Synthesis, characterization, and reactivity of monomeric, arylpalladium halide complexes with a hindered phosphine as the only dative ligand

Article abstract of DOI:10.1021/ja0264394

We report the isolation and structural characterization of several monomeric arylpalladium(II) halide complexes containing tri-tert-butyl phosphine, 1-adamantyl-di-tert-butylphosphine, or 2-adamantyl-di-tert-butylphosphine. X-ray diffraction, IR spectroscopy, and theoretical studies indicated that the complexes may be stabilized by agostic interactions. For example, the distance from the closest hydrogen atom to the palladium metal center in the X-ray structure of the 1-adamantyl-phenylpalladium bromide complex 1 was 2.26(3) A. The calculated Pd-H distance of 2.28 A and harmonic vibrational frequencies were in agreement with the measured distance, but Wiberg bond indices indicated only weak M-H-C interactions. Addition of 2-adamantyl-di-tert-butyl phosphine to 1 led to ligand exchange and formation of 2-adamantyl-di-tert-butyl phosphine complex 2. Addition of P(t-Bu)₃ generated free aryl bromide and Pd[P(t-Bu)₃]₂. Reactivity of complex 1 with nucleophiles provided evidence of the intermediacy of these complexes in palladium-catalyzed cross-coupling reactions. Complex 1 reacted with amine and base to form the corresponding arylamine, with tert-butoxide to form the corresponding ether, with boronic acid and fluoride to form the corresponding biaryl, and with styrene to form stilbene. This complex also catalyzed the reaction of bromobenzene with diphenylamine in the presence of base to produce 94% of the amination product in 45 min at room temperature. This rate is comparable to the reaction rate of in situ generated catalysts. Copyright

Full text of DOI:10.1021/ja0264394

Published on Web 07/17/2002
Synthesis, Characterization, and Reactivity of Monomeric, Arylpalladium
Halide Complexes with a Hindered Phosphine as the Only Dative Ligand
James P. Stambuli,‡ Michael Bu¨hl,§ and John F. Hartwig*,‡
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107, and
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany
Received April 5, 2002
Most organometallic reactions occur through intermediates that
possess an open coordination site.1 Such unsaturated intermediates
are generally detected by kinetic studies and are rarely observed
directly or isolated. For example, oxidative addition2,3 and reductive
elimination4,5 are often preceded by dissociation of a dative ligand,
and migratory insertion requires an open coordination site for
binding of a substrate.6 As a result, palladium-catalyzed cross
coupling is commonly proposed to involve 14-electron, three-
coordinate complexes.7-9 Three-coordinate, 14-electron palladium-
(II) complexes have not been isolated, but are believed to be favored
when the catalysts possess sterically hindered ligands.10-13 Three-
coordinate 14-electron nickel [Ni(mes)3]-14 and rhodium [Rh-
(PPh3)]+15 complexes have been isolated, but these unsaturated
species are not intermediates in catalytic chemistry. We report the
isolation of a series of monomeric arylpalladium(II) halide com-
plexes with a single dative ligand. These complexes are likely
intermediates in metal-catalyzed cross-coupling reactions.
The monophosphine, arylpalladium halide complexes were
synthesized as shown in Scheme 1. The combination of Pd(dba)2,
phosphine, and aryl halide in THF solvent formed arylpalladium
halide complexes 1-3b, as shown in Scheme 1.16 The monophos-
phine-ligated arylpalladium halides were also prepared by dissolu-
tion of Pd(dba)2 and the appropriate ligand in neat aryl halide. The
products were isolated by precipitation from THF after concentration
of the reaction solution or from the neat aryl halide by addition of
pentane. Formation of dimeric Pd(I) halides [LPdX]217 accompanied
the generation of the oxidative addition product, but the formation
of this dimeric material was suppressed by conducting reactions
with concentrations of palladium below 0.1 M in THF solvent or
below 0.5 M in neat aryl halide.
These complexes were also obtained by direct oxidative addition
of aryl halides to the L2Pd(0) precursors.18 Addition of 40 equiv of
PhBr to Pd[P(t-Bu)2(1-adamantyl)]2 formed 1 in 50% yield. Addi-
tion of 40 equiv of PhI to Pd[P(t-Bu)3]219 formed 3a in 50% yield
along with 36% of the Pd(I) dimer. Reaction of Pd[P(t-Bu)3]2 with
40 equiv of PhBr formed small amounts of the oxidative addition
product after 12 h, but many side products were formed at longer
reaction times. These results are consistent with reductive elimina-
tion of aryl bromides and chlorides and ortho-substituted aryl
iodides reported recently.20 These oxidative additions to isolated
L2Pd complexes were slower than additions to the 1:1 mixture of
Pd(dba)2 and ligand.
Figure 1. X-ray structure of 1 is shown on the left, and 3b is shown on
the right.
Scheme 1
structure was obtained (Figure 1). Complex 1 is a T-shaped
monomer. The phenyl group is located trans to the open coordina-
tion site of the metal. This geometry allows the ligand with the
greatest steric demand to bind in the least hindered position and
the covalent ligand with the largest trans effect23 to bind trans to
the open site. The angles about the palladium are 100.9°, 91.3°,
and 162.6°. Hydrogens H17 and H18 were refined isotropically,
and the distance between the palladium and the closest hydrogen
(H18) is 2.27(3) Å, which is within the distance of an agostic
interaction.24 Crystals of P(t-Bu)3-ligated 2-m-xylyl complex 3b
were suitable for X-ray diffraction (Figure 1). Again, the geometry
was T-shaped. The angles about palladium were 100.9°, 94.0°, and
164.6°. The calculated distance between the metal and H27 bound
to C16 was 2.33 Å, which could, again, reflect an agostic reaction.
31P and 13C NMR spectroscopy suggested that similar structures
were present in solution. The unusual coordination number was
indicated by an upfield, rather than downfield, change in chemical
shift upon oxidative addition to the L2Pd complex. Typical arylpal-
ladium halide complexes display 31P NMR chemical shifts that are
located downfield of the corresponding Pd(0) complex.21 A cis
disposition of aryl and phosphine groups was shown by a small
JP-C for the ipso carbon.
Infrared spectroscopy of complexes 1 and 3 in KBr did not
provide evidence for agostic interactions. However, infrared spectra
of complex 2 contained a medium-strong band at 2710 cm-1. This
stretching frequency is reduced from the C-H stretches of the free
ligand and indicates a stronger agostic interaction between the metal
and the ligand C-H bond in compound 2 than of compounds 1 or
3a,b.
1H NMR spectra of complexes 1 and 2 obtained at -115 °C
were broad, poorly resolved, and difficult to interpret with the large
number of adamantyl resonances. Low-temperature NMR spectra
Previous arylpalladium halide complexes with a single hindered
ligand, such as P(o-tol)3, have been dimeric,21,22 but X-ray diffrac-
tion of 1 and 3b showed that the complexes described here are
monomeric. Small, light orange crystals of complex 1 were grown
from a cold solution of pentane and THF (95/5), and its X-ray
* To whom correspondence should be addressed. E-mail: john.hartwig@yale.edu.
‡ Yale University.
§ Max-Planck-Institut fu¨r Kohlenforschung.
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10.1021/ja0264394 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
concentrations of added ligand showed clean, inverse first-order
rate behavior. No adduct between 1 and the added ligand was
detected by 31P NMR spectroscopy. Thus, the transition state of
the rate-determining step lacks any coordinated phosphine. Dis-
sociation of the large phosphine may be necessary to allow binding
of the olefin or to allow binding of the olefin cis to the aryl group.
Complex 1 is kinetically competent to be an intermediate in the
palladium-catalyzed amination of aryl halides.8,31 The reaction of
bromobenzene with diphenylamine occurred in 45 min at room tem-
perature in 98% yield. This rate is comparable to the rate of reaction
catalyzed by complexes of P(1-Ad)(t-Bu)2 generated in situ.
In summary, we have isolated a set of structurally unique aryl-
palladium halide complexes that are intermediates in cross-coupling
reactions induced by recently studied, highly active catalysts. These
complexes should allow detailed mechanistic studies of a variety
of palladium-catalyzed processes.
of P(t-Bu)3-complex 3 were better resolved, but the 1H NMR
spectrum at -115 °C did not contain any upfield signal that would
definitively demonstrate an agostic interaction.
Thus, we computed geometries and harmonic vibrational fre-
quencies at the BP86/ECP1 level;25 a gradient-corrected density
functional, effective core potentials on Pd, Br, and I,26 and polarized
split-valence basis sets were used to provide information on the
presence or absence of an agostic interaction in these complexes.
The calculations showed low-energy C-H vibrations decreasing
in the order 3a (2919 cm-1) > 1 (2812 cm-1) > 2 (2750 cm-1).
The optimized Pd-H distances decrease in the same sequence, 2.46
> 2.28 > 2.15 Å, while the concomitant Wiberg bond indices
(WBIs)27 that probe for covalent interactions increase in the same
order 0.015 < 0.023 < 0.041. For comparison, the WBI for the
Pd-Br bond in 2 was 0.42. Thus, we conclude from the static
calculations that the strength of the M-H interaction in 1-3 follows
the trend 3 < 1 < 2, and this trend parallels the stability of the
arylpalladium halide derivatives.
Calculations of 1H NMR chemical shifts28 using a hybrid density
functional and a large all-electron basis set of approximately
polarized triple-ú quality on all atoms in C1 symmetric 1 and 2
predicted resonances for the C-H closest to the metal of δ ) +0.6
and -0.4, respectively. Averaging of chemically equivalent hy-
drogens creates unremarkable δ(1H) values that are consistent with
the experimental spectra.
The formation of 1-3 from the Pd(dba)2 precursor most likely
occurs by displacement of one coordinated dba by phosphine and
oxidative addition of the aryl halide to either of the monophosphine
intermediates LPd or LPd(dba). This mechanism is depicted in
Scheme 2. Because aryl halides reacted with L2Pd(0) complexes
more slowly than with Pd(dba)2 and ligand, the reactions initiated
by Pd(dba)2 and ligand cannot occur through L2Pd(0). This
difference in the rate of formation of 1 parallels the differences
between the rate of catalytic reactions initiated by a 1:1 ratio of
Pd(dba)2 to phosphine and by isolated L2Pd(0).9,29,30
Acknowledgment. We thank the NIH (GM-58108) for support
of this work and Johnson-Matthey for a gift of PdCl2. M.B. thanks
Prof. W. Thiel and the Deutsche Forschungsgemeinschaft for
support.
Supporting Information Available: Experimental procedures and
spectroscopic data of new complexes; X-ray structural data for 1 and
3b (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; John Wiley & Sons: New York, 2000.
(2) Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314.
(3) Hartwig, J. F.; Paul, F. J. Am. Chem. Soc. 1995, 117, 5373.
(4) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933.
(5) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn.
1981, 54, 1868.
(6) Halpern, J.; Wong, C. S. J. Chem. Soc., Chem. Commun. 1973, 17, 629.
(7) Alcazar-Roman, L. M.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 12905.
(8) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(9) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020.
(10) Beare, N. A.; Hartwig, J. F. J. Org. Chem. 2002, 67, 541.
(11) Stambuli, J. P.; Stauffer, S. R.; Shaughnessy, K. H.; Hartwig, J. F. J. Am.
Chem. Soc. 2001, 123, 2677.
(12) Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295.
(13) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J. P.; Sadighi, J. P.;
Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 4369.
(14) Hay-Motherwell, R.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B.
Polyhedron 1996, 15, 3163.
(15) Knobler, C. B.; Marder, T. B.; Mizusawa, E. A.; Teller, R. G.; Long, J.
A.; Behnken, P. E.; Hawthorne, M. F. J. Am. Chem. Soc. 1984, 106, 2990.
(16) A product from Pd(dba)2, PtBu3, and 4-iodotoluene has been proposed,
but not isolated. see ref 9.
(17) Dura-Vila, V.; Mingos, D. M. P.; Vilar, R.; White, A. J. P.; Williams, D.
J. J. Organomet. Chem. 2000, 600, 198.
(18) Jutand and Brown recently reported reactions of PhI with Pd(0) complexes
of sterically varied alkylphosphines. However, 3a was not isolated, and a
typical dimeric structure was proposed: Galardon, E.; Ramdeehul, S.;
Brown, J. M.; Cowley, A.; Hii, K. K.; Jutand, A. Angew. Chem., Int. Ed.
2002, 41, 1760.
Preliminary reactivity of the three-coordinate arylpalladium halide
complexes is summarized in Scheme 3. Reaction between 1 and
2-adamantyl-di-tert-butylphosphine formed 2 quantitatively. Con-
versely, treatment of 2 with 1-adamantyl-di-tert-butylphosphine
produced only trace amounts of 1. Reaction between tri-tert-
butylphosphine and 1 consumed 1 completely and formed
Pd(P-t-Bu3)2 in 25% yield and bromobenzene in 39% yield.
Addition of diphenylamine and sodium tert-butoxide to 1 gave
triphenylamine in 99% yield, while addition of sodium tert-butoxide
to 1 gave tert-butyl phenyl ether in 53% yield. Reaction of
4-methoxyphenylboronic acid with 1 in the presence of KF in THF
formed 4-methoxybiphenyl in 94% yield at room temperature.
Complex 1 also reacted with styrene to form 83% yield of
stilbene and the hydridopalladium halide complex. Despite the open
coordination site present in complex 1, the formation of stilbene
was inhibited by added phosphine. Values of kobs at varying
(19) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem. Soc.
1976, 98, 5850.
(20) Roy, A. H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 1232.
(21) Paul, F.; Patt, J.; Hartwig, J. F. Organometallics 1995, 14, 3030.
(22) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. Organometallics 1996,
15, 2745.
(23) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles
of Structure and ReactiVity; Harper Collins: New York, 1993.
(24) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988,
36, 1.
(25) See, e.g.: Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474.
(26) Kaupp, M.; Schleyer, P. v. R.; Stoll, H.; Preuss, H. J. Am. Chem. Soc.
1991, 113, 6012.
(27) Wiberg, K. B. Tetrahedron 1968, 24, 1083.
(28) GIAO-B3LYP/II level as used in Bu¨hl, M.; Gaemers, S.; Elsevier, C. J.
Chem. Eur. J. 2000, 6, 3272.
(29) Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66, 3402.
(30) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(31) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem.
Res. 1998, 31, 805.
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