Synthesis, electrochemical, structural and theoretical study of new derivatives of O[sbnd]B[sbnd]N and O[sbnd]B[sbnd]O heterocycles

Article abstract of DOI:10.1016/j.ica.2016.08.009

Three new oxazaborine and two boron diketonate derivatives containing different structural motifs were synthesized and studied using NMR, electrochemistry (CV, RDV, and dc-polarography), UV–Vis spectra and X-ray structure analysis. The experimental data were correlated with quantum chemical calculations. The main attention was focused on determination of the first oxidation and the first reduction potentials, their relationship to the calculated HOMO and LUMO energies and to their UV–Vis spectra. The electrochemical data reflect the structure–redox properties relationship depending on location of oxidation and reduction center.

Full text of DOI:10.1016/j.ica.2016.08.009

Inorganica Chimica Acta xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, electrochemical, structural and theoretical study of new
derivatives of OABAN and OABAO heterocycles
a
b
c
c
c
d
ˇ
ˇ ˇ
Tomáš Mikysek , Hana Kvapilová , Hana Doušová , František Josefík , Petr Šimu˚ nek , Zdenka Ru˚ zicková ,
b,
⇑
ˇ
Jirí Ludvík
a University of Pardubice, Faculty of Chemical Technology, Department of Analytical Chemistry, Studentská 573, Pardubice CZ 532 10, Czech Republic
b J. Heyrovsky´ Institute of Physical Chemistry, ASCR, v.v.i., Dolejškova 3, 182 23 Prague 8, Czech Republic
c University of Pardubice, Faculty of Chemical Technology, Institute of Organic Chemistry and Technology, Studentská 573, Pardubice CZ 532 10, Czech Republic
d University of Pardubice, Faculty of Chemical Technology, Department of General and Inorganic Chemistry, Studentská 573, Pardubice CZ 532 10, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new oxazaborine and two boron diketonate derivatives containing different structural motifs were
synthesized and studied using NMR, electrochemistry (CV, RDV, and dc-polarography), UV–Vis spectra
and X-ray structure analysis. The experimental data were correlated with quantum chemical calculations.
The main attention was focused on determination of the first oxidation and the first reduction potentials,
their relationship to the calculated HOMO and LUMO energies and to their UV–Vis spectra. The electro-
chemical data reflect the structure–redox properties relationship depending on location of oxidation and
reduction center.
Received 13 April 2016
Received in revised form 4 August 2016
Accepted 5 August 2016
Available online xxxx
Keywords:
Boron heterocycles
Synthesis
Ó 2016 Elsevier B.V. All rights reserved.
X-ray structure analysis
Electrochemistry
HOMO-LUMO
DFT calculations
1. Introduction
[11–14]. In addition to the synthesis we have been studying the
properties of some of the heterocycles prepared as well. A number
Since the second half of the last century, when the chemistry of
boron-containing heterocycles was born [1–3], it has been experi-
encing a significant expansion. Boron heterocycles were promoted
from relative curiosities into compounds having a broad spectrum
of useful properties. The presence of boron in the molecules of
organic compounds substantially changes their electronic proper-
ties in comparison with the carbocyclic analogues [4]. Many
boron-containing heterocycles have great potential for applica-
tions in molecular electronics as non-linear optical (NLO) chro-
mophores and OLED materials [5,6]. A great chapter of boron
heterocycles are BODIPY, highly fluorescent and stable compounds
suitable as dyes, sensors and materials for OLEDs [7,8]. Some
NABAO heterocycles have been considered for using in BNCT
(boron-neutron capture therapy) [9]. Some compounds of this type
possess antibacterial activity [10].
of bicyclic triazaborines exhibit fluorescence in powder and 2-
MeTHF frozen state [14]. Recently we have performed electro-
chemical and DFT study of a homologous series of bicyclic triaza-
borines [15a] and their precursors, oxazaborines [15b]. The
obtained results describe and compare redox properties of both
series, their HOMO and LUMO energies, localization of redox
centers within the molecules and relationship between the
structure and properties, which could be used for both the
understanding the electronic situation inside the heterocycles
and for tuning-up their properties so as to be tailored for
particular application.
The present work represents a convenient extension of the pre-
vious contributions. It deals with synthesis, electrochemical and
quantum chemical study of three oxazaborines (boron ketoimi-
nates) being analogous to those in our previous study but without
the p-substituted phenyl azo-pendant (5a–c) and two similar
boron diketonates (4a,b). The oxazaborine 6 [15b] was taken for
During past several years we have developed a simple method-
ology for synthesis of various kinds of OABAN and NABAN hetero-
cycles with BPh2 moiety from corresponding polarized ethylenes
direct comparison with its homologue 5c as
a ‘‘bridging”
compound interconnecting the previous and the present series of
compounds. (Scheme 1).
⇑
Corresponding author.
E-mail address: jiri.ludvik@jh-inst.cas.cz (J. Ludvík).
http://dx.doi.org/10.1016/j.ica.2016.08.009
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Mikysek et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.08.009

2
T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
F
F
B-
+
B-
O
N+
N
B-
+
O
O
O
O
H3C
CH3
N
CH3
CH3
CH3
4a
4b
6
B-
B-
CH3
CH3
N+
B-
CH3
O
N+
O
O
N+
CH3
H3C
CH3
H3C
CH3
5a
5b
5c
Scheme 1. Oxazaborines and boron diketonates studied in this work.
2. Results and discussion
Fig. 1. The molecular structure (ORTEP 50% probability level) of 5a.
2.1. Synthesis
cell and compound 5b (Fig. 2) crystallizes in the orthorhombic
crystal system with Pbca space group with eight molecules within
the unit cell. In both compounds no intramolecular or intermolec-
ular contacts are present.
The boron atom in compound 5a lies out of the heteroatomic
ring plane (the torsion of 38.98(3)° (Fig. 3) where the rest of the
Boron diketonates 4a,b and ketoiminates 5a–c involved in the
study were prepared according to the Scheme 2. The BR2 fragment
was introduced by means of the reaction of appropriate trivalent
boron compound (BF3, BPh3 or Ph2BOH) with b-diketone or b-
enaminone. The synthesis of enaminone 3a was done using classic
condensation between 3-methylacetylacetone (1a) and methy-
lamine. An attempt to prepare in the same way also its benzoylace-
tone analogue 3b failed. Successful synthesis of 3b was achieved
after adopting the methodology published by Polanc [16] through
diketonatoboron difluoride 4a.
atoms forming this six-membered ring are involved in a p-electron
conjugated system. The boron atom has the geometry of a dis-
torted tetrahedron. The interatomic distances of BAO (1.508 Å)
and BAN (1.579 Å) in 5a are comparable to the previously reported
structurally related species [11–14]. In boron diketonates, the BAO
distances are comparable with those in 5a, on the other hand, in
the molecule where boron atom is substituted by two fluoride
atoms, the BAO bond is much shorter (1.466 Å) [17–19].
2.2. X-ray study
In the compound 5b the formation of the planar arrangement of
the OAC3AN atoms is suppressed probably by steric as well as
The compound 5a (Fig. 1) crystallizes in the monoclinic crystal
system with P21/c space group with four molecules within the unit
Ph Ph
B
O
O
BPh3
DCM
Ph
4b (70%)
Ph Ph
B
Ph3B or
Ph2BOH
O
NHMe
O
O
O
O
O
NMe
MeI
K2CO3
acetone
MeNH2
R
R
R
DCM
(65%)
(98%)
R
EtOH
for 2a
1a,b
3a
3b
2a (52%)
2b (85%)
5a
(52%)
1a
1b
R = Me (
R = Ph (
)
)
5b (38–45%)
F
F
MeNH2
THF
BF3.Et2O
DCM
B
O
O
Ph
4a (94%)
Ph
O
Ph
N
B
Ph3B
DCM, rt
O
HN
3c
5c
(40%)
Scheme 2. Synthesis of the boron heterocycles.
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T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
3
Fluorine-19 NMR spectrum of 4a consists of two broad singlets
(
10BF2 and 11BF2) whose chemical shifts (ca ꢀ142 ppm) are also
comparable with those described by Gardinier. In comparison with
fluorine-19 chemical shifts of difluoroboron ketoiminates e.g.
[17,18], b-diketonates possess certain upfield shift, which is
explained by the anisotropy of the chelate ring of the diketonate
causing greater shielding of the fluorines than in the ketoiminates
[18].
The substitution of the fluorines in 4a for phenyls leads to a
downfield shift in 11B NMR of 4b (0.18 ppm for 4a vs. 7.56 ppm
for 4b). This effect is common [20]. The chemical shift of 4b is com-
parable with those published e.g. by Chujo et al. [21].
Boron chemical shifts of diphenylboron ketoiminates 5 (3.54
and 3.76 ppm resp.) are more upfielded than those of N-cou-
marin-6-yl substituted oxazaborines ([17], d = 4.9–5.5) which can
be caused e.g. by lower steric effect of the N-methyl group in com-
parison with the coumarin-6-yl fragment. Greater degree of conju-
gation then cause larger shielding of the boron atom leading to the
observed upfield shift. An electron-donating effect of the C4 methyl
group can also play a role in the increased shielding of the boron
atom in 5.
2.4. Electrochemical data and quantum chemical correlations
The studied compounds within the series were selected in such
a manner enabling direct comparison of the electrochemical data
of at least two compounds differing only in one feature. In this
way the influence of individual structural motifs can be followed.
Experimental data of the first oxidation and the first reduction
potential, wavelengths of main bands in UV–Vis spectra and calcu-
lated HOMO and LUMO energies are presented in the Table 1. The
calculated distribution of HOMO and LUMO orbitals, respectively,
is depicted in the Fig. 4.
Fig. 2. The molecular structure (ORTEP 40% probability level) of 5b.
To interconnect the previous and the present series of com-
pounds, the oxazaborine 6 [15b] was taken for direct comparison
with its homologue 5c. The phenylazo-pendant in compound 6
represents
a
conjugated system extending the electron
delocalization of the heterocycle. In the absence of the phenylazo
substituent (5c) the oxidation and reduction centers are located
at the boron heterocycle: HOMO involves mostly the negatively
charged boron atom, the other two heteroatoms and at the
opposite C5AC6 bond of the heterocycle, LUMO is located at the
OAC and N@C bonds of the heterocycle (cf. Fig. 4). On the other
hand, oxidation of 6 as well as reduction of 6 proceeds more
Fig. 3. Angle between O1AB1AN1 plane and the rest of heteroatomic ring
O1AC1AC2AC3AN1 in compound 5a.
Table 1
Experimental data of the first oxidation and the first reduction potential, compared
with calculated HOMO and LUMO energies.
Compound E1/2
E1/2 (red
D
Ed
[V]
EHOMO
calculated
(eV)
ELUMO
calculated
(eV)
HOMO-
LUMO
gap
electronic influence of the phenyl substituent at the C1 atom
instead of the methyl substituent in 5a, O1 and N1 are thus located
slightly out of the C1AC2AC3 plane. Other interatomic distances
and angles are comparable to the corresponding parameters found
in 5a.
(ox 1) 1) [V]
[V]
4a
4b
5a
–
–
ꢀ1.14b
ꢀ1.31b
ꢀ2.25
ꢀ7.02
ꢀ6.54
ꢀ2.66
ꢀ2.46
ꢀ1.48
4.36
4.08
4.46
1.29a
3.51 ꢀ5.94
(85 mVc)
ꢀ1.89b
ꢀ2.21
5b
5c
1.34b
1.50a
3.23 ꢀ5.96
3.63 ꢀ6.14
ꢀ1.90
ꢀ1.62
4.06
4.52
2.3. NMR Study
(78 mVc)
ꢀ1.58b
1.27b
2.85 ꢀ6.00
ꢀ2.20
3.80
All boron chelates were characterized by their 11B chemical
shifts. Boron signal of difluoroboron diketonate 4a should be, in
principle, split to triplet. In reality, the signal of 4a is a broad sin-
glet. Gardinier et al. [18] explained this observation by quadrupo-
lar effect of the boron and by a magnetic anisotropy of the chelate
ring. Boron-11 chemical shift of 4a (d = 0.18) lies in the area typical
for four-coordinated BF2 compounds (Gardinier observed chemical
shifts for difluoroboron diaryldiketonates in range 0.23–0.99 ppm).
6
All potentials are given vs. SCE.
Potential of the irreversible anodic peak (Epa) taken from cyclic voltammetry at
Pt-electrode.
Half-wave potential (E1/2) taken from voltammetry at a platinum rotating disk
electrode (RDE).
a
b
c
Half-wave potential (E1/2
) taken from reversible cyclic voltammogram as
average value of oxidation and reduction peak potential, respectively (Epc ꢀ Epa/2).
d
Difference between the first oxidation and reduction potentials.
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T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
easily with respect to 5c, hence at ‘‘lower” (in absolute values)
potentials. The maps of LUMO and HOMO orbitals of 6 show that
the reduction center involves the azo group and the oxidation
Compound HOMO
4a
LUMO
occurs partly at the phenylazo-pendant, too. The limited
p-
electron system in 5c results in more positive oxidation potential
and more negative reduction potential, that means in larger gap
between Eox and Erec in comparison with 6.
4b
5a
For comparison of 5c and 5a, one should accept a legitimate
approximation, that the saturated [3,4-a]azepine ring would be
electronically similar to the two methyl groups in positions 3
and 4. Then the only significant difference is the 5-methyl group
in 5a. The above mentioned approximation is confirmed by very
close reduction potentials of both derivatives (see Table 1), where
the 5-methyl compound 5a has the reduction potential only by
40 mV more negative (according to expectations) than 5c. On the
other hand, the 5-methyl substituent in 5a is directly connected
with the HOMO at the C5AC6 bond. Therefore, the oxidation
potential of 5a is shifted by 210 mV less positively compared to
5c reflecting the donor character of the methyl group making the
oxidation easier.
The compounds 5a and 5b differ only in the substitution at the
position 6: The methyl derivative 5a is oxidized by 50 mV less pos-
itively (according to expectations) in comparison with the phenyl
derivative 5b. On the other hand, the reduction potential of 5a is
significantly (by 360 mV) more negative than that of 5b. The rea-
5b
son of such an easy reduction of 5b is the extended p-delocalized
system involving also the phenyl ring in the position 6 (cf. maps
of HOMO and LUMO orbitals at the Fig. 4).
5c
6
The derivatives 4b and 5b structurally interconnect the boron
diketonates and ketoiminates, where the heteroatom oxygen is
replaced by the positively charged quarternary iminium group
substituted by an additional methyl. As a result, the boron ketoim-
inate 5b is more easily oxidized but more hardly reduced (the imi-
nium methyl group here plays
a role) in comparison with
analogous diketonate 4b. However, due to different charge distri-
bution in the two types of heterocycles, a deeper discussion is
difficult.
In the case of diketonates, the replacement of two phenyl rings
in the position 2 of the boron diketonate 4b by two fluorine atoms
results in less negative reduction of the compound 4a by 170 mV
due to the electron-withdrawing effect of the two fluorine atoms.
Oxidation potentials of these two diketonates are out of the acces-
sible potential range and therefore they could not be determined.
The totally different location of HOMOs (cf. Fig. 4) is very notewor-
thy, especially that of 4b, which is located exclusively on the BPh2
part of the molecule.
Fig. 4. Calculated distribution of HOMO and LUMO in molecules under study.
Fig. 5. Correlation of a) the measured first oxidation potentials with calculated HOMO energies and b) the measured first reduction potentials with calculated LUMO energies.
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T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
5
From the Table 1 it is evident that the difference between the
first oxidation and the first reduction potentials correlates well
with calculated HOMO-LUMO gap. Similarly, the measured first
oxidation potentials correlate with calculated HOMO energies
and the measured first reduction potentials correlate with calcu-
lated LUMO energies (Fig. 5a and b), both with the slope close to
unity, confirming the appropriateness of the experimental as well
as theoretical approach. Similar good correlation offer the plots
of experimental versus calculated values of the first oxidation
and reduction potentials, respectively.
2.5. UV–Vis spectra and quantum chemical correlations
Optical properties of the title compounds were investigated by
the absorption electron spectra measured in acetonitrile in the UV–
Vis region (Fig. 6). The measured wavelengths of absorption max-
ima and their corresponding energies in eV are summarized in the
Table 2 together with calculated values yielding a good correlation
(Fig. 7). The longest-wavelength absorption bands with kmax
between 330 and 362 nm are dominating in the obtained spectra.
The HOMO-LUMO transitions of the compounds 4a, 4b, 5a, 5b
Fig. 7. Correlation of the measured kmax with the calculated kmax (from the Table 2).
studied compounds) where several transitions contribute (see
Table 2) most probably thanks to the azo pendant.
and 5c correspond to the pure lowest
p-
p⁄ transition on hetero-
cyclic ring. The observed spectra are of rather low intensity which
correlates with the Oscillator Strength values (dimensionless
quantity which is proportional to the integral intensity of the cor-
responding absorption band and points to the intensity of the tran-
sition). These values presented in the Table 2 fit well with the
intensities of the corresponding experimental spectra. The most
intensive one belongs to the di-fluorinated diketonate 4a, where
HOMO is located at the phenyl ring attached to the heterocycle
as distinct from the similar structures 4b and 5b (cf. Fig. 4). The
compound 6 has the second most intensive band (within the
3. Conclusions
A series of three new oxazaborine and two boron diketonate
derivatives containing different structural motifs were synthesized
and characterized using NMR, UV–Vis spectra and X-ray structure
analysis. Their oxidation and reduction abilities were investigated
electrochemically using rotating-disk voltammetry, cyclic voltam-
metry and dc-polarography. The experiments were focused mainly
to the first oxidation and first reduction potential, since their val-
ues are related to HOMO and LUMO energies.
For understanding of the relationship between the structure
and redox properties, the electronic influence of individual sub-
stituents on the reduction and oxidation potential was followed.
Whereas the oxazaborines 5a–c are reduced very negatively
(ꢀ1.89 to ꢀ2.25 V vs. SCE), the reduction potentials of boron dike-
tonates are about 800 mV less negative. However, the opposite sit-
uation is in oxidation: oxazaborines are oxidized at 1.29–1.50 V vs.
SCE, whereas oxidation of boron diketonates does not proceed
within the accessible potential range.
Using DFT method, the energies of HOMO and LUMO together
with the reduction and oxidation potentials were calculated and
compared with experimental values. The good correlation confirms
the credibility and reliability of experimental as well as theoretical
approach. Similarly, the energies of the longest-wavelength
absorption bands taken from UV–Vis spectra were compared with
the experimentally found differences Eox–Ered and with calculated
HOMO-LUMO gaps and transition energies.
Fig. 6. UV–Vis absorption spectra of OZB and OOB compounds measured in
acetonitrile.
4. Experimental
Table 2
4.1. Electrochemistry
Quantum-chemical calculations of electron transitions.
Compound kmax
experim.
[nm] (eV)
kmax
Oscillator Orbital transition
Electrochemical measurements were carried out in N,N-
dimethylformamide containing 0.1 M Bu4NPF6. DC polarography,
cyclic voltammetry (CV) and rotating disk voltammetry (RDV)
were used in a three electrode arrangement. The working electrode
was dropping mercury electrode (DME) for polarographic mea-
calculated strength
[nm]
(% contribution)
4a
4b
5a
5b
5c
6
330 (3.76) 313
362 (3.43) 359
338 (3.67) 318
353 (3.51) 354
334 (3.71) 317
345 (3.59) 356
0.533
0.0166
0.120
0.230
0.128
0.652
HOMO ? LUMO (98)
HOMO ? LUMO (94)
HOMO ? LUMO (98)
HOMO ? LUMO (99)
HOMO ? LUMO (97)
HOMO ? LUMO (43)
HOMOꢀ1 ? LUMO (30)
HOMO ? LUMO+1 (25)
surements (drop time
sD = 1 s, scan rate t
= 5 mV.sꢀ1), hanging
mercury drop electrode (HMDE) for CV and platinum or glassy car-
bon disk (2 mm in diameter) for oxidation and RDV experiments.
As the reference and auxiliary electrodes were used saturated calo-
mel electrode (SCE) separated by a bridge filled with supporting
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T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
electrolyte and a Pt wire, respectively. All potentials are given vs.
SCE. Voltammetric measurements were performed using a poten-
tiostat PGSTAT 128 N (AUTOLAB, Metrohm Autolab B.V., Utrecht,
The Netherlands) operated via NOVA 1.11 software.
4.5. Synthesis
All the solvents and reagents were commercial and used with-
out further treatment. Dry solvents were stored under argon using
AcroSealTM or Sure/SealTM technology.
4.2. Crystallography
5. General procedure for C-methylation of b-diketones
The X-ray data for colorless crystals of 5a and yellow of 5b ware
obtained at 150 K using Oxford Cryostream low-temperature
Appropriate b-diketone (100 mmol) was dissolved in anhydrous
acetone (80 mL). Subsequently, anhydrous potassium carbonate
(93 mmol) was added under inert. After stirring the solution for
5 min. at room temperature, methyl iodide (7.72 mL, 100 mmol)
was added dropwise during 10 min. The reaction mixture was then
refluxed for 12 h. Volatile components were distilled off, the resi-
due was diluted with ether (100 mL) and precipitated solid was
removed by suction. The filtrate was evaporated in vacuo and the
residue was further purified.
device on a Nonius KappaCCD diffractometer with Mo K radiation
a
(k = 0.71073 Å), a graphite monochromator, and the / and
v scan
mode. Data reductions were performed with DENZO-SMN [22].
The absorption was corrected by integration methods [23]. Struc-
tures were solved by direct methods (Sir92) [24] and refined by full
matrix least-square based on F2 (SHELXL97) [25]. Hydrogen atoms
were mostly localized on a difference Fourier map, however to
ensure uniformity of treatment of crystal, all hydrogen were
recalculated into idealized positions (riding model) and assigned
temperature factors Hiso(H) = 1.2 Ueq (pivot atom) or of 1.5 Ueq
(methyl). H atoms in methyl moieties and hydrogen atoms in
aromatic rings were placed with CAH distances of 0.96 and 0.93 Å.
5.1. 2-Methylacetylacetone (2a)
Rint
=
R
|F2o ꢀ F2o,mean|/
R
R
F2o, GOF = [
R
(w(F2o ꢀ F2c)2)/(Ndiffrs ꢀ Nparams)]00
Prepared from acetylacetone. Purification through its copper
diketonate using the methodology published e.g. in [34]. Yield
52% of yellow-brown liquid. In CDCl3 the product is 3:2 mixture
of enol-keto tautomers. 1H NMR (400 MHz, CDCl3): keto form
d = 3.71 (q, J = 7.1 Hz, 1H), 2.20 (s, 6H), 1.33 (d, J = 7.1 Hz, 3H); enol
form d = 16.46 (s, 1H), 2.12 (s, 6H), 1.85 (s, 3H). Data are in accor-
dance with those published in [35].
for all data, R(F) =
||Fo| ꢀ |Fc||/
R|Fo| for observed data,
wR(F2) = [
R
(w(F2o ꢀ F2c)2)/(
R
w(F2o)2)]00 for all data.
Crystallographic data for structural analysis have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC No.
1454995 and 1454996 for 5a and 5b. Copies of this information
may be obtained free of charge from the Director, CCDC, 12 Union
Road, Cambridge CB2 1EY, UK (fax: +44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
5.2. 2-Methylbenzoylacetone (2b)
4.3. NMR and UV–Vis spectroscopy and other instrumentation
Prepared from benzoylacetone. Purification by a column chro-
matography (hexane/EtOAc, 8:1 v/v). Yield 85% of yellow liquid.
The product is almost pure keto form in CDCl3. 1H NMR
(400 MHz, CDCl3) d = 7.99–7.97 (m, 2H), 7.62–7.58 (m, 1H), 7.52–
7.48 (m, 2H), 4.50 (q, J = 7.0 Hz, 1H), 2.17 (s, 3H), 1.46 (d,
J = 7.0 Hz, 3H) ppm. The data are in accordance with those pub-
lished in [36].
NMR Spectra were measured in CDCl3 using either Bruker
AVANCE III operating at 400.13 MHz (1H), 100.61 MHz (13C),
376.50 MHz (19F) and 128.38 MHz (11B) or Bruker AscendTM operat-
ing at 500.20 MHz (1H), 125.78 MHz (13C), 470.66 MHz (19F) and
160.48 MHz (11B). Proton NMR spectra were calibrated on an inter-
nal TMS (d = 0.00), carbon spectra on the middle signal of the sol-
vent multiplet (d = 77.23), fluorine spectra on
a,a,a-
trifluorotoluene (d = ꢀ63.9) and boron spectra on trimethoxybo-
rane (d = 18.1). Boron-11 NMR spectra were measured in 5 mm
quartz NMR tubes (Norell). Both carbon and fluorine spectra were
measured under proton decoupling. UV–Vis spectra were recorded
on a Shimadzu UV-2450 spectrophotometer in acetonitrile. Ele-
mental analyses were performed on a Flash EA 2000 CHNS auto-
matic analyser (Thermo Fisher Scientific). HRMS were measured
on a MALDI LTQ Orbitrap XL (Thermo Fisher Scientific).
5.3. 3-Methyl-4-methylaminopent-3-en-2-one (3a)
The mixture of b-diketone 2a (22 mmol, 2.46 g) in EtOH (41 mL)
and Me2NH (33 wt% solution in EtOH; 0.2 mol Me2NH) was
refluxed for 5 h. The volatile components were evaporated in vacuo
and the residue was separated between DCM (50 mL) and water
(50 mL). Organic layer was dried over Na2SO4 and evaporated
under reduced pressure to give oily product. Yield: 1.78 g, 65%.
1H NMR (400 MHz, CDCl3): d = 11.86 (br s, 1H), 2.94 (d, J = 5.1 Hz,
3H), 2.13 (s, 3H), 1.97 (s, 3H), 1.84 (s, 3H).
4.4. Quantum chemical calculations
Structures of the studied molecules were calculated using the
density functional theory (DFT) method with Becke’s three-param-
eter hybrid functional B3LYP [26,27] and triple-f polarized 6-311G
(d) [28] basis sets for all atoms. Geometries of all the compounds
were fully optimized with no symmetry constraints. Vibrational
analyses followed on the optimized structures to ensure proper
character of the stationary states. Electronic transitions were cal-
culated on the ground-state optimized structures with aid of the
time-dependent DFT (TDDFT) method [29]. Gaussian 09 software
package (G09) [30] was used for all the calculations. The effect of
solvation was accounted for by the polarizable continuum model
(PCM) [31,32] with solvent parameters according to the experi-
ments, as implemented in the G09 using the integral equation for-
malism [33]. Maps of the frontier molecular orbitals were plotted
using the GaussView software.
5.4. 2-Methyl-3-(methylamino)-1-phenylbut-2-en-1-one (3b)
To a stirred solution of a boron complex 4a (3.59 g, 16 mmol) in
anhydrous acetonitrile (15 mL), methylamine solution in THF
(8.4 mL, 48 mmol) was added. The reaction mixture was stirred
at room temperature for 1.5 h and volatile components were then
evaporated. The residue was dissolved in dichloromethane
(50 mL), washed with water (2 ꢁ 25 mL), dried over anhydrous
sodium sulfate and evaporated to dryness. Yield 2.97 g (98%) of
red solid with mp 59.5–61 °C (Ref. [37] gives 70–71 °C). 1H NMR
(400 MHz, CDCl3): d = 12.44 (br s, 1H), 7.31–7.39 (m, 5H), 3.04 (d,
J = 5.3 Hz, 3H), 2.07 (s, 3H) 1.82 (s, 3H) ppm. 13C NMR (100 MHz,
CDCl3): d = 192.4, 166.5, 143.3, 128.2, 127.8, 126.9, 97.6, 30.1,
16.4, 15.5 ppm.
Please cite this article in press as: T. Mikysek et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.08.009

T. Mikysek et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
7
5.5. 2,2-Difluoro-4,5-dimethyl-6-phenyl-1,3,2k4-dioxaborine (4a)
Crystallographic data for 5a: C19H22BNO, M = 291.19, mono-
clinic, P21/c, a = 8.3370(5) Å, b = 10.0640(9) Å, c = 19.8901(14) Å,
The method was adopted from [16]. To a solution of 1,3-dike-
tone 2b (3 g, 17 mmol) in dichloromethane (45 mL), boron trifluo-
ride etherate (6.43 mL, 51 mmol) was added at room temperature.
The reaction mixture was stirred at room temperature for 25 h.
Afterwards, volatile components were evaporated and the residue
was suspended in water (60 mL). Solid material was filtered off and
dried in a vacuum furnace. Yield: 3.59 g (94%) of yellow solid. The
sample for electrochemical study was further recrystallized from
ethanol to give white solid with mp 76–77 °C (Ref. [38] reports
153–154 °C). 1H NMR (400 MHz, CDCl3): d = 7.72–7.69 (m, 2H),
7.61–7.57 (m, 1H), 7.52–7.48 (m, 2H), 2.46 (s, 3H), 2.10 (s, 3H)
ppm. 13C NMR (100 MHz, CDCl3): d = 194.3, 184.1, 133.5, 133.1,
129.9, 128.7, 24.0, 14.2 ppm. 19F NMR (376.5 MHz, CDCl3):
d = ꢀ142.06 (10BF2), ꢀ142.13 (11BF2) ppm. 11B NMR (160.5 MHz,
CDCl3) d = 0.18 ppm. HRMS for C11H11BF2O2 calc. [MꢀF]+
205.08307 [M+Na]+ 247.07124 [M+K]+ 263.04518 [2M+Na]+
471.15271, found [MꢀF]+ 205.08322 [M+Na]+ 247.07143 [M+K]+
263.04540 [2M+Na]+ 471.15373.
b = 94.256(6) °, Z = 4, V = 1664.3(2) Å3, Dc = 1.162 g.cmꢀ3
,
l
= 0.070 mmꢀ1
,
Tmin/Tmax = 0.981/0.988;
ꢀ10 6 h 6 10,
ꢀ12 6 k 6 13, ꢀ23 6 l 6 25; 13938 reflections measured
(hmax = 27.49°), 3760 independent (Rint = 0.0279), 3066 with
I > 2r(I), 199 parameters, S = 1.070, R1(obs. data) = 0.0467, wR2
(all data) = 0.1143; max., min. residual electron density = 0.286,
ꢀ0.317 eÅꢀ3
.
5.9. 3,4,5-Trimethyl-2,2,6-triphenyl-1,3,2k4-oxazaborine (5b)
Method A, reaction time 5 days, purification by means of col-
umn chromatography (silica gel, DCM) followed by recrystalliza-
tion from ethanol, yield 38%. Method B: 1.5 eq. BPh3, reaction
time 5 days, purification by means of column chromatography (sil-
ica gel, DCM), yield 45%.
Bright yellow solid, mp 167–167.5 °C. 1H NMR (400 MHz,
CDCl3): d = 7.52–7.50 (m, 2H), 7.38–7.34 (m, 7H), 7.25–7.20 (m,
6H), 2.96 (s, 3H), 2.27 (s, 3H), 1.86 (s, 3H) ppm. 13C NMR
(100 MHz, CDCl3): d = 171.7, 169.2, 133.0, 131.4, 127.1, 126.2,
102.8, 38.2, 21.4, 17.2, 14.0 ppm. 11B NMR (160.5 MHz, CDCl3):
d = 3.76 ppm. Found C, 81.60; H, 6.87; N, 3.89. C24H24BNO requires
C, 81.60; H, 6.85; N, 3.96.
5.6. 4,5-Dimethyl-2,2,6-triphenyl-1,3,2k4-dioxaborine (4b)
The flask fitted with a calcium-chloride drying tube was
charged with 1,3-diketone 2b (1 g, 5.7 mmol) in anhydrous DCM
(25 mL). Afterwards BPh3 (1.65 g, 6.8 mmol) was added in one por-
tion. The reaction mixture was stirred for 4 days at laboratory tem-
perature and then concentrated under reduced pressure. The
residue was recrystallized from ethanol. Yield: 1.35 g (70%) of yel-
low solid, mp 107–108 °C. 1H NMR (500 MHz, CDCl3): d = 7.70–
7.73 (m, 2H), 7.51–7.54 (m, 5H), 7.44–7.48 (m, 2H), 7.26–7.29
(m, 4H), 7.18–7.22 (m, 2H), 2.38 (s, 3H), 1.91 (s, 3H) ppm. 13C
NMR (125 MHz, CDCl3): d = 193.4, 183.2, 148.1 (br), 134.8, 132.3,
131.5, 129.7, 128.5, 127.4, 126.6, 107.3, 24.0, 14.6 ppm. 11B NMR
(160.5 MHz, CDCl3) d = 7.56 ppm. Found C, 81.21; H, 6.27.
Crystallographic data for 5b:
orthorhombic, Pbca, a = 10.7690(14) Å,
c = 25.563(3) Å, b = 90°, Z = 8, V = 3923.7(8) Å3, Dc = 1.196 g.cmꢀ3
= 0.071 mmꢀ1
Tmin/Tmax = 0.981/0.987;
C
24H24BNO, M = 353.25,
b = 14.2530(12) Å,
,
l
,
ꢀ13 6 h 6 11,
ꢀ18 6 k 6 18, ꢀ33 6 l 6 30; 20639 reflections measured
(hmax = 27.50°), 4452 independent (Rint = 0.0438), 3202 with
I > 2r(I), 244 parameters, S = 1.092, R1(obs. data) = 0.0579, wR2
(all data) = 0.1189; max., min. residual electron density = 0.250,
ꢀ0.257 eÅꢀ3
.
5.10. 3-Methyl-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2-k4]
oxazaborino[3,4-a]azepine (5c)
C23H21BO2 requires C, 81.20; H, 6.22.
Method B, gradual addition of BPh3 under cooling (5 °C), reac-
tion time 24 h, purification by column chromatography (silica
gel, DCM). Yield 40% of white solid, mp 210.5–211.5 °C. 1H NMR
(400 MHz, CDCl3): d = 7.38–7.36 (m, 4H), 7.28–7.24 (m, 4H),
7.23–7.19 (m, 2H), 5.11 (s, 1H, @CH), 3.44–3.41 (m, 2H), 2.50–
2.47 (m, 2H), 2.00 (s, 3H, CH3), 1.68–1.63 (m, 4H), 1.22–1.18 (m,
2H). 13C NMR (100 MHz, CDCl3): d = 176.7, 173.8, 149.4 (br),
133.1, 127.1, 126.3, 98.6 (@CH), 50.3, 35.7, 30.1, 26.8, 24.0,
23.2 ppm. 11B NMR (117.5 MHz, CDCl3): d = 4.75 ppm. Found C,
79.56; H, 7.63; N, 4.31. C21H24BNO requires C, 79.51; H, 7.63; N,
4.42.
5.7. Synthesis of oxazaborines 5
5.7.1. Method A
The flask fitted with a calcium-chloride drying tube was
charged with enaminone 3 in anhydrous DCM (8 mL/mmol).
Afterwards diphenylborinic acid (2 eq.) was added gradually under
stream of argon. The reaction mixture was stirred for 5 days at
laboratory temperature and then concentrated under reduced
pressure. The residue was purified.
5.7.2. Method B
The flask fitted with a calcium-chloride drying tube was
charged with enaminone 3 in anhydrous DCM (8 mL/mmol). After-
wards BPh3 (1.1–2 eq.) was added gradually under stream of argon
during 30 min. The reaction mixture was stirred 1–5 days at labo-
ratory temperature and then concentrated under reduced pressure.
The residue was purified.
Acknowledgements
The authors H.K. and J.L. are grateful to the institutional support
RVO 61388955.
H.K. is further indebted to the Ministry of Education of the
Czech Republic (grant LD14129).
5.8. 3,4,5,6-Tetramethyl-2,2-diphenyl-[1,3,2-k4]oxazaborine (5a)
Appendix A. Supplementary data
Prepared using method B, 1.1 eq. BPh3, reaction time 3 days,
purification by a column chromatography (silica gel/DCM). Yield
52%, mp 93.5–94 °C. 1H NMR (400 MHz, CDCl3): d = 7.32–7.30 (m,
4H), 7.27–7.18 (m, 6H), 2.92 (s, 3H), 2.13 (s, 3H), 2.04 (s, 3H),
1.75 (s, 3H); 13C NMR (100 MHz, CDCl3): d = 171.7, 169.2, 133.1,
127.2, 126.2, 102.9, 38.3, 21.5, 17.3, 14.1 ppm. 11B NMR
(160.5 MHz, CDCl3) d = 3.54 ppm. Found C, 78.29; H, 7.53; N,
4.78. C19H22BNO requires C, 78.37; H, 7.62; N, 4.81.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2016.08.009.
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