Carbonate, acetate and phenolate phosphonium salts as catalysts in transesterification reactions for the synthesis of non-symmetric dialkyl carbonates

Article abstract of DOI:10.1039/c2ob25447f

Methyl trioctylphosphonium methyl carbonate [P₈₈₈₁] ⁺[MeOCO₂]^- was prepared by the alkylation of trioctyl phosphine with the non-toxic dimethyl carbonate. This salt was a convenient source to synthesize different ionic liquids where the methyl trioctylphosphonium cation was coupled to weakly basic anions such as bicarbonate, acetate, and phenolate. At 90-220 °C, all these compounds [P₈₈₈₁]⁺X^-; X = MeOCO₂; HOCO ₂; AcO; PhO were excellent organocatalysts for the transesterification of dimethyl and diethyl carbonate with primary and secondary alcohols, including benzyl alcohol, cyclopentanol, cyclohexanol, and the rather sterically hindered menthol. Conditions were optimized to operate with very low catalyst loadings up to 1 mol% and to obtain non-symmetric dialkyl carbonates (ROCO₂R′; R = Me, Et) with selectivity up to 99% and isolated yields >90%. The catalytic performance of the investigated ionic liquids was discussed through a cooperative mechanism of simultaneous activation of both electrophilic and nucleophilic reactants.

Full text of DOI:10.1039/c2ob25447f

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PAPER
Carbonate, acetate and phenolate phosphonium salts as catalysts in
transesterification reactions for the synthesis of non-symmetric dialkyl
carbonates†‡
Maurizio Selva,* Marco Noè, Alvise Perosa and Marina Gottardo
Received 29th February 2012, Accepted 14th June 2012
DOI: 10.1039/c2ob25447f
Methyl trioctylphosphonium methyl carbonate [P8881]+[MeOCO2]− was prepared by the alkylation of
trioctyl phosphine with the non-toxic dimethyl carbonate. This salt was a convenient source to synthesize
different ionic liquids where the methyl trioctylphosphonium cation was coupled to weakly basic
anions such as bicarbonate, acetate, and phenolate. At 90–220 °C, all these compounds [P8881]+X−;
X = MeOCO2; HOCO2; AcO; PhO were excellent organocatalysts for the transesterification of dimethyl
and diethyl carbonate with primary and secondary alcohols, including benzyl alcohol, cyclopentanol,
cyclohexanol, and the rather sterically hindered menthol. Conditions were optimized to operate with very
low catalyst loadings up to 1 mol% and to obtain non-symmetric dialkyl carbonates (ROCO2R′; R = Me,
Et) with selectivity up to 99% and isolated yields >90%. The catalytic performance of the investigated
ionic liquids was discussed through a cooperative mechanism of simultaneous activation of both
electrophilic and nucleophilic reactants.
Introduction
Catalytic transesterification reactions are among the most estab-
lished protocols for the synthesis of both diaryl and dialkyl car-
bonates (Scheme 1).1
These transformations, however, continue to fuel extensive
research activity due to the growing interest in eco-friendly car-
bonates as intermediates in the pharma, lubricant and polymer
industries,2 and as solvents.1
Scheme 1
Both patent and open literature claim that most of the trans-
esterification reactions of organic carbonates with alcohols
(Scheme 1) use basic catalysts including phosphines and tertiary
amines, alkali metal hydroxides, alkoxides, halides, and carbon-
ates, alkali metal exchanged faujasites, and hydrotalcites.1–3
However, acidic catalysts or co-catalysts,4 as well as thermal
(non-catalytic) procedures,5 have also been proposed for the
same reactions. All these methods show common issues: (i) reac-
tions rarely stop at the mono-transesterification products (II,
Scheme 1) unless at moderate conversions of the starting carbon-
ate I; (ii) although (solid) catalysts can be recycled, relatively
Scheme 2
high loadings are necessary resulting in tedious and costly
separations; (iii) when polyols are used, primary and secondary
OH groups are not discriminated and they simultaneously react
to produce mixtures of different carbonates (Scheme 2).
To overcome such problems, only a few specific solutions
have been described so far. For example, highly active La(III)-
based catalysts have been recently reported for the selective
mono-transesterification of dimethyl carbonate (DMC) to unsym-
metrical alkyl methyl carbonates (ROCO2Me).6
Another structure-specific reaction has been detailed via a
multistep sequence involving imidazole carboxylic esters (IV,
Scheme 3) as intermediates.7 The combination of the leaving
Dipartimento di Scienze Molecolari e Nanosistemi dell’Università Ca’
Foscari Venezia, Calle Larga S. Marta 2137, Venezia, Italy.
E-mail: selva@unive.it
†This article is part of the joint ChemComm–Organic & Biomolecular
Chemistry ‘Organocatalysis’ web themed issue.
‡Electronic supplementary information (ESI) available: NMR spectra
and a synopsis of major signals observed in MS spectra are reported for
all products. See DOI: 10.1039/c2ob25447f
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6569–6578 | 6569

Scheme 3
Scheme 5
This paper reports that, in the presence of salts 1a–1d, the
investigated DAlCs reacted with primary and secondary alcohols
to produce unsymmetrical dialkyl carbonates (ROCO2R′;
R = Me, Et) with a very high selectivity (over 99%) and com-
plete conversion (Scheme 5).
Compounds 1c and 1d, particularly 1c, showed a catalytic
performance better than 1a–1b. This indicates that excellent
IL-based transesterification catalysts can be obtained by coupling
alkyl phosphonium cations to weakly basic anions: not only
methyl carbonate and bicarbonate, but acetate as well.
Conditions could be optimized to operate with low catalyst
loadings (up to 1 mol%). Transesterification reactions were
explored over a wide temperature range (90–220 °C): as
expected, an increase in the temperature improves the reaction
rates, but it does not appreciably affect the mono-transesterifica-
tion selectivity nor does it degrade the catalysts, which proved to
be highly thermally stable.
Scheme 4
group ability and the steric bulk of imidazole allows the trans-
esterification of esters IV only with primary OH functions to
produce selectively unsymmetrical dialkyl carbonates V.
One last remarkable example concerns the use of an ionic
liquid-based catalyst, namely 1-n-butyl-3-methylimidazolium-
2-carboxylate (C2mim-2-CO2): this salt has been reported to be
so efficient that loadings as low as 1 mol% are enough to reach
quantitative yields in the synthesis of glycerol carbonate from
glycerol and DMC.8
In this context, our long-standing interest in the use and syn-
thesis of organic carbonates as safe reactants and solvents2d,9
prompted us to investigate the catalytic behaviour of new ionic
liquids (ILs) synthesized in our laboratories, namely methyl
trioctylphosphonium methyl carbonate ([P8881][MeOCO2]) and
methyl trioctylphosphonium bicarbonate [P8881][HOCO2]
(Scheme 4: 1a and 1b, respectively).10
Tertiary alcohols, however, reacted mainly through alkylation
and dehydration processes, yielding O-alkyl ethers and alkenes
as major products.
Results
Transesterification of DMC with cyclohexanol: effects of
The use of compounds 1a–1b offers practical advantages:
their preparation method via the methylation of an alkyl phos-
phine with non-toxic dimethyl carbonate (DMC) is not only
simple and green but it allows the preparation of halide-free and
very pure ionic liquids that can be used straight from the reaction
vessel, and are stable for months on the shelf. The most intri-
guing peculiarity, however, is that 1a and 1b exhibit an excep-
tionally high catalytic activity: for example, they are able to
catalyze C–C bond-forming reactions including Michael and
Henry additions, with performances comparable to those of the
superbase DBU (1,8-diazabicyclo[5.4.0]undec-7-ene),10,11 even
though both salts possess intrinsically poorly basic anions
(methyl carbonate and bicarbonate).
With the aim of further exploring their potential as organocata-
lysts, we decided to apply compounds 1a–1b to the transesterifi-
cation of dialkyl carbonates (DAlCs). Also, other ILs derived
from 1a were considered for this study: methyl trioctylphospho-
nium acetate (1c: [P8881][AcO]) and methyl trioctylphosphonium
phenolate (1d: [P8881][PhO]) were prepared by exchange reaction
of 1a with AcOH and PhOH, respectively. The reactions of
dimethyl and diethyl carbonate with model primary, secondary
and tertiary alcohols, including benzyl alcohol, cyclopentanol,
cyclohexanol, menthol, triphenylcarbinol and diphenyl ethanol,
were explored.
catalysts and temperature
An initial screening was carried out on the reaction of dimethyl
carbonate (DMC) and cyclohexanol (CyOH) in the presence of
four different catalysts: the salt 1a [P8881]+ [MeOCO2]-, a repre-
sentative transesterification base catalyst such as K2CO3, and the
non-ionic organic bases DMAP (N,N-dimethylaminopyridine)
and DBU.12
In a first set of experiments, a mixture of cyclohexanol
(CyOH: 1.10 g, 11.0 mmol), DMC (19.82 g, 220 mmol; the
molar ratio DMC : CyOH was 20 : 1 since DMC served also as a
solvent), and different amounts of the catalyst (the molar percen-
tage of catalyst ranged from 1 to 10 mol%) was set to react at
reflux (∼90 °C, bp of DMC), for 3.5 h.
All reactions were followed by GC/MS: only the formation of
the unsymmetrical carbonate, i.e. cyclohexyl methyl carbonate
(CyMC: CyOCO2Me), was observed. Selected mixtures of reac-
tions catalyzed by 1a were purified by flash column chromato-
1
graphy (FCC): H and 13C NMR of the isolated (crude) product
confirmed its structure. Also, isolated and GC yields of CyMC
were in substantial agreement with each other.
The results are reported in Table 1.
Among conventional catalysts, only DMAP allowed the reac-
tion to proceed to some extent (entries 1 and 2, 5th column:
6570 | Org. Biomol. Chem., 2012, 10, 6569–6578
This journal is © The Royal Society of Chemistry 2012

Table 1 The transesterification of DMC with cyclohexanol over
In the absence of catalyst, a thermal reaction could take place:
in particular, the formation of the transesterification product
(CyMC) was triggered by an increase of the temperature from
150 to 200 °C (entries 1–2: conversions of 3 and 31%, respec-
tively, after 6 h). This matched our recent findings and other
literature results for non-catalytic transesterifications of esters
and organic carbonates which were reported at T ≥ 170 °C.5,13
Under the same conditions, however, at 200 °C, the reaction
went substantially to completion when an amount as low as
1 mol% of the salt 1a was added (entry 4: conversion of 93%).
This high catalytic efficiency of 1a was further confirmed
by increasing the molar % of catalyst relative to CyOH from 1 to
10 (entries 5–6): at 200 °C, a 93% conversion was reached
after only 0.5 h. The comparison of entries 4 and 6 showed a
remarkable 12-fold reduction of the reaction time. In all cases,
the mono-transesterification product (CyMC) was obtained
with a selectivity of over 99% (by GC), and was isolated in a
≥90% yield.
different catalystsa
Conversion of CyOHc (%, GC) CyMCd (%)
Entry Cat (mol%)b K2CO3 DBU DMAP 1a Sel
Y
1
2
3
4
5
10
5
2
1
1
2
0
0
3
1
0
0
65
32
2
97 >99
93
90
90
60
32
0
no cat.
no react.
a All reactions were carried out at 90 °C for 3.5 h. b Molar percentage of
catalyst relative to cyclohexanol. c Conversion of cyclohexanol
determined by GC, using different catalysts. d CyMC: cyclohexyl methyl
carbonate (CyOCO2Me): % selectivity (Sel) and % isolated yield
(Y, crude compound). Y was determined after FCC purification of the
reaction catalyzed by 1a.
An experiment was carried out by reducing the DMC : CyOH
molar ratio from 20 : 1 to 5 : 1. Accordingly, a mixture of CyOH
(4.41 g, 44 mmol), DMC (19.82 g, 220 mmol) and 1a as a cata-
lyst (54 mg, 1 mol%) was set to react at 200 °C for 4 h. At a sub-
strate conversion of 87%, mono and bis-transesterification
products (CyOCO2Me: 79%; CyOCO2Cy, 8%) were obtained
with a 79% and 8% conversion, respectively. The reduced
amount of DMC caused a modest decrease in the mono-trans-
esterification selectivity (91%).
Table 2 Transesterification of DMC with cyclohexanol using catalyst
1a
CyMCc (%)
Conversion of
Entry T (°C) t (h) Cat (mol%)a CyOHb (%, GC)
Sel
Y
1
2
3
4
5
6
150
200
150
200
150
200
6
6
6
6
1.2
0.5
no cat.
no cat.
1
1
10
10
3
31
25
93
62
93
>99
>99
>99
>99
>99
>99
90
92
a Molar percentage of catalyst relative to cyclohexanol. b Conversion of
cyclohexanol determined by GC. c % Selectivity (Sel) and % isolated
yield (Y, crude compound) of cyclohexyl methyl carbonate (CyMC). Y
was determined after FCC purification of the reaction catalyzed by 1a.
Transesterification of DMC with cyclohexanol in the presence of
different IL catalysts
The transesterification of DMC with cyclohexanol was further
investigated to compare the catalytic performance of the salt
1a to three other phosphonium based ionic liquids. In particular,
methyl trioctylphosphonium bicarbonate (1b: [P8881][HOCOO]),
methyl trioctylphosphonium acetate (1c: [P8881][AcO]),
and methyl trioctylphosphonium phenolate (1d: [P8881][PhO]),
were considered. Salts 1b, 1c, and 1d were obtained from 1a.
The preparation of 1b took place as described in Scheme 4
(details are in ref. 10). Compounds 1c and 1d were synthesized
by exchange reactions of 1a (2.00 g, 4.06 mmol) with an
equimolar amount of AcOH and PhOH, respectively (AcOH:
0.244 g; PhOH: 0.382 g) (Scheme 6).
conversions of 65 and 32%, respectively). The salt 1a, however,
was by far the most efficient system for the synthesis of CyMC:
this was isolated in up to 93% yield. In particular, noteworthy
aspects were: (i) 1a catalyzed the reaction with a very high
mono-transesterification selectivity (>99%) with substantially
quantitative conversions of CyOH (entries 1 and 2, 6th column:
97 and 90%, respectively); (ii) the progress of the reaction was
significant even in the presence of low amounts (1–2 mol%) of
1a (entries 3 and 4). By contrast, under the same conditions,
even DMAP was not effective (entries 3 and 4, 5th column).
Additional reactions were then carried out using a similar pro-
cedure: the same amounts and molar ratio of reactants (CyOH:
1.10 g and DMC : CyOH = 20 : 1) were used, but different temp-
eratures (150 and 200 °C) and times (0.5 to 6 h) were
considered.
Products 1c and 1d were isolated in quantitative yields as
viscous liquids (1c: 1.81 g and 1d: 1.94 g) at rt (further details
are in the Experimental section).
In these cases, only 1a was the catalyst. The molar ratio
1a : CyOH was set to 1 : 1 and the molar percentage to 10 mol%.
Because of the relatively high temperatures, experiments took
place in autoclaves operating at (autogenic) pressures of up to 20
bar. Two control reactions were also performed without catalyst.
Cyclohexyl methyl carbonate (CyMC) was the sole product
observed in all experiments. Table 2 reports the results.
Scheme 6
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6569–6578 | 6571

Table 3 The transesterification of DMC with cyclohexanol using
catalysts 1a, 1b, 1c and 1da
(in an autoclave), the reaction was faster, though less selective:
after 2 h, benzyl alcohol was no longer present, but a
4 : 1 mixture of benzyl methyl carbonate and dibenzyl carbonate
(PhCH2OCO2CH2Ph) was observed (result not shown in the
Table). Secondary alcohols preferentially reacted at temperatures
greater than 150 °C. The transesterification of DMC with cyclo-
pentanol went to completion at 200 °C yielding cyclopentyl
methyl carbonate as the sole product (entries 2–3). The reaction
was remarkably quicker (2 h) than that of cyclohexanol (6 h:
entry 4, Table 2): this behaviour was consistent with other com-
parative results reported for the same cyclic alcohols,14 and it
suggested the importance of steric hindrance as well as confor-
mational mobility for such substrates. Even more severe con-
ditions were required by menthol, whose transesterification
proceeded efficiently only at 220 °C. After 6 h, however, the cor-
responding product (menthyl methyl carbonate) was isolated in
92% yield (entry 4).
Tertiary alcohols did not react at 220 °C unless the catalyst
loading and the reaction times were increased considerably to
10 mol% and 20 h, respectively. Under such conditions, the
transesterification was not observed. 1,1-Diphenyl ethanol was
almost quantitatively converted to1,1-diphenyl ethylene (95%)
by elimination of water.
Traces of 1,1-diphenylethyl methyl ether [Ph2C(OMe)CH3,
5%)] were also detected (entry 5). The reaction of triphenylcarbi-
nol proceeded with moderate conversions (40–54%) yielding the
corresponding O-methyl ethers, i.e. methyl triphenylmethyl
ether (Ph3COCH3, 40%) (entry 6).
Cat : CyOHb
(mol : mol,%)
Conv.c
(%, GC) (%, GC)
Selc
Entry Catalyst
1
2
3
4
1a: [P8881][CH3OCOO]
1
1
1
1
52
68
93
78
>99
1b: [P8881][HOCOO]
1c: [P8881][AcO]
1d: [P8881][PhO]
a All reactions were carried out at 200 °C for 3 h. b Molar percentage of
catalyst relative to cyclohexanol. c Conv.: conversion of cyclohexanol
determined by GC; Sel: the selectivity towards cyclohexyl methyl
carbonate (CyMC).
In the presence of salts 1a, 1b, 1c, and 1d, the transesterifica-
tion of DMC with cyclohexanol was examined under the con-
ditions of entry 4 of Table 2 (200 °C; CyOH: 1.10 g; molar
ratio DMC : CyOH = 20 : 1; molar % of catalyst relative to
CyOH = 1). Experiments were carried out for 3 h. Results are
reported in Table 3.
Salts 1b, 1c and 1d were not only able to catalyze the selective
formation of cyclohexyl methyl carbonate (CyMC), but their
activity was even better than that shown by compound 1a.
At 200 °C, after 3 h, the conversion of cyclohexanol was consist-
ently higher using 1b, 1c, and 1d with respect to 1a (compare
entries 2–3 to entry 1: conversions of 68–93% and 52%, respec-
tively). Particularly evident was the case of 1c, which allowed a
substantially complete reaction in about half the time required
with 1a (Table 2: entry 4), even with a catalyst loading of only
1 mol%.
To further compare the reactivity of primary and secondary
alcohols, a competitive reaction was investigated. At 90 °C, an
equimolar mixture of benzyl and cyclohexyl alcohols (11 mmol
of each substrate) was set to react with DMC (19.82 g,
220 mmol) in a 50 mL flask, in the presence of 1c as a catalyst
(49 mg, 1 mol%). Both alcohols gave only the corresponding
mono-transesterification products. Although primary and second-
ary OH groups were not fully differentiated, the product ratio
was 3.3 : 1 with a net preference for the benzyl derivative
(Scheme 8).
Transesterification of DMC with different alcohols
The good performance of salt 1c prompted us to explore its use
as a catalyst for the reaction of DMC with other alcohols such as
benzyl alcohol, cyclopentanol, menthol, 1,1-diphenylethanol and
triphenylcarbinol. Experiments were carried out using the same
reactant molar ratio as in Table 3 (alcohol, ROH: 11.0 mmol;
molar ratio DMC : ROH = 20 : 1), though different catalyst load-
ings (molar % of catalyst relative to ROH was from 1 to 10), and
reaction temperatures (90–240 °C) were considered. Under such
conditions, primary and secondary substrates allowed highly
selective mono-transesterification processes, while tertiary alco-
hols preferentially underwent O-methylation or elimination reac-
tions. The structures of the corresponding reaction products were
Transesterification of diethyl carbonate (DEC) with different
alcohols
In the presence of the salt 1c, the reactions of diethyl carbonate
with secondary alcohols such as cyclohexanol, cyclopentanol
and menthol were also investigated. Conditions were similar to
those of Table 4. The same reactant molar ratio was used
(alcohol, ROH: 11.0 mmol; molar ratio DEC : ROH = 20 : 1);
though different catalyst loadings (molar % of 1c relative to
ROH was from 1 to 5), and reaction temperatures (200–240 °C)
were considered. Under such conditions, all reactions proceeded
with the exclusive formation of the corresponding mono-trans-
esterification products (ROCO2Et). The structures of such com-
1
assigned by H and 13C NMR, MS spectra, and by comparison
to authentic commercial samples. The results are reported in
Scheme 7 and Table 4.
A rather clear trend emerged indicating that primary alcohols
underwent more rapid transesterifications than secondary ones,
which, in turn, were far more reactive than tertiary substrates.
The relative reactivity of different alcohols was reflected by the
conditions required for the experiments.
1
pounds were assigned by H and 13C NMR, and MS spectra.
The reaction of DMC with benzyl alcohol took place using
conventional laboratory glassware at 90 °C: after 6 h, the conver-
sion was complete with 97% of the mono-transesterification
product being generated (PhCH2OCO2Me: benzyl methyl car-
bonate, entry 1). When the temperature was increased to 110 °C
The results are reported in Table 5.
At 200 °C, in the presence of 1 mol% of 1c, the transesterifi-
cation of DEC with cyclohexanol was considerably slower than
the corresponding reaction with DMC (compare entries 1 and 3
of Tables 5 and 3, respectively).
6572 | Org. Biomol. Chem., 2012, 10, 6569–6578
This journal is © The Royal Society of Chemistry 2012

Scheme 7
Table 4 The reaction of DMC with different alcohols catalysed by salt 1c
Major productsc (%, GC)
Entry
1
Alcohol (ROH)
T (°C)
t (h)
1c (mol%)a
Conv.b (%, GC)
ROCO2Me
97
Ph2CvCH2
ROMe
Yd (%)
90
6
1
100
95
2
3
150
200
6
2
1
1
29
100
29
100
96
92
4
220
6
1
95
95
5
6
220
220
20
20
10
10
100
40
95
5
89
40
—
a Molar percentage of catalyst relative to alcohol. b Conv.: conversion of alcohol (ROH) determined by GC. c Major reaction products detected by GC.
d Y: Isolated yield of products of transesterification (ROCO2Me), O-methylation (ROMe), or elimination (Ph2CvCH2). Products were purified by FCC
on silica-gel.
Discussion
The role of ionic liquids 1a–1d as transesterification catalysts
The experiments of Tables 1 and 3 leave few doubts about the
superior catalytic activity of salts 1a–1d (with respect to conven-
tional organic and inorganic bases) for the model transesterifica-
tion of DMC with cyclohexanol. The reaction takes place
preferably at high temperatures (>150 °C). Under such con-
ditions, the contribution of a thermally induced process cannot
Scheme 8
Even when the temperature was increased to 220 °C, the time
for complete conversion was 15 h (entry 2), approximately 5
times longer than that required for DMC. Additional experiments
were then carried out with a higher catalyst loading of 5 mol%:
at 220 °C, this allowed acceleration of the reactions of both
cyclohexanol and cyclopentanol, which were complete in 10 and
5 h, respectively, with excellent isolated yields (>90%) of trans-
esterification products (entries 3 and 4). Harsher conditions were
necessary for menthol: after 18 h at 240 °C, a 72% conversion
was observed (entry 5).
be ruled out, but the evidence for the catalytic role of the chosen
ionic liquids is incontrovertible, especially considering that they
are used in amounts as low as 1% mol (Tables 2 and 4). The
comparison of compounds 1a–1d to other base catalysts offers
another remarkable aspect. At T > 180 °C, both solid bases
(K2CO3, MgO, Al2O3) and zeolites efficiently activate the reac-
tions of dialkyl carbonates with several nucleophiles;9 these
transformations, however, often proceed accompanied by exten-
sive decarboxylations of DAlCs themselves (ROCO2R; R = Me,
Et, Pr, and PhCH2), to produce the corresponding ethers (ROR)
and CO2.15 By contrast, in spite of the high reaction temperature,
transesterifications catalysed by compounds 1a–1d do not gene-
rate appreciable amounts of CO2.16 Not only are the investigated
Notwithstanding the relatively high temperature and long reac-
tion times needed for reactions of DEC, no traces of decompo-
sition products of the catalyst 1c were detected.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6569–6578 | 6573

Table 5 The reaction of diethyl carbonate with different alcohols in the presence of catalyst 1c
Entry
ROH
T (°C)
t (h)
1c (mol%)a
Conv.b (%, GC)
Productc (ROCO2Et, %GC)
Yd (%)
1
2
3
4
200
220
220
220
3
15
10
6
1
1
5
5
6
97
>99
>99
6
97
>99
>99
94
92
5
240
18
5
72
72
65
a Molar percentage of 1c relative to alcohol. b Conv.: conversion of the alcohol (ROH) determined by GC. c Amounts (% by GC) of the
transesterification products ROCO2Et. d Y: Isolated yield of ROCO2Et after purification by FCC on silica-gel.
ILs immune to decarboxylation side-processes, but they allow
the overall reaction sustainability to be improved since uncon-
verted dialkyl carbonates (DMC and DEC) can be fully recov-
ered and reused.17
complex mix of charge-to-charge interactions. This is a plausible
reason why the catalytic performance of a single IL or the
activity scale among different ILs is often difficult to predict.
Salts 1a–1d share the same cation structure and they are all
exchanged with weakly basic anions (methyl carbonate, bicar-
bonate, acetate and phenolate, respectively).
The reaction of different alcohols and carbonates
The cooperative catalysis effect also offers a basis to discuss the
The dissociation constants in water (pKa) of the corresponding
reaction
selectivity.
Mono-transesterification
products
acid precursors are:10,18
(ROCO2R′, Tables 1–5) are more hindered than DMC or DEC:
therefore, they (ROCO2R′) cannot undergo an efficient electro-
philic activation since the corresponding acid–base adducts of
Scheme 9 are disfavoured for steric reasons. The same holds true
for nucleophilic activation. In our case, the progressive crowding
at the OH group explains the observed reactivity of alcohols
(BnOH ≫ CpOH > CyOH > menthol, Tables 4 and 5), particu-
larly of menthol,22 which requires more severe conditions.
The effect of neighbouring groups becomes so relevant for ter-
tiary alcohols that the transesterification reactions is no longer
observed. In its place, other less sterically demanding transform-
ations take place: 1,1-diphenyl ethanol produces a highly conju-
gated alkene, 1,1-diphenyl ethylene, while triphenylcarbinol
preferentially attacks the methyl group of DMC. The same reac-
tions have been already observed at high temperature in the pres-
ence of faujasite catalysts;23 under such conditions, they have
been discussed according to the modes of interactions of alco-
hols and DMC on the solid catalytic surface.
However, the lack of reactivity of tertiary alcohols towards
transesterification is so common that t-BuOH, for example, is
often used as a co-solvent during the transesterification of bio-
diesel precursors.24
The analysis of Tables 4 and 5 also shows that, regardless of
the alcohol used, the reactions of DMC are always favoured over
those of DEC. This difference reflects the trend of relative reac-
tivity of DMC and DEC which has been observed in many com-
parative tests of the two dialkyl carbonates.1,2,9,15,25
19
4:76 ðAcOHÞ < 5:6 ðCH3OCO2HÞ < 6:4 ðH2CO3
< 9:89 ðPhOHÞ
Þ
If one compares such values to the activity trend observed for
the transesterification of DMC with cyclohexanol (Table 3: 1a
[P8881][CH3OCO2] < 1b [P8881][HOCO2] < 1d [P8881][ PhO] <
1c [P8881][CH3CO2]), then no correlation can be inferred. In fact,
the weakest base acetate salt (1c) is the most active catalyst, fol-
lowed by the phenolate salt (1d) which possesses the strongest
base anion. Then, bicarbonate and methyl carbonate follow. The
catalytic performance of salts 1a–1d does not depend only on
basicity. Rather, an ambiphilic (both nucleophilic and electrophi-
lic) catalysis appears more suitable to account for the results.
This is based on a recently proved concept for ionic liquids,20
which act as catalysts via ion pairs: anions and cations of ILs
may activate nucleophiles (as true nucleophiles or bases) and
electrophiles, respectively. According to this dual mechanism,
Scheme 9 has been formulated for the investigated transesterifi-
cation of dialkyl carbonates with alcohols. The model example
of catalyst 1c is considered. The strong phosphorus–oxygen
affinity favours the coordination of the P centre of the catalyst
(as a Lewis acid) to the basic carboxylic oxygen of the dialkyl
carbonate (top, right; in analogy with Henry and N-alkylation
reactions catalyzed by ILs11,20b). Another acid–base reaction
between the reactant alcohol and 1c likely accounts for the
nucleophilic activation (top, left): an alkoxide exchanged IL,
namely [MeP(octyl)3 RO−], forms. The existence of such salts
+
Conclusions
has been recently proved.21
Once both the electrophile and the nucleophile are triggered
by the catalyst, they may react according to the usual pattern of
nucleophilic acyl substitutions (NAcS) to yield the transesterifica-
tion product ROCO2R′ and restore the initial ionic liquid 1c.
Overall, a cooperative anion/cation effect goes through a
Ionic liquids 1a–1d based on carbonate, acetate and phenolate
exchanged phosphonium salts prove to be excellent organocata-
lysts for the transesterification of dimethyl and diethyl carbonate
with primary and secondary alcohols, to produce non-symmetri-
cal dialkyl carbonate with very good yields (>90%) and
6574 | Org. Biomol. Chem., 2012, 10, 6569–6578
This journal is © The Royal Society of Chemistry 2012

Scheme 9
selectivity (>99%). Reactions, especially for more hindered
using an acetonitrile flow of 0.050 mL min−1 (nebulizer
15.0 psi; dry gas 5.0 L min−1; dry temp 325 °C).
secondary substrates, are rather energy intensive and they are
conveniently carried out using a rather large excess (up to
20 mol) of dialkyl carbonate.
However, at 150–220 °C, appreciable reaction rates are
observed even operating with catalyst loadings as low as
1–5 mol%, and the surplus of light reactants (both DMC and
DEC) can be easily separated and recycled.
1H NMR spectra were recorded at 400 MHz, 13C NMR
spectra at 101 MHz, 31P NMR spectra at 162 MHz. Chemical
shifts were reported in δ values downfield from TMS; CDCl3
and DMSO-d6 were used as solvents. IR spectra were run on a
Perkin Elmer Spectrum-One FT-IR spectrophotometer.
The steric bulk of tertiary alcohols makes these compounds
not suitable for the transesterifications examined; however, they
follow other intriguing and selective pathways including
O-methylation reactions. Under the conditions investigated, two
aspects should be considered: (i) no trace of thermal degradation
of IL-catalysts was detected; (ii) in contrast to the behaviour
shown by several inorganic (solid) catalysts, neither the reactant
nor the product dialkyl carbonates were activated by ILs towards
decarboxylation side-processes. Overall, the investigated ILs
allow not only a robust and selective protocol, but also the com-
plete recycling of unconverted DMC or DEC.
The catalytic performance of compounds 1a–1d is not due to
a superbase nature; rather, thanks to their ionic structures, both
salts may display ambiphilic properties which operate synchro-
nously without interfering with each other. Accordingly, both
electrophilic and nucleophilic activation of the reactants can be
invoked under a cooperative catalysis mechanism.
Reaction procedures
Transesterification of DMC with cyclohexanol in the presence
of different catalysts at reflux. In a round bottomed flask
equipped with a reflux condenser, a mixture of cyclohexanol
(CyOH: 1.10 g, 11.0 mmol), DMC (19.82 g, 220 mmol) and the
appropriate amount of the selected catalyst (K2CO3, 15–152 mg;
DBU, 16.9–169 mg; DMAP, 13–133 mg; 1a, 54–542 mg; see
Table 1 for details) was set to react at reflux (∼90 °C, bp of
DMC) for 3.5 h.
Transesterification of DMC with cyclohexanol catalysed by
different phosphonium salts at high temperatures. A typical
procedure is described with the most active catalyst, compound
1c, and the substrates most used in this work, cyclohexanol and
DMC (Conditions of entry 3 in Table 3). A 120 mL stainless-
steel autoclave fitted with a pressure gauge and a thermocouple
for temperature control was charged with cyclohexanol (CyOH:
1.10 g, 11.0 mmol), DMC (19.82 g, 220 mmol) and methyl
trioctylphosphonium acetate (1c: 49 mg, 0,11 mmol, 1 mol%
with respect to CyOH). The autoclave was flushed with nitrogen,
closed, and then heated at 200 °C for 3 h with magnetic stirring.
The reactor was subsequently cooled to rt and vented. The
excess of DMC was removed by rotary evaporation. The GC/MS
analysis of the residual oily mixture (1.79 g) showed the pres-
ence of cyclohexyl methyl carbonate (93%) along with unreacted
cyclohexanol (7%). This mixture was purified by FCC on silica
gel (100 g) using a solution of ethyl acetate (EA) and petroleum
ether (PE) (EA–PE = 1 : 3 v/v; 400 mL). Rf of the reagent and
the product were 0.25 and 0.82, respectively. Cyclohexyl methyl
carbonate was obtained as a colourless liquid (1.56 g, 90%) with
Experimental
Materials
Dimethyl carbonate (DMC), cyclohexanol (CyOH), cyclopenta-
nol, benzyl alcohol, menthol, diphenylethanol, triphenylcarbinol,
phenol and acetic acid were ACS grade and were employed
without further purification. 1a and 1b were prepared accord-
ingly to a method recently reported by us.10,20b
GC/MS (EI, 70 eV) analyses were run using a HP5-MS capil-
lary column (30 m). Electrospray ionization mass spectra
(ESI-MS) were recorded on a quadrupole ion trap instrument
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6569–6578 | 6575

a GC-purity of 98%. (Further details on the product characteri-
zation are given later in this section, and in the ESI‡).
desired product 1c in the form of a viscous clear colourless
liquid (1.81 g, 100%). ESI-MS (CH3CN), positive ion
385 [C25H54P]. 1H NMR (400 MHz, CDCl3) δ 2.45–2.31
(m, 6H), 2.06 (d, J = 13.6 Hz, 3H), 1.95 (s, 3H), 1.57–1.40
(m, 12H), 1.36–1.17 (m, 24H), 0.87 (t, J = 6.7 Hz, 9H);
13C NMR (101 MHz, CDCl3) δ 176.5 (1C), 31.6 (3C), 30.6 (d,
J = 14.9 Hz, 3C), 28.9 (6C), 25.1 (1C), 22.5 (d, J(P,H) = 1.1 Hz,
3C), 21.7 (d, J(P,H) = 4.6 Hz, 3C), 20.0 (d, J(P,H) = 48.7 Hz,
3C), 14.0 (s, 3C), 4.2 (d, J(P,H) = 52.3 Hz, 1C); 31P NMR
(162 MHz, DMSO-d6) δ 30.0. Anal. Calcd for C27H57O2P:
C, 72.92; H, 12.92. Found: C, 72.89; H, 12.90%.
The same procedure was used to investigate the performance
of different ILs as catalysts. Accordingly, the above described
autoclave was charged with a mixture of cyclohexanol (CyOH:
1.10 g, 11.0 mmol), DMC (19.82 g, 220 mmol), and the desired
amount of phosphonium salt 1a (54 and 542 mg), or 1b
(49 mg), or 1d (53 mg). The mixture was set to react at different
temperatures (in the range of 150–200 °C) and times (0.5–6 h)
(see Tables 2 and 3 for details).
Transesterification of DMC with different alcohols catalysed
by 1c. The above described procedure for high temperature reac-
tions was also used for the transesterification of different alco-
hols with DMC. Accordingly, a 120 mL stainless-steel autoclave
was charged with a mixture of the selected alcohol (11.0 mmol:
cyclopentanol, 0.95 g; benzyl alcohol, 1.19 g; menthol, 1.72 g;
1,1-diphenylethanol, 2.18 g; triphenylmethanol, 2.86 g),
dimethyl carbonate (19.82 g, 220 mmol) and 1c (54 mg,
0.11 mmol). The mixture was set to react at different tempera-
tures (in the range of 150–220 °C) and times (2–20 h) (see
Table 4 for details).
The transesterification of DMC with benzyl alcohol was also
carried out at reflux (entry 1, Table 4). In this case, a mixture of
benzyl alcohol (11.0 mmol, 1.19 g), dimethyl carbonate
(19.82 g, 220 mmol) and 1c (54 mg, 0.11 mmol) was set to react
in a 50 mL round bottomed flask equipped with a reflux conden-
ser and a magnetic stirrer. The experiment was performed at
90 °C for 6 h. Further details on the purification and characteri-
zation of the product (benzyl methyl carbonate) are given later in
this section.
Methyl trioctylphosphonium phenate [P8881][PhO] 1d. A
50 mL round bottomed flask was charged with methyl trioctyl-
phosphonium methyl carbonate (1a; 2.00 g, 4.06 mmol). An
equimolar amount of phenol (0.382 g) was then added and the
resulting mixture was kept under magnetic stirring for 2 h at
50 °C. During this time forming methanol was continuously
removed by reduced pressure (150 mbar). Product 1d was
obtained as a brown oil (1.94 g, 100%). ESI-MS (CH3CN), posi-
1
tive ion 385 [C25H54P]. H NMR (400 MHz, DMSO-d6) δ 6.94
(dd, J = 8.5, 7.2 Hz, 2H), 6.58 (dd, J = 8.5, 1.1 Hz, 2H), 6.38
(tt, J = 7.3, 1.1 Hz, 1H), 2.21–2.07 (m, 6H), 1.77 (d, J =
14.0 Hz, 3H), 1.53–1.12 (m, 36H), 0.93–0.79 (m, 9H);
13C NMR (101 MHz, DMSO-d6) δ 163.3 (1C), 128.7 (2C), 116.7
(2C), 114.0 (1C), 31.2 (3C), 30.03 (d, J = 15.4 Hz, 3C), 28.4
(3C), 28.2 (3C), 22.1 (3C), 20.5 (d, J (P,H) = 4.3 Hz, 3C), 19.0
(d, J (P,H) = 48.9 Hz, 3C), 13.9 (3C), 3.08 (d, J (P,H) = 51.3 Hz,
1C); 31P NMR (162 MHz, DMSO-d6) δ 33.1. Anal. Calcd for
C31H59OP: C, 77.77; H, 12.42. Found: C, 77.81; H, 12.48%.
Isolation and characterization of the products
Transesterification of DEC with different alcohols catalysed
by 1c. The above described procedure for high temperature reac-
tions was used also for the transesterification of alcohols with
diethyl carbonate. Accordingly, a 120 mL stainless-steel auto-
clave was charged with a mixture of the selected alcohol
(11.0 mmol: cyclohexanol: 1.10 g; cyclopentanol, 0.95 g;
menthol, 1.72 g), diethyl carbonate (DEC, 25.99 g, 220 mmol),
and 1c (0.55 mmol; 5 mol%). The mixture was set to react at
different temperatures (in the range of 200–240 °C) and times
(3–18 h) (see Table 5). Further details on the purification and
characterization of the products (alkyl ethyl carbonates,
ROCO2Et) are given later on this section.
All mono-transesterification products (ROCO2Me and
ROCO2Et, respectively), and 1,1-diphenylethene were isolated
by FCC, and fully characterized by GC/MS, H and 13C NMR.
1
New compounds such as cyclopentyl methyl carbonate and
methyl menthyl carbonate were characterized also by IR and
elemental analysis (see below and ESI‡ section). In particular:
Cyclohexyl methyl carbonate.26 The product was isolated at
the end of five different reactions of cyclohexanol (1.10 g,
11.0 mmol) and DMC (19.82 g, 220 mmol) catalyzed by both
1a (entries 1, 2 of Tables 1 and 4, 6 of Table 2) and 1c (entry 3
of Table 3). In all cases, the final reaction mixtures were purified
by FCC: according to the above described method (see typical
procedure), silica gel and an eluent solution of ethyl acetate (EA)
and petroleum ether (PE), EA–PE = 1 : 3 v/v) were used. The
title compound was a colourless liquid (Y: 93, 90, 90, 92, and
90% respectively, Tables 1–3; GC-purity > 97%). 1H NMR
(400 MHz, CDCl3) δ 4.67–4.54 (m, 1H), 3.76 (s, 3H),
1.97–1.85 (m, 2H), 1.80–1.67 (m, 2H), 1.60–1.18 (m, 6H);
13C NMR (101 MHz, CDCl3) δ 155.4, 76.8, 54.5, 54.5, 31.6,
25.3, 23.7.
Synthesis of anion exchanged phosphonium salts
Methyl trioctylphosphonium acetate [P8881][AcO] 1c and methyl
trioctylphosphonium phenate [P8881][PhO] 1d were prepared by
anion exchange reactions of methyl trioctylphosphonium methyl
carbonate 1a. Both salts 1c and 1d were new compounds and
1
they were characterized by H-, 13C-, 31P-NMR, ESI-MS, and
elemental analysis (further details are in the ESI section‡).
Methyl trioctylphosphonium acetate [P8881][AcO] 1c. A
50 mL round bottomed flask was charged with an equimolar
mixture of methyl trioctylphosphonium methyl carbonate (1a;
2.00 g, 4.06 mmol) and acetic acid (0.244 g). The mixture was
kept under magnetic stirring for 2 h at 40 °C. Then, volatile
materials were removed by rotary evaporation yielding the
Benzyl methyl carbonate.27 The product was isolated from
the reaction of benzyl alcohol (1.19 g, 11 mmol) and DMC
(19.82 g, 220 mmol) carried out under the conditions of entry 1
of Table 4. The final mixture was purified by FCC: according to
the above described method (see typical procedure), silica gel
6576 | Org. Biomol. Chem., 2012, 10, 6569–6578
This journal is © The Royal Society of Chemistry 2012

and an eluent solution of ethyl acetate (EA) and petroleum ether
(PE) (EA–PE = 1 : 3 v/v) were used. The title compound was iso-
lated as a pale yellow liquid (Y: 1.73 g, 95%; GC-purity 98%).
1H NMR (400 MHz, CDCl3) δ 7.47–7.31 (m, 5H), 5.17 (s,
J = 3.3 Hz, 2H), 3.80 (s, 3H). 13C NMR (101 MHz, CDCl3)
δ 155.9, 135.4, 128.72, 128.66, 128.4, 69.8, 55.0.
(400 MHz, CDCl3) δ 4.64–4.55 (m, 1H), 4.17 (q, J = 7.1 Hz,
2H), 1.96–1.87 (m, 2H), 1.80–1.69 (m, 2H), 1.60–1.17 (m, 6H),
1.30 (t, J = 7.1 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 154.8,
77.3, 76.6, 63.6, 31.7, 25.4, 23.8, 14.4.
Cyclopentyl ethyl carbonate.31 The product was isolated from
the reaction of cyclopentanol (0.95 g, 11 mmol) and DEC
(26.0 g, 220 mmol) carried out under the conditions of entry 4
of Table 5. The final mixture was purified by FCC: according to
the above described method (see typical procedure), silica gel
and an eluent solution of ethyl acetate (EA) and petroleum ether
(PE) (EA–PE = 1 : 3 v/v) were used. The title compound was iso-
lated as a pale yellow liquid (Y: 1.60 g, 92%; GC-purity 98%).
1H NMR (400 MHz, CDCl3) δ 5.08–5.01 (m, 1H), 4.16 (t,
J = 7.1 Hz, 2H), 1.89–1.48 (m, 8H), 1.27 (t, J = 7.1 Hz, 3H).
13C NMR (101 MHz, CDCl3) δ 155.0, 80.9, 63.7, 32.7, 23.6,
14.4.
Cyclopentyl methyl carbonate. The product was isolated from
the reaction of cyclopentanol (0.95 g, 11 mmol) and DMC
(21.2 mL, 220 mmol) carried out under the conditions of entry 3
of Table 4. The final mixture was purified by FCC: according to
the above described method (see typical procedure), silica gel
and an eluent solution of ethyl acetate (EA) and petroleum ether
(PE) (EA–PE = 1 : 3 v/v) were used. The title compound was iso-
lated as a colourless liquid (Y: 1.52 g, 96%; GC-purity 97%).
1H NMR (400 MHz, CDCl3) δ 5.10–5.03 (m, 1H), 3.74 (s, 3H),
1.92–1.66 (m, 6H), 1.65–1.49 (m, 2H); 13C NMR (101 MHz,
CDCL3) δ 155.5, 81.1, 54.4, 32.6, 23.5. IR (Neat): ν = 2961,
2875, 1747 cm−1. Anal. Calcd for C7H12O3: C, 58.32; H, 8.39.
Found: C, 58.38; H, 8.45%.
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl ethyl carbonate.26
(ethyl menthyl carbonate).32 The product was isolated from the
reaction menthol (1.71 g, 11 mmol) and DEC (26.0 g,
220 mmol) carried out under the conditions of entry 5 of
Table 5. The final mixture was purified by FCC: according to the
above described method (see typical procedure), silica gel and
an eluent solution of ethyl acetate (EA) and petroleum ether (PE)
(EA–PE = 1 : 3 v/v) were used. The title compound was isolated
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl methyl carbonate
(methyl menthyl carbonate).28 The product was isolated from
the reaction of menthol (1.71 g, 11 mmol) and DMC (19.82 g,
220 mmol) carried out under the conditions of entry 4 of
Table 4. The final mixture was purified by FCC: according to the
above described method (see typical procedure), silica gel and
an eluent solution of ethyl acetate (EA) and petroleum ether (PE)
(EA–PE = 1 : 3 v/v) were used. The title compound was isolated
as a white low melting (mp < 30 °C) solid (Y: 2.17 g, 92%,
1
as a yellow liquid (Y: 1.63 g, 65%; GC-purity 95%). H NMR
(400 MHz, CDCl3) δ 4.50 (td, J = 10.9, 4.4 Hz, 1H), δ
4.24–4.08 (m, 2H), 2.11–2.02 (m, 1H), 2.01–1.90 (m, 1H),
1.71–1.62 (m, 2H), 1.60–1.44 (m, 2H), 1.40–1.33 (m, 1H), 1.29
(t, J = 7.1 Hz, 3H), 1.11–0.96 (m, 2H), 0.90 (d, J = 6.8 Hz, 3H),
0.89 (d, J = 7.2 Hz, 3H), 0.78 (d, J = 7.0 Hz, 3H). 13C NMR
(101 MHz, CDCl3) δ 155.0, 78.2, 63.7, 47.2, 40.9, 34.3, 31.5,
26.2, 23.4, 22.1, 20.9, 16.4, 14.4.
1
GC-purity 97%). H NMR (400 MHz, CDCl3) δ 4.51 (td, J =
10.9, 4.4 Hz, 1H), 3.77 (s, 3H), 2.11–2.04 (m, 1H), 1.96 (dtd,
J = 13.8, 6.9, 2.5 Hz, 1H), 1.72–1.63 (m, 2H), 1.56–1.34 (m,
2H), 1.12–0.98 (m, 2H), 0.91 (d, J = 6.9 Hz, 3H), 0.90 (d, J =
7.3 Hz, 3H), 0.79 (d, J = 7.0 Hz, 3H); 13C NMR (101 MHz,
CDCl3) δ 155.7, 78.6, 54.6, 47.2, 40.9, 34.3, 31.5, 26.2, 23.5,
22.1, 20.8, 16.4. IR (Neat): ν = 2958, 2872, 1747 cm−1. Anal.
Calcd for C12H22O3: C, 67.26; H, 10.35. Found: C, 67.30;
H, 10.42%.
Acknowledgements
MIUR (Italian Ministry of University and Research, Research
Grant PRIN 2008) is gratefully acknowledged for financial
support.
1,1-Diphenylethene.29 The product was isolated from the reac-
tion of 1,1-diphenylethanol (2.18 g, 11 mmol) DMC (19.82 g,
220 mmol) carried out under the conditions of entry 5 of
Table 4. The final mixture was purified by FCC: according to the
above described method (see typical procedure), silica gel and
an eluent solution of petroleum ether (PE) were used. The title
compound was isolated as a colourless oil (Y: 1.76 g, 89%;
GC-purity 99%) that turned to a white solid once refrigerated at
+4 °C. 1H NMR (400 MHz, CDCl3) δ 7.65–7.33 (m, 10H), 5.64
(s, 2H). 13C NMR (101 MHz, CDCl3) δ 150.2, 141.6, 128.4,
128.3, 127.8, 114.3.
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