Communications
Organometallics, Vol. 24, No. 2, 2005 191
Figure 1. ORTEP drawing of 2 with 30% ellipsoidal plots.
Selected bond distances (Å) and angles (deg): Pd1-N1 )
2.168(3), Pd1-N2 ) 2.177(3), Pd1-C1 ) 2.000(3), Pd1-
C8 ) 2.000(3); N1-Pd1-N2 ) 83.9(1), N1-Pd1-C1 )
94.6(1), N1-Pd1-C8 ) 178.1(1), N2-Pd1-C1 ) 178.5(1),
N2-Pd1-C8 ) 96.0(1), C1-Pd1-C8 ) 85.4(1).
Figure 2. First-order plots of the thermally induced
reductive elimination of 2,6-difluoro-4′-methoxybiphenyl
from complex 1: (a) at 45 °C; (b) with added diethyl
fumarate at 50 °C; (c) at 50 °C; (d) at 55 °C; (e) at 60 °C.
An Eyring plot is shown in the inset.
Pd-N2 bond (2.177(3) Å), indicating that the trans
influence of the 2,4,6-trifluorophenyl ligand is less than
that of the 4-methoxyphenyl ligand. The reactions in
eq 1 form the complexes in high yields after 3 h at room
temperature, but a longer reaction causes partial for-
mation of the biaryl caused by coupling of the two aryl
groups. Analogous reaction of (2,4-difluorophenyl)-
boronic acid with PdI(C6H4-4-OMe)(tmen) does not lead
to isolation of the diarylpalladium complex but forms
the coupling product, MeOC6H4-C6H3F2, directly.
Smooth transmetalation does not occur in the reaction
of (2,4,6-trifluorophenyl)boronic acid using AgBF4 in-
stead of Ag2O, suggesting that Ag2O activates not only
the Pd-I bond but also the C-B bond.
by addition of diethyl fumarate (8-fold molar amount
of Pd, 0.15 mM). The rate constants of the reactions
with addition of diethyl fumarate (1.12 × 10-4 s-1
)
and without the additive (1.24 × 10-4 s-1) do not
differ significantly from each other. Thus, the Pd metal
formed during the reaction does not affect the reaction
rate.6
The diarylpalladium complexes trans-PdAr2(PEt2Ph)2
(Ar ) Ph, m-C6H4Me) have been reported in the
literature.7 They are stable, due to the trans structure,
although addition of aryl halides to their solutions
results in formation of biaryls via the cis-diarylpalla-
dium intermediates, which undergo reductive elimina-
tion of the products. Coupling of aryl ligands at cis
positions of square-planar complexes of group 10 metals
was reported to take place much more easily than that
of cis-dialkyl complexes.8-10 The cis-diarylpalladium
complexes reported so far contain fluoro, chloro,11 and
nitro12 substituents at the ligand or are stabilized by a
chelating anchor group13 of the aryl ligands. In this
study, the combination of the fluorinated and non-
fluorinated aryl ligands and the tmen ligand enabled
both isolation of the cis-diarylpalladium complexes and
thermally induced reductive elimination of biaryl under
mild conditions.
Heating a C6D6 solution of 1 at 50 °C produces
4-methoxy-2′,6′-difluorobiphenyl via coupling of the two
aryl ligands in 94% yield, as shown in eq 2. The 1H NMR
(6) The reaction of 1 at 50 °C with added excess tmen (5-fold molar
amount of Pd) gives rise to the observed rate constant, 0.92 × 10-4
s-1. This result suggests that tmen influences the reaction rate to a
small extent.
(7) (a) Nakazawa, H.; Ozawa, F.; Yamamoto, A. Organometallics
1983, 2, 241. (b) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto,
A. Organometallics 1986, 5, 2144. (c) Ozawa, F.; Hidaka, T.; Yamamoto,
T.; Yamamoto, A. J. Organomet. Chem. 1987, 330, 253.
(8) Markies, B. A.; Canty, A. J.; Boersma, J.; van Koten, G.
Organometallics 1994, 13, 2053.
(9) (a) Braterman, P. S.; Cross, R. J.; Young, G. B. J. Chem. Soc.,
Dalton Trans. 1976, 1310. (b) Braterman, P. S.; Cross, R. J.; Young,
G. B. J. Chem. Soc., Dalton Trans. 1977, 1892.
(10) Komiya, S.; Abe, Y.; Yamamoto, A.; Yamamoto, T. Organo-
metallics 1983, 2, 1466.
(11) (a) Uson, R.; Fornie´s, J.; Navarro, R. J. Organomet. Chem. 1975,
96, 307. (b) Deacon, G. B.; Grayson, I. L. Transition Met. Chem. 1983,
8, 131. (c) Ruiz, J.; Rodriguez, V.; Lo´pez, G.; Casabo´, J.; Molins, E.;
Miravitlles, C. Organometallics 1999, 18, 1177. (d) Herberhold, M.;
Schmalz, T.; Milius, W.; Wolfgang, M.; Wrackmeyer, B. Inorg. Chim.
Acta 2003, 352, 51.
(12) Vicente, J.; Chicote, M. T.; Martin, J.; Artigao, M.; Solans, X.;
Font-Altaba, M.; Aguilo´, M. J. Chem. Soc., Dalton Trans. 1988,
141.
spectra of the reaction mixture indicate the formation
of the biaryl as the sole aromatic product. 4,4′-Dimeth-
oxybiphenyl, which could be formed by intermolecular
coupling or intermolecular scrambling of the aryl ligands,
does not exist in the reaction mixture. Figure 2 depicts
1
first-order plots of the reaction monitored by H NMR
spectra. The reaction of 1 obeys first-order kinetics with
the kinetic parameters ∆Hq) 101 kJ mol-1, ∆Sq ) -10
J mol-1 K-1, and ∆Gq ) 103 kJ mol-1 at 25 °C.
Formation of the biaryl from a thermal reaction of 2 is
complete after 3 days at 50 °C and is much slower than
the thermal decomposition of 1. The above reactions
accompany deposition of Pd metal, which is inhibited
(5) Crystal data and details of the structure refinement of 2:
C
19H25F3N2OPd, Mr ) 460.81, 1.2 × 1.1 × 0.7 mm, monoclinic, a )
8.730(4) Å, b ) 14.057(7) Å, c ) 16.033(8) Å, â ) 100.229(6)°, V )
1936.2(16) Å3, P21/c (No. 14), Z ) 4, Dcalcd ) 1.581 g cm-3, F(000) )
936.00, µ(Mo KR) ) 9.96 cm-1, Mo KR (λ ) 0.710 70 Å), 12 292 total
reflections measured, 4193 unique reflections (Rint ) 0.026), 3555
observations (I > 3.00σ(I)), 260 variables, R(I > 3.00σ(I)) ) 0.040, Rw(I
> 3.00σ(I)) ) 0.056, GOF ) 1.114.
(13) (a) Cornioley-Deuschel, C.; Ward, T.; von Zelewsky, A. Helv.
Chim. Acta 1988, 71, 130. (b) Fornie´s, J.; Navarro, R.; Sicilia, V.;
Tomas, M. Inorg. Chem. 1993, 32, 3675. (c) Edelbach, B. L.; Vicic, D.
A.; Lachicotte, R. J.; Jones, W. D. Organometallics 1998, 17, 4784.
Osakada, Kohtaro
