandwich complex [Pd2(µ-η3:η3-hexatriene)(CH3CN)4]-
[BF4]2 (3a) in 75% isolated yield (eq 1). The 1H NMR
monitoring experiments in CD3CN showed that complex 3a was
formed almost quantitatively. The similar product [Pd2(µ-η3:
η3-hexatriene)(CH3CN)2(PPh3)2][PF6]2 (3b) was obtained by the
reaction of [Pd2(µ-PPh3)2(CH3CN)2][PF6]2 (2′′)22 with 1,3,5-
hexatriene (a mixture of isomers) in CH2Cl2 (eq 2, 52% isolated
yield), and its structure was determined by X-ray structure
analysis (Figure 2). The two palladium atoms are bound on
opposite faces of the E-s-trans,s-trans-1,3,5-hexatriene ligand
through η3-allyl coordination mode. Because of the disorder of
the hexatriene ligand, it is difficult to discuss the structural
parameters with regard to the coordination of 1,3,5-hexatriene
ligand in 3b. The formation of the identical product from a
(29) Gebauer, T.; Frenzen, G.; Dehnicke, K. Z. Naturforsch., B: Chem. Sci.
1992, 47, 1505.
(30) The E/Z ratio of the released 1,3,5-hexatriene could not be determined due
to the overlap of those signals with the broad signals of 3b.
9
4380 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

+
Trans-olefination of the Dipalladium Moiety [Pd2Ln]2
A R T I C L E S
Scheme 7. 1H and 13C{1H} NMR Chemical Shifts (ppm) for
4-E,E,E-Antifacial and 8-E,E,E-Mesoa
a (a)Measured in CD3CN. (b) Measured in CD2Cl2.
7, left) are consistent with the bi-η3-allyl palladium(II) structure.
The allylic central carbons were less shielded than the allylic
terminal carbons as typically observed for mononuclear η3-allyl
palladium(II) complexes. The minor isomer showed 1H and 13
C
NMR signal patterns similar to those of 4-E,E,E-antifacial. The
structure of the minor isomer is assumed as 4-E,Z,E-antifacial,
by referring to the crystal structure of 5-E,Z,E-antifacial (Figure
3, right) derived from DBHT (vide infra). The reaction of 1
with (E,Z,E)-DPHT in CD3CN also resulted in the formation
of a mixture of 4-E,Z,E-antifacial and 4-E,E,E-antifacial
(38/62 after 1 h, 16/84 after 1 day at 25 °C). Unfortunately,
separation and isolation of 4-E,Z,E-antifacial failed, due in part
to the slow isomerization of 4-E,Z,E-antifacial to 4-E,E,E-
antifacial at ambient temperature, as well as the poor solubility
of DPHT in acetonitrile at -40 °C that caused low conversion
in the low-temperature experiments.
Figure 4. ORTEP drawing of 7-E,E,E-antifacial (50% probability
ellipsoids, BF4 anions were omitted for clarity). Selected bond distances
(Å): Pd1-N1 2.05(1), Pd1-P1 2.336(4), Pd2-N2 2.08(1), Pd2-P2
2.334(4).
Scheme 8 a
1.3. Dinuclear Addition Reactions Involving [Pd2(CH3CN)6]-
[BF4]2 (1) and 2,2,9,9-Tetramethyl-3,5,7-decatriene (DBHT).
Next, we chose 2,2,9,9-tetramethyl-3,5,7-decatriene (DBHT)27
as the triene ligand. It was found that the isomer ratio of products
was dependent on the reaction conditions including solvent and
reaction atmosphere.
In acetonitrile, the reaction of 1 with (E,E,E)-DBHT (1 equiv)
or (E,Z,E)-DBHT gave a mixture of two isomers, 5-E,Z,E-
antifacial and 5-E,E,E-antifacial (isomer ratio ) 76/24 from
(E,E,E)-DBHT, 85% conversion after 5 min; 56/44 from
(E,Z,E)-DBHT, 84% conversion after 5 min). The stereochem-
istry of these products was determined by X-ray crystallography
on 5-E,Z,E-antifacial (Figure 3, right) and the PPh3 adduct of
5-E,E,E-antifacial (Figure 4), as discussed later.
In CD2Cl2 under N2, a mixture of two isomers was formed
again (80/20 from (E,E,E)-DBHT, 69% conversion after 5 min;
61/39 from (E,Z,E)-DBHT, 49% conversion after 5 min)
(Scheme 8). However, when each reaction in CD2Cl2 was carried
out under aerobic conditions, the adduct was formed as the
single isomer. From (E,E,E)-DBHT, 5-E,Z,E-antifacial was
formed quantitatively after 1 day (>99/<1, 51% conversion after
5 min) (Scheme 8). From (E,Z,E)-DBHT, 5-E,E,E-antifacial
was formed exclusively after 5 min (<1/>99, 26% conversion),
while considerable mixing of the two isomers was observed
after 1 day (77/23, 44% conversion).
a (a) Under aerobic conditions.
right. The coordination mode of the triene is µ-η3:η3, and the
trend of C-C bond lengths for the triene part is similar to that
in 4-E,E,E-antifacial. However, the configuration around the
C3-C4 bond (cisoid) in 5-E,Z,E-antifacial is different from
that of 4-E,E,E-antifacial (transoid).
5-E,E,E-Antifacial was isolated by stirring the mixture of
5-E,Z,E-antifacial and free (E,E,E)-DBHT in CH2Cl2 overnight,
followed by recrystallization (54% yield). The isomerization
shown in eq 4 did not occur in the absence of free (E,E,E)-
DBHT, while the role of (E,E,E)-DBHT in eq 4 remains
elusive.31 The isolated 5-E,E,E-antifacial did not undergo
isomerization even in the presence of excess (E,E,E)-DBHT.
The product 5-E,Z,E-antifacial, which was exclusively
formed from (E,E,E)-DBHT under aerobic conditions, was
isolated in 76% yield, and its structure is shown in Figure 3,
Thus, two diastereomers derived from different series shown
in Scheme 3 were successfully isolated as 5-E,Z,E-antifacial
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4381

A R T I C L E S
Murahashi et al.
Scheme 9. PPh3-Induced Dinuclear Elimination from
5-E,Z,E-Antifacial
and 5-E,E,E-antifacial. The stereospecific nature of the di-
nuclear addition process involving 1 and DBHT was observed
only in the initial stage of the reactions carried out under aerobic
conditions in CD2Cl2. It may well be that the dinuclear addition
process is intrinsically stereospecific, but is competing with
some other processes leading to the formation of the opposite
series of products. The competitive occurrence of inter-series
isomerization during the dinuclear addition process particularly
under N2 atmosphere is probably induced by some palladium
species, for example, Pd0, which could be generated by
decomposition of reagent and product complexes.32a It is known
that the cationic η3-allylpalladium complex is subjected to
nucleophilic anti-attack of Pd0, which gives rise to the configu-
rational lability of η3-allylpalladium complexes (eq 5).25 Similar
anti-attack of a Pd0 species at 5 may result in the inter-series
isomerization, where a catalytic amount of Pd0 species works
effectively under inert condition. At present, involvement of
inter-series isomerization makes the evaluation of the stereo-
chemistry of the dinuclear addition process involving 1 and
1,3,5-trienes difficult.32b
treated with dppm (2 equiv) in the same condition, complex 6
and (E,E,E)-DBHT (>97%) were formed, where trace (E,Z,E)-
DBHT (<3%) was observed (eq 7).
When 5-E,Z,E-antifacial was treated with 2 equiv of PPh3
in CD3CN at 5-10 °C, a Pd-Pd bonded complex 2 and (E,E,E)-
DBHT were formed quantitatively (Scheme 9). The reaction
was monitored by 1H and 31P{1H} NMR spectroscopy at 5-10
°C. After 5 min, the starting materials disappeared and free
(E,E,E)-DBHT (45%), the Pd-Pd bonded complex 2 (44%),
and phosphine adduct 7-E,Z,E-antifacial (55%) were observed.
The phosphine adduct 7-E,Z,E-antifacial was then gradually
converted to free (E,E,E)-DBHT and 2, and the yields of
(E,E,E)-DBHT and 2 reached 97% and 96%, respectively, after
1 day, where a small amount of the phosphine adduct (3%)
remained. During the reaction, the formation of (E,Z,E)-DBHT
was not observed.
2. Phosphine-Induced Dinuclear Elimination from DBHT
Half-Sandwich Complexes. From the viewpoint of the micro-
scopic reversibility principle, elimination of a Pd-Pd bonded
moiety from bi-η3-allyl dipalladium(II) complexes is of great
interest in relation to the dinuclear addition reactions discussed
above. The major problem of the dinuclear addition processes
is that the rate of dinuclear addition is competitive with that of
inter-series isomerization particularly under Ν2 atmosphere.
However, as described in the Introduction, the stereochemical
study of the reverse reaction, dinuclear elimination, may provide
valuable information of the dinuclear processes, if the dinuclear
elimination is much faster than the inter-series isomerization.
We found that dinuclear elimination reactions from 5-E,Z,E-
antifacial or 5-E,E,E-antifacial occurred rapidly by adding
phosphines. Note that the choice of the solvent (CH3CN) was
important to the dinuclear elimination.
Treatment of 5-E,E,E-antifacial with 2 equiv of dppm (Ph2-
PCH2PPh2) in CD3CN at 0 °C afforded (E,Z,E)-DBHT and
[Pd2(dppm)2(CH3CN)2][BF4]2 (6)33 almost quantitatively after
10 min (eq 6). According to this reaction, (E,Z,E)-DBHT was
isolated in 81% yield. Thus, a thermodynamically unfavorable
isomerization from (E,E,E)-DBHT to (E,Z,E)-DBHT was at-
tained through the sequence of dinuclear addition of 1 to
(E,E,E)-DBHT to give 5-E,Z,E-antifacial, isomerization of
5-E,Z,E-antifacial to 5-E,E,E-antifacial, and dinuclear elimina-
tion from 5-E,E,E-antifacial. When 5-E,Z,E-antifacial was
When the same reaction was carried out at -40 °C, the
phosphine adduct 7-E,Z,E-antifacial was generated almost
quantitatively, and 7-E,Z,E-antifacial remained as the sole
species after warming the sample to 0 °C. 7-E,Z,E-Antifacial
existed as a mixture of two isomers (major/minor ) 95/5). The
31P{1H} NMR signal for the major isomer was observed at δ
1
20.6 ppm as a broad singlet at -40 °C. In H NMR spectra,
the triene protons for the major isomer appeared as three signals
at -40 °C, and the chemical shift pattern is similar to those
found in 5. Thus, it is reasonably assumed that the major isomer
retains the µ-η3:η3-bi-allyl structure, as drawn in Scheme 9. On
the other hand, the concentration of the minor isomer was too
low to lead to structural estimation. However, upon warming
the sample containing both isomers of the PPh3 adduct to 25
°C, free (E,E,E)-DBHT and 2 were gradually formed. Thus,
both isomers of 7-E,Z,E-antifacial would retain the stereogenic
identity of 5-E,Z,E-antifacial.
(31) The involvement of trans-olefination, which is described in section 4 in
this Article, does not result in such isomerization.
(32) (a) One reviewer suggested a possibility of the involvement of some
transient adduct between 1 and oxygen or water responsible for the
stereospecific dinuclear addition under aerobic conditions. (b) Another
transient active species Pd2+ may cause the isomerization32c of free (E,Z,E)-
DBHT to free (E,E,E)-DBHT during the dinuclear addition process,
resulting in slow formation of 5-E,Z,E-antifacial in the reaction of 1 and
(E,Z,E)-DBHT. (c) Maitlis, P. M. The Organic Chemistry of Palladium;
Academic Press: New York, 1971; Vol. 1, p 179, Vol. 2, p 111.
(33) Miedaner, A.; DuBois, D. L. Inorg. Chem. 1988, 27, 2479.
Treatment of 5-E,E,E-antifacial with 2 equiv of PPh3 in CD3-
CN at 5 °C resulted in the formation of two phosphine adducts,
7-E,E,E-antifacial and a structurally unspecified minor product
(7-E,E,E-antifacial/minor ) 78/22 in CD3CN at -40 °C, almost
quantitative after 5 min) (Scheme 10). The formation of a small
amount of 2 (2%) and free (E,Z,E)-DBHT (7%) was observed.
9
4382 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

+
Trans-olefination of the Dipalladium Moiety [Pd2Ln]2
A R T I C L E S
Scheme 10. PPh3-Induced Dinuclear Elimination from
5-E,E,E-Antifacial
Scheme 11. Three Possible Pathways for Dinuclear Elimination
from E,E,E-Antifacial Complexes
The phosphine adducts then decreased gradually. After 3 days,
the ratio of the phosphine adducts (7-E,E,E-antifacial + the
minor product), the Pd-Pd complex 2, and free (E,Z,E)-DBHT
reached 1:1:1, and the ratio did not change for 15 days.
7-E,E,E-Antifacial was isolated and structurally characterized
by X-ray structure analysis (Figure 4). Although the triene part
was considerably disordered, the configuration of the triene
moiety in the complex is reasonably concluded as E,E,E by
comparing the distance between quaternary carbons of t-Bu
groups (8.83 Å) with that in 5-E,Z,E-antifacial (7.89 Å) and
with the distance between ipso-carbons of Ph groups in 4-E,E,E-
antifacial (8.82 Å). When crystalline 7-E,E,E-antifacial was
dissolved in CD2Cl2, 1H and 31P{1H} NMR spectra showed the
presence of two species. The major set of the resonances was
consistent with the structure found in the crystalline state; three
trienyl proton signals and one phosphorus singlet were observed.
On the other hand, the minor isomer showed six trienyl protons
and two phosphorus doublet (JPP ) 13 Hz) signals. While the
structure of the minor isomer cannot be decisively assumed from
the NMR data,34 the stereogenic identity of the triene in
7-E,E,E-antifacial is retained in the minor isomer because
warming the sample of a mixture of 7-E,E,E-antifacial and
the minor isomer in CD3CN resulted in the selective conversion
to (E,Z,E)-DBHT and 2, as mentioned above. In CD2Cl2,
7-E,E,E-antifacial and the minor isomer did not undergo
dinuclear elimination at 25 °C.
When the complex 2 and free (E,Z,E)-DBHT were dissolved
in CD3CN, gradual formation of 7-E,E,E-antifacial and the
minor isomer was observed and the ratio reached (7-E,E,E-
antifacial + minor isomer):2:DBHT ) 1:1:1 after 1 day. In
this case, however, trace free (E,E,E)-DBHT was observed (3%).
Thus, the dinuclear elimination from the phosphine adducts
7-E,E,E-antifacial is reversible. On the other hand, no reaction
was observed when the mixture of compound 2 and free (E,E,E)-
DBHT in CD3CN was left for 1 day.
Thus, it is concluded that phosphine-induced dinuclear
elimination from the half-sandwich DBHT dipalladium(II)
complexes 5 is highly stereospecific. The observed stereochem-
istry is reasonably understood as the suprafacial (syn) elimina-
tion of the dipalladium(I) moiety from DBHT. The 180° rotation
at the central C-C bond of the DBHT ligand in 7-E,Z,E-
antifacial or 7-E,E,E-antifacial would give 7-E,E,E-synfacial
or 7-E,Z,E-synfacial, respectively (Scheme 11). In the synfacial
complexes, two Pd atoms are located at the juxtaposition with
each other, allowing the Pd-Pd bond formation. As summarized
in Scheme 11, there are three possible pathways for the
stereoretentive elimination from the synfacial complexes: (a)
retro-migratory insertion, (b) retro-dimetalla-[4π+2σ] reaction,
and (c) retro-dimetalla-[6π+2σ] reaction. However, it is difficult
to discriminate these three pathways on the basis of the
experimental observations. It is noticeable that µ-η2:η2-1,3-diene
dipalladium complexes that are related to the intermediate E in
the retro-dimetalla-[4π+2σ] reaction are known.21,35,36 Of
particular interest is the structure of the bis-1,3-butadiene
dipalladium(I) complex [Pd2(µ-η2:η2-1,3-butadiene)2(PPh3)2]-
[PF6]2, which shows an exceptionally long Pd-Pd length
(3.1852(6) Å).21 The long Pd-Pd separation may be regarded
as the result of trapping of an intermediate of the dimetalla-
[4π+2σ] process. The isolobal analogy between the dimetalla-
[4π+2σ] reaction and the parent [4π+2π] Diels-Alder reaction
can be understood qualitatively by considering the symmetry
of frontier orbitals as shown in Scheme 12. The related retro-
dimetalla-[4π+2π] process was proposed to be involved in the
o-xylylene elimination from [Co2(o-xylylene)Cp2(µ-CO)2]
(Scheme 2a).14
(34) The minor product showed six trienyl protons and two phosphorus doublet
(JPP ) 13 Hz) signals. The JH2-H3 coupling constant (7.2 Hz) indicated the
cis configuration at the C2-C3 bond in the minor product. It is also notable
that the carbons of both t-Bu groups were J-coupled with phosphorus centers
(JCP ) 3.8 Hz, JCP ) 3.8 Hz for methyl carbons, JCP ) 3.1 Hz, JCP ) 3.8
Hz for quaternary carbons). We assume a Pd-Pd bonded structure shown
below as the minor product.
The observed stereospecific dipalladium elimination from
DBHT is featured by the pairwise behavior of two palladium
(35) (a) Leoni, P.; Pasquali, M.; Sommovigo, M.; Albinati, A.; Lianza, F.;
Pregosin, P. S.; Ru¨egger, H. Organometallics 1993, 12, 4503. (b) Leoni,
P.; Pasquali, M.; Sommovigo, M.; Albinati, A.; Pregosin, P. S.; Ru¨egger,
H. Organometallics 1996, 15, 2047.
(36) Murahashi, T.; Kanehisa, N.; Kai, Y.; Otani, T.; Kurosawa, H. Chem.
Commun. 1996, 825.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4383

A R T I C L E S
Murahashi et al.
Scheme 12. Isolobal Analogy between the Dimetalla-[4π+2σ]
Process and the [4π+2π] Diels-Alder Process
antifacial, the trend of C-C bond lengths in each triene part
was quite different between the two complexes; the central C-C
bond length (C3-C4* 1.395(5) Å or C9-C10* 1.403(5) Å) in
8-E,E,E-meso is significantly shorter than that in 4-E,E,E-
antifacial (1.474(7) Å). The 13C{1H} NMR signal pattern of
the triene part in 8-E,E,E-meso was also considerably different
from that in 4-E,E,E-antifacial (Scheme 7). While 4-E,E,E-
antifacial showed 13C chemical shifts of the allylic central
carbons at lower field than those of allylic terminal carbons as
mentioned in section 1.2, the chemical shift of the allylic
terminal carbons in 8-E,E,E-meso appeared at the field similar
to or lower than that of the allylic central carbons (Scheme 7).
Thus, the structural and spectroscopic aspects give little support
to bi-η3-allyl dianion character in 8-E,E,E-meso and suggest
the considerable contribution of the electronic structure involving
less-reduced DPHT ligands and less-oxidized Pd-Pd moiety
in 8-E,E,E-meso.38
atoms sitting on the triene π-plane. The two Pd moieties in 5
are initially in the divalent state, and there is no Pd-Pd bond
between them. When those are subjected to a condition for
dissociation from DBHT, two palladium atoms come to the
synfacial positions to form a Pd-Pd bond prior to dissociation.
3.1. Formation of Sandwich Dipalladium Complexes of
1,6-Diphenyl-1,3,5-hexatriene (DPHT). When complex 1 was
treated with (E,E,E)-DPHT (excess) in dichloromethane, the
sandwich complex 8-E,E,E-meso was formed almost quanti-
tatively (eq 8).4 Although two isomers are possible for the
complex 8-E,E,E depending on the stacking manner of the two
zigzag (E,E,E)-DPHT ligands, only meso isomer was observed
throughout the monitoring experiments.37 The complex 8-E,E,E-
meso was isolated in 77% yield after recrystallization.
As mentioned in section 1.3, the reaction of 1 with DBHT
(excess) in CD2Cl2 afforded no sandwich complex but half-
sandwich complexes 5. Probably, stacking of two DBHT planes
required for formation of sandwich complexes is hampered by
the steric congestion between t-Bu groups.
The structure of 8-E,E,E-meso was determined by X-ray
structure analysis (Figure 5).4 Two independent molecules were
3.2. Interconversion between Half-Sandwich and Sand-
wich Complexes of 1,6-Diphenyl-1,3,5-hexatriene (DPHT).
When the sandwich complex 8-E,E,E-meso was dissolved in
CD3CN at -40 °C, one of the (E,E,E)-DPHT ligands was
replaced by four acetonitrile ligands to give the half-sandwich
complex 4-E,Z,E-antifacial quantitatively.4 Warming the sample
to 25 °C then resulted in the isomerization of 4-E,Z,E-antifacial
to a mixture of 4-E,Z,E-antifacial/4-E,E,E-antifacial (22/78)
within 1 day in CD3CN. When a solid sample of 4-E,Z,E-
antifacial obtained by evaporation of its CD3CN solution at
-40 °C was treated with (E,E,E)-DPHT (5 equiv) in CD2Cl2,
the four acetonitrile ligands were immediately replaced by
(E,E,E)-DPHT to generate the sandwich complex 8-E,E,E-rac
as a kinetic product.39 Ultimately, 8-E,E,E-rac isomerized to
8-E,E,E-meso within 3 h at 23 °C (8-E,E,E-rac/8-E,E,E-meso
) 2/98) (Scheme 13). Thus, the interconversion between half-
sandwich and sandwich complexes occurred smoothly.
Treatment of 4-E,E,E-antifacial with (E,Z,E)-DPHT (1
equiv) in CD2Cl2 gave a new sandwich complex 8-E,Z,E as a
single product, although the reaction was slow (13% conversion
after 5 min) probably due to the poor solubility of 4-E,E,E-
antifacial in CD2Cl2. After 0.5 h, a mixture containing 8-E,Z,E
(17%), 8-E,E,E-meso (21%), and 8-E,E,E-rac (3%) was
yielded, where a free DPHT existed as a mixture of (E,Z,E)-
DPHT (19%) and (E,E,E)-DPHT (27%). Once isomerization
of free (E,Z,E)-DPHT to free (E,E,E)-DPHT takes place due to
Figure 5. ORTEP drawings of 8-E,E,E-meso (two independent molecules
in a unit cell, 50% probability ellipsoids, BF4 anions were omitted for
clarity). Selected bond lengths (Å): Pd1-Pd1* 2.9156(6), Pd1-C1
2.378(4), Pd1-C2 2.176(3), Pd1-C3 2.193(3), Pd1-C4 2.231(3), Pd1-
C5 2.168(3), Pd1-C6 2.355(4), Pd2-Pd2* 2.9400(5), C1-C2 1.376(6),
C2-C3 1.424(5), C3-C4* 1.395(5), C4-C5 1.430(5), C5-C6 1.385(5),
Pd2-C7 2.345(4), Pd2-C8 2.177(3), Pd2-C9 2.223(3), Pd2-C10
2.216(3), Pd2-C11 2.152(4), Pd2-C12 2.352(4), C7-C8 1.374(5), C8-
C9 1.421(4), C9-C10* 1.403(5), C10-C11 1.431(4), C11-C12 1.383(5).
found in a unit cell, and each molecule shows similar structural
parameters. The Pd-Pd distances (2.9156(6) and 2.9400(5) Å)
are within the range of known Pd-Pd lengths (2.391-3.185
Å).28 Although the coordination mode (µ-η3:η3-mode) of DPHT
ligand in 8-E,E,E-meso is the same as that in 4-E,E,E-
(37) There are two isomers for 8, due to the two stacking modes (eclipsed and
(38) The results of theoretical investigation on the electronic structure of 8 will
be published elsewhere. It is notable that a related sandwich structure was
found in [Ni2(µ-η3:η3-pentadienyl)2], see: (a) Riena¨cker, R.; Yoshiura, H.
Angew. Chem., Int. Ed. Engl. 1969, 8, 677. (b) Kru¨ger, C. Angew. Chem.,
Int. Ed. Engl. 1969, 8, 678. (c) Bo¨hm, M. C.; Gleiter, R. Chem. Phys.
1982, 64, 183.
staggered) of zigzag conjugated polyenes. The existence of two isomers
was revealed on the related sandwich complex [Pd3(µ-η3:η2:η3-tetraene)]2+
;
see ref 5. In eq 8, 8-E,E,E-meso was isolated as a single isomer. The
structure of 8-E,E,E-rac was reasonably assumed from the 1H or 13C{1H}
NMR signal pattern, which was similar to that of 8-E,E,E-meso.
9
4384 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

+
Trans-olefination of the Dipalladium Moiety [Pd2Ln]2
A R T I C L E S
Scheme 13. Interconversion between Sandwich and
Half-Sandwich Complexes
some palladium species as mentioned earlier, the Pd22+ moiety
is trapped by free (E,E,E)-DPHT to form the stable sandwich
complex 8-E,E,E-meso.
The sandwich complex 8-E,Z,E was isolated by a different
method described in section 4, and its structure was determined
by X-ray structure analysis (Figure 6). The Pd-Pd bond length
in 8-E,Z,E (2.770(1) or 2.767(1) Å) is shorter than that in
8-E,E,E-meso (2.9156(6) or 2.9400(5) Å). In the crystal, the
dication of 8-E,Z,E held a pair of BF4 anion and H2O through
-
Figure 6. ORTEP drawings of 8-E,Z,E (two independent molecules in a
unit cell, 50% probability ellipsoids, BF4 anions and water molecules were
omitted for clarity). Selected bond lengths (Å): Pd1-Pd2 2.770(1), Pd1-
C1 2.398(10), Pd1-C2 2.18(1), Pd1-C3 2.19(1), Pd2-C4 2.196(9), Pd2-
C5 2.184(10), Pd2-C6 2.36(1), Pd2-C7 2.40(1), Pd2-C8 2.17(1), Pd2-
C9 2.19(1), Pd1-C10 2.20(1), Pd1-C11 2.17(1), Pd1-C12 2.35(1), C1-
C2 1.38(1), C2-C3 1.41(1), C3-C4 1.44(2), C4-C5 1.42(1), C5-C6
1.38(1), C7-C8 1.39(2), C8-C9 1.41(2), C9-C10 1.43(2), C10-C11
1.42(1), C11-C12 1.41(2), Pd3-Pd4 2.767(1), Pd3-C37 2.42(1), Pd3-
C38 2.18(1), Pd3-C39 2.18(1), Pd4-C40 2.18(1), Pd4-C41 2.19(1), Pd4-
C42 2.39(1), Pd4-C43 2.34(1), Pd4-C44 2.15(1), Pd4-C45 2.20(1), Pd3-
C46 2.20(1), Pd3-C47 2.18(1), Pd3-C48 2.36(1).
supramolecular interaction at its concave space (Figure 7). BF4
and H2O were connected through a hydrogen bond (F‚‚‚O
distance 2.82(1) or 2.84(1) Å). One of the four fluorine atoms
of BF4 and the oxygen atom of H2O are located near the four
hydrogen atoms of each trans,cis,trans-DPHT ligand (F‚‚‚C
distances 3.20-3.64 Å, O‚‚‚C distances 3.29-3.69 Å).
Treatment of 4-E,E,E-antifacial with (E,E,E)-DPHT (5
equiv) in CD2Cl2 at -40 °C afforded 8-E,E,E-rac.39 Interest-
ingly, however, the formation of free (E,Z,E)-DPHT was also
observed during the reaction (86% NMR yield).4 It is reasonably
assumed that (E,Z,E)-DPHT was released during the conversion
of 4-E,E,E-antifacial to 8-E,E,E-rac, in view of the fact that
(E,Z,E)-DPHT is thermodynamically less stable than (E,E,E)-
DPHT. As depicted in Scheme 13, dissociation of (E,Z,E)-DPHT
can be explained by the initial formation of 4-E,Z,E-synfacial,
which is a rotamer of 4-E,E,E-antifacial at the central C-C
bond of DPHT ligand. In 4-E,Z,E-synfacial, two Pd atoms that
are arranged co-facial are allowed to have a direct Pd-Pd
(39) The formation of another minor product (28% at 25 °C from 4-E,Z,E-
antifacial and 23% at 25 °C from 4-E,E,E-antifacial) was observed. A
slight amount of the minor product remained after 20 h during the reaction
of 4-E,E,E-antifacial and (E,E,E)-DPHT. Efforts to isolate this product
failed, and the structure remains elusive. Because the minor product showed
12 nonaromatic CH proton resonances, the product is assumed as a sandwich
complex having an unsymmetric structure. One possible structure is shown
below. 1H NMR signals (CD2Cl2, 23 °C) for -CH- of one DPHT, δ )
6.85 (H2), 6.42 (d, J ) 13.6 Hz, H6), 5.37 (dd, J ) 13.6 Hz, J ) 10.4 Hz,
H5), 5.18 (dd, J ) 12.8 Hz, J ) 7.2 Hz, H3), 4.78 (d, J ) 14.0 Hz, H1),
2.46 (dd, J ) 12.8 Hz, J ) 10.8 Hz, H4); for -CH- of another DPHT, δ
) 6.60 (H1), 6.58 (H6), 5.42 (dd, J ) 13.6 Hz, J ) 11.2 Hz, H2), 5.05 (dd,
J ) 13.8 Hz, J ) 11.2 Hz, H5), 4.28 (t, J ) 11.2 Hz, H3), 3.88 (t, J ) 11.2
Hz, H4).
Figure 7. Supramolecular binding of the [BF4‚‚‚H2O]- moiety by the
dication of 8-E,Z,E in the crystalline state.
interaction, as suggested in the dinuclear elimination reaction
involving DBHT. Two equivalents of (E,E,E)-DPHT then can
bind the Pd-Pd moiety to release (E,Z,E)-DPHT (Scheme 13).
This hypothesis is supported by the observation of the stereo-
retentive trans-olefination process described next.
4. Trans-olefination of [Pd2L4]2+ Moiety between 1,3,5-
Trienes. In the previous section, it is proposed that the
transformation of 4-E,E,E-antifacial to 8-E,E,E in the presence
of free (E,E,E)-DPHT may involve the replacement of (E,Z,E)-
DPHT ligand with (E,E,E)-DPHT, from the observation of the
release of free (E,Z,E)-DPHT during the reaction. The reaction
of 4-E,E,E-antifacial with the t-Bu analogue (E,E,E)-DBHT
was then examined. When 1 equiv of (E,E,E)-DBHT was added
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4385

A R T I C L E S
Scheme 14
Murahashi et al.
Experimental Section
General Considerations. Unless specified, all reactions were carried
out using standard Schlenk techniques under dinitrogen atmosphere.
1H and 13C NMR spectra were obtained on either 270 (JEOL GSX-
270), 400 (JEOL GSX-400, JEOL ECP-400), or 600 MHz (Varian
UNITY-INOVA 600) instruments. The chemical shifts were referenced
to the residual resonances of deuterated solvents. Elemental analyses
were performed at the Analytical Center, Faculty of Engineering, Osaka
University. UV-vis spectra were recorded on a Shimadzu UV-3100PC.
Unless specified, all reagents were purchased from commercial suppliers
and used without purification. CH2Cl2, CD2Cl2, CH3CN, and CD3CN
were distilled from CaH2 prior to use. [Pd2(CH3CN)6][BF4]2 (1),20
[Pd2(CH3CN)4(PPh3)2][BF4]2 (2),20 [Pd2(CH3CN)4(PPh3)2][PF6]2 (2′),21
[Pd2(µ-PPh3)2(CH3CN)2][PF6]2 (2′′),22 NaBArf,40 and (E,E,E)-DBHT41
were prepared according to the literature. (E,Z,E)-DPHT was generously
gifted from Prof. Tetsuro Majima (Osaka University). All monitoring
experiments using DPHT or DBHT were carried out in J. Young
resealable NMR tubes wrapped with aluminum foil.
to the solution of 4-E,E,E-antifacial in CD2Cl2 at 0 °C, the
formation of 5-E,Z,E-antifacial (37%) and 8-E,Z,E (33%) was
observed after 1 h (eq 9). After 3 h, the yield of 5-E,Z,E-
antifacial reached 46%, while 8-E,Z,E decreased to 23%
probably due to the partial precipitation of 8-E,Z,E (red
precipitates were observed), and approximately one-half the
amount of free (E,E,E)-DBHT remained.
Synthesis of [Pd2(µ-η3:η3-Hexatriene)(CH3CN)4][BF4]2 (3a). To
a solution of [Pd2(CH3CN)6][BF4]2 (1) (152.1 mg, 0.240 mmol) in CH3-
CN was added 1,3,5-hexatriene (31.4 µL, 0.289 mmol), and the mixture
was stirred for 1 h at room temperature. The yellow mixture was filtered,
and the volatiles were removed in vacuo. Recrystallization from CH3-
1
CN/Et2O gave microcrystals of 3a (113.9 mg, 75% yield). H NMR
(CD3CN) δ ) 6.01 (m), 4.37 (dd), 4.34 (d, J ) 6.8 Hz), 3.34 (d, J )
12.7 Hz). Anal. Calcd for Pd2C14H20N4B2F8: C, 26.66; H, 3.20; N,
8.88. Found: C, 26.04; H, 3.16; N, 8.45.
Synthesis of [Pd2(µ-η3:η3-Hexatriene)(CH3CN)2(PPh3)2][PF6]2
(3b). To a solution of [Pd2(µ-PPh3)2(CH3CN)2][PF6]2 (2′′) (70.0 mg,
0.0633 mmol) in CH2Cl2 was added 1,3,5-hexatriene (8.3 mL, 0.0760
mmol), and the mixture was stirred for 45 min at room temperature.
The resultant mixture was filtered, and hexane was added to give a
yellow powder. Recrystallization from CH2Cl2/hexane gave microc-
This result suggests the involvement of stereoretentive ligation
of (E,E,E)-DBHT and stereoretentive release of (E,Z,E)-DPHT
(Scheme 14). Selective formation of the transferred product
5-E,Z,E-antifacial indicates that the trans-olefination is medi-
ated via syn addition of a Pd2 unit. If the elimination of DPHT
ligand in 4-E,E,E-antifacial proceeds in a syn manner, (E,Z,E)-
DPHT should be released via 4-E,Z,E-synfacial. The formation
of 8-E,Z,E is reasonably understood by the immediate trap of
the released (E,Z,E)-DPHT by the remaining 4-E,E,E-antifacial,
on the basis of the result described in section 3.2.
1
rystals of 3b (39.5 mg, 52% yield). H NMR (CDCl3) δ ) 7.6-7.4
(m), 6.32 (m), 5.55 (m), 3.42 (br d), 3.03 (br d), 2.05 (s). 31P{1H}
NMR (CDCl3) δ ) 26.2 (s). Anal. Calcd for Pd2C46H44N2P4F12: C,
46.44; H, 3.73; N, 2.35. Found: C, 46.27; H, 3.71; N, 2.26.
Synthesis of 4-E,E,E-Antifacial. [Pd2(CH3CN)6][BF4]2 (1) (199.1
mg, 0.315 mmol) was dissolved in CH3CN (30 mL), and (E,E,E)-DPHT
(314.2 mg, 1.352 mmol) was added to the solution. The solution was
stirred for 1.5 h at room temperature. The orange solution was filtered,
and crystallization from CH3CN/Et2O afforded orange microcrystals
of 4-E,E,E-antifacial (136.0 mg, 55% yield). 1H NMR (CD3CN) δ )
7.70 (d, J ) 7.3 Hz), 7.6-7.3 (m), 6.57 (br t), 5.21 (d, J ) 11.1 Hz),
4.46 (br d). 13C{1H} NMR (CD3CN) δ ) 135.9 (s), 129.5 (s), 128.3
(s), 127.7 (s), 110.4 (s), 83.7 (s), 78.5 (s). Anal. Calcd for Pd2C26H28B2F8‚
H2O: C, 38.99; H, 3.78; N, 6.99. Found: C, 38.44; H, 3.60; N, 7.16.
Generation of 4-E,Z,E-Antifacial. The complex 4-E,Z,E-antifacial
was generated quantitatively by dissolving [Pd2((E,E,E)-DPHT)2][BF4]2
Conclusion
In summary, we have demonstrated the stereoretentive nature
of dinuclear elimination and transfer of [Pd2Ln]2+ moieties
between conjugated 1,3,5-trienes. From this study, several new
aspects are gained on the role of conjugated olefin as the
2+
multidentate ligands for a dipalladium moiety: (a) a Pd2
1
moiety derived from 1 adds to 1,3,5-trienes to form bi-η3-
allyldipalladium complexes; (b) dinuclear elimination from bi-
η3-allyldipalladium(II) complexes is induced by addition of
phosphine (PPh3 or dppm), and it proceeds in a stereospecific
(syn) manner; (c) bis-µ-η3:η3-triene dipalladium sandwich
complexes are reversibly formed from bi-η3-allyldipalladium-
(II) complexes and triene in a stereoretentive manner; and (d)
transfer of dipalladium moiety [Pd2L4]2+ between 1,3,5-trienes
(i.e., trans-olefination of [Pd2L4]2+) proceeds stereoretentively.
The observed highly stereoretentive dynamic behavior of Pd-
Pd moieties on 1,3,5-trienes represents the pairwise behavior
of a dinuclear metal moiety on the π-plane of sp2-carbon
framework; that is, multiple metal atoms behave in a correlated
way on an extended sp2-carbon framework through formation/
cleavage of metal-metal bonds.
(8-E,E,E-meso) in CD3CN or CH3CN at -40 °C. H NMR (CD3CN)
δ ) 7.78 (d, J ) 7.8 Hz), 7.6-7.3 (m), 6.89 (br t), 5.17 (d, J ) 11.1
Hz), 4.46 (br d). 13C{1H} NMR (CD3CN) δ ) 136.1 (s), 129.3 (s),
128.4 (s), 127.7 (s), 108.0 (s), 83.2 (s), 76.0 (s).
Synthesis of 5-E,Z,E-Antifacial. To a solution of [Pd2(CH3CN)6]-
[BF4]2 (1) (500.0 mg, 0.790 mmol) in CH2Cl2 (50 mL) was added
(E,E,E)-DBHT (159.6 mg, 0.830 mmol), and the mixture was stirred
for 50 min at room temperature under N2. The mixture was filtered,
(40) (a) Kobayashi, H.; Sonoda, A.; Iwamoto, H.; Yoshimura, M. Chem. Lett.
1981, 10, 579. (b) Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.;
Kobayashi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2600. (c) Brookhart, M.;
Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920. (d) Yakelis, N.
A.; Bergman, R. G. Organometallics 2005, 24, 3579.
(41) Horner-Wadsworth-Simmons modified Wittig-type reaction was used.
See: (a) Spangler, C. W.; McCoy, R. K.; Dembek, A. A.; Sapochak, L.
S.; Gates, B. D. J. Chem. Soc., Perkin Trans. 1 1989, 151. See for alternative
synthesis and NMR data: (b) Knoll, K.; Schrock, R. R. J. Am. Chem. Soc.
1989, 111, 7989.
9
4386 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

+
Trans-olefination of the Dipalladium Moiety [Pd2Ln]2
A R T I C L E S
5-E,Z,E-antifacial
Table 1. X-ray Crystallographic Data of 1, 3b, and
5-E,Z,E-Antifacial
and crystallization from CH2Cl2/Et2O gave a yellow powder (89.8 mg),
which contained 5-E,Z,E-antifacial and 5-E,E,E-antifacial. The
supernatant was separated from the powder, and Et2O was added.
Crystallization from this solution gave yellow microcrystals of 5-E,Z,E-
1
3b
formula
fw
C
12H18N6B2F8Pd2
C
46H44N2P4F12Pd2
C22H36N4B2F8Pd2
742.96
antifacial (446.8 mg, 76% yield). H and 13C{1H} NMR spectra of
1
632.72
1189.4
space group
a, Å
b, Å
P21/a (No. 14)
10.0193(6)
20.799(1)
12.0023(7)
C2/c (No. 15)
32.791(4)
8.962(2)
Pbca (No. 61)
17.2416(3)
12.1588(3)
29.5145(5)
this crystal in CD3CN showed the presence of two intra-series isomers
(93/7 at 25 °C).42,43 For major isomer: 1H NMR (CD3CN) δ ) 6.02
(m), 4.22 (d, J ) 11.6 Hz), 4.11 (m), 1.16 (s). 13C{1H} NMR
(CD3CN) δ ) 110.1 (s), 102.4 (s), 76.9 (s), 34.4 (s), 29.8 (s). For minor
isomer: 1H NMR (CD3CN) δ ) 5.53 (m), 4.68 (d, J ) 12.7 Hz), 4.35
(m), 1.15 (s). 13C{1H} NMR (CD3CN) δ ) 105.4 (s), 100.7 (s), 73.5
(s), 34.6 (s), 29.9 (s). Anal. Calcd for Pd2C22H36N4B2F8: C, 35.56; H,
4.88; N, 7.54. Found: C, 35.65; H, 4.84; N, 7.72.
c, Å
19.133(3)
R, deg
â, deg
γ, deg
V, Å3
112.696(2)
113.577(10)
2307.5(3)
4
1.821
5153(1)
4
1.533
8.99
296
6187.3(4)
8
1.595
12.29
277
Z
D(calcd), g/cm3
µ(Mo KR), mm-1 16.33
Synthesis of 5-E,E,E-Antifacial. To a solution of [Pd2(CH3CN)6]-
[BF4]2 (1) (500.0 mg, 0.790 mmol) in CH2Cl2 (40 mL) was added
(E,E,E)-DBHT (457.6 mg, 2.371 mmol), and the mixture was stirred
for 5 h at room temperature. The reaction mixture was filtered, and
the volatiles were removed from the filtrate in vacuo. To the residue
was added CH2Cl2 (200 mL), and the suspension was stirred overnight.
The mixture was then concentrated, and Et2O was added. The
supernatant was decanted, and the residue was washed with Et2O. The
residue was then dissolved in CH3CN, the solution was filtered, and
toluene was added to generate a yellow precipitate. Recrystallization
from CH3CN/CH2Cl2/toluene gave a yellow powder of 5-E,E,E-
antifacial (314.1 mg, 54% yield) at the bottom of the vessel. The yellow
crystalline materials on the wall of the vessel were a mixture of 5-E,Z,E-
temp, K
R1
273
0.041
0.077
0.036
H1), 5.03 (dd, J ) 13.6 Hz, J ) 9.2 Hz, H6), 4.42 (dd, J ) 11.2 Hz,
J ) 10.8 Hz, H4, 1H), 3.89 (dd, J ) 13.6 Hz, J ) 10.8 Hz, H5, 1H),
3.72 (dd, J ) 14.8 Hz, J ) 7.2 Hz, H2, 1H), 3.43 (dd, J ) 11.2 Hz, J
) 7.2 Hz, H3, 1H), 1.95 (s, CH3CN, 3H), 1.64 (s, CH3CN, 3H), 0.78
(s, t-Bu, 9H), 0.61 (s, t-Bu, 9H). 31P{1H} NMR (CD2Cl2, -40 °C) δ )
31.3 (d, J ) 12.5 Hz), 22.1 (d, J ) 12.5 Hz). 13C{1H} NMR (CD2Cl2,
25 °C) δ ) 135.3-129.7, 125.3 (s, CH3CN), 123.3 (s, C6), 113.5 (s,
C1), 108.2 (s, C2 or C5), 108.0 (s, C2 or C5), 80.3 (s, C3), 66.2 (s, C4),
34.7 (d, J ) 3.1 Hz, C(CH3)3), 34.1 (d, J ) 3.8 Hz, C(CH3)3), 30.3 (d,
J ) 4.6 Hz, C(CH3)3), 30.0 (d, J ) 3.8 Hz, C(CH3)3), 2.8 (s, CH3CN).
Anal. Calcd for Pd2C54H60N2P2B2F8: C, 54.71; H, 5.10; N, 2.36.
Found: C, 54.89; H, 5.15; N, 2.35.
1
antifacial and 5-E,E,E-antifacial. H NMR (CD3CN) δ ) 5.84 (m),
4.26 (d, J ) 12.4 Hz), 4.06 (m), 1.14 (s). 13C{1H} NMR (CD3CN) δ
) 112.6 (s), 102.0 (s), 79.7 (s), 34.3 (s), 29.8 (s). Anal. Calcd for
Pd2C22H36N4B2F8: C, 35.56; H, 4.88; N, 7.54. Found: C, 35.61; H,
4.88; N, 7.48.
Synthesis of (E,Z,E)-DBHT. (E,Z,E)-DBHT was conveniently
obtained by the dipalladium-mediated isomerization from (E,E,E)-
DBHT. To a solution of 5-E,E,E-antifacial (300.0 mg, 0.404 mmol)
in CH3CN (10 mL) was added dppm (294.9 mg, 0.767 mmol), and the
mixture was stirred for 10 min at 0 °C. The volatiles were removed in
vacuo, and cold pentane (-20 °C) was added to the residue. The
suspension was filtered immediately, and the volatiles were removed
in vacuo to give a colorless powder of (E,Z,E)-DBHT (49.5 mg, 81%
yield). NMR data were found in the literature.41b The compound that
remained on the filter funnel was [Pd2(dppm)2(CH3CN)2][BF4]2.
Generation of 7-E,Z,E-Antifacial. In an NMR tube, 5-E,Z,E-
antifacial (7.0 mg, 0.0094 mmol) and PPh3 (4.9 mg, 0.0188 mmol)
were added, and CD3CN (0.5 mL) was added. The temperature of the
tube was kept at -40 °C. After 5 min, 5-E,Z,E-antifacial disappeared,
and only 7-E,Z,E-antifacial was observed as a mixture of two isomers
(>95/<5). For major: 1H NMR (CD3CN) δ ) 7.6-7.4 (m), 5.79 (br),
3.92 (br), 1.69 (br), 0.92 (br s). 31P{1H} NMR (CD3CN) δ ) 20.6 (s).
For minor: 1H NMR (CD3CN) δ ) 7.19, 6.98, 4.70, 4.42, 4.31, 3.46,
3.06. Only these signals were detected. 31P{1H} NMR (CD3CN) δ )
18.2 (only one signal was detected).
Synthesis of 8-E,E,E-Meso. To a solution of [Pd2(CH3CN)6][BF4]2
(1) (243.4 mg, 0.385 mmol) in CH2Cl2 (30 mL) was added (E,E,E)-
DPHT (247.1 mg, 1.064 mmol), and the mixture was stirred for 2 h at
room temperature. The reaction mixture was filtered, and crystallization
from CH2Cl2/Et2O gave a red powder of 8-E,E,E-meso (251.8 mg,
77% yield). 1H NMR (CD2Cl2) δ ) 7.46 (t), 7.05 (t), 6.78 (d), 6.76 (d,
J ) 13.8 Hz), 5.26 (m), 3.98 (m). 13C{1H} NMR (CD2Cl2) δ ) 132.0
(s), 130.6 (br s), 130.0 (s), 129.7 (s), 129.4 (s), 98.9 (s), 93.1 (s). Anal.
Calcd for Pd2C36H32B2F8: C, 50.80; H, 3.79. Found: C, 50.51; H, 3.96.
A single crystal suitable for X-ray analysis was obtained by recrystal-
lization from CH2Cl2/t-Bu-benzene/n-hexane. 1H NMR data of 8-E,E,E-
rac: 1H NMR (CD2Cl2) δ ) 5.95 (d, J ) 13.5 Hz), 5.79 (m), 3.67
(m).
Synthesis of 7-E,E,E-Antifacial. To a solution of 5-E,E,E-antifacial
(100.0 mg, 0.135 mmol) in CH3CN (3 mL) was added PPh3 (70.6 mg,
0.269 mmol). The color immediately changed from yellow to orange.
The mixture was stirred for 5 min, and the reaction mixture was filtered
and poured into Et2O to afford pale yellow precipitates. The powder
was then washed with Et2O and dried in vacuo to afford 7-E,E,E-
antifacial (127 mg, 79% yield). A single crystal suitable for X-ray
analysis was obtained by recrystallization from CH2Cl2/benzene. For
major: 1H NMR (CD2Cl2, -40 °C) δ ) 7.7-7.2 (m), 4.69 (m, H2,
2H), 3.95 (dd, J ) 13.2 Hz, J ) 8.4 Hz, H1, 2H), 3.61 (m, H3, 2H),
1.58 (s, CH3CN, 6H), 0.59 (s, t-Bu, 18H). 31P{1H} NMR (CD2Cl2, -40
°C) δ ) 30.5 (s). 13C{1H} NMR (CD2Cl2, 25 °C) δ ) 135.3-129.7,
125.3 (s, CH3CN), 119.4 (s, C1), 111.6 (s, C2), 76.1 (s, C3), 34.4 (s,
C(CH3)3), 29.8 (s, C(CH3)3), 2.8 (s, CH3CN). For minor: 1H NMR
(CD2Cl2, -40 °C) δ ) 7.7-7.2, 5.24 (dd, J ) 14.8 Hz, J ) 10.4 Hz,
Synthesis of 8-E,Z,E. The complex 4-E,E,E-antifacial (15.0 mg,
0.0192 mmol) and (E,E,E)-DBHT (1.8 mg, 0.0095 mmol) were placed
in an NMR tube. To the tube was added CD2Cl2 (0.5 mL), and the
tube was supersonicated. After 2 h, the formation of 8-E,Z,E (37%
NMR yield) and 5-E,Z,E-antifacial (41% NMR yield) was observed
with trace (E,Z,E)-DPHT. The complex 8-E,Z,E was obtained as red
crystals together with 5-E,Z,E-antifacial (yellow crystals) from the
reaction of 4-E,E,E-antifacial and (E,E,E)-DBHT in CH2Cl2 with
stirring for 20 min followed by filtration and crystallization from
CH2Cl2/toluene. A red crystal suitable for X-ray diffraction analysis
1
(42) The H and 13C NMR signal patterns of the major and minor isomers are
quite similar (symmetric) to each other, and it is reasonably assumed that
both isomers are the intra-series products, although it cannot be concluded
that the isomer that was structurally determined by X-ray analysis is the
major isomer in CD3CN. For convenience, we abbreviate the two
equilibrated isomers as 5-E,Z,E-antifacial in this Article.
(43) The isolated 5-E,Z,E-antifacial was poorly soluble in CD2Cl2. In the
monitoring reactions of 1 and DBHT in CD2Cl2, however, 5-E,Z,E-
antifacial could be detected because 5-E,Z,E-antifacial becomes soluble
in the presence of free CH3CN released from 1. In these conditions, 5-E,Z,E-
antifacial existed as a mixture of two isomers (major/minor ) 9/1) as
observed in CD3CN.
1
was picked up from the crystalline mixture. H NMR (CD2Cl2) δ )
7.39 (m, p-Ph), 7.17 (d, J ) 13.5 Hz, H1), 6.93 (m, o-, m-Ph), 5.07 (m,
H2), 4.56 (m, H3).
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4387

A R T I C L E S
Murahashi et al.
Table 2. X-ray Crystallographic Data of 7-E,E,E-Antifacial and
8-E,Z,E
antifacial, and 8-E,Z,E) with graphite monochromated Mo KR
radiation. The calculations were carried out with the teXsan crystal
structure analysis software package of Molecular Structure Corp. The
structures were solved by heavy-atom Patterson methods and refined
by full-matrix least-squares procedures. The non-hydrogen atoms were
refined anisotropically. Detailed crystallographic data were collected
in the Supporting Information.
7-E,E,E-antifacial
8-E,Z,E
formula
fw
C54H60N2B2F8P2Pd2
1185.44
C36H34B2F8O1Pd2
869.07
space group
a, Å
b, Å
P21/n (No. 14)
14.1209(4)
17.2838(6)
P1h (No. 2)
14.4348(3)
14.8652(4)
16.9596(4)
72.4075(5)
90.278(1)
81.7435(7)
3428.5(1)
4
1.684
11.22
213
0.042
c, Å
22.5026(9)
Acknowledgment. This work was supported by PRESTO,
Japan Science and Technology Agency (JST), and Grants-in-
aid for Scientific Research, Ministry of Education, Culture,
Sports, Science, and Technology of Japan. Thanks are also due
to the Analytical Center, Faculty of Engineering, Osaka
University, for the use of NMR facilities.
R, deg
â, deg
97.679(2)
γ, deg
V, Å3
5442.8(3)
4
1.447
7.84
223
0.065
Z
D(calcd), g/cm3
µ(Mo KR), cm-1
temp, K
R1
Supporting Information Available: Details of the X-ray
single-crystal structural analyses for 1, 3b, 5-E,Z,E-antifacial,
7-E,E,E-antifacial, and 8-E,Z,E. This material is available free
of charge via the Internet at http://pubs.acs.org.
X-ray Crystallographic Studies. The crystallographic data were
summarized in Tables 1 and 2. All measurements were made on a
Rigaku AFC5R diffractometer (for 3b) or Rigaku RAXIS-RAPID
imaging plate diffractometer (for 1, 5-E,E,E-antifacial, 7-E,E,E-
JA0576740
9
4388 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006