Catalytic Decarboxylative Cross-Ketonisation of Aryl- and Alkylcarboxylic Acids using Magnetite Nanoparticles

Article abstract of DOI:10.1002/adsc.201000429

In the presence of catalytic amounts of magnetite nanopowder, mixtures of aromatic and aliphatic carboxylic acids are converted selectively into the corresponding aryl alkyl ketones. As by-products, only carbon dioxide and water are released. This catalytic cross-ketonisation allows the regioselective acylation of aromatic systems and, thus, represents a sustainable alternative to Friedel-Crafts acylations.

Full text of DOI:10.1002/adsc.201000429

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DOI: 10.1002/adsc.201000429
Catalytic Decarboxylative Cross-Ketonisation of Aryl- and Alkyl-
carboxylic Acids using Magnetite Nanoparticles
Lukas J. Gooßen,a,* Patrizia Mamone,a and Christoph Oppela
a
FB Chemie – Organische Chemie, TU Kaiserslautern, Erwin-Schrçdinger-Str. Geb. 54, 67663 Kaiserslautern, Germany
Fax : (+49)-631-205-3921; e-mail: goossen@chemie.uni-kl.de
Received: June 1, 2010; Published online: December 15, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000429.
Abstract: In the presence of catalytic amounts of
magnetite nanopowder, mixtures of aromatic and
aliphatic carboxylic acids are converted selectively
into the corresponding aryl alkyl ketones. As by-
products, only carbon dioxide and water are re-
leased. This catalytic cross-ketonisation allows the
regioselective acylation of aromatic systems and,
thus, represents a sustainable alternative to Friedel–
Crafts acylations.
remained for a broadly applicable, regioselective and
waste-minimised aryl alkyl ketone synthesis starting
from simple, inexpensive, and widely available chemi-
cals.
As a solution to this long-standing problem, we en-
visioned a catalytic process in which an aromatic and
an aliphatic carboxylic acid are cross-coupled under
release of CO2 and water, to selectively give the aryl
alkyl ketone (Scheme 1). In such a decarboxylative
cross-ketonisation, the carboxylate groups would pre-
define the position of bond formation, potentially al-
lowing a selective synthesis of any desired regioiso-
mer. If mediated by catalytic amounts of an inexpen-
sive metal with high selectivity for hetero- over homo-
coupling, the overall process would be advantageous
both from economical and ecological standpoints.
The decarboxylative homoketonisation of aliphatic
Keywords: aryl alkyl ketones; catalysis; cross-keto-
nisation; decarboxylation; iron
Aryl alkyl ketones are among the most important syn-
thons in the industrial manufacture of commodities,[1] carboxylic acids is an established strategy for the
fine chemicals, and pharmaceuticals.[2] Simple deriva- preparation of symmetrical dialkyl ketones or cyclic
tives, at early stages of the chemical value creation alkanones.[9] State-of-the-art protocols involve gas-
chain, are synthesised almost exclusively via Friedel– phase transformations at temperatures above 3508C
Crafts acylation of arenes, because the low cost of this at solid catalysts, for example, CaO, ZnO, MgO,[10]
reaction is unrivalled to date.[3] However, two main TiO2,[11] ZrO2,[12] MnO,[13] Fe2O3,[14] or rare earth metal
issues are associated with this approach. Its low regio- oxides[15] on SiO2-, Al2O3-, or pumice-based supports.
selectivity leads to the co-manufacture of large However, if equimolar mixtures of aromatic and ali-
amounts of isomeric by-products, and the use of acyl phatic carboxylic acids are subjected to the above cat-
chlorides as starting materials activated with super- alysts, dialkyl and aryl alkyl ketones are obtained at
stoichiometric amounts of salt mediators (e.g., AlCl3) best in a 1:1 ratio, using elaborate reaction technology
is responsible for the formation of enormous quanti- such as gas-phase reactions above 4008C,[16] or contin-
ties of salt waste.
uous-flow reactors.[17] Even when a metallic mediator
Regioselective and less waste-intensive acylations, is employed in overstoichiometric amounts and, thus,
e.g., cross-couplings[4] of sensitive arylmetal reagents the ecological advantage of this synthetic strategy is
with either preformed[5] or in situ-generated[6] carbox-
ylic acid derivatives are widely used to access higher-
value synthetic intermediates. However, their high
cost precludes their application in the manufacture of
base chemicals. This is also the case for alternative
approaches, such as Heck-type reactions,[7] or the Pd-
catalysed decarboxylative cross-coupling of a-oxocar-
Scheme 1. Decarboxylative cross-ketonisation of carboxylic
boxylate salts with aryl halides.[8] Thus, a clear need acids.
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Lukas J. Gooßen et al.
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both a high activity and a strong selectivity for the
cross-ketonisation product 3aa (full conversion, 23:1
selectivity, see entry 7).
At a much reduced reaction temperature of 2508C,
only iron continued to mediate the reaction, although
only in moderate yields (entry 8).[18] The iron powder
dissolved with formation of iron(II) carboxylates and
liberation of hydrogen gas, resulting in a barely stirra-
ble suspension. In order to improve the mixing of the
reagents, various high-boiling solvents were investi-
gated (entries 9–12). The addition of Dowtherm A (a
eutectic mixture of diphenyl ether and biphenyl)
proved to be particularly beneficial, leading to full
conversion of 2a (entry 12). Under these conditions,
the iron(II) carboxylates can also be generated from
basic iron(II) precursors and the arenecarboxylic
Scheme 2. Postulated mechanism for the cross-ketonisation.
sacrificed, aryl alkyl ketones are obtained only in un- acids, or from sodium carboxylates and iron salts (en-
satisfactory yields.[18]
tries 13–15). With iron(III) precursors, lower yields
To guide our search for a selective catalyst that were obtained (entry 16).
AHCTUNGTRENNUNG
would efficiently mediate the desired decarboxylative
For example, the cross-ketonisation product 3aa is
cross-ketonisation of aromatic and aliphatic carboxyl- obtained in high yields starting from iron(II) sulfate
ic acids, we used a modified literature mechanism for heptahydrate, two equivalents of sodium 3-methyl-
homoketonisations as the working hypothesis benzoate (10) and phenylacetic acid (2a) in Dow-
(Scheme 2).[19] A metal oxide 4 should initially react therm A (entry 13). For this experiment, high purity
with a mixture of aromatic 1 and aliphatic carboxylic iron sulfate (>99.999%) was used to make sure that
acid 2. Due to the higher acidity of 1, the carboxylate it is indeed the iron, rather than trace quantities of
5 derived from the aromatic carboxylic acid should another metal, that mediates this process. Control ex-
form preferentially. This carboxylate is not enolisable periments with 13C-labelled benzoic acid (1b) con-
because it posesses no a-protons. Therefore, the cata- firmed that the carbonyl group in the product 3ba is
lytic cycle should proceed only with the mixed car- derived from the aromatic carboxylic acid 1, whereas
boxylate 6 or the dialkyl carboxylate 8 present in the CO2 released originates from the aliphatic carbox-
equilibrium. The aromatic acyl residue in 6 can then ylic acid 2 (see Scheme 2).
shift to the b-position of the aliphatic carboxylate,
The decisive next step was to make the transforma-
forming a b-keto acid 7. This species should decarbox- tion catalytic in iron. However, whenever starting
ylate via a retro-oxo-ene reaction with formation of from only catalytic quantities of iron benzoates, little
the aryl ketone 3. The potential dialkyl ketone side more than a single turnover was achieved. No matter
product 9 may form analogously from the dialkyl car- which iron salt was used as precursor, strongly vary-
boxylate 8.
ing, usually low yields were obtained (entries 17–19).
Based on this mechanism, we reasoned that the The reaction solution darkened, and in some cases, a
best strategy to achieve a selective synthesis of aryl black precipitate formed. We reasoned that inter-
alkyl ketones would be to identify a metal oxide that mediately formed iron(II) oxide disproportionates
binds with strong preference to the more acidic, aro- with formation of catalytically less active ironACHTUNGTRENNUNG(III)
matic carboxylic acid 1 even at high reaction tempera- oxide modifications (entry 20).
tures. As a result, the equilibrium between species 5,
6 and 6, 8 (see Scheme 2) would be shifted as far as ing a finely divided, mixed Fe(II)/FeAHCNUTGTRENNUNG
The breakthrough was finally achieved by employ-
(III) oxide as the
possible away from 8. We began our investigations by iron source. When heating a mixture of the carboxylic
heating 2:1 mixtures of 3-toluic acid (1a) and phenyl- acids with a catalytic amount [15 mol%=^ 5 mol%
acetic acid (2a) in the presence of stoichiometric Fe(II)] of commercially available PVP-stabilised mag-
amounts of numerous metals and metal oxides at netite nanopowder [average 25–40 nm, see Figure 1,
3408C. As can be seen by the examples in Table 1, (a)], a clear yellow solution with tiny suspended parti-
several metals mediated the formation of the desired cles quickly formed, and full conversion of the ali-
cross-ketonisation product 3aa along with the sym- phatic carboxylic acid was achieved within 21 h
metrical dialkyl ketone 9aa. 3,3’-Dimethylbenzophe- (entry 21).[20] With this catalyst, the desired aryl alkyl
none was never detected even in traces. With Zr, Mn, ketone was isolated in almost 80% yield and 9:1 se-
and Co mediators, encouraging selectivities for the lectivity in favour of the cross-ketonisation product
cross-ketonisation product 3aa were observed, albeit over the dialkyl ketone. We assume that the reaction
at low conversions (entries 3–6). Solely iron displayed takes place either at iron(II) centres of solvated nano-
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Catalytic Decarboxylative Cross-Ketonisation of Aryl- and Alkylcarboxylic Acids
Table 1. Development of the catalyst system.[a]
Entry
Catalyst
mol%
T [8C]
t [h]
Solvent
Yield [%]
3aa
9aa
1
2
3
4
5
6
7
8
–
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
15
340
340
340
340
340
340
340
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
1
1
1
1
1
1
1
16
16
16
16
16
21
21
21
21
21
21
21
21
21
21
21
–
–
–
–
–
–
–
–
2
4
6
8
7
9
2
4
2
5
4
5
5
5
6
5
4
2
6
4
1
9
8
7
MgO
ZrO2
Co
Mn
MnO2
Fe
Fe
Fe
Fe
Fe
19
63
44
76
28
92
37
12
68
77
90
84
70
69
66
14
30
20
4
9
NMP
10
11
12
13[b]
14
15
16[d]
17[e]
18[e]
19[f]
20[g]
21[g]
22[g]
23[g]
sulfolane
tetraglyme
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Dowtherm A
Fe
FeSO4
FeCp2
1/3 Fe3O4
FeCl3
FeSO4
FeCl2
FeCl3
1/2 Fe2O3[h]
1/3 Fe3O4[c]
1/3 Fe3O4[i]
1/3 Fe3O4
[c]
15
15
15
15
15
15
80
64
57
[a]
Conditions: 2.00 mmol 1a, 1.00 mmol 2a, 2 mL solvent. Yields were determined by GC using n-undecane as standard.
2.00 mmol sodium 3-methylbenzoate (10), 1.00 mmol 2a.
<500 nm.
3.00 mmol 10, 1.00 mmol 2a.
0.30 mmol 10, 0.90 mmol 1a, 1.00 mmol 2a.
0.45 mmol 10, 0.75 mmol 1a, 1.00 mmol 2a.
1.20 mmol 1a, 1.00 mmol 2a.
<50 nm.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
<10 nm.
particles or via homogeneous iron(II) carboxylates, tron microscopy (SEM) revealed that magnetite
and that the role of the remaining nanoscale magnet- nanoparticles were present in the reaction mixtures
ite particles is to act as seeds for the reversible depo- even at a late stage of the reaction (16 h). At this
sition of intermediate iron(II) oxides, suppressing point, they displayed a size of 17–28 nm and a rather
their disproportionation and agglomeration. A large narrow size distribution [see Figure 1, (b)].[21] This
fraction of the magnetite particles could be recovered might explain why the best results were achieved
after the reaction by fixing them at the bottom of the when starting from a magnetite nanopowder with a
reaction flask with a strong magnet, while decanting similar particle size. In order to fully exclude that the
the reaction mixture. After rinsing with ethyl acetate reaction is catalysed by other metals present as con-
and drying under vacuum, the particles were reused taminants in the iron, we tested all trace metals de-
and displayed similar catalytic activity.
tected by AAS in the magnetite samples. None of
Magnetite powders with larger particle sizes them displayed comparable catalytic activity alone,
(<500 nm) remained undissolved to a greater extent, even at levels of 20 mol%, and when 2 mol% were
and only incomplete turnovers were achieved. The added to the magnetite, they did not substantially
use of even finer particles (<10 nm) also led to re- affect its catalytic activity (Tables S1 and S2 in the
duced yields (entries 22 and 23). Analysis by transmis- Supporting Information).
sion electron microscopy (TEM) and scanning elec-
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Lukas J. Gooßen et al.
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Figure 1. TEM (top) and SEM (bottom) images of the magnetite particles (a) before and (b) after the reaction.
After having identified an effective protocol for the in 80% yield demonstrates that decarboxylative cross-
decarboxylative cross-ketonisation, we investigated its ketonisation can easily be performed also on multi-
scope by combining various aromatic carboxylic acids gram scales, with isolation of the product by filtration
with phenylacetic acid in the presence of magnetite over silica gel or distillation.
nanopowder [15 mol%=^ 5 mol% Fe(II)]. The focus
was set on the synthesis of regioisomers that are inac- boxylative cross-ketonisation was identified that
cessible via Friedel–Crafts acylation. allows the regioselective, salt-free, and waste-mini-
In conclusion, an effective catalyst for the decar-
As can be seen from Table 2, both electron-rich mised synthesis of aryl alkyl ketones directly from
and electron-poor aromatic and heteroaromatic car- widely available carboxylic acids. This synthetic strat-
boxylic acids react in high yields, and many functional egy is an advantageous alternative to Friedel–Crafts
groups including halogens, phenols, ethers, tertiary acylations both from ecological and economical stand-
amines, and trifluoromethyl groups are tolerated. The points, as the substrates are available in a sustainable
scope with regard to the aliphatic carboxylic acids way from renewable sources or via air oxidation from
was also evaluated. As some of them displayed a alcohols or alkylarenes, a non-toxic and inexpensive
slightly lower reactivity than phenylacetic acid, we in- catalyst is used, and only CO2 and water are released
creased the iron loading to 20 mol% [=^ 7 mol% in the process.
Fe(II)] and the reaction temperature to 270–2808C.
Under these conditions, many linear aliphatic acids
were successfully coupled with 3-toluic acid (1a), in-
cluding fatty acids (Table 3). Slightly higher reaction
Experimental Section
times were required for the coupling of a-branched
carboxylic acids. The successful coupling of 50 mmol
Synthesis of 2-Phenyl-1-(3-tolyl)-ethanone (3aa)
phenylacetic acid (2a) and 3-toluic acid (1a) yielding
An oven-dried Schlenk flask was charged with magnetite
3aa in a 10:1 mixture with 1,3-diphenylacetone (9aa) nanopowder (<50 nm, 590 mg, 98% purity, 2.50 mmol), 3-
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Catalytic Decarboxylative Cross-Ketonisation of Aryl- and Alkylcarboxylic Acids
Table 2. Scope with regard to the aromatic carboxylic acid substrate.[a]
Product
Yield [%]
80
Product
Yield [%]
43
65
69
69
83
78
83
27
80
32
64[b]
[a]
Conditions: 1.20 mmol 1, 1.00 mmol 2a, 0.05 mmol Fe3O4, 21 h, 2508C, 2 mL Dowtherm A, isolated yields.
0.07 mmol Fe3O4.
[b]
Table 3. Scope with regard to the aliphatic carboxylic acid substrate.[a]
Product
Yield [%]
78
Product
Yield [%]
53[b]
80
86
51[c]
70
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Lukas J. Gooßen et al.
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Table 3. (Continued)
Product
Yield [%]
85
Product
Yield [%]
76[d]
76
74
59
41
[a]
Conditions: 1.20 mmol 1a, 1.00 mmol 2, 0.07 mmol Fe3O4, 2808C, 21 h, 2 mL Dowtherm A, isolated yields.
72 h.
Mixture of isomers.
2708C.
[b]
[c]
[d]
toluic acid (1a) (8.17 g, 60.0 mmol), phenylacetic acid (2a)
(6.88 g, 99% purity, 50.0 mmol), and degassed Dowtherm A
(100 mL). The apparatus was evacuated and flushed with ni-
trogen three times. The reaction mixture was heated to
reflux (2508C) overnight in a metal bath, while continuously
removing CO2 and water in a slow stream of nitrogen. The
mixture was then distilled trap-to-trap (60–1108C,
10À3 mbar) to give a clear, yellow distillate and dark brown
residue. Compound 3aa was separated from Dowtherm A
by fractional distillation over a Vigreux column (Dowtherm:
55–608C, 10À3 mbar) and obtained as a pale yellow oil that
solidified to an off-white solid upon cooling (100–1058C,
10À3 mbar); yield: 8.43 g (80%), as a 10:1 mixture with 1,3-
diphenylacetone (9aa). For spectroscopic characterisation,
the product was further purified by column chromatography
(SiO2, ethyl acetate/hexane gradient). The spectroscopic
data matched those reported in the literature for 2-phenyl-
1-(3-tolyl)-ethanone (3aa) (CAS 95606-81-8).
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Acknowledgements
We gratefully acknowledge Dr. C. Lehmann, H.-J. Bongard
and A. Dreier (MPI for Coal Research, Muelheim) for char-
acterising the nanoparticles by SEM and TEM. We thank
NanoKat, Novartis (Young Investigator Award for L.J.G.)
and the Landesgraduiertenfçrderung Rheinland-Pfalz (fel-
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