New approach to the generation of aryldifluoroboranes–prospective acid catalysts of organic reactions

Article abstract of DOI:10.1016/j.mencom.2018.07.009

A new approach for preparation of aromatic and fluoroaromatic difluoroboranes via the interaction between corresponding aryltrifluoroborates and ionic liquids containing tetrachloroaluminate-anion and aluminum chloride has been developed. Catalytic properties of obtained aryldifluoroboranes have been investigated in model reactions of phenols alkylation. The dependence of catalytic properties on both the nature of solvent used and the type of substituents in the aromatic ring of difluoroborane has been established.[Figure presented]

Full text of DOI:10.1016/j.mencom.2018.07.009

Mendeleev
Communications
Mendeleev Commun., 2018, 28, 369–371
New approach to the generation of aryldifluoroboranes –
prospective acid catalysts of organic reactions
Mikhail M. Shmakov,a,b Sergey A. Prikhod’ko,*a,b Vadim V. Bardinb,c and Nicolay Yu. Adonin*a,b
a G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk,
Russian Federation. Fax: +7 383 330 8056; e-mail: spri@catalysis.ru, adonin@catalysis.ru
b Novosibirsk State University, 630090 Novosibirsk, Russian Federation
c N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation
DOI: 10.1016/j.mencom.2018.07.009
A new approach for preparation of aromatic and fluoro-
aromatic difluoroboranes via the interaction between corre-
sponding aryltrifluoroborates and ionic liquids containing
tetrachloroaluminate-anion and aluminum chloride has been
developed. Catalytic properties of obtained aryldifluoro-
boranes have been investigated in model reactions of phenols
alkylation. The dependence of catalytic properties on both
the nature of solvent used and the type of substituents in the
aromatic ring of difluoroborane has been established.
OH
OH
R
Y
Xn
BF2
Hexane
BF3K
BF2
+
Xn
Ionic liquid
[bmim][AlCl4]·AlCl3
[bmim][AlCl4] + K[AlCl3F]
Xn
X, Y = H, F; n = 0–5
or X = F, Y = EtO
Hexane-soluble
weak Lewis acid
catalyst
R
Me
Despite a large number of studies on the reactions catalyzed
by Lewis acids and significant progress achieved in this field, a
lot of issues involving the production of soluble and stable in
organic solvents Lewis acids with the controlled acidity remain
relevant.1,2 Such Lewis acids are of importance as catalysts
for transformations of highly active substrates and compounds
containing several reaction centers with different reactivity. Among
the whole variety of known Lewis acids, of special interest are
fluorinated aryldihalogenoboranes, formal derivatives of boron
trifluoride, which are used for different processes in fine organic
synthesis.3 Fluorinated aryldihalogenboranes are relatively stable
and variation of the nature of substituents at the boron atom
allows one to control the Lewis acidity and catalytic activity.
However, complicated generation of these compounds is an
obstacle to their widespread use. For example, fluorinated aryl-
dihalogenoboranes obtained by reaction between corresponding
organotin compounds and boron halides are contaminated with
hardly separable tin-containing by-products.4,5 The synthesis
of fluorinated aryldifluoroboranes (ArFBF2), which is based on
potassium fluoride detachment from the corresponding potassium
organotrifluoroborate under the action of BF3, requires special
installation for handling gaseous boron trifluoride.6,7
corresponding potassium organotrifluoroborates and an ionic
liquid containing free aluminum chloride and investigate their
catalytic properties.
Aryldifluoroboranes (ArBF2) 1–4 were generated via the
elimination of KF from [ArBF3]K by the action of aluminum
chloride dissolved in ionic liquid 1-butyl-3-methylimidazolium
chloride [bmim]Cl (Scheme 1).† Reactant [bmim][AlCl4]·AlCl3 was
obtained as a low-viscosity transparent liquid by mixing anhydrous
AlCl3 and [bmim]Cl in a molar ratio of 1.5:1. The content of
†
General procedure for preparation of organofluoroboranes 1–4.
[bmim][AlCl4]·AlCl3 (224 mg, 0.3 mmol) and corresponding potassium
organotrifluoroborate (0.3 mmol) were placed in flame-dried flask upon
dry atmosphere, then the flask was closed by septum. Using a syringe,
hexane (3 ml) was added and the mixture was stirred at 25°C under
argon atmosphere for 2 h. After that a sample was analyzed by 19F NMR
spectroscopy using benzotrifluoride (3 ml, 24.6 μmol) as a quantitative
internal standard. Organofluoroboranes were used in catalytic experiments
as a solution in hexane.
Difluoro(pentafluorophenyl)borane 1. 19F NMR (hexane) d: –73.02 (br.s,
2F, BF2), –128.65 (br.s, 2F, 2-F, 6-F), –144.45 (tt, 1F, 4-F, 3JFF 19.6 Hz,
5JFF 7.3 Hz), –161.69 (m, 2F, 3-F, 5-F). 11B NMR (hexane) d: 23.21 (br.s,
1B, BF2).
Difluoro(4-fluorophenyl)borane 2: 19F NMR (hexane) d: –93.56 (q,
Here, we propose a convenient method for the preparation
of various aryldifluoroboranes via the interaction between the
1
2F, BF2, JBF 64.7 Hz), –104.12 (br.s, 1F, 4-F). 11B NMR (hexane) d:
24.50 (t, 1B, BF2, 1JBF 64.6 Hz).
Difluoro(4-ethoxy-2,3,5,6-tetrafluorophenyl)borane 3. 19F NMR (hexane)
d: –75.74 (br.s, 2F, BF2), –131.31 (br.s, 2F, 2-F, 6-F), –158.66 (dd, 2F,
3-F, 5-F, 3JFF 18.0 Hz, 5JFF 8.2 Hz). 11B NMR (hexane) d: 22.91 (br.s, 1B,
BF2). 1H NMR (benzene-d6) d: 3.91 (q, 2H, OCH2Me, 3JHH 7.1 Hz), 1.01
(t, 3H, OCH2Me, 3JHH 7.0 Hz). 13C NMR (benzene-d6) d: 151.42 (dddd,
2-C, 6-C, 1JCF 254.4 Hz, 2JCF 11.9 Hz, 3JCF 10.3 Hz, 4JCF 1.9 Hz), 140.66
(ddd, 3-C, 5-C, 1JCF 245.9 Hz, 2JCF 13.4 Hz, 3JCF 2.5 Hz), 137.98 (tt, 4-C,
2JCF 13.0 Hz, 3JCF 3.8 Hz), 70.33 (s, OCH2Me), 14.95 (s, OCH2Me), the
signal of 1-C atom, a weak broad multiplet at about 110–120 ppm, was
not identified.
R1
R1
R1
R1
R1
[bmim][AlCl4]·AlCl3,
hexane, 25 °C, 2 h
R2
BF3
R2
BF2
K
– KF
R1
R1
R1
1 R1 = R2 = F
2 R1 = H, R2 = F
3 R1 = F, R2 = OEt
4 R1 = R2 = H
Difluoro(phenyl)borane 4. 19F NMR (hexane) d: –92.89 (q, 2F, BF2,
1JBF 65.9 Hz). 11B NMR (hexane) d: 25.34 (t, 1B, BF2, 1JBF 66.2 Hz).
Scheme 1
© 2018 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 369 –

Mendeleev Commun., 2018, 28, 369–371
acids (for example, AlCl3) due to their participation in reactions.9
OH
OH
C6F5BF2 1
In our case, dichloromethane does not act as alkylation reagent in
the presence of compound 1 (Table 1, entries 4–7). Note that benzene
and toluene may also be used as solvents for alkylation reactions
in the presence of catalyst 1. This fact allows one to considerably
extend the range of solvents for the processes catalyzed by
Lewis acids. To determine the effect of the solvent nature on
organofluoroborane catalytic properties, we performed a series of
phenol alkylation experiments with various solvents (Table 2).
It was shown that the alkylation proceeds quantitatively in
dichloromethane within 30 min (see Table 2, entry 1). It may
be explained by high solvent polarity affecting the carbocation
stabilization and, as a consequence, lower activation energy of the
process. For the alkylation reaction in benzene medium (entry 2),
higher catalytic activity of borane 1 was demonstrated as compared
to that in hexane and tetrachloromethane. Apparently, it is caused
by partial carbocation stabilization due to p-stacking interactions.
A lower catalytic activity of compound 1 in tetrachloromethane
(entry 4) compared to that in hexane (entry 3) can be explained by
Hutman’s acceptor numbers (AN) scale, where different
solvents are located on a relative scale from 0 to 100.10 The larger
the acceptor number, the more effective the involved substance
is in the donor–acceptor interactions with Lewis acids. It may
be assumed that the use of CCl4 as a solvent results in partial
organofluoroborane deactivation due to its strong interaction
with the latter. Negative influence of this factor seems to be
balanced by positive effect of transition state stabilization at the
limiting stage of the alkylation process.
Y
(2 mol%)
Y
+
X
Me
X
X = H, 4-Br, 4-NH2, 2-Ph
Y = Bu, CH2Br, Ph
Scheme 2
Table 1 Catalytic activity of borane 1 in the alkylation reactions of
phenols.
Entry
X
Y
Solvent
T/°C t/min Conversion (%)
1
2
3
4
5
6
7
H
H
H
H
4-Br
4-NH2
2-Ph
Bu
CH2Br
Ph
Ph
Ph
Hexane
Hexane
Hexane
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
65
65
65
25
25
25
25
30
30
180
30
30
180
30
0
0
90
100
100
0
Ph
Ph
78
‘active’ aluminum chloride was 1.335 mmol g–1. The reagent is
immiscible with non-polar solvents such as hydrocarbons, benzene
and CCl4 and stable under dry atmosphere conditions, these
properties being very useful for our purpose.
The generation of aryldifluoroboranes was carried out in two-
phase ionic liquid–hexane system. The product has a non-ionic
structure and moves to a hexane phase. The aryldifluoroborane
concentration in solution was determined using 19F NMR spectro-
scopy and internal standard. We developed a new alternative rapid
method for determination of the aryldifluoroborane concentra-
tion in solution. It is based on the chromatographic determination
of the residual amount of 2-methylpyridine after the quantitative
formation of corresponding borane adduct insoluble in non-polar
solvents.4,5,‡ The results obtained by this method are in good
agreement with NMR spectroscopy data, which confirms its
suitability for the rapid determination of the organofluoro-
boranes concentration.
The catalytic properties of the difluoro(pentafluorophenyl)-
borane 1 obtained were investigated in phenol alkylation reac-
tions (Scheme 2, Table 1). Compound 1 did not exhibit catalytic
properties in the alkylation of phenol with hex-1-ene and allyl
bromide (Table 1, entries 1, 2). However, the acidity of 1 is
sufficient to promote the phenol reaction with styrene (entries 3, 4).
Exhaustive alkylation of 4-bromophenol proceeded in dichloro-
methane (entry 5). At the same time, the attempt to alkylate
4-aminophenol failed. It may be caused by catalyst deactivation
due to the formation of the adduct of borane and 4-aminophenol
(entry 6). It was of interest to study catalytic alkylation of
substrates containing several reaction centers. Thus, as result of
interaction between 2-phenylphenol and styrene in the presence
of catalyst 1, ortho- and para-products of alkylation of the phenolic
ring were obtained (entry 7). Meantime, no products of alkylation
of the unactivated ring were observed.
Thus, organofluoroborane catalyst activity depends on a
number of solvent characteristics, the main of which is the
solvent ability to stabilize reaction intermediates. Donor–acceptor
interactions with Lewis acids have a lower effect, but they
become noticeable in the absence of carbocation stabilizing
factors.
To study the influence of aromatic substituents in organo-
fluoroboranes on their catalytic activity, a series of phenol
alkylation experiments in the presence of organofluoroboranes
1–4 was carried out.‡ Hexane was chosen as a solvent, since its
solvating effect is minimized, thus the key factors affecting the
organofluoroborane catalyst activity are the electronic properties
of the aromatic substituents on the boron atom. We established a
well-traced dependence of catalytic properties of the considered
organofluoroboranes on the aromatic substituent electronic effects
(Table 3). According to published data, the presence of fluorine
substituents in the aromatic ring increases Lewis acidity of the
boron atom due to the electron density displacement by the
aromatic ring action and the absence of positive mesomeric
effects.11 This fact determines the maximal catalytic activity of
borane 1 (Table 3, entry 1). Compound 2 containing one strong-
accepting substituent in the aromatic ring has analogous properties
‡
To determine the organofluoroborane concentration, the reaction mixture
(1 ml) was added to 0.54 m solution of 2-methylpyridine in n-decane
(300 ml). The precipitate was separated, excess amount of 2-methyl-
pyridine was determined by gas chromatography using absolute calibration.
Catalytic experiments (general procedure). The substrate (20 mmol), viz.
phenol, 4-bromophenol, 4-aminophenol or 2-phenylphenol, 0.05 m aryl-
difluoroborane solution in hexane (1 ml, 0.05 mmol) and the solvent
(20 ml), viz. hexane, benzene, dichloromethane, were placed in flame-
dry flask filled with argon. The mixture was thermostatted at the desired
temperature for 15 min. Then an alkylating agent (2 mmol), viz. 1-hexene,
allyl bromide or styrene, was added. The conversion of the starting
compound and the mixture quantitative composition were determined
by gas chromatography. The qualitative composition of the mixture
was determined by GC-MS. The products were identified by comparison
of retention times and mass spectra with those of previously described
compounds.8
The examples are known when halogenated solvents are
inappropriate reaction media in the presence of classical Lewis
Table 2 Effect of the solvent nature on the phenol alkylation with styrene
catalyzed by compound 1.
Conversion ortho-
para-
Entry Solvent
AN t/min
(%)
product (%) product (%)
1
2
3
4
CH2Cl2
Benzene
Hexane
CCl4
20.4 30
8.2 30
100
79
57
67
46
65
43
33
54
35
0
30
65
8.6 30
56
– 370 –

Mendeleev Commun., 2018, 28, 369–371
Table 3 Catalytic activity of aryldifluoroboranes 1–4 in alkylation of phenol
both the nature of the solvent used and the electronic properties
of substituents in the aromatic ring of borane. These results open
new approaches to obtaining soft Lewis acids with controlled
acidity and catalytic activity, which are promising for the use
in selective transformations of activated and polyfunctional
substrates.
with styrene.
Conversion
of styrene (%)
Entry Catalyst
ortho-product (%) para-product (%)
1
2
3
4
5
1
2
3
4
90
22
20
0
58
44
53
0
42
56
47
0
The work was supported within the framework of budget
project for G. K. Boreskov Institute of Catalysis (no. AAAA-
A17-117041710082-8).
BF3·Et2O 27
54
46
(entry 2). Appending the electron-donor substituents such as
ethoxy group leads to increase in the electron density in the
perfluoroaryl ring and, as a consequence, a decrease in the boron
atom Lewis acidity and catalytic activity of borane 3 (entry 3).
In addition, the coordination between the oxygen atom of the
ethoxy group and the boron atom is possible, which may reduce
Lewis acidity. Apparently, a similar situation is observed for the
alkylation reaction in the presence of BF3·Et2O (entry 5), whose
catalytic activity is much lower as compared to that of compound 1.
The phenyl substituent in borane 4 does not have negative
electronic effects. Therefore, the effective charge on the boron
atom is diminished. Also, Lewis acidity and catalytic activity in
this case are the lowest due to the presence of a combined
p-system associated with the boron atom, which leads to an
additional displacement of the electron density in the active center
(entry 4).
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In conclusion, an efficient and convenient method for preparing
aryldifluoroboranes containing various substituents in the aromatic
ring was developed. It was found that obtained compounds are soft
Lewis acids and their catalytic activity in the reactions of phenol
alkylation with styrene was demonstrated. It was established that
the catalytic properties of aryldifluoroboranes are affected by
Received: 8th February 2018; Com. 18/5471
– 371 –