Organometallic chemistry of amidate complexes. Accelerating effect of bidentate ligands on the reductive elimination of N-aryl amidates from palladium(II)

Article abstract of DOI:10.1021/ja062333n

We report a series of arylpalladium complexes of acetamidate, sulfonamidate, and deprotonated oxazolidinone ligands that undergo reductive elimination with rates and yields that depend on the binding mode of the ancillary and amidate ligands. Complexes of the acetamidate ligands containing the bidentate phosphines DPPF and Xantphos as ancillary ligands undergo reductive elimination. The rate and yield were higher from the complex ligated by Xantphos, which contains a larger bite angle. In contrast, the analogous amidate complex containing a single sterically hindered monodentate ligand and a κ2-bound amidate ligand does not undergo reductive elimination. This trend of faster reductive elimination from complexes containing bidentate ancillary ligands than from a complex with a single monodentate ancillary ligand is unusual and is consistent with an effect of the denticity of the ancillary ligand on the binding mode of the amidate. Complexes of sulfonamidate ligands underwent reductive elimination faster than complexes of acetamidates, and reductive elimination occurred from complexes containing both bidentate and monodentate ancillary ligands. Like reductive elimination from the acetamidate complexes, reductive eliminations from the sulfonamidate complexes were faster when the complexes possessed bidentate Xantphos and κ¹-sulfonamidate ligands. Copyright

Full text of DOI:10.1021/ja062333n

Published on Web 06/23/2006
Organometallic Chemistry of Amidate Complexes. Accelerating Effect of
Bidentate Ligands on the Reductive Elimination of N-Aryl Amidates from
Palladium(II)
Ken-ichi Fujita, Makoto Yamashita, Florian Puschmann, Miguel Martinez Alvarez-Falcon,
Christopher D. Incarvito, and John F. Hartwig*
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107
Received April 5, 2006; E-mail: john.hartwig@yale.edu
Organometallic reactions of isolated transition metal amides, such
as reductive elimination1 and migratory insertion,2 have begun to
develop during the past decade, but the organometallic reactions
of transition metal amidates (deprotonated amides or sulfonamides)
have not. Several properties of amidate ligands are likely to limit
their reactivity. First, an amidate ligand should be less nucleophilic
than an amide ligand due to the electron-withdrawing carbonyl
group. Second, the M-N bond of an amidate complex would
contain more ionic character than that of an amide complex, and
this ionic character should increase the strength of the M-N bond.3
Third, amidates can bind in a κ2-fashion,4 and the additional metal-
ligand bond in a κ2-binding mode would be expected to inhibit
reactions that occur through a κ1-structure.
To determine how to overcome these limitations, we have begun
to study organometallic reactions of metal amidate complexes. We
have focused first on the isolation, structural characterization, and
reaction chemistry of arylpalladium amidate complexes possessing
both monodentate and bidentate ligands. We report the first results
methylsulfonamidate, and the potassium salt of 2-oxazolidinone to
of this work, which include the direct observation of reductive
Addition of potassium N-phenylacetamidate, potassium N-
Xantphos complex 1 generated the corresponding arylpalladium
elimination to form N-aryl amides. In contrast to reductive
amidate, sulfonamidate, and carbamate complexes 2-4 (eq 2).
Formation of the palladium sulfonamidate required excess of the
potassium salt and methylene chloride, instead of THF, as solvent,
but the sulfonamidate 3 was isolated in acceptable yield.
eliminations that form the C-N bond in amines and C-C bonds
in alkanes and alkylarenes,5-8 these reductive eliminations that form
the C-N bond in N-aryl amides occurred faster from the complexes
containing bidentate ligands than from complexes containing a
single hindered, monodentate ligand. Structural data imply that this
opposing trend results from the distinct bonding modes of the
amidate ligands in compounds containing the two classes of ligand.
To study the rates of reductive elimination of N-aryl amides,
we prepared arylpalladium amidate complexes containing the
bidentate ligand Xantphos (eq 1), the bidentate ligand 1,1-bis-
(diphenylphosphino)ferrocene (DPPF), and the hindered mono-
dentate ligand FcPBut2 (Fc ) ferrocenyl). Complexes of these
ligands were studied because catalysts containing Xantphos are
particularly reactive for the coupling of amides and sulfonamides
with bromoarenes,9,10 complexes of DPPF would allow a compari-
son of the rates of reaction of amidate complexes with those of
amido complexes reported previously,6 and the FcPBut2 ligand is
a representative hindered monophosphine that has previously
induced reductive elimination from typically unreactive malonate
complexes and electron-poor diarylamido complexes.11 The prepa-
ration of arylpalladium amidate complexes ligated by these phos-
phines is shown in eq 1-3.
The Xantphos-ligated amidate complexes were mixtures of cis
and trans isomers, with the cis isomers of 2 and 3 predominating.
The 31P NMR resonance of 2 at room temperature was a singlet
that proved to correspond to a rapidly interconverting mixture of
isomers. The singlet broadened at 0 °C and resolved at -80 °C
into signals consistent with a 65:35 ratio of the cis isomer and the
combination of two trans isomers from slow rotation about the
M-N bond. The 31P NMR resonance of 3 at room temperature
was also a broad resonance that corresponded to a rapidly
interconverting mixture of cis and trans isomers. The cis:trans ratio
was 75:25 in this case at -95 °C. The 31P NMR spectrum of
carbamate 4 at room temperature contained signals for both isomers,
but was broad. The resonances sharpened at 0 °C, and a cis:trans
ratio of 20:80 was measured. Complex 3 crystallized as the trans
isomer. The structure determined by X-ray diffraction (Figure 1)
confirmed that the sulfonamidate ligand is bound in a κ1-mode.
Arylpalladium amidate 5 ligated by DPPF was prepared in 70%
yield by similar procedures (eq 2). The NMR and IR spectra of
this complex indicated that the structure contains a standard square-
planar cis geometry with a κ1-amidate ligand.
The phenylpalladium bromide complex 1 ligated by Xantphos
was prepared by heating Pd(dba)2, Xantphos, and bromobenzene
at 80 °C for 4 h in benzene solvent (eq 1).10,12,13 The 31P NMR
spectrum of the arylpalladium bromide complex 1 displayed one
singlet (9.6 ppm) at room temperature that broadened below room
temperature and resolved at -95 °C into signals corresponding to
Analogous compounds ligated by the hindered monodentate
phosphine FcPBut2 were prepared as summarized in eq 3. The
FcPBut2-ligated arylpalladium iodide was converted to the N-
phenylacetamidate 6 and N-methylsulfonamidate 7 by addition of
potassium N-phenylacetamidate and N-methylsulfonamidate. These
two complexes were isolated in 55 and 74% yield, respectively.
X-ray diffraction of 6 and 7 (see Figure 1 for the structure of 6
1
a 98.5:1.5 ratio of trans-to-cis isomers. The H NMR spectrum at
-95 °C also contained resonances corresponding to cis and trans
isomers (see Supporting Information).
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C O M M U N I C A T I O N S
interaction in the κ2-amidate structure of the complex containing a
monodentate ligand adds to an already considerable barrier for
reductive elimination.
Finally, the N-methylsulfonamidate complexes underwent reduc-
tive elimination faster than the corresponding N-phenylacetamidate
complexes (eq 6). Heating of the Xantphos-ligated N-methyl-
sulfonamidate complex 3 in C6D6 with 2 equiv of added Xantphos
at 90 °C for 5 h induced reductive elimination of N-methyl
N-phenylsulfonamide in 88% yield, even though the N-methyl-
sulfonamido group contains â-hydrogens. The rate constant for this
process at 90 °C was (4.7 ( 0.2) × 10-4 s-1, whereas the rate
constant for elimination from amidate 2 at this temperature was
(2.5 ( 0.2) × 10-4 s-1. The sulfonamidate complex 7 ligated by
the ferrocenyl ligand underwent reductive elimination in 82% yield,
but this reaction required 30 h at 90 °C. This difference in rate of
reaction of Xantphos complex 3 and FcPBut2 is further consistent
with slower eliminations from κ2-amidate complexes containing
monodentate ligands than from κ1-amidate complexes containing
bidentate ligands.
Figure 1. ORTEP diagrams of 3 and 6 illustrating the κ1- and κ2-binding
modes. The Pd-O distances for 3, 6, and 7 (see Supporting Information)
are 2.798(2), 2.2678(17), and 2.303(2) Å, and the Pd-N distances are
2.139(3), 2.098(2), and 2.085(3) Å. The P-Pd-P angle in 3 is 143.93(3)°.
and Supporting Information for the structure of 7) revealed that
they are four-coordinate complexes with the acetamidate and
sulfonamidate ligands coordinated in κ2-binding modes.
The reductive elimination chemistry of the amidate complexes
is shown in eq 4. Heating of DPPF-ligated 5 at 110 °C in the
presence of additional DPPF (2 equiv) induced reductive elimination
to give N,N-diphenylacetamide, but this reaction required 29 h to
reach >90% conversion, and the yield of the reductive elimination
product was only 34%. This reductive elimination is, thus, much
slower and occurs in lower yield than reductive elimination from
DPPF-ligated arylpalladium amides.6 This comparison is consistent
with the trend of decreasing rate of reductive eliminations from
complexes containing ligands that are less basic, less nucleophilic,
and bound to the metal by more ionic M-N bonds.1,6
In general, these first direct observations of reductive eliminations
to form the C-N bonds in N-aryl amides and sulfonamides illustrate
that reductive elimination of N-aryl amides and sulfonamides from
complexes with bidentate ligands can be faster than those from
complexes with monodentate ligands. This trend is consistent with
the difference in ground state binding modes of the amidate ligands
in complexes of monodentate and bidentate phosphines. These
results also suggest that a large bite angle in the ancillary ligand
helps promote reductive elimination from complexes with weakly
nucleophilic amidate ligands bound to palladium by relatively strong
and ionic M-N bonds.
Acknowledgment. We thank the NIH (NIGMS GM-58108) for
support of this work.
Supporting Information Available: Experimental procedures and
characterization of all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
The analogous arylpalladium amidate complex 6 containing the
hindered FcPBut2 ligand was stable for hours below 110 °C and
decomposed over 2 h without formation of Ph2NC(O)Me at 110
°C. This high stability contrasts with the facile reductive elimination
of triarylamine below room temperature from a closely related
arylpalladium ditolylamido complex.7
In contrast to these slow reactions, heating of the Xantphos
complexes 2 and 4 containing the anions of N-phenylacetamide
(eq 4) and oxazolidinone (eq 5) with 2 equiv of added Xantphos
for 7 h at 90 °C formed the N,N-diphenylacetamide and N-phenyl-
2-oxazolidinone in 93 and 88% yield, respectively, along with the
palladium product Pd(Xantphos)2.14 The amount of added Xantphos
did not affect the rate of the reductive elimination, implying that
reductive elimination occurs directly from the starting 2 and 4 (see
Supporting Information).
The faster reductive elimination from Xantphos complexes 2 and
4 than from DPPF and FcPBut2 complexes 5 and 6 leads to two
conclusions. First, the differences in rates of reaction of Xantphos
complex 1 and DPPF complex 5 imply that a large bite angle15
helps promote reductive elimination from amidate complexes, which
are more stable than the analogous amide complexes.6 Second, the
faster reactions of the amidate complexes containing bidentate
ligands imply that the binding mode of the amidate ligand in the
ground state affects the reaction rate. Assuming reductive elimina-
tion occurs from a κ1-structure, then the cleavage of the M-O
References
(1) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852.
(2) (a) Zhao, P. J.; Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127,
12066. (b) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. J. Am. Chem.
Soc. 1988, 110, 6738.
(3) Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J. K.; Nolan, S.
P. J. Am. Chem. Soc. 1997, 119, 12800.
(4) Zhang, Z.; Schafer, L. L. Org. Lett. 2003, 5, 4733.
(5) For comparison, see refs 6-8.
(6) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 8232.
(7) Yamashita, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 5344.
(8) (a) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933. (b) Hayashi,
T.; Knoishi, M.; Kumada, M. Tetrahedron Lett. 1979, 21, 1871.
(9) Yin, J.; Buchwald, S. L. Org. Lett. 2000, 2, 1101.
(10) Yin, J.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 6043.
(11) Wolkowski, J. P.; Hartwig, J. F. Angew. Chem., Int. Ed. 2002, 41, 4289.
(12) For one previous arylpalladium complex ligated by Xantphos, see refs
10 and 13.
(13) Zuideveld, M. A.; Swennenhuis, B. H. G.; Boele, M. D. K.; Guari, Y.;
van Strijdonck, G. P. F.; Reek, J. N. H.; Kamer, P. C. J.; Goubitz, K.;
Fraanje, J.; Lutz, M.; Spek, A. L.; van Leeuwen, P. W. N. M. J. Chem.
Soc., Dalton Trans. 2002, 2308.
(14) Klingensmith, L. M.; Strieter, E. R.; Barder, T. E.; Buchwald, S. L.
Organometallics 2006, 25, 82.
(15) (a) Brown, J. M.; Guiry, P. J. Inorg. Chim. Acta 1994, 220, 249. (b) Kamer,
P. C. J.; van Leeuwen, P. W. N.; Reek, J. N. H. Acc. Chem. Res. 2001,
34, 895.
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