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calculations. The final R1 and wR2 were 0.0918 and 0.2271 (all data).
CCDC 699915.w
the use of a ten-fold amount of ligand led to a significant
decrease in the catalytic activity (see Table 3).
y General procedure for the Suzuki–Miyaura cross-coupling: An oven-
dried Schlenk tube equipped with a magnetic stirrer bar was charged
with potassium carbonate (3 mmol, 414.6 mg) and phenylboronic acid
(1.2 mmol, 146.3 mg). The Schlenk tube was sealed, and then
evacuated and backfilled with nitrogen (this sequence was repeated
three times). Freshly distilled toluene (1.9 mL) was added via a rubber
septum and a 0.02 M solution of 3 (0.002 mmol, 100 mL) in toluene
was added via a microsyringe. Aryl chlorides (1 mmol) were added in a
single portion and the Schlenk tube was sealed and placed into an oil
bath at 80 1C. Upon complete consumption of the starting material, as
judged by GC analysis, the reaction mixture was allowed to cool to
room temperature, diluted with diethyl ether (10 mL), filtered through
a thin pad of silica gel (eluting with diethyl ether) and concentrated
under reduced pressure. The crude material obtained was purified by
flash chromatography on silica gel when necessary. All of the biphenyl
products reported in Table 1, Table 2 and Table 3 are known
compounds, and were characterized by favorable comparison of their
1H and 13C NMR spectra with previously reported data in the
literature (see the ESIw).
Finally, the catalytic activity of 3 at room temperature was
also evaluated. Excellent yields were obtained after 12–24 h at
room temperature when operating at a L/Pd ratio of 1 : 1 for
activated aryl chlorides (Table 3, entries 4, 7 and 8). In
contrast to other catalytic systems that allow the use of aryl
chlorides at room temperature, catalyst 3 shows an important
activity with potassium carbonate as a base, meaning that
there is no need for expensive fluoride or caesium salts, or
strong alkoxide bases.15 Importantly, when using L/Pd ratios
of 2 : 1 or higher, the catalytic activity decreased significantly
(Table 3, entry 5), suggesting that a 12 VE complex may act as
the active species.16 Note also that in the absence of ligand,
only trace amounts of the desired biphenyl product were
formed (Table 3, entry 6).
Interestingly, ligand 2 showed the best performance with a
low L/Pd ratio, in contrast to several other bulky phosphine
ligands, where the best performances are obtained when
reactions are performed at higher L/Pd ratios of 2–6 : 1. Both
catalytic precursors 3 and 4 utilized herein efficiently catalyze
the Suzuki–Miyaura cross-coupling of aryl chlorides at rela-
tively low catalyst loadings (0.2%), with a good tolerance for
functional groups such as methoxy, ester, keto, fluoro, cyano
and nitro.
1 Phosphorus-Carbon Heterocyclic Chemistry: The Rise of
New Domain, ed. F. Mathey, Pergamon, New York, 2001.
a
2 G. Markl, F. Lieb and C. Martin, Tetrahedron Lett., 1971, 1249.
¨
3 E. Fuchs, M. Keller and B. Breit, Chem.–Eur. J., 2006, 12, 6930.
4 N. Me
5 (a) B. Breit and E. Fuchs, Synthesis, 2006, 2121; (b) B. Breit and
E. Fuchs, Chem. Commun., 2004, 694; (c) C. Muller and D. Vogt,
´
zailles and P. Le Floch, Curr. Org. Chem., 2006, 10, 3.
¨
Dalton Trans., 2007, 5505; (d) C. Muller, E. A. Pidko, D. Totev,
¨
M. Lutz, A. L. Spek, R. A. van Santen and D. Vogt, Dalton Trans.,
2007, 5372.
6 (a) N. Avarvari, P. Le Floch and F. Mathey, J. Am. Chem. Soc.,
1996, 118, 11978; (b) M. Blug, O. Piechaczyk, M. Fustier,
´
N. Mezailles and P. Le Floch, J. Org. Chem., 2008, 73, 3258.
7 Note: We recently showed that phosphabarrelene–phosphinosulfide
palladium complexes are active in the Suzuki–Miyaura coupling of
aryl bromides, but unreactive towards aryl chlorides: O. Piechaczyk,
M. Doux, L. Ricard and P. Le Floch, Organometallics, 2005,
24, 1204.
8 (a) J. P. Wolfe, R. A. Singer, B. H. Yang and S. L. Buchwald,
J. Am. Chem. Soc., 1999, 121, 9550; (b) J. P. Wolfe and
S. L. Buchwald, Angew. Chem., Int. Ed., 1999, 38, 2413;
(c) A. Zapf, R. Jackstell, F. Rataboul, T. Riermeier, A. Monsees,
C. Fuhrmann, N. Shaikh, U. Dingerdissen and M. Beller, Chem.
Commun., 2004, 1340; (d) A. Zapf, R. Jackstell, F. Rataboul,
T. Riermeier, A. Monsees, C. Fuhrmann, N. Shaikh,
U. Dingerdissen and M. Beller, Chem. Commun., 2004, 38;
(e) N. Kataoka, Q. Shelby, J. P. Stambuli and J. F. Hartwig,
J. Org. Chem., 2002, 67, 5553; (f) T. E. Barder, S. D. Walker,
J. R. Martinelli and S. L. Buchwald, J. Am. Chem. Soc., 2005, 127,
4685; (g) J. V. Kingston and J. G. Verkade, J. Org. Chem., 2007,
72, 2816.
In conclusion, we have synthesized a new bulky phosphine
ligand from a phosphinine derivative via a [4 + 2] Diels–Alder
reaction with benzyne. This method easily allows modification
of the bulkiness of the ligands via 2,6-disubstitution of the
starting phosphinine derivative. A relatively rare example of a
PdL2 (L = phosphine) complex, 4 has been synthesized and
structurally characterized. Additionally, an air-stable catalyst
precursor, 3, which has been shown to generate a room
temperature catalytic active species, has been synthesized. As
such, complex 3 ranks among the very best systems for this
cross-coupling reaction.
Therefore, 1-phosphabarrelenes should be regarded as a
promising class of ligands in catalyzed cross-coupling reac-
tions, due to the modular synthetic approach that allows fine
tuning of their steric properties.
´
The CNRS, the Ecole Polytechnique and the IDRIS (for
computer time, project no. 081616) are thanked for supporting
this work.
9 (a) A. F. Littke, C. Y. Dai and G. C. Fu, J. Am. Chem. Soc., 2000,
122, 4020; (b) A. F. Littke and G. C. Fu, Angew. Chem., Int. Ed.,
1998, 37, 3387.
10 N. Marion, O. Navarro, J. G. Mei, E. D. Stevens, N. M. Scott and
S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101.
Notes and references
11 O. Diebolt, P. Braunstein, S. P. Nolan and C. S. J. Cazin, Chem.
Commun., 2008, 3190.
12 Samples of 2 have been stored under ambient conditions for six
months without decomposition.
z Crystal data for 2: C19H29PSi2, M = 344.57, orthorhombic, space
group Pnma, a = 15.043(1), b = 16.246(1), c = 8.401(1) A, V =
2053.1(3) A3, T = 150.0(1) K, Z = 4, 12 956 reflections measured,
3085 unique (Rint = 0.0412), which were used in all calculations. The
final R1 and wR2 were 0.0707 and 0.1174 (all data). CCDC 699913.w
Crystal data for 3: C22H34ClPPdSi2, M = 527.49, triclinic, space
group P-1, a = 10.056(1), b = 15.490(1), c = 16.211(1) A,
a = 86.602(1), b = 83.472(1), g = 89.719(1)1, V = 2504.4(3) A3,
T = 150.0(1) K, Z = 4, 22 441 reflections measured, 11 297 unique
(Rint = 0.0598), which were used in all calculations. The final R1 and
wR2 were 0.0723 and 0.1479 (all data). CCDC 699914.w
13 (a) G. Frenking and N. Frohlich, Chem. Rev., 2000, 100, 717;
¨
(b) S. Dapprich and G. Frenking, J. Phys. Chem., 1995, 99, 9352;
(c) S. I. Gorelsky and A. B. P. Lever, J. Organomet. Chem., 2001,
635, 187; (d) S. I. Gorelsky, AOMix: Program for Molecular
Orbital Analysis, University of Ottawa, Ottowa, 2007
(http://www.sg-chem.net/).
14 A. Immirzi and A. Musco, J. Chem. Soc., Chem. Commun., 1974,
400.
15 S. P. Fantasia, Chem.–Eur. J., 2008, 14, 6987.
16 Z. Li, Y. Fu, Q.-X. Guo and L. Liu, Organometallics, 2008, 27,
4043.
Crystal data for 4: C38H58P2PdSi4, M = 795.54, orthorhombic,
space group Pbca, a = 10.310(1), b = 18.846(1), c = 43.610(1) A,
V = 8473.5(10) A3, T = 150.0(1) K, Z = 8, 44 281 reflections
measured, 10 810 unique (Rint = 0.0872), which were used in all
ꢀc
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Chem. Commun., 2009, 201–203 | 203