(NHC = N-heterocyclic carbene) catalysed alcohol oxidation using molecular oxygen

Article abstract of DOI:10.1039/c2dt30133d

A series of [Pd(NHC)(PR₃)] complexes quantitatively form [Pd(η²-O₂)(NHC)(PR₃)] products when submitted to an dioxygen atmosphere. Synthetic details and structural features of these Pd-peroxo complexes are described. The palladium compounds were found to be efficient catalysts in the oxidation of alcohols into corresponding carbonyl compounds using O₂ or air as oxidant. Reactions proceed at low catalyst loadings with a wide range of alcohols.

Full text of DOI:10.1039/c2dt30133d

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PAPER
[Pd(NHC)(PR3)] (NHC = N-heterocyclic carbene) catalysed alcohol oxidation
using molecular oxygen†‡
Václav Jurčík, Thibault E. Schmid, Quentin Dumont, Alexandra M. Z. Slawin and Catherine S. J. Cazin*
Received 18th January 2012, Accepted 22nd August 2012
DOI: 10.1039/c2dt30133d
A series of [Pd(NHC)(PR3)] complexes quantitatively form [Pd(η2-O2)(NHC)(PR3)] products when
submitted to an dioxygen atmosphere. Synthetic details and structural features of these Pd–peroxo
complexes are described. The palladium compounds were found to be efficient catalysts in the oxidation
of alcohols into corresponding carbonyl compounds using O2 or air as oxidant. Reactions proceed at low
catalyst loadings with a wide range of alcohols.
complex.16 Similarly, these complexes react cleanly with O2,
Introduction
forming peroxo-complexes (Scheme 1).17
The oxidation of alcohols to the corresponding carbonyl com-
pounds is a fundamental transformation in organic synthesis.1
Although a myriad of synthetic routes has been used to accom-
plish this fundamental functional group transformation, many of
the most common methods suffer from the use of forcing con-
ditions and/or toxic stoichiometric oxidants such as chromium
(VI) reagents.2,3 Transition metal-catalysed oxidations using
oxygen as terminal oxidant have therefore attracted considerable
attention from both academia and industry.4,5 Early reports based
on transition metal systems permitting the aerobic oxidation of
alcohols are those developed by the Uemura (Pd(OAc)2/
pyridine)6–8 and the Sheldon (Pd(OAc)2/phenantroline) groups.9
Sigman and co-workers have more recently described a
Pd(OAc)2/NEt3 system capable of room temperature oxidation of
alcohols at 1 atm of O2.10 The same group reported the use of
[Pd(H2O)(IPr)(OAc)2] (IPr = N,N′-bis-[2,6-(diisopropyl)phenyl]-
imidazol-2-ylidene) in combination with acetic acid and molecu-
lar sieves in the effective aerobic oxidation of primary and
secondary alcohols.11–13 Other systems involving NHCs in oxi-
dation catalysis have also been disclosed.14
With this reactivity and our interest in oxidation catalysis in
mind, we reasoned that such low-valent mixed-ligand systems
could possibly exhibit interesting catalytic behaviour in oxi-
dation reactions. Herein, we report on the reactivity of complexes
of the type [Pd(NHC)(PR3)] with O2 and their catalytic perform-
ance in oxidation reactions.18
Results and discussion
The synthesis of the peroxo complexes 2a–d, featuring IPr or
SIPr NHCs (SIPr = N,N′-bis-[2,6-(diisopropyl)phenyl]imidazoli-
din-2-ylidene) was readily achieved by reacting 1a–d with
dioxygen (in toluene solution and in the solid state) and leads to
quantitative yields of the corresponding dioxygen adducts
(Fig. 1). This transformation is typically accompanied by a clear
change in the 31P{1H} NMR spectrum (see Table 1).
While palladium(0) complexes 1a–d glows when exposed to
UV light (366 nm), exposure to oxygen leads to the complete
bleaching of this feature upon oxidation to 2a–d. Interestingly,
under these mild oxidation conditions, no phosphine decoordi-
nation or oxidation is observed by 31P{1H} NMR
spectroscopy.19,20
We have recently shown that Pd(0) complexes bearing mixed
ligand systems, NHC/PR3, were stable under hydrogenation con-
ditions and could lead to outstanding catalytic activity in alkene
reduction.15 These compounds react with H2 by oxidative
addition, leading to the formation of the corresponding dihydride
To unequivocally confirm the atom connectivity and structural
features of the two new complexes (2c and 2d), single crystals
were grown from CH2Cl2/pentane and subjected to single crystal
EaStCHEM School of Chemistry, University of St Andrews, St Andrews,
KY16 9ST, UK. E-mail: cc111@st-andrews.ac.uk;
Fax: +44 (0)1334 463 808
†Dedicated to Professor David J. Cole-Hamilton on the occasion of his
retirement and for his outstanding contributions to transition metal
catalysis.
‡Electronic supplementary information (ESI) available: NMR spectra of
2c and 2d, catalysts optimization (solvent and additives screening), cata-
lysis protocols and oxidation products characterization data. CCDC
CCDC-799172 (2c) and CCDC-799173 (2d). For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30133d
Scheme 1 Oxidative addition of H2 and O2 onto 1a.
Dalton Trans., 2012, 41, 12619–12623 | 12619
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Table 2 Selected bond lengths (Å) and angles (°) for complexes [Pd-
(η2-O2)(NHC)(PR3)] 2a–2d (esd)
NHC/PR3 IPr/PCy3 2a16 IPr/PPh3 2b17 SIPr/PCy3 2c SIPr/PPh3 2d
Pd–O(1)
Pd–O(2)
Pd–C
Pd–P
O–O
O–Pd–O
C–Pd–P
2.026(2)
2.0481(19)
2.057(3)
2.3295(7)
1.468(3)
42.23(8)
105.24(7)
1.9912(16)
2.0138(15)
2.0360(18)
2.2646(5)
1.430(2)
41.83(6)
104.74(5)
2.004(4)
2.015(4)
2.024(5)
2.3121(16)
1.448(5)
42.23(14)
105.80(14)
2.006(6)
2.058(7)
2.022(8)
2.281(2)
1.621(9)
47.0(3)
Fig. 1 Pd(0) complexes and peroxo analogues examined in this study.
102.0(2)
Table 1 31P{1H} NMR data for complexes of type 1 and 2 (recorded in
C6D6)
the peroxo-oxygen atoms, elimination of H2O2 and generation of
the corresponding diacetate species which then facilitates the
dehydrogenation of the alcohol substrate. Addition of 10 mol%
of acetic acid to the catalytic reaction resulted in a 36% conver-
sion to acetophenone in 23 h, while addition of 10 mol% AcOH
in the presence of molecular sieves (4 Å MS) provided quantita-
tive conversion to ketone in 2 h (Table 3, entries 2 and 3). The
beneficial role of molecular sieves has previously been dis-
cussed.23 The amount of AcOH also appears to play a crucial
role especially when lower catalyst loadings are used.24 Indeed,
when decreasing the Pd loading to 0.5 mol%, the use of 10 mol%
of AcOH appears deleterious to catalytic activity (Table 3, entry
4), whilst reducing it to 5 mol% then to 2.5 mol% appears
beneficial (Table 3, entries 5 and 6). The latter reaction con-
ditions were used to compare the catalytic efficiency of com-
plexes 2a–d (Table 3, entries 6–9). A general trend emerges:
complexes bearing IPr exhibit higher activity than their saturated
analogues and tricyclohexylphosphine-bearing compounds are
more reactive than their triphenylphosphine counterparts. Further
decreasing the catalyst loading to 0.25 mol% and 0.1 mol%
resulted in a decreased conversion to 87% and 26%, respectively
(Table 3, entries 11–12). The surprising performance of the
system at 0.25 mol% catalyst is noteworthy. Finally, the Pd(0)
precursor of the most efficient peroxo complex (2a), [Pd(IPr)-
(PCy3)] 1a, was tested under the optimised reaction conditions
in order to assess the importance of starting for the peroxo
derivative. Interestingly, at 0.5 mol% loading, [Pd(IPr)(PCy3)]
1a achieves, as its peroxo-analogue, the quantitative oxidation of
phenylethanol (Table 3, entry 13). Subsequent catalytic studies
were therefore carried out using the Pd(0) complex [Pd(IPr)-
(PCy3)] 1a.
Pd0 complex
δP (ppm) PdII complex
δP (ppm)
[Pd(IPr)(PCy3)] 1a17 49.4
[Pd(IPr)(PPh3)] 1b17 33.6
[Pd(SIPr)(PCy3)] 1c17 48.1
[Pd(SIPr)(PPh3)] 1d17 33.0
[Pd(η2-O2)(IPr)(PCy3)] 2a16 53.7
[Pd(η2-O2)(IPr)(PPh3)] 2b17 36.4
[Pd(η2-O2)(SIPr)(PCy3)] 2c 49.9
[Pd(η2-O2)(SIPr)(PPh3)] 2da 30.7
a Recorded in CD2Cl2.
Fig. 2 Molecular structures of [Pd(η2-O2)(SIPr)(PCy3)] 2c and
[Pd(η2-O2)(SIPr)(PPh3)] 2d (Hydrogen atoms omitted for clarity).
diffraction analysis. Fig. 2 presents graphical representations of
the molecular structures of 2c and 2d.
Bond lengths and angles for [Pd(η2-O2)(SIPr)(PR3)] com-
plexes 2c (PR3 = PCy3) and 2d (PR3 = PPh3) are similar to those
previously reported for the unsaturated NHC analogues
(Table 2).16,17 The most striking difference can be found in the
O–O distance (1.621(9) Å) in 2d compared to those found in
2a–c. When compared to other reported peroxo palladium com-
With these optimised conditions in hand,25 the generality of
the protocol was tested on various substrates. Pleasingly, the
reaction proceeds generally with excellent yields for a wide
range of secondary benzylic alcohols (Table 4, entries 1–9)
using 0.5 mol% of 1a. Isoborneol is oxidised to camphor in
good yield (Table 4, entry 10) and allylic alcohols such as cyclo-
hexen-2-ol and perillyl alcohol provide the corresponding carbo-
nyl compounds in moderate to modest conversions. (Table 4,
entries 11–12).
plexes, [Pd(η2-O2)(P(1-Ad)2 Bu] (1.395(4) Å)18 or [Pd(η2-O2)
n
(bc)] (1.412(4) Å, bc = bathocuproine),21 this bond distance is
abnormally long. On the other hand, the Pd–O bond distances
are comparable to those found for the complexes mentioned
above when the opposite bond distance trend could have been
expected based on charge transfer arguments.22a Additionally,
coordination of the metal centre by singlet oxygen can also be
ruled out as this would imply a short O–O distance.22b
The reactivity of complexes of type 1 with dioxygen prompted
us to examine the activity of mixed [Pd(η2-O2)(NHC)(PR3)]
complexes in oxidation catalysis.14 When 2 mol% of 2a was
used in the oxidation of phenylethanol using 1 atm of O2, no
reaction was observed, even after 19 h (Table 3, entry 1). Based
on proposed mechanisms for Pd-catalysed oxidation,11,13 the
addition of a protic acid was suggested to result in protonation of
The potential of complex [Pd(IPr)(PCy3)] 1a to promote the
oxidation of alcohols using air was then tested. This proved
slightly more difficult as longer reaction times were necessary to
obtain conversion to the oxidised product (Table 5). However,
benzylic alcohols such as phenylethanol and p-methoxyphenyl-
ethanol could be converted in excellent to quantitative yield to
the corresponding ketone.
12620 | Dalton Trans., 2012, 41, 12619–12623
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Table 3 Optimisation of the reaction conditionsa
Entry
Catalyst
Loading (mol%)
Additive (mol%)
Time (h)
Conv.b (%)
1
2
3
4
5
6
7
8
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PPh3)] 2b
[Pd(η2-O2)(SIPr)(PCy3)] 2c
[Pd(η2-O2)(SIPr)(PPh3)] 2d
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(η2-O2)(IPr)(PCy3)] 2a
[Pd(IPr)(PCy3)] 1a
2
2
2
—
19
23
2
0c
AcOH (10)
AcOH (10)
AcOH (10)
AcOH (5)
AcOH (2.5)
AcOH (2.5)
AcOH (2.5)
AcOH (2.5)
AcOH (1.0)
AcOH (1.25)
AcOH (0.5)
AcOH (2.5)
36c
>99
18
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
0.1
0.5
16
21
19
19
19
19
20
19
19
14
73
>99
91
90
86
83
87
26
>99
9
10
11
12
13
a Reaction conditions: phenylethanol (121 μL, 1.00 mmol), 4 Å molecular sieves (MS) (50 mg), toluene (2 mL), O2 (1 atm). b Conversion determined
by GC (average of two runs). c No molecular sieves added.
2 h. Substantial conversion of the alcohol to the corresponding
ketone was observed as well as the formation of palladium
black.
Mechanistic considerations
The exact mechanism at play in this transformation is a matter of
debate as two possible pathways can be envisaged: (1) involving
a HX reductive elimination step from a [Pd(H)(OAc)(IPr)-
(PCy3)] species leading to a Pd(0) species that then binds O2 to
then undergo protonolysis or (2) a H-abstraction by dioxygen on
the peroxo species followed by a rearrangement. Stahl has
recently addressed such mechanistic issues focusing on related
[Pd(NHC)2(H)(X)] complexes.26 We know that the reactions of
the Pd(0) species with dioxygen is extremely rapid (as fast as
diffusion controlled as is evident by colour changes, even in the
solid state). The peroxo complexes may revert back to Pd(0) as
exposure of 2a to high vacuum (10−6 mbar) at 80 °C for 72 h
resulted in 48% recovery of 1a showing that oxygen binding is
reversible. The necessary involvement of protic acid strongly
Finally, the reaction of [Pd(IPr)(PCy3)] with 1 equivalent of
acetic acid was performed at −78 °C. This reaction was analysed
by VT-NMR spectroscopy (−70 °C to room temperature) and
showed the presence of a complex mixture of Pd species. Only
minor quantities of hydride-containing species were detected.
In summary, the catalytic cycles involved in this reaction are
complex, the decoordination of the phosphine as well as the oxi-
dation of the latter were observed. The PCy3 ligand appears to
play the role of vacant site protecting group, but, at this point,
the beneficial role of OvPCy3 as a Pd(0) stabilising ligand
cannot be ruled out.
The formation of [Pd(OAc)2(IPr)(PCy3)], active in the oxi-
dation of alcohols to ketones, has also been evidenced. It then
appears that the present system leads to, amongst other uniden-
tified species, complexes that are similar to those published by
Sigman and co-workers.11 This formation appears to occur
through a combination of anion metathesis and phosphine
decoordination. However, the kinetics of these two steps could
theoretically be tuned. This could result in latent pre-catalysts for
oxidation reactions, which would be highly desirable. It would,
for example, allow the design of systems performing tandem
reactions. Nevertheless, the number of species formed during
catalysis does not allow us to unequivocally claim that the
previously reported catalytic cycles26,27 are the only possible
mechanisms involved in the present system.
supports the formation of palladium acetates as
intermediates.
a key
A number of tests were performed in order to obtain better
insights into the mechanism of this reaction. We first synthesised
[Pd(OAc)2(IPr)(PCy3)] (3) (see Experimental section for details),
which was characterised by NMR spectroscopy but seemed
unfortunately too unstable to be isolated in a microanalytically
pure form. The latter complex proved efficient for the stoichio-
metric oxidation of 1-phenylpropanol in toluene at 60 °C, where
a conversion of 95% was observed in 15 min, along with the for-
mation of tricyclohexylphosphine oxide.
The concomitant oxidation of [Pd(IPr)(PCy3)] under oxygen
in C6D6 followed by the addition of 2 equivalents of acetic acid
resulted in a complex mixture of four phosphorus-containing
compounds. Amongst them, the phosphorus peaks correspond-
ing to [Pd(OAc)2(IPr)(PCy3)] and OvPCy3 could be identified.
A catalytic reaction involving the oxidation of 1-phenylpropanol,
using [Pd(η2-O2)(IPr)(PCy3)] (2 mol%), acetic acid (10 mol%)
and molecular sieves (4 Å) in toluene at 60 °C was monitored
in situ. The NMR spectra clearly showed the formation of
OvPCy3 as the only phosphorus-containing compound after
Conclusions
Complexes of the type [Pd(NHC)(PR3)] react rapidly with
dioxygen to form the corresponding [Pd(η2-O2)(NHC)(PR3)].
Two such new complexes were prepared and fully characterised.
[Pd(NHC)(PR3)] complexes can be employed as pre-catalysts in
the oxidation of alcohols leading to the corresponding carbonyl
This journal is © The Royal Society of Chemistry 2012
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Table 4 Substrate scope of Pd-catalysed oxidation of alcoholsa
Table 5 Palladium-catalysed aerobic oxidation of alcoholsa
Entry Substrate
1
Product
Time (h) Conv.b (yield %)
Entry Substrate
1
Product
Time (h) Conversionb (%)
14
>99
14
>99
2
3
14
85 (85)
2
3
24, 44
63, 94
20
90 (73)
24
37
a Reaction conditions: catalyst (0.005 mmol), substrate (1.00 mmol),
AcOH (1.5 μL, 0.025 mmol), 4 Å MS (50 mg), toluene (2 mL) air
(1 atm). b Conversions determined by GC.
4
5
15
22
>99 (73)
>99 (78)
Experimental section
General considerations
6
7
15
14
>99 (90)
>99 (99)
Unless otherwise stated, all reactions were performed under
argon. Solvents were distilled from appropriate drying agents or
were dispensed from a solvent purification system. Other an-
hydrous solvents were purchased from Aldrich and degassed
1
prior to use. H, 13C{1H} and 31P{1H} Nuclear Magnetic Reso-
8
9
14.5
14
40
nance (NMR) spectra were recorded on Bruker Avance 400 and
500 Ultrashield NMR spectrometers. Gas chromatography (GC)
analyses were performed on an Agilent 7890A apparatus
equipped with a flame ionisation detector and a (5%-phenyl)-
methylpolysiloxane column (30 m, 320 μm, film: 0.25 μm). GC-
conversions are based on comparison of signals of starting
material and product. Product signals were assigned based on
comparison with commercially available authentic samples.
Complexes 1a–d and 2a–b were synthesised according to litera-
ture methods.16,17
>99 (96)
10
11
22
16
90 (59)
62
Synthesis of [Pd(η2-O2)(NHC)(PR3)] complexes
The [Pd(NHC)(PR3)] complex (0.05 mmol) was charged in a
Schlenk flask that was then connected to an oxygen gas manifold
(or alternatively to a dioxygen-filled balloon) where the inert
atmosphere was exchanged for oxygen by three evacuation-
backfill cycles. The solid rapidly turned from yellow-green to
white. After 10 min under dioxygen, the product was dissolved
in deuterated solvent (400 μL) and analysed by NMR spectro-
scopy. This reaction is simply and effectively performed in the
solid-state but can also be conducted in an organic solvent such
as toluene.
12
66
30
a Reaction conditions: [Pd(IPr)(PCy3)] 1a (0.005 mmol), substrate
(1.00 mmol), AcOH (1.5 μL, 0.025 mmol), 4 Å MS (50 mg), toluene
(2 mL) O2 (1 atm). b Conversion determined by GC, isolated yields
shown in parentheses, average of two runs.
compounds. These reactions proceed at low catalyst loading
(0.25 mol%) under mild conditions (60 °C and 1 atm of O2).
Additives such as acetic acid as well as molecular sieves have a
major effect on the reaction outcome. In the case of the most
reactive substrates, oxygen can be substituted with air. Further
studies examining the broader roles of these complexes in oxi-
dation catalysis and related chemistry are ongoing in our
laboratories.
[Pd(η2-O2)(SIPr)(PCy3)] (2c). Quantitative yield. 1H NMR
(300 MHz, C6D6, 333 K): δ (ppm) 1.08–1.11 (br m, 9 H,
3
CH-CH3), 1.17 (d, 12 H, JH–H = 6.8 Hz, CH-CH3), 1.27–1.30
(br m, 6 H, CH-CH3, cyclohexyl), 1.59–1.64 (br m, 12 H, cyclo-
hexyl), 1.68–1.73 (br m, 18 H, cyclohexyl), 3.55 (br s, 8 H, H4,
H5 and CH-CH3), 7.21 (s, 6 H, aromatic); 13C{1H} (75.5 MHz,
12622 | Dalton Trans., 2012, 41, 12619–12623
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C6D6, 298 K): δ (ppm) 24.3 (s, CHCH3), 26.4 (s, CH2 cyclo-
hexyl), 27.4 (s, CHCH3), 27.6 (d, JC–P = 10.7 Hz, CH2 cyclo-
hexyl), 30.3 (s, CH2 cyclohexyl), 33.6 (d, JC–P = 14.9 Hz, CH
cyclohexyl), 54.6 (s, C4 and C5), 129.4 (s, CH aromatic), 137.9
for financial support. Umicore AG is gratefully acknowledged
for gifts of palladium complexes.
2
(s, C aromatic), 221.8 (d, JC–P = 14.6 Hz, C2); 31P{1H}
Notes and references
(121.5 MHz, C6D6, 298 K): δ (ppm) 49.9 (s); Elem. Anal. Calcd
for C47H77N2O2PPd: C, 66.77; H, 8.84; N, 3.46. Found: C,
66.21; H, 8.82; N, 3.53.
1 M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph Series,
American Chemical Society, Washington, DC, 1990.
2 G. Cainelli and G. Cardillo, Chromium Oxidants in Organic Chemistry,
Springer, Berlin, 1984.
3 S. V. Ley and A. Madin, Comprehensive Organic Synthesis, ed.
B. M. Trost, I. Fleming and S. V. Ley, Pergamon, Oxford, 1991, vol. 7, 251.
4 (a) S. S. Stahl, Science, 2005, 309, 1824; (b) S. S. Stahl, Angew. Chem.,
Int. Ed., 2004, 43, 3400.
5 K. M. Gligorich and M. S. Sigman, Chem. Commun., 2009, 3854.
6 T. Nishimura, T. Onoue, K. Ohe and S. Uemura, J. Org. Chem., 1999, 64,
6750.
7 T. Nishimura, Y. Maeda, N. Kakiuchi and S. Uemura, J. Chem. Soc.,
Perkin Trans. 1, 2000, 1915.
8 T. Nishimura, N. Kakiuchi, T. Onoue, K. Ohe and S. Uemura, J. Chem.
Soc., Perkin Trans. 1, 2000, 4301.
9 G. J. Ten Brink, I. W. C. E. Arends and R. A. Sheldon, Science, 2000,
287, 1636.
10 M. J. Schultz, C. C. Park and M. S. Sigman, Chem. Commun., 2002, 3034.
11 D. R. Jensen, M. J. Schultz, J. A. Mueller and M. S. Sigman, Angew.
Chem., Int. Ed., 2003, 42, 3810.
12 J. A. Mueller, C. P. Goller and M. S. Sigman, J. Am. Chem. Soc., 2004,
126, 9724.
13 M. J. Schultz, S. S. Hamilton, D. R. Jensen and M. S. Sigman, J. Org.
Chem., 2005, 70, 3343.
[Pd(η2-O2)(SIPr)(PPh3)] (2d). Quantitative yield. 1H NMR
(300 MHz, CD2Cl2, 298 K): δ (ppm) 0.73–1.54 (m, 24 H,
CH-CH3), 3.31 (br s, 4 H, CH-CH3), 4.07 (br s, 4 H, H4 and
H5), 7.07–7.14 (m, 8 H, aromatic), 7.19–7.26 (m, 8 H, aromatic),
7.30–7.36 (m, 5 H, aromatic); 13C{1H} (75.5 MHz, CD3OD,
298 K): δ (ppm) 24.0 (s, CH-CH3), 26.9 (br s, CH-CH3), 30.3
(br s, CH-CH3), 56.3 (s, C4 and C5), 129.4 (s, CH aromatic),
129.6 (s, CH aromatic), 130.8 (s, CH aromatic), 131.6 (d, JC–P
=
1.5 Hz, CH aromatic), 132.4 (s, CH aromatic), 133.0 (s, CH aro-
matic), 135.0 (s, C aromatic), 135.1 (s, C aromatic), 137.4 (s, C
2
aromatic), 210.5 (d, JC–P = 20.6 Hz, C2); 31P{1H} (121.5 MHz,
CD2Cl2, 298 K): δ (ppm) 30.7 (s). Elem. Anal. Calcd for
C45H53N2O2PPd: C, 68.22; H, 6.87; N, 3.54. Found: C, 68.07;
H, 7.27; N, 3.60.
Synthesis of [Pd(OAc)2(IPr)(PCy3)] (3). In a Schlenk flask,
[Pd(OAc)(η2-OAc)(IPr)]28 (250 mg, 0.407 mmol) and PCy3
(125.8 mg, 0.448 mmol) were stirred in CH2Cl2 (10 mL) at
room temperature for 30 min. The volume reduced to 1 mL
in vacuo and pentane (5 mL) was added. The mixture was con-
centrated to 3 mL, the precipitate was collected by filtration and
washed with pentane (2 × 5 mL). The product was obtained as a
pale yellow powder (300 mg, 80%). 1H NMR (400 MHz,
CD2Cl2, 298 K): δ (ppm) 0.95–1.29 (m, 11 H, Cy), 1.07 (d,
3JH–H = 6.8 Hz, 12 H, CH-CH3), 1.32–1.45 (m, 6 H, Cy), 1.40
14 For NHC complexes in oxidation reactions, see: (a) V. Jurčík and
C. S. J. Cazin, in N-Heterocyclic Carbenes in Transition Metal Catalysis
and Organocatalysis, ed. C. S. J. Cazin, Springer, London, 2011, vol. 32,
p. 237; (b) M. M. Rogers and S. S. Stahl, in N-Heterocyclic Carbenes in
Transition Metal Catalysis, ed. F. Glorius, Springer, New York, 2006,
vol. 21, p. 21.
15 V. Jurčík, S. P. Nolan and C. S. J. Cazin, Chem.–Eur. J., 2009, 15, 2509.
16 S. Fantasia, J. D. Egbert, V. Jurčík, C. S. J. Cazin, H. Jacobsen,
L. Cavallo, D. M. Heinekey and S. P. Nolan, Angew. Chem., Int. Ed.,
2009, 48, 5182.
17 S. Fantasia and S. P. Nolan, Chem.–Eur. J., 2008, 14, 6987.
18 For a recent publication of Pd(PR3)2 complexes reacting with oxygen see:
A. G. Sergeev, H. Neumann, A. Spannenberg and M. Beller, Organo-
metallics, 2010, 29, 3368.
3
(d, JH–H = 6.6 Hz, 12 H, CH-CH3), 1.50–1.55 (m, 2 H, Cy),
1.58 (s, 6 H, CH3COO), 1.60–1.74 (m, 14 H, Cy), 3.19 (sept,
19 In addition to ref. 18, other examples of phosphine-containing palladium
complexes have shown stability towards oxygen: (a) M. Matsumoto,
K. Yoshioka, K. Nakatsu, T. Yoshida and S. Otsuka, J. Am. Chem. Soc.,
1974, 96, 3322; (b) T. Yoshida and S. Otsuka, J. Am. Chem. Soc., 1977,
99, 2134.
20 For reactions of homoleptic palladium–NHC complexes with air and
oxygen, see: (a) X. Cai, S. Majumdar, G. C. Fortman, C. S. J. Cazin,
A. M. Z. Slawin, C. Lehermitte, R. Prabhakar, M. Germain, T. Pallucio,
S. P. Nolan, E. Rybak-Akimova, M. Temprado, B. K. Captain and
C. D. Hoff, J. Am. Chem. Soc., 2011, 133, 1290; (b) M. Yamashita,
K. Goto and T. Kawashima, J. Am. Chem. Soc., 2005, 127, 7294;
(c) M. M. Konnick, I. A. Guzei and S. S. Stahl, J. Am. Chem. Soc., 2004,
126, 10212.
3JH–H = 6.6 Hz, 4 H, CH-CH3), 7.06 (s, 2 H, CH aromatic), 7.31
3
3
(d, JH–H = 7.6 Hz, 4 H, CH aromatic), 7.44 (t, JH–H = 7.7 Hz,
2 H, CH aromatic); 13C{1H} (100.6 MHz, CD2Cl2, 298 K): δ
(ppm) 23.2 (s, CH3), 23.9 (s, CH3), 26.6 (s, CH3), 26.9 (s, CH2),
28.4 (s, CH2), 28.5 (s, CH), 28.5 (s, CH2), 29.4 (s, CH2), 32.4
1
(d, JC–P = 16.5 Hz, P-CH), 124.1 (s, Ar-CH), 124.6 (d, J = 4.5
Hz, C4 and C5), 129.7 (s, Ar-CH), 136.8 (s, Ar-C), 147.0 (s,
Ar-C), 174.2 (s, C2); 31P{1H} (162.0 MHz, CD2Cl2, 298 K): δ
(ppm) 19.6 (s).
21 S. S. Stahl, J. L. Thorman, R. C. Nelson and M. A. Kozee, J. Am. Chem.
Soc., 2001, 123, 7188.
22 (a) C. J. Cramer, W. B. Tolman, K. H. Theopold and A. L. Rheingold,
Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 3635; (b) J. M. Praetorius,
D. P. Allen, R. Wang, J. D. Webb, F. Grein, P. Kennepohl and
C. M. Crudden, J. Am. Chem. Soc., 2008, 130, 3724.
23 B. A. Steinhoff, A. E. King and S. S. Stahl, J. Org. Chem., 2006, 71, 1861.
24 The presence of excess AcOH may inhibit catalysis and sequester the
active catalytic species in a less active form, namely, as a species of poss-
ible composition [Pd(PR3)(NHC)(OAc)2].
25 For solvent and other optimisation details, see ESI.†
26 B. A. Steinhoff, I. A. Guzei and S. S. Stahl, J. Am. Chem. Soc., 2004,
126, 11268.
27 M. J. Schultz, R. S. Alder, W. Zierkiewicz, T. Privalov and M. S. Sigman,
J. Am. Chem. Soc., 2005, 127, 8499.
General procedure for the oxidation catalysis
A Schlenk flask was charged with [Pd(IPr)(PCy3)] (1a) (3.9 mg,
0.005 mmol), 4 Å molecular sieves (50 mg) and the substrate
(if solid, 1 mmol). The inert atmosphere was exchanged for
oxygen by three evacuation-backfill cycles. Solvent (2 mL) and
AcOH (1.5 μL, 0.025 mmol) were then added, followed by the
substrate (if liquid, 1 mmol). The reaction mixture was stirred at
60 °C, monitored by GC, and purified by flash column
chromatography (SiO2, pentane).
We thank the EPSRC, the Royal Society (University Research
Fellowship to CSJC) and the EaStCHEM School of Chemistry
28 M. S. Viciu, E. D. Stevens, J. L. Petersen and S. P. Nolan, Organo-
metallics, 2004, 23, 3752.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12619–12623 | 12623