Masked N-Heterocyclic Carbene-Catalyzed Alkylation of Phenols with Organic Carbonates

Article abstract of DOI:10.1002/cssc.201600493

An easily prepared masked N-heterocyclic carbene, 1,3-dimethylimidazolium-2-carboxylate (DMI-CO₂), was investigated as a “green” and inexpensive organocatalyst for the alkylation of phenols. The process made use of various low-toxicity and renewable alkylating agents, such as dimethyl- and diethyl carbonate, in a focused microwave reactor. DMI-CO₂ was found to be a very active catalyst and excellent yields of a range of aryl alkyl ethers were obtained under relatively benign conditions. The observed difference in the conversion behavior of phenol methylation, in the presence of either the carbene or 1,8-diazabicycloundec-7-ene (DBU) catalyst, was rationalized on the basis of mechanistic investigations. The primary mode of action for the N-heterocyclic carbene is nucleophilic catalysis. Activation of the dialkyl carbonate electrophile results in concomitant evolution of an organo-soluble alkoxide, which deprotonates the phenolic starting material. In contrast, DBU is initially protonated by the phenol and thus consumed. Subsequent regeneration and participation in nucleophilic catalysis only becomes significant after some phenolate alkylation occurs.

Full text of DOI:10.1002/cssc.201600493

DOI: 10.1002/cssc.201600493
Communications
Masked N-Heterocyclic Carbene-Catalyzed Alkylation of
Phenols with Organic Carbonates
Matthew Y. Lui, Alexander K. L. Yuen, Anthony F. Masters, and Thomas Maschmeyer*
[
a]
An easily prepared masked N-heterocyclic carbene, 1,3-dime-
principle, be recycled. Examples of alkylation with other dialkyl
carbonates, such as diethyl carbonate (DEC) and dibenzyl car-
bonate (DBnC), are much less prevalent in the literature than
thylimidazolium-2-carboxylate (DMI-CO ), was investigated as
2
a “green” and inexpensive organocatalyst for the alkylation of
phenols. The process made use of various low-toxicity and re-
newable alkylating agents, such as dimethyl- and diethyl car-
[3]
are those describing alkylation with DMC. Nevertheless, these
longer chain dialkyl carbonates can be readily produced by
[4]
bonate, in a focused microwave reactor. DMI-CO was found to
transesterification of DMC with the corresponding alcohols,
2
[5]
be a very active catalyst and excellent yields of a range of aryl
alkyl ethers were obtained under relatively benign conditions.
The observed difference in the conversion behavior of phenol
methylation, in the presence of either the carbene or 1,8-diaza-
bicycloundec-7-ene (DBU) catalyst, was rationalized on the
basis of mechanistic investigations. The primary mode of
action for the N-heterocyclic carbene is nucleophilic catalysis.
Activation of the dialkyl carbonate electrophile results in con-
comitant evolution of an organo-soluble alkoxide, which de-
protonates the phenolic starting material. In contrast, DBU is
initially protonated by the phenol and thus consumed. Subse-
quent regeneration and participation in nucleophilic catalysis
only becomes significant after some phenolate alkylation
occurs.
as well as by catalytic oxidative carbonylation, opening up
the possibility of the synthesis of a wide range of alkyl aryl
ethers. One disadvantage of using organic carbonates for the
alkylation of phenols is that they are less reactive than other
common alkylating agents. Efficient alkylation with DMC, for
example, requires the reaction temperature to be considerably
higher than the reagent’s boiling point (908C) and, thus,
a high-pressure apparatus is usually required to drive methyla-
tion reactions to completion. Moreover, high temperatures,
often in conjunction with extended reaction times and an
excess amount of a promoter, are required for alkylation with
[3]
longer chain dialkyl carbonates.
Many catalysts have been studied for the alkylation of phe-
[3,6]
nols with DMC and other organic carbonates, with 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU) being one of the most
common for this kind of transformation, owing to its high ac-
[3a,6a–d]
The alkylation of phenols to form phenyl alkyl ethers is an im-
portant transformation for the synthesis of a wide range of
chemical products. Typically, phenyl alkyl ethers are synthe-
sized using harmful alkylating agents, such as alkyl halides and
tivity and selectivity.
It is reported that DBU does not
simply act as a base but also as a nucleophilic catalyst that
reacts with dialkyl carbonates to generate an intermediate N-
alkoxycarbonyl-amidinium adduct, which is a strongly activated
[
1]
[6a]
[7]
dimethyl sulfate, using the Williamson reaction. Dialkyl carbo-
nates, such as dimethyl carbonate (DMC), have been described
alkylating agent. However, DBU is highly toxic, thus an al-
ternative catalyst of potentially lower toxicity, retaining good
activity at low cost, is desirable for the construction of a “green-
er” reaction system.
[2]
as greener, alternative reagents for alkylation (Scheme 1).
DMC, for example, is considered a non-toxic, sustainable, bio-
degradable reagent, and is not classified as a volatile organic
compound (VOC) according to the US Environmental Protec-
tion Agency. Because DMC can be produced by the catalytic
oxidative carbonylation of methanol, the methanol by-product
produced under methylation conditions (Scheme 1) can, in
In numerous transformations N-heterocyclic carbenes (NHC)
[8]
have been utilized extensively as organocatalysts. More spe-
cifically, 1,3-dimethylimidazolium-2-carboxylate (DMI-CO ) is an
2
N-heterocyclic carbene precursor (a “masked carbene”) that
has been employed as a catalyst in the ring-opening polymeri-
[9]
zation and copolymerization of propylene oxide, and in the
[10]
synthesis of cyclic carbonates from bio-based diols, including
[
10,11]
glycerol.
DMI-CO is attractive, because it can be obtained
2
[12]
easily by heating DMC with 1-methylimidazole (Scheme 2),
[13]
which is itself a less expensive heterocycle than DBU.
Scheme 1. Catalytic alkylation of phenols with dialkyl carbonates.
Here, we report the investigation of this catalytic system for
the alkylation of phenolic compounds, such as guaiacol, syrin-
[
a] Dr. M. Y. Lui, Dr. A. K. L. Yuen, Prof. A. F. Masters, Prof. T. Maschmeyer
Laboratory of Advanced Catalysis for Sustainability, School of Chemistry
The University of Sydney
Sydney, NSW 2006 (Australia)
E-mail: thomas.maschmeyer@sydney.edu.au
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cssc.201600493.
2
Scheme 2. Synthesis of DMI-CO .
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gol, and cresol derivatives, known to be generated from the
Table 2. Alkylation of phenol with diethyl carbonate, diallyl carbonate,
and dibenzyl carbonate in acetonitrile using DMI-CO as precatalyst.
2
[
14]
depolymerization of lignin.
Specifically, the microwave-
[a]
assisted alkylation of phenolic compounds with organic carbo-
Entry
Carbonate
DMI-CO
mol%]
2
Conv.
[%]
O-alkylated
product [%]
nates catalyzed by the masked carbene, DMI-CO , is described
2
[
b]
[
in detail.
1
2
DEC
DEC
DEC
DAC
DAC
DBnC
10
50
50
10
10
10
16
71
93
78
93
15
70
90
78
93
97
Using an Ace Glass pressure tube, DMI-CO was synthesized
2
by heating 1-methylimidazole and DMC according to the pro-
[
c]
3
4
5
6
[
12b]
cedure of Crabtree and co-workers.
Using DMI-CO as a cata-
2
[
d]
lyst (10 mol% with respect to the phenolic substrate), phenolic
derivatives containing different ring substituents were methy-
lated with DMC in an Anton Paar 300 focused microwave reac-
tor at 1608C (Table 1).
100
[a] Reaction conditions: phenol (2.73 mmol), diethyl carbonate (DEC), dia-
llyl carbonate (DAC), and dibenzyl carbonate (DBnC, 13.1 equiv.) in aceto-
nitrile (3 mL) using DMI-CO
2
; the reaction was heated in an Anton Paar
3
00 focused microwave reactor at 1608C for 80 min. [b] Yield quantified
1
by H NMR. [c] Heated at 1708C. [d] Heated for 120 min.
Table 1. Methylation of different phenolic derivatives with DMC in aceto-
[
a]
2
nitrile using DMI-CO as precatalyst.
Entry
Phenolic
substrate
Time
[min]
Conv.
[%]
O-methylation
product [%]
[
b]
action resulted in only 15% yield of ethoxybenzene (Conver-
sion: 16%). Catalyst loading and temperature had to be in-
creased for higher conversion rates (Entries 2 and 3, Table 2).
Minor amounts of anisole were also detected in the reaction
mixture (Entries 1: 0.2%; 2: 1.3%; and 3: 3%). This anisole was
probably generated by the N-demethylation of the intermedi-
ate electrophile as shown below (Scheme 3).
1
2
3
4
5
6
7
8
9
phenol
guaiacol
4-methylguaiacol
syringol
3,5-dimethoxyphenol
m-cresol
p-cresol
eugenol
vanillin
4-chlorophenol
4-bromophenol
2,6-dichlorophenol
4-benzylphenol
1-naphthol
80
80
80
80
80
80
80
80
160
120
120
120
80
100
100
94
100 (71)
99 (95)
91 (90)
93 (93)
98 (96)
99 (84)
99 (88)
95 (91)
74 (77)
100 (94)
100 (100)
100 (97)
100 (99)
100 (99)
90 (90)
99
100
100
99
96
100
100
100
100
100
100
100
10
1
1
1
1
1
1
2
3
4
5
120
120
Scheme 3. Anisole by-product formation under ethylation conditions.
2-naphthol
[
1
a] Reaction conditions: phenolic derivatives (2.73 mmol), DMC (3 mL,
3.1 equiv.), acetonitrile (3 mL), and DMI-CO (0.273 mmol; 10 mol% with
respect to the phenolic substrate); the reaction mixture was heated at
608C in an Anton Paar 300 focused microwave reactor. [b] Yield was
2
For DAC, under standard conditions (Entries 4 and 5,
Table 2), 77% and 94% of phenol was converted after 80 min
(77% yield of allyloxybenzene) and 120 min (93% yield of ally-
loxybenzene), respectively. Besides allyloxybenzene and trace
amounts of anisole (observed using GC–MS, not observable
1
1
quantified by H NMR; isolated yield in parentheses; owing to the high
volatility of their corresponding products, large discrepancy in yield was
observed for Entries 1, 6, and 7.
1
with H NMR), no other aromatic products were observed. For
DBnC, under standard conditions (Entry 6, Table 2), phenol was
fully converted after 80 min to give 97% yield of benzyloxy-
benzene.
Most of the investigated phenols were successfully convert-
ed to the corresponding methylated products in 91–100%
yield after 80 min of heating. Substrates containing electron-
withdrawing groups required longer heating times to achieve
To compare the alkylation activity of DMI-CO versus that of
2
DBU, time-course studies were carried out for the conversion
of phenol to anisole with DMC in acetonitrile under similar
conditions as experiments reported in in Table 1.
>
90% conversion (4-halophenols and naphthol isomers), af-
fording the desired products with very high selectivity. Vanillin
however, was somewhat sluggish to react, requiring 160 min
for complete conversion (Entry 9, Table 1), with lower selectivi-
ty for the desired product (3,4-dimethoxybenzaldehyde).
The time-course of DBU-catalyzed methylation of phenol is
illustrated in Figure 1. An induction period of approximately
20 min was observed in the presence of 10 mol% (with respect
to phenol) DBU. Similar behavior for the DBU-catalyzed esterifi-
cation of benzoic acid with DMC was described by Shieh and
Alkylation of phenol with diallyl carbonate (DAC), diethyl car-
bonate (DEC), and dibenzyl carbonate (DBnC), was also at-
tempted using the masked NHC catalyst. Commercial sources
of DEC and DBnC were used; DAC was synthesized by transes-
terification of DMC with allyl alcohol, in the presence of cata-
lytic amounts of potassium carbonate. Initially, ethylation of
phenol with DEC was performed under the standard condi-
[15]
co-workers.
Several factors may contribute to the slow initial rate of
methylation in the presence of DBU. The primary cause is likely
to be the lack of free DBU available to react with DMC to gen-
erate the reactive N-methoxycarbonyl amidinium intermediate,
that was proposed by Shieh et al. to be the active electrophile
tions for methylation, that is, 10 mol% DMI-CO , 1608C,
2
[6a,15]
13.1 equivalent of organic carbonate (Entry 1, Table 2). The re-
(i.e., nucleophilic catalysis).
Even before the heating of the
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rate of conversion was considerably faster under analogous
conditions: more than 99% of the phenol was converted to
anisole after 60 min. No appreciable induction period was ob-
served and a steady increase in the conversion of phenol oc-
curred until the majority of the starting material was con-
sumed. In this case, the pre-catalyst is not initially protonated
by phenol and the active carbene catalyst is revealed only
upon heating, with evolution of CO . Despite the inherent ba-
2
[17]
sicity of the NHC (pK in DMSO=21.1) and the presence of
a
phenol, protonation at the C2-position of the NHC would lead
to an unstable imidazolium-phenoxide ion-pair, in which equi-
[18]
librium would favor the carbene. It is well established that
[19]
NHCs are relatively poor bases but strong nucleophiles, and
addition of carbene 3 to dialkyl carbonate, resulting in the
highly active 2-alkoxycarbonyl-1,3-dimethylimidazolium elec-
trophile, would account for the observed higher initial reaction
rate compared to the DBU case.
Figure 1. Methylation of phenol (2.73 mmol) with DMC (3 mL, 13.1 equiv.) in
acetonitrile (3 mL) using DMI-CO or DBU (0.273 mmol; 10 mol% with re-
spect to phenol) as (pre)catalysts. The reaction mixture was heated at 1608C
2
in an Anton Paar 300 focused microwave reactor. Yield quantified by
H NMR.
A plausible mechanism for this pathway is illustrated in
Figure 3. The free NHC catalyst, 3, which is generated from the
1
decarboxylation of DMI-CO , attacks the carbonyl group of di-
2
reaction mixture commenced, the vast majority of DBU would
be protonated by the excess phenol in a diffusion-controlled
reaction (shown in Figure 2). Furthermore, the cation of the
putative intermediate N-methoxycarbonyl amidinium methox-
ide (1, R=Me) is expected to be unstable and readily be de-
protonated (i.e., by methoxide or another equivalent of DBU)
to generate a ketene aminal (2), which is known to react with
methanol/phenol to regenerate DBU, as well as making DMC
alkyl carbonate at elevated temperature to generate 2-alkoxy-
carbonyl-1,3-dimethylimidazolium alkoxide 4. In the studies by
[10]
Bruijnincx and co-workers, 2-methylcarbonyl-1,3-dimethylimi-
dazolium methoxide could be generated at relatively low tem-
perature (748C) from 3 and dimethyl carbonate. The cation of
4 is rationalized as being a more activated alkylating agent
than dialkyl carbonate, owing to its positive charge.
[
16]
or methyl phenyl carbonate (MPC). MPC is expected to be
able to react analogously with DBU to generate a N-methoxy-
carbonyl amidinium phenoxide intermediate (1, R=Ph).
Figure 3. Proposed primary mechanism for the alkylation of phenols with di-
alkyl carbonates.
Figure 2. Basic and nucleophilic behavior of DBU.
To verify this hypothesis the salt 2-methoxycarbonyl-1,3-di-
methylimidazolium triflate (5) was synthesized (Scheme 4) and
reacted with phenol in the presence of the non-nucleophilic
base N,N-diisopropylethylamine (DIPEA).
The subsequent, apparent increase in reaction rate is less
straightforward to rationalize. One possibility is that the con-
version rate only starts to increase once the initial phenoxide
present reacts to produce anisole and methoxide ions. These
basic methoxides are then able to either liberate protonated
DBU or produce more phenoxide. A proportion of the liberat-
ed DBU would then be available to react with DMC, effecting
nucleophilic catalysis, increasing the overall methylation rate.
In contrast, the NHC liberated from the pre-catalyst DMI-CO2
Scheme 4. Synthesis of 2-methoxycarbonyl-1,3-dimethylimidazolium triflate
5.
displays very different behavior to DBU (Figure 1). Its initial
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When phenol was heated with 5 in the presence of 1 equiv-
alent of DIPEA for 5 min at 1608C, anisole and MPC were gen-
erated (Entry 1, Table 3). On the contrary, no products were
generated when phenol was heated with DIPEA and DMC
under analogous conditions (Entry 2, Table 3). To assess the al-
kylating ability of MPC, a reaction was conducted using it in
place of DMC (Entry 3, Table 3). Despite its inherent stability,
MPC was able to selectively produce anisole under NHC cataly-
sis (Entry 4, Table 3). This process is proposed to occur through
the reactive intermediate 2-methoxycarbonyl-1,3-dimethylimi-
dazolium phenoxide.
electrophile for alkylation and an alkoxide base simultaneously,
the latter being integral to the successful propagation of the
alkylation process.
Experimental Section
Conversion of phenols. A typical procedure for the conversion of
phenols with dialkyl carbonate using DMI-CO and DBU as catalysts
2
is as follows: Phenol derivative (2.73 mmol), catalyst, dialkyl car-
bonate (13.1 equiv.), and acetonitrile (3 mL) were heated in
a 30 mL glass tube, fitted with a pressure cap in an Anton Paar
Monowave 300 microwave synthesis reactor with a stirring rate of
6
00 rpm. After heating, the reaction tube was removed from the
microwave reactor and cooled to room temperature. Mesitylene
0.2 mL) was then added as an external standard. The solution was
[
a]
Table 3. Mechanistic investigation of phenol methylation.
(
Entry
Reagent A
Reagent B
Time [min]
Products
1
then mixed with [D ]DMSO for H NMR analysis. For GC–MS analy-
sis, the sample was passed through a short pad of silica and dilut-
ed with acetone before injection.
6
1
2
3
4
5
DIPEA
DIPEA
DIPEA
5
5
5
5
Anisole, MPC
None
None
DMC
MPC
MPC
DMI-CO
2
Anisole
Isolation of O-methylation products. Phenol substrate
[
a] Reaction conditions: phenol (0.49 mmol), reagents A/B (0.49 mmol),
and acetonitrile (2 mL) were heated at 1608C in an Anton Paar 300 fo-
(
(
2.73 mmol), DMI-CO2 (0.273 mmol), DMC (3 mL), and acetonitrile
3 mL) were heated in a 30 mL glass tube, fitted with a pressure
cused microwave reactor.
cap in an Anton Paar Monowave 300 microwave synthesis reactor,
with a stirring rate of 600 rpm. After cooling to room temperature,
the mixture was concentrated in vacuo. Purification by flash chro-
matography, eluting with 0–30% ethyl acetate/hexane or diethyl
ether/pentane (gradient), afforded the desired product.
The alkoxide anion of 4 also plays an important role in the
reaction progress, as it is a stronger base than 3 (pK values of
a
methanol and ethanol in DMSO are 29.0 and 29.8, respective-
[
12b]
[
20]
Synthesis of DMI-CO
was charged with dimethyl carbonate (9 mL), 1-methylimidazole
6 mL), and a stirrer bar. The mixture was heated for 72 hours at
08C. The solid was filtered and was washed thoroughly with
.
2
A screw-top 30 mL Ace pressure tube
ly) and, hence, the alkoxide can irreversibly deprotonate the
phenolic starting material. This phenoxide ion in turn reacts
with alkylating agent, 4, to generate the phenyl alkyl ether
product. In the cases where there is a large excess of the dia-
lkyl carbonate and only a small amount of carbene is present,
some of the phenoxide may also react directly, albeit slowly,
with dialkyl carbonate.
(
9
methylene chloride (3ꢁ40 mL), acetone (3ꢁ40 mL), and diethyl
ether (3ꢁ40 mL). The yield of solid DMI-CO2 was 8.2 g (78%).
H NMR (D O): d=7.30 (2H, s, 2ꢁCHN), 3.92 ppm (6H, 2ꢁCH N);
1
2
3
1
3
C NMR (D
O, ext. std. CH
OH): d=158.5 (CO
), 140.1 (CCO ), 123.4
2
2
3
2
(
2ꢁCHN), 37.1 ppm (2ꢁCH N).
It should be noted that alkoxide is generated continuously
after each round of substitution with dialkyl carbonates. The
alkoxides would most likely be the strongest base in the reac-
tion mixture and play a major role in the conversion, especially
when a small amount of basic catalyst is used. One key attri-
bute of this NHC catalyst is its ability to continuously generate
and maintain substantial amounts of soluble alkoxides in the
reaction mixture.
3
Synthesis of diallyl carbonate. A mixture of allyl alcohol (80 mL,
.18 mol), dimethyl carbonate (20 mL, 0.24 mol), and potassium
1
carbonate (7.7 g, 55.7 mmol) were stirred at 708C for 6 h in
a round-bottomed flask. Then the reaction mixture was concentrat-
ed slowly in vacuo at 608C under reducing pressure. The solid was
then filtered and washed with dichloromethane (100 mL). The or-
ganic solution was then extracted with distilled water (2ꢁ10 mL)
and concentrated in vacuo to give diallyl carbonate as a colorless
In summary, a DMC-derived, masked N-heterocyclic carbene
1
liquid (18 g, 53%). H NMR (CDCl ): d=5.95 (2H, ddt, J=17.2, 10.6,
3
(
DMI-CO ) was employed as the catalyst for the alkylation of
2
5
.5 Hz, 2ꢁCH =CH), 5.23–5.42 (4H, m, 2ꢁCH =CH), 4.64 ppm
2 2
phenolic derivatives with DMC and other organic carbonates.
13
(
(
4H, d, J=5.7 Hz, CH O); C NMR (CDCl ): d=155.0 (OCO ), 131.7
2 3 2
CHCH OCO ), 119.0 (CH CHCH OCO ), 68.6 ppm (CH OCO ).
The NHC arising from the decarboxylation of DMI-CO is very
2
2
2
2
2
2
2
2
active and selective, as excellent yields of O-alkylated products
could be generated in short reaction times and under relatively
benign conditions. This catalyst is potentially less expensive
than DBU, which is the established catalyst for this type of
transformation. In this investigation, the different conversion
behavior of phenol to anisole, catalyzed by DBU or NHC, was
observed and rationalized. The mechanism of the key transfor-
mation of this reaction catalyzed by the masked carbene was
also proposed: NHC is likely to act as a nucleophilic catalyst,
which reacts with the dialkyl carbonate to generate a 2-alkoxy-
carbonyl-1,3-dimethylimidazolium salt, which is a highly active
[
21]
Synthesis of 5. To a solution of methyl 1-methylimidazole-2-car-
boxylate (1.6 g, 11.4 mmol) in dried dichloromethane (100 mL) was
added neat methyl triflate (1.2 mL) dropwise at room temperature
under nitrogen. After stirring for 18 h, the solution was concentrat-
ed by rotary evaporation of the solvent and the resulting white
solid was recrystallized with acetone/diethyl ether mixture to give
the pure product (2.9 g, 84%). H NMR (D O): d=7.55 (2H, s,
CH N), 4.04 (6H, s, 2ꢁNCH ), 4.02 ppm (3H, s, OCH ); C NMR (D O,
ext. std. CF COOH): d=157.4 (COOCH ), 134.9 (CCOOCH ), 128.4
(2ꢁCHN), 122.0 (q, 317 Hz, SO CF ), 56.4 (COOCH ), 40.9 ppm (2ꢁ
CH N); F NMR (D O, ext. std. CF COOH): À79.6.
1
2
13
2
3
3
2
3
3
3
3
3
3
19
3
2
3
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Communications
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(
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1
give pure methyl phenyl carbonate (0.85 g, 40%). H NMR (CDCl ):
3
d=7.39 (2H, app t, J=7.5 Hz, C -H), 7.14–7.29 (3H, m, C2,4,6-H),
3,5
13
3
1
.90 ppm (3H, s, CH3); C NMR (CDCl ): d=154.4 (C=O), 151.3 (C ),
3 1
29.6 (C and C ), 126.2 (C ), 121,2 (C and C ), 55.5 ppm (CH ); GC-
3
5
+
4
2
6
3
MS: m/z 152 (M , 49%), 108 (39), 93 (19), 78 (100), 65 (92).
Synthesis of allyloxybenzene. Allyloxybenzene was synthesized
[22]
following a procedure reported previously. The compound was
1
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1
bonate (Entries 4 and 5, Table 2). H NMR ([D ]DMSO): d=7.28 (2H,
6
À1
app t, J=7.1 Hz, C -H), 6.89–6.99 (3H, m, C2,4,6-H), 5.96–6.13 (1H,
3
,5
[7] LD50 Oral—Rat: 215–681 mgkg (Safety Data Sheet from Sigma–Al-
drich).
m, ArOCH CH), 5.39 (1H, d, 17.3 Hz, ArOCH CHCH ), 5.25 (1H, d,
2
2
2
1
0.3 Hz, ArOCH CHCH ), 4.52–4.58 ppm (1H, m, ArOCH CHCH ); GC-
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007, 46, 2988–3000.
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[
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[
[
[23]
was used to verify the product of alkylation of phenol with diben-
1
zyl carbonate (Entry 6, Table 2). H NMR ([D ]DMSO): d=7.24–7.49
6
(
7H, m, other Ar-H), 7.01 (2H, d, 7.5 Hz, C -H), 6.94 (1H, 7.4 Hz, C -
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+
H), 5.10 ppm (3H, s, OCH ); GC-MS: m/z 184 (M , 9%), 91 (100), 65
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[
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Dollars per mole for 1-methylimidazole and 148 Australian Dollars per
mole for DBU (both based on 500 g pack size). In addition, LD50 Oral—
Acknowledgements
À1
Rat of 1-methylimidazole is 1144 mgkg (Safety Data Sheet from
The authors thank the Science and Industry Endowment Fund
Sigma–Aldrich), a significantly higher value than for DBU: see ref. [7]).
(SIEF, Grant no. RP03-028) for financial support for this research.
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Keywords: alkylation · carbenes · lignin · organic carbonates ·
organocatalysis
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Published online on && &&, 0000
ChemSusChem 2016, 9, 1 – 6
www.chemsuschem.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

COMMUNICATIONS
M. Y. Lui, A. K. L. Yuen, A. F. Masters,
T. Maschmeyer*
&& – &&
Masked N-Heterocyclic Carbene-
Catalyzed Alkylation of Phenols with
Organic Carbonates
The masked carbene: Organic carbo-
nates including dimethyl- and diethyl
carbonate are safer, renewable, but less
reactive alkylating agents than alkyl hal-
ides or sulfates. We unmask an N-heter-
ocyclic carbene for the catalyzed alkyla-
tion of phenols, many of which can be
derived from lignin, with these organic
carbonates. The resulting aryl alkyl
ethers are important for use in the
flavor, fragrance, and pharmaceutical in-
dustries.
&
ChemSusChem 2016, 9, 1 – 6
www.chemsuschem.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!