Reactivity of 2-formylphenylboronic acid toward secondary aromatic amines in amination-reduction reactions

Article abstract of DOI:10.1016/j.tetlet.2011.10.008

The synthesis of 2-(arylaminomethyl)phenylboronic acid via an amination-reduction reaction has been investigated within a model system comprising 2-formylphenylboronic acid and N-ethylaniline. Adoption of the appropriate reaction conditions influences the reactivity of 2-formylphenylboronic acid, enabling efficient synthesis of so-far unobtainable 2-(arylaminomethyl)phenylboronic compounds. The first crystal structure of the aromatic amine derivative has been determined and described.

Full text of DOI:10.1016/j.tetlet.2011.10.008

Tetrahedron Letters 52 (2011) 6639–6642
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reactivity of 2-formylphenylboronic acid toward secondary aromatic
amines in amination–reduction reactions
a,
⇑
Agnieszka Adamczyk-Wozniak , Raluca M. Fratila b, Izabela D. Madura a, Alicja Pawełko a,
´
a
a
b
a
_
´
Andrzej Sporzynski , Marta Tumanowicz , Aldrik H. Velders , Jacek Zyła
a Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
b Laboratory of Supramolecular Chemistry and Technology, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, MIRA Institute for Biomedical Chemistry
and Technical Medicine, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of 2-(arylaminomethyl)phenylboronic acid via an amination–reduction reaction has been
investigated within a model system comprising 2-formylphenylboronic acid acid and N-ethylaniline.
Adoption of the appropriate reaction conditions influences the reactivity of 2-formylphenylboronic acid,
enabling efficient synthesis of so-far unobtainable 2-(arylaminomethyl)phenylboronic compounds. The
first crystal structure of the aromatic amine derivative has been determined and described.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 13 July 2011
Revised 19 September 2011
Accepted 3 October 2011
Available online 8 October 2011
Keywords:
Boronic acids
Amination–reduction
Aromatic secondary amines
Benzoxaborole
There has been much interest over the past decade in
2-(arylaminomethyl)phenylboronic acids, mostly due to their
applications in sensor research.1–7 Derivatives of secondary aro-
matic amines are important due to their possible fluorescence
and resulting analytical utility in sensing. The amination–reduc-
tion reaction of 2-formylphenylboronic acid would appear to be
the cheapest and most straightforward preparative strategy for
these types of receptors, especially since some of the synthetic
obstacles have recently been circumvented.8 The success of the
amination–reduction reaction depends mainly on the reactivity
of both the carbonyl and amine groups as well as the selectivity
of the reducing agent employed.9 The so-far reported yields of
2-(arylaminomethyl)phenylboronic acids via amination–reduction
reaction are very low,10 probably because of the poor nucleophilic
character of secondary aromatic amines.11 There are many reports
on increasing the nucleophilicity of amines in amination–reduc-
tion reactions,9 however none of these consider the specific reac-
tivity of the boronic unit in such a system.8 Here we present an
extensive study providing appropriate conditions for the formation
of 2-(arylaminomethyl)phenylboronic acids.
character of the ortho and para positions of the aminal phenyl ring
manifested in the formation of 3-phenyl-substituted benzoxabo-
role.12 In order to optimize the reaction conditions for selectivity
and yield in 2-(arylaminomethyl)phenylboronic acid synthesis,
several parameters including reaction temperature, reducing
agent, solvent, the use of dehydrating agents, and an acidic catalyst
were investigated within the model reaction using 2-form-
ylphenylboronic acid and N-ethylaniline as a secondary aromatic
amine. According to our previous studies, the possible formation
of several products has been taken into account (Scheme 1, 3–
7).8,12 The unsubstituted benzoxaborole by-product 4 has been re-
moved successfully from the post-reaction mixture by adoption of
the previously developed procedure,8 that is, acidification of the
post-reaction mixture followed by selective extraction of 4 with
diethyl ether. Neutralization of the aqueous phase and subsequent
extraction resulted in pure 3. The presence of compounds 3–5 in
the resulting oily mixtures was monitored by 1H NMR spectros-
copy on the basis of the appropriate analytical signals, that is,
the 2-(arylaminomethyl)phenylboronic acid 3 (4.5 ppm), benzox-
aborole 4 (5.0 ppm) and the phenyl-substituted benzoxaborole 5
(6.1 ppm) in (CD3)2CO.
The differences in reactivity between the aliphatic and aromatic
amines toward 2-formylphenylboronic acid result from the lower
nucleophilic character of the nitrogen atom in the latter.11 Another
substantial difference is a consequence of the C-nucleophilic
The signal corresponding to the substituted benzoxaborole 6
was expected to appear at around 6 ppm11; however, the only
signal detected in this region was that corresponding to 5. The
identification of compound 5 as a constituent of the post-reaction
mixtures was confirmed by the addition of a standard sample,12
resulting in an increase of the considered signal intensity. In
⇑
Corresponding author. Tel.: +48 22 2346147; fax: +48 22 6282741.
E-mail address: agnieszka@ch.pw.edu.pl (A. Adamczyk-Woz´niak).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.008

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 52 (2011) 6639–6642
6640
B(OH)2
B(OH)2
HO
HO
B
O
HN
+
O
B
REDUCING AGENT
N
O
+
+
NH
2
3
1
4
5
HO
B
O
NH
DEHYDRATION
N
+
1
4
2
5
+
+
O
B
B
O
B
O
6
O
O
REDUCTION
REDUCTION
O
7
3
Scheme 1. The reactivity of 2-formylphenylboronic acid (1) with N-ethylaniline.
contrast to 6, which can easily be reduced to yield the desired
product 3, formation of benzoxaborole 5 results in a lower yield
of 3; therefore, avoiding its formation in the amination reaction
is highly desirable.
B(OH)2
B(OH)2
B(OH)2
N
N
N
An extensive study was conducted in order to find appropriate
conditions for the formation of 2-(arylaminomethyl)phenylboronic
acids (Table S1, Supplementarydata). The crude products 3 were of-
ten contaminated with unreacted amine, therefore the spectro-
scopic yield was used for the optimization experiments. The use
of NaBH4 as a reducing agent and methanol as the solvent, success-
ful in the case of aliphatic amines,8 failed to result in 3. The use of
acetonitrile resulted in the formation of 3 in 4% yield, therefore, fur-
ther research was carried out using acetonitrile as a solvent. One of
the easiest ways to increase the yield of the amination reaction was
to shift the equilibrium toward the formation of the product by
removing water formed during the reaction.9 In the case of boronic
acids however, the formation of complexed boroxin 7, which can
influence considerably the reactivity, must be taken into ac-
count.11,13 The use of MgSO4 or molecular sieves as dehydrating
agents did not result in satisfactory yields of 3. The yield of 3 in-
creased to 20% upon lowering the reaction temperature to 15 °C,
but this was still unsatisfactory. The positive influence of the reac-
tion temperature probably results from a decrease of the rate of
reduction of the starting material. Replacement of NaBH4 by a
milder reducing agent such as NaBH(OAc)3 resulted in a reasonable
rise in the yield of 3. Interestingly, lowering the temperature of the
reaction to 0 °C totally eliminated the formation of 5 with the yield
of 3 remaining satisfactory. Application of p-toluenesulfonic acid as
the catalyst improved slightly the yield of 3. The use of 2 equiv of
acetic acid afforded the desired product 3 in 46% yield, and by using
a twofold molar excess of 2-formylphenylboronic acid the yield in-
creased to 56%. Accordingly, the yield of 4 obtained as a by-product
by reduction of the starting material decreased. This result implies
that the amination–reduction reaction of 2-formylphenylboronic
acid is not as simple as it first appears and that the acid-boroxin
equilibrium may play an important role. A further increase in the
yield of 3 to 66% was achieved by the application of both excess of
2-formylphenylboronic acid and acetic acid catalysts. The use of a
milder reducing agent and lower reaction temperature limited the
reduction of the starting 2-formylphenylboronic acid. At the same
time, lowering the reaction temperature limited the irreversible for-
mation of 5. Application of a drying agent shifted the equilibrium to-
ward N-substituted benzoxaborole product 6 enabling subsequent
formation of 3. Having optimized the various reaction parameters,
2-(arylaminomethyl)phenylboronic acids (3, 3a, and 3b), Fig. 1)
3b
3
3a
Figure 1. Structures of the synthesized 2-(arylaminomethyl)phenylboronic acids.
were obtained in satisfactory isolated yields, that is, more than
43%. Compound 3b gave crystals suitable for X-ray measurements
enabling determination of the first example of an aromatic amine
derivative of phenylboronic acid.14
ꢀ
Molecules of 3b were crystallized in the p1 space group of the
triclinic system. The crystal data are summarized in Table S2 while
Table S3 (Supplementary data) lists selected geometrical parame-
ters. The boron atom in molecules of 3b showed trigonal coordina-
tion with two equidistant B–O bonds (within error margins) (Table
S3). The –B(OH)2 moiety adopts a syn–anti conformation and is
twisted with respect to the phenyl ring by 27.5(1)° (Fig. 2). This
may be attributed to the formation of an intramolecular O–HÁ Á ÁN
hydrogen bond. In 3b the anti-oriented OH donor interacts with
the naphthylamine nitrogen forming a seven-membered ring with
a relatively linear O–HÁ Á ÁN hydrogen bond. The OÁ Á ÁN distance of
2.671(1) Å, (HÁ Á ÁN distance = 1.78(2) Å) as well as the O–HÁ Á ÁN an-
gle of 168(2)° indicate a very strong interaction.15 The graph set16
related to this bond is given as S(7). It is worth noting that in the
syn–anti conformation, a twist of the –B(OH)2 versus the phenyl
ring and the O–HÁ Á ÁN intramolecular hydrogen bond are observed
in all structurally characterized aliphatic amine analogs.7,8,15,17–20
In all cases, slight pyramidalization of the nitrogen atom from
the plane defined by the surrounding carbon atoms was apparent,
ranging from 0.40 to 0.48 Å. In 3b the distance of the nitrogen atom
from this plane is found to be 0.422(1) Å. Further, the naphthyl ring
and the phenylboronic unit are perpendicular to each other with
the dihedral angle being equal to 89.96(5)° (Fig. 2b). This edge-
to-face orientation enables formation of a weak intramolecular
interaction of C–HÁ Á Á
p
type (HÁ Á ÁC distance = 2.79 Å) between the
naphthyl first ring, acting as a C–H donor, and the phenylboronic
ring being the
p-acceptor. According to examples collected re-
cently by Nishio,21 this interaction in cooperation with the strong
O–HÁ Á ÁN hydrogen bond may stabilize the geometry of molecules
of 3b. Moreover, the two molecules of 3b related by an inversion

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 52 (2011) 6639–6642
6641
C4
C3
C5
C6
C1
C2
O2
i
C8
B1
C7
i
C18
i
O2
i
O1
N1
i
B1
O1
i
C9
C5
C7
N1
C18
i
i
N1
O1
C8
i
C3
C7
C10
C11
i
B1
N1
C19
i
O2
C18
i
B1
C12
i
C8
O2
C13
C14
C15
C17
C16
(a)
(b)
Figure 2. (a) Hydrogen bonded dimer of 3b with the atom numbering scheme. Intramolecular O–HÁ Á ÁN and C–HÁ Á Á
p interactions are presented as dotted lines, while
intermolecular O–HÁ Á ÁO bonds appear as dashed lines. Thermal ellipsoids are drawn with 50% probability; (b) Side view of the centrosymmetric dimer showing the twist of
the –B(OH)2 moiety and the perpendicular orientation of the phenylboronic and naphthyl rings.
center form a dimer via two strong O–HÁ Á ÁO hydrogen bonds be-
tween the syn-oriented OH donor groups and oxygen atoms from
the anti OH groups acting as acceptors (Fig. 2). The OÁ Á ÁO distance
is 2.734(1) Å [HÁ Á ÁO distance = 1.85(2) Å] while the O–HÁ Á ÁO angle
reaches a value of 177(1)°. This centrosymmetric dimer is de-
scribed by the R22(8) graph set16 and together with the intramolec-
ular hydrogen bond motif constitute the first level graph set
N1 = S(7)R22(8).22 Such a graph set is also present in all aliphatic
analogs and can be treated as the first order supramolecular object
present in the crystal state of phenylboronic acids with ortho-ami-
nomethyl substituents.
The amination–reduction reaction conditions strongly influence
the reactivity of 2-formylphenylboronic acid toward secondary aro-
matic amines. Application of excess of the carbonyl reagent, acetic
acid as the protonating agent, MeCN as the solvent, molecular sieves
as the dehydrating agent, and NaBH(OAc)3 as the reducing agent
enables the synthesis of 2-(arylaminomethyl)phenylboronic acids
in reasonable yields. The first example of the crystal structure of
2-(arylaminomethyl)phenylboronic acid displaying intramolecular
contaminated with 3, which was confirmed on the basis of elemen-
tal analysis as well as the 1H NMR spectrum. The spectrum of the
pure acid was obtained after the addition of one drop of D2O.
Yellow crystals (mp = 84–89 °C for the anhydride contaminated
with 3).
Elemental analysis calculated for the anhydride: C45H48B3N3O3
(711.4) C, 75.98; H, 6.80; N, 5.91; and for 3: C15H18BNO2
(255.14): C, 70.62; H, 7.11; N, 5.49; found: C, 74.03; H, 6.84; N,
5.71. 1H NMR [(CD3)2CO with one drop of D2O, 400 MHz]: 7.72
(m, 1H, Ar); 7.26–7.13 (m, 5H, Ar); 6.91 (m, 2H, Ar); 6.75 (m, 1H,
Ar); 4.52 (s, 2H, CH2); 3.30 (q, J = 7.2 Hz, 2H, CH2); 1.03 (t,
J = 7.2 Hz 3H, CH3).
13C NMR [(CD3)2CO with one drop of D2O, 100 MHz]: 149.3,
143.6, 135.9, 130.1, 129.6, 128.6, 126.8, 120.0, 117.5, 56,7, 46.6,
11.5. 11B NMR [(CD3)2CO with one drop of D2O, 64 MHz]: 29.6,
20.0 (100:14).
Synthesis of (2-{[methyl(phenyl)amino]methyl}phenyl)boronic
acid (3a)
C–HÁ Á Áp interactions has been reported.
To a 250 ml flask equipped with a stir bar, MeCN (185 ml),
molecular sieves (3 Å, 18 g), 2-formylphenylboronic acid (1)
(5.600 g; 37.371 mmol), N-methylaniline (2.000 g, 18.665 mmol),
and Ac2O (3.815 g, 37.371 mmol) were added. The flask was placed
in an ice bath and the mixture was stirred for 3 h at 0 °C after
which NaBH(OAc)3 (7.900 g, 37.371 mmol) was added. Stirring
was continued for another 10 min, and the molecular sieves were
removed by filtration. The filtrate was concentrated under reduced
pressure and the residue was dissolved in 3 M HCl (8 ml) and H2O
(150 ml) and stirred for 5 min. Extraction was carried out with
Et2O (4 Â 25 ml). The combined organic layers were dried over
Na2SO4 and concentrated under reduced pressure to afford product
4. The pH of the aqueous phase was adjusted to 7 with 25% aq NH3
and the resulting white precipitate was filtered to afford 3a
(1.953 g, 43% yield).
Synthesis of (2-{[ethyl(phenyl)amino]methyl}phenyl)boronic
acid (3)
To a 50 ml flask equipped with a stir bar, MeCN (10 ml), molec-
ular sieves (3 Å, 1 g), 2-formylphenylboronic acid (1) (0.200 g,
1.334 mmol), N-ethylaniline (2) (0.0808 g, 0.667 mmol), and AcOH
(0.0801 g, 1.334 mmol) were added. The flask was placed in an ice
bath and the mixture was stirred for 3 h at 0 °C after which
NaBH(OAc)3 (0.283 g, 1.334 mmol) was added. Stirring was contin-
ued for another 10 min, and the molecular sieves were removed by
filtration. The filtrate was concentrated under reduced pressure
and the residue was dissolved in 3 M HCl (4 ml) and H2O (10 ml)
and stirred for 5 min. Extraction was carried out with Et2O
(4 Â 15 ml). The combined organic layers were dried over Na2SO4
and concentrated under reduced pressure to afford product 4.
The pH of the aqueous phase was adjusted to 7 with 25% aq NH3
and extraction was carried out with Et2O (4 Â 15 ml). The
combined organic layers were dried over Na2SO4 and concentrated
under reduced pressure to afford product 3 (highest yield was
66%). The product was isolated in the form of the anhydride
White powder (mp = 89–95 °C). Elemental analysis calculated
for the acid: C14H16BNO2 (241.10): C, 69.75; H, 6.69; N, 5.81;
found: C, 69.52; H, 6.62; N, 5.85.
The 1H NMR spectrum in CDCl3 revealed signals corresponding
to the acid as well as to boroxin. Addition of one drop of D2O re-
sulted in the spectrum of pure boronic acid.

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 52 (2011) 6639–6642
6642
1H NMR (CDCl3, 400 MHz): 7.93 (1H, Ar); 7.82 (d, J = 6.9 Hz, 1H,
is grateful for support within the Erasmus Program. I.D.M. kindly
acknowledges support by Warsaw University of Technology.
Ar); 7.36–7.33 (m, 6H, Ar); 7.26–7.20 (m, 7H, Ar); 6.93–6.83 (m,
2H, Ar); 4.68 (s, 6H, 3 Â CH2); 4.22 (s, 2H, CH2); 2.96 (s, 9H,
3 Â CH3); 2.66 (s, 3H, CH3).
Supplementary data
1H NMR (CDCl3 with one drop of D2O, 400 MHz): 7.95–7.93 (m,
1H, Ar); 7.36–7.31 (m, 4H, Ar); 7.22–7.18 (m, 3H, Ar); 7.08 (t,
J = 7.2 Hz, 1H, Ar); 4.22(s, 2H, CH2); 2.66 (s, 3H, CH3).
13C NMR (CDCl3 with one drop of D2O, 100 MHz): 150.5, 141.4,
136.3, 131.0, 130.4, 129.3, 127.6, 123.7, 120.3, 62.4, 40.4. 11B NMR
(CDCl3 with one drop of D2O, 64 MHz) 29.0 (br s), 20.0 (minor
intensity).
Supplementary data (a table describing the optimization exper-
iments. Details of X-ray measurements. Crystal data for 3b,
Selected geometrical parameters of crystals of 3b. Copies of 1H,
13C and 11B NMR spectra of 3, 3a and 3b) associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.tetlet.2011.10.008.
Synthesis of (2-{[ethyl(1-naphthyl)amino]methyl}phenyl)
boronic acid (3b)
References and notes
1. Hall, D.G.(Ed.), Boronic Acids. Preparation, Applications in Organic Synthesis and
Medicine, VCH: Weinheim: Germany, 2005.
To a 100 ml flask equipped with a stir bar, MeCN (24 ml),
molecular sieves (3 Å, 2.4 g), 2-formylphenylboronic acid (1)
(0.788 g, 5.259 mmol), and N-ethyl-1-naphthylamine (0.300 g,
1.752 mmol) were added. The flask was placed in an ice bath and
the mixture was stirred for 3 h at 0 °C after which the reducing
agent, NaBH(OAc)3, (1.112 g, 5.259 mmol) was added. Stirring
was continued for another 10 min, and the molecular sieves were
removed by filtration. The filtrate was concentrated under reduced
pressure and the residue was dissolved in 3 M HCl (4 ml) and H2O
(10 ml) and stirred for 10 min. Extraction was carried out with
Et2O (4 Â 15 ml). The organic layer was dried over Na2SO4 and con-
centrated under reduced pressure to afford 4. The pH of the aque-
ous phase was adjusted to 7 with 25% aq NH3 and the resulting
pink precipitate was filtered to afford 3b (0.207 g, 38% yield). An
analogous reaction carried out in the presence of an equimolar
amount of AcOH resulted in 0.303 g (56% yield) of the desired
product.
2. James, T. D.; Phillips, M. D.; Shinkai, S. Boronic Acids in Saccharide Recognition;
RSC Publishing: Cambridge, UK, 2006.
3. Springsteen, G.; Wang, B. Tetrahedron 2002, 58, 2002.
4. Fang, H.; Kaur, G.; Wang, B. J. Fluoresc. 2004, 14, 481.
5. Yang, W.; Gao, X.; Wang, B. Med. Res. Rev. 2003, 23, 346.
6. Duggan, P. J.; Houston, T. A.; Kiefel, M. J.; Levonis, S. M.; Smith, B. D.; Szydzik, M.
L. Tetrahedron 2008, 64, 7122.
´
´
´
7. Adamczyk-Wozniak, A.; Brzózka, Z.; Cyranski, M.; Filipowicz-Szymanska, A.;
_
_
Klimentowska, P.; Zubrowska, A.; Zukowski, K.; Sporzyn´ ski, A. Appl.
Organometal. Chem. 2008, 22, 427.
_
8. Adamczyk-Woz´niak, A.; Madura, I.; Pawełko, A.; Sporzyn´ ski, A.; Zubrowska, A.;
_
Zyła, J. Cent. Eur. J. Chem. 2011, 9, 199.
9. Baxter, E. W.; Reitz, A. B. In Organic Reactions; Wiley, 2002; Vol. 59, Chapter 1.
10. Tan, W.; Zhang, D.; Zhu, D. Bioorg. Med. Chem. Lett. 2007, 17, 2629.
´
´
11. Adamczyk-Wozniak, A.; Madura, I.; Velders, A. H.; Sporzynski, A. Tetrahedron
Lett. 2010, 51, 6181.
´
´
12. Adamczyk-Wozniak, A.; Cyranski, M. K.; Jakubczyk, M.; Klimentowska, P.; Koll,
_
_
A.; Kołodziejczak, J.; Pojmaj, G.; Zubrowska, A.; Zukowska, Z.; Sporzyn´ ski, A. J.
Phys. Chem. A 2010, 114, 2324.
13. Iovine, P. M.; Fletcher, M. N.; Lin, S. Macromolecules 2006, 39, 6324.
14. Crystallographic data (excluding structure factors) for the structure reported in
this Letter has been deposited with the Cambridge Crystallographic Data
Centre as deposit number CCDC 833148. Copies of the data can be obtained,
free of charge, on application to the CCDC, 12 Union Rd., Cambridge CB2 1EZ,
UK (fax: +44(1223)336033; e-mail: deposit@ccdc.cam.ac.uk)
15. Coghlan, S. W.; Giles, R. L.; Howard, J. A. K.; Patrick, L. G. F.; Probert, M. R.;
Smith, G. E.; Whiting, A. J. Organomet. Chem. 1990, 690, 4784.
16. Etter, M. C. Acc. Chem. Res. 1990, 23, 120.
17. Zhao, J.; Davidson, M. G.; Mahon, M. F.; Kociok-Kohn, G.; James, T. D. J. Am.
Chem. Soc. 2004, 126, 16179.
18. Zhu, L.; Shabbir, S. H.; Gray, M.; Lynch, V. M.; Sorey, S.; Anslyn, E. V. J. Am. Chem.
Soc. 2006, 128, 1222.
Pink powder (mp = 119–130 °C). Crystallization from CDCl3
afforded colorless crystals suitable for X-ray measurements.
Elemental analysis calculated for the acid:
C19H20BNO2
(305.18): C, 74.78; H, 6.61; N, 4.59; found: C, 74.66; H, 6.66; N,
4.60.
1H NMR (CDCl3 with one drop of D2O, 400 MHz): 8.13 (d,
J = 8.3 Hz, 1H, Ar); 7.86 (d, J = 6.8 Hz, 1H, Ar); 7.81 (d, J = 8.0 Hz,
1H, Ar); 7.65–7.63 (m, 1H, Ar); 7.53–7.50 (m, 1H, Ar); 7.48–7.40
(m, 3H, Ar); 7.33–7.27 (m, 3H, Ar); 4.44 (s, 2H, CH2); 3.28 (q,
J = 7.2 Hz, 2H, CH2); 0.97 (t, J = 7.2 Hz, 3H, CH3).
19. Arnold, K.; Batsanov, A. S.; Davies, B.; Whiting, A. Green Chem. 2008, 10, 124.
20. Zhang, L.; Kerszulis, J. A.; Clark, R. J.; Ye, T.; Zhu, L. Chem. Commun. 2009, 2151.
21. Nishio, M. Phys. Chem. Chem. Phys. 2011, 13, 13873.
13C NMR (CDCl3 with one drop of D2O, 100 MHz): 144.1, 141.2,
136.2, 134.8, 131.5, 130.2, 129.5, 128.7, 127.4, 126.1, 125.9, 125. 8,
125.0, 122.8, 119.7, 57.5, 49.3, 9.3. 11B NMR (CDCl3 with one drop
of D2O, 64 MHz): 29.3, 20.0 (minor intensity).
22. Etter, M. C.; MacDonald, J. C.; Bernstein, J. Acta Crystallogr. 1990, B46, 256.
Acknowledgments
Financial support by the Ministry of Science and Higher
Education (Grant No. N204 127938) is kindly acknowledged. A.P.