Straightforward heterogeneous palladium catalyzed synthesis of aryl ethers and aryl amines via a solvent free aerobic and non-aerobic dehydrogenative arylation

Article abstract of DOI:10.1039/c2gc36776a

Aryl ethers have been prepared from cyclohexanone derivatives and various alcohols in the presence of a catalytic amount of palladium on charcoal. The formation of an enol ether followed by an aerobic or non-aerobic dehydrogenation reaction, seem to be the key steps of this transformation. In addition, this new method was also adapted for the synthesis of arylamines.

Full text of DOI:10.1039/c2gc36776a

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Straightforward heterogeneous palladium catalyzed
synthesis of aryl ethers and aryl amines via a solvent
free aerobic and non-aerobic dehydrogenative
arylation†
Cite this: DOI: 10.1039/c2gc36776a
Received 25th October 2012,
Accepted 16th November 2012
DOI: 10.1039/c2gc36776a
www.rsc.org/greenchem
Marc Sutter,a Nicolas Sotto,a Yann Raoul,b Estelle Métaya and Marc Lemaire*a
Aryl ethers have been prepared from cyclohexanone derivatives workers15 described the synthesis of aryl ethers from 2-cyclo-
and various alcohols in the presence of a catalytic amount of hexenone in the presence of a stoichiometric amount of CuCl2
palladium on charcoal. The formation of an enol ether followed in toluene under O2. In addition, co-oxidant additives (KI and
by an aerobic or non-aerobic dehydrogenation reaction, seem to N-hydroxyphthalimide) were required when a catalytic amount
be the key steps of this transformation. In addition, this new of copper salts (10 mol%) was introduced, and this method
method was also adapted for the synthesis of arylamines.
was inefficient with cyclohexanone.
We previously developed acidic conditions to prepare
alcohol naphthyl ethers with the recyclable Nafion® catalyst;
however this methodology was not efficient enough with
phenol derivatives.16 We also recently described the reductive
alkylation of carbonyl derivatives with alcohols and (poly)gly-
cerol (Pd/C, Amberlyst 35, H2) but aryls were partially reduced
under the used conditions.17 In similar conditions reported by
Kita and co-workers18 and Linder and Gooßen19 only alkyl
ethers were prepared.
Based on our continuous interest in establishing new and
eco-efficient processes for etherification reactions which could
be applied to glycerol, we report herein a straightforward and
palladium-catalyzed direct aerobic and non-aerobic dehydro-
genative alkylation of cyclohexanone derivatives with alcohols
and polyols, including biosourced glycerol (Scheme 1).
In order to establish a green and safe process, our goal was
to develop a solvent-free system, using heterogeneous catalysis
and without any additive to facilitate the purification and to
limit the production of wastes. In addition, a reaction taking
place in an open reactor under air or in a closed system under
argon was preferred to avoid the utilization of O2, considering
flammability risks.
Aryl ethers are employed in diverse domains, like pharmaceuti-
cals, perfumes, cosmetics, paints, and varnishes, and some of
them are produced in several ten thousand tons per year.1 For
example, the global production of phenoxyethanol, which is
mainly used as a solvent, is around 170 000 t per year.2
Over the last century, numerous methods have been devel-
oped to synthesize aryl ethers. They can be obtained from
epoxides.3 The Williamson4 synthesis could be also applied to
basic media from halides. The Ullmann5 and Buchwald–
Hartwig6 coupling reactions are the most described pathways
from aromatic halides. Alternative routes using boronic7 or
bismuth8 coupling partners have also been reported. However,
the stoichiometric quantity of the base and the utilization of
solvent and toxic substrates in these processes implied the pro-
duction of a large amount of wastes.
From non-aromatic substrates, the preparation of phenols9
or anilines10 was previously described via dehydrogenation
pathways. Recently, the formation of diarylamines from cyclo-
hexanone using homogeneous palladium catalysis was
reported by Deng and co-workers,11 Li and co-workers,12 and
Yoshikai and co-workers.13 Maycock and co-workers14 pro-
posed the synthesis of diarylamines from cyclohexenones pro-
moted by iodine. Moreover, during our study, Li and co-
At the beginning of our study, cyclohexanone 2a and
hexanol 1a were used as model substrates in an open reactor,
aLaboratoire de Catalyse et Synthèse et Environnement, Institut de Chimie et
Biochimie Moléculaires et Supramoléculaires (ICBMS), CNRS, UMR5246, Université
Lyon 1, 43 Boulevard du 11 Novembre 1918, Bat CPE, 69622 Villeurbanne, France.
E-mail: marc.lemaire.chimie@univ-lyon1.fr; Fax: +33-472-43-14-08;
Tel: +33-472-43-14-07
bSofiprotéol, 11, rue de Monceau, CS 60003, 75378 Paris Cedex 08, France.
E-mail: sofiproteol@prolea.com; Fax: +33 147 230 288; Tel: +33 140 694 800
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2gc36776a
Scheme 1 Envisaged methodology for the synthesis of aryl ethers by a hetero-
geneous catalyzed solvent free dehydrogenative arylation using cyclohexanone
derivatives, alcohols and biosourced polyols.
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Table 1 Effects of the reactor type, the atmosphere and an additive (1-octene) on the alkylation reaction of hexanol 1a with cyclohexanone 2a or α-tetralone 2ja
Entry
1
Substrate (2)
Reactor type
Additive
Conv.b (2, %)
Yieldb (3, %)
Yieldb (4, %)
Sealed tube (argon)
—
>99
50
50
2c
3
Sealed tube (argon)
Open reactor (air)
Sealed tube (argon)
1-Octene (1 eq.)
>99
>99
>99
70
99
78
30
0
—
—
4
<1
5
Open reactor (air)
—
74
58
0
a Experimental conditions: molar ratio cyclohexanone 2a/hexanol 1a = 1/5, Pd/C (5%) 1 mol%, 130 °C, 24 h. b Conversions and the ratio were
determined by 1H NMR spectroscopy. c Reaction time = 48 h.
without any solvent. First, we supposed that the oxygen from compound 3 were observed. The by-products detected by
air will play the role of the final oxidant, in order to have an GC-MS were the deoxygenated starting material, the naphtha-
aerobic dehydrogenative arylation. After screening many sup- lene and the naphthol. In an open reactor, the same selectivity
ported metal catalysts and optimizing the experimental para- was observed, however the conversion was lower after 24 h. As
meters, we found that the desired aryl ether 3a was obtained a consequence, reactions performed with tetralone derivatives
in quantitative yield when 1 mol% of Pd/C (5%) was added to were realized in sealed tubes, and by increasing the tempera-
a molar ratio cyclohexanone 2a/hexanol 1a 1/5 at 130 °C in ture to 150 °C, the conversion was complete after 16 h. The fol-
24 h. Under these conditions, no by-products were observed.
lowing mechanisms can thus be proposed (Fig. 1).
Several experiments were performed in different reactors in
With cyclohexanone derivatives (A), the palladium activates
order to propose a mechanism for this transformation. As the carbonyl function, leading to the formation of hemiacetal
shown in Table 1, when the reaction was realized in a sealed 7 which is dehydrated to form enol ether 8. Indeed, when the
tube with model substrates cyclohexanone 2a and hexanol 1a, reaction was performed without a catalyst or with 10 wt% of
the conversion was excellent, but a 50/50 mixture of the aryl activated charcoal, no conversion was observed, indicating that
ether 3a and the hydrogenated ether 4a was obtained (entry 1). the palladium catalyst is necessary for the formation of enol
This result can be explained by the in situ formation of H2, ether 8. Then, this intermediate could be dehydrogenated a
which can reduce the double bonds of the intermediates first time by the Pd/C catalyst which may be regenerated by O2
before their aromatization. To emphasize this hypothesis, a from air. Thus, water is formed as the only by-product. Finally,
reaction was performed with 1-octene as an additive and aryl ether 3 is obtained after a second dehydrogenation reac-
afforded the expected aryl ether 3a with a better selectivity, in tion of 9. As described in the literature,9a the formation of
a 70/30 ratio between 3a and 4a (entry 2). Moreover, octane phenol 5 could be observed in these conditions when less reac-
was observed confirming the release of H2 in the medium or tive alcohols were used as the substrate as mentioned further
in the presence of hydrogen adsorbed on the palladium in this report. However, and as demonstrated with the results
surface. When the reaction was performed in an open reactor in Table 1 (entries 1 and 2), the oxygen in the air was clearly
under air, the selectivity was excellent for the desired aryl not necessary to obtain aryl ether 3, but its presence allows a
ether 3a (entry 3). We may conclude that in an open reactor better selectivity for the aromatic product by reacting quickly
and in the case of cyclohexanone, O2 from air consumes hydro- with the hydrogen released in the medium, avoiding the
gen adsorbed on the palladium surface, affording the desired hydrogenation reaction leading to hydrogenated ether
4
product in excellent yield. The reactivity of α-tetralone 2j was observed in the sealed tube in 50% yield (Fig. 1, B). Indeed,
compared to cyclohexanone’s 2a: in a sealed tube (entries 1 this observation was confirmed when the reaction was started
and 4) the conversion of the ketone was complete but the with α-tetralone 2j as the substrate (entry 4): the reaction
selectivity toward the desired product was better since 78% of afforded the desired aryl ether 3j with an excellent selectivity
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Fig. 1 Proposed mechanisms for (A) the dehydrogenative alkylation of cyclohexanone with an alcohol in an open reactor, (B) the formation of hydrogenated ether
4 in a sealed tube and (C) the dehydrogenative alkylation of a tetralone derivative with an alcohol in a sealed tube.
in a closed system, because in that case only one dehydrogena- Table 2 Aerobic dehydrogenative alkylation of cyclohexanone 2a with primary
alcohols, secondary alcohols and polyols, including glycerola
tion reaction has to be performed (Fig. 1, C). More precisely,
the aromatization of the intermediate enol ether was much
faster in the case of α-tetralone 2j than in the case of
cyclohexanone.
To evaluate the scope of this new method, the optimized
conditions were first applied for the synthesis of 1-O-arylethers
Conv.b Isolated yield
by catalytic aerobic dehydrogenative arylation of various alco-
hols and polyols with cyclohexanone 2a, without any solvent,
as can be seen from the results in Table 2. Aerobic dehydro-
genative arylation of hexanol 1a with cyclohexanone 2a
afforded the corresponding aryl ether 3a in a good 84% iso-
lated yield (entry 1). When starting with 3-methylbutanol 1b,
the corresponding 1-O-phenyl ether 3b was isolated in 75%
yield (entry 2). With a less reactive secondary alcohol 2-hexanol
1c, the yield of the desired aryl ether 3c decreased to 34%
(entry 3). In this last example, the conversion of the starting
cyclohexanone 2a was still complete, but phenol was observed
as the sole by-product of the reaction in 54% NMR-yield. With
ethylene glycol 1d, the conversion was around 92% after 60 h,
and 2-phenoxyethanol 3d was isolated in 43% yield (entry 4).
When starting with 1,2-propanediol 1e, a mixture of regio-
isomers 3e and 3e′ was isolated in 40% yield (entry 5). The
ratio between 1-O-phenyl ether 3e and 2-O-phenyl ether 3e′ was
about 61/39. In the two last examples, phenol was also
obtained as the sole by-product in 42% and 41% yield, respect-
ively. Finally, with glycerol as the biosourced and available sub-
strate, the desired 3-phenoxypropane-1,2-diol 3f was isolated
in 67% yield, with an excellent regioselectivity since no second-
ary aryl ether was detected (entry 6). To the best of our knowl-
edge, this is the first successful example of a direct, solvent
free arylation of glycerol by a heterogeneous catalysis. Because
of the particular reactivity of this natural polyol, the
Entry Substrate (1)
Product (3)
(2a, %) (arylether 3, %)
1
>99
>99
>99
92
84 (99)e
75 (96)e
34 (45)e
43 (48)e
2
3c
4c
5c
94
40 (44;e ratio
3e/3e′ = 61/39)f
6d
>99
67 (77)e
a Experimental conditions: molar ratio cyclohexanone 2a/alcohol = 1/5,
Pd/C (5%) 1 mol%, 130 °C, 24 h, open reactor. b Conversions were
determined by 1H NMR spectroscopy. c Reaction time
= 60 h.
d Experimental conditions: molar ratio cyclohexanone 2a/glycerol 1f =
1/20, Pd/C (5%) 2 mol%, 130 °C, 60 h, open reactor. e 1H NMR yields in
parentheses. f Ratio between 3e and 3e′ was determined by 1H NMR
spectroscopy.
experimental parameters had to be slightly reoptimized, with a
longer reaction time and a lower cyclohexanone 2a/glycerol 1f
molar ratio in order to achieve a complete conversion and thus
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to increase the yield for aryl ether 3f. In fact, acetal 6f was the formation of phenol as the only by-product of the reaction
detected as the major compound after a reaction time of 24 h in 28% yield. This original process was also efficient with tetra-
(Scheme 2), phenol 5 and the desired aryl ether 3f were also hydrothiophen-3-one 2i, yielding the corresponding hexyloxy-
identified as minor products. After 60 h, only traces of com- thiophene 3i in 56% (entry 3). Finally, this new method was
pound 6f were detected and the major product was the desired adapted to tetralone derivatives by increasing the temperature
aryl ether 3f. We may conclude that acetal 6f was in equili- to 150 °C in order to reach the complete conversion of the
brium with the intermediate enol ether and the driving force starting material after 16 h and using a sealed tube under an
of the reaction was the aromatization leading to ether 3f, as argon atmosphere as the reactor, as explained at the beginning
explained at the beginning of this report. The process in an of our report. Besides, working in a sealed tube overcomes the
open reactor was limited by the boiling point of the alcohols limitation with low-boiling alcohols. When the dehydrogena-
(>130 °C) in order to have a good conversion, but the reaction tive arylation was performed with α-tetralone 2j the desired
with volatile alcohols is actually under study in the laboratory.
naphthyl ether 3j was isolated in an acceptable 67% yield
As shown in Table 3, cyclohexanone derivatives were also (entry 4). With β-tetralone 2k, the reaction afforded product 3k
tested in order to afford functionalized aryl ethers. The dehy- in 80% isolated yield (entry 5). Finally, with a substituted α-tetra-
drogenative arylation of 3-methylcyclohexanone 2g afforded lone 2l, the corresponding naphthyl ether 3l was isolated in
aryl ether 3g in 81% isolated yield (entry 1). With 2-cyclohexe- 68% yield (entry 6). As explained at the beginning of the report
none 2h, phenyl ether 3a was isolated in 59% yield (entry 2). and contrary to cyclohexanone and despite using a sealed tube
When comparing this result with the reaction using cyclohexa- as the reactor, only traces of hydrogenated ether and naphthol
none 2a (Table 2, entry 1), this drop of yield was explained by were observed in the last three examples, indicating that the
aromatization was faster than the hydrogenation of the inter-
mediate enol ether by the released hydrogen.
These conditions were also applied to biosourced polyols in
order to have access to new molecules with possible surfactant
properties, as shown in Table 4. Thus, the reaction with gly-
cerol or diglycerol and α-tetralone 2j afforded the correspond-
ing naphthyl ethers 3m and 3n in 48% and 43% isolated
yields, respectively (entries 1 and 2). From β-tetralone 2k, the
desired naphthyl ethers 3o and 3p were obtained in 69% and
Scheme 2 Dehydrogenative alkylation of cyclohexanone 2a with glycerol 1f
52% yields (entries 3 and 4). In all these examples, the naphthyl
ethers were obtained with an excellent regioselectivity.
with all detected by-products after a reaction time of 24 h when using the opti-
mized conditions.
Table 3 Dehydrogenative alkylation of saturated substrate 2 with hexanol 1aa
Entry
1
Substrate (2)
Product (3)
Conv.d (2, %)
Isolated yield (arylether 3, %)
>99
81 (93)e
2
>99
59 (68)e
3b
4c
>99
>99
56 (59)e
67 (78)e
5c
6c
>99
>99
80 (88)e
68 (74)e
a Experimental conditions: molar ratio substituted cyclohexanone 2/hexanol 1a = 1 /5, Pd/C (5%) 1 mol%, 130 °C, 24 h, open reactor. b Reaction
time = 48 h. c Experimental conditions: molar ratio tetralone derivative 2/hexanol 1a = 1/5, Pd/C (5%) 1 mol%, 150 °C, 16 h, sealed tube.
d Conversions were determined by 1H NMR spectroscopy. e 1H NMR yields in parentheses.
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Table 4 Dehydrogenative arylation of α-tetralone 2k and β-tetralone 2l with glycerol 1f and diglycerol 1oa
Entry
1
Substrate (2)
Product (3)
Conv.b (2, %)
Isolated yield (arylether 3, %)
>99
48 (53)c
2
>99
43 (47)c
3
4
>99
>99
69 (72)c
52 (56)c
a Experimental conditions: molar ratio tetralone derivative 2/(di)glycerol = 1/5, Pd/C (5%) 2 mol%, 150 °C, 60 h, sealed tube. b Conversions were
determined by 1H NMR spectroscopy. c 1H NMR yields in parentheses.
aryl ethers with various hydrophilic/lipophilic balances can be
obtained. The key to the reaction mechanism seems to be the
formation of an enol ether which is dehydrogenated to afford
the final product. This process was successfully generalized for
the preparation of arylamines and may be extended to other
nucleophiles by slightly adapting the experimental parameters
in each case. Besides, the main limitation of this method is
the use of a relatively high temperature in an open reactor with
cyclohexanone derivatives. For low-boiling alcohols, we are cur-
rently working on specific reactors under air pressure in order
to overcome this limitation. Further investigations of this
transformation are in progress in our laboratory.
Scheme 3 Solvent free and Pd-catalyzed dehydrogenative alkylation of cyclo-
hexanone derivatives with hexylamine 10a. Experimental conditions: molar ratio
cyclohexanone derivative/hexylamine 10a = 1/1, Pd/C (5%) 1 mol%.
Finally, this new solvent free and heterogeneous palladium
catalyzed dehydrogenative arylation could be applied for the
preparation of aryl amines in a more eco-efficient way by opti-
mizing the experimental parameters (Scheme 3). Indeed, a
stoichiometric amount of hexylamine 10a and cyclohexanone
2a was reacted in an open reactor with 1 mol% of Pd/C (5%),
without any solvent and additive. Under these conditions, the
reaction afforded the corresponding arylamine 11a in a good
71% isolated yield. When starting with α-tetralone 2j, in a
sealed tube, the corresponding naphthylamine 11b was
obtained in 78% isolated yield after 16 h. The scope and limit-
ations of this reaction for amines are under study in our lab-
oratory and will be published further.
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