Preparation of a series of N-alkyl-3-boronopyridinium halides and study of their stability in the presence of hydrogen peroxide

Article abstract of DOI:10.2478/s11532-012-0006-5

A simple and efficient protocol for the preparation of a series of N-alkyl-3-boronopyridinium salts is described which requires exposure of 3-pyridineboronic acid neopentylglycol ester and corresponding alkyle halide to microwave irradiation followed by b

Full text of DOI:10.2478/s11532-012-0006-5

Cent. Eur. J. Chem. • 10(4) • 2012 • 1059-1065
DOI: 10.2478/s11532-012-0006-5
Central European Journal of Chemistry
Preparation of a series of N-alkyl-3-boronopyri-
dinium halides and study of their stability
in the presence of hydrogen peroxide
Research Article
Yevgen Karpichev1,2*, Hubert Matondo1, Illia Kapitanov2,
Oleksandr Savsunenko1,2, Marc Vedrenne3,
Verena Poinsot1, Isabelle Rico-Lattes1, Armand Lattes1
1Laboratory of IMRCP, UMR 5623 CNRS -
Paul Sabatier University, Toulouse 31062, France
2L.M.Litvinenko Institute of Physical Organic Chemistry & Coal Chemistry,
83114 Donetsk, Ukraine
3NMR Service, Paul Sabatier University, Toulouse 31062, France
Received 30 September 2011; Accepted 30 December 2011
Abstract:
A simple and efficient protocol for the preparation of a series of N-alkyl-3-boronopyridinium salts is described which requires exposure
of 3-pyridineboronic acid neopentylglycol ester and corresponding alkyle halide to microwave irradiation followed by boronic ester
hydrolysis. The technique employed drastically reduces the reaction time and prevents thermal degradation and the formation of side
products. Water solutions of the obtained boronopyridinium salts are shown to be stable at room temperature in wide pH range as well
as in the presence of hydrogen peroxide at pH 10.0 for 72 h.
Keywords: 3-Pyridineboronic acid • Green chemistry • Alkylation • Microwave activation • Peroxide activation
© Versita Sp. z o.o.
1. Introduction
peroxide, as it replaces peroxoborates [8] or oxone [9].
Surfactants functionalized with a boronate moiety were
shown recently [10] to be effective coupling reagents.
We are interested in exploring the possibility of using
N-alkylated pyridine-3-boronic acids for the design of
“mild” oxidative microheterogeneous systems [11],
polyol-recognition systems, and in the preparation
of N-alkylpyridinium aryl ketones. Despite the high
synthetic value of 4-pyridine boronic acid derivatives,
[7] it should be noted that they are much less soluble
in water and will thus be of less importance as the
precursors of micellar oxidative systems.
Recently, we have reported a simple method of
preparation of N-alkyl-3-boronopyridinium triflates [12],
in which the general method for the synthesis of N-alkyl
boronopyridinium halides involves the condensation
of 3-pyridylboronic acid and an alkylating agent with
nitromethane as the solvent under reflux for 3, 7, and
12 days [13].
The interest of chemists in boronic acids has risen
due to their unique properties such as mild organic
Lewis acids and acting as reactants under the mild
condition, coupled with their ease of handling [1].
All these properties make boronic acids a particularly
attractive class of synthetic intermediates [2],
scaffolds for functional materials [3] and self-assembly
[4], and molecular blocks for saccharide recognition
[5]. Because of the relatively low toxicity of boronic
acids and their degradation into the environmentally
benign boric acid they can be considered “green”
compounds. Yamamoto and coworkers recently
tested several reusable N-alkylated pyridineboronic
acid catalysts for the dehydrative amide condensation
between equimolar mixtures of acids and amines
[6,7]. Boronic acids combined with hydrogen peroxide
can be a source of peroxoboronate, a species of
higher oxidizing power than that of initial hydrogen
* E-mail: ekarpichev@gmail.com
1059

Preparation of a series of N-alkyl-3-boronopyridinium halides
and study of their stability in the presence of hydrogen peroxide
Figure 1. Scheme for N-alkyl-3-boronopyridinium halide preparation using microwave irradiation.
Among the several aspects of green chemistry, 8452ADiodeArrayspectrophotometerwiththermostated
the reduction/replacement of volatile organic solvents cell unit.
from the reaction medium is of utmost importance.
Each experiment on synthesis was performed by
The use of a large excess of conventional volatile both the classical reflux method and in the microwave
solvents required to conduct a chemical reaction oven. In each case, the same quantities of reagents
creates ecological and economic concerns [14]. were used and the products were worked up and
Organic reactions accelerated under the influence purified in the same way. Each reaction was run
of microwave (MW) irradiation have attracted several times to find the reaction times that gave
considerable attention in past decades for the the same yields by the conventional and microwave
efficient and relatively friendlier synthesis of
a
technique. This allowed the relative rate enhancement
variety of organic compounds [15-18]. In recent for each experiment to be estimated.
years, microwave irradiation using both laboratory
In a typical reaction, alkyl halide (1.25 mmol) and
monomode and commercially available multimode 1 (1 mmol) were mixed thoroughly, placed in an open
ovens is increasingly used for the optimization and pyrex container and the reaction mixture irradiated with
acceleration of organic synthesis under solvent free microwave oven at 300 W (30 s irradiation followed by
conditions [18-20]. Previously it was reported the mixing) until yellowish (iodide) or brownish (bromide)
synthesis of N-alkylated pyridinium boronic acids color was obtained. The progress of the reaction was
under microwave irradiations [21], starting from monitored by 1H NMR. The solid obtained was washed
3-alkoxy-2-bromopyridines.
with dry ether to remove unreacted starting materials.
We report herein an efficient method for the The resulting yellowish solid was added to a mixture
preparation of N-alkyl-3-boronopyridinium halides of THF/ H2O, and the solution was stirred at room
using microwave irradiation as energy source by temperature. The solvent was removed in vacuo,
simple exposure of reactants to microwave irradiation and the aqueous solution was washed with ether
under solvent free conditions, and studies of the until neopentyl glycol was completely extracted. The
stability of the boronopyridinium iodides towards water was removed by freeze drying and the resulting
degradation in the presence of hydrogen peroxide in yellow solid was washed thoroughly with dry ether
water.
and recrystallized from methanol/ether. Alternatively,
for obtaining 3f and 3g with good yield, the hydrolysis
technique was modified by using a mixture of
dichloromethane and water sodium bicarbonate (9:1).
The pH of the water phase was maintained at 9.5.
After stirring for 12 h under ambient conitions, the
organic phase was decanted and the water layer was
washed with an excess of dichlomethane. Organic
solvent was removed in vacuo, and final product was
recrystallized from methanol/ether.
Typical spectra of the compouds obtained are
given below.
N-methyl-3-boronopyridinium iodide neopentyl
ester (2a). 1H NMR (300 MHz, DMSO-d6) δH 1.00
(s, 6H, C(CH3)2), 3.82 (s, 4H, OCH2), 4.64 (s, 3H,
2. Experimental procedure
All chemicals were purchased as reagent grade from
Aldrich and used without further purification, unless
otherwise noted. 1H and 13C NMR spectra were recorded
on Bruker AVANCE 300 MHz and Bruker AVANCE
500 MHz instruments. 11B spectra were recorded in
quartz tubes on AVANCE 400 MHz instrument. The
ESI-MS were obtained from a Q-Tof Ultima (Waters).
A multimode microwave oven (Samsung GE107W;
microwave frequency 2450 MHz) was used for carrying
out the reactions. UV-Vis spectra recorded using HP
1060

Y. Karpichev et al.
CH3); 8.10 (t, 1 H, J= 6.5 Hz, Pyr-H5), 8.64 (d, 1H, of stable adducts, which strongly depends on the
J=6.5 Hz, Pyr-H6), 9.00 (d, 1H, J=6.5 Hz, Pyr-H6), alkyl halide chainlength. Under these conditions, PBA
9.04 (s, 1H, Pyr-H2); 13C NMR (75 MHz, DMSO-d6) is supposed to form the partially alkylated adduct
δC 21.1 (C(CH3)2), 31.5 (q C), 47.6 (Me), 71.5 (OCH2), with stoichiometry from 1:4 (R=CH3) to 1:1 (R=C12H25,
127.1 (Pyr-C5), 146.5 (Pyr-C6), 148.7 (Pyr-C4), 148.9 C16H33). Boronic acids are well known to form acyclic
(Pyr-C2); HR ESI MS m/z calcd for C11H17BNO2 (M-I) adducts with bases [22] or undergo dehydration
206.1347, found 206.1337.
to form anhydrides; some of these anhydrides
N-dodecyl-3-boronopyridinium iodide neopentyl are stable (an excellent example is cyclotrimeric
ester (2f). 1H NMR (300 MHz, DMSO-d6) δH 0.87 anhydrides boroxines [23] being of interest as ring
(t, 3H, J=6.5 Hz, CH3), 1.00 (s, 6H, C(CH3)2), 1.22- structures incorporated into functional materials
1.36 (br s, 18H, 9CH2), 1.95-2.05 (m, 2H, NCH2CH2), and macromolecular architectures [3]). However,
3.84 (s, 4H, OCH2), 4.64 (t, 2H, J=7 Hz, NCH2); 8.13 there were no peaks found in the ESI-MS spectra
(t, 1 H, J= 6.5 Hz, Pyr-H5), 8.68 (d, 1H, J=7 Hz, Pyr- corresponding to the primary molecular ion of the
H6), 9.11 (d, 1H, J=6.5 Hz, Pyr-H4), 9.14 (s, 1H, adducts of the supposed structure, probably due
Pyr-H2); 13C NMR (75 MHz, DMSO-d6) δC 13.9 (Me), to the fast decomposition under the electron spray
21.2 (C(CH3)2), 22.0 (CH3CH2), 25.3 (CH3CH2CH2), condition.
28.2, 28.6, 28.7, 28.8, 28.9, (dodecyl-CH2), 30.8
UV study of the adduct of 3-PBA and methyl iodide
(NCH2CH2CH2), 31.2 (NCH2CH2), 31.5 (q C), 60.4 in water demonstrates no changes in the pH range
(NCH2), 71.5 (OCH2), 127.4 (Pyr-C5), 145.6 (Pyr-C6), between pH 6 and 10 (Fig. 2a) whereas in the presence
148.1 (Pyr-C4), 149.1 (Pyr-C2); HR ESI MS m/z calcd of the excess of H2O2 the remarkable changes appear
for C22H39BNO2 (M-I) 360.3068, found 360.2850.
with increasing of pH (Fig. 2b) which is not shown for
N-methyl-3-boronopyridinium iodide (3a). 1H NMR N-methyl-3boronopyridinum iodide 3a at pH providing
(300 MHz, CD3OD) δH 4.42 (s, 3H, CH3), 8.05 (t, 1 H, complete transformation into tetrahedral boron. This
J= 7 Hz, Pyr-H5), 8.73 (d, 1H, J=7 Hz, Pyr-H4), 8.87 may be ascribed to the ionic equilibria with an adduct
(d, 1H, J=6.5 Hz, Pyr-H6), 8.95 (s, 1H, Pyr-H2); 13C as well as with products of its cleavage.
NMR (125 MHz, CD3OD) δC 49.5 (Me), 128.5 (Pyr-
An experiment using 1H NMR and 2D NMR
1
C5), 147.4 (Pyr-C6), 150.8 (Pyr-C4), 152.3 (Pyr-C2); techniques (HMBC, HSQC, and H-1H COSY NMR;
HR ESI MS m/z calcd for C6H7BNO2 (M-I) 138.0721, CD3OD) allows us to confirm the formation of adducts
found 137.0687
whose structure will be clarified in a separate work. On
1
N-dodecyl-3-boronopyridinium iodide (3f). H NMR increasing the microwave power level, degradation
(300 MHz, CD3OD) δH 0.90 (s, 3H, CH3), 1.24-1.38 (deboronation) products appear, suggesting that
(br s, 18H, 9CH2), 1.91-1.98 (m, 2H, NCH2CH2), 4.55 (t, these conditions are too harsh. Thus, target product
2H, J=7 Hz, NCH2CH2); 7.90 (t, 1 H, J= 6.5 Hz, Pyr-H5), (type 2) cannot be obtained under these conditions.
8.56 (d, 1H, J=7 Hz, Pyr-H4), 8.68 (s, 1H, Pyr-H6), 8.70
A protecting group on the boron moiety (pyridine
(d, 1H, J=6.5 Hz, Pyr-H2); 13C NMR (125 MHz, CD3OD) boronic acid neopentyl ester 1) prevents the risk of
δC 14.5 (Me), 23.8 (CH3CH2), 27.2 (CH3CH2CH2), formation of substantial by-product amounts and
30.1, 30.2, 30.5, 30.6, 30.6, 30.7 (dodecyl-CH2), 30.8 make it possible to complete the alkylation. Upon
(NCH2CH2), 62.5 (NCH2), 127.8 (Pyr-C5), 143.2 (Pyr- microwave irradiation, the N-alkyl-3-boronopyridinium
C6), 148.6 (Pyr-C4), 150.9 (Pyr-C2); HR ESI MS m/z ester halide 2a-g appears that increases the polarity
calcd for C17H37BNO2 (M-I) 292.2442, found 292.2291
of the reaction medium thereby increasing microwave
absorption. The formation products 2a-g can be
monitored visibly in the reaction when it turns from
a clear solution to yellow (iodide) or brownish
(bromide).
3. Results and discussion
We examined the effect of microwave irradiation and
organic solvent additives on a set of the reactions
of alkylhalides with 3-pyridine boronic acid (PBA)
and 3-pyridineboronic acid neopentylglycol ester 1
(Fig. 1).
Direct alkylation of PBA with RX under MW
irradiation of moderate power in the presence and
absence of an organic solvent leads to the formation
Typical procedure includes reaction of 1 with
dodecyl iodide. It is observed that, at elevated
power levels, evaporation of alkyl halide and partial
decomposition/charring of the salt occurs, which
eventually results in lower yields. To circumvent this
problem, we conducted the reaction with intermittent
heating and mixing at a moderate power level to
provide a better yield and cleaner salt formation. After
1061

Preparation of a series of N-alkyl-3-boronopyridinium halides
and study of their stability in the presence of hydrogen peroxide
Figure 2. UV spectra of the adduct of 3-PBA (22 mg/l) with CH3I at different pH in water (a) and in the presence of 0.5 mM H2O2 (b);
1 M KCl; water, 250C.
unreacted starting materials are removed by washing
with ether and the product dried under vacuum at
40°C.
A series of salts 2a-g were prepared by microwave
heating and the protocol is then compared with
similar preparation using conventional heating (room
tempeture stirring for iodomethane and heating on
the oil bath unfer reflux for the rest alkyl halides).
The results are summarized in Table 2.
The 2a-g salts were initially prepared using a 10%
haloalkane molar excess, but the amount was later
increased. Due to evaporative loss, the preparation
1
Figure 3. H NMR (DMSO-d6) spectra for the reaction of 1
salts 2a-g in open vessels often requires a large
molar excess of haloalkane to obtain good yields.
For example, 50% molar excess of 1-bromobutane
haw previously been used to prepare N-butyl-3-
boronopyridinium ester bromide (89% yield). The
rate at which the quaternization of pyridine proceeds
follows the conventional order, R-I > R-Br > R-Cl [24].
Due to the high reactivity of the iodides excellent
yields are obtained in all cases with minimum
exposure time.
The progress of the reaction was monitored by 1H
NMR (Fig. 3). Further evidence for characterization of
synthesized compounds was obtained from the mass
spectral data. Furthermore, conversions (based on
1H NMR spectroscopy) of 99.8% for 1 were obtained,
making the current synthetic route more cost- and
reagent efficient, and hence greener.
with iodododecane at different times of microwave
irradiation (300 W).
Table 1. Optimization of reaction conditions for microwave-assisted
preparation of 12f iodide; Loading: 1.25 mmol C12H25I;
1.0 mmol 1.
N°
MW Power, W
Time, min
Yield, %
1
180
300
300
300
450
300
300
300
300
5
9
<5
10
45
90
56
10
80
88
96
2
3
25
40
5
4
5a
6b
7
4
10
15
20
8b
9
a Partial decomposition occurred at higher power
b 0.2 mL of ethanol added
the first irradiation for 30 s at 300 W the homogeneity
of the reaction mixture changes due to formation of
small amounts of type 2 salts.
The reaction mixture is then taken out, mixed again
for 90 s and then heated at the same power level for
additional 30 s (Table 1). This step is repeated until
the formation of a colored product. At this stage, the
An experiment was performed to determine if the
reaction time in a polar solvent would be reduced
in a microwave oven. A comparison of the heating
rate of a mixture of 1 and RX in ethanol with that of
mixture of 1 and RX without solvent in an open vessel
in the microwave oven at 300 watts, shows that the
1062

Y. Karpichev et al.
1H NMR (H2O-D2O mixture, 250C, water suppression) spectra of 3a iodide (0,01 M phosphate buffer solution) in the presence
of 10-fold excess of H2O2 vs time: 2 h (bottom line); 72 h (medium line); 1-methyl-3-hydroxyl pyridinium iodide (upper
spectrum).
Figure 4.
Table 2. Microwave-assisted preparation of compounds 2a-g.
a
Compound
RX
[RX], mmol
MW Power, W
Time, min
Yield %
Yield %b (time/days)
2a
2b
2c
2d
2e
2e
2f
CH3I
C4H9I
4
180
180
180
180
180
300
300
300
3
94
82
86
84
76
90
75
96
75 (7)
50 (5)
2.5
18
12
25
33
18
18
20
C6H11I
2.5
75 (7)
C8H17I
2.5
75 (7)
C10H11Br
C10H21I
C12H25Br
C12H25I
1,25
1.25
1.25
1.25
65 (10
75 (10)
75 (15)
75 (15)
2f
2g
C16H33I
1.25
300
20
96
75 (15)
a Using MW, with 0.2 mL of ethanol.
b Alternative heating method (oil bath )
dilute solutions of substrate in ethanol are heated deboronation indicative for most aryl boronic acids is
very rapidly. This occurs because only the polar of high interest. Some of amphiphilic boronopyridinium
molecules in solution absorb the microwave energy salts were reported recently [10] to be reagents for
[25], superheating the solvent rapidly.
cross-coupling reactions but the stability of alkaline
Aryl boronic acid are known to be unstable solutions and an interaction with hydrogen peroxide
in the presence of H2O2 [26] and small inorganic have not been reported.
nucleophiles [27]. At the same time, 3-pyridine boronic
1H NMR spectra recorded for system 3a:H2O2
acid is one of the most acidic of the known boronic (1:10) indicate (Fig. 4) that in the presence of large
acids, with a pKa of ca. 4.0 [28], and possesses the excess of H2O2 a small amount of deboronaton
most electrophilic boron atom that can best form product 3-hydroxypyridinium methiodide appears
and stabilize a hydroxyboronate anion. Unlike most (see chemical shifts developing upon a time, Fig. 4).
boronic acids, it exists in the anionic tetrahedral During the first 2 h, ca.3% of starting boronic acid
species at a pH ranging from slightly acidic to strong undergoes deboronation with hydrogen peroxide,
alkaline, which may facilitate peroxoacid formation and after 72h, less than 15% of the initial compound
in the presence of hydroperoxide anion under “mild” undergoes deboronation. Thus, in the case of 3a, the
conditions. If they are stable in a water/H2O2 solution, rearrangement of peroxoarylboronic acid to the boric
N-alkyl derivatives of 3-pyridine boronic acid may acid aryl ester reported for phenylboronic acid [30]
be of interest as peroxide activators, particularly as proceeds much slowly, and salts of type 3 are stable
the components of “surfoxidants” [29] for oxidative enough to be investigated as peroxide activators and
detoxication of organic sulfides [11]. Thus, the key components for oxidative systems.
problem of how stable these compounds are against
1063

Preparation of a series of N-alkyl-3-boronopyridinium halides
and study of their stability in the presence of hydrogen peroxide
1
Figure 5. B NMR (H2O-D2O mixture, BF3OEt2 is a reference) spectra of (a) 3a iodide and (b) boric acid in the presence and absence
of hydrogen peroxide, pH 10.0; 0.01 M phosphate buffer solution. The bottom spectrum in (a) corresponds to pH 3.3.
11B spectra of salt 3a and boric acid alone were
recorded at different pH’s and in the presence of an
4. Conclusions
In conclusion, we have developed a general and rapid
method for the synthesis of N-alkyl-3-boronopyridinium
acid halide salts in nearly quantitative yield, under
microwave irradiation. The high efficiency and use of
only cosolvent without other additives makes the method
usefulandattractiveforthesynthesisofN-alkylpyridinium
boronic acids. The stability of obtained salts was studied
excess of hydrogen peroxide. All samples were freshly
prepared and run within 30 min. At low pH, salt 3a
exist in its trivalent (cationic boronic acid salt) form,
characterized by a broad peak with chemical shift at
23.69 ppm (Fig. 5a, bottom spectrum) similarly to that
of methylboronic acid reported in [30]. With increasing
pH (10.0) the addition of OH to the boronic acid
takes place, and tetrahedral (zwitter-ionic boronate
form) species appears following by signal shifting
to 1.64 ppm (Fig. 5a, medium spectrum).
1
using UV-Vis spectroscopy and 11B, H and 13C NMR
at different pH, salt concentration, and H2O2 content.
At 10 times excess of hydrogen peroxides (pH 10.0),
less than 15% of degradation products appear after
72 h of exposure. The N-alkyl-3-boronopyridinium
halides are proposed to be used as peroxide activators
and precursors to mild oxidants for the oxidation of
organic sulfides.
In the case of boric acid, there is
a signal
corresponding to borate anion at pH 10 (2.94 ppm).
The presence of excess of H2O2 does not affect main
chemical shift drastically (less that 1 ppm towards
strong field) but is characterized by appearance of
additional peak, with chemical shift of 4.33 ppm and
3.14 ppm for boric acid and 3a, respectively.
1
Acknowledgement
As we can conclude from H data, the secondary
signal (Fig. 5a) unlikely reflects deboronation that
could take place in the presence of peroxide. The
new signals are similar for boric and boronic acid
and are likely correspond to peroxoacids formation,
peroxoboronic (Fig. 5a, upper spectrum) or
peroxoboric (5b, upper spectra), and is generally in
agreement with the investigation of mixture of boric
acid and H2O2 [31].
This research was made possible in part by French-
Ukrainian International Network on Molecular Chemistry
(GDRI).
1064

Y. Karpichev et al.
References
[17]
[18]
[1]
D.G. Hall (Ed.), Boronic Acids. Preparation
and Applications in Organic Synthesis
and Medicine (Wiley-VCH, Weinheim, 2005) 549
P.N. Craig, In: C.J. Drayton (Ed.), Comprehensive
Medicinal Chemistry (Pergamon Press, New York,
1991) Vol. 8
J.T.
Tierney,
P.
Lindström
(Eds.),
Microwave-assisted Organic Synthesis (Blackwell,
Oxford, 2005)
N.E. Leadbeater, Microwave heating as a tool for
sustainable chemistry (CRC Press, Boca Raton,
2011) 278
[2]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[3]
[4]
[5]
A.L. Korich, P.M. Iovine. Dalton Trans. 39, 1423
(2010)
N. Fujita, S. Shinkai,T.D. James. Chem. Asian J. 3,
1076 (2008)
T.D. James, M.D. Phillips, S. Shinkai, Boronic
Acids in Saccharide Recognition (RSC Publishing,
Cambridge, 2006) 174
T. Maki, S. Ishihara, H. Yamamoto, Org. Lett. 7(22),
5047 (2005)
T.Maki, S. Ishihara, H. Yamamoto, Tetrahedron 63,
8645 (2007)
D.M. Davies, M.E. Deary, K. Quill, R.A. Smith,
Chem. Eur. J. 11(12), 3552 (2005)
C.A. Bunton, H.J. Foroudian, A. Kumar. J. Chem.
Soc. Perkin Trans. 2, 33, (1995)
H. Matondo, J.C. Garrigues, I. Rico-Lattes,
A. Lattes, Appl. Organometal. Chem. 23, 191
(2009)
A.F. Popov. Pure Appl. Chem. 80, 1381 (2008)
O. Savsunenko et al., J. Surfact. Deterg.,
DOI: 10.1007/s11743-011-1315-y
L.K. Mohler, A.W. Czarnik, J. Am. Chem. Soc. 115,
2998 (1993)
G.W.V. Cave, C.L. Raston, J.l. Scott, Chem.
Commun. 2159 (2001)
S. Chatti, M. Bortolussi, A. Loupy, Tetrahedron, 57,
4365 (2001)
H.P. Nguyen, H. Matondo, M. Baboulene, Green
Chem. 5, 303 (2003)
J. Kua, M.N. Fletcher, P.M. Iovine, J. Phys. Chem.
A 110, 8158 (2006)
J.C. Peprin, R.N. Keller. J. Am. Chem. Soc. 80, 182
(1958)
M. Deetflets, K.R. Seddon, Green Chem. 5, 181
(2003)
R.N. Gedye, F.E. Smith, K.C. Wetaway, Can.
J. Chem. 66, 17 (1988)
H. Kuivila. J. Am. Chem. Soc. 76, 870 (1954)
R. Glaser, N. Knotts J. Phys. Chem. A 110, 1295
(2006)
[6]
[7]
[8]
[9]
[26]
[27]
[10]
[28]
[29]
[30]
[31]
F.C. Fisher, E. Havinga, Recl. Trav. Chim.
Pays-Bas 93, 21 (1974)
[11]
[12]
D.E. Richardson, H. Yao, K.M. Frank, D.A. Bennett,
J. Am. Chem. Soc. 122, 1729 (2000)
Y. Kono, K. Ishira, A. Nagasawa, K. Umemoto,
K. Saito, Inorg. Chim. Acta 262, 91 (1997)
R. Pizer, Ch. Tihal. Inorg. Chem. 26(21), 3639
(1987)
[13]
[14]
[15]
[16]
R.S.Varma, V.V. Namboodiri, Pure Appl. Chem.
73(8), 1309 (2001)
C.O. Kappe, Angew. Chem., Int. Ed. 43, 6250
(2004)
D. Dallinger, H. Lehmann, J.D. Moseley, A. Stadler,
C.O. Kappe, Org. Process Res. Dev. 15(4), 841
(2011)
1065