Four-coordinate organoboron compounds from β-enaminonitriles and diazonium salts

Article abstract of DOI:10.1016/j.tet.2011.12.082

β-Enaminonitriles react with substituted benzenediazonium tetraphenylborates to form 1,2,4,3λ⁴-triazaborines or 1,3,2λ⁴-oxazaborines. The formation of either the first or the second product is affected by the reaction conditions, esp

Full text of DOI:10.1016/j.tet.2011.12.082

Tetrahedron 68 (2012) 2052e2060
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Four-coordinate organoboron compounds from
salts
b-enaminonitriles and diazonium
,
a
a
b
ꢀ a
ꢂ
ꢀꢁ
ꢀ a
ꢁ ꢂꢁ ꢁ
ꢁ
ꢁ
*
ꢀ
Marketa Svobodova , Petr Simunek , Vladimír Machacek , Lucie Struncova , Ales Ruzicka
a
ꢀ
Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 532 10 Pardubice, Czech Republic
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 532 10 Pardubice, Czech Republic
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
l
4-tri-
Article history:
Received 7 October 2011
Received in revised form 20 December 2011
Accepted 29 December 2011
Available online 5 January 2012
b
-Enaminonitriles react with substituted benzenediazonium tetraphenylborates to form 1,2,4,3
azaborines or 1,3,2
l
4-oxazaborines. The formation of either the first or the second product is affected by
the reaction conditions, especially by the presence of water in the reaction components and in the
solvent. If the reaction is performed under anhydrous conditions, the major product is the triazaborine.
When ‘wet’ diazonium salts are used or a small amount of water is added into the reaction mixture,
oxazaborine is the product.
Dedicated to Professor Josef Panchartek on
occasion of his 80th birthday
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Triazaborines
Enaminonitriles
Diazonium tetraphenylborate
Boron heterocycles
1. Introduction
There are very few mentions of the reactions of b-enaminoni-
triles with diazonium ions as an electrophilic reagent in the liter-
ature. The products of such reactions are azo or hydrazo
compounds;8 only one work mentions the formation of a hetero-
cyclic compound.9
So-called polarized ethylenes, that is, compounds with their
double bond substituted with a strongly electron-withdrawing
group at the carbon C-1 and a strongly electron-donating group
at the carbon C-2, represent, thanks to their versatile reactivity,
important building blocks for the syntheses of a wide variety of
different heterocyclic systems of practical usability (pharmaceuti-
The aim of this work is to study the reaction of b-enaminoni-
triles with substituted benzenediazonium tetraphenylborates. In
this way it is possible to prepare new heterocyclic compounds,
oxazaborines and triazaborines with four-coordinate boron atoms
(an arrangement of the heteroatoms in the cycles NeB(Ph2)eO and
NeB(Ph2)eNeN).
Various kinds of heterocyclic compounds containing boron
atoms in their molecules are interesting in terms of their photo-
physical and electrochemical properties and biological activity,10 by
which they differ substantially from their carbocyclic analogues.
The properties of chiral compounds containing the (Ph2)B group
have also been studied.11 Four-coordinate boron compounds having
two phenyl groups adjacent to boron are interesting with regard
to their potential photoluminescent and electroluminescent
properties12 and are potentially available as materials for organic
light-emitting diodes (OLEDs). Some of them undergo reversible
photochromic switching13 or can show NLO properties.14
cals, fungicides and solvatochromic dyes1).
b-Enaminonitriles (3-
aminoalkenenitriles, Fig. 1), including 3-aminobut-2-enenitrile
(Fig. 1, R1¼Me, R2¼H) and its N-substituted derivatives, belong
among simple representatives of the polarized ethylenes. Enami-
nonitriles react with a number of reagents1,2 to form substituted
pyridines,3 dihydropyridines,3i,k,4 pyrazoles,5 quinolines4e,6 and
other heterocyclic compounds.5f,6,7
Fig. 1. The general structure of b-enaminonitriles.
In our previous works we described a new preparation of pol-
ysubstituted oxazaborines, diazaborines and triazaborines by the
reaction of diazonium tetraphenylborates with b-enaminones and
* Corresponding author. Tel.: þ420 46 603 7039; fax: þ420 46 603 7068; e-mail
ꢀ
address: marketa.svobodova@upce.cz (M. Svobodova).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.12.082

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2053
b
-enaminoamides.15 All the compounds prepared by means of the
then analogical as in the case of the enamino ketones17 and op-
posite to the products of the azo coupling to propanedinitrile,
methyl cyanoacetate,19 ethyl 3-oxobutanoate, pentane-2,4-dione,
or dimethyl propanedioate, which exist as pure hydrazo forms.20
Upon reaction of 3-aminobut-2-enenitrile with 4-methyl-
benzenediazonium tetraphenylborate (dried in air) in dry dichloro-
methane, a mixture of compounds is formed (according to TLC). 6-
Amino-4-methyl-5-(4-methylphenyldiazenyl)-2,2-diphenyl-3H-1,3,
method contain the structural motif e(Ph2)BꢀeNþe. This reaction,
described by us, enables both preparation of the compounds with
various combinations of substituents and modification of the sub-
stituents already present on them to be conducted to achieve the
desired properties.
2. Results and discussion
2l
4-oxazaborine (4a) was isolated from the mixture by means of
column chromatography, with a yield of 30% (after crystallization).
The compound was identified on the basis of 1H, 15N and 11B NMR
spectra.
One of the first mentions of the reactions of b-enaminonitriles
with diazonium salt is from 1964. The reaction was performed in
aqueous pyridine and, according to the authors, the products were
mixtures of azo/hydrazono tautomers.8d,e This assumption, how-
ever, has not been confirmed experimentally.
From the 1H NMR spectrum (Fig. 3) it is clear that the product
has three acidic hydrogens. All belong to NeH groups, which was
proved by reading the coupling constants 1J (15N, 1H) from 0.18% 15
N
According to the literature, the reactions of b-enaminonitriles
satellites in the proton spectrum as well as by measuring the 1D
1He15N gsHMBC NMR spectrum (Fig. 4). The values of these three
coupling constants are 1JNeH¼90.3, 81.5 and 94.4 Hz. The chemical
shifts of the nitrogen atoms are shown in Table 1. Positive values of
have been studied hitherto in protic environments only and under
such conditions that hydrolysis of either the starting nitriles or the
primarily formed products of the azo coupling took place to give 2-
arylhydrazono-3-oxobutanenitriles.8f,g 3-Amino-2-phenyldiazenyl-
3-phenyl-prop-2-enenitrile or its imino-phenylhydrazono tautomer,
d (
15N), 22.2 and 86.4, correspond to the presence of an azo
group.17a,21 The value of the 11B chemical shift corresponds to the
values obtained for six-membered heterocyclic compounds with an
NeBeO arrangement.15 The structure of the oxazaborine 4a was
subsequently confirmed by X-ray analysis. ORTEP plot is shown in
Figs. 5 and 6.
respectively, that is, the real product of the azo coupling to b-enam-
inonitrile, was prepared by another method: the reaction of phenyl-
magnesium bromide with 2-phenylhydrazonopropanedinitrile.16
The tautomeric form of the compound was not determined. By
means of the reaction of b-enaminonitriles with diazonium tetra-
fluoroborates17 or tetraphenylborates in dichloromethane in the
presence of remelted sodium acetate it is possible to prepare the
products of the azo coupling without the risk of hydrolysis of the
enamino group. We found that the products of the azo coupling
prepared in this way are a mixture of two isomers, which differ with
regard to the existence of an intramolecular hydrogen bonding.
The azo coupling product 5a exists in DMSO-d6 solution as
a mixture of two E/Z isomers (Fig. 2). The ratio of the isomers was
estimated on the basis of 1H NMR data and was found to be 1:0.37.
To distinguish between the isomers, selectively 15N enriched iso-
topomer was prepared and the stereochemical significance of
13Ce15N coupling constants was used.
Fig. 3. 500 MHz 1H NMR spectrum of 4a in CDCl3.
Fig. 2. Geometric isomers of the compound 5a.
The geminal coupling constant 2J (13C, 15N) for C3 is observable
only in the minor form, having a relatively large value, 9.5 Hz. It
corresponds to the syn arrangement of the nitrogen lone pair rel-
ative to the carbon C3, which evidences that the minor form is the E
isomer (Fig. 2; minor).18 The value of the coupling constant 3J (13C,
15N) is 2.8 Hz for the minor form, whereas no splitting was ob-
served for the major form. This is also in agreement with the minor
form being an E-isomer because the anti arrangement of C and N
with the orientation of the electron lone pair, as shown in Fig. 2 (5a,
major form), has a lower value of the coupling constant 3J (13C, 15N)
than any syn arrangement.18
15N Chemical shifts of both the forms correspond to the prac-
tically pure azo form. The position of the tautomeric equilibrium is
Fig. 4. 1D 1He15N gsHMBC NMR spectrum of the compound 4a in CDCl3.

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2054
Table 1
If the diazonium salt was dried for about 1 h in vacuo at room
temperature before being added to the reaction mixture, the major
product was triazaborine 3a (Scheme 1). The reaction was initially
performed in dichloromethane at room temperature for 72 h. The
yield was 48% after chromatography and crystallization. When the
reaction was carried out in dichloromethane/toluene mixture (3:5
v/v) under reflux, shortening of the reaction time to 3 h was ach-
ieved. The yield was comparable (Table 2).
The reaction leading to the triazaborines proceeds with enam-
inonitriles both with a primary amino group and with an N-methyl
group (Scheme 1). When the enaminone 1e with an N-phenyl
group was used, no heterocyclic compound was obtained but the
azo coupling product 5b only (Scheme 2).
We have suggested a possible mechanism of formation of the
oxazaborines (Scheme 3). We assumed that in situ formation of
diphenylborinic acid and its subsequent reaction with the azo
coupling product 5a at amino group nitrogen took place. To be able
to confirm the mechanism, we experimented with preparing the
compound 4a independently. We prepared 3-amino-2-(4-
methylphenyldiazenyl)but-2-enenitrile by the azo coupling re-
action and left it to react with freshly prepared diphenylborinic acid
(Scheme 4). However, according to NMR spectra, the product was
not the oxazaborine 4a but the triazaborine 3a. This was then
confirmed by means of X-ray diffraction (Fig. 10). Hence, the oxa-
zaborines are not formed according to the suggested mechanism
(Scheme 3).
15N and 11B chemical shifts of compounds 3aef and 4 in CDCl3 (For notation of ni-
trogen see Scheme 1)
Product
15N1
15N2
15N3
11B
3aa
3b
ꢀ141.9
ꢀ148.3
ꢀ137.4
ꢀ145.6
ꢀ141.3
ꢀ148.0
ꢀ137.1
ꢀ145.0
ꢀ142.9
ꢀ136.7
25.5
21.2
31.8
ꢀ204.2
ꢀ202.1
ꢀ214.9
ꢀ206.0
ꢀ207.5
ꢀ203.7
ꢀ217.1
ꢀ207.8
ꢀ203.9
ꢀ211.8
ꢀ168.3 (CN)
ꢀ1.2
1.26
3cb
3d
3ec
3f
ꢀ1.11
1.16
ꢀ0.99
1.01
20.2
3gd
3h
3ie
3jf
ꢀ0.94
0.94
ꢀ176.7 (CN)
ꢀ186.2 (CN)
ꢀ330.6 (NMe2)
ꢀ277.9 (NH2)
ꢀ1.02
ꢀ1.14
4ag
4b
22.2
21.1
27.5
86.4
82.3
ꢀ222.3
ꢀ223.5
ꢀ225.3
2.12
1.99
2.19
4ch
ꢀ276.3 (NH2)
a
1J (15N3, 1H)¼83.2 Hz.
1J (15N3, 1H)¼83.5 Hz.
1J (15N3, 1H)¼82.6 Hz.
1J (15N3, 1H)¼82.5 Hz.
1J (15N3, 1H)¼81.8 Hz.
1J (15N3, 1H)¼81.7 Hz.
b
c
d
e
f
g
h
1J (15N3, 1H)¼81.3 Hz, 1J (15N4, 1H)¼89.5 Hz, 1J (15N4, 1H)¼90.8 Hz.
1J (15N3, 1H)¼81.6 Hz, 1J (15N4, 1H)¼89.6 Hz, 1J (15N4, 1H)¼90.7 Hz.
Nitriles react with a number of Lewis acids.22 Upon co-
ordination of a Lewis acid on the electron lone pair of nitrogen of
the nitrile group, an increase in the polarity of the group takes
place. Subsequent addition of water to form amide is then ac-
celerated by several orders of magnitude. From the literature23
nitriles are known to form coordination compounds
ReC^NþeꢀBPh3 with triphenylborane. Triphenylborane is
formed in the course of the reaction of diazonium tetraphe-
nylborate with protons introduced during azo couplings to po-
larized ethylenes (Scheme 3).15a After coordination of the
triphenylborane with the eC^N nitrogen, the addition of water,
present in a small amount, can easily take place to form amide.
The isolated oxazaborines 4 (Figs. 5e8) are then probably formed
not directly from enaminonitriles but from their hydration
products (Scheme 3). The oxazaborines are really formed as the
main products from enaminoamides.15b
Fig. 5. ORTEP view of oxazaborine 4a displaying the thermal ellipsoids at 30% proba-
bility; hydrogen atoms, except those involved in hydrogen bonds, are omitted for clarity.
The reaction of the enaminonitriles with diazonium tetraphe-
nylborates to give triazaborines probably proceeds by the same
mechanism as we have suggested before for other polarized eth-
ylenes.15 In the case of enaminones it was possible to prove (by
means of NMR experiments) that the reaction goes through a non-
isolable intermediate, hydrazone Ima (Ref. 15a, Fig. 9). The forma-
tion of an analogous intermediate Imb was observed for the
enaminonitriles, as well. It was possible to isolate the intermediate
if the reaction was done at room temperature in dichloromethane.
The intermediate Imb was cyclized in toluene under reflux to the
triazaborine 3a.
The intermediate Imb was, in the case of the reaction of 3-
aminobut-2-enenitrile with 4-methylbenzenediazonium tetra-
phenylborate, identified on the basis of comparison of its 13C
chemical shifts with those of the major isomer of 3-amino-2-(4-
methylphenyldiazenyl)but-2-enenitrile (for the part of the in-
termediate Imb coming from the enaminonitrile and the diazonium
ion) and a hitherto unpublished 13C NMR spectrum of the isolated
coordination compound of triphenylborane with ethyl 3-amino-2-
(4-methylphenyl-hydrazono)butanoate24 (for the part of the mol-
ecule containing the Ph3Bꢀ group). The 13C spectrum, combined in
this manner, is practically identical with the spectrum of the in-
termediate Imb (Table S1). It was not possible to purify the com-
pound for microanalysis due to its cyclization when crystallization
Fig. 6. Chain of hydrogen bonded molecules in crystal packing of oxazaborine 4a.
By modification of the reaction conditions (CH2Cl2/Et2O/H2O in
ratio 30:10:0.3, vol, under reflux) the yield of the oxazaborine 4a
increased up to 45%.

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2055
Scheme 1. Reaction of b-enaminonitriles 1 and 4-substituted benzenediazonium tetraphenylborate.
Table 2
Reaction conditions and yields for reaction of 1aee with diazonium salts
Product Solventa
Temperature Time (h) Yieldb (%) Yieldc (%)
a
3a
3a
3a
3b
3c
3d
3e
3f
3g
3h
3i
3i
3j
CH2Cl2
CH2Cl2
rt
rt
1
72
3
3
3
Traces
76
72
63
75
55
85
60
80
65
70
74
42
61
d
d
a
48
53
49
60
35
62
47
62
40
50
50
30
51
30
45
21
24
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
rt
Reflux
Reflux
Reflux
rt
4
3
15.5
1.5
3
72
2
4.5
2
72
4
a
CH2Cl2
CH2Cl2/toluene
CH2Cl2/toluene
CH2Cl2/toluene
5b
4a
4a
4b
4c
a
CH2Cl2
CH2Cl2/Et2O/H2O Reflux
d
CH2Cl2/Et2O/H2O Reflux
3
72
d
a
CH2Cl2
rt
d
a
Dichloromethane was used (35 mL/5 mmol).
Isolated yield (after column chromatography).
Yield after recrystallization.
b
c
Scheme 3. The proposed (and finally rejected) mechanism for preparation of ox-
azaborines 4 from diphenylborinic acid.
Scheme 2. Reaction of
b-enaminonitrile 1e and 4-methylbenzenediazonium tetra-
phenylborate in mixture of CH2Cl2/toluene at reflux.
Scheme 4. Reaction of azo compound 5a with diphenylborinic acid in CH2Cl2 at room
temperature.
was attempted. The structure of the intermediate Imb was con-
firmed by means of HRMS.
intermolecular H-bond. The triazaborines 3h and 3j exhibit the
HN1]C2eC1]N3 (HN5D]C48eC47]N7 and HN9D]C25eC24]
N11 for 3a) heterodienic systems display a good delocalisation
within the cyano groups. Triazaborines 3a, 3h and 3j contain the
cyano substituent at C1 for 3h and 3j, and, C47 and C24 for 3a,
respectively, which forms the usual intermolecular NeH/NC (for
3a) hydrogen bonded chain, but rather unusual N1eH1/C6i
(i¼½ꢀx, ½þy, ½ꢀz) intermolecular connection between the amino
group and the aromatic system from adjacent molecule, as a short
ORTEP views of the compounds 3a, 3h, 3j, 4a and 4b are shown
in Figs. 5e8,10e14. The oxazaborines 4a and 4b display intra-
molecular short NeH/N resonance assisted hydrogen bonds25
(Table S3) between the enamino and the diazenyl group.15b,17d
The HN2eC1]C2eN3]N4 heterodienic systems exhibit ex-
tended conjugation within the C2]C1eN2eH enamino moiety and
weaker delocalisation within the C2eN3]N4 diazenyl one. In both
the compounds 4a and 4b the NeH groups are not involved in any

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2056
Fig. 7. ORTEP view of oxazaborine 4b displaying the thermal ellipsoids at 50% prob-
ability; some hydrogen atoms are omitted for clarity.
Fig. 10. ORTEP view of triazaborine 3a displaying the thermal ellipsoids at 50%
probability.
Fig. 8. Chain of hydrogen bonded molecules in crystal packing of oxazaborine 4b.
Fig. 11. Chain of hydrogen bonded molecules in crystal packing of triazaborine 3a.
Fig. 9. The structure of intermediate on the route to triazaborines.
intramolecular contact. The six boron heterocycles assume quite
similar conformations, as evidenced by the puckering parameters26
in Table S4.
3. Conclusion
By means of the reaction of b-enaminonitriles with substituted
benzenediazonium tetraphenylborates in the absence of water,
substituted triazaborines were formed. If water was present in the
reaction mixture, oxazaborines, whichformed afterhydrolysis of the
nitrile groupin one of the reaction stages, werethe major products. If
the amino group of the starting b-enaminonitrile was substituted
with the phenyl group, only the azo coupling product was isolated.
The compounds were characterized by means of NMR spectroscopy
(1H,13C,15N and 11B, see Table 1 and Experimental section) and some
were also characterized by means of X-ray diffraction.
Fig. 12. ORTEP view of triazaborine 3h displaying the thermal ellipsoids at 50%
probability; hydrogen atoms are omitted for clarity.

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2057
ratios were 70:30:50.1. Experiments were performed with the NH
one-bond coupling 90 Hz, and NH long-range coupling 5 Hz,
2 Kꢁ160 zero filled to 2 Kꢁ1 K, sinebell squared in both dimensions.
The 11B NMR spectra were calibrated on external B(OCH3)3 placed in
a coaxial capillary (d
18.1). In order to suppress the signals of 11B
nuclei from NMR tube glass, the measurements were carried out in
Teflon sample tube liners (Aldrich) inserted into 5 mm tubes whose
bottom part of about 25 mm length was cut off.
The X-ray data for crystals of 3a, 3h, 3j, 4a and 4b were obtained
at 150 K using Oxford Cryostream low-temperature device on
a
Nonius Kappa CCD diffractometer with Mo
¼0.71073 A), a graphite monochromator, and the
K
f
a
radiation
c
ꢂ
(
l
and scan
mode. Data reductions were performed with DENZO-SMN.27 The
absorption was corrected by integration methods.28 Structures
were solved by direct methods (Sir92)29 and refined by full matrix
least-square based on F2 (SHELXL97).30 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)¼1.2 Ueq (pivot atom) or of 1.5 Ueq for the methyl
ꢂ
ꢂ
moiety with CeH¼0.96 A and 0.93 A for methyl and hydrogen
Fig. 13. ORTEP view of triazaborine 3j displaying the thermal ellipsoids at 40% proba-
bility; hydrogen atoms, except those involved in hydrogen bonds, are omitted for clarity.
ꢂ
atoms in aromatic ring, respectively, and 0.86 A for NeH bonds.
Rint¼SjFo2ꢀFo2,meanj/SFo2, GOF¼[S(w(Fo2ꢀFc2)2)/(NdiffrsꢀNparams)] for
½
all data, R(F)¼SjjFojꢀjFcjj/SjFojfor observed data, wR(F2)¼
[S(w(Fo2ꢀF2c)2)/(Sw(Fo2)2)] for all data.
½
Crystallographic data for structural analysis have been de-
posited with the Cambridge Crystallographic Data Centre, CCDC no.
779055e779059 for 3a, 3h, 3j, 4a and 4b, respectively. Copies of
this information may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EY, UK (fax: þ44
1223 336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
HRMS spectrum was measured with ESI technique under neg-
ative ionization mode using LTQ Orbitrap Velos (Thermo Scientific)
instrument.
Melting points were determined with a Kofler hot stage mi-
croscope and were not corrected. The microanalyses were carried
out with a FISONS EA 1108 automatic analyser.
4.2. Starting compounds
Anhydrous dichloromethane was purchased commercially
(Fluka) and distilled from P2O5 before use. Toluene and diethylether
were dried over sodium. 3-Aminobut-2-enenitrile (1a) was
purchased from Aldrich and crystallized from ethyl acetate.
3-Methylaminobut-2-enenitrile31 (1b), 3-amino-3-phenylprop-2-
enenitrile32 (1c) and 3-phenylaminobut-2-enenitrile31 (1e) was
prepared as previously reported.
Fig. 14. Chain of hydrogen bonded molecules in crystal packing of triazaborine 3j.
4. Experimental section
4.1. General
NMR spectrawere measured at room temperature using a Bruker
AVANCE 500 spectrometer equipped with a 5 mm broadband probe
with a gradient of magnetic field in the direction of z axis operating
at the frequencies 500.13 MHz (1H), 125.77 MHz (13C), 50.69 MHz
4.2.1. 3-Methylamino-3-phenylprop-2-enenitrile (1d). Methylamine
(32 mL, 33% ethanolic solution) was added to solution of 3-oxo-3-
phenylpropanenitrile33 (4.05 g, 28 mmol) in ethanol (55 mL). The
reaction mixture was heated to reflux and stirred 10 h at the same
temperature. Ethanol was evaporated and dark yellow viscous oil
was obtained. The oil was refluxed with charcoal (0.5 g) and
methanol (20 mL). Then the mixture was filtrated and methanol
was evaporated. Yield of the title compound was 3.1 g (70%). 1H
NMR spectrum was in accordance with that reported in Ref. 34.
(
15N) and a Bruker AVANCE III 400 operating at 400.13 MHz (1H),
100.62 MHz (13C), 128.38 MHz (11B) and 40.55 MHz (15N). The 1H
NMR spectra were calibrated in CDCl3 on internal standard (hex-
amethyldisiloxane (
NMR spectra were calibrated on the central signal of the solvent
multiplet (
76.9). The 13C NMR spectra were measured in standard
d 0.05) or tetramethylsilane (d C
0.00)) and the 13
d
way and by means of the APT pulse sequence (spectral width
26.455 kHz, acquisition time 1.238 s, zero filling to 64 K and line
broadening 1 Hz prior Fourier transformation). The 15N NMR spectra
were calibrated on external neat 15N nitromethane placed in a co-
4.2.2. Substituted benzenediazonium tetraphenylborates. Aniline or
4-substituted aniline (6.84 mmol) was dissolved in boiling aqueous
hydrochloric acid (3 mL, 1:1). The solution was cooled to ꢀ5 ꢂC and
diazotised by gradual addition of a cold solution of sodium nitrite
(0.49 g, 7.11 mmol) in water (1.5 mL). The temperature during the
diazotation was maintained between ꢀ5 and 0 ꢂC. The excess of
nitrous acid (presence tested by starch-iodide paper) was
axial capillary (d 0.0). The d (
15N) values were measured with the
help of techniques with inversion detection (gradient selected 2D
1He15N HMBC) processed in the magnitude mode. The gradient

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M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2058
decomposed using the required amount of sulfamic acid. A solution
of sodium tetraphenylborate (6.84 mmol) in water (75 mL) was
added at once. The precipitated diazonium salt was filtered and
washed with cold ethanol (1ꢁ30 mL) and diethylether (1ꢁ30 mL).
The diazonium salts were prepared immediately before use and
dried in vacuo at room temperature in a desiccator for about 1 h
(CAUTION: dry diazonium tetraphenylborates can undergo violent
decomposition when the crystalline material is ground!). The
yields of diazonium salts were about 79e87%.
toluene mixture. Yield 1.17 g (62%), yellow crystals, mp
180e182.5 ꢂC; [found: C, 72.73; H, 5.84; N, 14.48. C23H21BN4O re-
quires C, 72.65; H, 5.57; N 14.73%]. 1H NMR (500 MHz, CDCl3):
d 2.17
(s, 3H), 3.62 (s, 3H), 6.54e6.57 (m, 2H), 7.15e7.22 (m, 6H), 7.25e7.27
(m, 2H), 7.31e7.33 (m, 4H), 7.49 (s, 1H). 13C NMR (125 MHz, CDCl3):
d
20.4, 55.1, 109.8, 112.9, 117.0, 124.3, 126.7, 127.4, 133.2, 140.5, 145.9
(br), 157.5, 158.7.
4.3.6. 2-(4-Methoxyphenyl)-4,5-dimethyl-3,3-diphenyl-1,2,4,3l
4-tri-
azaborine-6-carbonitrile (3f). Recrystallization from cyclohexane/
toluene mixture. Yield 0.92 g (47%), orange crystals, mp
177e179 ꢂC; [found: C, 73.09; H, 6.00; N, 14.01. C24H23BN4O re-
4.3. General procedure for reaction of 1aee with 4-
substituted benzenediazonium tetraphenylborates
quires C, 73.11; H, 5.88; N 14.21%]. 1H NMR (500 MHz, CDCl3):
d 2.24
To a cold (5 ꢂC) solution of
b
-enaminonitriles 1aee (5 mmol) in
(s, 3H), 2.90 (s, 3H), 3.56 (s, 3H), 6.46e6.49 (m, 2H), 7.08e7.11 (m,
dry dichloromethane (15 mL) and toluene (25 mL, dried over so-
dium) was added freshly prepared corresponding benzenediazo-
nium tetraphenylborate (5 mmol). The reaction mixture was
stirred 1 h at room temperature and then 2e15.5 h at reflux (See
Table 2). The reaction mixture was cooled to room temperature
and solvents were evaporated. The crude residue was chromato-
graphed on silica gel with dichloromethane to give compounds
3ae3j and 5b.
2H), 7.15e7.18 (m, 2H), 7.20e7.24 (m, 4H), 7.36e7.38 (m, 4H). 13C
NMR (125 MHz, CDCl3):
d 17.4, 38.4, 54.9, 110.7, 112.6, 117.5, 124.3,
126.6, 127.4, 133.5, 140.6, 144.0 (br), 157.4, 158.1.
4.3.7. 2-(4-Methoxyphenyl)-3,3,5-triphenyl-4H-1,2,4,3l
4-tri-
azaborine-6-carbonitrile (3g). Recrystallization from toluene.
Yield 1.37 g (62%), orange crystals, mp 226e229 ꢂC; [found: C,
76.26; H, 5.50; N, 12.38. C28H23BN4O requires C, 76.03; H, 5.24; N
12.67%]. 1H NMR (500 MHz, CDCl3):
d 3.70 (s, 3H), 6.63e6.66 (m,
4.3.1. 5-Methyl-2-(4-methylphenyl)-3,3-diphenyl-4H-1,2,4,3
l
4-tri-
2H), 7.07 (br s, 1H), 7.21e7.27 (m, 6H), 7.35e7.38 (m, 6H),
azaborine-6-carbonitrile (3a). Recrystallization from cyclohexane.
Yield 0.97 g (53%), yellow crystals, mp 187e189 ꢂC; [found: C, 75.74;
H, 6.09; N,15.23. C23H21BN4 requires C, 75.84; H, 5.81; N 15.38%]. 1H
7.49e7.52 (m, 2H), 7.58e7.61 (m, 1H), 7.69e7.70 (m, 2H). 13C
NMR (125 MHz, CDCl3):
d 55.2, 109.8, 113.1, 117.3, 125.0, 126.9,
127.5, 128.6, 129.2, 131.0, 133.0, 133.3, 140.6, 145.7 (br), 156.1,
159.2.
NMR (500 MHz, CDCl3):
2H), 7.13e7.21 (m, 8H), 7.32e7.34 (m, 4H), 7.67 (br s, 1H). 13C NMR
(125 MHz, CDCl3): 20.7, 20.9, 110.0, 116.9, 122.7, 126.7, 127.3, 128.4,
d 2.19 (s, 3H), 2.22 (s, 3H), 6.83e6.84 (m,
d
4.3.8. 2-(4-Methoxyphenyl)-4-methyl-3,3,5-triphenyl-1,2,4,3l
4-tri-
133.2, 137.5, 144.5, 145.9 (br), 157.7.
azaborine-6-carbonitrile (3h). Recrystallization from toluene. Yield
0.92 g (40%), orange crystals, mp 228e231 ꢂC; [found: C, 76.44; H,
5.73; N, 12.22. C29H25BN4O requires C, 76.33; H, 5.52; N 12.28%]. 1H
4.3.2. 4,5-Dimethyl-2-(4-methylphenyl)-3,3-diphenyl-1,2,4,3l
4-tri-
azaborine-6-carbonitrile (3b). Recrystallization from cyclohexane.
Yield 0.93 g (49%), orange crystals, mp 194e197 ꢂC; [found: C,
76.28; H, 6.27; N, 14.69. C24H23BN4 requires C, 76.20; H, 6.13; N
NMR (500 MHz, CDCl3):
2H), 7.19e7.21 (m, 2H), 7.23e7.32 (m, 9H), 7.41e7.43 (m, 4H),
7.50e7.51 (m, 3H). 13C NMR (125 MHz, CDCl3):
40.2, 55.2, 111.4,
112.8,117.2, 124.9, 126.9, 127.2, 127.6, 129.2, 130.6, 131.0, 133.7, 140.9,
144.0 (br), 158.6, 158.8.
d 2.81 (s, 3H), 3.67 (s, 3H), 6.57e6.60 (m,
d
14.81%]. 1H NMR (500 MHz, CDCl3):
d
2.15 (s, 3H), 2.30 (s, 3H), 2.94
(s, 3H), 6.80e6.82 (m, 2H), 7.05e7.07 (m, 2H), 7.18e7.26 (m, 6H),
7.37e7.39 (m, 4H). 13C NMR (125 MHz, CDCl3):
17.6, 20.7, 38.5,
d
111.1, 117.5, 122.9, 126.8, 127.5, 128.2, 133.6, 136.8, 144.1 (br), 144.8,
157.6.
4.3.9. 5-Methyl-2,3,3-triphenyl-4H-1,2,4,3l
4-triazaborine-6-
carbonitrile (3i). Recrystallization from cyclohexane/toluene mix-
ture. Yield 0.88 g (50%), light orange crystals, mp 216.5e218.5 ꢂC;
[found: C, 75.17; H, 5.67; N, 15.72. C22H19BN4 requires C, 75.45; H,
4.3.3. 2-(4-Methylphenyl)-3,3,5-triphenyl-4H-1,2,4,3l
4-triazaborine-
6-carbonitrile (3c). Recrystallization from cyclohexane. Yield
1.28 g (60%), orange crystals, mp 219e222 ꢂC. [found: C, 78.84; H,
5.71; N, 12.88. C28H23BN4 requires C, 78.88; H, 5.44; N 13.14%]. 1H
5.47; N 16.00]. 1H NMR (500 MHz, CDCl3):
d
2.21 (s, 3H), 7.09e7.11
(m, 3H), 7.18e7.27 (m, 6H), 7.33e7.39 (m, 6H), 8 (br s, 1H). 13C NMR
(125 MHz, CDCl3): 20.3, 110.5, 116.8, 122.9, 126.7, 127.3, 127.4,
d
NMR (500 MHz, CDCl3):
d
2.22 (s, 3H), 6.92e6.94 (m, 2H), 7.19 (br
127.8, 133.3, 145.9 (br), 146.6, 157.9.
s, 1H), 7.20e7.26 (m, 6H), 7.29e7.32 (m, 2H), 7.35e7.37 (m, 4H),
7.46e7.49 (m, 2H), 7.55e7.58 (m, 1H), 7.66e7.68 (m, 2H). 13C
4.3.10. 5-Methyl-2-(4-dimethylaminophenyl)-3,3-diphenyl-4H-
NMR (125 MHz, CDCl3):
d
20.9, 110.0, 117.1, 123.4, 126.9, 127.5,
1,2,4,3l
4-triazaborine-6-carbonitrile (3j). Recrystallization from
128.5, 128.6, 129.2, 130.9, 133.0, 133.3, 138.1, 144.6, 145.8 (br),
156.3.
ethanol. Yield 0.60 g (30%), brown crystals, mp 193e195.5 ꢂC;
[found: C, 73.43; H, 6.39; N, 17.67. C24H24BN5 requires C, 73.29; H,
6.15; N 17.81%]. 1H NMR (500 MHz, CDCl3):
d
2.15 (s, 3H), 2.83 (s,
6H), 6.31e6.33 (m, 2H), 7.07 (br s, 1H), 7.16e7.25 (m, 8H), 7.31e7.33
(m, 4H), 13C NMR (125 MHz, CDCl3):
20.3, 39.9, 109.1, 110.6, 117.5,
4.3.4. 4-Methyl-2-(4-methylphenyl)-3,3,5-triphenyl-1,2,4,3l
4-tri-
azaborine-6-carbonitrile (3d). Recrystallization from toluene.
Yield 0.78 g (35%), orange crystals, mp 262e264 ꢂC; [found: C,
79.33; H, 5.95; N, 12.57. C29H25BN4 requires C, 79.10; H, 5.72; N
d
124.2, 126.6, 127.3, 133.3, 137.1, 146.5 (br), 149.5, 156.7.
12.72%]. 1H NMR (500 MHz, CDCl3):
d
2.20 (s, 3H), 2.81 (s, 3H),
4.3.11. 2-(4-Methylphenyldiazenyl)-3-phenylaminobut-2-enenitrile
(5b). Recrystallization from cyclohexane. Yield 0.71 g (51%), yellow
crystals, mp 139e141 ꢂC; [found: C, 73.95; H, 5.91; N, 20.27.
C17H16N4 requires C, 73.89; H, 5.84; N 20.27%]. 1H NMR (400 MHz,
6.87e6.89 (m, 2H), 7.14e7.16 (m, 2H), 7.22e7.25 (m, 2H),
7.26e7.32 (m, 6H), 7.42e7.44 (m, 4H), 7.50e7.51 (m, 3H). 13C
NMR (125 MHz, CDCl3):
d 20.8, 40.3, 111.7, 117.1, 123.4, 126.8,
127.2, 127.5, 128.3, 129.2, 130.6, 131.0, 133.7, 137.2, 143.9 (br),
144.9, 158.9.
CDCl3, 45 ꢂC):
d 2.23 (s, 3H), 2.31 (s, 3H), 6.96e6.97 (br m, 2H),
7.12e7.14 (m, 2H), 7.21e7.23 (br m, 1H), 7.31e7.32 (br m, 2H),
7.37e7.41 (br m, 2H), 14.27 (br s, 1H). 13C NMR (100 MHz, CDCl3,
4.3.5. 2-(4-Methoxyphenyl)-5-methyl-3,3-diphenyl-4H-1,2,4,3
l
4-tri-
45 ꢂC):
d
18.21 (br), 20.77, 109.6 (br), 117.8 (br), 112.1 (br), 125.7 (br),
azaborine-6-carbonitrile (3e). Recrystallization from cyclohexane/
128.1, 129.1, 129.6, 136.6 (br), 143.3 (br), 160.9.

ꢀ
M. Svobodova et al. / Tetrahedron 68 (2012) 2052e2060
2059
4.4. General procedure for synthesis of 4a and 4b
116.7 (br, CN, maj.), 121.4 (d, JC,N¼4.1 Hz, C6, min.), 122.2 (d,
JC,N¼4.1 Hz, C6, maj.), 130.0 (C7, maj.), 130.1 (C7, min.), 137.7 (C8,
min.), 138.4 (C8, maj.),151.1 (d, JC,N¼5.3 Hz, C5, maj.), 151.3 (d,
JC,N¼6.2 Hz, C5, min.), 164.6 (br s, C3, maj.), 168.4 (d, JC,N¼9.5 Hz, C3,
To a cold (5 ꢂC) solution of 3-aminobut-2-enenitrile (1a) (0.41 g,
5 mmol) or 3-phenyl-3-aminoprop-2-enenitrile (1c) (0.72 g,
5 mmol) in dichloromethane (30 mL) was added water (0.3 mL)
and diethylether (10 mL) and then freshly prepared corresponding
benzenediazonium tetraphenylborate (5 mmol). The reaction
mixture was stirred either 72 h at room temperature or 1.5 h at
room temperature and then 3e4 h at reflux. The reaction mixture
was cooled to room temperature and solvents were evaporated.
The crude residue was chromatographed on silica gel with
dichloromethane to give compounds 4aec.
min.). 15N NMR (40, 55 MHz, DMSO-d6):
d
(
15Nb)¼87.8 (maj.), 92.7
(min.).
*Although this signal belongs to both the isomers, this is singlet
due to an accidental equivalence of the chemical shifts.
4.6. Preparation of 3a from 5a and diphenylborinic acid
To a stirred solution of azo coupling product 5a (1 g, 5 mmol) in
dichloromethane (35 mL) was added diphenylborinic acid (1 g,
5.5 mmol) (prepared according to Ref. 23). The reaction mixture
was stirred at room temperature for 17 h. Dichloromethane was
evaporated and the residue was chromatographed (silica gel/
dichloromethane). The crude product was crystallized from tolu-
ene/cyclohexane to give 1 g (55%) yellow crystals. 1H NMR data
were in accordance with those obtained in Section 4.3.1.
4.4.1. 6-Amino-4-methyl-5-(4-methylphenyldiazenyl)-2,2-diphenyl-
3H-1,3,2l
4-oxazaborine (4a). Recrystallization from cyclohexane/
toluene mixture. Yield 0.86 g (45%), yellow crystals, mp
215e217 ꢂC; [found: C, 72.49; H, 6.31; N, 14.58. C23H23BN4O re-
quires C, 72.27; H, 6.06; N 14.66%]. 1H NMR (500 MHz, CDCl3):
d
2.34 (s, 3H), 2.53 (s, 3H), 6.13 (s,1H), 6.83 (s,1H), 7.15e7.16 (m, 2H),
7.19e7.22 (m, 2H), 7.26e7.29 (m, 4H), 7.38e7.42 (m, 6H), 11.33 (br s,
1H). 13C NMR (125 MHz, CDCl3):
21.1, 21.3, 114.4, 120.4, 126.4,
d
4.7. Preparation of intermediate Imb
127.3, 129.5, 131.7, 137.5, 148.9 (br), 150.2, 161.9, 170.0.
All components were dried before use dichloromethane was
4.4.2. 6-Amino-5-(4-methoxyphenyldiazenyl)-4-methyl-2,2-
stirred
2 days with P2O5 and then distilled, 3-aminobut-2-
diphenyl-3H-1,3,2l
4-oxazaborine (4b). Recrystallization from tolu-
enenitrile was dried under vacuum for 14 h after recrystallization
from toluene. The reaction proceeded under argon atmosphere. To
ene. Yield 0.42 g (21%), yellow crystals, mp 180e183.5 ꢂC; [found: C,
69.61; H, 5.88; N, 14.35. C23H23BN4O2 requires C, 69.36; H, 5.82; N
a
solution of
dichloromethane
b
-enaminonitrile (1a) (0.74 g, 9 mmol) in dry
(55 mL) freshly prepared 4-methyl-
14.07%]. 1H NMR (400 MHz, CDCl3):
d 2.54 (s, 3H), 3.81 (s, 3H), 6.14
(s, 1H), 6.80 (s, 1H), 7.21e7.25 (m, 2H), 7.27e7.30 (m, 4H), 7.40e7.42
benzenediazonium tetraphenylborate (3.95 g, 9 mmol) was added.
The reaction mixture was stirred for 72 h at room temperature. The
solid was filtered to give 1.93 g yellow crystals. MS (ESI neg): calcd
for [MꢀH]ꢀ 441.2250, found for C29H27BN4 m/z 441.2242. The fil-
trate was evaporated to dryness and the crude residue was chro-
matographed on silica gel with dichloromethane to give 0.74 g
(23%) triazaborine 3a.
(m, 4H), 7.48e7.51 (m, 2H), 11.25 (br s, 1H). 13C NMR (100 MHz,
CDCl3): d 21.3, 55.4,114.0,114.2,121.8,126.4,127.3,131.7,146.4, 149.1
(br), 159.2, 161.9, 169.8.
4.4.3. 6-Amino-5-(4-methylphenyldiazenyl)-2,2,4-triphenyl-3H-
1,3,2l
4-oxazaborine (4c). The reaction proceeded with equimolar
amount of enaminonitrile and undried diazonium salt in
dichloromethane at room temperature (see Table 2). Re-
crystallization from toluene. Yield 0.53 g (24%), yellow crystals, mp
227e230 ꢂC; [found: C, 75.73; H, 5.90; N, 12.54. C28H25BN4O re-
quires C, 75.69; H, 5.67; N 12.61%]. 1H NMR (500 MHz, CDCl3):
Acknowledgements
ꢁ
The authors thank to Prof. Antonín Lycka for his help with some
NMR measurements. The work was financially supported by Min-
istry of Education, Youth and Sports of the Czech Republic (project
MSM 002 162 7501).
d
2.31 (s, 3H), 6.38 (br d, 1H), 6.94 (br s, 1H), 7.09e7.10 (m, 2H),
7.22e7.25 (m, 4H), 7.29e7.32 (m, 4H), 7.46e7.50 (m, 6H), 7.52e7.54
(m, 1H), 7.61e7.63 (m, 2H), 11.58 (br s, 1H). 13C NMR (125 MHz,
CDCl3):
d 21.1, 114.4, 120.7, 126.5, 127.4, 128.0, 129.4, 129.5, 131.0,
Supplementary data
131.8, 135.6, 137.7, 148.8 (br), 150.1, 163.6, 169.4.
The table for comparison of intermediate’s (Imb) 13C chemical
shifts (Table S1). The crystallographic data (Table S2eS5). Supple-
mentary data related to this article can be found online at
doi:10.1016/j.tet.2011.12.082.
4.5. 3-Amino-2-(4-methylphenyldiazenyl)but-2-enenitrile
(5a) and (15N-5a)
To a stirred solution of enaminonitrile 1a (0.82 g, 10 mmol) in
dichloromethane (70 mL) were added remelted sodium acetate
(2.46 g, 30 mmol) and 4-methylbenzenediazonium tetra-
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