Organometallics
Article
tions. The proportion of products can be understood as a
function of the rates of the different processes involved, as
discussed above.
ACKNOWLEDGMENTS
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Financial support is gratefully acknowledged from the Junta de
Castilla y Leon (Projects GR169 and VA256U13), the Spanish
́
MINECO (CTQ2013-48406-P), and European Commission
(2010-2401/001-001-EMA2; EU Mobility Program, EADIC II
Erasmus Mundus Scholarship to E.G.).
EXPERIMENTAL SECTION
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General Methods. All the manipulations were performed under
an atmosphere of argon using standard Schlenk techniques unless
otherwise stated. Solvents were dried using an SPS PS-MD-5 solvent
purification system or distilled from appropriate drying agents under
nitrogen, prior to use. The compounds [PdCl2(P-L1)] and
[PdCl2(PPh3)2] were prepared by literature methods.4 Solutions of
ZnEt2 1.0 M in dry hexane were prepared from commercial salt-free
liquid ZnEt2 (Sigma-Aldrich Zn wt ≥ 52.0%).17 Solutions of ZnEtCl
were prepared by asymmetrization rearrangement of ZnEt2 and ZnCl2.
All other reagents were commercially available and used as received.
1H, 19F, and 31P{1H} spectra were recorded on a Bruker AV-400 or
a Varian Inova 500 spectrometer. Chemicals shifts (in δ units, parts per
million) were referenced to the residual solvent signal, to CFCl3, and
to 85% H3PO4, respectively. The spectral data were recorded at 293 K
unless otherwise noted. GC-mass spectra were recorded on a Thermo
Scientific Focus DSQII system. Elemental analyses were performed on
a PerkinElmer 2400B CHN analyzer.
REFERENCES
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(1) (a) Handbook of Organopalladium Chemistry for Organic Synthesis,
Vol. 1, part III; Negishi, E., Ed.; Wiley-Interscience: New York, 2002.
(b) Negishi, E.; Zeng, X.; Tan, Z.; Qian, M.; Hu, Q.; Huang, Z. In
Metal-Catalyzed Cross-Coupling Reactions; de Meijere, A., Diederich, F.,
Eds.; Wiley-VCH: New York, 2004; Chapter 15. (c) Negishi, E.; Hu,
Q.; Huang, Z.; Qian, M.; Wang, G. Aldrichim. Acta 2005, 38, 71−88.
(d) Negishi, E. Bull. Chem. Soc. Jpn. 2007, 80, 233−257. (e) Phapale,
V. B.; Cardenas, D. J. Chem. Soc. Rev. 2009, 38, 1598−1607.
́
(2) (a) Davidson, P. J.; Lappert, M. F.; Pearce, R. Chem. Rev. 1976,
76, 219−242. (b) Zaera, F. Chem. Rev. 1995, 95, 2651−2693.
(c) Car
(d) Espinet, P.; Alben
Chemistry; Fundamentals of Molecular Catalysis; Kurosawa, H.,
Yamamoto, A., Eds.; Elservier: Amsterdam, 2003; Vol. 3, Chapter 6,
p 300.
́
denas, D. J. Angew. Chem., Int. Ed. 2003, 42, 384−387.
́
iz, A. C. In Current Methods in Inorganic
[PdCl2(PPh2(6-HC6F4))2]. Phosphine PPh2(6-HC6F4) (50.9 mg,
0.152 mmol) was added to a solution of [PdCl2(NCMe)2] in 4 mL of
THF (phosphine:Pd molar ratio = 2:1). The reaction mixture was
stirred for 2 h at RT. The volatiles were evaporated. A pale yellow solid
was obtained, washed with pentane, and dried under vacuum (59.2 mg,
92%). 1H NMR (400.13 MHz, δ, CDCl3): 7.86−7.76 (m, 8H), 7.58−
7.52 (m, 4H), 7.51−7.45 (m, 8H), 6.59 (m, 2H). 19F NMR (376.46
MHz, δ, CDCl3): −120.16 (m, 2F), −137.56 (m, 2F), −149.76 (m,
2F), −153.29 (m, 2F). 31P{1H} NMR (161.97 MHz, δ, CDCl3): 17.76
(dd, JP−F = 7.0, 7.0 Hz, 2P). Anal. Calcd for C36H22Cl2F8P2Pd: C,
51.12; H, 2.62. Found: C, 50.93; H, 2.86.
(3) (a) Luo, X.; Zhang, H.; Duan, H.; Liu, Q.; Zhu, L.; Zhang, T.;
Lei, A. Org. Lett. 2007, 9, 4571−4574. (b) Zhang, H.; Luo, X.;
Wongkhan, K.; Duan, H.; Li, Q.; Zhu, L.; Wang, J.; Batsanov, A. S.;
Howard, J. A. K.; Marder, T. B.; Lei, A. Chem.−Eur. J. 2009, 15, 3823−
3829.
(4) Gioria, E.; Martínez-Ilarduya, J. M.; García-Cuadrado, D.; Miguel,
J. A.; Genov, M.; Espinet, P. Organometallics 2013, 32, 4255−4261.
(5) Perez-Rodríguez, M.; Braga, A. A. C.; García-Melchor, M.; Perez-
́ ́
Temprano, M. H.; Casares, J. A.; Ujaque, G.; de Lera, A. R.; Alvarez,
R.; Maseras, F.; Espinet, P. J. Am. Chem. Soc. 2009, 131, 3650−3657.
(6) The Ar−Et/Ar−H/Ar−Ar ratios of the products observed in
these reactions follow the same trends reported previously,4 although
there are quantitative differences probably due to the use of preformed
catalyst and to the faster reaction rates produced by the higher
concentrations of reagents employed here. In fact the reaction rates of
the competing processes discussed below have different concentration
dependence, and the proportion of products changes with the
concentration or with the mixture of solvents. These are kept identical
within this study.
(7) It is not unusual that in long-time manipulations of ZnEt2 traces
of ethylene can be observed. We suspect that it can be related to
catalysis by traces of metals. In such case the slow “noncatalyzed”
reaction would in fact be a catalyzed one with very little catalyst.
(8) Some broadening is observed for the signals of 5 and 6 in all the
́
General Procedure for the Catalysis. Preformed palladium
complex (0.015 mmol) was weighed and put into an oven-dried 10 mL
Schlenk, which was evacuated and refilled with argon. Ethyl 2-
iodobenzoate (50.8 μL, 0.3 mmol) was added by microsyringe, and the
Schlenk was sealed. Finally, a solution of ZnEt2 (0.75 mL, 1.0 M in
hexane) in 0.25 mL of THF was added slowly with gentle stirring of
the mixture, which turned immediately to dark brown. After 3 h a
sample was taken and checked by NMR (before hydrolysis spectrum).
The sample was hydrolyzed with a 2 M solution of HCl, extracted with
diethyl ether, dried with magnesium sulfate, and filtered through silica
gel. This final solution was checked by GC-MS and NMR (after
hydrolysis spectrum). For the experiment in THF-d8 liquid ZnEt2 was
directly used to avoid the presence of hexane.
1
Ethyl 2-Ethylbenzoate (2). H NMR (400.13 MHz, δ, CDCl3):
7.84 (dd, J = 7.7 Hz, 1.4 Hz, H6), 7.42 (ddd, J = 7.7, 7.7, 1.4 Hz, H4),
7.24 (ddd, J = 7.7, 7.7, 1.5 Hz, H5), 7.27 (dm, J = 7.7 Hz, H3), 4.36 (q,
J = 7.1 Hz, CH2 from COOEt), 2.97 (q, J = 7.5 Hz, CH2 from Et) 1.39
(t, J = 7.1 Hz, CH3 from COOEt) 1.24 (t, J = 7.5 Hz, CH3 from Et).
1
spectra commented on in this work. H NMR experiments prove that
the broadening is due to exchange of the Et groups between 5, ZnEtI,
and ZnEt2 and the exchange of the Ar groups between 5 and 6.
(9) Note that [PdCl2(P-L1)] is in fact a precatalyst that has to be
transformed into [Pd(P-L1)] to enter the catalytic cycle. This requires
double ethylation to [PdEt2(P-L1)] and reductive elimination
producing butane. For 5% of Pd-catalyst, 5% of ZnEt2 (or 10% of
ZnEtCl, when this is the ethylating agent) will be consumed in this
precatalytic process.
1
Ethylbenzoate (3). H NMR (400.13 MHz, δ, CDCl3): 8.05 (m,
H6 + H2), 7.55 (tt, J = 7.4, 1.3 Hz, H4), 7.44 (m, H5 + H3), 4.38 (q, J =
7.1 Hz, CH2 from COOEt), 1.40 (t, J = 7.1 Hz, CH3 from COOEt).
Diethyl Biphenyl 2,2′-Dicarboxylate (4). 1H NMR (400.13
MHz, δ, CDCl3): 8.01 (ddd, J = 7.7, 1.4, 0.4 Hz, H6), 7.52 (ddd, J =
7.7, 7.7, 1.4 Hz, H4), 7.43 (ddd, J = 7.7, 7.7, 1.4 Hz, H5), 7.21 (dm, J =
7.7 Hz, H3), 4.04 (q, J = 7.1 Hz, CH2 from COOEt), 0.98 (t, J = 7.1
Hz, CH3 from COOEt).
(10) Observed shifts are due to different solvent-mixture polarities.
While before hydrolysis there was a 3:1 mixture of THF/hexane, after
hydrolysis the solvent was only THF.
1
(11) By checking the H NMR spectrum of the preformed solution
of ZnEt2 in THF-d8 it was confirmed that it contained about 53 ppm
of ethane.
(12) Note that other Pd/Zn transmetalations (secondary trans-
metalations) can occur, as we and others have studied thoroughly in
other papers (see refs 13−15). Only some examples (eqs 1 and 2) are
shown.
AUTHOR INFORMATION
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Corresponding Authors
Notes
(13) (a) Casares, J. A.; Espinet, P.; Fuentes, B.; Salas, G. J. Am. Chem.
The authors declare no competing financial interest.
Soc. 2007, 129, 3508−3509. (b) Fuentes, B.; García-Melchor, M.;
F
dx.doi.org/10.1021/om5005379 | Organometallics XXXX, XXX, XXX−XXX