Benzoquinone-promoted reaction of O2 with a Pd II-hydride

Article abstract of DOI:10.1021/ja200957n

Benzoquinone (BQ) and O₂ are among the most common stoichiometric oxidants in Pd-catalyzed oxidation reactions. The present study provides rare insights into mechanistic differences between BQ and O₂ in their reactivity with a well-defined Pd-hydride complex, Pd(IMes) ₂(H)(O₂CPh) (1). BQ promotes the reductive elimination of PhCO₂H from 1 and catalyzes the formation of a Pd^II-OOH complex when this reaction is carried out under aerobic conditions. These results have important implications for Pd-catalyzed oxidation reactions.

Full text of DOI:10.1021/ja200957n

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Benzoquinone-Promoted Reaction of O2 with a PdIIꢀHydride
Nattawan Decharin and Shannon S. Stahl*
Department of Chemistry, University of Wisconsin—Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S Supporting Information
b
Scheme 1. Proposed Mechanism for PdII-Catalyzed Oxida-
ABSTRACT: Benzoquinone (BQ) and O2 are among the
most common stoichiometric oxidants in Pd-catalyzed
oxidation reactions. The present study provides rare insights
into mechanistic differences between BQ and O2 in their
reactivity with a well-defined Pdꢀhydride complex, Pd-
(IMes)2(H)(O2CPh) (1). BQ promotes the reductive
elimination of PhCO2H from 1 and catalyzes the formation
of a PdIIꢀOOH complex when this reaction is carried out
under aerobic conditions. These results have important
implications for Pd-catalyzed oxidation reactions.
tion Reactions with O2 or BQ as the Stoichiometric Oxidant
Scheme 2. Aerobic Oxidation of trans-Pd(IMes)2(H)-
(O2CPh) (1)
alladium(II)-catalyzed oxidation reactions have been the
P
focus of extensive recent development and investigation.1
These reactions typically proceed via a PdII/Pd0 catalytic
cycle in which the substrate (SubH2) is oxidized by PdII, and
Pd0 is oxidized by a secondary oxidant, such as O2, benzo-
quinone (BQ), or CuCl2. The preferred oxidant for individual
catalytic reactions varies, but relatively little is known about
the factors that contribute to the success or failure of different
oxidants in these reactions. Here, we report the reaction of
BQ with a well-defined PdIIꢀhydride complex, trans-Pd-
(IMes)2(H)(O2CPh) (1), under anaerobic and aerobic con-
ditions. The results provide the first fundamental insights into
the differences between BQ and O2 in their reactivity with
PdIIꢀhydride species.2
only 1 equiv of BQ was used in this reaction, 5 and 6 formed in 50%
yield, with 0.5 equiv of 1 remaining unreacted in solution.6
Pd-catalyzed oxidation reactions often feature β-hydride elim-
ination from PdIIꢀalkyl, PdIIꢀalkoxide, or related intermediates
as the final step of substrate oxidation. The resulting
PdIIꢀhydride intermediate is generally believed to undergo
reductive elimination of HX from the PdIIꢀhydride species
(X = halide, carboxylate, etc.), followed by reaction of the LnPd0
intermediate with BQ, O2, or another stoichiometric oxidant
(Scheme 1). Recent experimental and computational studies of
PdIIꢀhydride complexes with O2 provide strong support for this
pathway.3,4 A specific example is the reaction of molecular
oxygen with trans-Pd(IMes)2(H)(O2CPh) (1) to afford the
PdIIꢀhydroperoxide complex 4 (Scheme 2), which has been
shown to proceed by a stepwise pathway initiated by rate-limiting
reductive elimination of PhCO2H from 1.3b
The formation of 5 in eq 1 reflects the net reductive elimination
of PhCO2H from 1 (cf. step 1, Scheme 2) and displacement of
IMes from Pd0(IMes)2 (2) by BQ. Independent synthesis of 2 and
subsequent reaction with BQ supports this proposed sequence.
When BQ and 2 were combined in benzene at room temperature,
animmediate reactionoccurredtoafford [Pd(IMes)(BQ)]2 (5) in
quantitative yield based on 1H NMR spectroscopic analysis (eq 2).
We envisioned that BQ could react with 1 via insertion of a
CdC or CdO bond into the PdIIꢀH; however, BQ instead reacts
with 1 under anaerobic conditions (benzene, 50 °C) to produce a
previously characterized dimeric Pd0ꢀquinone complex,
[Pd(IMes)(BQ)]2 (5), and the salt 6 (eq 1).5 The latter product,
which precipitated from the reaction mixture, arises from the
reaction of 1 equiv of BQ, benzoic acid, and an IMes ligand. When
Received: January 31, 2011
Published: March 25, 2011
r
2011 American Chemical Society
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Figure 1. Time courses for the reactions of 1 with O2, BQ, and O2 þ
BQ. Reaction conditions: [1] = 0.45ꢀ0.5 mM, [BQ] = 9.1 mM, pO2 = 1
atm, 0.65 mL of C6D6, 50 °C.
On the basis of these observations, we carried out a competi-
tion study involving BQ and O2. When PdIIꢀhydride 1 was
added to a solution of BQ under an oxygen atmosphere (18 equiv
of dissolved BQ and O2),7 the PdIIꢀOOH complex 4 was
obtained as the sole product; none of the BQ-derived product
5 was observed (eq 3).
Figure 2. Kinetic data for the oxygenation of 1 in the presence of BQ.
Conditions: (A) [1] = 0.32ꢀ2.48 mM, [BQ] = 23.4 mM, pO2 = 1 atm,
0.65 mL of C6D6, 50 °C; (B) [1] = 0.5 mM, [BQ] = 9.1 mM, pO2 =
0.67ꢀ3.5 atm, 0.65 mL of C6D6, 50 °C; (C) [1] = 0.5 mM, [BQ] =
0ꢀ9.1 mM, pO2 = 1 atm, 0.65 mL of C6D6, 50 °C.
A similar competition experiment was carried out with Pd0
complex 2. This complex was added slowly to a benzene solution
of BQ, O2, and PhCO2H (18:18:1 equiv) at room temperature.
The bright yellow color characteristic of complex 2 bleached
immediately upon contact with the BQ, O2, and PhCO2H
mixture and once again yielded only the O2-derived product 4
(eq 4). That PdIIꢀhydride 1 reacts slowly with O2 and BQ under
these conditions indicates the formation of 4 arises from rapid
reaction of 2 with O2, followed by addition of PhCO2H to the
Pd(IMes)2(O2) complex 3.
Figure 3. Effect of BQ on the 1H NMR chemical shift of the hydride
resonance of 1. Conditions: [1] = 0.47 mM, [BQ] = 0ꢀ4.16 mM, under
N2 atm, 0.65 mL of C6D6, 24 °C.
previously in the absence of BQ.3c The rate exhibited a saturation
dependence on [BQ] (Figure 2), with the maximum rate
observed with ∼1ꢀ2 equiv of BQ relative to 1. Spectroscopic
studies of 1 in the presence of BQ provided evidence for a
ground-state interaction between 1 and BQ. Titration of BQ into
a solution of 1 in C6D6 led to a small upfield shift in the PdꢀH
resonance of 1 (Figure 3), with the [BQ] dependence of the
chemical shift similar to the dependence observed in the kinetic
study (cf. Figure 2C).9
These data support pre-equilibrium coordination of BQ to
PdIIꢀhydride 1. The nearly identical rates observed for forma-
tion of the Pd0ꢀBQ complex 5 and oxygenation of 1 in the
presence of BQ suggest that these reactions have the same rate-
limiting step, and the data suggest this step involves reductive
Each of the three reactions of PdIIꢀhydride 1 with BQ and/or
O2 was monitored by 1H NMR spectroscopy: 1 þ O2
(Scheme 2), 1 þ BQ (eq 1), and 1 þ BQ þ O2 (eq 3). A
nonexponential decay of 1 was observed in each of these
reactions, similar to previous observations of reactions initiated
by reductive elimination of PhCO2H from 1.8 The reactions
carried out in the presence of BQ exhibited nearly identical half-
lives, and both proceeded more rapidly than the reaction with O2
alone (Figure 1).
elimination of PhCO2H from a BQ adduct of 1 (1 BQ). A
3
The data in Figure 1 reveal that BQ reacts with 1 more rapidly
than O2 and it promotes the reaction of 1 with O2. These results
have important implications for catalysis (see below), and kinetic
studies were carried out to gain further insight into these effects.
Use of initial-rates methods avoided complications associated
with the nonexponential time course. The oxygenation of 1 in the
presence of BQ exhibited a first-order dependence on [1] and a
zero-order dependence on [O2], matching results obtained
mechanism consistent with all of the data, including those
obtained in the presence and absence of BQ, is shown in
Scheme 3. The reaction of PdIIꢀhydride 1 with O2 in the
absence of BQ has been characterized previously (Scheme 2),
and it proceeds via rate-limiting reductive elimination of
PhCO2H (left side of Scheme 3). The present data support
a similar reaction pathway in the presence of BQ, with the
exception that the reaction proceeds via pre-equilibrium
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Scheme 3. Proposed Mechanism for the Reaction of 1 with
BQ and the Oxygenation of 1 in the Presence of BQ
Pd0, and, in many catalytic reactions, Pd0 is unstable and
decomposes via aggregation into inactive Pd black ([Pd0]m in
Scheme 1). Coordination of BQ to the PdIIꢀhydride not only
enhances the rate of HX reductive elimination but also incorpo-
rates a stabilizing ligand into the Pd coordination sphere prior to
formation of Pd0. This feature should enhance the stability of the
resulting zero-valent Pd species,1b potentially giving it sufficient
lifetime to undergo bimolecular reaction with O2 and oxidation
to PdII. A catalytic cycle based on these considerations, shown in
Scheme 4, provides a rationale for the observation that BQ can be
a beneficial cocatalyst in Pd-catalyzed aerobic oxidation reac-
tions.13 An important future research direction is the identifica-
tion of ancillary ligands for Pd that enable aerobic catalytic
turnover to be achieved without BQ as a requisite additive.14
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedure and
b
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
stahl@chem.wisc.edu
Scheme 4. Proposed Catalytic Cycle for Pd(II)-Catalyzed
Oxidation Utilizing Both BQ and O2 as Oxidants
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(CHE-9629688 and CHE-0543585).
’ REFERENCES
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formation of the BQ adduct 1 BQ. Reductive elimination of
3
PhCO2H from 1 BQ is more facile than from 1 and generates
3
a Pd0ꢀBQ adduct, Pd0(IMes)2(BQ) (2 BQ). Displacement
3
of BQ from 2 BQ by O2 affords the same PdII(η2-O2)
complex 3 observed in the absence of BQ and can proceed
3
to the observed PdꢀOOH product. If O2 is not present, 2 BQ
3
undergoes dissociation of an IMes ligand to form the dimeric
Pd0 complex 5.
BQ has been shown to promote reductive elimination from a
number of organopalladium(II) species, especially π-allyl-PdII
complexes.10ꢀ12 The present results represent the first example
of BQ-promoted reductive elimination from a PdIIꢀhydride.
Although the precise structure of the BQ adduct 1 BQ is not
3
known (e.g., whether the BQ is coordinated via an oxygen
lone pair or the alkene), formation of a five-coordinate complex
could have at least two beneficial effects on reactivity. First, the
π-acidity of BQ could remove electron density from PdII and
make it more susceptible to reductive elimination. In addition,
formation of a trigonal bipyramidal species would reduce the
distance between the carboxylate and hydride ligands, thereby
lowering the barrier for this “intramolecular deprotonation” type
of reductive elimination reaction.
The ability of BQ to promote the oxygenation of
PdIIꢀhydride 1 (eq 3) has important implications for catalysis.
Reductive elimination of HX from a PdIIꢀhydride species forms
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(6) See Supporting Information for details.
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(7) Under the conditions of these NMR experiments, mass transfer
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dissolved O2 concentration is relevant, and 1 atm O2 corresponds to a
dissolved O2 concentration of ∼9 mM in benzene ([PdꢀH] = 0.5 mM).
See Supporting Information for additional discussion.
(8) Previous studies demonstrated that the reductive elimination of
PhCO2H from 1 is accelerated by the presence of protic additives or
products from the reaction (e.g., PdꢀOOH 4, PhCO2H). See ref 3b for
discussion. The reaction with BQ forms the protic product 6, which
could similarly affect the reaction rate and lead to a nonexponential time
course.
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