Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

Article abstract of DOI:10.1021/acscatal.1c01074

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O2) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd0 during catalytic turnover. We previously showed that Pd(OAc)2 and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O2 in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)2 catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)2 catalyst systems under three different oxidation conditions: (1) O2 alone, (2) O2 with cocatalytic BQ, and (3) O2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O2 and BQ, both of which are capable of oxidizing Pd0 to PdII. The reaction of O2 generates H2O2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of PdII. NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

Full text of DOI:10.1021/acscatal.1c01074

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Research Article
Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2‑Catalyzed
Aerobic Allylic Acetoxylation in the Absence and Presence of a
Co(salophen) Cocatalyst
Caitlin V. Kozack, Stephen J. Tereniak, Jonathan N. Jaworski, Bao Li, David L. Bruns,
Spring M. M. Knapp, Clark R. Landis, and Shannon S. Stahl*
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ABSTRACT: Palladium(II)-catalyzed allylic acetoxylation has
been the focus of extensive development and investigation.
Methods that use molecular oxygen (O2) as the terminal oxidant
typically benefit from the use of benzoquinone (BQ) and a
transition-metal (TM) cocatalyst, such as Co(salophen), to
support oxidation of Pd0 during catalytic turnover. We previously
showed that Pd(OAc)2 and 4,5-diazafluoren-9-one (DAF) as an
ancillary ligand catalyze allylic oxidation with O2 in the absence of
cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)2
catalytic activity, nearly matching the performance of reactions that
include both BQ and Co(salophen). These observations are
complemented by mechanistic studies of DAF/Pd(OAc)2 catalyst
systems under three different oxidation conditions: (1) O2 alone,
(2) O2 with cocatalytic BQ, and (3) O2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of
Co(salophen) is traced to the synergistic roles of O2 and BQ, both of which are capable of oxidizing Pd0 to PdII. The reaction of O2
generates H2O2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of PdII. NMR spectroscopic studies,
however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while
inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.
KEYWORDS: allylic acetoxylation, catalysis, palladium, oxidation, benzoquinone
product formation at the start of the reaction, coinciding with
the formation of a PdI dimer,36 followed by steady-state
catalytic turnover in which the catalyst resting state consists of
an allyl-PdII species (Scheme 3).37 The catalytic rate exhibits a
dependence on pO2 during steady-state turnover, implicating
catalyst reoxidation by O2 as the turnover-limiting step.
Recognizing that BQ is capable of promoting both C−O
bond formation (formally, reductive elimination) from allyl-
PdII species and reoxidation of Pd0,38 we evaluated BQ
derivatives as additives in a recent study of allylic acyloxylation
with diverse carboxylic acids.30 In the course of this work, we
found that the use of 5 mol % 2,6-dimethylbenzoquinone
(Me2BQ) in the absence of a TM cocatalyst more than
doubled the yield relative to a reaction with O2 in the absence
INTRODUCTION
■
Palladium(II)-mediated allylic C−H acetoxylation was first
reported nearly 60 years ago.1 Since then, many other catalytic
applications of this method have been reported that employ
various stoichiometric oxidants, including benzoquinone
derivatives,1−10 peroxides,11−14 copper salts,15,16 and hyper-
valent iodine reagents17,18 (Scheme 1). Benzoquinone (BQ) is
commonly used in cocatalytic quantities with a stoichiometric
oxidant, such as MnO2
19−21
or a peroxide.11,14,22 Molecular
oxygen (O2) has been used effectively, typically in combination
with BQ and a transition-metal (TM) complex, such as iron
phthalocyanine or Co(salophen), as a cocatalyst to support
catalytic turnover (Scheme 2).20,21,23−30 In rare cases, O2 is an
effective oxidant for Pd-catalyzed allylic acetoxylation without
any cocatalysts.31,32
Our initial studies of Pd-catalyzed allylic acetoxylation were
motivated by an interest in developing a ligand-based strategy
to support the use of O2 as an oxidant and led to the discovery
of 4,5-diazafluoren-9-one (DAF) as an effective ancillary ligand
for these reactions.30,32−35 Subsequent mechanistic studies
revealed that these reactions exhibit a kinetic “burst” of
Received: March 8, 2021
Revised: April 26, 2021
Published: May 13, 2021
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© 2021 American Chemical Society
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cyclohexadiene with tBuOOH, complementing other Pd/BQ
cocatalyst systems that use peroxides as the stoichiometric
oxidant,11 including one case using DAF/Pd(OAc)2 as the Pd
catalyst.14 The relevance of these insights is apparent upon
recognizing that oxidation of Pd0 by O2 in the presence of
AcOH generates H2O2 (Scheme 4),35,43−45 and we postulated
that this in situ-generated H2O2 could promote oxidation of
H2Q to BQ, thereby accounting for the beneficial effect of BQ
on Pd-catalyzed aerobic allylic oxidation, even in the absence
of a TM cocatalyst.
Scheme 1. Pd-Catalyzed Allylic Acetoxylation With Various
Oxidants
The present study surveys the influence of various quinones
on Pd-catalyzed aerobic allylic acetoxylation in the absence and
presence of Co(salophen) as a cocatalyst. We then investigate
the relative rates of H2Q oxidation by O2 in the presence of
different components of the catalyst system and employ NMR
spectroscopy to probe the quinone speciation in the absence
and presence of cocatalytic Co(salophen). Collectively, the
results provide unique mechanistic insights into commonly
used multicomponent catalyst systems in Pd-catalyzed
oxidation reactions.
of quinone. This result is not readily understood from prior
studies, which suggest that the cocatalyst is needed to reoxidize
hydroquinone (H2Q).23−30,39
RESULTS AND DISCUSSION
■
Influence of Quinones on Pd-Catalyzed Allylic
Oxidation. We initiated our study by investigating Pd-
catalyzed acetoxylation of allylbenzene (1) with 5 mol %
loading of quinones 3a−3m, which have different steric
properties and redox potentials (Figure 1A). Reactions carried
out in the absence and presence of 0.5 mol % Co(salophen)
show a “volcano”-like dependence of the cinnamyl acetate (2)
yield on the quinone 2e−/2H+ redox potential (Figure 1B).46
Reaction times were shortened from 14 to 7 h when using
Co(salophen) to enhance the contrast among different
quinones. The variations in yields among the best quinones
probably reflect the superposition of steric and electronic
contributions to the outcome. But, optimal yields of 2 were
generally obtained with BQs with redox potentials ranging
from ∼550 to 635 mV (3d−3h) in the absence of
Co(salophen). The range of optimal quinone redox potentials
expands for reactions conducted in the presence of Co-
The mechanistic model commonly invoked for multi-
component catalyst systems of this type features a redox
cascade in which O2 oxidizes the TM cocatalyst, the TM
cocatalyst oxidizes H2Q, BQ oxidizes Pd0, and PdII oxidizes the
organic substrate (Scheme 2).23,24,40 This cascade sequence is
widely invoked, but few studies have interrogated mechanistic
features of the cocatalyst system40,41 and prior studies do not
provide a rationale for the beneficial role of BQ in the absence
of a TM cocatalyst. Two recent studies seemed to offer
possible insights. DAF/Pd(OAc)2 was found to undergo redox
equilibration with H2Q, generating BQ and reduced Pd
species;39 however, this reactivity does not provide a clear
basis for improved catalyst performance. A separate study
showed that PdII salts catalyze oxidation of H2Q to BQ
derivatives using tBuOOH as the oxidant.42 This reactivity
contributes to the Pd/BQ-cocatalyzed 1,4-diacyloxylation of
Scheme 2. Proposed Mechanism for Aerobic Allylic Acetoxylation With Multicomponent Catalyst Systems (ETM = Electron-
Transfer Mediator)
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Scheme 3. Mechanistic Steps in DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation
Scheme 4. Reaction of Pd0 With O2 Resulting in the
Formation of H2O2
with tBuBQ in the absence of Co(salophen) are noteworthy.
Similar results were obtained with a series of other substrates,
including electronically differentiated allylbenzene derivatives
(1a−1c) and aliphatic substrates (1d, 1e). In each case,
addition of tBuBQ to the reaction substantially improves the
yield, and further improvements are evident when both tBuBQ
and Co(salophen) are included as cocatalysts in the reaction
mixture (Figure 3).
Analysis of H2Q Oxidation and Quinone Speciation
under Catalytic Conditions. BQ cocatalysts can play at least
two roles in these reactions: (a) promote C−O bond
formation/reductive elimination from allyl-PdII spe-
cies5,38,47−49 and (b) promote reoxidation of Pd0 to PdII.50
The first role does not alter the redox state of the BQ,51 while
the second will generate H2Q, which must be reoxidized to BQ
to continue serving as a cocatalyst. The latter process is
typically thought to require Co(salophen) or a related
cocatalyst,23,24 raising questions about the beneficial effect of
BQ in the absence of a TM cocatalyst.
Experiments were undertaken to probe methods for
oxidation of tBuH2Q to tBuBQ that could be operative
under the Pd-catalyzed oxidation conditions (Figure 4). The
reaction temperature for these studies was lowered to 40 °C to
facilitate data collection at early time points for some of the
rapid reactions. O2 alone does not promote oxidation of
tBuH2Q to tBuBQ (Figure 4a) and only minor oxidation was
observed with H2O2 in the absence of metal (co)catalysts (4%
yield after 6 h, Figure 4b). Similar moderate rates of tBuH2Q
oxidation were observed with Co(salophen) under O2 and
DAF/Pd(OAc)2 under O2 (52 and 55% yields after 6 h, Figure
4c,d, respectively). The DAF/Pd(OAc)2/O2 outcome is
somewhat unexpected considering BQ derivatives are typically
used to oxidize Pd0; however, it aligns with the recent
(salophen), accommodating quinones with potentials of
∼550−700 mV (3d−3k).
The poor performance of low- and high-potential quinones
in both cases is rationalized by offsetting electronic effects on
the redox processes involving quinones: (a) BQ promotion of
Pd-catalyzed substrate oxidation, including C−O bond
formation from allyl-PdII species and reoxidation of Pd0, is
expected to benefit from high-potential quinones, while (b)
reoxidation of H2Q will be more facile for low-potential
quinones. The positive shift in quinone redox potentials for
reactions with Co(salophen) is consistent with the ability of
this cocatalyst to promote H2Q oxidation (see below).
The effect of quinone on DAF/Pd(OAc)2-catalyzed
acetoxylation of 1 was evaluated further by monitoring the
reaction time course under three different aerobic oxidation
conditions: (1) with 1 atm O2 in the absence of cocatalysts,
(2) with 1 atm O2 and 5 mol % 2-tert-butyl-1,4-benzoquinone
(tBuBQ) as a cocatalyst, and (3) with 1 atm O2, 5 mol %
tBuBQ, and 0.5 mol % Co(salophen) as cocatalysts (Figure 2).
The reaction conducted under O2 in the absence of cocatalysts
exhibits a rapid burst of product formation during the first ∼10
min followed by slower steady-state turnover, consistent with
previous reports.36,37 Both reactions with tBuBQ exhibit higher
rates and lead to high yields of 2 after 15 h, at which time the
O2-alone reaction has reached only ∼40% yield. The presence
of both tBuBQ and Co(salophen) leads to the best
performance, as expected,23 but the good results observed
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Figure 2. Time course of cinnamyl acetate formation during the
aerobic allylic acetoxylation of allylbenzene without additives, with 5
mol % tBuBQ, and with 5 mol % tBuBQ and 0.5 mol %
Co(salophen). Trendlines reflect empirical fits to guide the eye (see
the Supporting Information for details).
Figure 1. Allylic acetoxylation of allylbenzene with different
benzoquinone derivatives. (A) Benzoquinone additives and their
redox potentials. (B) Observed yields relative to the 2H+/2e− redox
potential of the quinone for conditions without Co(salophen) (14 h)
and with Co(salophen) (7 h).
observation of redox equilibria between DAF/Pd(OAc)2 and
H2Q derivatives39 and indicates that net O2 reduction to H2O
drives tBuH2Q oxidation under these conditions. In this
context, DAF/Pd(OAc)2 mediates rapid oxidation of tBuH2Q
by H2O2 under anaerobic conditions (75% yield after 6 h,
Figure 4e). Oxidation of tBuH2Q is most rapid when both
DAF/Pd(OAc)2 and Co(salophen) are used as cocatalysts
under an O2 atmosphere (Figure 4f).
The redox state of the quinone cocatalyst during the
catalytic acetoxylation reaction was then interrogated for
reactions conducted in the absence and presence of the
Co(salophen) cocatalyst. The quinone speciation was
determined by analyzing aliquots of the reaction mixture
throughout the reaction time course by 1H NMR spectroscopy
(samples were quenched with pyridine and diluted in CDCl3
for analysis; see the Supporting Information for details). In the
absence of Co(salophen), tBuBQ is completely consumed
during the first 30 min with concomitant formation of tBuH2Q.
Then, tBuH2Q is slowly oxidized to tBuBQ as the allylic
oxidation reaction proceeds to completion over the next 15−
20 h (Figure 5A). The reaction conducted in the presence of
Co(salophen) also shows an initial reduction of tBuBQ to
1
Figure 3. H NMR yields of allylic acetates 2a−2e obtained from
substrates 1a−1e under three different aerobic reaction conditions:
without cocatalysts (black), with 5 mol % tBuBQ (red), and with 5
mol % tBuBQ and 0.5 mol % Co(salophen) (blue).
tBuH2Q; however, this progression is reversed after 25−30%
conversion to tBuH2Q, and tBuBQ is present in a high steady-
state concentration throughout most of the reaction (Figure
5B).
We then used the Wisconsin High-Pressure NMR Reactor
(WiHP-NMRR)52 to conduct operando analysis of the
reactions. This apparatus enables reactions to be run under a
continuous supply of a gaseous reagent while recording NMR
spectra,37,53−55 and it enabled us to probe the Pd catalyst
under three different reaction conditions: (1) O2 alone, (2) O2
+ 5 mol % tBuBQ, and (3) O2 + 5 mol % tBuBQ + 0.5 mol %
Co(salophen) (Figure 6). These experiments exhibit relatively
low signal-to-noise but still provide valuable insights into the
Pd speciation.
Previous investigation of DAF/Pd(OAc)2-catalyzed allylic
oxidation under aerobic conditions (i.e., in the absence of
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Figure 4. Time course plots obtained from the oxidation of tBuH2Q
to tBuBQ under a series of different conditions.
Figure 6. Palladium speciation monitored by operando WiHP-NMRR
during the acetoxylation of 4-fluoroallylbenzene 1f to the allyl acetate
1
2f. (A) Formation/decay of [PdI dimer 5] analyzed by H NMR
spectroscopy and (B) appearance of [π-cinnamyl-PdII 6] (or lack
thereof) determined by 19F NMR analysis. Trendlines reflect
empirical fits to guide the eye (see the Supporting Information for
details).
progresses, π-cinnamyl-PdII species (6) become evident in the
reaction mixture lacking cocatalysts (Figure 6B, black data),
implicating an allyl-PdII resting state during steady-state
turnover. This species is not evident, however, in both reaction
mixtures containing cocatalysts (Figure 6B, red and blue data).
The reduced formation of the PdI dimer in the presence of
tBuBQ (Figure 6A) is rationalized by the ability of tBuBQ to
coordinate to Pd0 and reoxidize it to PdII,50 thereby
minimizing Pd0/PdII comproportionation to form the PdI
dimer. The lack of allyl-PdII species when cocatalysts are
present (Figure 6B) is consistent with the ability of BQ
cocatalysts to promote C−O bond formation from allyl-
PdII.5,38,47−49 These data provide direct operando evidence for
the two commonly invoked roles of quinones in Pd-catalyzed
allylic oxidation reactions.
Summary and Catalytic Mechanism. The data pre-
sented herein illuminate a number of features that have not
been characterized previously for multicomponent Pd/
cocatalyst systems for aerobic oxidation. The results show a
synergistic role of O2 and BQ, in which the H2O2 byproduct of
Pd0 aerobic oxidation supports oxidation of the H2Q species.
This feature allows BQ to be an effective cocatalyst, even in the
absence of a Co(salophen) cocatalyst (cf. Figure 2). None-
theless, the catalytic reaction is more effective when both
quinone and Co(salophen) are included in the reaction. This
mutual benefit reflects improved rates of H2Q oxidation when
Co(salophen) is present (cf. Figure 4), resulting in oxidized
BQ species being available throughout the catalytic reaction
(cf. Figure 5) to support C−O bond formation and reoxidation
of the Pd catalyst.
Figure 5. Time courses of product formation and starting material
depletion (top plot) and tBuBQ speciation (bottom plot) during the
aerobic allylic acetoxylation of allylbenzene (A) without Co-
(salophen) and (B) with 0.5% Co(salophen). Trendlines reflect
empirical fits to guide the eye based on functions described in the
Supporting Information.
cocatalysts) revealed formation of the dimeric PdI species 5 at
early stages of the reaction, arising from comproportionation of
Pd0 and PdII species present in the reaction mixture (cf.
Scheme 3).37 These observations were reproduced under the
O2-alone conditions (black data, Figure 6A). Inclusion of
tBuBQ leads to reduced build-up of the PdI dimer intermediate
early in the reaction (red data, Figure 6A) and even less
formation of PdI when both tBuBQ and Co(salophen) are
included in the reaction (blue data, Figure 6A). As the reaction
The catalytic mechanism in Scheme 5 accounts for many of
the observations described herein. In the absence of
cocatalysts, DAF/Pd(OAc)2 activates the allylic C−H bond
of the olefin substrate, leading to the formation of an allyl-PdII
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Scheme 5. Proposed Catalytic Mechanism for Pd-Catalyzed Aerobic Allylic Acetoxylation
intermediate that can undergo (reversible32) C−O bond
formation to afford a DAF/Pd0 species that can be reoxidized
by O2 via a peroxo-PdII intermediate (right catalytic cycle,
Figure 5). The presence of BQ promotes C−O bond
formation from the allyl-PdII species and provides an
alternative pathway for reoxidation of Pd0 via a DAF/Pd0-
BQ adduct. The latter step will generate H2Q, which can
undergo reoxidation to BQ, using H2O2 from the aerobic
pathway or a cocatalytic process involving DAF/Pd/Co-
(salophen)/O2 (cf. Figure 4).
States; orcid.org/0000-0002-9000-7665; Email: stahl@
chem.wisc.edu
Authors
Caitlin V. Kozack − Department of Chemistry, University of
Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0001-9052-4673
Stephen J. Tereniak − Department of Chemistry, University of
Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0003-4978-0020
Jonathan N. Jaworski − Department of Chemistry, University
of Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-9509-3769
Bao Li − Department of Chemistry, University of Wisconsin-
Madison, Madison, Wisconsin 53706, United States
David L. Bruns − Department of Chemistry, University of
Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0003-2455-502X
Spring M. M. Knapp − Department of Chemistry, University
of Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-5771-1634
Clark R. Landis − Department of Chemistry, University of
Wisconsin-Madison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-1499-4697
CONCLUSIONS
■
This study provides new insights into the cooperative
contributions of O2, BQ, and Co(salophen) in promoting Pd
catalyst reoxidation in allylic acetoxylation reactions. O2 can
reoxidize the Pd catalyst directly when a suitable ancillary
ligand, such as DAF, is present to ensure catalyst stability and
promote the reaction with O2. The beneficial effect of BQ
reflects its ability to promote both C−O bond formation in the
substrate oxidation half-reaction and its ability to oxidize Pd0
under acidic reaction conditions. But, a method is needed to
ensure reoxidation of H2Q after each catalytic cycle. Hydro-
quinone does not react readily with O2, but we show that O2/
BQ are effective together in the Pd-catalyzed conditions.
Under these conditions, O2 reduction generates H2O2, which
then supports efficient oxidation of H2Q in the presence of the
Pd catalyst. The best conditions feature a combination of
quinone and Co(salophen) cocatalysts, reflecting the ability of
the Co(salophen) to promote rapid oxidation of the H2Q
species, maximizing the steady-state quantity of quinone
available to support the substrate and catalyst oxidation
steps. The results described herein provide new insights and
direct evidence for previous mechanistic proposals.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01074
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the NSF (CHE-1665120 and CHE-
1953926). NMR instrumentation was supported by NSF
CHE-1048642, NIH S10 OD012245 and a generous gift from
Paul J. and Margaret M. Bender. The WiHP-NMRR was
supported by the Dow Chemical Company. C.V.K. was
supported in part by the National Science Foundation
Graduate Research Fellowship Program under Grant No.
DGE-1256259. S.J.T. was funded by the NIH (NRSA
Fellowship Award F32GM119214). Support was also provided
by the Graduate School and the Office of the Vice Chancellor
for Research and Graduate Education at the University of
Wisconsin-Madison with funding from the Wisconsin Alumni
Research Foundation.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01074.
■
sı
Experimental details and NMR data (PDF)
AUTHOR INFORMATION
Corresponding Author
Shannon S. Stahl − Department of Chemistry, University of
Wisconsin-Madison, Madison, Wisconsin 53706, United
■
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