Selective catalytic etherification of glycerol formal and solketal with dialkyl carbonates and K2CO3

Article abstract of DOI:10.1039/c1gc15796e

At T ≥ 200 °C, in the presence of K₂CO₃ as a catalyst, an original etherification procedure of non-toxic acetals such as glycerol formal (GlyF) and solketal has been investigated by using dialkyl carbonates as safe alkylating agents. The effects of parameters including the temperature, the reaction time, and the loading of both the catalyst and the dialkyl carbonate have been detailed for the model case of dimethyl carbonate (DMC). Both GlyF and solketal were efficiently alkylated by DMC to produce the corresponding O-methyl ethers with selectivity up to 99% and excellent yields (86-99%, by GC). The high selectivity could be accounted for by a mechanistic study involving a combined sequence of methylation, carboxymethylation, decarboxylation and hydrolysis processes. The O-methylation of GlyF and solketal could be successfully scaled up for multigram synthesis even operating with a moderate excess (5 molar equiv.) of DMC and in the absence of additional solvent. Notwithstanding the advantageous reduction of the process mass index, scale up experiments provided evidence that prolonged reaction times may induce the decomposition of DMC mainly by the loss of CO₂. The K ₂CO₃-catalyzed etherification of solketal with other carbonates such as dibenzyl and diethyl carbonate (DBnC and DEC, respectively), proceeded with the same good selectivity observed for DMC. However, at 220 °C, the solketal conversion did not exceed 81% since both DBnC and DEC were extensively consumed in competitive decarboxylation and hydrolysis reactions.

Full text of DOI:10.1039/c1gc15796e

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PAPER
Selective catalytic etherification of glycerol formal and solketal with dialkyl
carbonates and K2CO3†
Maurizio Selva,* Vanni Benedet and Massimo Fabris
Received 5th July 2011, Accepted 7th October 2011
DOI: 10.1039/c1gc15796e
At T ≥ 200 ◦C, in the presence of K2CO3 as a catalyst, an original etherification procedure of
non-toxic acetals such as glycerol formal (GlyF) and solketal has been investigated by using
dialkyl carbonates as safe alkylating agents. The effects of parameters including the temperature,
the reaction time, and the loading of both the catalyst and the dialkyl carbonate have been
detailed for the model case of dimethyl carbonate (DMC). Both GlyF and solketal were efficiently
alkylated by DMC to produce the corresponding O-methyl ethers with selectivity up to 99% and
excellent yields (86–99%, by GC). The high selectivity could be accounted for by a mechanistic
study involving a combined sequence of methylation, carboxymethylation, decarboxylation and
hydrolysis processes. The O-methylation of GlyF and solketal could be successfully scaled up for
multigram synthesis even operating with a moderate excess (5 molar equiv.) of DMC and in the
absence of additional solvent. Notwithstanding the advantageous reduction of the process mass
index, scale up experiments provided evidence that prolonged reaction times may induce the
decomposition of DMC mainly by the loss of CO2. The K2CO3-catalyzed etherification of solketal
with other carbonates such as dibenzyl and diethyl carbonate (DBnC and DEC, respectively),
proceeded with the same good selectivity observed for DMC. However, at 220 ◦C, the solketal
conversion did not exceed 81% since both DBnC and DEC were extensively consumed in
competitive decarboxylation and hydrolysis reactions.
particularly light compounds such as glycerol formal (GlyF) and
solketal (1a–b and 1c, respectively) recently caught our attention
Introduction
Glycerol is a non-toxic biodegradable compound whose rel-
(Scheme 1). Compounds 1 are non-toxic substances synthesized
evance as a chemical is recognized worldwide in a number
by acid-catalysed condensations of glycerol with formaldehyde
of applications for the pharma, polymer, and food sectors.1
and acetone.4 GlyF is commercially available as a mixture of
Not by chance, in 2004, the National Laboratories of the US
two 5- and 6-membered ring isomers (1a and 1b in a 2 : 3 molar
Department of Energy, have classified glycerol among the twelve
ratio, respectively), while solketal is obtained exclusively as a
most attractive platform chemicals that can be produced from
five-membered ring product.5
biomass via biological or chemical conversions.2 However, due
to the exponential increase of biodiesel production and demand,
the global market is experiencing a massive surplus of glycerol:
in the EU for example, some additional 1.0 ¥ 106 tonnes of such
a compound have been available in 2010 (almost tripled since
2005).3
These considerations have fuelled a growing interest of both
academic and industrial communities for intensive research
programs aimed at the conversion of glycerol and its derivatives
into energy and chemicals. In this context, glycerol acetals,
Scheme 1 Light glycerol acetals.
Dipartimento di Scienze Molecolari e Nanosistemi dell’Universita` Ca’
Foscari Venezia, Calle Larga S. Marta, 2137-30123, Venezia, Italy.
As such, both acetals are mainly used in the field of solvents
E-mail: selva@unive.it; Fax: +39 041 234 8958; Tel: +39 041 234 8687
for injectable pharmaceutical preparations, paints and water-
† Electronic supplementary information (ESI) available: Synthesis of
based inks,4c,6 and for the preparation of additives for biodiesel
dibenzyl carbonate (DBnC); GC calibration curves; MS spectra of
componds 6c, 7c, 2a + 2b and 3a + b. See DOI: 10.1039/c1gc15796e
formulations.3a,5d,7 GlyF and solketal however, are also valuable
188 | Green Chem., 2012, 14, 188–200
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glycerol synthons whose reactivity involves almost exclusively
the conversion of their free hydroxyl functionality by conven-
tional esterification and etherification agents (carboxylic acids
and alkyl halides, respectively).8,9 This aspect prompted us to
investigate whether eco-friendly protocols could be devised for
such processes, especially for the synthesis of O-alkyl derivatives
of both 1a–b and 1c. To this purpose, dialkyl carbonates
(DAlCs) appeared attractive compounds since they offer genuine
green advantages over classical alkylating reagents:10 i) they are
non-toxic compounds; ii) their alkylation mediated reactions
are always catalytic processes, with no by-products except for
alcohols recyclable to the synthesis of DAlCs, and CO2 which
does not involve disposal problems; iii) very often, (light) organic
carbonates can be used not only as reactants, but also as
reaction solvents. Moreover, their alkylation reactivity can be
tuned for a variety of nucleophiles. This behaviour has been
extensively investigated by our research group in the past fifteen
years.11 Consider, for example, the model case of base-catalysed
methylations mediated by dimethyl carbonate (Scheme 2).
An in-depth investigation has been carried out to analyze the
course of the overall transformation. Beyond the O-methylation
process which affords methyl ethers 2 in yields up to 90%,
results provide evidence that also the transesterification of
acetals 1a–b and 1c with DMC takes place, producing the
corresponding carboxymethyl derivatives (Scheme 2, path b).
These compounds however, disappear as the reaction proceeds:
both the reversible nature of the transesterification and the
occurrence of competitive processes of decarboxylation and
hydrolysis of carboxymethyl products account for this result.
Accordingly, a mechanistic hypothesis has been formulated.
The same mechanism plausibly holds true for higher ho-
mologues of DMC: both diethyl- and dibenzyl- carbonate
react with solketal to yield the related carboxyalkyl and O-
alkyl derivatives, the latter compounds being the final reaction
products. Although an excellent alkylation selectivity (up to
100%) can be achieved, these transformations appear somewhat
limited by the consumption of the starting dialkyl carbonates
via concurrent decarboxylation and hydrolysis processes.
Overall, the catalytic O-alkylation of renewable acetals of
glycerol with safe dialkyl carbonates, particularly DMC, not
only represents a genuinely green procedure, but it unlocks a
potential for preparative purposes: this study demonstrates that
the reaction can be scaled up for multigram synthesis without
any loss of selectivity.
Results
Scheme 2
Since previous studies on base catalysed reactions of GlyF and
solketal with dialkyl carbonates were not available, a full set
of experiments was necessary to examine the feasibility of the
process. Our experience on the alkylation reactivity of DMC
steered us on the choice of reaction conditions, particularly
on the range of major parameters such as catalyst amount,
temperature, time, and reactants molar ratios.
At 150–200 ◦C, in the presence of solid alkaline carbonates as
catalysts, the reaction of CH2-active compounds, phenols, and
thiols (NuH) with DMC proceeds with an exceptionally high
selectivity (>99%) towards the corresponding methyl derivatives
(NuMe, path a).
Under these conditions, a different scenario is offered by alco-
hols: plausibly due to their hard nucleophilic character, a number
of primary and secondary alcohols including benzyl substrates,
react with DMC to produce predominantly the corresponding
transesterification products (path b).11h,12 Notwithstanding the
alcohol-like reactivity expected for glycerol acetals, this paper
demonstrates that reaction conditions can be found for the
selective etherification of the hydroxyl group of both GlyF
and solketal with DMC. In particular, this unprecedented
reaction takes place at 200–220 ◦C, in the presence of potassium
carbonate as a catalyst (Scheme 3).
The reaction of glycerol formal with DMC in the presence of
K2CO3
Initial experiments were carried out in a stainless steel autoclave
(200 mL) charged with a mixture of commercial glycerol formal
(isomers 1a–1b in a 2 : 3 ratio; 0.80 g; 7.7 mmol), dimethyl
carbonate (DMC, 19 mL, molar ratio W = DMC : GlyF =
30) and K2CO3 (molar ratio Q = K2CO3 : GlyF = 1.2). DMC
was used in a large excess, acting simultaneously as a reagent
and a solvent.◦The autoclave was purged with N2, electrically
heated at 200 C, and kept under magnetic stirring. Reactions
were followed for different time intervals of 6, 10, 20 and 24
h, respectively. At the end of each experiment, the reactor
was cooled to rt and vented, and the reaction mixture was
analyzed by GC-MS. In all cases, major products (≥95%) derived
from O-methylation and carboxymethylation reactions of GlyF
(compounds 2a–2b and 3a–3b, respectively; Scheme 4).
Products 2 and 3 were isolated and fully characterized by GC-
1
MS, H and 13C NMR (further details are in the experimental
section). The structure of isomers 3a–3b was also confirmed
by comparison to authentic samples. Other unidentified by-
products (£5% in total, by GC), were also detected. The results
Scheme 3
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ring analogues (O-methyl and carboxymethyl derivatives 2a and
3a). After 24 h, the amount of 2b was doubled than 2a (24 and
48%, respectively); while, compound 3b and 3a were present in
24 and 3% amounts, respectively.
Once the last experiment (24 h reaction) of Fig. 1 was con-
cluded, K2CO3 was filtered and dried under vacuum overnight
(8 mbar, 100 ◦C). The solid was finely powdered and reused to
catalyze the reaction of GlyF and DMC under the conditions
of Fig. 1 (24 h). The recycle of K2CO3 was repeated once more.
GC-MS analyses of experiments with both fresh and recycled
catalyst showed conversions and product distributions that did
not differ by more than 5% from each other. This confirmed
that K2CO3 was a recyclable catalyst. The result matched the
behaviour already reported for DMC mediated carboxylations
and methylations of different nucleophiles.14
Scheme 4 Major products of the K2CO3-catalysed reaction of glycerol
formal with DMC.
are shown in Fig. 1a–b where the amount (%, by GC) of both
reagents and products is plotted vs. the reaction time.13
Two additional experiments were carried out in the absence
of K2CO3. A mixture of GlyF and DMC was set to react for
10 and 24 h under the conditions of Fig. 1. The conversions of
GlyF were 60 and 96%, respectively, but only a transesterification
reaction took place yielding carboxymethylated compounds
(3a+3b) as exclusive products. This in line with literature results
for the thermal (non-catalytic) transesterifications of esters and
organic carbonates which were reported at T > 170 ◦C.15
Alkali metal exchanged faujasites could also act as efficient
catalysts for O-alkylation of alcohols with dialkyl carbonates.11
However, attempts to use faujasites in the reaction of GlyF
and solketal with DMC, were not successful. For example, at
200 ◦C, the conversion of GlyF was complete after only 3 h on
NaX (weight ratio zeolite : GlyF = 1), but the formation of O-
methyl derivatives 2a+2b (72%, by GC) was accompanied by the
aperture of the cyclic structures of both GlyF and products 2,
which yielded an extensive amount of by-products (28%). This
competitive reaction likely followed the general mechanism of
acetal methanolysis,16 where Brønsted acidity on solid faujasites
was the catalyst.17
Catalyst amount. Under the conditions of Fig. 1 (200 ◦C,
molar ratio DMC : GlyF = 30), the reaction of GlyF and DMC
was carried out using different molar ratios K2CO3 : GlyF (Q) of
0.5, 2, and 3, respectively. Experiments were run for 10 h. Fig. 2
shows the composition (%, by GC) of the reaction mixtures and
Fig. 1 The reaction of GlyF with DMC carried out at 200 ◦C: (a) total
amounts of each couple of isomers (1a+1b, 2a+2b, 3a+3b, respectively).
“Other” were unidentified by-products; (b) amounts of each single
isomer of methyl (2a–2b) and carboxymethyl (3a–3b) products.
Four main aspects emerged from Fig. 1a: i) after 10 h,
the reaction conversion was substantially quantitative (97%);
ii) the amount of O-methylation products (2a+2b) increased
progressively with time, with a methylation selectivity up to
73% after 24 h; iii) the amount of carboxymethylation products
(3a+3b) went up to a maximum in the first 6 h, and then,
it dropped to 27% as the reaction proceeded further; iv) by-
products were negligible. Fig. 1b showed that six-membered ring
products (O-methyl and carboxymethyl derivatives 2b and 3b)
formed andreacted, respectively, faster thantheir five-membered
Fig. 2 Effect of the K2CO3 amount in the reaction of GlyF with DMC.
Q is the molar ratio K2CO3 : GlyF.
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the O-methylation selectivity. The figure also includes the results
obtained with Q = 1.2 (Fig. 2).
The conversion of GlyF remained above 94%, even at a Q ratio
of 0.5, indicating that the reaction was a truly catalytic process.
The base amount however, affected the distribution of O-methyl
and carboxymethyl derivatives: as the Q ratio increased (from 0.5
to 3.0), the quantity of products (2a+2b) and (3a+3b) steadily
increased and decreased, respectively, from 39 to 57% (light gray
bar) and from 55 to 39% (gray bar). Accordingly, the O-methyl
selectivity went up from 41 to 60% (black bar). Although the
relative increments/decrements of compounds 2 and 3 tended
to level off at high catalyst loadings (2.0 £ Q £ 3.0), the trend
agreed with that of Fig. 1: as the reaction proceeded, O-alkyl
derivatives were the ultimate products, while carboxymethyl
species progressively decreased.
Fig. 4 Effect of DMC loading in the reaction of GlyF and DMC.
Temperature and reaction time. The effect of the temperature
was examined in three subsequent reactions of GlyF with DMC
carried out at 200, 210 and 220 ◦C. Other conditions where
those of Fig. 2 (molar ratio DMC : GlyF = 30; molar ratio
K2CO3 : GlyF = 1.2); though, a longer reaction time of 20 h
was chosen to possibly maximise the formation of products 2.
Results are reported in Fig. 3.
alkylation of GlyF was apparently disfavoured by a larger excess
of DMC.
Mass balance and scale up of the O-methylation of GlyF.
A mixture of commercial glycerol formal (0.80 g; 7.7 mmol),
dimethyl carbonate (DMC, 19 mL, molar ratio
W =
DMC : GlyF = 30), K2CO3 (molar ratio Q = K2CO3:GlyF = 1.2)
was set to react in the presence of n-dodecane (C12; 0.175 mL,
0.131 g, molar ratio, Z, GlyF : C12 = 10) as an internal standard.
Conditions were those of Fig. 3 (220 ◦C, 20 h). The analysis of the
final reaction mixture gave conversion and selectivity towards
O-methyl derivatives (2a+2b) of 98% and 95%, respectively. The
calculated yield of (2a+2b) was 95%. No significant mass loss
took place during the reaction.
The reaction of GlyF with DMC was then scaled up. At
220 ◦C, two experiments (A and B) were performed multiplying
by a factor of 5 the amounts of reagents and catalyst. In the
first reaction (A), conditions were those of Fig. 3 (W = 30
and Q = 1.2), while in the second test (B), the DMC volume
was significantly decreased to reach a W ratio (molar ratio
DMC : GlyF) of 5. Accordingly, a mixture of glycerol formal
(4.0 g, 38 mmol) and K2CO3 (6.3 g, 46 mmol) was set to react
with 96 and 17 mL of DMC, respectively (A and B).
Fig. 3 Reactions of GlyF with DMC at different temperatures.
Under the conditions of experiment (B), a further reaction
was carried out in the presence of n-dodecane (0.91 mL, molar
ratio GlyF : C12 = 10) as the internal standard. This experiment
was repeated three times: calculated conversions and product
distributions did not differ more than 5% from one test to
another. Also, the yield of O-methyl products 2a+2b was
determined. Results are summarized in Table 1.
The reaction proved to be feasible on a multigram scale. In
analogy to results of Fig. 1b, C6 isomers of compounds 2 and
3 were more reactive than the corresponding C5 isomers: for
example, after 26 h at 220 ◦C, the amounts of 2a and 2b were 20
and 41% respectively, while those of 3a and 3b were 27 and 5%
(conditions of entry 1, Table 1). However, once the reaction was
complete, O-methyl derivatives (2a+2b) were always obtained
with a high selectivity (up to 97%) and an excellent yield (99%,
by GC).
The increase of the temperature induced a remarkable im-
provement of the O-methylation selectivity (up to 95%, black
bar): from 200 to 220 ◦C, the amount of 2a+2b (light gray bar)
increased progressively from 68 to 96%, while carboxymethyl
derivatives (3a+3b, gray bar) decreased from 30% to 2%. Further
to that, high reaction temperatures did not favour other by-
products (£3% in total). The conversion was substantially
quantitative in all cases.
Reactants molar ratio. The reaction of GlyF and DMC was
explored using different molar ratios DMC : GlyF (W) of 20, 30,
and 40 respectively. According to Fig. 3, the temperature was set
to 220 ◦C. However, experiments were run for 10 h to appreciate
possible effects on the product distribution. The molar ratio
K2CO3 : GlyF was 1.2. Fig. 4 reports the composition (%, by
GC) of reaction mixtures and the corresponding O-methylation
selectivity.
The alkylation rate was considerably altered by the DMC
volume: reaction times were of 52 and 20 h at W = 30 and 5,
respectively (entries 1 and 2–3). Apparently, a lower amount of
DMC (W = 5) had not only the positive effect of reducing the
Surprisingly, the increase of the W ratio from 20 to 40 brought
about a remarkable decrease of the selectivity from 98 to 80%
(black bar) due to unconverted carboxymethyl derivatives 3: the
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Table 1 Scale up of the reaction of GlyF and DMC
Yield (2,%)c
(by GC)
Entry
Wa (mol : mol)
t (h)
ISb (n-C12)
Conv. (%)c
Selectivity (%)c
Isolatedd
1
2
3
30
5
5
52
20
20
None
None
Yes
99
98
99
94
96
97
65
64
67
99
a Molar ratio DMC : GlyF. b IS: internal standard (n-dodecane). c Conversion of GlyF and selectivity/yield of O-methyl products 2a+2b. Entry 3:
values of both conversion, selectivity and yield were averaged over three different runs and they were calculated based on GC calibration curves.
d Isolated yield of distilled products 2a+2b.
mass index of the process, but it allowed an even faster reaction
thereby confirming the trend of Fig. 4.
exceeding 2%, by GC), indicating that the starting carbonate
was also subjected to hydrolysis.20
Final mixtures from experiments of entries 1–3 of Table 1,
were filtered and O-methylation products 2 were distilled under
vacuum (further details are in the experimental section). Pure
(≥96%) liquid mixtures of isomers 2a and 2b were obtained,
though isolated yields did not exceed 67%. Technical limitations
of the available distillation apparatus always brought to middle
fractions containing the work-up solvent, DMC, and not
negligible amounts of products 2.
Then, the reactivity of isomers 3a–3b was examined. Com-
pounds 3 were prepared from GlyF and DMC according to
a conventional method for base-catalysed transesterification
reactions.21 The products were isolated as a mixture of 3a and
3b (in the same 2 : 3 ratio of the starting isomers of GlyF) in a
79% yield. They were fully characterized by 1H and 13C NMR,
and GC-MS.
The reaction of 3a–3b was performed under the conditions
of previous experiments (Fig. 1–4), by replacing DMC with
dimethoxyethane (glyme) as the reaction solvent.22 In particular,
a mixture of 3a–3b (1.25 g, 7.7 mmol), K2CO3 (1.28 g, molar ratio
Q1 = K2CO3 : 3a–3b = 1.2) and glyme (19 mL) was set to react at
220 ◦C, in an autoclave, for 10 h. Once cooled to rt, a residual
pressure up to 3 bar inside the reactor, indicated that gaseous
products were originated. These were recovered and analysed by
GC-MS: the exclusive formation of CO2 was observed (details
are in the experimental section). This proved unambiguously
that, alike to DMC, compounds 3 underwent a decarboxylation
reaction. In addition, the (weighed) amount of CO2 (0.20 g)
allowed to estimate a 63% conversion of carboxymethyl reagents
(3a–3b). The result was in good agreement with the GC analysis
of the residual liquid mixture in the autoclave, by which the
conversion of 3 resulted 73%. However, the analysis of the liquid
phase also indicated that the expected methyl derivatives (2a–
2b) were present in a minor amounts [8%, Scheme 6, path (a)],
while major products were GlyF isomers (65%). The presence of
water in the reaction mixture likely accounted for this behaviour
[Scheme 6, path (b)].
Decarboxylation of DMC and carboxymethyl derivatives 3a–
3b. Dimethyl carbonate is thermally stable at temperatures up
to 390 ◦C.18 However, very recent studies by us and by other have
◦
demonstrated that at T ≥ 200 C, dialkyl carbonates partially
decomposed through a decarboxylation reaction catalysed by
solid bases.19 Scheme 5 illustrates the case of DMC where
dimethyl ether and CO2 were the observed products.
Scheme 5
Since conditions for such a process encompassed those of Fig.
1–4, a decarboxylation was expected in the reaction of GlyF
with DMC, involving not only DMC, but also non symmetrical
carbonates such as carboxymethyl isomers 3a and 3b (Scheme
4). Bearing this in mind, a more in-depth analysis of this aspect
was undertaken.
Initially, the decarboxylation of DMC was investigated.
Experiments were carried out under the conditions of Fig. 3. In
an autoclave, a mixture of DMC (19 mL, 0.23 mol) and K2CO3
(1.28 g, 9.4 mmol, the molar ratio, W1, DMC : K2CO3 was 24),
was set to react at temperatures of 200, 210 and 220 ◦C, and
for 20 h. Once the reactor was cooled to rt, a residual pressure
up to 5 bar indicated the formation of gaseous products. GC-
MS analyses of such gas-phase confirmed the presence of CO2
and dimethyl ether (DME) (Scheme 5). These compounds were
recovered by a cold-trap apparatus,19a and weighed: their total
amounts were 0.36, 1.26, 2.43 g, at 200, 210, 220 ◦C, respectively.
According to the stoichiometry of Scheme 5, the corresponding
conversions of DMC were 2, 6, and 12%. Although of minor
entity, the decarboxylation of DMC took place under the
conditions used for the methylation of GlyF. The analysis of the
residual DMC in the autoclave showed traces of methanol (not
Scheme 6 Decarboxylation of the carboxymethyl derivatives (3a–3b)
over K2CO3.
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Table 2 The mass balance and the scale up of the reaction of solketal and DMC
Entry
Solketal 1c (g)
DMC : 1c (W, mol : mol) a
T/◦C
t (h)
ISb (n-C12)
Conv. (%)c
Sel. (%)c
Yield (2c,%)
1
2
1.02
3.50
30
5
220
80
50
Yes
None
95
92
92
99
86c
66d
The K2CO3 : 1c molar ratio was 1.2.a Molar ratio DMC : GlyF. b IS: internal standard (n-dodecane, C12). The 1c : C12 molar ratio was 10. c Conversion
of solketal and selectivity/yield of O-methyl product 2c. Entry 1: values of both conversion, selectivity and yield were calculated based on GC
calibration curves. d Isolated yield of 2c.
To reduce/eliminate the competitive hydrolysis of compounds
3, an additional experiment was carried out using anhydrous
◦
glyme and K2CO3. The latter was dried under vacuum (70 C,
18 mm, overnight). The reaction was performed under the
conditions of Scheme 6. However, modest variations were
observed: the conversion was 70%, and the amounts of O-
methyl products (2a–2b) and GlyF isomers were 19 and 51%,
respectively. Overall, the decarboxylation of compounds 3a–3b
occurred, but this reaction took place along with a competitive
hydrolysis of reactants themselves.
The reaction of solketal with DMC in the presence of K2CO3
The reaction of solketal with DMC was studied based on
the results previously obtained for GlyF. Initial reactions were
performed in a stainless steel autoclave (200 mL) charged with
Fig. 5 The reaction of solketal with DMC carried out at 220 ◦C.
commercial solketal (1c: 1.02 g; 7.7 mmol), dimethyl carbonate
(DMC, 19 mL, molar ratio, W, DMC : 1c = 30) and K2CO3
(lowered) to reach a molar ratio DMC : solketal = 5. After 50
(molar ratio, Q, K2CO3 : 1c = 1.2). Five experiments were carried
◦
h, conversion and O-methyl selectivity were excellent (92 and
99%, respectively; entry 2), thereby proving the feasibility of the
reaction on a multigram scale. The final mixture appeared as
a thick slurry whose analysis showed that only minor amounts
of DMC (20% with respect to product 2c) were still present.
Not only the occurrence of the decarboxylation/hydrolysis of
dimethyl carbonate was confirmed, but the process became
rather extensive at long reaction times.
out at 220 C for 20, 30, 50, 60, and 80 h, respectively. Major
products (≥95% by GC-MS) were from O-methylation and
carboxymethylation reactions of solketal (Scheme 7: compounds
2c and 3c, respectively).
The mixture was diluted with diethyl ether and filtered,
and the O-methylation product 2c was distilled under vacuum
(56 ◦C, 3 mmHg) as a pure (98%, by GC) colourless liquid.
The isolated yield of 2c was of 2.50 g (66%). As for GlyF, the
available distillation apparatus brought also to middle fractions
containing the work-up solvent, DMC and MeOH, and not
negligible amounts of 2c.
Scheme 7 Major products of the reaction of solketal with DMC
catalysed by K2CO3.
These compounds were isolated and fully characterized by
GC-MS, 1H and 13C NMR (further details are in the experimen-
tal section). Other unidentified by-products whose total amount
was £5% (by GC), were also detected. The results are reported
in Fig. 5.
The K2CO3-catalyzed reaction of solketal with dibenzyl- and
diethyl-carbonate
As for the case of GlyF, the evaluation of the mass balance
and the scale up of the methylation of solketal with DMC were
investigated. Results are reported in Table 2.
The mass balance was examined in the presence of n-dodecane
as internal standard (entry 1). Reaction conditions were those
of Fig. 5 (80 h). Calculated conversion, O-methyl selectivity, and
yield on 2c were 95, 92 and 86%, respectively. With respect to
solketal, these values confirmed that no significant mass loss
took place during the reaction.
In order to expand the scope of the alkylation of glycerol
acetals with dialkyl carbonates, the reaction of solketal with
both dibenzyl- and diethyl-carbonate (DBnC and DEC, respec-
tively) was investigated.23 Experiments were carried out under
conditions similar to those used for DMC. In a stainless steel
autoclave, a mixture of solketal (1c, 1.8 mL, 14.6 mmol), K2CO3
(2.4 g, molar ratio, Q, K2CO3 : 1c = 1.2) and different amounts
of DBnC or DEC (molar ratios, W, dialkyl carbonate:1c = 5 and
8 were used) was set to react in the range of 190–220 ◦C, for 18–
66 h. Final reaction mixtures were analysed by GC-MS. Both
products of O-alkylation and carboxyalkylation of solketal were
observed (Scheme 8: derivatives 4c–5c, and 6c–7c, respectively).
The reaction was then scaled up. With respect to conditions of
Fig. 5, the amounts of solketal and the catalysts were multiplied
by a factor of 3.5. Instead, the volume of DMC was adjusted
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Table 3 The reaction of solketal with dibenzyl- and diethyl-carbonate
DAlC : 1c, W
Products (%, by GC)b
O-alkyl
selectivity (%)
Entry
DAlC
(mol : mol)
T/◦C
t (h)
Conv. (%)a
4c
5c
6c
7c
Others
Yield (%)c
1
2
DBnC
5
8
190
18
85
83
22
8
19
7
62
71
4
5
3
4
5
5
8
8
220
220
18
18
32
78
80
81
97
85
100
76
68
81
1
12
1
80
6
7
DEC
5
18
66
80
54
1
100
1
54
76
3
a Conversion of solketal. b 4c and 6c: O-benzyl and O-ethyl derivatives of solketal, respectively. 5c and 7c: O-carboxybenzyl and O-carboxyethyl
derivatives of solketal, respectively. c Isolated yield of product 4c.
Scheme 9 Hydrolysis and decarboxylation of DBnC.
solketal did not exceed 81% (entry 5). Again, not traces of DBnC
were detected in the final mixture.
Diethylcarbonate. DEC was remarkably less active than
DBnC. At 220 ◦C, after 18 h, the reaction of solketal with
DEC produced almost exclusively the carboxyethyl derivative
7c (entry 6). The selective formation of O-ethyl product 6c
Scheme 8 Major products of the K2CO3-catalysed reaction of solketal
with DBnC and DEC.
was achieved only after 66 h (entry 7); though, a simultaneous
decrease of the reaction conversion (54%) was observed. As
for DBnC, the hydrolysis of 7c along with the decarboxyla-
tion/hydrolysis of DEC offered an explanation for the result.
Compounds 4c and 5c were isolated and characterized by GC-
MS, 1H and 13C NMR, while the structures of products 6c and
7c were assigned from their mass spectra. Results are reported
in Table 3.
DEC in fact, was no longer present in the final mixture of
entry 7.
Dibenzylcarbonate. As for DMC (Fig. 4), the alkylation of
solketal was disfavoured by an excess of DBnC: at both 190
and 220 ◦C, after 18 h, an appreciable reduction of O-benzyl
selectivity was observed when the W ratio was increased from 5
to 8 (entries 1–2).
Discussion
The mechanism of DMC-mediated methylations of GlyF and
solketal
The increase of the temperature from 190 to 220 ◦C boosted
the formation of product 4c: the O-benzyl selectivity rose from 8–
22% to 97% (entries 1–2 and 3–4). Contrary to our expectations,
the conversion of solketal remained rather constant in all
experiments (around 80%, entries 1–4).
In the presence of K2CO3 as a catalyst, the reactions of dimethyl
carbonate with glycerol formal (GlyF) and with solketal show
remarkable analogies. First of all, experimental conditions can
be found for the formation of O-methyl ethers of both substrates
with a very high selectivity (>90%) at complete conversion (Fig.
3 and 5, Tables 1 and 2). This unexpected and never previously
reported result fulfils the synthetic goal of the present work. It
should be noted that under basic catalysis, alcohols are claimed
to react with dialkyl carbonates to form transesterification and
not alkylation products (Scheme 2). Other important similarities
between the methylation of GlyF and of solketal emerge when
the effects of the temperature (T) and of the reaction time (t) on
the product distribution are considered (Fig. 1, 3, and 5).
Both alkylations of GlyF and solketal yield O-methyl and
carboxymethyl derivatives (2 and 3, respectively); however,
as T and/or t are increased, the amount of carboxymethyl
This incongruity was plausibly due to: i) a partial hydrolysis
of the carboxybenzyl product 5c which brought back to the
starting reactant 1c (as in Scheme 6); ii) extensive decar-
boxylation/hydrolysis reactions of DBnC. Processes (ii) were
confirmed by the analysis of the final reaction mixture: at 220 ◦C,
after 18 h (W = 8), DBnC was absent and sizeable amounts of
dibenzyl ether and benzyl alcohol (41 and 46%, respectively,
with respect to O-methyl product 4c, 13%) were detected (entry
4).24 (Scheme 9).
Under the same conditions (220 ◦C, W = 8), an additional
reaction carried out for a prolonged time (32 h) proceeded with
total selectivity towards compound 4c, but the conversion of
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compounds rises to a maximum and then, it drops to zero. At
the same time, O-methyl compounds always increase (though
at different rates with respect to 3) to become final reaction
products.
Two plausible explanations can be offered for the forma-
tion/disappearance of compounds 3: i) these products are
originated by a reversible reaction (transesterification), i.e. an
addition/elimination at the DMC carbonyl;25 ii) compounds
3a–3b (from GlyF) are consumed by competitive decarboxy-
lation/hydrolysis processes which are triggered by temperature
(Fig. 3 and Scheme 6).26
The catalytic cycle terminates when the protonated base (BH+)
reacts with an alcoholate anion (e.g. MeO-) to restore the free
base (B) and produce MeOH [eqn (4)].
Overall, several concurrent processes take place, but if tem-
perature is high enough and sufficient time is allowed, the system
equilibrates to thermodynamically favoured products: the result
is the selective formation of O-methyl derivatives of both GlyF
and solketal.
In the case of GlyF, the higher rate of forma-
tion/disappearance of 6-membered ring isomers (2b and 3b)
with respect to their 5-membered ring analogues (2a and 3a)
is an additional aspect (Fig. 1b and Table 1). Despite the
huge literature on glycerol formal, very few papers describe
fundamental studies on the behaviour of isomers of glycerol
acetals and ketals. Among them, a work by Aksnes et al. has
investigated 5- and 6-membered ring acetals obtained by the
reaction of glycerol with acetaldehyde (Scheme 11: compounds
a and b, respectively).27
Based on these observations and on the sequence for base-
catalyzed DMC-mediated reactions,10,11 Scheme 10 illustrates
a mechanistic hypothesis for the O-methylation of GlyF and
solketal with DMC.
Scheme 11 (a) 4-Hydroxymethyl-2-methyl-1,3-dioxolane; (b) 5-
hydroxy-2-methyl-1,3-dioxane. Dashed lines: intramolecular H-bonds.
Authors provide evidence for intramolecular H-bonds
(dashed lines of Scheme 11) between the hydroxyl proton and
endocyclic oxygen atoms of both acetals (a) and (b). Moreover,
stereochemical reasons particularly, the mutual interactions
between axial hydrogen atoms, offer an explanation for the
different stability of (a) with respect to (b). This seems a
promising basis also for the investigated reaction even though
our experimental conditions, above all the high temperature
(≥200 ◦C), plausibly tend to smooth energetic differences due to
hydrogen bonds or to conformational barriers. Further studies
will be devoted to a more in-depth analysis of such aspects.
Scheme 10 The mechanism for the methylation of GlyF and solketal
with DMC.
In the first step, the base (B) converts the acetal/ketal (ROH:
GlyF or solketal) in the corresponding alcoholate anion [RO-,
eqn (1)] that reacts with both electrophilic centers of DMC
[paths a and b, BAc2 and BAl2 mechanisms of eqn (2) and
(3)]. Since the carbonyl carbon is a stronger electrophilic site
than the methyl one, a lower activation energy is expected for
the transesterification [path (a)] with respect to the methylation
reaction [path (b)]. This may account for the faster formation
of carboxymethyl- (3: ROCO2Me) with respect to methyl- (2:
ROMe) derivatives (Fig. 1 and 5). However, the onset of
the methylation process makes the reversible transesterification
backtrack: after an initial accumulation of compounds 3, they
decrease to zero, while ROMe become final products.
A further contribution to the disappearance of carboxymethyl
products is given by competitive decarboxylation and hydrolysis
reactions,20 shown in paths (e) and (f), respectively. For a
complete view, also the decarboxylation/hydrolysis of DMC
is indicated [paths (c)–(d)]: the extent of such transformations
become relevant only for long reaction times.
The role of the catalyst (K2CO3)
Fig. 2 shows that the reaction of GlyF with DMC takes place
even when a substoichiometric amount of K2CO3 is used (Q =
0.5). Increase of the catalyst loading speeds up the reaction until
a sort of plateau is reached: at Q ≥ 2, further improvements of
rate and/or selectivity are less, if at all, appreciable. Although
a kinetic analysis is beyond the scope of this work, it should
be considered that ◦K2CO3 is only moderately soluble in DMC
(0.58 g L-1 at 180 C,11a). Therefore, the investigated reaction
is plausibly partitioned between a liquid homogeneous phase
saturated with the base and the solid catalytic surface where
organic reactants/products may adsorb/desorb. As the Q
ratio increases, the homogeneous contribution should remain
constant, while the heterogeneous reaction over K2CO3 should
be favored until the solid surface accommodates the maximum
allowed quantity of organic substrates. After that, the reaction
rate cannot increase any further and the selectivity levels off as
well.
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A similar behavior has been observed also for other base-
catalysed reactions of DMC, for example in the methylation of
CH2-active compounds.28
properties of K2CO3 or the reactants.33 Whatever its origin, the
presence of water in our system is witnessed by the extensive hy-
drolysis observed for carbonates 3a–3b (Scheme 6) and DBnC as
well. It should be finally noted that the solid basicity per se is not
a sufficient requisite for a rapid decarboxylation reaction: solid
K2CO3 is by far a poorer catalyst with respect to amphoteric
faujasites. A possible reason lies in a inefficient adsorption of
dialkyl carbonates on K2CO3 with respect to zeolites.34
The DMC amount
Fig.
4 and Tables 1–2 indicate that the decrease of
DMC : substrate molar ratio (W) favors the alkylation of both
GlyF and solketal. This is particularly evident in scale-up
experiments which show that the reaction time halves when the
W ratio is reduced from 30 to 5. The reasons of this behaviour
are not clear, but some speculations can be made. A dilution
effect is hardly responsible for the result since DMC is also
a reactant. However: i) at W = 5, the high concentration of
GlyF or solketal may favour the base dissolution. It should
be noted that the solubility of K2CO3 boosts by a factor
of 22 (from 0.58 to 12.7 g L-1) when DMC is compared
to methanol,29 and both acetals (GlyF or solketal) possess
polar alcohol-like structures. An increase of the reaction rate
is plausible if the catalyst solubility is improved. Moreover, as
the reaction proceeds, GlyF (or solketal) is gradually converted
to the corresponding methyl and carboxymethyl derivatives, but
an equivalent amount of methanol is originated to restore a
medium polarity similar to that of the starting solution. The
effect of decarboxylation/hydrolysis of DMC may also play a
role, especially for prolonged experiments (above 50 h), where
such transformations occur to a sizeable extent. ii) As mentioned
above, the investigated reaction is likely to occur both in solution
and over solid K2CO3. For a number of reasons including
solvation effects, competitive adsorption, alteration of diffusion
constants, etc., the modes of (physical/chemical) interactions of
reagents (GlyF/solketal and DMC) on the catalytic surface as
well as the regime of the reaction can be modified by varying the
volume of the solvent (DMC).30
Other dialkyl carbonates
Previous studies by our group have demonstrated that in
the alkylation of several nucleophiles (including CH2-active
compounds and amines), dibenzyl carbonate (DBnC) displays a
reactivity comparable if not higher than DMC;35 while, diethyl
carbonate (DEC) is a less active alkylation agent than DMC.36
The same behavior holds true for the alkylation of solketal
investigated here (compare Tables 2 and 3). Moreover, although
the study of such a reaction is at a preliminary stage, the
formation of O-alkyl and carboxyalkyl compounds (4c–6c, and
5c–7c, respectively) from DBnC and DEC, is plausibly explained
by a mechanism analogue to that outlined in Scheme 10 for
DMC. Table 3 also shows that excellent O-alkyl selectivities
(up to 100%) are achieved; in all cases, notwithstanding the
excess of dialkyl carbonates, conversions are never quantitative
and they level off, if not decrease, at high temperature and for
prolonged reactions. As for DMC, the occurrence of decarboxy-
lation/hydrolysis processes decompose the starting carbonates:
results indicate that DBnC is a more efficient alkylating agent,
but it also disappears at a higher rate with respect to DEC.
Resonance effects similar to those invoked for SN2 displacements
likely enhance the reactivity of the benzyl system.37
The K2CO3-catalyzed decarboxylation of DMC and
carboxymethyl derivatives 3
Conclusions
The set up of innovative transformations of glycerol and
its derivatives represents an appealing challenge of modern
chemistry. Not only for the fact that glycerol is among the most
valuable biomass derived platform chemicals, but also for the
market surplus of glycerol due to biodiesel productions. This
work demonstrates that in the presence of K2CO3 as a base
catalyst, dimethyl carbonate reacts with acetals of glycerol such
as glycerol formal and solketal, to give the selective formation
of the corresponding methyl ethers: at complete conversion,
these products are obtained with selectivity up to 99%. These
never previously reported reactions can be also scaled up for
multigram synthesis without loss of selectivity; interestingly,
under such conditions, the use of a reduced volume of DMC
allows a consistent decrease of the reaction time and of the
mass index of the process. Although the reaction is rather
energy intensive, the procedure has a genuine potential for its
low environmental impact. It is a catalytic process that couples
the use of non-toxic dimethyl carbonate with renewable-derived
acetals/ketals. Also, no additional solvents are required and
major by-products are methanol and CO2. An accurate analysis
of the mass balance indicates that excellent GC yields (>90%)
of O-methylation products (2) can be achieved, meaning that
no mass loss occurs with respect to starting acetals. However,
We have recently reported that a class of zeolites (alkali metal
exchanged faujasites) are efficient catalysts for the decarboxyla-
tion of dialkyl carbonates.19a The more basic the zeolite (NaX,
CsY), the higher its activity. This feature has been explained by
the excellent potential for CO2 capture/storage of these catalysts
that are able to adsorb reversibly up to 5.5 mmol of CO2 per g
of solid.31 Accordingly, the breakdown of dialkyl carbonates
is favoured and, at the same time, the reversibility of the CO2
uptake prevents the saturation of the catalytic surface by the gas.
This leaves a sufficient solid basicity to allow the decarboxylation
to proceed. An similar interpretation may be offered for the
activity of K2CO3 in the decarboxylation reactions of both DMC
and compounds 3a–3b and 3c (Schemes 5–6 and Fig. 5). In fact,
K2CO3 as such or supported on various porous materials (C or
silica gel), is a very good CO2 sorbent system on condition that
water acts as a co-reagent (Scheme 12).32
Scheme 12 The chemical adsorption of CO2 over K2CO3.
This process may take place under the conditions described
in this work where water is plausibly present due to hygroscopic
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due to technical limitations of the purification (distillation) step,
compounds 2 are isolated in moderate yields not exceeding 67%.
Beyond the methylation process, the overall transformation
includes the transesterification of both GlyF and solketal with
DMC to produce the corresponding carboxymethyl deriva-
tives. These compounds follow an intermediate-like behaviour
which makes them disappear as the reaction proceeds: the
reversible nature of the transesterification and the decarboxy-
lation/hydrolysis of carboxymethyl products account for this
result. The latter processes become significant also for DMC
when prolonged reactions are performed. To the best of our
knowledge, this behavior has never been previously described
for reactions of alcohols and dialkyl carbonates under basic
catalysis conditions, and it may open interesting synthetic
perspectives. Also, a mechanistic hypothesis has been formulated
on this basis.
The preliminary investigation of the reaction of solketal with
both dibenzyl- and diethyl-carbonate suggests that the alkyla-
tion of glycerol acetals with dialkylcarbonates may be of a gen-
eral scope. However, the extensive decarboxylation/hydrolysis of
the starting carbonates does not allow the complete conversion
of solketal. To limit side-reactions, studies are in progress with
other base catalysts able to operate at lower temperatures than
K2CO3.
the autoclave was cooled to rt and then to -60 ◦C. The reactor
was slowly vented until it returned to rt. The reaction mixture
was finally analyzed by GC.
This procedure was adapted to examine the effect of reaction
parameters on the alkylation of both GlyF and solketal with
DMC: accordingly, the temperature, the time, the catalyst
loading and DMC amount were modified as reported in Fig.
1–5 and Table 2.
Scale up of reactions. The procedure above described was
used to scale up of the reaction of GlyF with DMC. The amounts
of reagents and GlyF were multiplied by a factor of 5: a mixture
of GlyF (1a+1b isomers, 4.0 g, 38 mmol), DMC [96 mL, 103 g,
the molar ratio W = DMC : GlyF = 30] and K2CO3 [6.3 g,
46 mmol, the molar ratio Q = K2CO3 : GlyF = 1.2] was set to
react at 220 ◦C (Table 1). Since a sizeable quantity of CO2 was
generated during the reaction, once a pressure of 60 bar was
reached (after ~25 h), the autoclave was rapidly cooled to rt,
and then to -60 ◦C in a liquid nitrogen bath, and slowly vented.
After that, the autoclave was re-purged with N2 and re-heated
◦
at 220 C. The release of CO2 overpressure was repeated at 37
and 43 h, respectively. The reaction was finally stopped after 52
h (Table 1, entry 1).
The same procedure was used to scale up the reaction in the
presence of: i) a reduced volume DMC (17 mL, molar ratio, W,
DMC : GlyF = 5; Table 1, entry 2), and ii) n-dodecane as an
internal standard (0.861 mL, 0.646 g, molar ratio 1 : C12 = 10;
Table 1, entry 3).
Experimental
General
The same procedure was used to scale up the reaction of
solketal with DMC. In this case, the amounts of solketal and
K2CO3 were multiplied by a factor of 3.5, while DMC was used
in a5-molar excess over the substrate: a mixture of solketal (1c,
3.5 g, 26 mmol), DMC (11 mL) and K2CO3 [4.3 g, 31 mmol, the
molar ratio, Q, K2CO3 : 1c = 1.2) (Table 2, entry 2) was set to
react at 220 ◦C for 50 h.
Glycerol formal (GlyF, 1a+1b), solketal (1c), benzyl alco-
hol, dimethyl carbonate (DMC), diethyl carbonate (DEC),
dimethoxyethane (glyme), n-dodecane (C12), K2CO3, ethyl ac-
etate (EA), petroleum ether (PE) and diethyl ether (Et2O),
were ACS grade from Aldrich. If not otherwise specified, they
were employed without further purification. Dibenzyl carbonate
(DBnC) was prepared via the transesterification reaction of
benzyl alcohol with DMC, by adjusting a method reported in
the literature18 (further details are in the ESI†). NaX faujasite
was from Aldrich: before its use, it was dehydrated under vacuum
(70 ◦C, 18 mm, overnight). GC-MS (EI, 70 eV) analyses were run
using a FFAP capillary column (30 m), and CG/FID analyses
Reactions of solketal (1c) with DBnC and DEC. Experiments
were carried out according to the general procedure described
above. The autoclave was charged with a mixture of solketal
(1.8 mL, 14.6 mmol), K2CO3 (2.4 g, molar ratio, Q, K2CO3:1c =
1.2) and different amounts of DBnC [16.5 and 26.4 g, molar
ratio, W, DBnC:1c = 5 and 8, respectively) or DEC [8.0 g, molar
ratio, W, DEC:1c = 5]. Reactions were carried out in the range
of 190–220 ◦C, for 18–66 h (see Table 3).
1
were run using a DBWAX capillary column (30 m). H NMR
were recorded at 400 MHz, 13C NMR spectra at 100 MHz, and
chemical shifts were reported in d values downfield from TMS;
CDCl3 was used as solvent.
Decarboxylation of DMC and carboxymethyl derivatives 3a–
3b. The study of K2CO3-catalysed decarboxylation of DMC
and of carboxymethyl derivatives of GlyF 3a–b, was carried out
in an autoclave, using a procedure already reported by us.19a In
the case of DMC, a mixture of DMC (19 mL, 0.23 mol) and
K2CO3 (1.28 g, 9.4 mmol, the molar ratio, W1, DMC : K2CO3
was 24), was set to react for 20 h at temperatures of 200, 210 and
220 ◦C. In the case of carboxymethyl derivatives 3a–3b, a mixture
of isomers 3a+3b (1.25 g, 0.0077 mmol), K2CO3 (1.28 g, molar
ratio, Q1, K2CO3 : 3a–b = 1.2), and dimethoxyethane (glyme,
Reaction procedures
Reactions of 1a–1b, 1c with DMC in the presence of K2CO3.
In a typical procedure, a stainless steel autoclave (200 mL of
internal volume) equipped with a pressure gauge, a thermo-
couple, and two needle valves, was charged with a mixture of
7.7 mmol of GlyF (1a+1b, 0.80 g; solketal, 1c, 1.02 g), dimethyl
carbonate (DMC, 19 mL, molar ratio, W, DMC : 1 = 30) and
K2CO3 (1.28 g, molar ratio, Q, K2CO3 : 1 = 1.2). Where indicated,
reactions with internal standard were carried out by adding n-
dodecane (0.175 mL, 0.131 g, molar ratio, Z, 1 : C12 = 10) to the
mixture. The autoclave was purged with N2, electrically heated at
the desired temperature (200–220 ◦C) and kept under magnetic
stirring throughout the reaction. At the end of the experiment,
◦
19 mL) as solvent, was set to react at 220 C for 10 h. Under
these conditions, an additional experiment was carried out
using anhydrous solvent and catalyst: glyme was dried through
conventional methods,38 while K2CO3 was dried under vacuum
(70 ◦C, 18 mm, overnight).
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At the end of experiments, gaseous products generated
from decarboxylation reactions, were recovered by a cold-trap
apparatus,19a weighed and analysed by GC-MS. In the case
of DMC, mixtures of CO2 and dimethyl ether (DME) were
obtained:39 the corresponding amounts were 0.36, 1.26 and 2.43
at 200, 210 and 220 ◦C, respectively. In the case of carboxymethyl
derivatives 3a–3b, only CO2 was recovered: the amount was 0.20
and 0.23 g for experiments in non-anhydrous and anhydrous
glyme/catalysts, respectively.
#4) to remove K2CO3, and the solid was washed diethyl ether
(3 ¥ 50 mL). The resulting solution was concentrated by rotary
evaporation. The residual oil was purified by FCC (flash column
chromatography) over silica gel, using a mixture of petroleum
ether (PE)/diethyl ether (Et2O)/ethyl acetate (EA) as eluant
solution [PE:Et2O:EA = 4 : 1 : 1 v/v]. Isolated yield of 4c was of
80% (2.58 g). Product 4c was fully characterized by GC-MS, 1H
and 13C NMR.
GC-MS (relative intensity, 70 eV) m/z: 222 (M+, <1%), 207
([M-H3C]+, 12), 105 (11), 101 ([M-H2COCH2Ph]+, 28), 92 (12),
91 ([CH2Ph]+, 100), 65 (11), 43 ([H3CCO]+, 35). 1H NMR
(CDCl3, 400 MHz) d (ppm): 7.40–7.27 (m, 5H), 4.62–4.53 (m,
2H), 4.34–4.26 (m, 1H) 4.05 (dd, 1H, J1 = 8.2 Hz, J2 = 6.4 Hz),
3.74 (dd, 1H, J1 = 8.3 Hz, J2 = 6.3 Hz), 3.55 (dd, 1H, J1 = 9.8 Hz,
J2 = 5.7 Hz), 3.47 (dd, 1H, J1 = 9.8 Hz, J2 = 5.5 Hz), 1.42 (s,
3H), 1.36 (s, 3H). 13C NMR (CDCl3, 100 MHz) d (ppm): 138.0,
128.4, 127.7, 109.4, 74.7, 73.5, 71.1, 66.9, 26.8, 25.4.
Isolation and characterization of O-alkyl products
4-(Methoxymethyl)-1,3-dioxolane (2a) and 5-methoxy-1,3-
dioxane (2b). 9a,40 These products were isolated at the end of
experiments reported in Table 1. The final reaction mixtures
were filtered (Gooch #4) to remove K2CO3, and the solid was
washed with diethyl ether (3 ¥ 50 mL). The resulting solution
was concentrated by rotary evaporation (40 ◦C, 600 mbar),
and then vacuum distilled. Two cold traps and a dephlegmator
were used during the distillation. O-Methylated products 2a–
2b were isolated (58 ◦C, 3 mmHg) as mixtures of isomers in
approximately the same 2 : 3 ratio of the starting isomers of
GlyF. Yields of 2a+2b were 2.91, 2.87 and 3.00 g (65, 64 and
67%; Table 1 entries 1, 2, and 3, respectively). Compounds 2a–
2b were fully characterized by GC-MS, 1H and 13C NMR.
GC-MS (relative intensity, 70 eV) m/z: 2a 118 (M+, <1%), 117
([M - H]+, 1), 88 [M-H2CO]+, 15), 73 ([M-H2COCH3]+, 22), 72
(14), 59 (11), 45 ([CH2 OCH3]+, 100), 44 (17), 43 (11), 31 (15),
29 (29), 28 (12), 27 (10); 2b 118 ([M]+, 2%), 88 ([M-H2CO]+, 11),
58 ([M-(H2CO)2]+, 100), 43 ([M-(H2CO)2-CH3]+, 34), 31 (14), 29
(30), 28 (20). 1H NMR (CDCl3, 400 MHz) d (ppm): 2a 5.01 (s,
1H), 4.86 (s, 1H), 4.22–4.15 (m, 1H), 3.94 (dd, 1H, J1 = 8.1 Hz,
J2 = 6.9 Hz), 3.66 (dd, 1H, J1 = 8.2 Hz, J2 = 6.0 Hz), 3.47–3.41
(m, 2H), 3.38 (s, 3H); 2b 4.83 (d, 1H, J = 6.1 Hz), 4.73 (d, 1H,
J = 6.1 Hz), 4.02 (dd, 2H, J1 = 11.6 Hz, J2 = 3.5 Hz), 3.71 (dd,
2H, J1 = 11.6 Hz, J2 = 6.1 Hz), 3.40 (s, 3H), 3.32–3.26 (m, 1H).
13C NMR (CDCl3, 100 MHz) d (ppm): 2a 95.3,74.3, 68.9, 66.9,
56.8; 2b 93.6, 72.9, 71.3, 54.8.
Synthesis, isolation and characterization of carboxyalkyl
products
Carboxyalkyl products were prepared by adjusting a conven-
tional method for base-catalyzed transesterification reactions.21
(1,3-Dioxolan-4-yl)methyl methyl carbonate (3a) and 1,3-
dioxan-5-yl methyl carbonate (3b). A 250 mL glass flask
equipped with a condenser was charged with GlyF (1a+1b, 10 g,
0.096 mol), DMC (162 mL, 173 g, molar ratio, W, DMC : GlyF =
20) and K2CO3 (15.9 g, molar ratio, Q, K2CO3 : GlyF = 1.2).
The flask was immersed in a thermostated oil bath, heated
to the reflux temperature of DMC (90 ◦C) and kept under
magnetic stirring. The reaction was monitored by GC. After
55 h, the mixture was cooled to rt, and filtered to remove K2CO3
(Gooch#4). The solid was washed with diethyl ether (3 ¥ 30 mL).
The◦resulting solution was concentrated by rotary evaporation
(30 C, 600 mbar), and then vacuum distilled. Compounds 3a
and 3b were isolated (65 ◦C, 3 mmHg) as a mixture of isomers
in approximately the same ratio of the starting isomers of GlyF.
Yield of 3a–3b was 79% (12 g, purity 98%, by GC). The products
were fully characterized by GC-MS, 1H and 13C NMR.
GC-MS (relative intensity, 70 eV) m/z: 3a 162 (M+, <1%),
161 ([M - H]+, 4), 103 ([M-CO2CH3]+, 14), 102 (34), 86 ([M-
H-OCO2CH3]+, 49), 77 (14), 73 ([M-CH2OCO2CH3]+, 44), 59
([CH3OC O]+, 40), 58 (43), 57 (38), 55 (11), 45 ([CH OCH3]+,
100), 44 (36), 43 (50), 31 (52), 30 (10), 29 ([CH O]+, 98), 28
(34), 27 (25); 3b 162 (M+, <1%), 161 ([M - H]+, 4%), 102 ([M-
(H2CO)2]+, 72), 86 ([M-H-CO2-OCH3]+, 49), 59 ([CH3OC O]+,
41), 58 ([M-(H2CO)2-CO2]+, 65), 57 (33), 55 (16), 45 (63),
44 (21), 43 ([M-(H2CO)2-CO2CH3]+, 72), 31 (59), 30 (14), 29
([CH O]+,100), 28 (44), 27 (27). 1H NMR (CDCl3, 400 MHz)
d (ppm): 3a 5.03 (s, 1H), 4.89 (s, 1H), 4.32–4.25 (m, 1H), 4.21
(dd, 1H, J1 = 11.3 Hz, J2 = 5.7 Hz), 4.16 (dd, 1H, J1 = 11.3 Hz,
J2 = 5.2 Hz), 3.97 (m, 1H), 3.79 (s, 3H), 3.72 (dd, 1H, J1 = 8.5 Hz,
J2 = 5.5 Hz); 3b 4.89 (d, 1H, J = 6.3 Hz), 4.81 (d, 1H, J = 6.2 Hz),
4.60–4.57 (m, 1H), 4.03 (dd, 2H, J1 = 12.2 Hz, J2 = 2.9 Hz), 3.96
(dd, 2H, J1 = 12.1 Hz, J2 = 4.2 Hz). 13C NMR (CDCl3, 100 MHz)
d (ppm): 3a+3b 155.5, 155.1, 95.5, 95.4, 72.5, 67.2, 66.6, 54.9,
54.9.
9c–e,41
4-(Methoxymethyl)-2,2-dimethyl-1,3-dioxolane (2c).
Compound 2c was isolated at the end of the reaction reported
in Table 2 (entry 2). The final mixture was filtered (Gooch #4)
to remove K2CO3, and the solid was washed diethyl ether (3 ¥
50 mL). The resulting solution was concentrated by rotary
evaporation (40 ◦C, 600 mbar), and then vacuum distilled
(56 ◦C, 3 mmHg) to give product 2c in a 66% yield (2.47 g,
purity 98% by CG).
GC-MS (relative intensity, 70 eV) m/z: 146 (M+, <1%), 131
([M-H3C]+, 26), 101 ([M-H2COCH3]+,4), 71 (24), 59 (8), 58
([M-H3C-(H2CO)-(H3CCO), 100), 45 ([CH2 OCH3]+, 4), 43
([H3CCO]+, 48), 42 (11), 41 (11). 1H NMR (CDCl3, 400 MHz) d
(ppm): 4.28–4.22 (m, 1H), 4.03 (dd, 1H, J1 = 8.2 Hz, J2 = 6.4 Hz),
3.67 (dd, 1H, J1 = 8.2 Hz, J2 = 6.5 Hz), 3.44 (dd, 1H, J1 = 9.8 Hz,
J2 = 6.1 Hz), 3.38 (dd, 1H, J1 = 9.8 Hz, J2 = 5.1 Hz), 3.37 (s,
3H), 1.40 (s, 3H), 1.34 (s, 3H). 13C NMR (CDCl3, 100 MHz) d
(ppm): 109.3, 74.5, 73.7, 66.6, 59.2, 26.6, 25.3.
4-[(Benzyloxy)methyl]-2,2-dimethyl-1,3-dioxolane (4c). 9c–e,41
Compound 4c was isolated at the end of the reaction reported in
Table 3 (entry 5). The final reaction mixture was filtered (Gooch
198 | Green Chem., 2012, 14, 188–200
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Methyl (2,2-dimethyl-1,3-dioxolan-4-yl)methyl carbonate (3c).
A 100 mL glass flask equipped with a condenser was charged
with solketal (1c, 3.0 g, 0.023 mol), DMC (39 mL, 41 mL, molar
ratio, W, DMC : 1c = 20) and K2CO3 (3.9 g, molar ratio, Q,
K2CO3 : 1c = 1.2). The flask was immersed in a thermostated oil
bath, heated to the reflux temperature of DMC (90 ◦C) and kept
under magnetic stirring. The reaction was monitored by GC.
After 95 h, the mixture was cooled to rt, and filtered to remove
K2CO3 (Gooch#4). The solid was washed with diethyl ether (3 ¥
30 mL). The resulting solution (~100 mL) was concentrated by
rotary evaporation (30 ◦C, 600 mbar), and then vacuum distilled.
Product 3c was isolated by distillation under reduced pressure
(63 ◦C, 3 mmHg), in 88% yield (3.85 g, purity 98% by GC). The
product was fully characterized by GC-MS, 1H and 13C NMR.
GC-MS (relative intensity, 70 eV) m/z: 190 (M+, <1%),
175 ([M-H3C]+,18), 115 (M-OCO2CH3]+, 2), 101 (M-
CH2OCO2CH3]+, 8), 71 (19), 59 ([CH3OC O]+, 16), 57 (11),
Future of Glycerol: New Usages for a Versatile Raw Material, 2008,
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43 ([H3CCO]+, 100), 41 (17). H NMR (CDCl3, 400 MHz) d
(ppm): 4.36–4.31 (m, 1H), 4.19–4.15 (m, 2H), 4.08 (dd, 1H, J1 =
8.6 Hz, J2 = 6.4 Hz), 3.78 (dd, 1H, J1 = 8.6 Hz, J2 = 5.8 Hz), 3.79
(s, 3H), 1.43 (s, 3H), 1.36 (s, 3H). 13C NMR (CDCl3, 100 MHz)
d (ppm): 155.5, 190.8, 73.2, 67.9, 66.2, 54.9, 26.6, 25.3.
Benzyl(2,2-dimethyl-1,3-dioxolan-4-yl)methyl carbonate (5c).
A 50 mL glass flask equipped with a condenser was charged with
solketal (1c, 0.94 g, 7.1 mmol), DBnC (3.6 g, molar ratio, W,
DBnC : 1c = 2), K2CO3 (1.3 g, molar ratio, Q, K2CO3 : 1c = 1.2)
and dimethoxyethane (glyme, 20 mL) as the solvent. The flask
was immersed in a thermostated oil bath, heated to the reflux
temperature of glyme (85 ◦C) and kept under magnetic stirring.
The reaction was monitored by GC-MS. After 12 h, the mixture
was cooled to rt, and filtered to remove K2CO3 (Gooch#4). The
solid was washed with diethyl ether (3 ¥ 10 mL). Compound
5c was purified by FCC (flash column chromatography) over
silica gel, using gradient petroleum ether (PE)/diethyl ether
(Et2O) elutions [initial PE : Et2O = 1 : 0 v/v, final PE:Et2O =
1 : 1 v/v], and isolated in a yield of 65% (1.2 g). Product was
fully characterized by GC-MS, 1H and 13C NMR.
GC-MS (relative intensity, 70 eV) m/z: 266 (M+, <1%), 101
([M-H2COCO2CH2Ph]+,11), 91 ([CH2Ph]+, 100), 65 (10), 59 (10),
43 ([H3CCO]+, 64). 1H NMR (CDCl3, 400 MHz) d (ppm): 7.39–
7.33 (m, 5H), 5.17 (s, 2H), 4.35–4.31 (m, 1H), 4.21 (dd, 1H, J1 =
9.2 Hz, J2 = 3.2 Hz), 4.16 (dd, 1H, J1 = 9.2 Hz, J2 = 3.9 Hz), 4.07
(dd, 1H, J1 = 8.5 Hz, J2 = 6.4 Hz), 3.78 (dd, 1H, J1 = 8.5 Hz, J2 =
5.9 Hz), 1.42 (s, 3H), 1.36 (s, 3H). 13C NMR (CDCl3, 100 MHz)
d (ppm): 149.7, 129.8, 123.4, 123.2, 104.7, 68.1, 64.6, 62.7, 61.1,
21.4, 20.1.
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200 | Green Chem., 2012, 14, 188–200
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