Detection of Palladium(I) in Aerobic Oxidation Catalysis

Article abstract of DOI:10.1002/anie.201700345

Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric Pd^I species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C?H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric Pd^I complex, [Pd^I(μ-DAF)(OAc)]₂, which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this Pd^I complex and its implications for catalytic reactivity are discussed.

Full text of DOI:10.1002/anie.201700345

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International Edition: DOI: 10.1002/anie.201700345
German Edition: DOI: 10.1002/ange.201700345
Palladium Catalysts
Hot Paper
Detection of Palladium(I) in Aerobic Oxidation Catalysis
Jonathan N. Jaworski, Scott D. McCann, Ilia A. Guzei, and Shannon S. Stahl*
Abstract: Palladium(II)-catalyzed oxidation reactions exhibit
broad utility in organic synthesis; however, they often feature
high catalyst loading and low turnover numbers relative to
non-oxidative cross-coupling reactions. Insights into the fate of
the Pd catalyst during turnover could help to address this
limitation. Herein, we report the identification and character-
ization of a dimeric PdI species in two prototypical Pd-
nanoparticles and/or bulk metallic Pd. Use of ancillary ligands
can help to prevent catalyst decomposition and/or promote
reoxidation of Pd0 by O2, thereby avoiding a requirement for
undesirable oxidants, such as stoichiometric CuII, AgI, or
benzoquinone (BQ). Nevertheless, the ligand-modulated
aerobic oxidation reactions often feature high catalyst loading
and low turnover numbers. The basis for this limitation is
poorly understood, and efforts to overcome this issue are
complicated by limited understanding of catalyst speciation
under aerobic oxidation conditions. Herein, we report the
first observation of a PdI intermediate in ligand-modulated
Pd-catalyzed oxidation reactions and assess its role in the
catalytic mechanism.
À
catalyzed aerobic oxidation reactions: allylic C H acetoxyla-
tion of terminal alkenes and intramolecular aza-Wacker
cyclization. Both reactions employ 4,5-diazafluoren-9-one
(DAF) as an ancillary ligand. The dimeric PdI complex,
[PdI(m-DAF)(OAc)]2, which features two bridging DAF
ligands and two terminal acetate ligands, has been character-
ized by several spectroscopic methods, as well as single-crystal
X-ray crystallography. The origin of this PdI complex and its
implications for catalytic reactivity are discussed.
À
Allylic C H acetoxylation of alkenes and aza-Wacker
reactions are two prototypical Pd-catalyzed aerobic oxidation
reactions.[8] In previous studies of these reactions, we found
that 4,5-diazafluoren-9-one (DAF) is especially effective in
promoting aerobic catalytic turnover [Eqs. (1) and (2)][9,10]
and that DAF adopts diverse coordination modes, including
k1, k2, and bimetallic bridging (m) ligation, under the catalytic
conditions.[11,12]
T
he development of new palladium-catalyzed oxidation
reactions has been the focus of extensive research in recent
years.[1] Reactions that proceed via a PdII/Pd0 cycle are
appealing because they are often compatible with the use of
O2 as the terminal oxidant.[2] Prominent examples include the
Wacker process,[3] alcohol oxidation,[4] Wacker-type cycliza-
tion[5] and the Fujiwara–Moritani (dehydrogenative/oxidative
Heck) reaction.[6] The majority of Pd-catalyzed aerobic
oxidations proceed through a catalytic cycle wherein PdII
mediates substrate oxidation and Pd0 is reoxidized by O2
(Scheme 1).[7] The Pd0 intermediate in this cycle is metastable
and susceptible to decomposition via aggregation into Pd
Our initial mechanistic studies of DAF/Pd(OAc)2-cata-
lyzed allylic acetoxylation revealed unusual kinetic behavior.
The reaction time course exhibits two kinetic phases: an
initial burst of product formation, followed by slower steady-
state behavior (Figure 1). The initial color of the reaction
mixture changes rapidly from a yellow solution to red upon
addition of allyl benzene to a solution of the catalyst. When
the beginning of the acetoxylation reaction was monitored by
1H NMR spectroscopy (3:1 dioxane-[D8]/AcOD-[D4]), new
DAF ligand peaks appeared during the burst phase, consistent
with the formation of a symmetrical DAF-ligated Pd species.
Scheme 1. Simplified catalytic cycle for homogeneous palladium-cata-
lyzed aerobic oxidation reactions.
1
[*] J. N. Jaworski, S. D. McCann, Dr. I. A. Guzei, Prof. S. S. Stahl
Department of Chemistry
This new DAF species had well-dispersed H chemical shifts
(o-/m-/p-CH 1H d = 8.83, 7.54, and 8.21 ppm) and a 15N
chemical shift of 217 ppm (Figure S2 in the Supporting
Information).
University of Wisconsin—Madison
1101 University Ave., Madison, WI 53706 (USA)
E-mail: stahl@chem.wisc.edu
Complementary observations were made with the Pd-
catalyzed aza-Wacker cyclization of (Z)-4-hexenyltosylamide.
At 5 mol% catalyst loading, the reaction time course again
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700345.
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Figure 3. Catalyst and O2 dependence on the aza-Wacker cyclization
reaction. A) Catalyst dependence [DAF/Pd(OAc)2]=2 mm–10 mm.
B) O2 dependence with [DAF/Pd(OAc)2] at 5 mm and 0.5 mm. All
reactions performed with [(Z)-4-hexenyltosylamide]=0.1m and
730 Torr O2 and reactions monitored by O2 gas consumption.
Figure 1. A 20 h time course of DAF/Pd-catalyzed allylic acetoxylation
of allyl benzene and expansion of the burst phase observed during the
first 30 min of the reaction at 808C.
O2 pressure-dependence data support this hypothesis. The
rate exhibits no dependence on pO2 at low [DAF/Pd(OAc)2]
(0.5 mm) but a linear dependence at high [DAF/Pd(OAc)2]
(5 mm) (Figure 3B). Changes in the mixing technique (stir-
ring versus orbital agitation) and use of different reaction
vessels under otherwise identical conditions resulted in
different rates. The rate plateau correlates with a color
change from yellow to deep red and rapid precipitation of the
Pd catalyst as a red solid.[14]
The NMR data for the new DAF-ligated Pd species
observed in the catalytic reactions do not match the data for
any characterized DAF/Pd(OAc)2 species.[10,11] For example,
the ortho and para protons of mononuclear k2-DAF- ligated
PdII complexes overlap,[10] while the corresponding peaks of
the new species are well resolved (Dd > 0.5 ppm). Dimeric
PdII species with bridging DAF ligands have similarly well
resolved chemical shifts for the ortho and para protons;
however, the o-CH chemical shifts appear much further
downfield (d = 9.9–10.6 ppm) than those in the new species
(d = 8.6–8.7 ppm). The downfield 15N chemical shift of DAF
in the new species, detected by 1H-15N HMBC (d = 217–
219 ppm), more closely resembles that of the dimeric PdII
complex [Pd(m-DAF)(k1-OPiv)2]2 (15N d = 218 ppm) than
those of mononuclear (k2-DAF)PdII complexes (15N d =
199–213 ppm).
exhibits two-phase kinetic behavior in which a burst of
product formation is followed by a slower steady-state rate
(Figure 2). Moreover, analysis of the aza-Wacker reaction by
[13]
NMR spectroscopy in CDCl3 revealed formation of a new
symmetrical DAF species at early stages of the reaction. The
1H and 15N chemical shifts are similar to those observed in the
acetoxylation reaction, with differences likely arising from the
different reaction solvents (o-/m-/p-CH 1H d = 8.71, 7.49, and
8.15 ppm and 15N d = 219 ppm; see Figure S5).
Further studies of the aza-Wacker reaction yielded
valuable insights. The initial rate of the reaction exhibited
a nearly linear dependence on [Pd] at low catalyst loading, but
shifted abruptly to a zero-order dependence at [Pd] = 2.5 mm
(Figure 3A). This behavior implicated the onset of gas-liquid
mass transport limitations at [DAF/Pd(OAc)2] ꢀ 2.5 mm, and
The prominent color change associated with formation of
the new Pd species facilitated monitoring of the reactions by
UV/Vis spectroscopy. Early stages of the acetoxylation
reaction (Figure 4A) revealed the appearance of an absorb-
ance at lmax = 475 nm. The aza-Wacker reaction exhibited
similar UV/Vis spectroscopic features, again showing appear-
ance of an absorption band with lmax = 475 nm upon addition
of (Z)-4-hexenyltosylamide to a toluene solution DAF/Pd-
(OAc)2 (Figure 4B). The absorbance maximized at 1 min and
then decayed as red precipitate formed in the cuvette.
Collectively, the NMR and UV/Vis spectroscopic data
obtained from the two different reactions suggest that the
same species is forming in both reactions, with the minor
Figure 2. A 250 min time course of DAF/Pd-catalyzed aza-Wacker and
expansion of the burst phase observed during the first 30 min of the
reaction at 248C.
2
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Figure 5. Two views of the structure of [PdI(m-DAF)(OAc)]2. H-atoms,
two AcOH molecules and dioxane are omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Pd1-Pd1i 2.4764(7), Pd1-O2 2.2136(2),
Pd1-N1 2.022(2), Pd1-N2 2.044(2). N1-Pd1-Pd1i-N2i 52.549(7). Sym-
metry code (i): 1Àx, y, 1.5Àz.[25]
carbonyl groups are oriented in the same direction. The
[PdI(m-DAF)(OAc)]2 complex co-crystallizes with one equiv-
alent of dioxane and two equivalents of acetic acid (not shown
in Figure 5), with the acetic acid hydrogen bonding to the
coordinated acetate ligands. This structure is an unusual
example of a PdI dimer that does not have a bridging anionic
(e.g., halide, allyl) or carbonyl ligand.[16,17]
The dimeric structure of the [PdI(m-DAF)(OAc)]2 com-
plex was anticipated to be responsible for the UV/Vis
absorption band at 475 nm. But, to probe the origin of this
feature, time-dependent density functional theory (TD-DFT)
calculations were carried out.[18] The calculations revealed an
excitation at 497 nm, which is in good agreement with the
experimental value. Electronic structural analysis of this
transition show that is arises from excitation of an electron in
À
the Pd Pd s bond as the HOMO into p* orbitals associated
with the DAF ligand (Figure 6). The calculations predict an
extinction coefficient of e = 0.99 mmÀ1 cmÀ1, which is consis-
Figure 4. UV/Vis spectra of acetylation and aza-Wacker reactions at
room temperature under air. A) First 120 min of UV/Vis time course
5 mm DAF/Pd(OAc)2 in 3:1 dioxane/AcOH with 0.27m allylbenzene.
B) First 60 s of UV/Vis time course of a reaction of 5 mm DAF/
Pd(OAc)2 in toluene with 0.1m (Z)-4-hexenyltosylamide.
differences attributed to the different solvents used in the
reactions.
When the acetoxylation reaction was performed at room
temperature, crystals suitable for single-crystal X-ray diffrac-
tion (XRD) analysis were obtained from the red reaction
mixture (see Experimental Details in the Supporting Infor-
mation). XRD analysis of the red crystals revealed a PdI
dimer with two bridging DAF ligands and a k1-acetate ligand
À
capping each of the Pd centers (Figure 5). The Pd Pd distance
of 2.4764(7) ꢀ is consistent with the presence of a bond
between the two metals.[15] The 78.04(4)8 dihedral angle
between the planes of the two coordinated DAF ligands
Figure 6. Calculated TD-DFT excitation that correspond to the
observed 475 nm absorbance of [PdI(m-DAF)(OAc)]2. Excitation from
the Pd HOMO to different ligand antibonding orbitals: metal-to-ligand
charge transfer (MLCT). Pd light blue, N dark blue, O red, C gray.
À
presumably allows Pd N bonding to be maximized while
À
allowing the Pd centers to optimize Pd Pd bonding. The
acetate ligands exhibit a syn relationship, in which the
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tent with the experimental value from UV/Vis spectroscopic
data: e475nm = 1.44 mmÀ1 cmÀ1 (see Figure S11 in the Support-
ing Information).
The collective observations above provide clear evidence
for a palladium(I) intermediate in these Pd-catalyzed aerobic
oxidation reactions, and they show that the PdI species arises
from comproportionation of Pd0 and PdII (left equilibrium,
Scheme 2). These results may be compared to the involve-
Formation of the PdI dimer may be rationalized by
a reaction sequence involving oxidation of the substrate by
PdII, followed by rapid reaction of the resulting Pd0 inter-
mediate with another equivalent of the PdII catalyst. To probe
this mechanism of PdI formation, we performed the allylic
acetoxylation and aza-Wacker reactions under anaerobic
conditions [Eqs. (3) and (4)].
In both cases, approximately 50% yield of product was
observed with respect to the original [Pd]. This result
indicates that the PdII species reacts much faster with Pd0
than with the substrate, resulting in formation of the PdI
dimer. As an additional control experiment, Pd(OAc)2 and
0.5 equivalents of Pd2dba3 were combined with 2 equivalents
of DAF (DAF:Pdtotal = 1) in acetonitrile [Eq. (5)]. The
dimeric PdI complex [Pd(m-DAF)(OAc)]2 formed rapidly
and was isolated in 94% yield from this reaction. The NMR
and UV/Vis spectroscopic properties of this compound are
identical to those of the species observed in the catalytic
reactions.
Scheme 2. Pd0 pathways in DAF-promoted reactions.
ment of PdI species in carbonylation[19] and cross-coupling
reactions.[20,21] For example, dimeric PdI complexes have been
used as pre-catalysts or observed as intermediates in diverse
cross-coupling reactions.[20,22] We speculate that O2 reacts with
the reversibly generated Pd0 species; however, direct reaction
of O2 with the PdI dimer cannot be excluded and will be the
focus of future studies.[23]
These results have several implications for catalysis. On
one hand, the PdI dimer appears to be an off-cycle species that
slows catalytic turnover. This behavior is evident in the aza-
Wacker reaction. The PdI dimer is not only off-cycle in this
reaction, but it precipitates from the reaction mixture, thereby
removing some of the Pd catalyst from solution. At suffi-
ciently low catalyst concentration, oxidation of Pd0 by O2 can
outcompete the bimolecular PdI dimer formation and
accounts for the observed steady-state turnover (cf.
Figure 2). In the allylic acetoxylation reaction, involvement
of the PdI dimer is somewhat different. The PdI dimer is
soluble under these reaction conditions, and it is a kinetically
competent source of the active catalyst. The kinetic burst in
this reaction (cf. Figure 1) indicates that the DAF/Pd(OAc)2
pre-catalyst is more active than the PdI dimer; however, we
speculated that formation of PdI has a net beneficial effect
that partly accounts for the unique effectiveness of DAF as
a ligand that promotes aerobic allylic acetoxylation. Specif-
ically, the ability of DAF to promote comproportionation of
Pd0 and PdII into PdI by serving as a stabilizing bridging ligand
could help to prevent aggregation of Pd0 into inactive metallic
Pd.[24]
Additional control experiments demonstrate that the PdI
dimer is a kinetically competent precatalyst for the acetox-
ylation reaction. No kinetic burst is observed, and the reaction
time course matches the steady-state profile of the reaction
initiated with DAF/Pd(OAc)2 as the pre-catalyst (see Fig-
ure S12 in the Supporting Information). Furthermore, the PdI
dimer reacts with O2 in AcOH/dioxane to afford a product
mixture that is spectroscopically indistinguishable from a 1:1
solution of DAF and Pd(OAc)2 (see [Eq. (6)] and Figure S13
in the Supporting Information). The insolubility of the PdI
dimer in toluene precluded kinetic competency tests for the
aza-Wacker reaction.
The results described herein provide an important foun-
dation for future efforts to probe the role and reactivity of PdI
species in diverse Pd-catalyzed aerobic oxidation reactions.
We anticipate that such efforts will contribute to the develop-
ment of new highly effective ligands and practical catalyst
systems.
Acknowledgements
We thank Drs. Paul White and Damian Hruszkewycz for
insightful discussions and Professor Thomas Brunold for his
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2012, 134, 16496 – 16499; h) A. Vasseur, C. Laugel, D. Harakat, J.
help in performing TD-DFT computations. We are grateful to
the NIH (R01 GM067173 and R01 GM100143) for financial
support of this work. Scott McCann is grateful for support
from the NIH (F31-GM116443). The NSF provided partial
support for the computational resources (CHE-0840494).
NMR spectroscopic instrumentation was partially funded by
the NSF (CHE-1048642, CHE-0342998, CHE-9208463, S10
RR13866-01).
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Conflict of interest
The authors declare no conflict of interest.
Keywords: aerobic · homogeneous catalysis · oxidation ·
palladium · Wacker cyclization
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Angewandte
Communications
Chemie
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[25] CCDC 1531309 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via The
Cambridge Crystallographic Data Centre.
Manuscript received: January 11, 2017
Final Article published: && &&, &&&&
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Angew. Chem. Int. Ed. 2017, 56, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Palladium Catalysts
Catalyst entanglement: Catalyst dimeri-
zation is observed in both the aerobic
À
J. N. Jaworski, S. D. McCann, I. A. Guzei,
palladium-catalyzed allylic C H acetoxy-
S. S. Stahl*
&&&& — &&&&
lation and aza-Wacker cyclization. A novel
PdI species is rigorously characterized in
catalysis which has implications for pal-
ladium catalyst design and presents
insights into catalyst deactivation.
Detection of Palladium(I) in Aerobic
Oxidation Catalysis
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!