New triazaborine chromophores: Their synthesis via oxazaborines and electrochemical and DFT study of their fundamental properties

Article abstract of DOI:10.1021/om500219g

Eight new and stable triazaborine chromophores featuring various substituents as donor and acceptor moieties were prepared and investigated. An interpretation of the measured electrochemical data (CV, RDV, and dc polarography) and UV - vis spectra and quantum chemical calculations are presented. In the homologous series the first reduction proceeds as a one-electron reversible process localized at the - N=C - C=N - part of the central heterocycle being in conjugation with the attached carbonyl. The first oxidation of triazaborines proceeds as a two-electron irreversible process, most probably of the ECE type, localized at the negatively charged boron atom and surrounding unsaturated structures, including the substituted phenyl ring. For a better understanding of the relationship between the structure and redox properties, the LFER approach was applied for the first oxidation as well as reduction potential using the Hammett σ (para) substituent constants. The energies of the longest-wavelength absorption bands taken from UV-vis spectra were compared with the experimentally found differences E_ox - E_red and with calculated HOMO-LUMO gaps, and no systematic influence of substitution was found. The calculated optimized structures and displacement of the frontier orbitals confirmed the interpretations presented above. (Figure Presented)

Full text of DOI:10.1021/om500219g

Article
pubs.acs.org/Organometallics
New Triazaborine Chromophores: Their Synthesis via Oxazaborines
and Electrochemical and DFT Study of Their Fundamental Properties
†
,‡
†
†
§
̌
Frantisek Josefík, Tomas Mikysek,* Marketa Svobodova, Petr Simunek, Hana Kvapilova,
̌
́
̌
́
́
̊
́
and Jirí Ludvík§
̌
†Institute of Organic Chemistry and Technology, Faculty of Chemical Technology and ‡Department of Analytical Chemistry, Faculty
of Chemical Technology University of Pardubice, Studentska
́
573, CZ-53210 Pardubice, Czech Republic
§J. Heyrovsky Institute of Physical Chemistry ASCR, v.v.i., Dolejsk
̌
ova 3, CZ-182 23 Prague 8, Czech Republic
́
S
* Supporting Information
ABSTRACT: Eight new and stable triazaborine chromophores featuring various
substituents as donor and acceptor moieties were prepared and investigated. An
interpretation of the measured electrochemical data (CV, RDV, and dc polarography)
and UV−vis spectra and quantum chemical calculations are presented. In the homologous
series the first reduction proceeds as a one-electron reversible process localized at the
−NC−CN− part of the central heterocycle being in conjugation with the attached
carbonyl. The first oxidation of triazaborines proceeds as a two-electron irreversible
process, most probably of the ECE type, localized at the negatively charged boron atom
and surrounding unsaturated structures, including the substituted phenyl ring. For a better
understanding of the relationship between the structure and redox properties, the LFER approach was applied for the first
oxidation as well as reduction potential using the Hammett σ (para) substituent constants. 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 no systematic influence of substitution was found. The calculated optimized
structures and displacement of the frontier orbitals confirmed the interpretations presented above.
Some of them are applicable to the construction of organic
light-emitting devices (OLED) and organic thin-film transistors
(OTFT).12−14 The research on dipyrromethane boron
derivatives (BODIPY) which exhibit excellent thermal,
chemical, and photochemical stability and high quantum yields
of fluorescecence9,15−17 also played an important role. Another
wide group of boron heterocycle structures consists of the
scorpionates, which showed suitable properties for OLED.18 In
addition to optoelectronics some of the boron heterocycles
were also tested for their biological activity. Notably, the
derivatives of diazaborine exhibited antibacterial and anticancer
activity.3,7,19,20
INTRODUCTION
■
In recent years, part of materials chemistry has been devoted to
the development of new organic compounds for modern
technologies.1 In general, charge-transfer chromophores that
exhibit solid-state fluorescence, good optical transparency,
solubility, thermal stability, or biological activity are currently
in demand.2,3
The history of boron-containing heterocyclic compounds
started in 1958, when Dewar reported on 9-aza-10-
boraphenanthrene and related derivatives.4,5 Such compounds
exhibited high chemical stability in warm acidic and alkaline
media. Later on, boron derivatives of purine and quinazoline
were reported,6 initially targeted to their use in boron-neutron
capture therapy (BNCT).7 At the beginning of new millennium
various types of boron heterocycles with common features
started to appear; boron was coordinated between two
heteroatoms, mainly nitrogen and oxygen, and moreover
substituted by two phenyl groups or two fluorine atoms.
Such an arrangement is often called “super-acceptor” thanks to
the deficit of electrons at the nitrogen atom due to boron
coordination and the presence of electron-acceptor groups at
the boron atom. An increase of publications was also registered
after the year 2000, when modern technologies dealing with
boron-based materials started to play a significant role. Papers
related to boron heterocyclic compounds were more focused
on descriptions of their physicochemical and chemical
properties, and they were tested for photoluminescence,
electroluminescence, and nonlinear optical properties.8−11
This paper continues previously published work21−24 which
relates to triazaborine and oxazaborine compounds. The
present contribution deals with synthesis, electrochemical
characterization of redox properties, and HOMO−LUMO
characterization of a new series of triazaborine chromophores
recently synthesized via oxazaborines (Scheme 1). The
experimental data and their interpretation can be further
employed in the design and development of new structures of
organic boron compounds and in “tuning” of their properties.
Special Issue: Organometallic Electrochemistry
Received: March 3, 2014
© XXXX American Chemical Society
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Scheme 1. Triazaborines Prepared and Studied in This Work
Scheme 3. Synthesis of Oxazaborines 3
RESULTS AND DISCUSSION
■
Synthesis. The starting β-enaminone 1 (1-azepan-2-
ylideneacetone) was prepared by a two-step synthesis, starting
from acetylacetone and 7-methoxy-3,4,5,6-tetrahydro-2H-aze-
pine. Acetylacetone reacts with the 7-methoxy-3,4,5,6-tetrahy-
dro-2H-azepine in the presence of a catalytic amount of
Ni(acac)2 at 100 °C to form 3-azepan-2-ylidenepentane-2,4-
dione,25 which undergoes deacetylation by sodium methanolate
in methanol26 to form 1-azepan-2-ylideneacetone (1) (Scheme
2).
Scheme 4. Thermal Rearrangement of Oxazaborines 3 to
Triazaborines 4
Two strategies (A and B) were used for the synthesis of
oxazaborines 3 (Scheme 3). Procedure A is suitable for the
oxazaborines having X as electron donating or slightly electron
withdrawing (halogen) groups.24 The method, however, cannot
be used for the synthesis of oxazaborines bearing electron-
withdrawing groups due to the instability of the corresponding
diazonium tetraphenylborates. In this case, the two-step
reaction (B) has to be applied (Scheme 3). The approach
consists of the preparation of the intermediary azo compound
2, which can be transformed to oxazaborine 3 via its reaction
with triphenylborane.21 This trick allowed us to prepare
oxazaborines having X = COOEt, CN, NO2 in acceptable
yields of 50−68%. The mechanism of the formation of
oxazaborines is suggested in ref 21. Oxazaborines 3 were
then subjected to thermal rearrangement to the corresponding
triazaborines 4 (Scheme 4). The rearrangement is discussed in
more detail in ref 21.
Electrochemistry. The fundamental electrochemical char-
acterization of oxidation and reduction abilities of the newly
synthesized triazaborine molecules 4 were performed in dried
dimethylformamide (DMF).27 To discern the possible
influence of electrode material on the redox processes, platinum
and mercury electrodes were used and the data were compared.
To identify reversible and irreversible processes, “steady-state”
techniques such as rotating disk voltammetry (RDV) and dc
polarography (dc-P) as well as a “transient” method (cyclic
voltammetry, CV) were applied. Four types of electrodes were
used: Pt stationary electrode and hanging mercury drop
electrode (HMDE) for CV, Pt rotating disk electrode (Pt-
RDE) for RDV, and dropping mercury electrode (DME) for
dc-P. To distinguish diffusion-controlled processes from
adsorption phenomena, dependences of limiting or peak
currents on concentration, scan rate (in CV), and rotation
speed (in RDV) were determined and evaluated.
As a result, electrochemical data from dc-P, RDV, and CV on
Pt as well as on HMDE for each respective compound are
consistent: that is, the material of the electrode plays no
significant role and the first reduction and oxidation processes
are transport-controlled. The complete electrochemical data are
summarized in Table 1. The main attention was given to the
first oxidation and reduction potentials and their reversibility,
since their values can be related to HOMO and LUMO
energies; their difference reflects the HOMO−LUMO gap and
influences the low-energy band of the UV−vis spectra.
For an understanding of the structure−redox properties
relationship and for confirmation/exclusion of the analogous
redox mechanisms, the electronic influence of individual
substituents on the reduction and oxidation potential was
followed. For this reason the LFER (linear free energy
relationship) approach of the Hammett type was applied28
for the first oxidation and reduction potentials with utilization
of the σ (para) substituent constants (Table 1 and Figure 1).
Reduction. The first reduction of all compounds 4a−h
proceeds at potentials of −1.0 to −1.55 V (vs SCE) as a one-
electron reversible process. The cathodic/anodic peak separa-
tion 60−75 mV, the cathodic/anodic peak current ratio close to
unity, and the independence of the peak potential on scan rate
(in CV) as well as the half-wave potentials on rotation speed
(in RDV) confirm this interpretation.
Scheme 2. Synthesis of the Starting β-Enaminone 1
B
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Table 1. Electrochemical Data of Studied Compounds
g
g
oxidation
reduction
a
c
d
c
e
compd
Epa(CV), V
E1/2(RDV), V
E°(CV), V
ΔEp, mV
E°(HMDE), V
ΔEp, mV
E1/2(RDV), V
E1/2(dc-P), V
σp
b
4a
4b
4c
4d
4e
4f
0.64
0.65
1.20
1.33
1.41
1.445
f
−1.53
−1.47
−1.435
−1.39
−1.325
−1.23
−1.165
−1.00
75
65
75
65
70
90
65
−1.52
−1.47
−1.44
−1.39
−1.33
−1.23
−1.16
65
60
70
75
80
90
75
60
−1.52
−1.47
−1.44
−1.39
−1.32
−1.022
−1.17
−1.00
−1.53
−1.47
−1.43
−1.39
−1.32
−1.20
−1.17
−1.00
−0.63
−0.28
−0.13
0
1.25
1.42
1.51
1.54
f
0.23
0.43
0.71
4g
4h
f
1.62
1.65
f
b
60
−0.99
0.81
g
a
c
d
e
f
Anodic peak. E° of the reversible couple ((Epa + Epc)/2). (Epa + Epc)/2. Epa − Epc Hammett σ (para) constant. Nonevaluable shoulder. Vs
SCE.
Figure 1. Dependence of the first reduction (a) and oxidation potentials (b) of compounds 4a−h on the respective Hammett σ (para) substituent
constants.
From the Ered − σp plot (Figure 1a) it is evident that the
reduction potentials of 4a−g follow well a straight line with the
slope 0.29 V/σp unit; this means that all of these compounds
are reduced according to an analogous mechanism, the
reduction centers in the molecules are the same, and the
localizations of the LUMO should be similar.
However, the only exception is compound 4h, which is
reduced at a much less negative potential; thus, the data for 4h
do not fit this graph. It is evident that the nitro group is reduced
more easily than the rest of the molecule; the nitro group itself
represents the reduction center, and the reduction mechanism
is therefore completely different.
oxidation of the diethylaniline moiety preceding the oxidation
of the core. This type of reversible oxidation was observed for
example in the case of bis(dimethylanilino)imidazoles.29
In all cases, the oxidation behaviors (on the cyclic
voltammogram) are identical when the potential is first
scanned to positive potentials and when the reduction proceeds
first. The same situation occurs with reductions. This means
that the oxidation and reduction occur independently at
different parts of the triazaborine molecules.
In addition to this, a higher slope (reaction constant,
Hammett ρ value) is observed for oxidation than for reduction.
The substitution thus influences the oxidation center more
strongly: in other words, the oxidation center should be closer
to the substituent than the reduction center and/or some kind
of delocalized system must interconnect the oxidation center
and the substituent.
Quantum Chemical Calculations. In order to correlate
the experimental electrochemical data with the theory, quantum
chemical calculations of HOMO and LUMO energies were
performed. The results are collected in Table 2. Since the
experimental values of the first reduction potential should
correlate with the LUMO energy and the first oxidation
potential should correlate with the HOMO energy, the
calculated values were plotted versus the experimental data
(Figure 4). The good correlation confirms the credibility and
reliability of the calculations. (This is especially important in
the anomalous cases of compounds 4a,h.)
A typical electrochemical behavior, analogous for compounds
4b−g, is presented in Figure 2, and the CV and RDV curves of
4a are shown in Figure 3.
Oxidation. From a comparison of the limiting oxidation
current with the one-electron-reduction current of the same
compound recorded at the Pt-RDE (Figure 2b), it follows that
the first oxidation of triazaborines 4b−h proceeds as a two-
electron irreversible process, most probably of the ECE type.
The LFER treatment revealed that the first oxidation
potentials follow a linear dependence on σp (Figure 1b),
pointing to the fact that the oxidation of 4b−e,g,h proceeds
analogously at the same oxidation center and therefore a similar
displacement of HOMO can be expected. (In the case of 4f,
only a nonevaluable anodic shoulder in CV and RDV was
obtained in the same potential region.)
The anomalous case not fitting the Hammett plot is the
diethylamino derivative 4a, where a reversible one-electron
oxidation proceeds at much lower potentials (Figure 3). This
oxidation was attributed to the one-electron reversible
UV−Vis Spectrometry. Optical properties of the target
compounds were investigated by the absorption electron
spectra measured in acetonitrile in the UV−vis region (Figure
5). The wavelengths of absorption maxima and their energies in
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Figure 2. Electrochemical behavior of 4b: (a) cyclic voltammetry, scan rate v = 100 mV/s; (b) Pt-RDV, rotation frequencies f = 500 and 1500 min−1;
(c) dc polarography, concentrations 0.5 and 1 mM.
Figure 3. Electrochemical behavior of 4a: (a) cyclic voltammetry, scan rate v = 100 mV/s; (b) Pt-RDV, rotation frequencies f = 500 and 1500 min−1.
Table 2. Quantum Chemical Calculations and Optical and Electrochemical Data of Compounds 4a−h
calcd values
LUMO, eV
exptl data
a
compd
HOMO, eV
HOMO−LUMO gap, eV
Eox − Ered
,
V
λmax, nm (eV)
4a
4b
4c
4d
4e
4f
−5.21
−5.80
−5.96
−6.05
−6.08
−6.16
−6.22
−6.29
−2.26
−2.36
−2.41
−2.44
−2.52
−2.63
−2.71
−3.02
2.95
3.44
3.55
3.61
3.56
3.53
3.51
3.27
2.17
2.67
2.77
2.80
2.77
-
470 (2.64)
425 (2.92)
420 (2.95)
417 (2.97)
420 (2.95)
422 (2.94)
427 (2.90)
436 (2.84)
4g
4h
2.79
2.65
a
E1/2(ox1) − E1/2(red1) based on the data from Table 1.
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Figure 4. Correlations between (a) calculated HOMO energy and first oxidation potential and (b) calculated LUMO energy and first reduction
potential.
Figure 6. Influence of the substitution of compounds 4b−g on the
▲
calculated HOMO−LUMO gap ( ), energy of the longest-wave-
■
length band in the UV−vis spectrum ( ), and experimental difference
●
Eox − Ered ( ). The respective data for compounds 4a,h are
Figure 5. UV−vis absorption spectra of compounds 4a (1), 4d (2),
and 4h (3) measured in acetonitrile. The spectra of 4b,c,e−g are very
similar to that of 4d.
□
.
○
,
△
, and
represented by the open symbols
has been studied previously.24 No fluorescence in the solution
state at room temperature has been observed.
The calculated optimized structures and displacement of the
frontier orbitals shown in Figure 7 confirmed the interpreta-
tions presented above. (In addition to the anomalous
compounds 4a,h, the localization of the HOMO and LUMO
of 4d is presented for comparison as a typical case.)
It follows from the HOMO and LUMO “maps” that the
oxidation center in the 4a is localized prevalently at the
diethylaniline moiety (Figure 7a), whereas the oxidation center
in all other derivatives involves the negatively charged boron
atom and surrounding unsaturated structures including the
substituted phenyl ring (Figure 7c,e). The reduction center in
4h is localized at the nitro group and the π-electron system is
extended to the phenyl ring under formation of a quinone-like
system (Figure 7f). The other derivatives, however, have their
reduction center localized at the −NC−CN− part of the
central heterocycle being in conjugation with the attached
carbonyl (acetyl) (Figure 7b,d).
The reduction center is distant and separate from the
substituents X; therefore, their influence on the reduction
potential is low. On the other hand, the oxidation center is
closer to the substitution and is interconnected through the
phenyl ring. This explains the higher Hammett ρ value for
reductions.
eV are summarized in Table 2. The intense longest-wavelength
absorption bands with λmax between 417 and 470 nm dominate
in the spectra. The most bathochromically shifted band was
obtained for compound 4a (λmax 470 nm). It reflects the
presence of the strong N,N-diethylamino donor which is
engaged more efficiently in D−A (donor−acceptor) inter-
actions than the other substituents. The energies of the longest-
wavelength absorption bands were compared with the
calculated HOMO−LUMO gaps and with the experimentally
acquired difference between the first oxidation and reduction
potentials, respectively (Table 2). The plot of the three
aforementioned energies versus the Hammett σ constants
(Figure 6) shows two results. (a) In the series 4b−g, the
substitution has no systematic influence on the λmax, HOMO−
LUMO gap and Eox − Ered values (solid symbols in Figure 6).
On the other hand, compounds 4a,h having a different
oxidation (diethylaniline) or reduction center (nitro group),
exhibit different energies and spectra (open symbols in Figure
6). (b) The energies of the λmax, HOMO−LUMO gap, and Eox
− Ered values correlate mutually very well, differing only in
additive constants.
Structural Aspects. X-ray diffraction data of some relevant
oxazaborines and triazaborines have been reported previously.24
In addition to this, all of the prepared triazaborines fluoresce in
the solid state upon illumination with a UV lamp (254/366
nm). The fluorescence behavior of some compounds (4b,c,e)
in both the solid state and frozen 2-Me THF solution (at 77 K)
It is noteworthy that the two phenyl rings attached at the
boron atom are not involved in any π system due to their nearly
perpendicular position toward the rest of the molecule.
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Figure 7. Calculated distribution of the HOMO and LUMO in molecules 4a,d,h.
series 4b−g no systematic influence of the substitution exists
and the energies of the λmax, HOMO−LUMO gap, and Eox
red values correlate mutually very well. On the other hand, the
compounds 4a,h, having a different oxidation (diethylaniline)
or reduction center (nitro group), exhibit bathochromically
shifted spectra and lower HOMO−LUMO gaps.
The calculated optimized structures and displacement of the
frontier orbitals confirmed the interpretations presented above
that the oxidation center of 4a is localized prevalently at the
diethylaniline moiety, whereas the reduction center of 4h is
localized at the nitro group. The oxidation center in all other
derivatives involves the negatively charged boron atom and
surrounding unsaturated structures, including the substituted
phenyl ring; the reduction center is localized at the −NC−
CN− part of the central heterocycle being in conjugation
with the attached carbonyl. All aforementioned derivatives of
triazaborine exhibit solid-state fluorescence, which points to
their potential use in optoelectronics.
CONCLUSIONS
■
−
A new series of substituted triazaborine chromophores was
synthesized via oxazaborines and their thermal rearrangement.
Their oxidation and reduction abilities were investigated
electrochemically using rotating-disk voltammetry, cyclic
voltammetry, and dc polarography. The electrode material
plays no role, and all processes are transport-controlled. The
main attention was given to the reversibility of the process and
to the first oxidation and reduction potentials, since their values
can be related to HOMO and LUMO energies.
For an understanding of the relationship between the
structure and redox properties, the electronic influence of
individual substituents on the reduction and oxidation
potentials was followed. For this reason the LFER approach
was applied to the first oxidation as well as the reduction
potential using the Hammett σ (para) substituent constants.
The first reduction of all compounds 4a−h proceeds at
potentials of −1.0 to −1.55 V (vs SCE) as a one-electron
reversible process. It was found from the Hammett plot that the
reduction mechanism is analogous for all studied compounds,
except for the nitro derivative, where the nitro group itself is
reduced at less negative potentials.
The first oxidation of triazaborines 4b−h proceeds as a two-
electron irreversible process, most probably of the ECE type.
The only anomalous case is for the diethylamine derivative,
where the one-electron reversible oxidation of the diethylaniline
moiety precedes the oxidation of the core.
Using the DFT method, the energies of the HOMO and
LUMO were calculated and compared with experimental values
of oxidation and reduction potentials, respectively. The good
correlation confirms the credibility and reliability of the
calculations.
E
EXPERIMENTAL SECTION
■
NMR spectra were measured in CDCl3 using a Bruker AVANCE III
spectrometer operating at 400.13 (1H) and 100.62 MHz (13C). All of
the pulse sequences were taken from the Bruker software library.
Proton spectra measured were calibrated on the basis of internal TMS
(δ 0.00). Carbon NMR spectra were calibrated on the middle signal of
the solvent multiplet (δ 77.16). Elemental analyses were performed on
a Flash 2000 CHNS Elemental Analyzer. Melting points were
measured on a Kofler Boetius PHMK 80/2644 hot-stage microscope
and were not corrected. MALDI HRMS were measured using a
MALDI LTQ Orbitrap XL instrument (Thermo Fisher Scientific) with
DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methylprop-2-en-1-ylidene]-
malononitrile) as the matrix dissolved in acetonitrile.
UV−vis spectra were recorded on a Shimadzu UV-2450
spectrophotometer in acetonitrile. All solvents and reagents were
purchased from commercial suppliers and were used without further
purification. Diazonium tetrafluoroborates were prepared according to
an ordinary procedure (diazotization using cold NaNO2/HCl with
subsequent precipitation, from the filtered solution of the diazonium
chloride, by aqueous sodium tetrafluoroborate). Precipitated diazo-
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. The plot of the three aforementioned
energies versus the Hammett σ constants shows that in the
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nium tetrafluoroborates were dried in vacuo in a desiccator and stored
in a refrigerator. Diazonium tetraphenylborates were prepared
immediately before use according to the literature procedure.21
Caution! Diazonium tetraphenylborates in the crystalline state can
undergo explosive decomposition even with very slight mechanical
stimulation! Enaminone 1 was prepared according to the procedures
reported in ref 24.
23.1. Anal. Calcd for C27H27BBrN3O (500.24): C, 64.83; H, 5.44; N,
8.40. Found: C, 64.93; H, 5.47; N, 8.19.
4-(4-Ethoxycarbonylphenyldiazenyl)-3-methyl-1,1-diphenyl-
6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3f).
Yield: 50%. Mp: 158−160 °C. 1H NMR (400 MHz, CDCl3): δ
8.10−8.06 (m, 2H), 7.66−7.62 (m, 2H), 7.40−7.20 (m, 10H), 4.42−
4.34 (m, 2H), 3.69−3.57 (m, 2H), 3.30−3.26 (m, 2H), 2.60 (s, 2H),
1.80−1.70 (m, 4H), 1.43−1.30 (m, 5H). 13C NMR (101 MHz,
CDCl3): δ 180.5, 173.6, 166.4, 155.8, 133.2, 131.9, 130.9 130.6, 130.3,
127.4, 126.8, 121.5, 61.2, 51.4, 29.9, 29.7, 26.0, 23.4, 23.3, 14.5. Anal.
Calcd for C30H32BN3O3 (493.40): C, 73.03; H, 6.54; N, 8.52. Found:
C, 72.81; H, 6.59; N, 8.29.
General Procedures for the Preparation of Oxazaborines 3.
Method A. This methodology was adopted from ref 24.
Method B. To a cold (5 °C) mixture of β-enaminone 1 (5 mmol)
and sodium acetate (5 mmol) in dry dichloromethane (15 mL) was
added benzenediazonium tetrafluoroborate (5 mmol). The reaction
mixture was stirred at room temperature for 24 h. Sodium acetate was
removed by suction, and to the crude reaction mixture was added
BPh3 (10 mmol). The solution was stirred for 24 h at room
temperature. The solvent was evaporated in vacuo. The crude residue
was chromatographed on silica gel with dichloromethane as the mobile
phase to give the corresponding oxazaborines 3.
4-(4-Cyanophenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tetra-
hydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3g). Yield: 69%. Mp:
1
213−215 °C. H NMR (400 MHz, CDCl3): δ 7.72−7.61 (m, 4H),
7.38−7.33 (m, 4H), 7.33−7.20 (m, 6H), 3.71−3.58 (m, 2H), 3.34−
3.22 (m, 2H), 2.61 (s, 3H), 1.80−1.70 (m, 4H), 1.42−1.32 (m, 2H).
13C NMR (101 MHz, CDCl3): δ 181.5, 173.6, 155.3, 134.9, 133.2,
Compound 3h was too unstable to be characterized properly. It
decomposed upon an attempt to purify it; hence, the crude reaction
mixture was subjected to thermal rearrangement (vide infra) and the
corresponding triazaborine 4h was then purified and identified.
The following compounds were prepared according to the
aforementioned procedure.
4-(4-Diethylaminophenyldiazenyl)-3-methyl-1,1-diphenyl-
6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3a).
Yield: 26%. Mp: 193−195 °C. 1H NMR (400 MHz, CDCl3): δ
7.61−7.52 (m, 2H), 7.40−7.34 (m, 4H), 7.29−7.17 (m, 6H), 6.70−
6.60 (m, 2H), 3.63−3.53 (m, 2H), 3.45−3.31 (m, 4H), 3.22−3.16 (m,
2H), 2.48 (s, 3H), 1.80−1.65 (m, 4H), 1.40−1.30 (m, 2H), 1.23−1.12
(m, 6H). 13C NMR (101 MHz, CDCl3): δ 175.8, 173.3, 148.8, 143.4,
134.8, 133.3, 131.8, 127.2, 126.4, 123.6, 111.2, 51.1, 44.7, 29.8, 29.8,
26.3, 23.7, 22.3, 12.8. Anal. Calcd for C31H37BN4O (492.46): C, 75.61;
H, 7.57; N, 11.38. Found: C, 75.42; H, 7.51; N 11.20.
131.9, 128.1, 127.4, 126.9, 122.2, 119.0, 111.6, 51.5, 30.0, 29.6, 26.0,
23.5, 23.3. Anal. Calcd for C28H27BN4O (446.35): C, 75.34; H, 6.10;
N, 12.55. Found: C, 75.27; H, 6.18; N, 12.29.
3-Methyl-4-(4-nitrophenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetrahy-
dro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3h). Yield: 61%. 1H
NMR (400 MHz, CDCl3): δ 8.29−8.27 (m, 2H), 7.71−7.69 (m, 2H),
7.39−7.33 (m, 4H), 7.33−7.22 (m, 6H), 3.70−3.61 (m, 2H), 3.36−
3.26 (m, 2H), 2.63 (s, 3H), 1.80−1.74 (m, 4H), 1.40−1.35 (m, 2H).
HRMS (MALDI): calcd for C27H27BN4O3 [M + H]+ 467.22490 Da;
found [M + H]+ 467.22547 Da.
General Procedure for the Preparation of Triazaborines 4.24
Oxazaborines 3 (2 mmol) were heated at reflux in DMF (10 mL) for
the appropriate time (shown for the particular compound). The
reaction was monitored by TLC. The solvent was evaporated in vacuo,
and the crude reaction mixture was chromatographed on silica gel with
dichloromethane as the mobile phase to give the corresponding
triazaborines. The following compounds were prepared using the
aforementioned procedure.
4-Acetyl-2-(4-diethylaminophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexa-
hydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4a). Yield: 1% (10
h). Mp: 194−196 °C. 1H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m,
4H), 7.25−7.17 (m, 6H), 7.14−7.09 (m, 2H), 6.38−6.28 (m, 2H),
3.53−3.46 (m, 2H), 3.28−3.20 (m, 6H), 2.52 (s, 3H), 1.65−1.55 (m,
4H), 1.28−1.23 (m, 2H), 1.13−1.02 (m, 6H). 13C NMR (101 MHz,
CDCl3): δ 196.4, 164.3, 146.3, 137.6, 134.1, 134.1, 133.1, 127.5, 126.5,
124.5, 110.3, 51.5, 44.4, 29.5, 29.5, 27.5, 27.0, 23.1, 12.7. HRMS
(MALDI): calcd for C31H37BN4O [M]+ 492.30549 Da, [M + Na]+
515.29526 Da; found [M]+ 492.30507 Da, [M + Na]+ 515.29486 Da.
4-Acetyl-2-(4-methoxyphenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahy-
dro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4b). Yield: 6% (20
h). Mp: 143−145 °C. 1H NMR (400 MHz, CDCl3): δ 7.42−7.38 (m,
4H), 7.26−7.16 (m, 8H), 6.59−6.52 (m, 2H), 3.66 (s, 3H), 3.56−3.50
(m, 2H), 3.25−3.19 (m, 2H), 2.52 (s, 3H), 1.66−1.59 (m, 4H), 1.31−
1.22 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 164.8, 157.9,
145.5, 141.8, 134.0, 133.8, 127.6, 126.7, 124.4, 113.0, 55.4, 51.8, 29.8,
29.4, 27.4, 26.9, 22.8. Anal. Calcd for C28H30BN3O2 (451.37): C,
74.51; H, 6.70; N, 9.31. Found: C, 74.30; H, 6.69; N, 9.10.
4-(4-Methoxyphenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tet-
rahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3b). Yield: 42%.
1
Mp: 160−162 °C. H NMR (400 MHz, CDCl3): δ 7.65−7.57 (m,
2H), 7.40−7.35 (m, 4H), 7.30−7.20 (m, 6H), 6.96−6.89 (m, 2H),
3.83 (s, 3H), 3.64−3.57 (m, 2H), 3.28−3.18 (m, 2H), 2.53 (s, 3H),
1.80−1.70 (m, 4H), 1.41−1.32 (m, 2H). 13C NMR (101 MHz,
CDCl3) δ 177.8, 173.4, 160.6, 147.5, 147.3, 133.3, 131.6, 127.2, 126.6,
123.2, 114.2, 55.6, 51.2, 29.8, 29.8, 26.2, 23.6, 22.7. Anal. Calcd for
C28H30BN3O2 (451.37): C, 74.51; H, 6.70; N, 9.31. Found: C, 74.79;
H, 6.76; N, 9.02.
3-Methyl-4-(4-methylphenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetra-
hydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3c). Yield: 29%. Mp:
1
135−137 °C. H NMR (400 MHz, CDCl3): δ 7.56−7.51 (m, 2H),
7.39−7.33 (m, 4H), 7.30−7.17 (m, 8H), 3.64−3.54 (m, 2H), 3.26−
3.20 (m, 2H), 2.55 (s, 3H), 2.37 (s, 3H), 1.78−1.68 (m, 4H), 1.40−
1.30 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 178.4, 173.5, 151.1,
139.4, 134.8, 133.3, 131.6, 129.7, 127.3, 126.6, 121.7, 51.2, 29.9, 29.8,
26.2, 23.5, 22.8, 21.5. Anal. Calcd for C28H30BN3O (435.37): C, 77.24;
H, 6.95; N, 9.65. Found: C, 77.49; H, 7.03; N, 9.51.
3-Methyl-4-(phenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetrahydro-5H-
[1,3,2 λ4]oxazaborino[3,4-a]azepine (3d). Yield: 57%. Mp: 137−139
1
°C. H NMR (400 MHz, CDCl3): δ 7.66−7.61 (m, 2H), 7.42−7.35
4-Acetyl-2-(4-methylphenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro-
1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4c). Yield: 18% (9 h).
(m, 7H), 7.30−7.21 (m, 6H), 3.70−3.54 (m, 2H), 3.32−3.23 (m, 2H),
2.57 (s, 3H), 1.78−1.70 (m, 4H), 1.42−1.32 (m, 2H). 13C NMR (101
MHz, CDCl3) δ 179.0, 173.5, 153.1, 134.9, 133.3, 131.7, 129.0, 127.3,
127.1, 126.7, 121.8, 51.3, 29.9, 29.8, 26.1, 23.5, 22.9. HRMS
(MALDI): calcd for C27H28BN3O [M + H]+ 422.23982 Da, [M +
Na]+ 444.22177 Da; found [M + H]+ 422.24027 Da, [M + Na]+
444.22214 Da.
1
Mp: 141−143 °C. H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m,
4H), 7.25−7.17 (m, 6H), 7.14−7.10 (m, 2H), 6.87−6.82 (m, 2H),
3.58−3.50 (m, 2H), 3.24−3.19 (m, 2H), 2.52 (s, 3H), 2.19 (s, 3H),
1.65−1.58 (m, 4H), 1.31−1.24 (m, 2H). 13C NMR (101 MHz,
CDCl3): δ 196.4, 165.0, 145.8, 145.5, 136.0, 134.1, 128.4, 127.6, 126.7,
123.0, 51.9, 29.8, 29.5, 27.4, 26.9, 22.7, 21.0. Anal. Calcd for
C28H30BN3O (435.37): C, 77.24; H, 6.95; N, 9.65. Found: C, 77.41;
H, 7.02; N, 9.39.
4-(4-Bromophenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tetra-
hydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3e). Yield: 48%. Mp:
1
163−165 °C. H NMR (400 MHz, CDCl3): δ 7.55−7.47 (m, 4H),
4-Acetyl-2-phenyl-1,1-diphenyl-2,5,6,7,8,9-hexahydro-1H-[1,2,4,3
λ4]-triazaborino[4,5-d]azepine (4d). Yield: 7% (6 h). Mp: 159−161
7.38−7.34 (m, 4H), 7.30−7.20 (m, 6H), 3.68−3.52 (m, 2H), 3.30−
3.19 (m, 2H), 2.56 (s, 3H), 1.80−1.65 (m, 4H), 1.40−1.33 (m, 2H).
13C NMR (101 MHz, CDCl3): δ 179.6, 173.5, 151.9, 133.2, 132.2,
131.6, 128.1, 127.3, 126.7, 123.2, 122.9, 51.3, 29.9, 29.7, 26.1, 23.5,
1
°C. H NMR (400 MHz, CDCl3): δ 7.44−7.39 (m, 4H), 7.25−7.21
(m, 6H), 7.20−7.17 (m, 2H), 7.08−6.99 (m, 3H), 3.60−3.49 (m, 2H),
3.24−3.19 (m, 2H), 2.54 (s, 3H), 1.65−1.59 (m, 4H), 1.32−1.23 (m,
G
dx.doi.org/10.1021/om500219g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2H). 13C NMR (101 MHz, CDCl3): δ 196.4, 165.1, 148.0, 134.3,
134.1, 127.8, 127.6, 126.7, 126.1, 123.2, 52.0, 29.9, 29.4, 27.3, 26.8,
22.7. Anal. Calcd for C27H28BN3O (421.34): C, 76.97; H, 6.70; N,
9.97. Found: C, 77.10; H, 6.68; N, 9.79.
states. Maps of the frontier molecular orbitals were constructed using
GaussView software.
ASSOCIATED CONTENT
■
4-Acetyl-2-(4-bromophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro-
S
* Supporting Information
1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4e). Yield: 20% (14 h).
1
Figures giving H and 13C NMR spectra for 3a−h and 4a−h
1
Mp: 136−137 °C. H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m,
and an xyz file giving molecular coordinates for the calculated
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
4H), 7.27−7.18 (m, 6H), 7.17−7.09 (m, 4H), 3.58−3.51 (m, 2H),
3.22−3.16 (m, 2H), 2.53 (s, 3H), 1.65−1.59 (m, 4H), 1.33−1.22 (m,
2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 165.4, 147.0, 144.9,
134.4, 134.0, 130.9, 127.8, 126.9, 124.5, 119.6, 52.1, 30.0, 29.4, 27.3,
26.7, 22.5. Anal. Calcd for C27H27BBrN3O (500.24): C, 64.83; H, 5.44;
N, 8.40. Found: C, 65.03; H, 5.44; N, 8.12.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.M.: tomas.mikysek@upce.cz.
■
4-Acetyl-2-(4-ethoxycarbonylphenyl)-1,1-diphenyl-2,5,6,7,8,9-
hexahydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4f). Yield:
30% (4 h). Mp: 131−133 °C. 1H NMR (400 MHz, CDCl3): δ
7.77−7.70 (m, 2H), 7.45−7.40 (m, 4H), 7.33−7.28 (m, 2H), 7.27−
7.17 (m, 6H), 4.35−4.23 (m, 2H), 3.67−3.49 (m, 2H), 3.23−3.15 (m,
2H), 2.56 (s, 3H), 1.68−1.60 (m, 4H), 1.39−1.21 (m, 5H). 13C NMR
(101 MHz, CDCl3): δ 196.5, 166.3, 165.6, 151.3, 144.8, 134.8, 134.0,
129.4, 127.8, 127.3, 127.0, 122.5, 60.9, 52.3, 30.1, 29.4, 27.3, 26.7, 22.5,
14.4. Anal. Calcd for C30H32BN3O3 (493.40): C, 73.03; H, 6.54; N,
8.52. Found: C, 72.73; H, 6.45; N, 8.22.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank Prof. Vladimır Machac
■
́
́
e
̌
k for consultations
during synthesis. J.L. is grateful for institutional support from
RVO 61388955. This research was supported by the project
CZ.1.07/2.3.00/30.0021 (Ministry of Education, Youth and
Sports of the Czech Republic).
4-Acetyl-2-(4-cyanophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro-
1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4g). Yield: 50% (4 h).
1
Mp: 136−138 °C. H NMR (400 MHz, CDCl3): δ 7.45−7.40 (m,
4H), 7.34−7.29 (m, 4H), 7.27−7.20 (m, 6H), 3.62−3.51 (m, 2H),
3.20−3.15 (m, 2H), 2.56 (s, 3H), 1.66−1.61 (m, 4H), 1.30−1.22 (m,
2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 165.9, 151.0, 144.5,
135.1, 133.9, 131.9, 128.0, 127.2, 123.0, 119.0, 108.4, 52.5, 30.3, 29.4,
27.3, 26.6, 22.3. Anal. Calcd for C28H27BN4O: C, 75.34; H, 6.10; N,
12.55. Found: C, 75.60; H, 6.17; N, 12.30.
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1
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dx.doi.org/10.1021/om500219g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published on the Web on July 9, 2014, with an
error in the third paragraph from the end. The corrected
version was reposted on July 16, 2014.
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dx.doi.org/10.1021/om500219g | Organometallics XXXX, XXX, XXX−XXX