Metal-free transesterification catalyzed by tetramethylammonium methyl carbonate

Article abstract of DOI:10.1039/c7gc03858e

Environmentally benign metal-free tetramethylammonium methyl carbonate is effective as a catalyst for the chemoselective, scalable, and reusable transesterification of various esters and alcohols in common organic solvents. In situ-generated highly active species, tetramethylammonium alkoxides, can greatly avoid self-decomposition at ≤110 °C, and are reusable. In particular, chelating substrates, such as amino alcohols, diols, triols, sugar derivatives, alkaloids, α-amino acid esters, etc., which deactivate conventional metal salt catalysts, can be used. A 100 gram scale biodiesel production was also demonstrated.

Full text of DOI:10.1039/c7gc03858e

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Metal-Free Transesterification Catalyzed by
Tetramethylammonium Methyl Carbonate
Received 00th January 20xx,
Accepted 00th January 20xx
Manabu Hatano , Yuji Tabata, Yurika Yoshida, Kohei Toh, Kenji Yamashita, Yoshihiro Ogura and
Kazuaki Ishihara
*
DOI: 10.1039/x0xx00000x
rsc.li/greenchem
Environmentally benign metal-free tetramethylammonium methyl equimolar amounts of esters and alcohols in common organic
carbonate is effective as a catalyst for the chemoselective, solvents, quaternary ammonium salt catalysts, which would be
scalable, and reusable transesterification of various esters and regardless of chelation, might be promissing.15–17 However, it
alcohols in common organic solvents. In situ-generated highly is known that ammonium alkoxides ([R4N]+[OR’]–) are usually
active species, tetramethylammonium alkoxides, can greatly avoid unstable and moisture sensitive due to their strong basicity.18
the self-decomposition at ≤110 °C, and reusable. In particular, Therefore, it is difficult to apply them directly as catalysts for
chelating substrates, such as amino alcohols, diols, triols, sugar transesterification. In contrast, [R4N]+[OCO2Me]– are easy to
derivatives, alkaloids, α-amino acid esters, etc., which deactivate handle due to their stability.10e
conventional metal salt catalysts, can be used. A 100 gram scale transesterification by a catalytic use of in situ-generated
Here we describe a
biodiesel production was also demonstrated.
[R4N]+[OR’]– from [R4N]+[OCO2Me]–
1b).
Although [Me(n-octyl)3P]+[OCO2Me]– 2a19,20a–c and
3 and R’OH (substrate) (Fig.
DABCO-derived ammonium methyl carbonate20d have already
been known as effective catalysts for the transesterification of
DMC (dimethyl carbonate) as a solvent, no one has yet
reported the substrate generality including functionalized
esters and alcohols,20e since the instability of conventional
phosphonium salts (and ammonium salts) without excess DMC
would not allow the application to transesterification in
common solvents. Nevertheless, we made a great effort to
investigate the stability and activity of onium salts, and finally
found a great potential of the onium salts as transesterification
The transesterification of carboxylic esters with alcohols is an
indispensable synthetic method in organic chemistry.1
However, modern transesterification still often requires the
use of harmful, colored, and/or expensive metal salt catalysts,
such as highly regarded Al(III),2 Sb(III),3 Ti(IV),4 Sn(IV),5 Sm(III),6
Hf(IV),7 Zr(IV),7 Y(III),8 La(III),9,10 Zn(II),11 Fe(III),12 and Co(II)13
species.14 Moreover, these metal salt catalysts have another
serious drawback; i.e., they mostly cannot be used with
chelating substrates (Fig. 1a), and thus alcoholysis in alcohol-
solvents has been usually conducted.1–14 To overcome this
obstacle to the synthesis of highly functionalized esters from
catalysts. In particular, a new catalyst [Me4N]+[OCO2Me]– 3d
,
which is much more active than 2a, successfully addressed the
challenging problems so far unsolved by metal salt catalysts.
Table 1 shows a probe reaction of methyl salicylate 4a and
benzyl alcohol 5a in a Soxhlet thimble under azeotropic reflux
conditions of n-hexane (bp. 69 °C), where MS 5Å was used to
remove methanol (see the Electronic supplementary
information (ESI) for a reactor). Our previous La(III) catalyst
system
with
was not effective with chelating 4a, and product 6a
was obtained in 1% yield (entry 1). In contrast, phosphonium
salt 2a alone was much more effective than 2a2, and 6a was
La(NO3)3·H2O–[Me(n-octyl)3P]+[OCO2Me]–
10e
(1•2a2)
1•
Fig. 1 Outline of catalysts for transesterification.
obtained in 87% yield (entry 2). This excellent catalytic
performance might strongly depend on chelating 4a, since
1
•
2a2 was much better than 2a alone for non-chelating
substrates.10e Moreover, when we used ammonium salt
[Me(n-octyl)3N]+[OCO2Me]– 3a in place of 2a
6a was obtained
,
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Table 1 Screening of catalystsa
Table 2 Screening of ammonium salt catalysts 3a
Entry
Catalyst
Yield (%)
Yield
(%)
Entry
Catalyst
1
2
3
4
5
6
7
8
[Me(n-octyl)3N]+[OCO2Me]– (3a)
[Me2(n-octyl)2N]+[OCO2Me]– (3b)
[Me3(n-octyl)N]+[OCO2Me]– (3c)
[Me4N]+[OCO2Me]– (3d)
[Et4N]+[OCO2Me]– (3e)
26
57
64
85
0
1
2
3
4
1/2 La(NO3)3•H2O (1) + [Me(n-octyl)3P]+[OCO2Me]– (2a)
[Me(n-octyl)3P]+[OCO2Me]– (2a)
[Me(n-octyl)3N]+[OCO2Me]– (3a)
3a (1 mol%)
1
87 (71)b
90 (78)b
90c (90)c,d
[Me4N]+[OCO2H]– (3f)
[Me4N]+[OH]– (3g)
[Me4N]+[Cl]– (3h)
6
a
The reaction was carried out with 4a (2 mmol), 5a (2 mmol), and
catalyst (6 mol%) in n-hexane (bp. 69 °C) at 90 °C (bath temperature)
17
0
for 2 h. A Soxhlet extractor containing MS 5Å was used under
azeotropic reflux conditions. Yield of 6a for 30 min. Reaction was
b
c
a
d
carried out with 1 mol% of 3a for 5 h. MS 5Å was reused in another
reaction after removal of MeOH under reduced pressure.
The reaction was carried out with 4b (2 mmol), 7 (2 mmol), and
catalyst (6 mol%) in toluene (bp. 110 °C) at 140 °C (bath temperature)
for 1 h. The same reactor system was used as in Table 1.
in 90% yield (entry 3). Based on the yields for a shorter
reaction time (0.5 h), 3a was more active than 2a (entries 2
and 3). The catalyst loading of 3a could be reduced to 1 mol%
(entry 4).21,22 Moreover, MS 5Å could be reused without
deterioration in a next reaction after removal of MeOH under
reduced pressure (entry 4, parenthesis).
Fig. 3 Decomposition of 3a (Table 2, entry 1).
In this reaction, the [OCO2Me]– moiety of 2a and 3a would
quickly react with 5a, and irreversibly provide methanol, CO2,
and [Me(n-octyl)3P]+[OBn]– or [Me(n-octyl)3N]+[OBn]– as active
species in situ (Fig. 2, also see the ESI for 1H NMR
experiments). [Me(n-octyl)3N]+[OBn]– would form a simple
ion-pair, whereas [Me(n-octyl)3P]+[OBn]– would form a stable
pentacoordinated phosphorane23 (Fig. 2). Therefore, the
nucleophilicity of [Me(n-octyl)3P]+[OBn]– would be weakened.
A cationic nitrogen center of [Me(n-octyl)3N]+ cannot directly
activate an ester, but α-H moieties might act as Brønsted acids
(Fig. 2, right).24
[Et4N]+[OCO2Me]– 3e, which is slightly more sterically hindered
than 3d but has twelve β-H, showed no catalytic activity due to
the severe Hofmann elimination (entry 5). Although we
cannot completely exclude the effect of the steric factor of the
alkyl moieties in catalysts 3a–e, the Me group without β-H on
the nitrogen should be essential for increasing the yields.25
Moreover, the [OCO2Me]– moiety should also be important,
since [Me4N]+[OCO2H]– 3f, [Me4N]+[OH]– 3g, and [Me4N]+[Cl]–
3h showed poor catalytic activity (entries 6–8). Catalyst 3f
might be led to 3g in situ, which would react with 4b to
generate PhCO2H and then neutralized inactive species
[Me4N]+[OCOPh]–.
[Me4N]+[Cl]– 3h would be inherently
neutralized and unlikely to play a role in this catalysis.
Notably, white powdery 3d was dissolved and showed
homogeneous states in the reaction mixture. However, when
the reaction mixture was cooled at 0 °C, the 3d-derived viscous
deposit was generated. In this regard, we examined the reuse
of catalyst 3d (1 mol%) in the reaction of 4b (6 mmol) with 5b
(6 mmol) in n-hexane (Scheme 1). After the reaction, a clear
solution was divided from the mixture at 0 °C. Then, n-hexane,
Fig. 2 Active species in situ from 2a or 3a
.
However, during the initial investigation21 of the substrate
scope regardless of chelating or non-chelating substrates,
catalyst 3a was not effective for some low-reactive 2°-alcohols,
4b, and
5
such as sterically hindered 7, as shown in Table 2. Indeed, 3a
decomposed to Me(n-octyl)2N (81%) via the Hofmann
elimination of β-H and (n-octyl)3N (19%) via an SN2 reaction on
the Me group (entry 1, also see Fig. 3).18 To avoid these
decomposition pathways, we examined [Me2(n-octyl)2N]+
[OCO2Me]– 3b (entry 2), [Me3(n-octyl)N]+[OCO2Me]– 3c (entry
3), and [Me4N]+[OCO2Me]– 3d (entry 4), which have fewer β-H
moieties than 3a. In particular, 3d has no β-H moiety. As a
result, the yield of
8 was increased in the order 3a (26%) < 3b
(57%) < 3c (64%) < 3d (85%). This order also might seem to be
due to the bulkiness of the catalysts. However,
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Scheme 1 Recovery and reuse of the catalyst in gram scale
synthesis of 6b
were added, and the reaction restarted. Overall, 3d was
.
b
recovered and reused for the same reaction three times. Thus,
all crude products were combined and purified to give 5.48 g
(96% yield) of 6b 26
.
With the optimized catalyst 3d in hand, we demonstrated the
substrate scope by taking advantage of tolerance for highly
chelating substrates, which are difficult to apply in metal salt
catalysis (Scheme 2). As a result, amino alcohols could be
used, and chemoselective O-acylation proceeded in toluene to
give 10a in 94% yield and 10b in 78% yield. A pyridine alkaloid-
Scheme 3 Ten gram (10 g) scale synthesis of α-tocotrienol
analogue 10c
,
(+)-D
-isosorbide-derivative 10d
,
and α-
10e
.
tocotrienol (vitamin E) 10e27 could also be obtained in high
yields with the use of 3d in toluene. Remarkably, 25 mmol-
scale synthesis could be achieved (Scheme 3), and 10e was
obtained in 91% yield (10.6 g). Moreover, catalyst 3d was
effective for the synthesis of cholesterol-derivative 10f, but the
reaction stopped within 3 h when the yield was moderate
(74%), probably due to the decomposition of 3d. To avoid a
possible SN2 reaction on the Me group of catalyst 3d, we used
n-hexane (bp. 69 °C) in place of toluene (bp. 110 °C) under
azeotropic reflux conditions. As a result, the yield was
improved to 94%, although a prolonged reaction time (16 h)
was needed. For the synthesis of phosphoric acid ester 10g, n-
hexane was again better than toluene. Moreover, nucleic
acid-analogue 10h, which is hardly soluble in toluene and n-
hexane, was successfully obtained in solvating DMF (N,N-
dimethylformamide) at 90 °C. Indol/prolinol-derivative 10i
,
citric acid-derivative 10j, quinine- and quinidine-derivatives
10k and 10l were obtained in improved yields when the
solvent was changed to solvating THF (tetrahydrofuran).
Moreover, the chemoselective transesterification of chelating
N-unprotected optically active α-amino acid esters has been
scarcely
reported.28
Fortunately,
3d-catalyzed
-leucine and L-
transesterification of methyl esters of
L
methionine proceeded, and 10m and 10n were obtained
respectively without a serious loss of enantio-purity. Overall,
unlike with conventional metal salt catalysts, we could use
common solvents in the present transesterification.
We next examined the 3d-catalyzed transesterification in
substrates used as solvents (Scheme 4), since inexpensive ethyl
acetate (EtOAc), methyl acetate (MeOAc), DMC, methyl meth-
acrylate (MMA), and 2-propanol (i-PrOH) are often used as
solvents in transesterification, particularly in industry. EtOAc
and MeOAc could be used for chelating sugar-derivatives and a
Scheme 2 3d-Catalyzed transesterification. a Powdered MS 5Å
was used.
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Scheme 4 3d-Catalyzed transesterification with ester or catalyst and released glycerol were completely removed by
alcohol as a solvent. a Powdered MS 5Å was used.
routine extraction without silica gel column chromatography.
Remarkably,
a >100 gram scale synthesis of 12a was
performed without serious problems.30
Finally, we could control the chemoselective di- and mono-
transesterification of unsymmetrical 1,3-diols, by using
ammonium and phoshponium salts, respectively. 1,3-Diol-
derived crosslinkable bis(methacrylate) monomers have wide
applications to electronic, coating, and photoresist materials.31
However, since the intramolecular transesterification would
occur reversibly, chemoselective control is difficult.32 In this
regard, we finally used unsymmetrical 1°/3°-, 1°/2°-, and 2°/3°-
diols 13 with MMA (Scheme 6). Highly active ammonium salt
3d promoted the transesterification reaction of diols 13a
–c to
give diesters 15a–c exclusively. In contrast, much less active
phosphonium salt 2a instead of 3d provided monoesters 14a–c
exclusively.
Conclusions
Scheme 5 Biodiesel production with model substrates 11
.
In
summary,
we have developed
metal
free
[Me4N]+[OCO2Me]– for the transesterification of various
functionalized esters and alcohols. In particular, unlike
conventional metal salt catalysts, chelating substrates could be
used in common solvents. Moreover, biodiesel production,
chemoselective transesterification, gram-scale reaction, and
catalyst recycling were demonstrated for environmentally
benign use in the laboratory as well as in industrial process
chemistry.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Scheme 6 Selective transesterification of with MMA.
triol (see 10o ). DMC was used for tertiary alcohols (see 10r
Financial support was partially provided by JSPS KAKENHI Grant
Numbers 17H03054, 15H05755, and JP15H05810 in Precisely
Designed Catalysts with Customized Scaffolding.
–
q
and 10s). Synthetically useful borneol-, isoborneol-, and
tropine-derived acrylates, which can be used in synthetic
resins and refractive lenses, were successfully synthesized in
Notes and references
MMA (see 10t–v). Moreover, i-PrOH could be used as asolvent
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DOI: 10.1039/C7GC03858E
COMMUNICATION
Adv., 2014, , 32127; Moreover, BnEt3NCl was used as a
Journal Name
4
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21 The generality of 3a-catalysis for chelating substrates was
briefly examined (See Scheme S1 in the ESI).
22 When 1 mol% of catalyst 3d was used in place of 3a, the
reaction proceed more smoothly (90% yield within 1 h).
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25 Reaction of 4b with acyclic and cyclic alcohols with the use of
3a or 3d were systematically examined in Scheme S2 in the
ESI.
26 Recovered catalysts might be [Me4N]+[OMe]– and/or
[Me4N]+[OR]–. In the next new batch reaction of 4b and 5b
,
MS 5Å might remove methanol systematically under
azeotropic reflux conditions and hence shall increase the
amount of Me4N]+[OR]– at the expense of [Me4N]+[OMe]–.
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amino acid esters has been reported in refs. 10e, 11b, 11f,
11h, 11i, and 12d. An example with an NH2-free substrate
was reported in ref. 11h.
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.
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6 | Green Chem., 2018, 20, xxxx-xxxx
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