Chemoselective aerobic diol oxidation by palladium(II)-pyridine catalysis

Article abstract of DOI:10.1002/ejic.201001300

Neutral and cationic palladium complexes that bear pyridine ligands [i.e., pyridine (Py), 4-ethylpyridine (4-EtPy) and 2,4,6-trimethylpyridine (2,4,6-Me₃Py)] have been isolated and characterized in solution by ¹H and ¹³C{¹H} NMR spectroscopy, cyclic voltammetry (CV) and in the solid state by elemental analysis and single-crystal structure analysis. All palladium compounds have been scrutinized as a precursor to catalyze the aerobic oxidation of diols either in the presence or in the absence of an external base (i.e., K₂CO₃). As a result, the chemoselective production of the corresponding hydroxy ketones has been achieved. The bis-cationic palladium complex of the formula [Pd(4-EtPy)₄](OTs)₂ (OTs = p-toluenesulfonate) [5b(OTs)₂] emerged as the most promising precursor; it outperformed the neutral precursor that consisted of trans-[Pd(OAc)₂(4-EtPy) ₂] (OAc = acetate) and 4-EtPy [3b/2(4-EtPy)] (2 mol-equiv.). An operando high-pressure (HPNMR) spectroscopic study with the precursor 5b(OTs)₂ combined with the results obtained from catalytic reactions has provided insight into the catalytic mechanism that is operative in 5b(OTs)₂-catalyzed aerobic diol oxidation reactions. Neutral and cationic palladium(II) complexes with pyridine ligands were synthesized and employed as catalyst precursors for the aerobic K₂CO ₃-assisted oxidation of unprotected diols to chemoselectivelygive hydroxy ketones. Within the series of catalyst precursors studied, the bis-cationic compound [Pd(4-EtPy)₄](OTs)₂ (Py = pyridine, OTs = p-toluenesulfonate) emerged as the most promising.

Full text of DOI:10.1002/ejic.201001300

FULL PAPER
DOI: 10.1002/ejic.201001300
Chemoselective Aerobic Diol Oxidation by Palladium(II)–Pyridine Catalysis
[a]
[a]
[a]
[a]
Lorenzo Bettucci, Claudio Bianchini, Jonathan Filippi, Alessandro Lavacchi, and
Werner Oberhauser*[
a]
Keywords: Oxidation / Palladium / N ligands / Chemoselectivity / Diols
Neutral and cationic palladium complexes that bear pyridine
ligands [i.e., pyridine (Py), 4-ethylpyridine (4-EtPy) and
dium complex of the formula [Pd(4-EtPy)
toluenesulfonate) [5b(OTs) ] emerged as the most promising
precursor; it outperformed the neutral precursor that con-
sisted of trans-[Pd(OAc) (4-EtPy) ] (OAc = acetate) and 4-
EtPy [3b/2(4-EtPy)] (2 mol-equiv.). An operando high-pres-
sure (HPNMR) spectroscopic study with the precursor
4 2
](OTs) (OTs = p-
2
2
3
,4,6-trimethylpyridine (2,4,6-Me Py)] have been isolated
1
13
1
and characterized in solution by H and C{ H} NMR spec-
troscopy, cyclic voltammetry (CV) and in the solid state by
elemental analysis and single-crystal structure analysis. All
palladium compounds have been scrutinized as a precursor
to catalyze the aerobic oxidation of diols either in the pres-
2
2
5b(OTs)
2
combined with the results obtained from catalytic
reactions has provided insight into the catalytic mechanism
ence or in the absence of an external base (i.e., K
2
CO
3
). As
2
that is operative in 5b(OTs) -catalyzed aerobic diol oxidation
a result, the chemoselective production of the corresponding
hydroxy ketones has been achieved. The bis-cationic palla-
reactions.
Pd(OAc)
and Py,[5i] which is the best compromise between
2
Introduction
optimal turnover rate and catalyst stability with time, as
confirmed by Stahl and co-workers’ theoretical study on
Since the discovery of palladium-catalyzed aerobic
alcohol oxidation by Schwartz et al.,[ increasing research
has being carried out to develop palladium-based catalysts
for the oxidation of alcohols to either ketones or aldehydes,
which represents a key transformation in many organic syn-
1]
[6]
the latter catalytic system.
Since the chemoselective aerobic conversion of unprotec-
ted diols into the corresponding hydroxy ketone is impor-
[5d,7]
tant from an atom-economic point of view,
we report
[
2]
thesis protocols. The circumvention of traditionally ap-
plied organic oxidants, currently used in stoichiometric
herein the synthesis, characterization and application of
neutral and cationic pyridine-based palladium compounds
for the aerobic oxidation of unprotected diols in a 19:1
[
3]
[4]
amounts, or of toxic metals is mandatory for a green
chemistry approach to organic synthesis. Oxygen or air have
already found wide application as final hydrogen acceptors
(v/v) toluene–DMSO solvent mixture.
[
5]
in palladium-catalyzed alcohol oxidation reactions.
Among the various palladium-based homogeneous com-
plexes used as precursors for aerobic alcohol oxidation,
Results and Discussion
[5b,5g]
Sheldon and co-workers’ palladium-neocuproin
palladium-bathocuproin system as well as Uemura and
and
Synthesis and Characterization of Palladium(II) Complexes
[5f]
co-workers’ palladium-pyridine precatalyst[ emerged as
5i]
Palladium complexes with different palladium-to-ligand
the most promising precursors for these oxidation reactions. ratios have been synthesized and characterized in solution
Uemura’s precatalyst is generally prepared in situ by mixing and in the solid state (Scheme 1).
3
Pd(OAc) and pyridine (Py) in a 1:2 ratio directly in the
reaction medium (i.e., toluene). Attempts to optimize the X = OAc (1a) and Cl (2a) have been obtained by the reac-
The palladium complexes [Pd(X)(η -allyl)(Py)] in which
2
[5i]
3
[8a]
[8b]
efficiency of this precatalyst by varying the molar ratio be- tion of [{Pd(X)(η -allyl)} ] (X = Cl, OAc ) with Py in
2
tween Pd(OAc) and Py led to the optimized Uemura pre- a 1:2 ratio in dichloromethane (Scheme 1, a). Both com-
2
catalyst that is characterized by a 1:4 molar ratio between plexes, which show a dynamic behaviour on the NMR spec-
[
5d,9]
troscopic timescale,
were obtained as yellowish powders
1
[
a] Istituto di Chimica dei Composti Organometallici (ICCOM- in around 90% yield. The 13C{ H} NMR spectra, acquired
CNR),
at room temperature, showed only one singlet for both allyl-
CH groups centred at δ = 57.06 (1a) and 61.10 ppm (2a),
2
whereas H and C{ H} NMR spectra acquired at –60 °C
showed for both latter compounds well-defined NMR spec-
troscopic signals for the chemically inequivalent allyl-CH2
Area di Ricerca CNR di Firenze, via Madonna del Piano 10,
5
0019 Sesto Fiorentino, Italy
Fax: +39-055-225-203
1
13
1
E-mail: werner.oberhauser@iccom.cnr.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001300.
Eur. J. Inorg. Chem. 2011, 1797–1805
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1797

L. Bettucci, C. Bianchini, J. Filippi, A. Lavacchi, W. Oberhauser
FULL PAPER
1
showed for the ortho-methyl hydrogen atoms two H NMR
spectroscopic singlets centred at δ = 3.71 and 3.72 ppm and
two 13C NMR spectroscopic singlets at δ = 24.81 and
25.00 ppm.
The bis-cationic palladium complexes of the type [Pd(L)4]-
(
X) {in which L = Py, X = tetra[bis(3,5-trifluoromethyl)-
2
phenyl]borate (BArЈ ) [5a(BArЈ ) ], OTs [5a(OTs) ]; L = 4-
4
4 2
2
EtPy, X = BArЈ4 [5b(BArЈ ) ], OTs [5b(OTs) ], SbF
4 2
2
6
[
5b(SbF ) ]} were straightforwardly obtained in 85–94%
6 2
yield by the reaction of PdCl with the corresponding li-
2
gand in the presence of either NaBArЈ , AgOTs or
4
Ag(SbF ) as shown in Scheme 1 (c). The structural formu-
6
lation of these complexes was unambiguously confirmed by
1
integration of the corresponding H NMR spectroscopic
signals, by elemental analysis and by a single-crystal struc-
ture analysis of 5b(BArЈ ) and 5b(OTs) ·2H O (Figure 1).
4
2
2
2
Scheme 1.
groups. Moreover, the 13C cross-polarization (CP) magic
angle spinning (MAS) spectra of 1a and 2a (Figure S1 in
the Supporting Information), which are in agreement with
1
3
1
the corresponding C{ H} NMR spectra in CD Cl at
2
2
–
60 °C, and a single-crystal X-ray structure analysis of 2a
3
confirmed the asymmetric coordination of the η -allyl
group to palladium in the solid state (Figure S2).
The neutral palladium acetate complexes of the general
1
[10]
formula [Pd(κ -O-OAc) (L) ] in which L = Py (3a)
and
2
2
4 2 2 2
Figure 1. ORTEP plot of 5b(BArЈ ) (a) and 5b(OTs) ·2H O (b).
4
-EtPy (3b) were straightforwardly obtained at room tem-
Thermal ellipsoids are drawn at the 30% probability level and only
the asymmetric part of the cations is labelled, whereas the sym-
metry-related counterion is omitted for clarity. Selected bond
perature by the reaction of Pd(OAc) with the correspond-
ing ligand and applying a 1:2 stoichiometry. Analogously,
2
the neutral palladium–trifluoroacetate complexes of the for- lengths [Å] and angles [°] for 5b(BArЈ ) : Pd1–N1 2.026(3), Pd1–
4
2
1
N2 2.020(3), N1–Pd1–N2 90.14(12); for 5b(OTs)
.021(2), Pd1–N2 2.020(2) and N1–Pd1–N2 90.28(6). Intramolecu-
lar distances [Å] for 5b(OTs) ·2H O: Pd1···O1 3.096, O1···H1
.650, O1···H12 2.589.
2 2
·2H O: Pd1–N1
mula [Pd(κ -O-tfa) (L) ] (tfa = trifluoroacetate) with L =
2
2
2
4-EtPy (4b) and 2,4,6-(Me) Py (4c) were obtained by the
3
2
2
reaction of Pd(tfa) with the corresponding ligand. All these
neutral palladium compounds, isolated as yellow or brown-
ish powders in 74 to 80% yield, share a square-planar coor-
2
2
The crystal structure of 5b(BArЈ ) and 5b(OTs) ·2H O
4
2
2
2
dination geometry around palladium with the anionic li- is centrosymmetric with palladium occupying a centre of
gand (i.e., OAc or TFA) located trans to each other as con- inversion. Both molecular structures show an almost per-
firmed by single-crystal X-ray structure analysis carried out pendicular tilt of the pyridine ring with respect to the metal
for 3b, 4b and 4c (Figures S3 and S4 in the Supporting In- coordination plane defined by N1, N2 and Pd1 of
formation).
80.85(19), 89.57(14)° [5b(BArЈ ) ] and of 82.92(8),
4 2
The unsymmetric coordination of 2,4,6-(Me) Py to pal- 87.03(10)° [5b(OTs) ]. Such a ligand orientation is known
3
2
ladium in the crystal structure of 4c is retained in solution. for related pyridine-bearing palladium complexes and re-
1
13
1
Accordingly, H and C{ H} NMR spectra of the latter sults from minimizing steric repulsion between the metal-
[
11]
compound acquired in CD Cl at room temperature coordinating pyridine ligands.
The most important fea-
2
2
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1797–1805

Chemoselective Aerobic Diol Oxidation
ture of the latter crystal structures is the different displace- Catalytic Study
ment of the counterions with respect to the coordination
Compounds 1a–5b(SbF ) were tested as catalysts for the
6
2
plane of palladium. Whereas in 5b(BArЈ ) the anions are
located almost in the palladium coordination plane, in
4
2
aerobic oxidation, i.e., air (300 psi) of unprotected 1,2- and
,3-diols (Scheme 2, A–E) to give exclusively the corre-
sponding hydroxy ketones (Scheme 2, AЈ–EЈ) in a 19:1
v/v) toluene/DMSO mixture, which solubilizes all sub-
strates employed in this study. Catalytic reactions carried
out in other organic solvents such as THF and CH CN led
to much lower substrate conversion. The catalytic reactions
were carried out either in the presence or in the absence of
an external base, i.e., K CO . The substitution of K CO
1
5
b(OTs) ·2H O both anions are localized symmetry-related
2
2
above and below the metal coordination plane as shown in
Figure 1. As a result, the tosylate oxygen atom O1 and its
symmetry-related counterpart occupy axial positions of a
pseudo-octahedral metal coordination sphere and show in-
tramolecular distances to Pd1 and to the ortho-hydrogen
atoms H1 and H12 of 4-EtPy (Figure 1). Similar intramo-
lecular distances between cation and anion have been ob-
(
3
2
3
2
3
by either Na CO or Cs CO gave no benefit in terms of
2
3
2
3
served in related palladium complexes that bear weakly co-
substrate conversion or catalyst stability with time.
ordinating oxygen-donor anions.[
11b] 1
H NMR spectra of
5
a/b(OTs) acquired in CD Cl or in a 19:1 (v/v) [D ]tolu-
2 2 2 8
ene/[D ]DMSO solvent mixture (i.e., the reaction medium
6
of the aerobic oxidation reactions) showed for the ortho-
hydrogen atoms of 4-EtPy a significant high-frequency
chemical shift of δ = 1.44 ppm relative to 5a/b(BArЈ ) ,
4
2
thereby proving that the solid-state conformation is re-
tained in solution. Furthermore, conductivity measure- Scheme 2.
ments of nitroethane solutions of 5a/b(OTs) behave as a
2
1
:1 electrolyte, whereas 5a/b(BArЈ ) behaves as a 1:2 elec-
The catalytic activity of 1a–5b(SbF ) was screened by
4
2
6 2
trolyte.
employing 1,2-propanediol as test substrate, and the results
To determine the relative σ-electron donor property of of this comparative catalytic study are compiled in Table 1.
the pyridine ligands (i.e., Py, 4-EtPy, 2,4,6-Me Py) in neu- Regardless of the precursor employed for the aerobic oxi-
3
tral (i.e., 3a/b, 4b and 4c) and cationic palladium complexes, dation of 1,2-propanediol, hydroxyacetone was chemoselec-
i.e., 5a/b(BArЈ ), a CV study was carried out using a 19:1 tively obtained. The catalyst derived from 3a converted
4
(v/v) toluene/DMSO solvent mixture (see the Supporting 46% of 1,2-propanediol to hydroxyacetone after 1 h reac-
Information). As a result, the cathodic half-wave potential tion time (Table 1, entry 4). An analogous reaction in the
(
E½c), which is directly correlated to the σ-electron donor presence of 2 mol-equiv. of K CO showed only a slight in-
2 3
[12]
property of the Py ligands,
increased, as expected, with crease of substrate conversion (Table 1, entry 4 vs. 5). This
the increasing σ-electron donation property of the pyridine experimental result confirms the efficiency of acetate as in-
ligand: Py Ͻ 4-EtPy Ͻ 2,4,6-(Me) Py.
ternal base. A comparison of the catalytic activity of 3a
3
6 2
Table 1. Aerobic oxidation of 1,2-propanediol catalyzed by 1a–5b(SbF ) .
Entry[a]
Precursor
K
2
CO
[mol-equiv.]
Conversion [%][b]
1 h
3
0.5 h
2 h
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1a
1a
2a
3a
3a
3b
3b
3b
3b
4b
4b
4c
–
2
2
–
2
–
2
4
–
–
2
2
2
2
2
–
2
4
2
40
38
37
40
38
–
–
–
–
–
–
–
–
–
–
–
46
–
–
41
51
47
46
49
50
53
48
34
11
65
56
19
55
29
1.1
80
61
60
–
52
50
48
59
62
73
63
72
12
80
67
30
69
36
2.1
90
74
79
[
c]
0
1
2
3
4
5
6
7
8
9
5a(BArЈ
5a(OTs)
5b(BArЈ
4
)
2
2
4
)
2
5b(OTs)
5b(OTs)
5b(OTs)
2
2
2
5b(SbF
6
)
2
[
(
a] Catalytic conditions: catalyst precursor (0.01 mmol), 1,2-propanediol (1.00 mmol), 19:1(v/v) toluene/DMSO (20 mL), 80 °C, p(air)
300 psi), mol-equiv. of K CO with respect to the amount of catalyst precursor. [b] Conversion given as average value of 3 runs using
decane as internal standard for GC analysis. [c] Addition of 4-EtPy (0.02 mmol).
2
3
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L. Bettucci, C. Bianchini, J. Filippi, A. Lavacchi, W. Oberhauser
FULL PAPER
with that of 1a, which bears only one coordinated acetate 1,2-octanediol (C), (Ϯ)-1,3-butanediol (D) and a 1:1 mix-
and one pyridine ligand, shows an identical substrate con- ture of meso-/rac-2,4-pentanediol (E) (Scheme 2). The cata-
version for a short reaction time (i.e., 0.5 h; Table 1, entries lytic performance of 5b(OTs) was compared with that of
2
II
4
and 5 vs. 1 and 2). In this context, it is important to 3b/2(4-EtPy) with which it shares a Pd -to-ligand ratio of
mention that 1a enters the catalytic cycle by a substrate- 1:4. The results of the comparative catalytic study are listed
[
5d]
mediated protonation of the coordinated allyl moiety,
in Table 2.
thereby resulting in a Pd–alkoxy species under the concomi-
tant release of propene, the presence of which in the cata-
lytic solution has been proven by H NMR spectroscopy
and GC–MS analysis. To underscore the fact that acetate
in 1a does not function as an internal base, an identical
catalytic reaction with 2a, which contains chloride instead
of acetate (Table 1, entry 3 vs. 2), was carried out and gave
almost identical substrate conversion.
Table 2. Aerobic oxidation of diols catalyzed by 3b/2(4-EtPy) and
5
b(OTs)
2
.
1
Entry[a] Precursor
Substrate
Conversion [%]
[b]
1
h (TOF)[c]
2 h (TOF/Yield)[c]
1
3b/2(4-EtPy)
5b(OTs)
5b(OTs)
3b/2(4-EtPy)
5b(OTs)
5b(OTs)
3b/2(4-EtPy)
5b(OTs)
5b(OTs)
A
A
A
B
B
B
C
C
C
D
D
E
E
34
80(80)
30(120)
37
71(71)
25(100)
30
80(80)
35(140)
46
72
90(45)
45(90/43)
57
95(47)
37(74/34)
55
95(95)
55(110/53)
56
[
d]
2
3
4
2
[
d,e]
2
The increasing substrate conversion in the presence of
K CO obtained either by substituting Py in 3a for the
[d]
5
2
[
d,e]
6
7
8
9
2
2
3
stronger σ-electron-donating 4-EtPy (3b) or by substituting
OAc in 3b for TFA (4b) confirms that the observed catalytic
activity is the result of the easier anion dissociation (i.e.,
TFA vs. OAc) and the stabilization of the catalytic species 11
by a stronger σ-electron donor (i.e., 4-EtPy vs. Py). It is
[
d]
2
[
d,e]
2
10
3b/2(4-EtPy)
5b(OTs)2
5b/2(4-EtPy)
[d]
65
45
59
84
49
75
1
2
[d]
13
5b(OTs)
2
worth mentioning that neutral palladium complexes that
contain a weaker base than OAc, such as TFA or chloride, [a] Catalytic conditions: catalyst precursor (0.01 mmol), substrate
(
8
1.00 mmol), 19:1 (v/v) toluene/DMSO (20 mL), p(air) 300 psi,
0 °C. [b] Conversion given as average value of 3 runs using decane
as internal standard for GC analysis. [c] Turnover frequency (TOF)
need an external base to be converted into the catalytically
active Pd–alkoxy species.[ Further increasing the σ-elec-
tron donor property of the pyridine ligand by using 2,4,6-
7]
–1
values are expressed as (mmol substrate)ϫ[(mmol Pd)ϫhour] ;
CO (0.02 mmol).
Me Py (4c) clearly showed that a more reluctant ligand isolated yield given in %. [d] Addition of K
2
3
3
[e] Substrate (4.00 mmol).
dissociation from the precursor molecule results in a de-
crease of the catalytic activity observed for 4c relative to 4b
Both catalyst precursors chemoselectively converted the
diols into the corresponding hydroxy ketones as shown in
(Table 1, entry 12 vs. 11).
In accordance with Uemura’s optimized precatalyst, the
Scheme 2. A perusal of Table 2 shows 5b(OTs) to be more
2
contemporary presence of 3b and 4-EtPy (2 mol-equiv.) in
the catalytic solution [i.e., 3b/2(4-EtPy)], which does not
lead to the formation of a bis-cationic palladium complex
efficient than 3b/2(4-EtPy), regardless of the substrate em-
ployed. The catalytic performance of the former catalyst
precursor was particularly pronounced with 1,2-diols and
showed for a substrate-to-precursor ratio of 400 a TOF of
up to 140 (Table 2, entry 9). On the other hand, using 1,3-
diols instead of 1,2-diols caused a drop of the catalytic ac-
1
of the formula [Pd(4-EtPy) ](OAc) , as proven by H NMR
4
2
spectroscopy, led to an increased catalyst stability with time
relative to 3b (Table 1, entry 9 vs. 6).
Within the series of bis-cationic palladium precursors in-
vestigated for the aerobic oxidation of unprotected diols,
tivity of 5b(OTs) as is evident from the results compiled in
2
Table 2.
5
b(OTs) in conjunction with external base (2 mol-equiv. of,
2
i.e., K CO ) has emerged as the most active catalyst precur-
2
3
sor and outperforms 3b/2(4-EtPy) under comparable cata-
lytic conditions (Table 1, entry 17 vs. 9). It is important to
mention that the bis-cationic precursors need the presence
1
Operando H HPNMR Spectroscopic Study
In an attempt to spectroscopically follow the 5b(OTs)2-
of an external base to enter the catalytic cycle. In this re- catalyzed aerobic oxidation reaction of 1,2-propanediol, an
spect, 2 mol-equiv. of base (in reference to the amount of operando HPNMR experiment was carried out in a [D8]-
precursor) is the optimal amount (Table 1, entry 17 vs. 16 toluene/[D ]DMSO (19:1 v/v) solvent mixture. This cata-
6
1
and 18).
lytic reaction was followed by variable-temperature
A comparison of the catalytic performance of 5b(OTs)2 NMR spectroscopy and selected H NMR spectra in the
and 5b(SbF ) with that of 5b(BArЈ ) clearly showed the pertinent chemical-shift range from δ = 6.4 to 10.4 ppm are
H
1
6
2
4 2
presence of an anion effect on the catalytic performance of shown in Figure 2.
the bis-cationic precursors. Indeed, within the series of bis-
The 1H NMR spectrum of a 19:1 (v/v) [D ]toluene/
cationic catalyst precursors, the tosylate precursor showed [D ]DMSO solution of 5b(OTs) showed for the ortho-hy-
8
6
2
the highest and the BArЈ counterpart the lowest catalytic drogen atoms of 4-EtPy a high-frequency doublet at δ =
4
3
activity (Table 1, entry 17 vs. 15 and 19).
10.25 ppm ( J
= 8.8 Hz) (Figure 2, trace a). An analo-
H,H
Since 5b(OTs)2 in combination with 2 mol-equiv. of gous spectrum of 5b(BArЈ ) under identical experimental
4 2
K CO was the most promising precursor, its application conditions showed for the same hydrogen atoms a doublet
2
3
was extended to other diols such as 2R,3R-butanediol (B), centred at δ = 9.18 ppm. This spectroscopic finding is in-
1800
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Eur. J. Inorg. Chem. 2011, 1797–1805

Chemoselective Aerobic Diol Oxidation
pattern occurred (trace d), whereas on heating the sapphire
1
tube to 80 °C for 1 h, two new broad H NMR spectro-
scopic doublets centred at δ = 8.56 and 6.75 ppm appeared
1
in the corresponding NMR spectrum (trace e). These H
NMR spectroscopic doublets were safely attributed to un-
0
coordinated 4-EtPy, due to the formation of Pd in the
course of the catalytic oxidation reaction. It is important to
mention at this point that 5b(OTs) is stable in [D ]toluene/
2
8
[
D ]DMSO even upon heating the latter solution at 80 °C,
6
1
as proven by H NMR spectroscopy.
Based on batch catalytic reactions and HPNMR spectro-
scopic experiments, we propose for the aerobic oxidation of
diols mediated by bis-cationic Pd–pyridine-based catalysts
1
Figure 2. Variable-temperature H HPNMR spectroscopic study of a catalytic cycle as shown in Scheme 3.
the aerobic oxidation of 1,2-propanediol with 5b(OTs)
tube, 19:1 (v/v) [D ]toluene/[D ]DMSO, 200.13 MHz, δ values
at room temperature; (b) addition of K CO
2 mol-equiv.); (c) addition of 1,2-propanediol (50 μL); (d) after
2
[sapphire
II
The bis-cationic Pd -based catalyst precursors (5) are
8
6
converted into the catalytically active monocationic Pd–alk-
oxy species (7) by removing two pyridine ligands from the
charging with air (300 psi) at room temperature; and (e) after 1 h metal centre and the successive deprotonation reaction of
(
(
ppm)]: (a) 5b(OTs)
2
2
3
at 80 °C. Asterisks denote signals of uncoordinated 4-EtPy.
the coordinated substrate by an external base (i.e., K CO )
2 3
(Scheme 3). The substrate-mediated ligand-substitution re-
dicative of a hydrogen-bond-based interaction between OTs action is most likely an equilibrium reaction. The extent
and the ortho-hydrogen atoms of the metal-coordinated 4- to which this latter equilibrium is shifted to the substrate-
EtPy. The addition of K CO (2 mol-equiv.) to the latter coordinated species depends on: (i) the “bite angle” of the
2
3
solution did not change the NMR spectroscopic pattern diol employed (i.e., 1,2-diols more favoured than 1,3-diols;
trace b), whereas by addition of 1,2-propanediol to the lat- Table 2); (ii) the bulkiness of the counterion of the catalytic
(
1
ter solution, the H NMR spectroscopic doublet assigned precursor involved, as experimentally proven by a compari-
to the ortho-hydrogen atoms of 4-EtPy shifted to 10.03 ppm son of the catalytic activity of 5b(BArЈ ) , 5b(SbF ) and
4
2
6 2
1
and also the H NMR spectroscopic doublet assigned to 5b(OTs) (Table 1). In this context, the single-crystal X-ray
2
OTs shifted to slightly lower frequency (trace c). This evi- structure analysis of 5b(BArЈ ) clearly shows the localiza-
4
2
dence is indicative of adduct formation between 1,2-pro- tion of the counterions in the coordination plane of palla-
panediol and the ortho-hydrogen atoms of 4-EtPy, which is dium, which contrasts with the crystal structure of 5b-
similar to that reported by Stahl et al. between 3a and (OTs) ·2H O (vide infra). In addition, the experimental fact
2
2
[
6]
benzyl alcohol. In fact, an adduct formation between the that 5b(OTs) showed a higher catalytic activity when com-
2
substrate and the catalyst precursor is a prerequisite for a pared to 5b(SbF ) (Table 1) might be ascribed to the stabi-
6
2
substrate-induced substitution reaction of 2-EtPy. By lizing effect of OTs through coordination to palladium
II
charging the sapphire tube with air (300 psi) at room tem- (Scheme 3) fostering the oxygen insertion into the Pd –hy-
1
[13]
perature no alteration of the H NMR spectroscopic dride bond due to its hydrogen-bond acceptor property.
Scheme 3. Proposed catalytic cycle for the chemoselective aerobic oxidation of unprotected diols by cationic Pd–pyridine-based catalysts.
1801
Eur. J. Inorg. Chem. 2011, 1797–1805
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org

L. Bettucci, C. Bianchini, J. Filippi, A. Lavacchi, W. Oberhauser
FULL PAPER
II
Monocationic Pd –alkoxy species 7 undergoes a β-hy- pend on (i) the coordination property of the diol employed
II
II
dride elimination reaction to give monocationic Pd –hy- to Pd (i.e., 1,2-diols show higher conversion than 1,3-di-
dride 8 under the concomitant release of hydroxy ketone. ols); and (ii) the bulkiness and coordination property of the
Then, 8 inserts oxygen to yield Pd–hydroperoxide species 9. counterion (i.e., the OTs precursor showed the highest and
The catalytic cycle is then closed by the reaction of the sub- the BArЈ counterpart the lowest catalytic activity).
4
strate with 9 to release H O . The presence of H O in the
2
2
2
2
catalytic solution has been proven upon interception of di-
methylsulfone by GC analysis of the catalytic solution. Im- Experimental Section
portantly, the oxidation of DMSO to dimethylsulfone by
General Procedure: All synthetic reactions were carried out in air
by using round-bottomed flasks. Solvents were either distilled or
passed through columns that contained dehydrating agents. Rea-
gents were used as received from Aldrich unless stated otherwise.
[
5d]
H O has been proven by carrying out a blank reaction.
2
2
The most significant differences between the bis-cationic
Pd–pyridine- and the 3b/2(4-EtPy)-based catalytic cycle
3
[8a]
2
3
[8b]
(
Scheme 4) can be summarized as follows: (i) unlike the bis- Compounds [PdCl(η -allyl)] ,
[Pd(κ -O-OAc)(η -allyl)]
and
2
2
cationic Pd precursors, 3b/2(4-EtPy) enters the catalytic cy- 3a[ were prepared according to literature methods. Deuterated
10]
cle by an intramolecular reaction of one coordinated acet- solvents for routine NMR spectroscopic measurements were dried
1
13
1
with activated molecular sieves. H and C{ H} NMR spectra
were obtained with a Bruker Avance DRX-400 spectrometer ac-
quiring spectra at 400.13 and 100.62 MHz, respectively. Chemical
shifts are reported in ppm (δ) with reference to TMS as an internal
ate molecule with the substrate, whereas the second acetate
molecule remains coordinated to palladium throughout the
catalytic cycle (Scheme 4); (ii) the Pd–alkoxy species formed
from 3b/2(4-EtPy) is neutral and may thus undergo a much
slower β-hydride elimination reaction compared to the
monocationic counterpart formed from bis-cationic Pd pre-
cursors (Scheme 3); and (iii) the Pd–hydroperoxy species in
1
13
1
1
standard ( H and C{ H} NMR spectra). H HPNMR spectro-
scopic experiments were carried out with a Bruker Avance II-200
spectrometer equipped with a 10 mm BB probe using a 10 mm sap-
phire tube [Saphikon (Milford, NH), equipped with a titanium
[
15]
the 3b/2(4-EtPy)-based catalytic cycle can alternatively be high-pressure charging head constructed at ICCOM-CNR]. CP-
0
13
formed by an aerobic, acetate-mediated oxidation of Pd
MAS C NMR spectra were acquired with a Bruker Avance DRX-
[
5a,14]
(Scheme 4).
400 spectrometer equipped with a 4 mm BB CP-MAS probe at a
working frequency of 100.62 MHz and a spinning rate of 10 kHz.
Microanalyses were performed with a Carlo–Erba Model 1106 ele-
mental analyzer. Conductivities were measured with an ORION
Model 990101 conductance cell connected to a conductivity meter.
The conductivity data were obtained at sample concentrations of
around 0.001 m in nitroethane.[ Oxidation reactions were per-
formed with 65 mL stainless steel autoclaves, constructed at IC-
COM-CNR, equipped with magnetic stirring, oil-bath heating and
a temperature and pressure controller. GC analyses were performed
with a Shimadzu 2010 gas chromatograph equipped with a flame
ionization detector and a 30 m (0.25 mm i.d., 0.25 μm film thick-
ness) VF-WAXms capillary column. Decane was used as internal
standard. GC–MS analyses were performed with a Shimadzu QP
16]
5000 apparatus, equipped with a 30 m (0.32 mm i.d., 0.50 μm film
thickness) CP-WAX 52 CB WCOT-fused silica column.
2
3
Synthesis
0
2
of
.848 mmol) was dissolved in CH
.050 mmol) was added to this solution under vigorous stirring at
1a:
[{Pd(κ -O-OAc)(η -allyl)}
2
]
(350.0 mg,
2
Cl (10 mL). Py (167.0 μL,
2
room temperature. The resulting solution was allowed to stir at
room temperature for 1 h. Afterwards, the solution was concen-
trated to a small volume (2 mL) and on addition of diethyl ether
Scheme 4. Proposed catalytic cycle for the chemoselective aerobic
oxidation of unprotected diols by 3b/2(4-EtPy).
(
15.0 mL) the product precipitated as brownish powder that was
separated by filtration and washed with diethyl ether (3ϫ5 mL).
The solid was then dried in a stream of nitrogen; yield 445.5 mg
Conclusion
(
2
92%). C10H13NO Pd (285.51): calcd. C 42.07, H 4.55, N 4.90;
1
2 2
A series of neutral and cationic pyridine-based palladium found C 41.95, H 4.40, N 4.73. H NMR (400.13 MHz, CD Cl ,
3
2
1 °C): δ = 1.95 (s, 3 H, CH
3 2
CO ), 3.15 (d, JH,H = 11.6 Hz 2 H,
complexes have been synthesized, completely characterized
and tested as catalyst precursors for the aerobic oxidation
of unprotected diols to chemoselectively yield the corre-
sponding hydroxy ketone. The comparative catalytic study
showed that the bis-cationic precursor 5b(OTs) in combi-
nation with an external base (i.e., K CO ) outperforms the
allyl-C ), 3.87 (br. s, 2 H, allyl-C
a
H
2
b
H
2
), 5.62 (quintet, 3
H,H
J =
6
.4 Hz, 1 H, allyl-CH), 7.43 (m, 2 H, m-Ar–H), 7.86 (m, 1 H, p-
1
3
1
Ar–H), 8.69 (m, 2 H, o-Ar–H) ppm. C{ H} NMR (100.62 MHz,
CD
2 2 3 2 2
Cl , 21 °C): δ = 23.60 (s, CH CO ), 57.06 (br. s, allyl-CH ),
2
1
13.51 (s, allyl-CH), 124.99 (s, Ar–C), 137.90 (s, Ar–C), 151.91 (s,
2
3
3 2
Ar–C), 176.00 (br. s, CH CO ) ppm. H NMR (400.13 MHz,
CD
1
neutral optimized Uemura-type precatalyst 3b/2(4-EtPy).
3
2
Cl
2
, –60 °C): δ = 1.93 (s, 3 H, CH
3
CO
2
), 2.96 (d,
H,H
J =
3
The efficiency of bis-cationic catalyst precursors in the aero- 11.6 Hz, 1 H, allyl-C HHЈ), 3.07 (d, J
= 12.4 Hz, 1 H, allyl-
HHЈ), 3.96 (d, JH,H
a
H,H
3
3
bic oxidation of unprotected diols has been found to de-
C
b
HHЈ), 3.67 (d, JH,H = 6.8 Hz, 1 H, allyl-C
a
1802
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1797–1805

Chemoselective Aerobic Diol Oxidation
HHЈ), 5.64 (quintet, 3JH,H = 6.0 Hz, 1 H,
7.28 (d, 3JH,H = 8.0 Hz, 4 H, m-Ar–H), 8.36 (d, JH,H = 8.0 Hz, 4
3
=
6.8 Hz, 1 H, allyl-C
b
1
3
1
allyl-CH), 7.43 (m, 2 H, m-Ar–H), 7.87 (m, 1 H, p-Ar–H), 8.59 (m,
H, o-Ar–H) ppm. C{ H} NMR (100.62 MHz, CD
= 13.50 (s, CH CH ), 28.08 (s, CH CH ), 113.98 (q,
289.9 Hz, CF CO ), 125.10 (s, m-Ar–C), 150.06 (s, o-Ar–C), 157.92
(s, p-Ar–C), 162.22 (q, JC,F = 36.2 Hz, CF
2 2
Cl , 21 °C): δ
H, o-Ar–H) ppm. 13C{ H} NMR (100.62 MHz, CD
δ = 24.10 (s, CH CO ), 57.39 (s, allyl-C HHЈ), 58.00 (s. allyl-
HHЈ), 114.52 (s, allyl-CH), 125.51 (s, Ar-C), 138.51 (s, Ar–C),
52.10 (s, Ar–C), 177.18 (s, CH CO
) ppm. CP-MAS 13C NMR: δ
28.30 (s, CH CO ), 60.30 (s, allyl-C HHЈ, C HHЈ), 113.20 (s,
allyl-CH), 128.20 (s, Ar–C), 139.10 (s, Ar–C), 153.20 (s, Ar–C),
78.10 (s, CH CO ) ppm.
1
Cl
, –60 °C):
1
2
2
2
3
2
3
2
C,F
J =
3
2
a
3
2
2
C
1
=
b
3 2
CO ) ppm.
3
2
Synthesis of 4c: Pd(TFA)
CH Cl (8 mL). 2,4,6-Me
2
(179.4 mg, 0.540 mmol) was dissolved in
Py (144.0 μL, 1.079 mmol) was added to
3
2
a
b
2
2
3
this solution. The obtained solution was allowed to stir at room
temperature for 1 h, followed by its concentration to a small vol-
ume (2 mL). On addition of diethyl ether the product precipitated
as yellow-brownish powder that was separated by filtration, washed
with diethyl ether and then dried in a stream of nitrogen; yield
1
3
2
3
Synthesis of 2a: [{Pd(Cl)(η -allyl)}
dissolved in CH Cl (10 mL). Py (167.0 μL, 2.050 mmol) was added
to this solution under vigorous stirring at room temperature. The
obtained yellow solution was allowed to stir at the same tempera-
ture for 1 h, followed by its concentration to a small volume
2
] (310.2 mg, 0.848 mmol) was
2
2
2
29.6 mg (74%). C20
N 4.87; found C 41.72, H 3.78, N 4.80. H NMR (400.13 MHz,
CD Cl , 21 °C): δ = 2.33 (s, 6 H, p-CH ), 3.71 (s, 6 H, o-CH ), 3.72
s, 6 H, o-CH ), 6.94 (s, 2 H, m-Ar–H), 6.97 (s, 2 H, m-Ar–H) ppm.
C{ H} NMR (100.62 MHz, CD Cl , 21 °C): δ = 20.38 (s, p-CH ),
4.81 (s, o-CH ), 25.00 (s, o-CH ), 113.54 (q, C,F = 290.0 Hz,
CF CO ), 123.75 (s, m-Ar–C), 124.24 (s, m-Ar–C), 151.75 (s, p-Ar–
22 6 2 4
H F N O Pd (574.62): calcd. C 41.80, H 3.83,
1
(2 mL). Addition of diethyl ether (10 mL) caused the precipitation
2
2
3
3
(
3
of the product as yellow powder that was separated by filtration
washed with diethyl ether (3ϫ5 mL) and dried in a stream of nitro-
gen; yield 399.8 mg (90%). C
H 3.82, N 5.34; found C 36.81, H 3.90, N 5.15. H NMR
1
3
1
2
2
3
1
2
3
3
J
8
H10ClNPd (261.96): calcd. C 36.68,
1
3
2
2
3
C), 160.52 (s, o-Ar–C), 160.76 (s, o-Ar–C), 162.16 (q, JC,F
36.6 Hz, CF CO ) ppm.
=
(
400.13 MHz, CD
allyl-C HHЈ), 4.04 (d, JH,H = 6.8 Hz, 2 H, allyl-C
H, allyl-CH), 7.45 (m, 2 H, m-Ar–H), 7.87 (m, 1 H, p-Ar–H),
2 2
Cl , 21 °C): δ = 3.13 (d, JH,H = 12.2 Hz, 2 H,
3
HHЈ), 5.67 (m,
3
2
a
b
2
8
2
4 2 2 2
Synthesis of 5a(BArЈ ) and 5a(OTs) : PdCl (56.4 mg, 0.318 mmol)
was suspended in CH Cl (10 mL). Py (109.0 μL, 1.332 mmol) was
2 2
.84 (m, 2 H, o-Ar–H) ppm. 13C{ H} NMR (100.62 MHz, CD
1
Cl
,
2
2
1 °C): δ = 61.10 (br. s, allyl-CH ), 114.23 (s, allyl-CH), 125.02 (s,
2
added to this latter suspension and the obtained yellow solution
was allowed to stir for 1 h. Afterwards, NaBArЈ4 (567.2 mg,
0.640 mmol) or AgOTs (178.6 mg, 0.640 mmol) was added to the
solution, which caused the precipitation of NaCl and AgCl, respec-
tively. The obtained suspension was allowed to stir in the dark at
room temperature for 1 h, followed by its filtration through a plug
of celite. The obtained solution was then concentrated to dryness.
5a(BArЈ ) : Yield 580.8 mg (85%). C H B F N Pd (2148.86):
1
Ar–C), 137.99 (s, Ar–C), 152.54 (s, Ar–C) ppm. H NMR
3
(400.13 MHz, CD
2 2
Cl , –60 °C): δ = 2.98 (d, JH,H = 12.4 Hz, 1 H,
3
allyl-C HHЈ), 3.24 (d, JH,H = 12.4 Hz, 1 H, allyl-C
a
b
HHЈ), 3.95 (d,
HHЈ), 4.03 (d, JH,H = 5.6 Hz, 1 H,
HHЈ), 5.67 (m, 1 H, allyl-CH), 7.44 (m, 2 H, m-Ar–H), 7.87
3
3
J
H,H = 6.8 Hz, 1 H, allyl-C
a
allyl-C
b
13
1
(m, 1 H, p-Ar–H), 8.75 (m, 2 H, o-Ar–H) ppm. C{ H} NMR
(100.62 MHz, CD Cl , –60 °C): δ = 58.07 (s, allyl-C HHЈ), 64.74
(s. allyl-C
2
2
a
4
2
84 44
2
48
4
b
HHЈ), 114.99 (s, allyl-CH), 125.40 (s, Ar–C), 138.45 (s, calcd. C 46.95, H 2.05, N 2.60; found C 46.80, H 1.98, N 2.45.
Ar–C), 152.60 (s, Ar–C) ppm. CP-MAS C NMR: δ = 58.10 (br. 5a(OTs) : Yield 210.9 mg (90%). C H N O PdS (764.91): calcd.
13
2
34 34
4
6
2
s, allyl-C
a
HHЈ), 68.31 (s, allyl-C
b
HHЈ), 118.40 (s, allyl-CH), 125.40
C 53.39, H 4.44, N 7.32; found C 53.20, H 4.50, N 7.20.
(
s, Ar–C), 138.21 (s, Ar–C), 153.80 (s, Ar–C) ppm.
1
NMR Spectroscopic Data for 5a(BArЈ
4
)
2
: H NMR (400.13 MHz,
3
3
Synthesis of 3b: Pd(OAc)
2
(119.9 mg, 0.534 mmol) was dissolved in
2 2
CD Cl , 21 °C): δ = 7.35 [dd, JH,H = 8.0, JH,H = 6.4 Hz, 8 H, m-
CH Cl
4
(8 mL). Afterwards, 4-EtPy (124 μL, 1.070 mmol) was Ar–H(Py)], 7.59 [s, 8 H, p-Ar–H(BArЈ )], 7.76 [br. s, 16 H, o-Ar–
2
2
3
4
added to the solution and stirred for 1 h. The latter solution was
then concentrated to a small volume (2 mL) by means of vacuum
and on addition of diethyl ether (10.0 mL) the product was precipi-
tated as light brown powder that was separated by filtration,
washed with diethyl ether (3ϫ5 mL) and dried in a stream of nitro-
H(BArЈ
4
)], 8.01 [tt, JH,H = 8.0, JH,H = 1.2 Hz, 4 H, p-Ar–H(Py)],
3
H,H = 6.4, 4JH,H = 1.2 Hz, 8 H, o-Ar–H(Py)] ppm.
C{ H} NMR (100.62 MHz, CD Cl , 21 °C): δ = 117.52 [s, p-Ar–
C,F = 272.5 Hz, CF C), 128.81 [s, m-Ar–
C(Py)], 129.99 [q, JC,F = 38.7 Hz, m-Ar–C(BArЈ )], 134.77 [s, o-
Ar–C(BArЈ )], 142.78 [s, p-Ar–C(Py)], 150.14 [s, o-Ar–C(Py)],
161.72 [m, ipso-Ar–C(BArЈ )] ppm. This complex behaves as an 1:2
electrolyte in nitroethane (Λ
8.32 [dd,
J
1
3
1
2
2
1
C(BArЈ
4
)], 124.54 (q,
J
3
2
4
gen; yield 175.6 mg (75%). C18
H 5.47, N 6.38; found C 49.19, H 5.39, N 6.30. H NMR
400.13 MHz, CD Cl H,H = 8.0 Hz, 6 H,
, 21 °C): δ = 1.27 (t, 3
), 1.83 (s, 6 H, CH
H
24
N
2
O
4
Pd (438.61): calcd. C 49.29,
4
1
4
–1
2
–1 [16]
(
2
2
J
M
= 113 Ω cm mol ).
3
CH
CH
3
CH
2
3
CO
2
), 2.72 (q, JH,H = 8.0 Hz, 4 H,
), 7.22 (d, JH,H = 6.4 Hz, 4 H, m-Ar–H), 8.46 (d, JH,H
: 1H NMR (400.13 MHz,
), 7.29 [d, JH,H = 6.4 Hz, 4
NMR Spectroscopic Data for 5a(OTs)
CD Cl , 21 °C): δ = 2.43 (s, 6 H, CH
2
3
3
3
CH
2
=
3
2
2
3
1
3
1
6
.4 Hz, 4 H, o-Ar–H) ppm. C{ H} NMR (100.62 MHz, CD
1 °C): δ = 13.63 (s, CH CH ), 22.95 (s, CH CO ) 28.08 (s,
CH ), 124.51 (s, m-Ar–C), 150.71 (s, o-Ar–C), 156.62 (s, p-Ar–
C), 177.51 (s, CH CO ) ppm.
2
Cl
2
,
3
3
H, Ar–H(OTs)], 7.37 [dd, JH,H = 7.6 , JH,H = 6.4 Hz, 8 H, m-Ar–
2
CH
3
2
3
2
3
H,H = 7.6, 4
JH,H = 1.2 Hz, 4 H, p-Ar–H(Py)],
H(Py)], 7.75 [tt,
J
3
3
3
2
7
.99 [d, JH,H = 6.4 Hz, 4 H, Ar–H(OTs)], 9.76 [dd, JH,H = 6.4,
4
13
1
3
2
J
H,H
=
1.2 Hz,
100.62 MHz, CD
8
H, o-Ar–H(Py)] ppm. C{ H} NMR
3
, 21 °C): δ = 21.04 (s, CH ), 125.98 [s, m-Ar–
C(Py)], 126.43 [s, Ar–C(OTs)], 128.60 [s, Ar–C(OTs)], 139.33 [s, p-
Ar–C(Py)], 139.54 [s, Ar–C(OTs)], 144.62 [s, Ar–C(OTs)], 152.36 [s,
o-Ar–C(Py)] ppm. This complex behaves as an 1:1 electrolyte in
(
2
Cl
2
2
Synthesis of 4b: Pd(TFA) (197.9 mg, 0.595 mmol) was dissolved
in CH Cl (10 mL), followed by the addition of 4-EtPy (130 μL,
.190 mmol). The obtained solution was allowed to stir at room
2
2
1
temperature for 1 h. Afterwards the solvent was completely re-
moved by vacuum and the obtained solid was suspended in diethyl
ether (10 mL). The brownish powder was separated from solution
by filtration, washed with diethyl ether (3ϫ5 mL) and dried in a
stream of nitrogen; yield 260.1 mg (80%). C18
(
–
1
2
–1 [16]
nitroethane (Λ
Synthesis of 5b(BArЈ
0.299 mmol) was suspended in CH
addition of 4-EtPy (139.5 μL, 1.226 mmol) and the obtained solu-
tion was allowed to stir for 1 h. Afterwards, NaBArЈ (556.5 mg,
0.628 mmol), AgOTs (175.2 mg, 0.628 mmol) or Ag(SbF
(215.8 mg, 0.628 mmol) was added to the solution, which caused
M
= 58 Ω cm mol ).
, 5b(OTs) and 5b(SbF
Cl (10 mL), followed by the
4
)
2
2
6 2 2
) : PdCl (53.0 mg,
2
2
H
18
F
6
N
2
O
4
Pd
546.60): calcd. C 39.55, H 3.29, N 5.12; found C 39.48, H 3.20, N
4
1
3
5
7
.01. H NMR (400.13 MHz, CD
2
3
Cl
2
, 21 °C): δ = 1.28 (t, JH,H
=
6
)
.8 Hz, 6 H, CH
3
CH
2
), 2.75 (q, JH,H = 7.8 Hz, 4 H, CH CH ),
3
2
Eur. J. Inorg. Chem. 2011, 1797–1805
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1803

L. Bettucci, C. Bianchini, J. Filippi, A. Lavacchi, W. Oberhauser
FULL PAPER
the precipitation of NaCl and AgCl, respectively. The obtained sus-
pension was allowed to stir in the dark at room temperature for
Catalytic Oxidation Reactions: Typically, the amount of precursor
(0.01 mmol) and of K CO (0.02 mmol), if used, was put at room
2 3
1
h. The suspension was then filtered through a plug of celite and
the obtained solution was concentrated to dryness. 5b(BArЈ
Yield 608.3 mg (90%). C92 Pd (2260.95): calcd. C 48.87,
H 2.65, N 2.48; found C 48.77, H 2.55, N 2.30. 5b(OTs) : Yield
46.5 mg (94%). C42 PdS (877.00): calcd. C 57.52, H 5.70,
N 6.38; found C 57.40, H 5.55, N 6.22. 5b(SbF : Yield" 267.8 mg
89%). C28 PdSb
.57; found C 33.15, H 3.49, N 5.33.
temperature in a stainless steel autoclave (65 mL) equipped with a
homemade temperature and pressure controller and a magnetic
stirrer. Then a 19:1 (v/v) solvent mixture of toluene and DMSO
(20 mL), substrate (1.00 mmol) and decane (100 μL) (i.e., standard)
were added and the autoclave was sealed. The autoclave was then
heated to 80 °C by means of an oil bath. Once the desired tempera-
4 2
) :
60 2 48 4
H B F N
2
2
H
50
N
4
O
6
2
6 2
)
(
5
H
36
F
12
N
4
2
(1006.24): calcd. C 33.42, H 3.58, N ture had been reached, the autoclave was pressurized with air
(300 psi) and stirring was started (600 rpm). After the desired reac-
tion time the autoclave was successively removed from oil bath,
cooled to 10 °C by means of a water/ice bath, followed by the re-
lease of the air pressure from the autoclave. Afterwards, the cata-
lytic solution was subjected to GC and GC–MS analyses. Isolated
products where obtained chromatographically.
NMR Spectroscopic Data for 5b(BArЈ
4
)
2
: 1H NMR (400.13 MHz,
, 21 °C): δ = 1.18 (t, JH,H = 7.6 Hz, 12 H, CH CH ), 2.70
), 7.33 [d, JH,H = 8.8 Hz, 8 H,
m-Ar–H(4-EtPy)], 7.59 [s, 8 H, p-Ar–H(BArЈ )], 7.75 [s, 16 H, o-
)], 8.08 [d, JH,H = 8.8 Hz, 8 H, o-Ar–H(4-EtPy)] ppm.
C{ H} NMR (100.62 MHz, CD Cl
CH CH ), 28.24 (s, CH CH ), 117.47 [s, p-Ar–C(BArЈ
C,F = 272.5 Hz, CF C), 128.19 [s, m-Ar–C(4-EtPy)], 129.29
q, 2JC,F = 38.7 Hz, m-Ar–C(BArЈ
)], 134.77 [s, o-Ar–C(BArЈ )],
49.46 [s, o-Ar–C(4-EtPy)], 161.70 [s, p-Ar–C(4-EtPy)], 161.71 [m,
3
CD
q, 3JH,H = 7.6 Hz, 8 H, CH
CH
3 2
2
Cl
2
3
2
3
(
4
3
Ar–H(BArЈ
4
13
1
2
2
, 21 °C): δ = 12.81 (s, Crystal-Structure Determination of 5b(BArЈ ) and 5b(OTs) ·2H O:
4 2 2 2
3
2
3
2
4
)], 124.54 Single crystals of 5b(BArЈ ) and 5b(OTs) ·2H O suitable for a sin-
4 2 2 2
1
(q,
J
3
gle-crystal X-ray structure analysis were obtained by diffusion of a
toluene-layered solution of the corresponding compounds in
dichloromethane at room temperature. Typically, a suitable crystal
was mounted on a glass fibre with silicon grease and placed into
the cold nitrogen stream. Diffraction data were collected with an
[
1
4
4
4
ipso-Ar–C(BArЈ )] ppm. This complex behaves as an 1:2 electrolyte
in nitroethane (Λ
= 115 Ω–1 cm mol ).
: 1H NMR (400.13 MHz,
CH ), 2.41
), 7.19 [d,
2
–1 [16]
M
α
Oxford Diffraction CCD diffractometer, using Mo-K radiation (λ
= 0.71069 Å) and corrected for Lorentz and polarization effects.
NMR Spectroscopic Data for 5b(OTs)
CD
2
3
2
Cl
2
, 21 °C): δ = 1.16 (t, JH,H = 7.6 Hz, 12 H,CH
JH,H = 7.6 Hz, 8 H, CH CH
H,H = 8.8 Hz, 8 H, m-Ar–H(4-EtPy)], 7.29 [d, JH,H = 7.6 Hz, 4
3
2
), 2.61 (q, 3
Absorption corrections were performed using the XABS2 pro-
(
s, 6 H, CH
J
3
3 2
gram.[
17a]
All structures were solved by direct methods using
3
3
[17b]
3
SHELXS-97
and refined by full-matrix least-squares methods
H, Ar–H(OTs)], 7.99 [s,
JH,H = 7.6 Hz, 4 H, Ar–H(OTs)], 9.52
2
[17c]
3
13
1
against F using the WINGX
software package. All non-hydro-
[
(
d,
J
H,H = 8.8 Hz, 8 H, o-Ar–H(4-EtPy)] ppm. C{ H} NMR
100.62 MHz, CD Cl , 21 °C): δ = 13.26 (s, CH CH ), 21.03 (s,
), 27.95 (s, CH CH ), 125.85 [s, m-Ar–C(4-EtPy)], 126.02 [s,
gen atoms were refined anisotropically, whereas hydrogen atoms
were added at calculated positions and refined applying a riding
model with isotropic U values depending on the Ueq. of the adja-
2
2
3
2
CH
3
3
2
Ar–C(OTs)], 128.75 [s, Ar–C(OTs)], 139.36 [s, Ar–C(OTs)], 144.85
s, Ar–C(OTs)], 151.52 [s, o-Ar–C(4-EtPy)], 157.19 [s, p-Ar–C(4-
EtPy)] ppm. This complex behaves as an 1:1 electrolyte in nitroeth-
[
= 58 Ω–1 cm mol ).
2
–1 [16]
ane (Λ
M
Table 3. Crystallographic data and structure refinement details for
: 1H NMR (400.13 MHz, 5b(BArЈ
, 21 °C): δ = 1.23 (t, JH,H = 7.6 Hz, 12 H, CH CH ), 2.71
), 7.35 (d, JH,H = 6.4 Hz, 8 H,
)
2
and 5b(OTs)
·2H
O.
NMR Spectroscopic Data for 5b(SbF
CD
6
)
2
4
2
2
3
2
Cl
2
3
2
5
b(BArЈ
4
)
2
5b(OTs)
Pd C42
913.43
white prism
0.20ϫ0.20ϫ0.10 0.30ϫ0.20ϫ0.10
2
·2H
2
O
q, 3JH,H = 7.6 Hz, 8 H, CH
CH
H,H = 6.4 Hz, 8 H, o-Ar–H) ppm. C{ H}
Cl , 21 °C): δ = 13.17 (s, CH CH ), 28.11
), 126.84 (s, m-Ar–C), 150.35 (s, o-Ar–C), 159.07 (s, p-
Ar–C) ppm.
3
(
3
2
3
13
1
Formula
C
92
H
60
B
2
F
48
N
4
H
54
N
4
O
8
PdS
2
m-Ar–H), 8.73 (d,
J
–
1
M
r
[gmol ]
2261.46
white prism
NMR (100.62 MHz, CD
s, CH CH
2
2
3
2
Crystal habit
Crystal size [mm]
T [K]
(
3
2
150(2)
170(2)
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
monoclinic
monoclinic
P2 /c
1
10.6103(5)
11.4837(4)
18.3511(9)
Operando HPNMR Experiment: A solution of 5b(OTs)
2
(17.0 mg,
P2
1
/n
0
.019 mmol) in a [D ]toluene/[D ]DMSO (19:1 v/v; 2.0 mL) was
8
6
14.0605(4)
15.4722(4)
23.1075(5)
transferred into a 10 mm sapphire tube that was placed into a
NMR spectroscopy probe at room temperature followed by the ac-
quisition of a 1H NMR spectrum at the same temperature. The
sapphire tube was then removed from the NMR spectroscopy
100.021(3)
106.619(5)
2 3
probe and K CO (5.4 mg, 0.038 mmol) was added, followed by γ [°]
1
3
the acquisition of an H NMR spectrum at room temperature. The
V [Å ]
4950.3(2)
2
1.517
2256
4.21–29.68
0.324
2142.6(2)
2
1.416
952
4.23–30.52
0.586
Z
sapphire tube was then charged with 1,2-propanediol (156.5 μL,
–
3
1
Dcalcd. [gcm ]
F(000)
1
.90 mmol) and again a H NMR spectrum was acquired at the
latter temperature. The sapphire was then removed again from the
NMR spectroscopy probe and charged with air (300 psi), replaced
in the NMR spectroscopy probe, followed by the acquisition of an
Θ range [°]
Absorption coefficient
–1
[
mm ]
1
H NMR spectrum at room temperature. The NMR spectroscopy
Range of transmission
factors
Reflections collected
Independent reflections
Parameters
0.330–0.892
0.457–0.889
probe was then heated to 80 °C and maintained at this temperature
1
for 1 h. During this period of time, H NMR spectra were acquired
36662
12272
666
0.0603, 0.1586
0.1061, 0.1779
0.981/–0.540
5332
5332
270
0.0321, 0.0733
0.0479, 0.0773
0.355/–0.505
in a time interval of 15 min. Afterwards the NMR spectroscopy
1
probe was cooled to room temperature and a final H NMR spec-
R
R
1
, wR
, wR
2
[IϾ2σ(I)]
3(all data)
trum was acquired before removing the sapphire tube from the
probe and venting off the air pressure. The NMR spectroscopic
solution was then analyzed by GC–MS.
1
2
–
Δρ [eÅ ]
1804
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1797–1805

Chemoselective Aerobic Diol Oxidation
cent carbon atom. Relevant crystallographic data for 5b(BArЈ
4
)
2
64, 6750–6755; j) M. J. Schultz, S. S. Hamilton, D. R. Jensen,
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and 5b(OTs)
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·2H
2
O are compiled in Table 3.
CCDC-790283 [for 5b(BArЈ
4
)
2
] and -790284 [for 5b(OTs) ·2H O]
2 2
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge
Crystallographic
data_request/cif.
Data
Centre
via
www.ccdc.cam.ac.uk/
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Received: December 10, 2010
Published Online: March 2, 2011
Eur. J. Inorg. Chem. 2011, 1797–1805
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1805