Highly efficient carbene and polycarbene catalysis of the transesterification reaction

Article abstract of DOI:10.24820/ark.5550190.p010.034

Highly efficient carbene and polycarbene catalysis of the transesterification reaction of ethyl benzoate in methanol has been observed and results in values of TON as high as 4000–6150 at a molar ratio of ester to methanol of 1:18 at room temperature. The most effective catalysts were found to be the individual carbenes or in situ generated carbenes, namely adamantyl and aromatic substituted cyclic compounds and polymeric carbenes. None of the reactions requires the use of molecular sieves. The polymeric imidazol-2-ylidene catalyst was used for the efficient production of biodiesel fuel from sunflower oil.

Full text of DOI:10.24820/ark.5550190.p010.034

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Arkivoc 2017, part iv, 365-376
Organic Chemistry
Highly efficient carbene and polycarbene catalysis
of the transesterification reaction
Konstantin A. Marichev,a Nikolai I. Korotkikh,*b Alan H. Cowley,c Vagiz Sh. Saberov,b
Nataliya V. Glinyanaya,a Gennady F. Rayenko,a and Oles P. Shvaika a
a The L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, Ukrainian Academy of Sciences,
50 Kharkiv road, Kiev 02160, Ukraine
b The Institute of Organic Chemistry, Ukrainian Academy of Sciences, 5, Murmanskaya Str.,
Kyiv-94, 02660, Ukraine
c Department of Chemistry, The University of Texas at Austin, 1 University Station A5300,
Austin, Texas 78712-0165, U.S.A.
Email: nkorotkikh@ua.fm
Received 01-27-2017
Accepted 03-12-2017
Published on line 06-27-2017
Abstract
Highly efficient carbene and polycarbene catalysis of the transesterification reaction of ethyl benzoate in
methanol has been observed and results in values of TON as high as 4000–6150 at a molar ratio of ester to
methanol of 1:18 at room temperature. The most effective catalysts were found to be the individual carbenes
or in situ generated carbenes, namely adamantyl and aromatic substituted cyclic compounds and polymeric
carbenes. None of the reactions requires the use of molecular sieves. The polymeric imidazol-2-ylidene
catalyst was used for the efficient production of biodiesel fuel from sunflower oil.
Keywords: Carbenes, transesterification, catalysis, efficiencies
DOI: https://doi.org/10.24820/ark.5550190.p010.034
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Introduction
The transesterification reaction is one of the mildest and most effective methods for the preparation of
esters.1 However, the traditional catalysts are either insufficiently effective due to their basic nature or cause
corrosion of the metallic equipment (inorganic acids). Since stable carbenes are highly basic compounds (pKa
21–28) they have also been suggested as catalysts for the transesterification reaction.2-8 The latter reaction is
typically carried out at room temperature in the presence of 0.5–5.0 mol% of carbenes and molecular sieves.
The efficiencies of the catalysts can be estimated on the basis of the values of TON and TOF. TON is a turnover
number that means a number of catalytic cycles made (the ratio of a mol number of the product to a mol
number of the catalyst). TOF is a turnover frequency that means a number of catalytic cycles per a unit of
time, the most often per one hour (TON per a unit of time). In the case of reactions that are catalysed by
imidazol-2-ylidenes and their 4,5-dihydro analogues, the TON values typically fall in the range of 20-60, TOF
30–220 h-1.2 However, larger values of 200 and 2300 respectively have been observed for the reactions of vinyl
esters due to the irreversibility of the acetaldehyde release process.4
The transesterification reaction serves as an important tool for the production of biodiesel fuel. In the
latter case the reaction is carried out in an excess of alcohol. To the best of our knowledge, there has only
been one attempt to carry out a similar catalytic process using carbenes on a model compound.3 In this paper
the transesterification of methyl benzoate with ethanol and isopropanol in a 1:20 molar ratio was carried out
in the presence of 3.6–4.4 mol% of 1,3-disubstituted imidazol-2-ylidenes and resulted in 79–85% yields of
ethyl esters (TON 17-19, TOF 1 h-1).
The catalytic activities of other classes of stable carbene have not been studied yet.
The goal of the present work was to study the catalytic properties of a variety of types of stable carbenes
for use in transesterification reactions.
It was particularly important taking into account moderate or low efficiences of carbenes in many other
organic reactions (in most cases TON is no more than 100).9,10
Results and Discussion
Preparation of carbenes and precarbenes
Ethyl benzoate was used as the model compound for the transformation to methyl benzoate in the
presence of a catalyst in methanol solution (Scheme 1). The stable carbenes 1-11 were used as catalysts for
this reaction (Schemes 1 and 2), and were generated in situ from the corresponding azolium salts (BH+X-), or
alternatively used as individual compounds (1a, 2d, 3c, 7). The requisite compounds were the monocarbenes
1-6, the biscarbenes 7,8 and the polycarbenes 9-11. In alcohol solutions all of these compounds are in
equilibrium with alkoxyazolines and H-complexes of carbenes (see, for example, the equilibrium 1 ⇄ 1A ⇄ 1B
in Scheme 3).
The requisite carbenes 1-11 were generated in situ using the appropriate precarbenes (BH+X) and
potassium methoxide in anhydrous methanol solution. The corresponding organic perchlorate salts were used
as precarbenes.
For comparison purposes the catalysis was also studied with ammonium (12) and phosphonium (13)
methoxides which together with sodium methoxide can clear up the role of alkoxide ions in catalysis.
However, it should be noted that the first compound can exist in equilibrium with ammonium ylides,11 as can
the latter compound with the appropriate phosphorane 13A (Scheme 3).
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1a R = t-Bu, Ar = p-BrC6H4; 1b R = p-BrC6H4CH2, Ar = p-BrC6H4; 2a R = Bn, R′ = H; 2b R = 1-Ad, R′ = H; 2c 1-Ad,
R′ = Ph; 2d R = dbep, R′ = H; 3a R = Me, 3b R = Bn, 3c R = 1-Ad. dbep = 2,6-dibenzhydryl-4-ethylphenyl
Scheme 1. Monocarbene catalysts for transesterification of ethyl benzoate in methanol
Scheme 2. Biscarbene and polycarbene catalysts for transesterification of ethyl benzoate in methanol
Scheme 3. Equilibria of 1,2,4-triazol-5-ylidenes in methanol. Ammonium and phosphonium methoxides as
catalysts for transesterification of ethyl benzoate in methanol
Methods for the preparation of precarbenes for compounds 1-7 are well known and are indicated in the
Supporting Information (SI).12-28 The known carbenes 1a, 2d, 3c, 7 were used in an isolated state. Methods for
their preparation are also given in the SI.
The structures of the carbenes 1a, 2d, 3c, 7 were confirmed on the basis of single crystal X-ray diffraction
studies.12, 15, 20, 22, 23, 25
The bisazolium salts 8a,b.2НХ were synthesized by quaternization of 1,4-di-(1-imidazolyl)butane 8A or
p-xylylenedi-(1-imidazole) 8B with dimethyl sulfate, followed by the anion exchange to form the perchlorate
(Scheme 4). The polymeric salts for carbenes 9-11 were obtained by quaternization of 4,4′-dipyridyl, p-bis-(1-
imidazolyl)benzene 10A and p-xylylenedi-(1-imidazole) 8B, respectively, with p-xylylene dichloride (for
compounds 10.2НClO4 and 11.2НClO4 see Scheme 5). Experimental procedures for the preparation of
compounds 8a,b.2НClO4, 9-11.2НClO4 are presented below.
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Scheme 4. Synthesis of bisimidazolium salts 8a,b.2НClO4
Scheme 5. Synthesis of polymeric imidazolium salts 10, 11.2НClO4.
.
.
The compositions of compounds 9, 10 2НClO4, 10A, 11A and 11a,b 2НClO4 have been confirmed by
1
elemental analysis and the structures by H NMR spectroscopy and liquid chromatography. The characteristic
signal of meso-proton of the imidazole ring (δ 8.37 ppm) was observed in the H NMR spectra of compound
1
1
10A. The H NMR spectrum of compound 11A shows the characteristic signals of the methylene (δ 5.10 ppm)
and the meso-proton of the imidazole ring (δ C2H 7.52 ppm).
Compound 11.2НClO4 is easily soluble in polar organic solvents, such as dimethylformamide,
dimethylsulfoxide and insoluble in other organic solvents. According to the liquid chromatography data the
polymer 11.2НClO4 has a narrow molecular weight distribution (Mw 68800; Mn 63100; Mw / Mn = 1.09; naver
=
1
257). The low degree of polydispersity of the polymer is also confirmed by the narrow single signals in the H
NMR spectrum. Particularly, there are narrow signals of methylene (δ 5.44 ppm), aromatic protons (δ 7.50 -
7.78 ppm) and meso-protons (C2H) of the imidazolium ring (δ 9.37 ppm).
It should be noted that the polymeric salt 10.2НClO4 has conjugated fragments and a monomeric unit
comprises two carbenoid centers unlike the polyxylyleneimidazolium polymer 11.2НClO4 that includes only
one carbenoid center in a monomeric unit.
The singlet proton signals for polymeric salts 9.2НClO4 and 10.2НClO4 are expressed rather clearly. The
signals of methylene group (δ 5.95 ppm), aromatic proton and pyridinium phenylene nuclei (δ 7.70 – 8.73
ppm) were found in the spectrum of the polymer salt 9.2НClO4 along with C2H-signals of the pyridinium nuclei
that are shifted downfield (δ 9.33 and 9.47 ppm). The signals of meso-protons of salt 10.2НClO4 (C2H) is
strongly shifted downfield (δ 10.11 ppm) as compared with bisimidazolium salt 8a (δ 9.05 ppm), to a lesser
extent, but also shifted downfield the characteristic signals of the methylene (δ 5.58 ppm) and aromatic
protons (δ 7.65 - 8.44 ppm).
Catalytic results
The overall results of the catalytic transesterification of compounds 1-11 are presented in the Table.
The data in the table revealed that the efficiency of a sodium methoxide in these reactions is minimal
(TON 7–17, TOF 8–22 h-1). As a consequence a significant concentration of the catalyst and an increase in
temperature are necessary for generating high yields of the product under the conditions that were selected
for this experiment (the ratio of the reactants was chosen to be 1:9, 0.04 mol% of the catalyst and 4 h at room
temperature).
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Table. The catalytic properties of carbenes 1-11 and compounds 12 and 13 in the transesterification reaction
of ethyl benzoate in methanol solution (molar ratio of reactants 1:9) at 23 °С
Catalyst Catalyst loading, (mol%) Time, h
Yield, %
TON
TOF, h-1
CH3ONa
CH3ONa
1а
1b
2а
2b
2b
2c
5.00
2.00
0.04
0.04
0.04
0.04
0.01 a
0.04
0.01 a
0.04
0.04
0.04
0.04
0.01 a
0.01 b
0.04
0.04
0.04
0.04
0.04
0.04
0.5
2.00
0.33
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
0.33
0.33
4.00
4.00
4.00
4.00
4.00
85
14,5
7
17
8,5
21,8
44
7,3
175
9
225
56
65
93.5
35
94
43
0
1625
2338
3500
2350
4300
0
406
584
875
588
1075
0
2c
2d
3a
3b
3c
72
70
93,5
26
61,5
84
88,5
90
25
52
87
94
84
95
94
88
19
40
94
88
46
65
1800
1750
2338
2600
6150
2100
2213
2250
625
450
438
585
650
1538
525
553
563
156
325
544
47
3c
3c
4
5
6
7
8a
8b
9
1300
2175
188
9
0.04
0.5
2100
190
525
48
10
10
11
11
11
11
11
12
13
0.04
2350
880
588
2667
5758
1000
588
667
288
394
0.1
0.01 a
0.01 a
0.04
0.033 c
0.04
0.04
1900
4000
2350
2667
1150
1575
a
b
The experiment was carried out at the molar ratio ester-methanol 1:18, for 4 h at room temperature; with
c
individual carbene 3с; with sunflower oil. The catalyst loading, TON and TOF are indicated on the basis of one ester
group of the oil.
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The efficiency of the triazolylidene catalysts 1a, b, 7 (either in an individual state or in an situ generated
form) was found to be higher than that for sodium methoxide. The carbenes 1a and 1 b were found to be less
active (TON 175–225, TOF 44–56 h-1) than those of the biscarbene 7 (TON 625, TOF 156 h-1). It is interesting to
note that under identical conditions, the increase in the number of catalytic centers in one molecule of
compound 7 relative to that of 1a resulted in a higher yield of methyl benzoate by almost four times (25%). In
the case of the highly sterically shielded carbene with branched aromatic substituents (2d) the catalytic effect
was not evident.
The efficiencies of carbenes 1 and 7 of the triazole series were found to be significantly inferior to those of
carbenes 2a, 3a and 3b of the imidazole and benzimidazole series (TON 1620–1800, TOF 270–450 h-1). Even at
loadings of 0.04 mol% for compounds 2a, 3a and 3b yields of methyl benzoate can be achieved in the region of
65–72% when exposed to room temperature for 4 hours. However, on the other hand, the arylene substituted
biscarbene 8b exhibited an even higher efficiency (TON 2183, TOF 546 h-1), by contrast the biscarbene 8a with
an aliphatic bridge is significantly lower in terms of efficiency (TON 1300, TOF 325 h-1).
In the cases of the diaminocarbenes 4 and 5 and the cycloheptatrienylidene 6 the efficiencies were found
to be even higher (TON 2100–2250, TOF 525–563 h-1) than those for the imidazol-2-ylidene 2a and the
benzimidazol-2-ylidenes 3a and 3b. Overall, the adamantyl substituted imidazol-2-ylidenes 2b and 2c and the
benzimidazol-2-ylidene 3c were found to achieve the highest levels of efficiency (TON 2338–2350, TOF 584–
588 h-1).
The carbenes 9-11 were also found to be highly efficient polymeric catalysts and exhibited high TON
values in the range of 2100–2350.
In terms of relative efficiencies RTON represents a TON value that is comparable to that for the reference
catalyst (taken as a unit). Under identical experimental conditions, the following series for the efficiencies
emerged for the carbene catalysts that were examined: 2d (0) < 1a (1,0) < 1b 1,5) < 7 (1,8) < 8a (3,7) < 8b
(6,2) < 9 (6,0) < 10 (6,7) < 2a (9,3) < 3b (10,0) < 3a (10,3) < 4 (12,0) < 5 (12,6) < 6 (12,9) < 3c = 2b (13,3) < 2c =
11 (13,4).
An additional increase in efficiency was observed for the carbenes (TON 2600–4300 and TOF 650–1075
h-1) using a 0.01 mol% loading and a ratio of the reagents of 1:18. In this case a series of efficiencies was
evident from the values of TON and TOF,in h-1: 3c (2600, 650) < 2b (3500, 875) < 11 (4000, 1000) < 2c (4300,
1075)
Compounds 2b, c, 3c, 11 exhibited the highest catalytic efficiencies for the transesterification reaction.
Moreover, in this case the method does not require the use of molecular sieves to produce high yields of the
end products. Among the individual carbenes (1a, 2d, 3c, 7) compound 3c was found to have the highest
efficiency (TON 6150, TOF 1538 h-1).
The following conclusions can be made with respect to the different factors that influence the catalytic
efficiencies.
1) The largest influence on the catalytic efficiencies of the carbenes is exhibited by carbenes that feature N-
adamantyl- and N-aromatic groups (even if they are not connected to a heterocyclic nucleus such as p-
xylylene) (2b, c, 3c, 4, 5, 8b, 10, 11). The carbocyclic aromatic carbene 6 is also very efficient. In some cases
the TON values can be very large (for 3c up to 6150 cycles).
The high effect of adamantyl substituted compounds 2b,c, 3c, 4, 5 can be explained by their largest
nucleophilicities. For compounds 8b, 10, 11 not only carbene centers can play an important role but also their
ylidic forms (being in an equilibrium).
2) The steric shielding of carbenes (for example, by the use of adamantyl groups) increases the efficiency of
the catalysis. It should be recalled, however, that some results depend on the extent of steric shielding. For
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example, carbene 2d has a buried volume of %Vbur 53% (for the evaluation of analogues) 29,30 but, by contrast,
does not show a notable catalytic effect.
3) Individual carbenes are considerably more efficient than those of the in situ generated compounds (in the
case of carbene 3c by 2,6 times). Furthermore, in the latter case the efficiency is also dependent on the
method that was employed for carbene generation.
The foregoing results indicate that catalyst 11 would be one of the best choices for the transesterification
reaction of plant oils (esters of fatty acids and glycerol). In fact, when the catalytic green process was carried
out on sunflower oil in the presence of compound 11 [0,1 mol% or 0.033 mol% for one ester function] this
catalyst worked with high efficiency (TON 2667, TOF 667 h-1) and afforded an 88% yield of the distilled
colorless mixture of methyl esters of fatty acids that are predominantly linoleic (75%) and oleic (23%), that in
turn can be used in industry for the production of e.g. perfume additives or biodiesel fuel. In the latter case,
only technical oils should be used (from e.g. palm, rapeseed, algae, etc.).
It should be noted that until now many catalysts have been proposed for the preparation of biodiesel fuel
(as reviews see, for example, the papers 31-34). Among them special attention was paid to guanidine derivatives
(see also 35-38), which having basicities close to carbenes do not form soaps during the reaction and their
catalytic efficiencies are close to those for sodium hydroxide and methoxide (for 1,5,7-triazabicyclo[4.4.0]dec-
5-ene TBD TON is no more than 270 at 70 C, 1 mol% of a catalyst, the molar ratio of rapeseed oil : methanol is
1:2.3). As can be seen from the data obtained in the present work carbenes are considerably more efficient
(TON 2667 at room temperature, 0,1 mol% of a catalyst, the molar ratio of sunflower oil : methanol is 1:20).
The mechanism of the catalysis of the transesterification reaction by carbenes has not yet been studied in
detail.27 However, the low efficiency of sodium methoxide in the reaction indicates that the role of the
carbene catalyst is not limited by the reaction of alkoxide ions with the ester. The ammonium (12) and
phosphonium (13) alkoxides that are generated from the corresponding salts are significantly more effective
as stronger bases (TON 1150, 1575 and TOF 288, 394 h-1, respectively) than sodium methoxide (probably due
to their ylidic forms playing a part in the reaction). As a consequence, the phosphonium ylide, easily obtained
from salt 13, is more effective than the ammonium ylide from salt 12. However, carbenes 2-6, 8b, 9-11 are
significantly more efficient than those of the ammonium and phosphonium catalysts derived from salts 12 and
13. The increase of the efficiencies of the heterocyclic carbene catalysts relative to those of the onium ylides
from 12 and 13 points to an important role of the carbene centers for catalysis, particularly multiplet ones to
activate the ester molecules.
Several possible versions of the mechanism have been discussed and presented. Examples include the
work described in research reports39,40 or in monographs9,10. However, a common feature of these
mechanisms is the participation of carbenes that are connected to alcohols by means of H-bonds such as those
in 1B or in azoline forms such as 1A, with which they are in equilibrium. These particles are bonded by a
carbene transfer from the alcohol component to the activated ester carbon atom and accept the leaving
molecule of alcohol.
Conclusions
As a consequence of the foregoing, a highly efficient catalysis by mono-, bis- and polyheterocyclic and
carbocyclic carbenes was observed for the transesterification reaction when carried out in an excess of
alcohol. Furthermore, the TON and TOF indices were found to exceed significantly those of the known
catalytic effect of carbenes that have close to stoichiometric quantities of the reactants in the presence of
molecular sieves. They also exceed those of the known process by two orders of magnitude in an excess of
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alcohol. The possibility of using polymer 11 with high efficiency to produce biodiesel fuel has in fact been
demonstrated in an experiment on sunflower oil.
Experimental Section
General information
1
13
1
The H NMR and C NMR spectra were recorded using a Bruker Avance II 400 spectrometer (400 MHz for H
NMR spectra and 100 MHz for 13C NMR spectra) or Gemini 200 spectrometer in DMSO-d6 or CDCl3 solutions or
in the solid state.
1
The H NMR and 13C NMR chemical shifts are reported relative to those of tetramethylsilane (TMS) (solution)
and sodium 2,2-dimethyl-2-silapentan-5-sulfonate (DSS) (solid state). The thin-layer chromatography was
performed on silica gel with chloroform or a 10 : 1 mixture of chloroform and methanol as an eluent, followed
by development with iodine. All elemental analyses were carried out at the Analytical Laboratory of the
Litvinenko Institute of Physical Organic and Coal Chemistry.
The molecular characteristics of the oligomers and polymers were studied on a Du Pont device for liquid
chromatography that was equipped with a set of bimodal columns Zorbax PSM-100, and 1000, each of which
can give a linear calibration in the molecular weight range 102 – 106. The chromatograph was calibrated with
a polystyrene standard Du Pont PS with molecular weights of Mw 1,000 and 50,000 (Mw / Mn 1.01) along with
a sample of the known salt 2a.НClO4. The polymer yield from the column was found by a UV sensor that was
tuned to a wavelength of 282 nm.
3,3'-Dimethyl-1,4-butylenedi-(1-imidazolium) perchlorate (8a.2НClO4). To
a solution of 1,4-bis-(1-
imidazolyl)butane 8A (X = (CH2)4) (0.3 g, 1.58 mmol) in acetonitrile (2 mL) dimethyl sulfate (0.4 g, 3.16 mmol)
was added upon stirring. The reaction mixture was heated to reflux for 4 h, diluted with acetonitrile (5 mL),
and sodium perchlorate (0.39 g, 3.16 mmol) was added thereby forming a precipitate of sodium methyl sulfate
that was filtered off. The mother liquor was concentrated to 2 mL and the perchlorate 8a was precipitated by
1
diethyl ether (10 mL). Yield 0.5 g (76 %). Mp 86–88 С (acetonitrile–diethyl ether). Н NMR (400MHz, DMSO-
d6), H: 1.76 (4Н, s, CH2C), 3.84 (6H, s, CH3N), 4.19 (4Н, s, CH2N), 7.71 (4Н, d, J 9.6 Hz, C4,5Н), 9.05 (2Н, s, C2Н).
Anal. Calcd for С12Н20Cl2N4О8 (419.22): С 34.38, Н 4.81, Cl 16.91, N 13.36. Found: С 34.47, Н 4.73, Cl 16.84, N
13.40%.
3,3'-Dimethyl-1,4-xylylenedi-(1-imidazolium) perchlorate (8b.2НClO4)
а) (1,4-Xylylene)-1,1'-diimidazole (8B, X = CH2C6H4CH2). To a reaction mixture containing imidazole (1.55 g,
22.8 mmol), sodium hydroxide (6.8 g, 170 mmol), triethylbenzylammonium chloride (0.26 g, 1.14 mmol) and
water (7 mL), a solution of 1,4-xylylene dichloride (2 g, 11.4 mmol) in acetonitrile (10 mL) was added
portionwise upon stirring. The mixture was stirred at 50 ºС for 7 h thus forming the solid. After cooling, the
reaction mixture was treated with water (50 mL). The precipitate was filtered off, washed with water (50 mL)
1
and dried. Yield 2.3 g (85%). Mp 134–136 С (toluene). Н NMR (400MHz, DMSO-d6), H: 5.10 (4Н, s, СH2N),
6.88 (2Н, s, C4,5H), 7.07 (2Н, s, C4,5H), 7.13 m (4Н, Ar), 7.52 (2Н, s, C2HN). The product 8B was identical to that
reported27 from imidazole and 1,4-xylylene dichloride in the presence of NaH in DMF.
b) 3,3'-Dimethyl-1,4-xylylenedi-(1-imidazolium) perchlorate (8b.2НClO4). To a solution of (1,4-xylylene)-1,1'-
diimidazole 8B (0.84 g, 3,36 mmol) in acetonitrile (20 mL) dimethyl sulfate (0.85 g, 6.72 mmol) was added
portionwise upon cooling, stirred additionally for 20 min at room temperature and then heating to reflux for
4 h. The solvent was evaporated, the oily product was dissolved in hot water, a small amount of activated
charcoal was added, and the solution was filtered. Sodium perchlorate (0.82 g, 6.72 mmol) was added to the
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1
filtrate, and the precipitate formed was filtered off. Yield 0.93 g (59%). Mp 222–225 С (water). Н NMR
(400MHz, DMSO-d6), H: 3.86 (6Н, s, CH3N), 5.43 (4Н, s, СH2N), 7.48 (4Н, s), 7.69 (2H, s), 7.75 (2Н, s) (Ar), 9.18,
9.23 (2Н, s, C2Н). Anal. Calcd for C16H20Cl2N4O8 (467.26): С 41.13, Н 4.31, Cl 15,17, N 11.99. Found: С 41.04, Н
4.39, Cl 15.28, N 12.02%.
Poly-,'-(1,4-xylylene)-4,4'-dipyridinium perchlorate (9.2НClO4). The mixture of 4,4'-dipyridyl (1,35 g, 8,65
mmol) and 1,4-xilylene dichloride (1.51 g, 8.65 mmol) was heated at 120 С in a melt for 1 h. DMF (5 mL) was
added and heated at 120 С for 8 h. The precipitate was filtered off, washed with DMF (5 mL) and diethyl
ether (20 mL). The chloride salt was dissolved in water, the solution was treated with activated carbon, filtered
and treated with sodium perchlorate (2.2 g, 18 mmol). Polydipyridinium perchlorate 9.2НClO4 was filtered off
and dried at 120 ºС. Yield 2.8 g (71 %). Mp > 300 С. 1Н NMR (400MHz, DMSO-d6), H: 5.95 (4Н, s, CH2N), 7.70,
8.02 (4Н, s), 8.64, 8.73, 8.87 (4Н, s) (Ar), 9.33, 9.47 (4Н, s) (C-2-HN). Anal. Calcd for (C18H16Cl2N2O8)n (459.24): С
47.08, Н 3.51, Cl 15.44, N 6.10. Found: С 47.23, Н 3.44, Cl 15.24, N 6.20 %
Poly-[,'-1,4-xylylene-1,4-di-(1-imidazolium-3,3'-diyl)benzene] perchlorate (10.2НClO4).
а) 1,4-Phenylene-1,1'-diimidazole (10A). To a solution of 1,4-dibromobenzene (5 g, 21 mmol) and anhydrous
imidazole (3.6 g, 53 mmol) in dimethylsulfoxide (25 mL) potassium carbonate (7.31 g, 53 mmol) and copper(II)
o
oxide (0.42 g, 5,3 mmol) were added. The reaction mixture was heated at 150 С for 50 h with stirring. The
product together with the catalyst was treated with water (100 mL), the precipitate formed was filtered off
and washed with water (30 mL). The product was separated from the catalyst by extraction with chloroform.
The solution was filtered through a thin layer of silica gel (the eluent is chloroform-methanol, 5:1) and was
evaporated. Yield 3 g (68 %). Mp 206–208 С (benzene). 1Н NMR (400MHz, CDCl3), H: 7.16 (2Н, s, C4,5H), 7.82
(2Н, s, C4,5H), 7.83 (4Н, s, Ar), 7.52 (2Н, s, C2HN). Anal. Calcd for С12Н10N4 (210.24): С 68.56, Н 4.79, N 26.65.
Found: С 68.50, Н 4.87, N 26.63%. The product 10A was identical to that obtained from 1,4-dibromobenzene,
imidazole, K2CO3 and CuSO4 without a solvent.28
b) Poly-[,′-1,4-xylylene-1,4-di-(1-imidazolium-3,3′-diyl)benzene] perchlorate (10.2НClO4). A solution of
1,4-phenylene-1,1'-diimidazole 10A (0.5 g, 2.38 mmol) and 1,4-xylylene dichloride (0.42 g, 2.38 mmol) in DMF
(2 mL) was refluxed for 2 h. The precipitate was formed after 10 min of refluxing. The product was filtered off,
washed with DMF (3 mL) and diethyl ether (15 mL). It was dissolved in water (20 mL) and added an equimolar
amount of sodium perchlorate. The precipitate of polyimidazolium perchlorate 10.2НClO4 was filtered off and
1
dried at 120 ºС. Yield 1.22 g (100 %). Mp > 300 С. Н NMR (400MHz, DMSO-d6), H: 5.57 (4Н, s, CH2N), 7.65
(4Н, s), 8.14 (4Н, s) (Ar), 8.07 (2Н, s), 8.44 (2Н, s) (C4,5), 10.11 (2Н, s, C2H). Anal. Calcd for (C20H18Cl2N4O8)n
(513.29): С 46.80, Н 3.53, Cl 13.81, N 10.92. Found, %: С 46.63, Н 3.54, Cl 14.01, N 10.88.
Poly-,′-1,4-xylyleneimidazolium perchlorate (11.2НClO4). A mixture of 1,4-xylylene-1,1'-diimidazole 8B
(0.31 g, 1.26 mmol) and 1,4-xylylene dichloride (0.22 g, 1.26 mmol) was heated in a melt at 150 ºС for 1 h.
DMF (2 mL) was added and heated at 150 ºС for 8 h. The reaction mixture was diluted with diethyl ether and
the solvent was decanted. The chloride salt was dissolved in water (15 mL) and treated with an excess amount
of sodium perchlorate (0.17 g, 1.4 mmol). The precipitate formed was filtered off and dried at 120 ºС. Yield 0.5
g (74 %). Mp 256–258С (toluene). 1Н NMR (400MHz, DMSO-d6), H: 5.44 (4Н, s, СH2N), 7.49 (4Н, m, Ar), 7.78
(2Н, s, C4,5H), 9.37 (1Н, s, C2HN). Anal. Calcd for (С11Н11ClN2O4)2n (270.57): С 48.81, Н 4.10, Cl 13.10, N 10.35.
Found, %: С 48.82, Н 4.19, Cl 13.02, N 10.34. According to the liquid chromatography data Мw 68800; Mn
63100; Mw/Mn = 1.09; naver = 257.
Procedure for catalytic measurements.
The catalytic transesterification reaction of ethyl benzoate to methyl benzoate with carbenes 1-11 and onium
alkoxides 12,13 were carried out in an anhydrous methanol solution (using a molar ratio of ester-methanol of
1:9 or 1:18). Molecular sieves were not used in this process.
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To a solution of ethyl benzoate (3.15 mL, 22 mmol) in a methanolic solution (8 mL) of potassium methoxide
(0.616 g, 0.0088 mmol) a salt precatalyst (0.0088 mmol) was added and stirred for 4 h at room temperature.
Diethyl ether (30 mL) was added to the reaction solution, the organic layer was washed with water (320 mL),
separated and dried with anhydrous sodium sulfate. The ether was evaporated and the residue was distilled in
1
a vacuum. The distillate was analyzed by H NMR spectroscopy on the contents of methyl benzoate and ethyl
benzoate ( СН3О 3.85 ppm, ethyl benzoate  СН3С 1.34 ppm and  СН2С 4.34 ppm in CDCl3).
Procedure for the preparation of the sunflower biodiesel fuel
The sunflower oil has been preliminary dried upon stirring with CaH2 in hexane for a night. The polymeric salt
11.2НClO4 (2,66 mg, 9,84.10-3 mmol, 0.033 mol% to one ester function of oil) was added to a solution of
stoichiometric amount of potassium methoxide (0,689 mg, 9,84.10-3 mmol) in anhydrous methanol (6 mL) and
stirred for 10 min. The obtained solution was added to a solution of anhydrous sun flower oil (10 g) in
anhydrous methanol (18 mL) and stirred at room temperature (23 C) for 4 h. Diethyl ether (30 mL) was added
to the reaction solution, the organic layer was washed with water (320 mL), separated and dried with
anhydrous sodium sulfate. The ether was evaporated and the residue was distilled in a vacuum up to dryness
collecting a fraction b.p. 166-172 C /2 mm Hg as a colorless mobile liquid. Yield 7.75 g (88 % according to the
ester function contents in oil 2.98 mmol/1g that corresponds to a saponification number 167 mg/1g). Contains
1
predominantly methyl esters of oleic and linoleic acids. Н NMR (400MHz, CDCl3), H: 0.87 (3H, s, CH3C), 1.24
(16H, s, CH2C), 1.61 (2H, m, CH2C), 2.02 (3H, m, CH2C), 2.28 (2H, t, CH2C, J 7,6 Hz), 2.75 (1H, dd, CH2C, J 6.0 Hz,
methyl linoleate, CH2-CH=), 3.64 (3H, s, CH3O), 5.33 (3H, m, CH=CH).
Acknowledgements
We thank the Ukrainian Academy of Sciences for financial support (grant No. III-02-2015), and the generous
financial support provided by the Robert A. Welch Foundation (grant F-0003) (A.H.C.).
Supplementary Material
Supplementary data to this article can be found online. These data include basic methods and schemes for the
preparation of the known carbenes and precarbenes used in the work, and 1H NMR spectra of the synthesized
compounds.
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