Preparation and C-X reductive elimination reactivity of monoaryl Pd IV-X complexes in water (X = OH, OH2, Cl, Br)

Article abstract of DOI:10.1021/ja107185w

Monohydrocarbyl palladium(IV) complexes bearing OH, OH₂, Br, and Cl ligands at the metal and supported by facially chelating 1-hydroxy-1,1-bis(2-pyridyl)methoxide were readily prepared in water at 0 °C. These complexes reductively eliminate Ar-

Full text of DOI:10.1021/ja107185w

Published on Web 09/24/2010
Preparation and C-X Reductive Elimination Reactivity of Monoaryl PdIV-X
Complexes in Water (X ) OH, OH2, Cl, Br)
Williamson Oloo, Peter Y. Zavalij, Jing Zhang, Eugene Khaskin, and Andrei N. Vedernikov*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742, United States
Received August 10, 2010; E-mail: avederni@umd.edu
Scheme 2
Abstract: Monohydrocarbyl palladium(IV) complexes bearing OH,
OH2, Br, and Cl ligands at the metal and supported by facially
chelating 1-hydroxy-1,1-bis(2-pyridyl)methoxide were readily pre-
pared in water at 0 °C. These complexes reductively eliminate
Ar-X (X ) OH, Br, Cl) in water at room temperature in high yield,
and the corresponding first-order rate constants kOH, kCl, and kBr
are on the same order of magnitude.
droxymethoxide ligands: L derived from di-2-pyridyl ketone (dpk)
and L′ derived from 2-aroylpyridine (aroyl ) benzoyl or 3-meth-
ylbenzoyl) (Scheme 3). Both L and L′ were expected to provide
better support for a MIV center than analogous sulfonate ligands
L′′ (Scheme 3) used in our previous works,19 which feature the
poorer O-donor sulfonate group. Complexes 3 and 4 were prepared
using 2 mM aqueous solutions of ionic PdII-2-aroylpyridine adducts
1(OAc) and 2(OAc), respectively, and 2 equiv of hydrogen peroxide
as an oxidant (Scheme 3, step a). In both cases, a dark-yellow color
typical of 3 and 4 developed. Both reactions were quantitative,
according to NMR spectroscopy. The oxidation was faster for the
methyl-substituted analogue 2(OAc),20 where the transformation
was complete in 30 min at room temperature. The cations 3 and 4
are stable in neutral aqueous solutions for an indefinitely long time
and were characterized by means of 1H and 13C NMR spectroscopy,
selective nuclear Overhauser effect (NOE) experiments, and elec-
trospray ionization mass spectrometry (ESI-MS). Complex 3(OAc)
was also characterized by X-ray diffraction (XRD) (Figure 1a).
The use of acetate as the counterion imparted relatively high
solubility in water to these and similar Pd(IV) complexes, but their
isolation and purification were greatly facilitated by precipitation
of cations 3 and 4 in the form of the corresponding less soluble
tetrafluoroborate or trifluoroacetate salts 3(Z) and 4(Z) (Z ) BF4,
CF3COO). These BF4 and CF3COO salts are stable in the solid
state and could be characterized by elemental analysis. In contrast,
the acetates decompose readily in the solid phase and in aprotic
polar solvents such as dimethyl sulfoxide. This reactivity may be
High-valent organopalladium complexes were introduced into
organometallic chemistry about 30 years ago1 and have recently
received much attention as intermediates in a variety of Pd-catalyzed
oxidative functionalization reactions.2-7 Hydrocarbyls of PdIV and/
or PdIII have been proposed as key intermediates in oxidative Pd-
catalyzed ligand-directed C-H functionalization (Scheme 1),6-9
olefin difunctionalization,3,4 heterocyclization,10 and others.7 Many
of these processes exploit the remarkable ability of high-valent
palladium hydrocarbyls to undergo facile C-Y bond reductive
elimination (e.g., see Scheme 1, step c; Y ) C, N, O, halogen,
etc.). Synthesis and C-Y bond elimination reactivity studies of a
number of model tri- and dihydrocarbyl PdIV complexes have been
reported.7,11-13 At the same time, PdIV monohydrocarbyls remain
elusive, and their reactivity is poorly known. In particular, it is
unknown whether PdIV monohydrocarbyls can be generated in
water,13 a solvent of choice for “green” chemistry. Few PdIV
monohydrocarbyls have been isolated to date14-17 (Scheme 2). C-F
and C-Cl bond reductive elimination reactivity in organic solvents
has been reported for some of these complexes.
We disclose here a simple protocol for the preparation in aqueous
solution of a number of cationic and neutral zwitterionic monoaryl
palladium(IV) complexes supported by 1-hydroxy-1,1-bis(2-py-
ridyl)methoxide (L)18 (Scheme 3) with various ligands X ) OH,
OH2, Cl, Br, or OC(OH)(2-pyridyl)(2-phenylene) at the metal. We
also report some results from studies of their reactivity, including
C-O, C-Cl, and C-Br bond-forming reactions. These results may
be important for development of PdII/PdIV-mediated C-H func-
tionalization chemistry in aqueous media.
Scheme 3
We first targeted the more stabilized19 cationic monohydrocarbyl
PdIV complexes 3 and 4 having two facially chelating 1-hy-
Scheme 1
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10.1021/ja107185w  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 5
The reactivity pattern that was observed for complexes 3 and 4
helped us design and accomplish the synthesis of the more reactive
monoaryl PdIV complexes 15-18 possessing only one facially chelating
ligand L. The monoaryl hydroxo PdIV complex 15(OAc) could be
generated quantitatively from palladacycle 14(OAc) in aqueous solu-
tions at 0 °C using 10 equiv of H2O2 (Scheme 5, step a). The complex
was characterized in the same solvent by means of 1H and 13C NMR
spectroscopy, NOE experiments, and ESI-MS. An attempt to isolate
15 from these solutions as a trifluoroacetate salt in the presence of
excess CF3COOH led instead to the poorly soluble dicationic monoaryl
PdIV aqua complex 16 (Scheme 5, step b; X ) H2O+) which was
fully characterized by means of NMR spectroscopy, ESI-MS, elemental
analysis, and single-crystal XRD (Figure 2a).
Similar low solubilities in cold water were observed for the
dichloride 16(Cl)2 and dibromide 16(Br)2 derived from the same
monoaryl PdIV aqua dication 16, which were isolated upon addition
of excess HCl or HBr to aqueous 15(OAc) at 0 °C. The aqua ligand
in 16 can be readily substituted with the halide anions. In particular,
upon careful drying under vacuum, the aqua complexes 16(Cl)2
and 16(Br)2 were cleanly converted into the monoaryl chloro and
bromo PdIV derivatives 17(Cl) and 18(Br), respectively (Scheme
5). The latter two compounds were characterized by 1H NMR
spectroscopy, NOE studies, ESI-MS, and, in the case of 18(Br),22
elemental analysis. Importantly, all three substances 16(Z)2 (Z )
OOCCF3, Cl, Br) exhibited identical 1H NMR and ESI-MS spectra in
water that are distinct from those of 17(Cl) and 18(Br). Independent
syntheses of the monoaryl chloro and bromo PdIV complexes 17 and
18 could be accomplished by reacting 14(OAc) with N-chloro- and
N-bromosuccinimide, respectively, in water at 0 °C.
Figure 1. ORTEP drawings (50% probability ellipsoids) of (a) cation 3 in
3(OAc)·MeOH and (b) complex 6.
attributed in part to the presence in these salts of the relatively
basic acetate counterion and acidic hydroxo groups of the ligands
L and L′ (e.g., cation 3 has pKa1 ) 8.15 ( 0.02).
Reactions of 3 and 4 with 1 equiv of NaOH in water led to the
formation of yellow precipitates of the poorly water-soluble neutral
zwitterionic monoaryl PdIV complexes 5 and 6, respectively
(Scheme 3, step b), and full characterization of these complexes,
including single-crystal XRD (Figure 1b), was performed.
Both 5 and 6 dissolve in the presence of 1 equiv of aqueous
alkali metal hydroxide to form clear solutions that can eliminate
phenols 8 and 9, respectively (Scheme 4, step a). In particular, the
phenolate derived from 8 was formed in 53% yield21 after 6 h at
50 °C; the reaction rate followed first-order kinetics with an
observed first-order rate constant k ) (2.04 ( 0.02) × 10-4 s-1 at
50 °C. The observed rate constant was virtually independent of
the excess amount of base present in the solution, suggesting the
involvement of the same predominant anionic species such as 7
(Scheme 4, step a) in the C-O bond elimination step.
Though 3(OOCCF3) is stable in neutral aqueous solutions up to
∼90 °C, in neat acetic acid it formed phenol 8 and the corresponding
arylacetate 10 in an ∼1:2 ratio. The combined NMR yield of 8
and 10 was 94% after 4 h at 50 °C. The reaction exhibited clean
first-order kinetics over the temperature range 20-70 °C with the
following activation parameters: ∆Hq ) 23.4 ( 0.2 kcal/mol and
∆Sq ) -6 ( 6 cal mol-1 K-1
.
The reductive elimination of C-Y bonds (Y ) Cl, Br) was also
efficient in aqueous solutions of 3(OOCCF3) containing 20 equiv
of HCl or HBr at 70 °C. The corresponding aryl halides 12 and 13
were produced in 89-92% yield after 8 h (Scheme 4, b). Formation
of 12 and 13 presumably proceeds via the intermediacy of
corresponding PdIV-X complexes such as 11-X in Scheme 4, step
The higher reactivity of complexes 15-18 in C-X reductive
elimination in comparison with 3-6 is illustrated in Scheme 6. The
C-O bond reductive elimination reaction of cation 15 leading to
virtually quantitative formation of aryloxide 17 (see the X-ray
structure in Figure 2b) occurred even at room temperature in water.
The reaction followed first-order kinetics with rate constant kOH
)
1
(2.52 ( 0.03) × 10-5 s-1 at 22 °C (Scheme 6, step a). Interestingly,
addition of 5 equiv of pyridine did not inhibit the reaction.23 The
corresponding phenol, 2-(2-hydroxy-4-methylphenyl)pyridine, could
be isolated in pure form upon treatment of aqueous solutions of
19(OAc) with excess CF3COOH. C-O reductive elimination of
arylacetates from some PdIV diaryl dicarboxylate complexes was
studied previously, and a conclusion was made that five-coordinate
b, which were detected in the reaction mixtures using H NMR
spectroscopy and ESI-MS.
Scheme 4
Figure 2. ORTEP drawings (50% probability ellipsoids) of (a) dication
16 in 16(OOCCF3)2 ·H2O and (b) PdII aryloxide cation 19 in 19(OAc).
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C O M M U N I C A T I O N S
Scheme 6
facially chelating ligands for tuning reactivity of the metal was
demonstrated. Though the oxidation of the dpk-supported complex
14 to give 15 can be fast enough (Scheme 5, step a), the C-O
reductive elimination from 15 is slow (Scheme 6). Therefore, we
used the sulfonate ligand L′′ (Scheme 3, R′ ) R′′ ) Me) to enhance
the reactivity19 of the expected high-valent metal intermediates.
The reaction between 2-p-tolylpyridine and 8 equiv of 30% aqueous
H2O2 was performed in AcOH solvent in the presence of 10%
Pd(OAc)2 and L′′ or dpk at 35 °C. Formation of 2-(2-hydroxy-4-
methylphenyl)pyridine as the only reaction product was observed
after 7 h in yields of 61% (L′′), 19% (no ligand), and 10% (dpk),
1
respectively, according to H NMR spectroscopy.
In summary, we have developed a procedure for the preparation
of monohydrocarbyl PdIV complexes from the corresponding monoaryl
PdII precursors in water using H2O2 as an oxidant, and we have fully
characterized a series of compounds with alkoxide, OH, OH2, Cl, and
Br ligands at the PdIV center. The C-Y (Y ) O, Br, Cl) bond reductive
elimination reactions from these species have also been documented.
These include the first C-Br and C-O bond reductive eliminations
from isolated monohydrocarbyl PdIV complexes. We also demonstrated
the possibility of Pd-catalyzed C-H oxidative functionalization with
H2O2 in the presence of a facially chelating bis(2-pyridyl)methane-
sulfonate ligand. A much greater variety of monohydrocarbyl PdIV
complexes may be accessible using the approach described here.
Studies of its substrate scope, the C-X reductive elimination reactivity
of complexes of this class, and applications in catalysis involving C-H
bond breaking and C-Y bond making in aqueous media are currently
underway in our lab.
PdIV intermediates are most likely involved in the reaction.24 While a
more detailed study of the C-X elimination reactions reported here
is required in order to explore their mechanisms, some comment can
be made now. The absence of inhibition of step a in Scheme 6 by
pyridine is consistent with C-O reductive elimination directly from
the starting six-coordinate PdIV complex 15. This reaction path
involving the “six-coordinate” transition state TS6 was studied by us
computationally using density functional theory (Scheme 7; all data
shown are for the gas-phase reactions; the same trend was seen when
aqueous solutions were modeled20). In TS6, the pyridyl nitrogen atom
trans to the aryl carbon is only partially dissociated, with a Pd-N
distance of 2.518 Å (vs 2.239 Å in 15). Another reaction pathway via
five-coordinate PdIV intermediate 22 and transition state TS5 was also
analyzed. According to these results, the latter pathway is less
competitive than the direct mechanism via TS6.
Acknowledgment. We thank the NSF (CHE-0614798) and the
U.S.-Israel Binational Science Foundation for the financial support
of this work.
Supporting Information Available: Experimental details; CIF files
for 3(OAc), 5, 6, 16(OOCCF3)2, and 19(OAc); and computational details. This
material is available free of charge via the Internet at http://pubs.acs.org.
Elimination of aryl halides 20 and 21 from halogeno PdIV
complexes 17(Cl) and 18(Br), respectively, was also facile at room
temperature (Scheme 6, step b). These products formed in 90-95%
NMR yield after 12-38 h. The first-order rate constants for the
C-X elimination step at 22 °C could be estimated from our kinetics
modeling performed for these reactions as kBr ) (2.50 ( 0.10) ×
10-5 s-1 and kCl ) (1.60 ( 0.05) × 10-5 s-1. Surprisingly, both
rate constants are on the same order of magnitude as kOH. The aryl
halides ArX could also be produced in 51-59% yield along with
31-37% ArOH by heating aqueous solutions of 15(OAc) and 10
equiv of the corresponding HX at 70 °C.
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