Imidazolylphosphine Ligands on Pd(0) and Pd(II)
A R T I C L E S
rides20 and even alkyl halides and related substrates.21-27 In
many cases, it is clear that even minor changes in ligand can
make the difference between a high-yielding catalyst and an
inactive one.21,22,28 The extent of interest in new types of ligands
for Suzuki coupling catalysts is exemplified by at least 10
references in 2004 alone on new phosphines,29-38 with focus
on ones which are sterically demanding and/or hemilabile,
favoring more rapid catalysis. In addition, other catalytic C-C,
C-O, and C-N bond-forming reactions on C-X (X ) halide
or sulfonate) are facilitated by using sterically demanding
phosphines.39-44
ability of the N donor in the hybrid complex to dissociate from
the Ni(0) center during the catalytic cycle. In addition, the
hybrid, hemilabile ligands in Rh complex A were found to
The large phosphines used have included P(t-Bu)3, which can
be used to make the isolable, air-stable species Pd[P(t-Bu)3]2.45,46
There is considerable evidence that even this coordinatively
unsaturated species loses a ligand, producing a catalytically
active 12-electron monophosphine intermediate.18,19,47
Hybrid ligands offer a way to stabilize low-coordinate species
with a weakly coordinating ligand which can readily be removed
from the metal, facilitating catalytic reactions. The most
intensive study has been on hybrid P-O- and P-N-based
ligands, which can show large and useful differences in binding
affinity of the soft P and hard O or N donors for a given metal,
which may itself be hard or soft.48 For example, on related Ni-
(0) complexes, the hybrid ligand (i-Pr)2PCH2CH2PNMe2 af-
forded good yields of organic products, whereas the analogous
chelating bis(phosphine) (i-Pr)2PCH2CH2P(i-Pr)2 either gave no
or little product.49,50 These differences were ascribed to the
facilitate the carbonylation of methyl iodide in comparison with
conventional monodentate trialkylphosphines in related sys-
tems.51 Effects of oxygen substituents on rate and selectivity
of hydrogenation and hydroformylation have been noted.6,52
Among complexes with P-N-based hybrid ligands, those with
pyrid-2-yl-modified phosphorus ligands were found to improve
selectivity in hydroformylation of some olefins.53,54
In this contribution we describe a new group of phosphine
ligands which are sterically demanding because of two large
alkyl groups on P and which are hybrid because the third
substituent is an imidazol-2-yl unit. For comparison, in recent
years, a great deal of attention has been drawn to 2-pyridylphos-
phines,55 mainly as bridging ligands; however, despite synthesis
of literally hundreds of their complexes there have been
surprisingly few improved catalysts using such bifunctional
ligands.56-59 By contrast, complexes of imidazolylphosphines
have been much less-studied. Ligands with three imidazole
groups have been used as tridentate (N)3 ligands, bioinorganic
models for three histidines in a protein.60-64 In these species
the phosphorus is not bound to the metal and only serves as a
connecting element. There have been a few reports of imida-
zolylphosphines as bridging ligands,65-73 bound through both
(20) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176-4211.
(21) Netherton, M. R.; Dai, C.; Neuschu¨tz, K.; Fu, G. C. J. Am. Chem. Soc.
2001, 123, 10099-10100.
(22) Kirchhoff, J. H.; Dai, C.; Fu, G. C. Angew. Chem., Int. Ed. Engl. 41, 1945-
1947.
(23) Kirchhoff, J. H.; Netherton, M. R.; Hills, I. D.; Fu, G. C. J. Am. Chem.
Soc. 2002, 124, 13662-13663.
(24) Netherton, M. R.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 3910-3912.
(25) Menzel, K.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 3718-3719.
(26) Lee, J.-Y.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 5616-5617.
(27) Zhou, J.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 12527-12530.
(28) Hills, I. D.; Netherton, M. R.; Fu, G. C. Angew. Chem., Int. Ed. 2003, 42,
5749-5752.
(50) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Simhai, N.; Iverson, C. N.;
Mu¨ller, C.; Satoh, T.; Jones, W. D. J. Mol. Catal. A: Chem. 2002, 189,
157-168.
(51) Lindner, E.; Andres, B. Chem. Ber. 1988, 121, 829-832.
(52) Horner, L.; Simons, G. Z. Naturforsch. B: Chem. Sci. 1984, 39, 497-
503.
(29) DeVasher, R. B.; Moore, L. R.; Shaughnessy, K. H. J. Org. Chem. 2004,
69, 7919-7927.
(30) Smith, R. C.; Woloszynek, R. A.; Chen, W.; Ren, T.; Protasiewicz, J. D.
Tetrahedron Lett. 2004, 45, 8327-8330.
(53) Basoli, C.; Botteghi, C.; Cabras, M. A.; Chelucci, G.; Marchetti, M. J.
Organomet. Chem. 1995, 488, C20-C22.
(31) Baillie, C.; Zhang, L.; Xiao, J. J. Org. Chem. 2004, 69, 7779-7782.
(32) Brenstrum, T.; Gerristma, D. A.; Adjabeng, G. M.; Frampton, C. S.; Britten,
J.; Robertson, A. J.; McNulty, J.; Capretta, A. J. Org. Chem. 2004, 69,
7635-7639.
(54) Kurtev, K.; Ribola, D.; Jones, R. A.; Cole-Hamilton, D. J.; Wilkinson, G.
J. Chem. Soc., Dalton Trans. 1980, 55-58.
(55) Newkome, G. R. Chem. ReV. 1993, 93, 2067-2089.
(56) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J. Organomet. Chem. 1994,
475, 57-63.
(33) Colacot, T. J.; Shea, H. A. Org. Lett. 2004, 6, 3731-3734.
(34) Kwong, F. Y.; Lam, W. H.; Yeung, C. H.; Chan, K. S.; Chan, A. S. C.
Chem. Commun. 2004, 1922-1923.
(57) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J. Organomet. Chem. 1993,
455, 247-253.
(35) Weng, Z.; Teo, S.; Koh, L. L.; Hor, T. S. A. Organometallics 2004, 23,
4342-4345.
(58) Drent, E.; Van Broekhoven, J. A. M.; Doyle, M. J. J. Organomet. Chem.
1991, 417, 235-251.
(36) Weissman, H.; Shimon, L. J. W.; Milstein, D. Organometallics 2004, 23,
3931-3940.
(59) 3- and 4-Pyridylphosphines have been used recently as bridging ligands to
assemble polynuclear catalysts: Slagt, V. F.; Kamer, P. C. J.; Van Leeuwen,
P. W. N. M.; Reek, J. N. H. J. Am. Chem. Soc. 2004, 126, 1526-1536.
Slagt, V. F.; Reek, J. N. H.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.
Angew. Chem., Int. Ed. 2001, 40, 4271-4274. Kleij, A. W.; Lutz, M.; Spek,
A. L.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Chem. Commun.
(Cambridge) 2005, 3661-3663.
(37) Wang, A.-E.; Zhong, J.; Xie, J.-H.; Li, K.; Zhou, Q.-L. AdV. Synth. Catal.
2004, 346, 595-598.
(38) Harkal, S.; Rataboul, F.; Zapf, A.; Fuhrmann, C.; Riermeier, T.; Monses,
A.; Beller, M. AdV. Synth. Catal. 2004, 346, 1742-1748.
(39) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852-860.
(40) Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046-2067.
(41) Hartwig, J. F. In Modern Amination Methods; Ricci, A., Ed.; Wiley-VCH:
Weinheim, Germany, 2000.
(60) Slebocka-Tilk, H.; Cocho, J. L.; Frackman, Z.; Brown, R. S. J. Am. Chem.
Soc. 1984, 106, 2421-2431.
(42) Hartwig, J. F. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley: Hoboken, NJ, 2002; pp 1051-1096.
(43) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res.
1998, 31, 805-818.
(61) Brown, R. S.; Zamkanei, M.; Cocho, J. L. J. Am. Chem. Soc. 1984, 106,
5222-5228.
(62) Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg. Chem. 1995, 34, 952-
960.
(44) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125-146.
(45) Matsumoto, M.; Yoshioka, H.; Nakatsu, K.; Yoshida, T.; Otsuka, S. J. Am.
Chem. Soc. 1974, 96, 3322-3324.
(63) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Dalton 2000,
2191-2194.
(64) Kimblin, C.; Murphy, V. J.; Hascall, T.; Bridgewater, B. M.; Bonanno, J.
B.; Parkin, G. Inorg. Chem. 2000, 39, 967-974.
(46) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem. Soc.
1976, 98, 5850-5858.
(65) Burini, A.; Pietroni, B. R.; Galassi, R.; Valle, G.; Calogero, S. Inorg. Chim.
Acta 1995, 229, 299-305.
(47) Hartwig, J. F.; Paul, F. J. Am. Chem. Soc. 1995, 117, 5373-5374.
(48) Pearson, R. G.; Scott, A. In SurVey of Progress in Chemistry; Academic
Press: New York, 1969; Chapter 1.
(66) Burini, A.; Galassi, R.; Pietroni, B. R.; Rafaiani, G. J. Organomet. Chem.
1996, 519, 161-167.
(49) Mu¨ller, C.; Lachicotte, R. J.; Jones, W. D. Organometallics 2002, 21, 1975-
(67) Bachechi, F.; Burini, A.; Galassi, R.; Macchioni, A.; Pietroni, B. R.; Ziarelli,
F.; Zuccaccia, C. J. Organomet. Chem. 2000, 593-594, 392-402.
1981.
9
J. AM. CHEM. SOC. VOL. 128, NO. 2, 2006 439