Acid-catalysed carboxymethylation, methylation and dehydration of alcohols and phenols with dimethyl carbonate under mild conditions

Article abstract of DOI:10.1039/c6gc01826b

Dimethyl carbonate (DMC) chemistry has been extended to include acid-catalysed reactions of different aliphatic alcohols and phenols. For the first time, p-toluenesulfonic acid (PTSA), H₂SO₄, AlCl₃ and FeCl₃ have been shown to aid carboxymethylation for primary aliphatic alcohols at catalytic loadings with quantitative conversion and selectivity. For carboxymethylation of secondary alcohols, stoichiometric PTSA and catalytic AlCl₃ both gave quantitative conversion and selectivity. Stoichiometric FeCl₃ and H₂SO₄ promoted dehydration of linear aliphatic alcohols. Additionally FeCl₃ catalysed methylation of cyclohexanol, whilst AlCl₃ resulted in methylation of phenolic compounds. This research expands the range of potential application for DMC in green chemistry.

Full text of DOI:10.1039/c6gc01826b

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Acid-catalysed carboxymethylation, methylation
and dehydration of alcohols and phenols with
dimethyl carbonate under mild conditions†
Cite this: DOI: 10.1039/c6gc01826b
Saimeng Jin, Andrew J. Hunt, James H. Clark and Con Robert McElroy*
Dimethyl carbonate (DMC) chemistry has been extended to include acid-catalysed reactions of different
aliphatic alcohols and phenols. For the first time, p-toluenesulfonic acid (PTSA), H2SO4, AlCl3 and FeCl3
have been shown to aid carboxymethylation for primary aliphatic alcohols at catalytic loadings with
quantitative conversion and selectivity. For carboxymethylation of secondary alcohols, stoichiometric
PTSA and catalytic AlCl3 both gave quantitative conversion and selectivity. Stoichiometric FeCl3 and
H2SO4 promoted dehydration of linear aliphatic alcohols. Additionally FeCl3 catalysed methylation of
cyclohexanol, whilst AlCl3 resulted in methylation of phenolic compounds. This research expands the
range of potential application for DMC in green chemistry.
Received 4th July 2016,
Accepted 18th August 2016
DOI: 10.1039/c6gc01826b
www.rsc.org/greenchem
with a strong base.16 Potassium tert-butoxide was also dis-
covered to give excellent conversion and selectivity towards
Introduction
Dimethyl carbonate (DMC) is an established solvent and a unsymmetrical carbonates from primary and secondary alco-
green reagent which continues to attract attention. It readily hols and DMC.17 1,5,7-Triazabicyclo[4.4.0] dec-5-ene (TBD) was
biodegrades in the atmosphere1 and is non-toxic.2 It can be also applied as a basic catalyst for the carboxymethylation of
synthesised by many methods and processes.3–7 DMC has primary through to tertiary alcohols and for the production of
shown extensive applications as a solvent,8 including in polycarbonates from diols and DMC.18 The f-block base
pharmaceutically relevant synthesis9 and in biocatalysis.10 In lanthanum(III) isopropoxide has been reported as an excellent
fact, owing to its properties, a recent solvent guide has classi- catalyst for the carboxymethylation of primary, secondary and
fied DMC in the greenest “recommended” bracket.11 tertiary alcohols in high yield.19 Beyond base catalysis, the
Additionally, supercritical DMC has been shown to give a non- ionic liquid 1-(3-trimethoxysilylpropyl)-3-methylimidazolium
catalytic, reagent free route to biodiesel and glycerol carbon- chloride was found to be an activated reaction medium for the
ate.12 Significantly, DMC is a green substitute for highly toxic synthesis of non-symmetrical dialkyl carbonates,20 while enzy-
and hazardous compounds such as (a) dimethyl sulphate matic carboxymethylation of alcohols has also been reported.21
(DMS) and halohydrocarbon (CH3X, X = I, Br, Cl) in methyl- However, there is very limited literature on acid catalysed
ation reactions and (b) phosgene (COCl2) in carboxymethyl- (Brønsted or Lewis acid) carboxymethylation or methylation of
ation (methoxycarbonylation) reactions.
aliphatic alcohols/phenolic compounds with DMC.
Nowadays, most DMC mediated carboxymethylation reac-
Herein, for the first time Brønsted and Lewis acids includ-
tions are base catalysed. Potassium carbonate is one of the ing sulfuric acid, iron(III) chloride, p-toluenesulfonic acid
most common basic catalysts used in DMC carboxymethyl- (PTSA) and aluminium chloride have been used to catalyse the
ation and methylation reactions with alcohols and carboxymethylation and/or methylation reactions for various
phenols.13–15 Reaction of the highly hindered secondary alco- alcohols and phenols. Scheme 1 shows the general carboxy-
hols of isosorbide with DMC under reflux results in carboxy- methylation reaction used in this study, while Scheme 2 illus-
methylation in the presence of a weak base or methylation
Green Chemistry Centre of Excellence, Department of Chemistry, The University of
York, Heslington, York, YO10 5DD, UK. E-mail: rob.mcelroy@york.ac.uk;
Tel: +44 (0)1904324456
†Electronic supplementary information (ESI) available: Experiment process,
1H-NMR spectra, FT-IR spectrum and GC-MS profile of products. See DOI:
10.1039/c6gc01826b
Scheme 1 The acid-catalysed DMC carboxymethylation reaction for
different ROHs at 90 °C under normal pressure.
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carboxymethylation, however, for secondary alcohols only de-
hydration to the resultant alkene was observed (GC-MS spec-
trum is provided in the ESI†). This is as a result of increased
stability of the intermediate carbonium ion of secondary alco-
hols as compared to primary alcohols during acid catalysed de-
hydration.22 Under such conditions, stoichiometric H2SO4 had
been shown to be an excellent reagent for the dehydration of sec-
ondary alcohols. This is significant as dehydration of secondary
aliphatic alcohols is generally conducted at higher temperatures.
Specifically, dehydration of 2-octanol has been reported utilising
a molybdenum complex at 150 °C,23 while heterogeneous Al2O3-
ZrO2 required very high temperatures of 280 °C.24
Traditional base catalysed DMC chemistry with OH func-
tionality proceeds through deprotonation to form a strong
nucleophile prior to attack upon the carbonyl (carboxymethyl-
ation) or methyl (methylation) carbon of the alkyl carbonate. A
possible mechanism of Brønsted acid catalysed carboxymethyl-
ation reaction for alcohols is given in Scheme 3 and is based
on the proposed Brønsted acid catalysed transesterification
mechanism.25 Under acidic conditions, the lone pair of the
DMC carbonyl picks up a free proton from solution, before
undergoing nucleophilic attack from the alcohol. After losing
a proton, the hemiketal intermediate forms and then elimi-
nates methanol to yield the carboxymethylation product.
Scheme 2 The acid-catalysed DMC methylation reaction for different
ROHs at 90 °C under normal pressure.
trates the possible general methylation reaction equation at
90 °C. Crucially use of acid catalysts will aid in expanding the
potential range of substrates for DMC chemistry to include
those that cannot currently be investigated using traditional
base catalysed DMC chemistry, such as acidic substrates.
Results and discussion
Carboxymethylation products obtained by Brønsted acid
catalysis
In order to investigate acid catalysed DMC carboxymethylation
chemistry, PTSA and H2SO4 were selected as typical Brønsted
acid catalysts. Table 1 shows the conversion and selectivity
towards carboxymethylation products synthesised from
different primary and secondary alcohols in the presence of
DMC and catalytic (0.05 equivalents) or stoichiometric
(1.00 equivalent) loadings of catalyst.
Carboxymethylation products obtained by Lewis acid catalysis
According to Table 1, catalytic loading of PTSA resulted in
carboxymethylation of primary alcohols (entries 1 and 3,
Table 1). Under similar conditions, PTSA also promoted
carboxymethylation of secondary alcohols with high selectivity
but reduced conversion (entries 2 and 4, Table 1). When stoi-
chiometric loading of PTSA was employed, the conversion and
selectivity towards carboxymethylation for all primary and
secondary alcohols became quantitative (entries 1–4, Table 1).
Sulphuric acid was selected as a representative low cost in-
organic acid catalyst. When catalytic laoding of sulphuric acid
was applied, reactions of all alcohols occurred with high
selectivity, with good conversions for primary alcohols (entries
1 and 3, Table 1), but only moderate conversion for secondary
alcohols (entries 2 and 4, Table 1). Under stoichiometric
loading of H2SO4, the conversion of both primary and second-
ary alcohols became quantitative (entries 1–4, Table 1). In the
case of primary alcohols, selectivity was exclusively towards
AlCl3 and FeCl3 were selected as common and low cost Lewis
acids for the study of the effects of such catalysts in DMC
chemistry.
Scheme 3 The possible mechanism of Brønsted acid catalysed
carboxymethylation between DMC and alcohols.
Table 1 The experimental results of carboxymethylation products (ROCO2Me) synthesised from different ROHs with DMC promoted by catalytic
amount (0.05 equivalent) or stoichiometric amount (1.00 equivalent) of Brønsted acid PTSA or H2SO4 at 90 °Ca
Catalytic PTSA
Stoichiometric PTSA
Catalytic H2SO4
Conv.b (%)
Stoichiometric H2SO4
Entry
ROH
Conv.b (%)
Sel.b (%)
Conv.b (%)
Sel.b (%)
Sel.b (%)
Conv.b (%)
Sel.b (%)
1
2
3
4
1-Butanol
2-Butanol
1-Octanol
2-Octanol
>99
65
>99
28
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
94
52
>99
41
2
4
>99
>99
>99
>99
>99
>99
>99
>99
>99
0c
3
2
>99
3
0c
a Reaction conditions: ROH/DMC/PTSA or H2SO4 = 6.00 mmol : 240.00 mmol : 0.30/6.00 mmol; T = 90 °C; reaction time 19 h. b Conversions and
selectivity were calculated by 1H-NMR and GC. c Selectivity >99% towards dehydration products.
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Table 2 The experimental results of carboxymethylation products (ROCO2Me) synthesised from different ROHs with DMC promoted by catalytic
amount or stoichiometric amount (1.00 equivalent) of Lewis acid AlCl3 or FeCl3 at 90 °Ca
AlCl3
FeCl3
Conv./Sel.b (%)
(0.01 equivalent)
Conv./Sel.b (%)
(0.05 equivalent)
Conv./Sel.b (%)
(0.20 equivalent)
Conv./Sel.b (%)
(1.00 equivalent)
Conv./Sel.b (%)
(0.05 equivalent)
Conv./Sel.b (%)
(1.00 equivalent)
Entry
ROH
1
2
3
4
5
6
7
1-Butanol
2-Butanol
1-Octanol
2-Octanol
3-Nitrophenol
Phenol
>99/99
77 4/>99
99 1/>99
>99/>99
>99/98
>99/99
>99/>99
0/0
0/0
0/0
>99/>99
>99/>99
>99/>99
>99/>99
20 3/76
42 5/94
14 4/97
>99/>99
>99/99
>99/>99
>99/51d
83 1/29
85 2/33
90 2/56
>99/>99
88 2/>99
>99/>99
>99/>99
>99/0c
>99/>99
8
1/>99
74 3/99
>99/0c
0/0
0/0
0/0
9
4/>99
9
3/>99
0/0
0/0
0/0
0/0
m-Cresol
a Reaction conditions: ROH/DMC = 6.00 mmol : 240.00 mmol; T = 90 °C; reaction time 19 h. b Conversions and selectivity were calculated by
1H-NMR and GC. c Selectivity >99% towards dehydration products. d Selectivity 49% towards dehydration products.
Table 2 illustrates the conversion and selectivity of different
alcohols towards carboxymethylation in the presence of cata-
lytic and stoichiometric loading of AlCl3 or FeCl3. Selectivity
and conversion of primary alcohols is quantitative regardless
of acid loading being catalytic or stoichiometric (entries 1 and
3, Table 2). In the case of secondary alcohols, selectivity
remains high at catalytic loadings, but conversion drops
(entries 2 and 4, Table 2). At a stoichiometric loading however
FeCl3, and to a lesser extent AlCl3, result in dehydration of the
secondary alcohol to the relevant alkenes (entries 2 and 4,
Table 2).
Table 3 The experimental results of methylation products (ROMe) syn-
thesised from different phenols with DMC promoted by catalytic
amount (0.20 equivalent) or stoichiometric amount (1.00 equivalent) of
AlCl3 at 90 °Ca
Catalytic amount
(0.20 equivalent)
of AlCl3
Stoichiometric amount
(1.00 equivalent)
of AlCl3
Entry
ROH
Conv./Sel.b (%)
Conv./Sel.b (%)
1
2
3
3-Nitrophenol
Phenol
m-Cresol
20 3/24
42 5/6
14 4/3
83 1/71
85 2/67
90 2/44
In addition to the aliphatic alcohols, three phenols were
also investigated; phenol, m-cresol and 3-nitrophenol. The
methyl group on m-cresol has electron-donating hyperconjuga-
tion effect while the nitro group on 3-nitrophenol has electron-
withdrawing inductive effect. These three substrates were
a Reaction conditions: ROH/DMC/AlCl3 = 6.00 mmol : 240.00 mmol :
1.20/6.00 mmol; T = 90 °C; reaction time 19 h. b Conversions and
selectivity were calculated by 1H-NMR and GC.
selected to observe the effect of electron density upon the reac- reactions between DMC and phenols in the presence of stoi-
tivity of phenol in these methylation reactions. chiometric AlCl3 gave similar methylation results to base cata-
When reacting phenols with DMC in the presence of AlCl3 lysed reactions but with much improved conversion.14
at 0.01 and 0.05 molar equivalents, no reaction was observed, However when employing 0.2 equivalent of AlCl3, selectivity is
however at 0.20 equivalents or stoichiometric loading, both significantly towards carboxymethylation for all three reactions
carboxymethylation and methylation took place (entries 5–7, regardless of the softness of the nucleophile (entries 5–7,
Table 2 and entries 1–3, Table 3). At 0.2 equivalents of AlCl3 Table 2). This is in contrast to base catalysed reactions.14
conversion is moderate to low but selectivity is high towards
In order to investigate the impact of extended reaction time
carboxymethylation, while increasing to stoichiometric load- the stoichiometric AlCl3 reactions were repeated but run for
ings drastically increases conversion but decreases selectivity 60 h (Table 4). In comparison to 19 h reactions, conversion
(entries 5–7, Table 2). However, with regard to methylation, remains constant (within statistical error) while selectivity
employing stoichiometric AlCl3 enables good selectivity towards methylation increases. This suggests carboxymethyl-
towards 3-nitroanisole and anisole, and reasonable selectivity ation of the phenols occurs first but this reaction is reversible
to 3-methylanisole (entries 1–3, Table 3).
so constantly reforming small quantities of the substrates
Employing hard–soft acid–base (HSAB)26,27 theory to (Scheme 1). Methylation proceeds much more slowly but is
explain the experimental results, softer nucleophiles prefer to irreversible (Scheme 2) and results in the production of metha-
react with the softer methyl carbon (lower positive charge) on nol, pushing the carboxymethylation equilibrium to the left.
the DMC to obtain the methylation product, while the harder Therefore, the longer a reaction is run, the greater the degree
nucleophilic reagents prefer to react with the harder centre of methylation observed.
carbon (higher positive charge) of DMC to form the corres-
As AlCl3 can lead to highly acidic and toxic contaminated
ponding carboxymethylation product.14 In this study the order aqueous wastes,28,29 a second safer Lewis acid, FeCl3 was also
of hardness for the phenols is m-cresol > phenol > 3-nitro- investigated. According to Table 2, quantitative conversions
phenol, with selectivity towards methylation products follow- and selectivity of primary alcohols were attained under both
ing this trend (entries 1–3, Table 3). This indicated that catalytic and stoichiometric conditions (entries 1 and 3,
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Table 4 The experimental results of methylation products (ROMe) and
carboxymethylation products (ROCO2Me) synthesised from different
phenols with DMC promoted by stoichiometric amount (1.00 equi-
valent) of AlCl3 at 90 °C for 60 ha
hexanol and 2-decanol were selected as the substrates and the
reaction conducted in either DMC or n-heptane. n-Heptane
was selected as comparable reactions could be conducted at
90 °C while still solubilising the substrates.
Conv.b
(%)
Sel. (%)
ROMe
Sel. (%)
ROCO2Me
Table 5 demonstrates the results for the dehydration of
cyclohexanol. When employing FeCl3 as a reagent in DMC,
conversion of cyclohexanol was quantitative, although the
major product was methoxycyclohexane, with little dehydra-
tion to cyclohexene observed (entry 1, Table 5). In heptane,
cyclohexene is the only product but with low conversion (entry
Entry
ROH
1
2
3
3-Nitrophenol
Phenol
m-Cresol
84
88
92
3
1
3
88
85
62
12
15
38
a Reaction conditions: ROH/DMC/AlCl3 = 6.00 mmol : 240.00 mmol :
6.00 mmol; T = 90 °C; reaction time 60 h. b Conversions and selectivity 2, Table 5). It is obvious that under such conditions, dehydra-
were calculated by 1H-NMR and GC.
tion of cyclohexanol in either DMC or n-heptane is difficult. Of
interest is the methylation of an aliphatic, cyclic alcohol at low
temperature with no carboxymethylation products observed.
Table 2). Good conversion and high selectivity of secondary
alcohols were also obtained under catalytic loading (entries 2
and 4, Table 2). Reactions of secondary alcohols and DMC in
the presence of stoichiometric loading of FeCl3 resulted in
quantitative conversion, but with selectivity exclusively towards
dehydration to the respective alkene (entries 2 and 4, Table 2),
as observed with stoichiometric H2SO4. Although H2SO4 is a
well-known dehydrating agent,30 stoichiometric FeCl3 is not
normally considered as such.
When forming alkyl carbonates through Brønsted or Lewis
acid catalysed reactions between an aliphatic alcohol and
DMC, the ease of attack follows the trend primary alcohol >
short chain secondary alcohol > long chain secondary alcohol
> tertiary alcohol. This is consistent with previously published
base catalysed studies.31 Regardless of the system employed,
no acid catalysed reaction occurred between a tertiary alcohol
(tert-butanol) and DMC (detail as shown in the ESI†).
These reactions typically require the use of highly toxic methyl-
ation reagents such as CH3I and DMS,32,33 or very high reac-
tion temperature (>170 °C) in the case of DMC.34
Consequently, FeCl3 in DMC under 90 °C can be used as a
green system for the methylation of cyclohexanol to replace
these undesirable reagents or conditions. Base catalysed reac-
tions under similar conditions with cyclohexanol as the reac-
tant generally gave poor conversion unless an ionic liquid was
employed, with selectivity solely towards the carboxymethyl-
ation product.35 This demonstrates that Lewis acids promote
different reactions with DMC and alicyclic alcohols in com-
parison with base catalysts. Strangely stoichiometric loading of
H2SO4 did not result in either methylation or carboxymethyl-
ation of cyclohexanol in DMC despite showing good activity in
Table 1. However, in both solvents the dehydration product
was obtained with high selectively but low conversion (entries
1 and 2, Table 5).
During the investigation of reactions of phenols and DMC
in the presence of catalytic or stoichiometric FeCl3 (entries
5–7, Table 2), only very low conversion of 3-nitrophenol was
observed with >99% selectivity towards methyl 3-nitrophenyl
carbonate. Stoichiometric loading of H2SO4 resulted in
m-cresol reacting with DMC to give methylation (selectivity
90%) and carboxymethylation (selectivity 10%) products, but
at low conversion (16%). As such, phenols catalysed by stoi-
chiometric amount of H2SO4 or FeCl3 did not follow the HSAB
theory, where the hardest m-cresol had a high selectivity
towards its methylation product promoted by H2SO4, and the
In the case of 2-decanol, either acidic reagent in DMC
resulted in quantitative conversion of 2-decanol with selectivity
exclusively towards a mixture of C10 alkenes (entry 1, Table 6,
GC-MS and 1H-NMR (400 MHz) spectrums provided in the
ESI†). With heptane as the solvent, selectivity remained the
same but with a lower conversion (entry 2, Table 6). Overall,
DMC resulted in greater conversion of an aliphatic secondary
alcohol than heptane during this process. This may be a
simple solvent effect or as a result of the specific chemistry
associated with DMC such as carboxymethylation followed by
decarboxylation as opposed to straight dehydration.
softest 3-nitrophenol had
a near quantitative selectivity
towards its carboxymethylation product under FeCl3 (entry 5,
Table 2). The mechanistic study of Lewis acid catalysed DMC
chemical reactions is currently in progress. Of note is the need
to run these Lewis acid reactions under anhydrous conditions
to limit chlorination.
Table 5 The experimental results of products synthesised from cyclo-
hexanol with stoichiometric FeCl3 or H2SO4 in DMC or n-heptane at
90 °Ca
b
b
Stoichiometric FeCl3
Conv./Sel.c/Sel.d(%)
Stoichiometric H2SO4
Conv./Sel.c/Sel.d(%)
Lewis and Brønsted acid dehydration of cyclohexanol and
2-decanol in DMC and n-heptane
Entry
Solvent
1
2
DMC
n-Heptane
>99/12/88
35 4/99/0
9
6
3/99/0
2/99/0
As mentioned above, stoichiometric FeCl3 and H2SO4 both
exhibited the ability to dehydrate secondary alcohols to their
respective alkene with DMC as the reaction medium. In order
to further investigate their ability to dehydrate secondary ali-
phatic alcohols as well as the influence of the solvent, cyclo-
a Reaction conditions: cyclohexanol/solvent/reagent
=
6.00 mmol :
240.00 mmol : 6.00 mmol; T = 90 °C; reaction time 19 h. b Conversions
and selectivity were calculated by 1H-NMR and GC. c Towards cyclohex-
ene. d Towards methoxycyclohexane.
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Table 6 The experimental results of products synthesised from
2-decanol with stoichiometric FeCl3 or H2SO4 in DMC or n-heptane at
90 °Ca
at low loadings suggesting that greener reusable, hetero-
geneous aluminium catalysts may be practical. Stoichiometric
loading of FeCl3 or H2SO4 resulted in dehydration of secondary
alcohols, with better results in DMC than in n-heptane.
Overall, the system of FeCl3 and DMC has the potential to be
applied to substrates that have previously been difficult to
dehydrate. In addition, FeCl3 and DMC have been found to be
a good green methylation system for cyclohexanol under mild
conditions.
Stoichiometric FeCl3
Conv./Sel.b (%)
Stoichiometric H2SO4
Conv./Sel.b (%)
Entry
Solvent
1
2
DMC
n-Heptane
>99/99
80 4/99
>99/99
90 3/99
a Reaction conditions: 2-decanol/solvent/reagent = 6.00 mmol : 240.00
mmol : 6.00 mmol; T = 90 °C; reaction time 19 h. b Conversions and
selectivity were calculated by 1H-NMR and GC, selectivity towards decene.
Acknowledgements
For the purpose of study, control reactions for every sub-
strate and solvent were carried out in the absence of any cata-
lyst. In each case negligible conversion of alcohols or phenols
was observed. Therefore, the acid played a vital role for the
reaction between OH functionality and DMC (details are pro-
vided in the ESI†).
The authors gratefully acknowledge Dr Duncan J. Macquarrie
(University of York) who gave many helpful suggestions about
this research. The authors also wish to thank Karl Heaton
(University of York) and Julia Sarju (University of York) for
their support in gas phase GC-MS analysis.
This work highlights several major advantages of acid-cata-
lysed DMC chemistry including, expanding the range of sub-
strates that can be reacted with DMC to include acids. This
was previously impossible due to acid–base neutralisation
reactions of the substrate and catalyst which would occur. The
acid catalysts used in this study have comparable or lower
market prices than the common base catalysts used in existing
DMC chemistry.36 This is especially true in the case H2SO4
which is a widely available, cheap inorganic catalyst that will
offer a cost effective alternative to base catalysed DMC pro-
cesses. The Lewis acids AlCl3 and FeCl3 while employed homo-
geneously in this work, could potentially be used to form
heterogeneous ion-exchanged clays which would open up
numerous options to further green this chemistry.37,38
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