Trinuclear Pd3O2 intermediate in aerobic oxidation catalysis

Article abstract of DOI:10.1002/anie.201400134

The activation of O₂ is a key step in selective catalytic aerobic oxidation reactions mediated by transition metals. The bridging trinuclear palladium species, [(LPd^II)₃(μ³- O)₂]²⁺ (L=2,9-dimethylphenanthroline), was identified during the [LPd(OAc)]₂(OTf)₂-catalyzed aerobic oxidation of 1,2-propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd₃O₂ species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes. Catalytic menage a trois: A catalytically active trinuclear Pd ₃O₂ complex was identified during Pd-mediated aerobic oxidation of alcohols. Synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. These results illuminate a new pathway for O₂ reduction by Pd complexes.

Full text of DOI:10.1002/anie.201400134

.
Angewandte
Communications
DOI: 10.1002/anie.201400134
Catalytic Oxidation
Hot Paper
Trinuclear Pd3O2 Intermediate in Aerobic Oxidation Catalysis**
Andrew J. Ingram, Diego Solis-Ibarra, Richard N. Zare,* and Robert M. Waymouth*
Abstract: The activation of O2 is a key step in selective catalytic
aerobic oxidation reactions mediated by transition metals. The
bridging trinuclear palladium species, [(LPdII)3(m3-O)2]2+ (L =
2,9-dimethylphenanthroline), was identified during the [LPd-
(OAc)]2(OTf)2-catalyzed aerobic oxidation of 1,2-propane-
diol. Independent synthesis, structural characterization, and
catalytic studies of the trinuclear compound show that it is
a product of oxygen activation by reduced palladium species
and is a competent intermediate in the catalytic aerobic
oxidation of alcohols. The formation and catalytic activity of
the trinuclear Pd3O2 species illuminates a multinuclear path-
way for aerobic oxidation reactions catalyzed by Pd com-
plexes.
for O2 activation by reduced palladium complexes, thereby
providing new insights into strategies for selective aerobic
oxidation catalysis.
Palladium catalysts are an important class of oxidation
catalysts for both commodity and fine chemical intermedi-
ates.[9,12–18] Experimental and theoretical investigations have
illuminated several pathways for the activation of O2 by
reduced Pd complexes (Figure 1), the role of ligand environ-
ment, and h2-peroxo and hydroperoxy intermediates in the
reoxidation of LPd0 or LPd-H+ with O2.[7,9,10,19]
S
elective oxidation reactions are critical to all aerobic
organisms and are among the most useful chemical trans-
formations on an industrial scale.[1–5] Air is convenient,
thermodynamically potent, and readily available as a terminal
oxidant; however, high rates and selectivity require a catalyst
to bind and activate O2. If reduced to water, oxygen is a 4-
electron oxidant, but most desired oxidations are 2-electron
oxidations (alkanes to alcohols, alcohols to carbonyls, olefins
to epoxides, etc.). Moreover, partially reduced oxygen species
are typically more reactive and potent oxidants than O2 itself:
a major challenge for selective oxidation reactions in air or
O2.[1–6]
Figure 1. Pathways for the aerobic oxidation of LPd complexes.[9,10]
Herein, we describe the discovery, identification, and
isolation of a trinuclear [(LPd)3(m3-O)2]2+ compound (1; L =
neocuproine = 2,9-dimethylphenanthroline) that illuminates
an additional pathway for O2 activation with Pd complexes
(Figure 1). The formation of the trinuclear complex 1 was
detected by electrospray ionization mass spectrometry (ESI-
MS) during the aerobic oxidation of vicinal diols catalyzed by
[LPd(OAc)]2OTf2 (22(OTf)2).[20–22] Synthetic, mechanistic,
and catalytic studies provide key insights into the role of
multinuclear Pd complexes during aerobic oxidation catalysis.
The cationic PdII catalyst 22(OTf)2 catalyzes the selective
oxidation of primary and secondary aliphatic alcohols,[20–23]
vicinal diols,[21] polyols,[22] and carbohydrates.[24] For vicinal
diols and polyols, room temperature oxidation with 22(OTf)2
is chemoselective to afford the a-hydroxyketones.[21,22] Air or
benzoquinone can be used as terminal oxidants; in the
presence of air or O2, competitive oxidation of the ligand
limits the number of turnovers to approximately 15.[20] With
benzoquinone, more than 500 turnovers can be realized.[22]
ESI-MS is a powerful technique for illuminating the
speciation of intermediates in catalytic reactions.[25–28] Unex-
pected species can be identified and characterized from
reaction mixtures and their roles interrogated.[26,27] We have
previously reported a desorption electrospray ionization
(DESI)-MS study of the oxidation of 1,2-propanediol by
22(OTf)2 at millisecond timescales.[22] In the present study, we
employed the longer timescales (minutes to hours) accessed
Mechanistic studies of the reactions of oxygen with
transition metal compounds have illuminated many of the
pathways for O2 activation with reduced metal centers,[4,5,7–11]
as well as strategies for circumventing undesired free-radical
auto-oxidation reactions.[2,5] Herein, we report a new pathway
[*] A. J. Ingram, Dr. D. Solis-Ibarra, Prof. R. N. Zare,
Prof. R. M. Waymouth
Department of Chemistry, Stanford University
333 Campus Drive, Stanford, CA 94305-5080 (USA)
E-mail: zare@stanford.edu
waymouth@stanford.edu
[**] R.M.W. acknowledges support from the Department of Energy (DE-
FG02-10ER16198). R.N.Z. gratefully acknowledges the financial
support from a subcontract with the University of Utah (Agreement
no. 10029173-S2) for which the Air Force Office of Scientific
Research (Grant FA9550-12-1-0481) is the prime sponsor. A.J.I. is
grateful for a Robert M. Bass and Anne T. Bass Stanford Graduate
Fellowship, a National Science Foundation Graduate Research
Fellowship, and a Center for Molecular Analysis and Design
Graduate Fellowship. We thank Wilson Ho for providing the
22(OTf)2 used in this work and Stephen R. Lynch for assisting with
the diffusion NMR experiments. We are grateful to E. I. Solomon
and J. W. Ginsbach for the gift of the 18O2 used in these experiments.
We also acknowledge Kevin Chung, T. Daniel P. Stack, and
Hemamala Karunadasa for fruitful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400134.
5648
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 5648 –5652

Angewandte
Chemie
by electrospray ionization (ESI) to identify species formed
during the catalytic aerobic oxidation of 1,2-propanediol with
22(OTf)2.
Aerobic oxidation of a 0.14m solution of 1,2-propanediol
in MeCN was carried out at room temperature under air with
7 mm 22(OTf)2 (10 mol% Pd) and monitored by sampling
diluted aliquots at various time points with ESI-MS.[29]
Analysis of the resulting mass spectra revealed a doubly
charged species at m/z 486.9995 (m/z reported for 106Pd
isotopologues), which we assign as the trinuclear dication
[(LPdII)3(m3-O)2]2+ (1; Figure 2). Also detected were a number
Scheme 1. Synthesis of 1(BF4)2.
We devised an independent synthesis of 1(BF4)2 to
confirm its structure and characterize its reactivity
(Scheme 1).[29] The addition of a pale yellow MeCN solution
of [LPd(MeCN)2](BF4)2 (7(BF4)2), to a brown MeCN slurry
containing 1.8 equivalents of the h2-peroxo LPd(O2) (8)[30]
under anhydrous conditions and under nitrogen at À308C
resulted in gas evolution; after stirring for 90 min at À308C,
a bright orange solution containing 1(BF4)2 as the major
product was obtained. The complex is unstable in this solution
at room temperature but can be precipitated with cold Et2O
and isolated as an orange powder in 87% yield. ESI-MS of
this compound shows the same envelope of peaks about m/z
486.9995. 1H NMR spectra are consistent with a diamagnetic
compound in a symmetric neocuproine environment. Solid
phase IR shows no major stretches above 600 cmÀ1 other than
those of metallated neocuproine and BF4À. The stoichiometry
of this reaction was confirmed by performing the reaction
with an excess of 7(BF4)2, where 2 equivalents of 8 were
converted into 1 for each equivalent of 7 consumed.
The crystal structure of 1 is shown in Figure 3. The Pd3O2
core is evenly spaced with approximately 1208 between Pd
atoms, and the neocuproine ligands are bent slightly out of the
Figure 2. Representative species detected by mass spectrometry and
their proposed structures.
of previously proposed[20,21] or detected intermediates[22] and/
or inactive products of ligand oxidation.[20] Representative
ions are shown in Figure 2 (see the Supporting Information
for complete assignments). Comparisons of the exact masses
and relative intensities of the observed ions to simulated
Figure 3. Crystal Structure of 1(BF4)2 (O,N light gray (labeled); Pd
medium gray (labeled); and C dark gray; H atoms, solvent, and
counter ions omitted for clarity; thermal ellipsoids set at the 90%
probability level).
isotope patterns revealed
a
molecular formula of
[C42H36N6O2Pd3]2+ for 1. This composition corresponds to
three neocuproine ligands, three Pd centers, and two oxygen
atoms. Performing the alcohol oxidation in 9:1 MeCN/H216O
solvent under 18O2 yielded ions shifted by four mass units and
centered about m/z 489.0042 (z = 2); a result consistent with
the molecular formula [C42H36N618O2Pd3]2+. When the reac-
tion was carried out under air (16O2) in 9:1 MeCN/H218O
PdO2 planes. The O–O distance (2.393 ꢀ) suggests there is no
À
O O bond. The average Pd–Pd distance is 2.824 ꢀ. The
methyl groups of the neocuproine ligands are positioned
above and below the Pd-oxo[31–33] core and are in close
contact, with an average C–C distance between the methyl
groups of 3.64 ꢀ. This structure is related to known Cu3O2,
Ni3O2, Pd3S2 and Pt3O2 analogues.[34–36] Gas-phase DFT
calculations are consistent with the solid-state structure and
diamagnetism of 1 (see the Supporting Information). Diffu-
solvent,
the
species
detected
corresponded
to
[C42H36N616O2Pd3]2+ (see Table S1 in the Supporting Informa-
tion for the isotope-labeling data for compounds 1, 5, and 6).
These data demonstrate that the oxygen atoms in 1 are
derived from O2.
Angew. Chem. Int. Ed. 2014, 53, 5648 –5652
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 5649

.
Angewandte
Communications
sion NMR experiments were performed to determine the
Stokes radius of 1 and to confirm its structure in solution.[29]
The diffusion coefficient for 1 in [D3]MeCN at À108C was
(0.541 Æ 0.087) ꢁ 10À5 cm2 sÀ1. Assuming that the shape of
1 can be approximated as a sphere, this yields a radius of
(7.0 Æ 1.1) ꢀ, a value in good agreement with the crystal
structure of 1, in which the outermost hydrogen atoms lie
between 5 and 8 ꢀ from the center of the molecule.
A series of mass spectrometry and catalysis experiments
were performed to assess the role of 1 in catalytic alcohol
oxidations with the dimeric precursor 22(OTf)2. The speci-
ation of the Pd compounds was monitored during the
oxidation of 0.14m 1,2-propanediol catalyzed by 22(OTf)2
(7 mm, 10 mol% Pd) in MeCN by measuring the ESI-MS of
diluted aliquots at various time points (species that showed
significant variation during catalysis are plotted in Figure 4,
observed are present in the reaction mixture rather than being
generated in the course of microdroplet evaporation.
To assess the chemical and kinetic competence of 1 as an
intermediate in the catalytic oxidation reaction, the isolated
complex 1(BF4)2 was investigated for its activity in the
catalytic oxidation of 1,2-propanediol at room temperature.
Oxidation of 1,2-propanediol (0.3m, MeCN) occurred readily
under 1 atm of air in the presence of 0.83 mm of 1(BF4)2
(2.5 mm Pd, 0.83% loading). The initial rate of alcohol
oxidation and TON (TON = mmol diol consumed/mmol Pd,
see
Table S2)
with
1(BF4)2
[ratei = (0.77 Æ 0.08) ꢁ
10À1 mm minÀ1] are lower than those with 22(BF4)2 [ratei =
(6.06 Æ 0.26) ꢁ 10À1 mm minÀ1] under similar conditions.[40]
Furthermore, a significant amount of a black precipitate,
likely Pd black, formed during catalysis with 1(BF4)2, thus
suggesting that LPd0 oxidation was not efficient under these
conditions. By contrast, no Pd black is observed for the
oxidation of 1,2-propanediol with 22(BF4)2 under these
conditions. The addition of 3 equiv of HOAc relative to
1 led to an increase in the initial rate and TON [rate = (0.98 Æ
0.11) ꢁ 10À1 mm minÀ1] of diol oxidation with 1, however the
activity was lower than that of 22(BF4)2 and Pd black still
formed. By contrast, when a 2:1 cocatalytic mixture of 1(BF4)2
and [LPd(MeCN)2](BF4)2 (7(BF4)2) was used (0.55 mm 1-
(BF4)2, 0.83 mm 7(BF4)2, 2.5 mm total Pd,), the initial rate of
1,2- propanediol conversion and TON per Pd atom [rate =
(5.84 Æ 0.23) ꢁ 10À1 mm minÀ1] were indistinguishable to those
for 22(BF4)2 at the same Pd loading.[41] Under these conditions,
the formation of Pd black was significantly reduced (see the
Supporting Information). This result suggests that some
amount of LPdII with exchangeable ligands is required for
efficient oxidation of LPd0 when 1 is used as a catalyst
precursor.
Figure 4. ESI-MS reaction monitoring. [Pd] for each sample=0.2 mm,
ESI potential=2.0 kV. Intensities for each time point are normalized to
the total ion current at that time point. Peaks areas for related species
are summed and plotted against the left axis. The rate of 1,2-propane-
diol conversion (black circles), measured between two time points, is
plotted against the right axis. Lines connecting data points are
provided merely to illustrate the trends in the data.
The catalytic activity of 1(BF4)2 in combination with
7(BF4)2 indicates that the trinuclear Pd3O2 complex 1(BF4)2 is
both a chemically and kinetically competent intermediate in
the catalytic oxidation of 1,2-propanediol. That both 1(BF4)2
and 7(BF4)2 are needed to achieve a similar rate to that
achieved with the cationic dimer 22(BF4)2 implies that the
efficient activation and reduction of O2 to H2O is facilitated
by the cooperative behavior of multiple Pd centers.[11,34,42]
In Figure 5, we present a mechanistic hypothesis in which
others are described and plotted in Figure S4). An independ-
ent reaction was carried out under identical conditions except
that the aliquots were passed through silica plugs and
analyzed by gas chromatography with flame ionization
detection (GC–FID) to measure the rate of 1,2-propanediol
oxidation. As shown in Figure 4, the appearance and decay of
the trinuclear compound 1 correlate with the rate of 1,2-
propanediol consumption. While this correlation is only
qualitative, given the challenges in quantifying complex
mixtures by ESI-MS, the data nevertheless imply that 1 is
associated with the catalytic oxidation reaction in MeCN. To
assess whether 1 might have been formed during the ESI
ionization process, a similar monitoring experiment was
carried out with nanoelectrospray ionization (nanospray)
MS[37] and provided similar results to those from the ESI-MS
experiments (see the Supporting Information). Because
ionization by nanospray occurs ca. 103 times faster than by
ESI,[38,39] the observation of similar species in both the ESI-
MS and nanospray-MS implies that 1 and the other Pd species
the Pd3O2 complex 1 liberates the catalytically active m-OH
[15,20]
compound 92
and [LPdII]2+ (7) in the presence of acid.
Oxidation of the alcohol would generate [LPd-H]+ and
subsequently the h2-peroxo Pd complex LPd(O2) (8).[30]
Regeneration of the Pd3O2 complex 1 could occur in
a manner similar to that utilized in its synthesis (Scheme 1),
where coupling of the peroxo complex 8 with [LPdII]2+ (7)
generates the proposed dicationic m-peroxo 10, which reacts
with another unit of 8, thereby displacing O2 in order to form
1 (see the Supporting Information for DFT calculations).[43]
Alternatively, 10 might also be generated from hydrogen
peroxide generated from protonolysis of peroxo 8.[30] That
complex 1 can be formed in aprotic solvents indicates that
hydrogen peroxide is not necessary to generate 10 or 1, but
these intermediates may be relevant to the proposed dis-
proportionation of hydrogen peroxide.[45,46]
5650
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 5648 –5652

Angewandte
Chemie
[9] S. S. Stahl, Angew. Chem. 2004, 116, 3480; Angew.
Chem. Int. Ed. 2004, 43, 3400.
[10] K. M. Gligorich, M. S. Sigman, Angew. Chem. 2006,
118, 6764; Angew. Chem. Int. Ed. 2006, 45, 6612.
[11] E. I. Solomon, J. W. Ginsbach, D. E. Heppner, M. T.
Kieber-Emmons, C. H. Kjaergaard, P. J. Smeets, L.
Tian, J. S. Woertink, Faraday Discuss. 2011, 148, 11.
[12] B. M. Stoltz, Chem. Lett. 2004, 33, 362.
[13] K. M. Gligorich, M. S. Sigman, Chem. Commun.
2009, 3854.
[14] C. N. Cornell, M. S. Sigman, Inorg. Chem. 2007, 46,
1903.
[15] R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink, A.
Dijksman, Acc. Chem. Res. 2002, 35, 774.
[16] X. Chen, K. M. Engle, D.-H. Wang, J.-Q. Yu, Angew.
Chem. 2009, 121, 5196; Angew. Chem. Int. Ed. 2009,
48, 5094.
[17] R. I. McDonald, G. Liu, S. S. Stahl, Chem. Rev. 2011,
111, 2981.
Figure 5. Proposed multinuclear catalytic mechanism.
[18] C. S. Yeung, V. M. Dong, Chem. Rev. 2011, 111, 1215.
[19] J. M. Keith, R. J. Nielsen, J. Oxgaard, W. A. Goddard, J. Am.
Chem. Soc. 2005, 127, 13172.
[20] N. R. Conley, L. A. Labios, D. M. Pearson, C. C. L. McCrory,
R. M. Waymouth, Organometallics 2007, 26, 5447.
[21] R. M. Painter, D. M. Pearson, R. M. Waymouth, Angew. Chem.
2010, 122, 9646; Angew. Chem. Int. Ed. 2010, 49, 9456.
[22] K. Chung, S. M. Banik, A. G. De Crisci, D. M. Pearson, T. R.
Blake, J. V. Olsson, A. J. Ingram, R. N. Zare, R. M. Waymouth, J.
Am. Chem. Soc. 2013, 135, 7593.
According to this mechanism, the dicationic [LPdII]2+ (7)
plays a key role in reacting with the peroxo complex 8 to
generate 10. This hypothesis is related to that proposed for O2
activation by multinuclear copper complexes,[11] where the
À
second Pd center facilitates the activation of the O O bond in
10 to form the trinuclear m3-oxo complex 1. The species
responsible for ligand oxidation to form the inactive species 5
and 6 are not yet clear but may be associated with
intermediates 8 or 10. The mechanism by which 1 forms
from 8 and 7 warrants further investigation; nevertheless, the
observation that significant concentrations of the catalytically
active Pd3O2 complex 1 form during the aerobic oxidation of
alcohols with cationic Pd catalysts highlights the potential role
of multinuclear m-oxo Pd complexes in aerobic oxidation
reactions.
[23] D. M. Pearson, N. R. Conley, R. M. Waymouth, Organometallics
2011, 30, 1445.
[24] M. Jꢃger, M. Hartmann, J. G. de Vries, A. J. Minnaard, Angew.
Chem. 2013, 125, 7963; Angew. Chem. Int. Ed. 2013, 52, 7809.
[25] M. N. Eberlin, Eur. J. Mass Spectrom. 2007, 13, 19.
[26] Reactive Intermediates: MS Investigations in Solution (Ed.: L. S.
Santos), Wiley-VCH, Weinheim, 2010).
[27] K. L. Vikse, Z. Ahmadi, C. C. Manning, D. A. Harrington, J. S.
McIndoe, Angew. Chem. 2011, 123, 8454; Angew. Chem. Int. Ed.
2011, 50, 8304.
[28] D. Schrçder, Acc. Chem. Res. 2012, 45, 1521.
[29] See the Supporting Information for experimental procedures.
[30] S. S. Stahl, J. L. Thorman, R. C. Nelson, M. A. Kozee, J. Am.
Chem. Soc. 2001, 123, 7188.
[31] T. Hosokawa, M. Takano, S. Murahashi, J. Am. Chem. Soc. 1996,
118, 3990.
In summary, the trinuclear Pd3O2 complex 1 is generated
in the course of aerobic alcohol oxidation by cationic
neocuproine Pd complexes. This compound was shown to
be a chemically and kinetically competent intermediate in
catalytic alcohol oxidation reactions. The indication that
multinuclear Pd complexes are involved in the activation of
O2 presents a new mechanistic hypothesis for aerobic Pd
oxidation reactions.
´
[32] Y. Zhang, R. J. Puddephatt, L. Manojlovic-Muir, K. W. Muir,
Chem. Commun. 1996, 2599.
Received: January 7, 2014
Revised: February 2, 2014
Published online: April 7, 2014
[33] A. Singh, P. R. Sharp, Dalton Trans. 2005, 2080.
[34] L. M. Mirica, X. Ottenwaelder, T. D. P. Stack, Chem. Rev. 2004,
104, 1013.
[35] A. Singh, U. Anandhi, M. A. Cinellu, P. R. Sharp, Dalton Trans.
2008, 2314.
[36] V. K. Jain, L. Jain, Coord. Chem. Rev. 2010, 254, 2848.
[37] M. S. Wilm, M. Mann, Int. J. Mass Spectrom. Ion Processes 1994,
136, 167.
Keywords: aerobic oxidation · homogeneous catalysis ·
mass spectrometry · palladium · reaction mechanisms
.
[38] A. Wortmann, A. Kistler-Momotova, R. Zenobi, M. C. Heine, O.
Wilhelm, S. E. Pratsinis, J. Am. Soc. Mass Spectrom. 2007, 18,
385.
[1] F. Cavani, J. H. Teles, ChemSusChem 2009, 2, 508.
[2] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 2005, 105,
2329.
[39] M. Girod, E. Moyano, D. I. Campbell, R. G. Cooks, Chem. Sci.
2011, 2, 501.
[40] The rates of diol oxidation with 22(BF4)2 or 22(OTf)2 were
indistinguishable under the conditions employed, thus indicating
that the nature of the anion (OTf to BF4) does not influence the
catalytic reactivity.
[41] The rate of diol oxidation with a mixture of 1(BF4)2 and 7(BF4)2
did not change upon the addition of 1 equiv (relative to Pd) of
acetic acid (Table S2).
[3] J. R. Monnier, Appl. Catal. A 2001, 221, 73.
[4] “Catalysis by Metal Complexes”: L. I. Simꢂndi, Advances in
Catalytic Activation of Dioxygen by Metal Complexes, Kluwer,
Dordrecht, 2002)
[5] L. Que, W. B. Tolman, Nature 2008, 455, 333.
[6] B. N. Ames, M. K. Shigenaga, T. M. Hagen, Proc. Natl. Acad.
Sci. USA 1993, 90, 7915.
[7] L. Boisvert, K. I. Goldberg, Acc. Chem. Res. 2012, 45, 899.
[8] Z. Shi, C. Zhang, C. Tang, N. Jiao, Chem. Soc. Rev. 2012, 41, 3381.
Angew. Chem. Int. Ed. 2014, 53, 5648 –5652
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5651

.
Angewandte
Communications
[42] J. T. York, A. Llobet, C. J. Cramer, W. B. Tolman, J. Am. Chem.
Soc. 2007, 129, 7990.
[45] B. A. Steinhoff, S. R. Fix, S. S. Stahl, J. Am. Chem. Soc. 2002, 124,
766.
[43] The displacement of O2 from 8 in the formation of 1 is likely an
associative process since O2 does not readily dissociate from 8
(Ref. [44]).
[46] We have been unable to detect hydrogen peroxide in aerobic
alcohol oxidations catalyzed by [LPd(OAc)]2OTf2 (22(OTf)2) at
room temperature (Ref. [20]), but we cannot rule out that it is
rapidly disproportionated under these conditions.
[44] B. V. Popp, S. S. Stahl, J. Am. Chem. Soc. 2006, 128, 2804.
5652
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 5648 –5652