Straightforward access to oxazaborines, diazaborinones and triazaborines by reactions of β-enaminoamides with 4-methylbenzenediazonium tetraphenylborate

Article abstract of DOI:10.1016/j.jorganchem.2008.10.004

The reaction of substituted β-enaminoamides with 4-methylbenzenediazonium tetraphenylborate in dichloromethane produces besides the primary products of azo coupling reaction at the α-carbon atom of β-enaminoamides, also mixtures of heterocyclic compounds

Full text of DOI:10.1016/j.jorganchem.2008.10.004

Journal of Organometallic Chemistry 694 (2009) 63–71
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Journal of Organometallic Chemistry
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Straightforward access to oxazaborines, diazaborinones and triazaborines by
reactions of b-enaminoamides with 4-methylbenzenediazonium tetraphenylborate
a
a
b
a
Markéta Svobodová a, , Jan Bárta , Petr Šimu˚ nek , Valerio Bertolasi , Vladimír Machácek
*
ˇ
a
ˇ
Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice, nám. Cs. legií 565, Pardubice 532 10, Czech Republic
b Università di Ferrara, Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Via L. Borsari 46, Ferrara 441 00, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of substituted b-enaminoamides with 4-methylbenzenediazonium tetraphenylborate in
dichloromethane produces besides the primary products of azo coupling reaction at the -carbon atom
Received 5 September 2008
Accepted 7 October 2008
Available online 11 October 2008
a
of b-enaminoamides, also mixtures of heterocyclic compounds of boron: 1,3,2k4-oxazaborines, 1H-
1,3,2k4-diazaborine-4-ones and 4H-1,2,4,3k4-triazaborines. Proportions of the products change depend-
ing on the reaction conditions, particularly depending on the presence or absence of base (sodium ace-
tate) in the reaction mixture. The heterocyclic compounds were separated chromatographically and
identified by means of X-ray, 1H, 11B, 13C and 15N NMR spectra and elemental analyses.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
b-Enaminoamides
Diazonium tetraphenylborate
Oxazaborines
Diazaborinones
Triazaborines
1. Introduction
(e.g. diphenylborinic acid or its ester, boron trifluoride or triphen-
ylborane) [21].
Boron together with carbon, oxygen and nitrogen forms a num-
ber of types of heterocyclic compounds: oxazaborolidines, oxazab-
orines, oxazaborinanes etc. Some representatives of this group of
substances exhibit miscellaneous types of biological activity [1–
4]: antibacterial, fungicidal, insecticidal, and herbicidal.
Some of them have been considered as potential BNCT agents
for targeting brain tumors [5]. In organic synthesis, heterocyclic
compounds of boron have been adopted for protection of amino
acids [6], in the synthesis of isoquinoline [7], isoindole [7]. They
are used in asymmetrical hydroborations [8], to separate diaste-
reomeric and racemic methoxyborolane mixtures [9], as the cata-
lysts for enantioselective reduction of ketones [10], or in the
Diels–Alder reaction [11]. Literature describes application of
1,3,2-oxazaborinides to dyeing of polyester, polyamide and poly-
propylene fibres [12].
Recently we published a new method of preparation of substi-
tuted 1,3,2k4-oxazaborines by reaction of the corresponding
b-enaminones with 4-substituted benzenediazonium tetraphe-
nylborates in dichloromethane at room temperature [22]. We also
suggested a mechanism of formation of oxazaborines. Thermal
rearrangement of the mentioned oxazaborines leads to substituted
4H-1,2,4,3k4-triazaborines [22].
Various polarized ethylenes were applied in the development of
novel synthetic methodologies in our laboratory. In this paper, we
report on novel synthetic transformations leading to N–B–O and
N–B–N heterocyclic systems using reactions of b-enaminoamides
and diazonium salts.
2. Results and discussion
The most common methods for preparing heterocycles with O–
B–N grouping are reactions of 1,2- and 1,3-aminoalcohols with al-
kyl- and arylboronic acids [10e,13] or dialkyl- and diarylborinic
acids [14], boronate [15] or borane [16] and borane-dimethylsul-
fide [17]. Another method [18] consists in aminoboration of ketene
with dialkyl(dimethylamino)boranes or bromodimethylborane
and amine, or reactions of substituted vinyloxyboranes with ni-
triles [19,20]. Another possibility is the synthesis from b-enami-
nones and various derivatives of boron containing compounds
The reaction of equimolar amounts of substituted benze-
nediazonium tetraphenylborates with b-enaminoamides in anhy-
drous dichloromethane at room temperature gives a mixture of
boron containing heterocyclic compounds. The decisive factor
affecting the composition of this mixture is the presence or ab-
sence of base – anhydrous sodium acetate. If sodium acetate is
present, then the reaction products contain (in two cases; 1c,d),
besides the heterocyclic compounds, also the primary product 5
of azo coupling reaction with b-enaminoamide. The presence of
this base also changes the proportion of heterocyclic compounds
in the reaction mixture (Scheme 1).
* Corresponding author. Tel.: +420 466 037 039; fax: +420 466 037 068.
E-mail address: marketa.svobodova@upce.cz (M. Svobodová).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.004

64
M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
R1
N
O
R2
R1
B
N
R2
R1
B
N
R2
N
N
N
N
O
N
N
B
N
H
N
O
H
N
H
CH2Cl2
rt, 4 d
N2 BPh4
R2
H
N
O
R1
N
H
2a—d
2a,b
4a—c
4a
3a, b
R2
R1
H
N
N
O
N
1
a: R1 = Ph, R2 = H
N
H
b: R1 = Me, R2 = H
AcONa
CH2Cl2
rt, 4 d
: R1 = Ph, R2 = Me
c
d
: R1 = Me, R2 = Me
5b, d
Scheme 1. Reaction of b-enaminoamides 1 with 4-methylbenzenediazonium tetraphenylborate.
In the case of the reaction of 3-amino-N,3-diphenylprop-2-ena-
When suggesting the structure of compounds 2a, 3a, 4a, we
started from the previously described mechanism of the reaction
of b-enaminones with diazonium tetraphenylborates [22]: the pri-
mary product of the azo coupling reaction reacts with tetraphenyl-
borate anion with gradual splitting off of two molecules of benzene
and ring closure giving the heterocycle. The primary product of the
azo coupling reaction can exist in several forms stabilised by the
formation of intramolecular hydrogen bonds. These forms were
observed spectroscopically in the case of azo coupling reaction
products obtained from b-enaminones [23]. Theoretically, the
coordination between boron atom and the atoms originally linked
by means of the intramolecular hydrogen bond can lead to five
structures with six-membered rings: A–E (Scheme 2).
In the case of compound 2a we rejected triazaborines B and C,
because the positive value of d(11B) corresponds to the arrange-
ment N–B–O (Ref. [22]) (Table 2). 1H–15N gsHMBC spectrum of
the compound 2a is presented in Fig. 1 (top); the values of chem-
ical shifts of nitrogen atoms are given in Table 2. The positive val-
ues of d(15N) 12.2 and 86.9 ppm indicate the presence of an azo
group [23,24], which is not present in the suggested structures D
and E. Hence, the compound 2a has structure A.
mide (1a) with 4-methylbenzenediazonium tetraphenylborate
without the presence of sodium acetate the reaction products in-
clude oxazaborine (2a), diazaborinone (3a) and triazaborine (4a)
in the molar ratio of 5:2:1 (determined from integration of 1H
NMR spectrum of the crude reaction mixture). The ratios of the
products obtained with other combinations of substituents are
presented in Table 1. The reaction products were separated chro-
matographically on a silica gel (dichloromethane as the mobile
phase) and further purified by recrystallization.
The products were identified by 1H, 13C, 11B and 15N NMR, and
in several cases also by means of X-ray diffraction. The 1H NMR
spectra of the compounds 2a, 3a and 4a are shown in Fig. 1. The
proton spectra of these compounds are of similar character. In
the aromatic part, there are multiplets belonging to three types
of monosubstituted benzene rings, one 1,4-disubstituted ring,
and broadened signals of N–H groups. In the aliphatic part, there
is only a singlet of methyl group from the starting diazonium salt.
The protons of the N-phenyl group were identified by measuring
the substances prepared from partially deuterated amide 1a (using
aniline D5). The multiplets of the phenyl groups linked with the
boron atom exhibit a twofold intensity.
With regard to the positive values of d(15N), a potential tau-
tomeric equilibrium of this compound is shifted in favour of
the azo compound (Scheme 3), which is also in accordance with
the found value of the coupling constant 1J(15N1,1H) = 84.3 Hz
[25] (for notation of nitrogen atoms, see the structure X in
Scheme 2). The structure of the compound 2a suggested on
the basis of NMR parameters was confirmed by X-ray diffraction
(Fig. 2).
Table 1
Proportions of products 2, 3, 4 and 5 in a crude reaction mixture.
Starting
compound
Products The ratio of the
productsa,b
Yieldc The ratio of the
(%)
Yieldc
(%)
productsa,d
1a
1b
1c
2a
3a
4a
2b
3b
4b
2c
3c
4c
5c
2d
3d
4d
5d
5
2
1
10
7
4
10
–
13
–
10
Traces
Traces
Traces
54
21
8
43
28
11
41
–
45
–
74
–
10
traces
0.7
1
–
–
1
–
–
5
1
–
–
86
–
4
64
–
–
15
–
–
51
20
–
In the 11B NMR spectrum of the compound 3a, the boron atom
exhibits a negative chemical shift (Table 2), which excludes struc-
tures A and E (Scheme 2). The measured value of coupling constant
1J(15N4, 1H) = 93.7 Hz corresponds to the presence of pure hydra-
zone form. The 1H–15N gsHMBC spectrum of the compound 3a is
given in Fig. 1 (middle); the values of chemical shifts of nitrogen
atoms are presented in Table 2. The ORTEP view is in Fig. 3.
The chemical shift of boron atom in the compound 4a
(d(11B) = À1.46 ppm (Table 2)) is comparable with the measured
values for the earlier-studied triazaborines [22] prepared from b-
enaminones. Scheme 2 suggests two possible structures for tri-
azaborines, namely B and C. Fig. 1 (bottom right) presents the
1H–15N HMBC spectrum of this compound, wherefrom it is obvious
that the triazaborine prepared possesses two different nitrogen
atoms with directly linked hydrogen atoms (single-bond interac-
1d
–
–
–
57
5
From integration of 1H NMR.
Without the presence of sodium acetate.
Isolated yield (after column chromatography).
In the presence of sodium acetate.
a
b
c
d

M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
65
Fig. 1. 500.13 MHz 1H a 1H–15N gsHMBC spectra of compounds 2a, 3a and 4a in CDCl3.
tion N–H). This does not correspond with the suggested structure
B, whose 1H–15N HMBC spectrum would exhibit the single-bond
interaction N–H belonging to one nitrogen atom only. We also
measured the 1H–13C HMBC spectrum (Fig. 4), which shows a
two-bond interaction C–H of amidic group that is not present in
structure B.
The reaction of b-enaminoamide 1a with equimolar amount
of diazonium salt in the presence of sodium acetate gave only
two (isolable) products: the major product – oxazaborine 2a
(yield 68% after recrystallization) and the minor product – tri-
azaborine 4a (yield 3% after recrystallization) (Scheme 1). Diaza-
borinone 3a was only formed in a non-isolable amount. Its

66
M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
Scheme 2. The possibility of the coordination of BPhÀ4 to the azo compound X.
presence in the reaction mixture was confirmed by the 1H NMR
spectrum.
spond to any of the structures suggested. This signal is present in
the spectrum even if the substance was twice recrystallized before
the measurement. It could belong to the third isomer, whose pos-
sible structure is presented in Fig. 6 (bottom). However, this is only
hypothesis whose confirmation would need more experimental
data.
Similar results were also obtained from the reaction of b-enami-
noamide 1b: if no sodium acetate was present, then three product
were isolated: 2b, 3b and 4b; if the reaction took place in the pres-
ence of sodium acetate, then the major product obtained was
oxazaborine 2b. The 11B a 15N chemical shifts of these compounds
are presented in Table 2.
2.1. X-ray diffraction study
b-Enaminoamides 1c and 1d differ from the previous ones by
the presence of secondary N-methyl enamino group (C@C–NHMe)
in the molecule. When the reaction of these enaminoamides took
place without the presence of sodium acetate, then two products
(from b-enaminoamide 1c) or one product (from b-enaminoamide
1d) were obtained. Whereas 3-methylaminobut-2-enamide (1d)
reacts with 4-methylbenzenediazonium tetraphenylborate to give
oxazaborine 2d only, the reaction of the same diazonium salt with
3-methylamino-N,3-diphenylprop-2-enamide (1c) produces both
oxazaborine 2c and triazaborine 4c (for the ORTEP diagram, see
Fig. 5). Diazaborinone was not formed under these reaction
conditions.
The primary products 5 of the azo coupling reactions of 4-
methylbenzenediazonium tetraphenylborate were identified in
the case of the reactions with enaminoamides 1c and 1d. In CDCl3
solution, these products exist as mixtures of E- and Z-isomers dif-
fering in the arrangement of intramolecular hydrogen bond. The
tautomeric form of the major E-isomer is opposite to that of the
minor Z-isomer (Fig. 6). The positions of tautomeric equilibria
were determined on the basis of the 15N NMR parameters (Table
2) [23] The 1H–15N HMBC spectrum of the compound 5c exhibits
a signal with chemical shift d(15N) = À220.4, which does not corre-
ORTEP [26] views of the compounds 2a, 3a and 4c are shown in
Figs. 2, 3 and 5. ORTEP views of the compounds 2b, 2c and 3b are
shown in Figs. S42–44. The oxazaborines 2a–c display intramolec-
ular short N–HÁ Á ÁN resonance assisted hydrogen bonds [27] (Table
S3) between an enamino and
a diazenyl group [28]. The
Á Á ÁHN2–C3@C2–N3@N4Á Á Á heterodienic systems exhibit extended
conjugation within the C2@C3–N2–H enamino moiety and weaker
delocalisation within the C2–N3@N4 diazenyl one. In both
compounds 2a and 2b the N–H groups are not involved in any
intermolecular H-bond.
The diazaborinones 3a and 3b exhibit intramolecular H-bond
assisted by resonance between a keto and a hydrazone group
[29]. The HN4–N3@C2–C3@O1 heterodienic systems display a
good delocalisation within both the C3@O1 keto and HN4–
N3@C2 hydrazone groups. In both compounds the N1–H moiety
forms intermolecular H-bond with O1 oxygen.
Triazaborine 4c contains an amidic substituent at C2 which
does not form the usual intermolecular N–HÁ Á ÁO hydrogen bonded
chain but only N4–HÁ Á ÁN2 intramolecular H-bond which can be
rather considered, owing to the narrow H-bond angle of 112(1)°,
as a short intramolecular contact.

M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
67
Table 2
15N and 11B chemical shifts of compounds 2a–d, 3a–b, 4a–c and 5c,d in CDCl3 (for
notation of nitrogens see Scheme 2).
Product
15N1
15N2
15N3
85.9
78.9
84.3
133.9
3.3
15N4
12.3
5.4
0.5
À11.4
11B
2.51
2.37
4.41
2aa
À245.2
À246.8
À244.8
À243.4
À228.8
À228.0
À226.7
À225.6
À262.7
À262.9
À262.8
À262.0
À223.5
À219.9
À218.9
À216.0
À183.1
À179.3
À178.4
À177.6
À206.1
À199.3
À200.3
À195.5
2bb
2cc
2dd
4.08
3ae
À189.1
À194.0
À193.8
À197.8
À159.9
À163.4
À167.7
À171.3
À0.50
À0.93
À2.54
À2.78
À1.46
À1.20
À3.32
À3.65
3bf
À4.9
3cg
1.9
3dh
À8.2
9.3
5.4
3.7
1.0
4ai
4bj
4ck
4dl
5cm
Major*
Minor#
5dn
À246.4
À265.1
À92.4
96.1
À7.3
14.1
À212.3
–
o
Major*
Minor#
À247.2
À239.5
À260.5
À93.4
85.3
À17.7
3.4
À215.3
–
a
1J(15N1,1H) = 84.3 Hz, 1J(15N2,1H) = 81.8 Hz.
b
c
d
e
f
1J(15N2,1H) = 80.1 Hz.
1J(15N1,1H) = 81.4 Hz.
1J(15N1,1H) = 79.5 Hz.
1J(15N4,1H) = 93.7 Hz, 1J(15N2,1H) = 78.9 Hz.
1J(15N4,1H) = 94.2 Hz, 1J(15N2,1H) = 79.8 Hz.
1J(15N4,1H) = 96.4 Hz.
g
h
i
1J(15N4,1H) = 96.4 Hz.
1J(15N1,1H) = 89.3 Hz, 1J(15N2,1H) = 81.5 Hz.
1J(15N1,1H) = 89.6 Hz, 1J(15N2,1H) = 80.3 Hz.
1J(15N1,1H) = 89.4 Hz.
Fig. 2. ORTEP view of compound 2a. The thermal ellipsoids are drawn at 30%
probability level.
j
k
l
1J(15N1,1H) = 89.3 Hz.
m*
J(15N1,1H) = 85 Hz, 1J(15N2,1H) = 92 Hz, 2J(15N2,CH3) = 6.6 Hz.
1J(15N4,1H) = 95.2 Hz.
m#
n*
n#
o
1J(15N1,1H) = 86 Hz, 1J(15N2,1H) = 92.6 Hz, 2J(15N2,CH3) = 5.5 Hz.
1J(15N1,1H) = 82.7 Hz, 1J(15N4,1H) = 96 Hz.
Not detected.
Scheme 3. Azo-hydrazone tautomerism of the oxazaborines.
The six boron heterocycles assume quite different conforma-
tions, as evidenced by the puckering parameters [30] in Table S4,
due to the different atoms forming the cycles and electronic and
steric features of the substituents.
2.2. Rearrangement of the oxazaborines
When oxazaborines 2a–d were refluxed in N,N-dimethylform-
amide, a recyclization occurred to give the diazaborinones 3a,c,d,
triazaborines 4a–d and unreacted oxazaborines 2a–d (Scheme 4;
reaction conditions and yields are shown in Table 3). Although
diazaborinone 3b was identified in the reaction mixture (1H
NMR), its amount was insufficient for isolation.
It cannot be decided whether the amounts represented an
equilibrium proportion of the products or further heating would
completely transform the oxazaborines into a mixture of diazabo-
rinones and triazaborines, because the substances undergo decom-
Fig. 3. ORTEP view of compound 3a. The thermal ellipsoids are drawn at 30%
probability level.
position on long-term heating. The hypothesis that the mixture
represents an equilibrium between the three products mentioned
is supported by the fact that also diazaborinone 3b is slowly
transformed into a mixture of oxazaborine and triazaborine on
heating. Hence, by increasing temperature it is possible to obtain

68
M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
at all or undergo it but very unwillingly, the yields of triazaborines
being poor.
3. Conclusion
We have described a new, simple method of synthesis of six-
membered heterocyclic compounds containing oxygen, nitrogen
and boron. Besides the earlier-described reaction of b-enaminones
with substituted benzenediazonium tetraphenylborates, here are
now described reactions of b-enaminoamides having primary or
secondary enamino group with 4-methylbenzenediazonium tetra-
phenylborate. This reaction at mild conditions (r.t.) produces mix-
tures of oxazaborines, diazaborinones and triazaborines. The
primarily formed oxazaborines can be thermally rearranged to give
a mixture of diazaborinones and triazaborines, which is possible
even in the cases of the starting b-enaminoamides that have a sec-
ondary amino group (C@C–NHR). In the case of the oxazaborines
derived from b-enaminones, an analogous rearrangement was only
possible in the presence of primary amino group.
4. Experimental
3-Amino-N,3-diphenylprop-2-enamide (1a) and 3-amino-N-
phenylbut-2-enamide (1b) were synthesized according to the liter-
ature procedures [31] from corresponding b-oxoamide and
NH4OAc.
Fig. 4. 500.13 MHz 1H–13C gsHMBC spectrum of compound 4a in CDCl3.
3-Methylamino-N,3-diphenylprop-2-enamide (1c). To a solution
of benzoylacetanilide (6.95 g, 26 mmol) in ethanol (50 mL) was
added methylamine (33% solution in ethanol, 30 mL). The resulting
solution was refluxed for 7 h and concentrated in vacuo. The resi-
due was a yellow oil which solidified on standing overnight at
room temperature. Recrystallization of the solid from toluene
afforded enaminoamide 1c as a white powder (4.61 g, 63%): m.p.
110–113 °C; 1H NMR (500 MHz, CDCl3) d 2.66 (d, J = 5 Hz, 3H),
4.54 (s, 1H), 6.95–6.99 (m, 1H), 7.03 (br s, 1H), 7.20–7.23 (m,
2H), 7.28–7.30 (m, 2H), 7.34–7.36 (m, 3H), 7.44–7.45 (m, 2H),
9.07 (q, J = 5 Hz, 1H); 13C NMR (125 MHz, CDCl3) d 31.1, 87.4,
119.6, 122.7, 127.6, 128.1, 128.6, 128.8, 136.0, 138.9, 164.0,
168.7. Anal. Calc. for C16H16N2O: C, 76.16; H, 6.39; N, 11.10. Found:
C, 76.16; H, 6.43; N, 11.07%.
3-Methylamino-N-phenylbut-2-enamide (1d). This compound
was prepared from acetoacetanilide according to the same proce-
dure as 1c. Recrystallization of the crude product from ethanol
gave white solid (4.42 g, 45%): m.p. 142.5–144 °C (Ref. [32] 145 °C).
4.1. General procedures for the synthesis of oxazaborines,
diazaborinones and triazaborines
Method A. A stoichiometric amount of freshly prepared 4-meth-
ylbenzenediazonium tetraphenylborate [22] (4.38 g, 10 mmol) was
added to a stirred solution of b-enaminoamide (10 mmol) in dry
dichloromethane (70 mL). The reaction mixture was stirred at
room temperature for 96 h. Then it was filtered and the solvent
was removed in vacuo. The crude residue containing one to three
compounds was separated by column chromatography over silica
gel (dichloromethane) and the fractions containing the individual
compounds were recrystallised. The following compounds were
prepared according to the procedure described.
5-(4-Methylphenyldiazenyl)-2,2,4-triphenyl-6-phenylamino-3H-
1,3,2k4-oxazaborine (2a). Recrystallization from toluene, yellow
crystals (2.23 g, 43%): m.p. 199–201 °C; 1H NMR (500 MHz, CDCl3)
d 2.32 (s, 3H), 6.99 (s, 1H), 7.11–7.12 (m, 2H), 7.19–7.23 (m, 3H),
7.27–7.30 (m, 6H), 7.37–7.40 (m, 2H), 7.45–7.48 (m, 2H), 7.51–
7.55 (m, 5H), 7.64–7.69 (m, 4H), 14.70 (s, 1H); 13C NMR
(125 MHz, CDCl3) d 21.0, 114.9, 120.4, 121.9, 125.6, 126.3, 127.3,
Fig. 5. ORTEP view of compound 4c. The thermal ellipsoids are drawn at 30%
probability level.
diazaborinone also from b-enaminoamides having a secondary
enamino group. The difference in thermal rearrangement of
oxazaborines 2a–d as compared with the oxazaborines prepared
from b-enaminones [22] lies in the fact that compounds 2c and
2d (derived from enamioamides with N-methyl group) give tri-
azaborines already at room temperature or at enhanced tempera-
ture, while the oxazaborines with N-methyl group derived from
b-enaminones [22] either do not undergo thermal rearangement

M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
69
2
H
N
R
N
O
N
O
N
R
N
O
1
N
H
N
H
R
N
N
H
N
H
N
H
N
3
4
E-isomer
major
Z-isomer
minor
H
N
N
N
N
O
H
another suggested
structure for 5c
Fig. 6. The structure of compounds 5c and 5d.
R1
N
O
R2
B
R2
R1
B
N
R2
Ph
Ph
N
O
N
N
N
N
B
N
H
Ph
DMF
R1
N
H
N
unreacted
O
H
2a d
—
reflux
N
N
2a—d
3a,c,d
Scheme 4. Rearrangement of oxazaborines 2a–d.
4a—d
127.9, 129.2, 129.4, 129.5, 130.9, 131.7, 135.5, 136.4, 137.6, 148.9,
149.2, 159.4, 169.3. Anal. Calc. for C34H29BN4O: C, 78.47; H, 5.62; N,
10.77. Found: C, 78.61; H, 5.61; N, 10.65%.
(500 MHz, CDCl3) d 2.28 (s, 3H), 2.54 (s, 3H), 2.93 (s, 3H), 6.98–
7.02 (m, 1H), 7.12–7.14 (m, 2H), 7.18–7.23 (m, 4H), 7.26–7.28
(m, 4H), 7.41–7.48 (m, 8H), 14.99 (s, 1H); 13C NMR (125 MHz,
CDCl3) d 15.1, 21.0, 37.5, 116.1, 119.8, 121.9, 125.1, 126.4, 127.1,
129.0, 129.6, 132.9, 136.8, 137.0, 147.4, 149.0, 157.3, 170.5. Anal.
Calc. for C30H29BN4O: C, 76.28; H, 6.19; N, 11.86. Found: C,
76.50; H, 6.25; N, 11.78%.
4-Methyl-5-(4-methylphenyldiazenyl)-2,2-diphenyl-6-phenylami-
no-3H-1,3,2k4-oxazaborine (2b). Recrystallization from toluene, yel-
low crystals (1.32 g, 29%): m.p. 206–208 °C; 1H NMR (500 MHz,
CDCl3) d 2.36 (s, 3H), 2.58 (s, 3H), 6.96 (br s, 1H), 7.15–7.22 (m,
5H), 7.26–7.29 (m, 4H), 7.33–7.36 (m, 2H), 7.43–7.45 (m, 4H),
7.48–7.50 (m, 2H), 7.59–7.61 (m, 2H), 14.58 (s, 1H); 13C NMR
(125 MHz, CDCl3) d 21.1, 21.2, 114.9, 120.1, 121.9, 125.5, 126.3,
127.3, 129.1, 129.6, 131.6, 136.5, 137.5, 148.9, 149.2, 157.9,
170.0. Anal. Calc. for C29H27BN4O: C, 75.99; H, 5.94; N, 12.22.
Found: C, 75.93; H, 6.11; N, 12.29%.
3-Methyl-5-(4-methylphenyldiazenyl)-2,2,4-triphenyl-6-phenyla-
mino-1,3,2k4-oxazaborine (2c). Recrystallization from cyclohexane,
yellow crystals (1.67 g, 31%): m.p. 204–207 °C; 1H NMR
(500 MHz, CDCl3) d 2.27 (s, 3H), 2.81 (s, 3H), 6.70–7,04 (m, 4H),
7.12–7.15 (m, 1H), 7.24–7.27 (m, 2H), 7.29–7.33 (m, 6H), 7.35–
7.37 (m, 2H), 7.45–7.53 (m, 7H), 7.55–7.57 (m, 2H), 14.67 (s,
1H); 13C NMR (125 MHz, CDCl3) d 21.0, 39.3, 116.9, 119.9, 121.8,
125.3, 126.5, 127.2, 127.8, 128.0, 128.7, 129.1, 129.4, 132.9,
134.1, 136.9, 136.9, 147.3, 149.1, 158.2, 171.7. Anal. Calc. for
C35H31BN4O: C, 78.65; H, 5.85; N, 10.48. Found: C, 78.56; H, 5.97;
N, 10.52%.
5-[(4-Methylphenyl)hydrazono]-2,2,3,6-tetraphenyl-1H-1,3,2k4-
diazaborine-4-one (3a). Recrystallization from ethanol, yellow
crystals (0.84 g, 16%): m.p. 216–219 °C; 1H NMR (500 MHz,
CDCl3) d 2.30 (s, 3H), 6.81–6.83 (m, 2H), 6.00–7.02 (m, 2H),
7.04–7.06 (m, 1H), 7.08–7.11 (m, 4H), 7.18–7.25 (m, 6H),
7.42–7.44 (m, 4H), 7.48–7.51 (m, 2H), 7.56–7.61 (m, 3H),
8.19 (s, 1H), 15.66 (s, 1H); 13C NMR (125 MHz, CDCl3)
d
20.8, 115.7, 121.8, 125.6, 126.2, 127.1, 127.9, 128.1, 128.2,
129.0, 129.9, 131.7, 133.4, 133.7, 135.4, 139.1, 141.1, 147.5,
164.3, 169.1. Anal. Calc. for C34H29BN4O: C, 78.47; H, 5.62;
N, 10.77. Found: C, 78.47; H, 5.69; N, 10.71%.
6-Methyl-5-[(4-methylphenyl)hydrazono]-2,2,3-triphenyl-1H-1,3,
2k4-diazaborine-4-one (3b). Recrystallization from ethanol, yellow
crystals (0.91 g, 20%): m.p. 212–215 °C; 1H NMR (500 MHz, CDCl3)
d 2.32 (s, 3H), 2.44 (s, 3H), 6.66–6.68 (m, 2H), 6.98–7.04 (m, 3H),
7.13–7.21 (m, 10H), 7.32–7.33 (m, 4H), 8.19 (br s, 1H), 15.46 (s,
1H); 13C NMR (125 MHz, CDCl3) d 20.8, 20.9, 115.6, 122.4, 125.6,
126.2, 127.0, 127.9, 128.3, 130.0, 133.5, 135.2, 139.1, 141.0,
147.3, 163.6, 170.2. Anal. Calc. for C29H27BN4O: C, 75.99; H, 5.94;
N, 12.22. Found: C, 76.05; H, 6.14; N, 12.38%.
3,4-Dimethyl-5-(4-methylphenyldiazenyl)-2,2-diphenyl-6-phenyla-
mino-1,3,2k4-oxazaborine (2d). Recrystallization from cyclohexane,
yellow crystals (2.08 g, 44%): m.p. 175.5–178.5 °C; 1H NMR

70
M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
Table 3
6.96–7.08 (m, 6.18H, maj. + min.), 7.17–7.20 (m, 2.3H, maj. + min.),
7.29–7.33 (m, 3H, maj.), 7.36–7.39 (m, 4H, maj.), 7.64–7.67 (m,
2.72H, maj. + min.), 11.84 (br q, 1H, 3J = 5.1 Hz, maj.), 13.36 (br s,
1H, maj.), 13.63 (s, 0.26 H, min.), 14.08 (s, 0.26 H, min.); 13C NMR
(125 MHz, CDCl3) major form d 20.8, 31.9, 114.2, 120.1, 120.2,
120.6, 123.3, 127.5, 128.1, 128.6, 129.1, 133.2, 136.5, 138.6,
150.0, 164.3, 172.8; minor form d 20.4, 39.7, 115.7, 120.5, 123.9,
125.8, 127.3, 127.5, 128.4, 128.7, 129.4, 132.4, 135.9, 137.8,
140.3, 163.7, 171.2. Anal. Calc. for C23H22N4O: C, 74.57; H, 5.99;
N, 15.12. Found: C, 74.40; H, 6.20; N, 15.20%.
3-Methylamino-2-(4-methylphenyldiazenyl)-N-phenylbut-2-ena-
mide (5d). The title compound was obtained (1.2 g, 39%) as yellow
crystals: recrystallization from cyclohexane, m.p. 154–155.5 °C;
1H NMR (500 MHz, CDCl3) major form d 2.30 (s, 3H), 2.34 (s,
3H), 2.88 (d, 3H, 3J = 5.2 Hz), 6.99–7.04 (m, 1.38H (maj. + min.)),
7.10–7.12 (m, 2H), 7.26–7.30 (m, 2.32H (maj. + min.)), 7.41–7.43
(m, 2H), 7.59–7.61 (m, 2H), 12.00 (q, 1H, 3J = 5.2 Hz), 13.59 (s, 1
H); minor form d 2.07 (s, 0.37H), 2.24 (s, 0.41H), 3.16 (s, 0.42H),
6.99–7.04 (m, 1.38H (maj. + min.)), 7.07–7.09 (m, 0.26H), 7.26–
7.30 (m, 2.32H (maj. + min.)), 7.54–7.55 (m, 0.35H), 13.86 (s,
1H), 14.14 (s, 1 H); 13C NMR (125.77 MHz, CDCl3) major form d
14.0, 20.9, 30.5, 119.5, 120.1, 122.8, 123.2, 128.6, 129.4, 136.4,
138.8, 150.3, 164.0, 172.2; minor form d 13.6, 20.6, 37.9, 120.5,
123.8, 128.7, 129.8, 132.4, 139.0, 140.6, 163.9, 169.6. Anal. Calc.
for C18H20N4O: C, 70.11; H, 6.54; N, 18.17. Found: C, 70.36; H,
6.44; N, 18.13%.
Reaction times, products and yields of the rearrangement of oxazaborines 2a–d in
DMF at reflux.
Reaction Products of
time
(min)
The ratio of Yield after
Yield after
the
rearrangement in the
the product chromatography crystallization
(%)
(%)
reaction
mixturea
2a 140
2a
3a
4a
2b
3b
4b
2c
3c
4c
2d
3d
4d
3
2
10
2
0.1
10
10
10
7
10
15
8
17
10
54
10
–
64
28
31
14
25
39
18
10
5
35
7
2b 140
2c 370
2d 400
–
47
20
17
7
20
28
14
a
From integration of 1H NMR.
2-(4-Methylphenyl)-3,3,5-triphenyl-4H-1,2,4,3k4-triazaborine-6-
carboxanilide (4a). Recrystallization from cyclohexane-toluene, yel-
low crystals (0.21 g, 4%): m.p. 201–204 °C; 1H NMR (500 MHz,
CDCl3) d 2.29 (s, 3H), 7.01–7.05 (m, 3H), 7.12 (br s, 1H), 7.19–
7.28 (m, 8H), 7.30–7.32 (m, 2H), 7.35–7.37 (m, 4H), 7.42–7.45
(m, 2H), 7.50–7.55 (m, 5H), 8.98 (s, 1H); 13C NMR (125 MHz, CDCl3)
d 20.9, 119.4, 123.4, 123.5, 126.7, 127.4, 128.1, 128.3, 128.4, 128.7,
128.8, 131.4, 133.4, 134.5, 137.1, 137.9, 144.8, 146.1, 157.3, 160.3.
Anal. Calc. for C34H29BN4O: C, 78.47; H, 5.62; N, 10.77. Found: C,
78.47; H, 5.87; N, 10.57%.
5-(4-Methylphenyldiazenyl)-2,2,4-triphenyl-6-[2H5]phenylamino-
3H-1,3,2k4-oxazaborine (2a). The title compound was prepared
according to the procedure B. 3-Amino-N-[2H5]phenyl-3-phenyl-
prop-2-enamide was used instead of enamide 1a. It was obtained
as yellow crystals (1.32 g, 63%): recrystallization from cyclohex-
ane-toluene, m.p. 200.5–202.5 °C; 1H NMR (500 MHz, CDCl3) d
2.24 (s, 3H), 6.93 (s, 1H), 7.04–7.06 (m, 2H), 7.16–7.20 (m, 2H),
7.23–7.28 (m, 6H), 7.37–7.40 (m, 2H), 7.42–7.45 (m, 1H), 7.50–
7.52 (m, 4H), 7.57–7.58 (m, 2H), 14.67 (s, 1H); 13C NMR
(125 MHz, CDCl3) d 21.0, 121.6 (t, 1J(13C, 2H) = 25 Hz), 125.2 (t,
1J(13C, 2H) = 25 Hz), 126.3, 127.3, 127.9, 128.7 (t, 1J(13C,
2H) = 25 Hz),129.4, 129.5, 131.0, 131.7, 135.6, 136.3, 137.7, 148.9,
149.2, 159.5, 169.3. Anal. Calc. for C34H24D5BN4O: C, 77.72; H,
6.52; N, 10.66. Found: C, 77.95; H, 6.81; N, 10.57%.
5-Methyl-2-(4-methylphenyl)-3,3-diphenyl-4H-1,2,4,3k4-triazab-
orine-6-carboxanilide (4b). Recrystallization from toluene, orange
crystals (0.17 g, 4%): m.p. 215–218.5 °C; 1H NMR (500 MHz, CDCl3)
d 2.20 (s, 3H), 2.49 (s, 3H), 6.91–6.93 (m, 2H), 7.01–7.04 (m, 1H),
7.13–7.21 (m, 8H), 7.24–7.27 (m, 2H), 7.31–7.33 (m, 4H), 7.38 (br
s, 1H), 7.50–7.52 (m, 2H), 8.91 (s, 1H); 13C NMR (125 MHz, CDCl3)
d 20.8, 23.6, 119.7, 122.7, 123.6, 126.6, 126.7, 127.3, 128.6, 128.8,
133.4, 136.6, 137.8, 145.0, 146.5, 159.1, 161.8. Anal. Calc. for
C29H27BN4O: C, 75.99; H, 5.94; N, 12.22. Found: C, 76.07; H, 6.06;
N, 12.18%.
4-Methyl-2-(4-methylphenyl)-3,3,5-triphenyl-4H-1,2,4,3k4-triazab-
orine-6-carboxanilide (4c). Recrystallization from toluene, orange
crystals (1.24 g, 23%): m.p. 225–229 °C; 1H NMR (500 MHz, CDCl3)
d 2.26 (s, 3H), 2.80 (s, 3H), 6.94–6.97 (m, 3H), 7.14–7.24 (m, 12H),
7.35–7.36 (m, 4H), 7.40–7.45 (m, 5H), 8.93 (s, 1H); 13C NMR
(125 MHz, CDCl3) d 20.9, 40.4, 119.4, 123.2, 123.6, 126.3, 126.6,
127.4, 128.4, 128.6, 128.9, 129.3, 133.8, 134.0, 136.4, 138.0,
144.8, 145.4, 159.1, 160.6. Anal. Calc. for C35H31BN4O: C, 78.65;
H, 5.85; N, 10.48. Found: C, 78.63; H, 6.05; N, 10.47%.
4.2. Rearrangement of oxazaborines
The appropriate oxazaborines (3 mmol) was dissolved in DMF
(8 mL) and the solution was refluxed (the reaction times and
resulting yields are shown in Table 3). DMF was evaporated in va-
cuo and the residue was subjected to the column chromatography
over silica gel (dichloromethane).
Method B. The procedure was the same as for the method A,
excepting re-melted and finally grounded sodium acetate (2.46 g,
30 mmol) was added to a stirred solution of b-enaminoamide
and 4-methylbenzenediazonium tetraphenylborate. The following
compounds were prepared by the procedure described.
Compound 2a. Yellow crystals; 3.54 g (68%).
4.2.1. 2,2,3,6-Tetraphenyl-1-methyl-5-[(4-methylphenyl)hydrazono]-
1H-1,3,2k4-diazaborine-4-one (3c)
Recrystallization from cyclohexane-toluene, yellow crystals:
m.p. 232–235 °C; 1H NMR (500 MHz, CDCl3) d 2.21 (s, 3H), 2.80
(s, 3H), 6.55–6.56 (m, 2H), 6.70–6.72 (m, 2H), 6.95–6.96 (m, 2H),
6.98–7.00 (m, 3H), 7.20–7.26 (m, 8H), 7.48–7.50 (m, 7H), 15.27
(s, 1H); 13C NMR (125 MHz, CDCl3) d 20.7, 41.0, 115.3, 124.3,
125.6, 127.0, 127.1, 127.9, 128.3, 128.5, 129.3, 129.8, 132.9,
133.9, 134.6, 139.4, 141.0, 145.9, 163.1, 171.8. Anal. Calc. for
C35H31BN4O: C, 78.65; H, 5.85; N, 10.48. Found: C, 78.91; H, 6.13;
N, 10.49%.
Compound 2b. Yellow crystals; 2 g (44%).
Compound 2c. Yellow crystals; 0.23 g (4%).
Compound 2d. Yellow crystals; 0.6 g (13%).
Compound 4a. Yellow crystals; 0.17 g (3%).
3-Methylamino-2-(4-methylphenyldiazenyl)-N,3-diphenylprop-2-
enamide (5c). The title compound was obtained chromatographi-
cally as yellow crystals from the same reaction mixture as the
compound 2c (1.13 g, 31%): recrystallization from ethanol, m.p.
123.5–127 °C; 1H NMR (500 MHz, CDCl3) d 2.15 (s, 0.87H, min.),
2.22 (s, 3H, maj.), 2.81 (d, 3H, 3J = 5.2 Hz, maj.), 3.12 (s, 0.83H,
min.), 6.61–6.66 (m, 0.52H, min.), 6.88–6.90 (m, 0.69H, min.),
4.2.2. 1,6-Dimethyl-5-[(4-methylphenyl)hydrazono]-2,2,3-triphenyl-
1H-1,3,2k4-diazaborine-4-one (3d)
Recrystallization from ethanol, yellow crystals: m.p. 205.5–
208 °C; 1H NMR (500 MHz, CDCl3) d 2.30 (s, 3H), 2.56 (s, 3H),
3.00 (s, 3H), 6.48–6.50 (m, 2H), 6.95–6.96 (m, 3H), 7.12–7.14 (m,

M. Svobodová et al. / Journal of Organometallic Chemistry 694 (2009) 63–71
71
2H), 7.16–7.21 (m, 8H), 7.37–7.38 (m, 4H), 15.41 (s, 1H); 13C NMR
(125 MHz, CDCl3) d 15.2, 20.8, 39.8, 115.3, 123.2, 125.5, 126.1,
127.0, 127.8, 128.4, 130.0, 133.9, 134.5, 139.5, 141.0, 145.9,
162.8, 170.9. Anal. Calc. for C30H29BN4O: C, 76.28; H, 6.19; N,
11.86. Found: C, 76.42; H, 6.41; N, 11.82%.
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triazaborine-6-carboxanilide (4d)
Recrystallization from cyclohexane, orange crystals: m.p. 180–
182.5 °C; 1H NMR (500 MHz, CDCl3) d 2.21 (s, 3H), 2.67 (s, 3H),
3.00 (s, 1H), 6.88–6.89 (m, 2H), 7.03–7.06 (m, 3H), 7.15–7.22 (m,
6H), 7.27–7.33 (m, 6H), 7.54–7.56 (m, 2H), 9.03 (s, 1H); 13C NMR
(125.77 MHz, CDCl3) d 17.7, 20.8, 39.0, 119.6, 123.1, 123.4, 126.5,
127.4, 128.2, 128.3, 128.8, 133.7, 135.8, 145.0, 145.5, 159.8,
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Found: C, 76.24; H, 6.34; N, 11.89%.
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We thank to the Ministry of Education, Youth and Sports of the
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Appendix A. Supplementary material
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Experimental section such as NMR, crystallography and general,
copies of 1H and 13C NMR for all new compounds, 15N NMR spectra
for selected compounds (2b–d, 3b–d, 4b–d, 5c,d), ORTEP views for
2b,c and 3b, X-ray tables and crystalographic information files.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.10.004.
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