Oxidation of adamantane by palladium acetate systems

Article abstract of DOI:10.1016/S0020-1693(99)00276-5

Only low yields have been found for the oxidative substitution of adamantane catalysed by diacetatopalladium(II) in trifluoroacetic acid. Addition of copper(II) acetate produced a significant increase in conversion, exclusively to 1-adamantanol. The addition of potassium persulfate gave even higher yields, but at the expense of selectivity, with some 2-adamantanol product.

Full text of DOI:10.1016/S0020-1693(99)00276-5

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Inorganica Chimica Acta 294 (1999) 99–102
Note
Oxidation of adamantane by palladium acetate systems
James K. Beattie *, Susan Macleman, Anthony F. Masters
School of Chemistry, Uni6ersity of Sydney, Sydney, NSW 2006, Australia
Received 28 January 1999; accepted 14 June 1999
Abstract
Only low yields have been found for the oxidative substitution of adamantane catalysed by diacetatopalladium(II) in
trifluoroacetic acid. Addition of copper(II) acetate produced a significant increase in conversion, exclusively to 1-adamantanol.
The addition of potassium persulfate gave even higher yields, but at the expense of selectivity, with some 2-adamantanol product.
©
1999 Elsevier Science S.A. All rights reserved.
Keywords: Oxidation; Adamantane; Alkane activation; Palladium acetate
1
. Introduction
sodium wire; diethyl ether (BDH) and benzene (Merck)
from sodium wire and benzophenone. Adamantane
Adamantane has been used by numerous researchers
(
(
(
(
(
(
Aldrich), 1-adamantanol (Merck), 2-adamantanol
Aldrich), 2-adamantanone (Aldrich), biphenyl
Aldrich), chloroform (Prolabo), copper(II) acetate
Univar), copper(II) chloride (Aldrich), decalin
Merck), deuterated chloroform (Aldrich), ethyl acetate
Prolabo), glacial acetic acid (Aldrich), hydrogen perox-
to investigate alkane activation. It is a non-volatile
substrate, experimentally more convenient to use than
lighter alkanes. With 12 equivalent secondary carbons
and four equivalent tertiary carbons, it is a potential
probe for regioselectivity, although the interpretation of
that selectivity is a matter of continuing controversy
ide (Pacific), nitric acid (Univar), palladium(II) acetate
Aldrich), palladium(II) chloride (Matthey Garrett),
[1,2]. Sen et al. have used adamantane to model activa-
(
tion of methane with palladium acetate [3,4]. Reaction
in trifluoroacetic acid at 80°C was reported to give
\
We have been unable to reproduce this work. Our
efforts are described here, together with the observation
of significant enhancement of the reaction in the pres-
ence of copper acetate.
palladium sponge (Johnson Matthey), potassium per-
oxydisulfate (Univar), spectroscopic grade potassium
bromide (Univar), tetramethylsilane (Aldrich), tri-
fluoroacetic acid (Aldrich), trifluoroacetic anhydride
50% yields of 1-adamantyltrifluoroacetate in 1 h [3].
(
(
Aldrich) and trifluoromethanesulfonic (triflic) acid
Aldrich) were used as received.
2.2. Syntheses
2. Experimental
2.2.1. Diacetatopalladium(II) from palladium(II)
chloride
Diacetatopalladium(II) was prepared following the
method of Hausman et al. [5]. Palladium(II) chloride
2.6 g, 0.015 mol) was dissolved in a solution of
hydrochloric acid (10 M, 2.7 ml) and water (5.0 ml), by
heating in an ethanol bath. Sodium hydroxide (20%
w/v, 12 ml) was added dropwise and as quickly as
possible (ca. 10 min), without the reaction temperature
2
.1. Materials
Analytical grade solvents were dried and distilled
(
under nitrogen. Heptane (Merck) was distilled from
*
Corresponding author. Tel.: +61-2-9351 3797; fax: +61-2-9351
329.
E-mail address: beattiej@chem.usyd.edu.au (J.K. Beattie)
3
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00276-5

100
J.K. Beattie et al. / Inorganica Chimica Acta 294 (1999) 99–102
exceeding 80°C, which would cause polymerisation of
the hydrous palladium oxide. The hydrous palladium
oxide was filtered off, washed with water (4×35 ml)
and partially dried at the pump. Complete drying also
can cause polymerisation. The product was dissolved in
glacial acetic acid (18 ml) and stirred at 80°C for 2 h.
The solvent was removed under vacuum at room tem-
perature (r.t.) over 10 h. The resultant solid was dried
over potassium hydroxide under vacuum at 85°C to
yield a red–brown powder (2.9 g, 0.013 mol, 88%). The
product was characterised by its vibrational spectrum
and melting point [6].
Stephenson et al. report that it is possible to recrys-
tallise the product from benzene/acetic acid [7]. How-
ever, in the present work, this often led to decom-
position to Pd(0). Decomposition occurred even if diac-
etatopalladium(II) were merely left standing at r.t. in
some solvents (methanol, ethanol, chloroform, benzene)
from several hours to several days.
the solution showed a maximum at 350 nm. The solvent
was removed under vacuum at r.t. over anhydrous
potassium hydroxide. The solid products were extracted
with chloroform, the solution filtered and the chloro-
form removed. The chloroform soluble solid was then
extracted with ethyl acetate. Some blue–green solid,
reportedly [Cu Pd(MeCO ) ] [8], was insoluble in ethyl
2
2 6
acetate. The ethyl acetate was removed to give a dark
green product, reportedly [CuPd (MeCO ) ]. When the
2
2 6
[CuPd (MeCO ) ] was again extracted with chloroform,
2
2 6
a yellow–brown solution resulted. The electronic ab-
sorption spectrum of the yellow–brown solution exhib-
ited the same maximum at 350 nm as the initial
reaction mixture.
1-Adamantyltrifluoroacetate was prepared by reflux-
ing 1-adamantanol (1.4801 g, 9.722 mmol) in trifluoro-
acetic anhydride (10 ml) for 3 h. The reaction mixture
was allowed to cool and then extracted with heptane.
The heptane extracts were dried over anhydrous cal-
cium carbonate and the solvent removed to yield a
white crystalline powder (1.689 g, 70%).
2.2.2. Diacetatopalladium(II) from palladium sponge
Palladium sponge (2 g, 0.019 mol) was boiled gently
under reflux in a solution of glacial acetic acid (50 ml)
and concentrated nitric acid (1.5 ml) until the evolution
of brown fumes ceased. A small amount of palladium
sponge usually remained undissolved; if this were not
the case, a little more sponge was added and reflux
continued until no trace of brown fumes was observed.
This process was necessary to avoid contamination of
the product with Pd(NO )(OOCCH ). The boiling
brown solution was filtered and allowed to cool where-
upon most of the complex separated as orange–brown
crystals. The crystals were washed with acetic acid and
water and air dried. The yield was virtually quan-
titative.
Bis(trifluoroacetato)palladium(II) was prepared fol-
lowing the method of Stephenson et al. [7]. Diacetato-
palladium(II) (330 mg, 1.47 mmol) was dissolved in
trifluoroacetic acid (15 ml) and the solvent evaporated
on a steam bath. More trifluoroacetic acid (7 ml) was
added and the process repeated. The product was dried
under vacuum at 40°C for 7 h. Bis(trifluoroacetato)-
palladium(II) can also be prepared by the method of
Hausman et al. [5] for the preparation of diacetatopal-
ladium(II). However, instead of stirring the hydrous
palladium oxide in acetic acid, the hydrous palladium
oxide is stirred in trifluoroacetic acid for 2 h.
2.3. Analyses
Gas chromatographic analyses were performed on a
Hewlett–Packard 5890 GC fitted with an SGE 25 m
QC2/BP1 0.2 m capillary column and a flame ionisation
detector with a 3393A Hewlett–Packard integrator.
The GC was operated in split/splitless mode with a 90%
rejection. In a typical analysis a helium carrier gas flow
of 4191 ml min was maintained and a column head
pressure of 80 KPa was used. The injector and detector
ports were set at 270°C since adamantane and deriva-
tives sublime at ꢀ240°C. The temperature program
consisted of an initial temperature of 80°C for one min,
2
3
−
1
−
1
then a ramp of 20°C min
to 200°C held for 5 min.
The injected sample size was 0.5 ml. Initially, a type A
SGE ‘plunger-in-needle’ graduated 1.0 ml syringe was
used for sample injections which gave inconsistent re-
sults and was replaced with a series II SGE graduated
5.0 ml syringe to obtain reproducible results.
Adamantane and relevant adamantyl derivatives
were identified from their retention times (90.1 min)
by comparison with authentic samples: biphenyl, 8.1;
adamantane,5.7;1-adamantanol,7.1;1-adamantyl triflu-
oroacetate, 7.2; 2-adamantanol, 7.7; 2-adamantanone,
7.6. A standard solution of known quantities of
biphenyl, adamantane, 1-adamantanol, 2-adamantanol
and 2-adamantanone was used daily. Neither decalin
nor decafluorodecalin were found to be as suitable as
biphenyl as an internal standard.
2
.2.3. Attempted preparation of palladiumꢀcopper
acetate species
Sloan and Thornton [8] reported a preparation of
mixed palladium(II)ꢀcopper(II) acetates, which in our
hands gave inconclusive results. Diacetatopalladium(II)
2.4. Procedures
(
0.6312 g, 2.812 mmol) and copper(II) acetate (1.0239
g, 5.128 mmol) were heated in acetic acid (80 ml) at
0°C for 30 min. An electronic absorption spectrum of
In a typical experiment, adamantane (75.0 mg, 0.550
mmol) and an equimolar amount of a palladium(II)
5

J.K. Beattie et al. / Inorganica Chimica Acta 294 (1999) 99–102
101
species were heated under reflux in trifluoroacetic acid
for 6 h. Teflon sleeves were used as silicon grease is
soluble in trifluoroacetic acid. Adamantane dissolved
completely in the trifluoroacetic acid only at reflux.
After 6 h, the cooled reaction mixture was diluted with
water (10 ml) and then with chloroform (10 ml), which
was added to ensure that none of the adamantyl species
sublimed at this point or were carried out with the
gases. A sodium carbonate solution (20% w/v, 50 ml)
was added cautiously to adjust the pH to 9–10. The
reaction mixture was extracted with additional chloro-
form (2×50 ml) and the combined chloroform extracts
dried over sodium carbonate, filtered and the solvent
removed. Biphenyl (approximately 30 mg, accurately
weighed) was added as a GC calibrant and the mixture
dissolved in chloroform (25 ml) for GC analysis.
For a blank standard adamantane (75 mg, 0.550
mmol) and biphenyl (38.6 mg, 0.250 mmol) were heated
under reflux in trifluoroacetic acid (10 ml) for 6 h. The
reaction solution remained clear and was analysed in
the usual manner. Adamantane and biphenyl were the
only species identified from the GC traces, i.e. there was
no observable reaction. A mass balance of about 80%
was usually obtained, typical of reports of other work-
ers [9–11].
of 1-adamantanol and unreacted adamantane. We con-
sider that the missing mass is due to losses in handling
and not to non-extractable products such as carboxylic
acids, because in the blank process of stirring adaman-
tane with biphenyl only 78% of the material was
recovered.
For the reaction of Pd(OAc) with adamantane (Exp.
2
1), we have been unable to reproduce the conversions
of greater than 50% reported by Sen et al. [3]. Regard-
less of the source of palladium acetate (purchased, or
prepared by two different methods) the conversion to
1-adamantanol was consistently only about 12% after 6
h. Although variability in results has been ascribed to
the ‘freshness’ of the palladium acetate [3], the repro-
ducibility of our results with different sources of the
reagent indicates that this is probably not the reason
for the low conversions which we observe.
Sen does not describe his method of workup of the
reaction, apart from extraction into pentane, but does
report that 1-adamantyl trifluoroacetate was the only
organic product [4]. It is not stated whether any unre-
acted adamantane was observed in the GC trace nor
what mass balance was observed. In the absence of a
description of procedures and the use of standards, it is
difficult to account for the differences between Sen’s
results and the work reported here. Other workers have
also reported conversions of methane with palladium
acetate much lower than 50% [12–14].
3. Results
When palladium trifluoroacetate was used in place of
palladium acetate as the oxidant, a similar conversion
of 11% was observed. This is perhaps not surprising, if
it is assumed that in refluxing trifluoroacetic acid palla-
dium acetate is converted by ligand exchange into
palladium trifluoroacetate.
Ten different types of experiments were performed.
In each case a 10 ml solution of 55 mM adamantane
was refluxed for 6 h in the presence of 55–56 mM
oxidant or oxidant and co-oxidant(s). In each case,
except in the presence of K S O (Exps. 8–10, Table 1),
2
2
8
The use of copper complexes as co-catalysts with
palladium is well established in the Wacker process.
1
-adamantanol was the only product detected by GC
and a mass balance of 75–85% was obtained as the sum
Table 1
a
Conversion of adamantane to 1-adamantanol with various oxidants
Exp.
Oxidant
Conversion (%)
Original colour
Final colour
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(TFA)2
Cu(OAc)2
1293
1192
0
993
1092
992
4091
591
8592
8692
red–brown
red–brown
green
blue–brown
blue–brown
blue–brown
green–brown
clear
darkened, mirror, some black ppt
darkened, mirror, some black ppt
green
blue–black
blue–black
blue–black
green–black
clear
Pd(OAc) , CuSO
2
4
Pd(OAc) , CuCl
2
2
Pd(TFA) , CuCl
2
2
Pd(OAc) , Cu(OAc)
2
2
e,f
K S O
2
2
8
b,c
Pd(OAc) , Cu(OAc) , K S O
green–brown
brown
yellow–grey
dark brown
2
2
2
2
8
b,d
1
0
Pd(OAc) , K S O
2
2
2
8
a
b
c
d
e
f
Reaction conditions: 0.55 mmol adamantane, 0.55–0.56 mmol of each of other oxidants, 10 ml TFA, reflux for 6 h.
1% 2-adamantanol in addition to 1-adamantanol.
Mass balance 7393%.
1
Mass balance 77%.
3
% 2-adamantanol.
Mass balance 6796%.

102
J.K. Beattie et al. / Inorganica Chimica Acta 294 (1999) 99–102
Hence a series of experiments was conducted with the
addition of various copper(II) salts to palladium acetate
in refluxing trifluoroacetic acid (Exps. 3–7). There was
no reaction in the presence of copper acetate alone
complexes. Copper(II) chloride and copper(II) sulfate
did not have any effect on the conversion of adaman-
tane by the palladium(II) complexes. The conversion of
adamantane to 1-adamantanol by diacetatopalladi-
um(II), however, increased approximately fourfold
when copper(II) acetate was added. The selectivity of
tertiary to secondary substitution was not altered; the
sole product was the tertiary derivative 1-adamantanol.
Copper(II) acetate did not activate adamantane alone.
Potassium peroxydisulfate has also been used to oxi-
dise palladium(0) to palladium(II) [3]. When one equiv-
alent each of adamantane, diacetatopalladium(II) and
potassium peroxydisulfate were reacted in trifluoro-
acetic acid, a conversion of \90% was achieved.
However, this was at the expense of selectivity, which
decreased to approximately 90%, with some 2-adaman-
tanol formed as well. The results indicate that the
palladium acetateꢀcopper acetate system is a promising
one for the controlled oxidation of alkanes. Fujiwara
and co-workers have made very similar observations on
the carboxylation of gaseous alkanes with CO [15].
Further work is required to determine the optimum
conditions for selectivity and conversion, to extend the
range of alkanes that can be oxidised and to investigate
the possibility of a catalytic cycle, in place of the
stoichiometric reactions of the present work.
(
Exp. 3). The addition of copper sulfate to palladium
acetate (Exp. 4) or copper chloride to palladium acetate
or to palladium trifluoroacetate (Exps. 5 and 6) pro-
duced no additional conversion over that observed in
the absence of copper.
A dramatic increase in conversion to 40% is ob-
served, however, when equimolar quantities of palla-
dium acetate and copper acetate are used together
(
1
Exp. 7). The product is exclusively the tertiary isomer
-adamantanol. Mixed palladiumꢀcopper acetate
trimers have been described in the literature, both
Cu Pd and CuPd species having been reported [8].
2
2
Even more dramatic effects are observed when potas-
sium persulfate is added to the system. Potassium per-
sulfate alone causes only a minor (5%) amount of
oxidation, with both 1- and 2-adamantanol produced
(
Exp. 8). But in the presence of palladium acetate and
copper acetate (Exp. 9) or with just palladium acetate
alone (Exp. 10), about 85% conversion occurs. This
increase is at the expense of selectivity; however, in
contrast to the systems without persulfate, about 11%
2-adamantanol is also formed (as the percentage of the
total amount of organics detected).
References
4. Discussion
[
[
[
1] D.H.R. Barton, Chem. Soc. Rev. (1996) 237.
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adamantane with diacetatopalladium(II) in trifluoro-
acetic acid gave \50% of 1-adamantyl trifluoroacetate
when equimolar mixtures of adamantane (0.030 g,
.223 mmol) and diacetatopalladium(II) (0.050 g, 0.223
mmol) were stirred in trifluoroacetic acid (3–4 ml) at
0°C over 2 h [4]. In our hands, over 6 h only 1193%
of the adamantane is converted to the ester. Other
groups also have reported much lower yields for the
related activation of methane [12–14]. A possible
source of experimental error might be the small vol-
umes which were used in the previous work.
Both palladium acetate and palladium trifluoroac-
etate activate adamantane to approximately the same
limited extent, presumably because the acetate is con-
verted into the trifluoroacetate in the trifluoroacetic
acid solvent. The only product of the adamantane
oxidation is 1-adamantanol, with 100% selectivity to
the tertiary product in each case.
8
109.
[4] A. Sen, E. Gretz, T.F. Oliver, Z. Jiang, New J. Chem. 13 (1989)
55.
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[
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[
[
1
994, p. 533.
Copper(II) has been used extensively in Wacker-type
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[
[
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