Novel 5-(4-Substituted-phenyldiazenyl)-1,3,2λ 4-oxazaborines and their rearrangement to 1,2,4,3λ 4-triazaborines

Article abstract of DOI:10.1021/om051078x

The reaction of substituted benzenediazonium tetraphenylborates with the β-enaminones derived from pentane-2,4-dione, 1-phenylbutane-1,3-dione, and 1,4-diphenylbutane-1,3-dione with a primary or secondary (N-methyl, N-phenyl) amino group in CH₂Cl₂ gives 5-(substituted-phenyldiazenyl)-2,2-diphenyl-4,6-disubstituted-1,3,2λ ⁴-oxazaborines or 5-(substituted-phenyldiazenyl)-2,2-diphenyl-3,4,6-trisubstituted-1,3, 2λ⁴-oxazaborines, respectively. The reaction intermediate of these compounds has been identified, and a mechanism for the reaction has been suggested. Substituted 1,3,2λ⁴-oxazaborines gradually rearrange into 1,2,4,3λ⁴-triazaborines at temperatures above 100°C.

Full text of DOI:10.1021/om051078x

Organometallics 2006, 25, 2025-2030
2025
Novel 5-(4-Substituted-phenyldiazenyl)-1,3,2λ4-oxazaborines and
Their Rearrangement to 1,2,4,3λ4-Triazaborines
Marke´ta Pesˇkova´,† Petr Sˇimu˚nek,*,† Valerio Bertolasi,‡ Vladim´ır Macha´cˇek,† and
Anton´ın Lycˇka§
Department of Organic Chemistry, Faculty of Chemical Technology, UniVersity of Pardubice,
Na´m Cˇ s. Legi´ı 565, CZ-532 10 Pardubice, Czech Republic, Dipartimento di Chimica and Centro
di Strutturistica Diffrattometrica, UniVersita` di Ferrara, Via L. Borsari 46, I-441 00 Ferrara, Italy, and
Research Institute for Organic Syntheses (VUOS), RybitV´ı 296, CZ-532 18 Pardubice 20, Czech Republic
ReceiVed December 19, 2005
The reaction of substituted benzenediazonium tetraphenylborates with the â-enaminones derived from
pentane-2,4-dione, 1-phenylbutane-1,3-dione, and 1,4-diphenylbutane-1,3-dione with a primary or
secondary (N-methyl, N-phenyl) amino group in CH2Cl2 gives 5-(substituted-phenyldiazenyl)-2,2-diphenyl-
4,6-disubstituted-1,3,2λ4-oxazaborines or 5-(substituted-phenyldiazenyl)-2,2-diphenyl-3,4,6-trisubstituted-
1,3,2λ4-oxazaborines, respectively. The reaction intermediate of these compounds has been identified,
and a mechanism for the reaction has been suggested. Substituted 1,3,2λ4-oxazaborines gradually rearrange
into 1,2,4,3λ4-triazaborines at temperatures above 100 °C.
phenylborates react with â-enaminones in an entirely different
Introduction
way. The aim of this work is to study this reaction.
In earlier communications1-7 we published results on the
reactions of substituted benzenediazonium tetrafluoroborates and
hexafluorophosphates with â-enaminones.
Results and Discussion
Depending on the structure of the starting enaminone and
substitution in the benzenediazonium used, three situations have
been described so far.
1. The reaction takes place between only one molecule of
diazonium and one molecule of enaminone at the methine
R-carbon atom (the tautomeric form of the azo coupling product
depends particularly upon the structure of enaminone).1-3
2. The reaction takes place between two molecules of
diazonium and one molecule of enaminone at its two different
carbon atoms.4,5
3. The reaction takes place between two molecules of
diazonium and one molecule of enaminone at its two different
carbon atoms, and subsequently, the substituted pyridazinium
salt is formed6,7 via intramolecular cyclization.
The pyridazinium salts are formed in good yields, particularly
from substituted benzenediazonium hexafluorophosphates. We
wanted to find out what effect upon the reaction result would
be observed after replacing the hexafluorophosphate counterion
by another noncoordinating anion. We have found that tetra-
We have found that the reaction of 4-(methylamino)pent-3-
en-2-one (1) with 4-methoxybenzenediazonium tetraphenylbo-
rate in CH2Cl2 in the presence of remelted sodium acetate gives
a 58% yield of 5-((4-methoxyphenyl)diazenyl)-3,4,6-trimethyl-
2,2-diphenyl-1,3,2λ4-oxazaborine (9). The structure of com-
pound 9 was proved by means of X-ray diffraction (Figure 1).
The formation of oxazaborines is not limited to the aforemen-
tioned starting compounds: substituted 5-(phenyldiazenyl)-2,2-
diphenyl-1,3,2λ4-oxazaborines are also formed by the reaction
of â-enaminones 2-6 with 4-(dimethylamino)-, 4-methoxy-, and
4-methylbenzenediazonium tetraphenylborate as the only iso-
lated products (Scheme 1). Benzenediazonium tetraphenylbo-
rates are relatively stable only if the benzenediazonium ion
carries an electron-donor substituent or hydrogen. The literature8
also describes the preparation of 4-(ethoxycarbonyl)benzene-
diazonium tetraphenylborate, which is stable for a period of 2
min at 0 °C. Our attempt at preparing the 4-nitro derivative
failed. The molecules of starting â-enaminones can be varied:
the amino group present can be primary or secondary (N-methyl,
N-phenyl), the acetyl group can be replaced by benzoyl, and
the 5-methyl group can be replaced by benzyl. The oxazaborines
prepared were identified by means of X-ray diffraction and 1H,
13C, 15N, and 11B NMR spectra.
* To whom correspondence should be addressed. E-mail: petr.simunek@
upce.cz. Tel: +420 466 037 039. Fax: +420 466 037 068.
† University of Pardubice.
‡ Universita` di Ferrara.
§ Research Institute for Organic Syntheses (VUOS).
(1) Macha´cˇek, V.; Lycˇka, A.; Sˇimu˚nek, P.; Weidlich, T. Magn. Reson.
Chem. 2000, 39, 293.
The 1H and 13C NMR parameters are given in the Supporting
Information, and selected 11B and 15N NMR data of compounds
8-16 are presented in Table 1.
(2) Sˇimu˚nek, P.; Bertolasi, V.; Macha´cˇek, V. J. Mol. Struct. 2002, 642,
41.
The phenyl groups attached to boron in oxazaborines 8-10
and 12-16 are chemically equivalent, at least on the NMR time
scale in CDCl3. In the case of oxazaborine 11, which is
substituted at nitrogen N3 with the 2,4-dimethoxyphenyl group,
the two phenyl groups are chemically nonequivalent on the
NMR time scale, due probably to steric reasons. The nonequiva-
(3) Sˇimu˚nek, P.; Bertolasi, V.; Pesˇkova´, M.; Macha´cˇek, V.; Lycˇka, A.
Org. Biomol. Chem. 2005, 3, 1217.
(4) Sˇimu˚nek, P.; Lycˇka, A.; Macha´cˇek, V. Eur. J. Org. Chem. 2002,
2764.
(5) Sˇimu˚nek, P.; Bertolasi V.; Macha´cˇek, V.; Lycˇka, A. Org. Biomol.
Chem. 2003, 1, 3250.
(6) Sˇimu˚nek, P.; Pesˇkova´, M.; Bertolasi, V.; Lycˇka, A.; Macha´cˇek, V.
Eur. J. Org. Chem. 2004, 5055.
(7) Sˇimu˚nek, P.; Pesˇkova´, M.; Bertolasi, V.; Macha´cˇek, V.; Lycˇka, A.
Tetrahedron 2005, 61, 8130.
(8) Lindeman, S. V.; Kosynkin, D.; Kochi, J. K. J. Am. Chem. Soc. 1998,
120, 13268.
10.1021/om051078x CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/14/2006

2026 Organometallics, Vol. 25, No. 8, 2006
PesˇkoVa´ et al.
Scheme 1
Figure 1. ORTEP view of compound 9 displaying thermal
ellipsoids at 30% probability. Selected bond distances (Å):
B1-N1 ) 1.596(2), N1-C1 ) 1.304(2), C1-C2 ) 1.442(2),
C2-C3 ) 1.383(2), O1-C3 ) 1.307(2), B1-O1 ) 1.522(2). The
six-membered ring B1-N1-C1-C2-C3-O1 adopts a mixed boat-
twisted conformation, B4,1/3T1, with the following puckering
parameters:9 æ ) 171.9(2)°, Q ) 0.437(2) Å, and θ ) 107.9(2)°.
Scheme 2
Table 1. 11B and Selected 15N Chemical Shifts of the
Compounds 8-18 in CDCl3
chem shift (ppm)
compd
NR
Nâ
Nγ
B
8
9
88.4
85.5
85.6
86.9
not detected
99.5
not detected
not detected
-204.6
-200.9
-201.6
-196.0
1.8
3.7
1.4
3.6
3.5
3.5
4.1
1.9
10
11
12
13
14
15
16
17
18
Ima,b
not detected
95.3
-208.3
4.0
-152.7
-151.1
-197.6
27.5
-198.5
-205.9
-157.1
-1.9
-1.5
-2.4
27.5
the reverse sequence of these reaction steps was answered by
the finding that 3,4-dimethyl-2,2,6-triphenyl-1,3,2λ4-oxazaborine
(16) does not react with diazonium salt.
-12.8c
1
a Data measured in CD2Cl2 (see Scheme 3). b 1J(15NR, H) ) 96.1 Hz,
1J(15Nγ, 1H) ) 72.8 Hz. c From the 15N-enriched compound Im-15N (see
Scheme 3).
Hence, the first elementary step is the reaction of â-enami-
none with diazonium ion, which produces a protonated species
with the character of a Brønsted N acid (Scheme 3). The reaction
takes place in heterogeneous phases, the base used being sodium
acetate, which is virtually insoluble in the reaction medium
(CH2Cl2). The proton transfer from the nitrogen atom of the N
acid to the acetate ion through the phase interface is slower
than the reaction of this N acid with tetraphenylborate. The
reaction of tetraphenylborate anion with heterocyclic N acids
is known;11 it leads to the formation of stable isolable coordina-
tion compounds of the type HetN+-B-Ph3 and benzene. The
reaction obviously goes through a Wheland-type intermediate,12
and this is in SEAr referred to as protodeboronation. An
lence is manifested by formation of complex multiplets of
1
aromatic protons in the H NMR spectra.
It is known10 that substituted 1,3,2λ4-oxazaborines are formed
by the reaction of â-enaminones with diphenylborinic acid or
its derivatives. The question of whether in our case the first
process is formation of 3,4-dimethyl-2,2,6-triphenyl-1,3,2λ4-
oxazaborine (16) from 3-(methylamino)-1-phenylbut-2-en-1-one
(4) (Scheme 2), followed by azo coupling of compound 16, or
(9) Cremer, D.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 1354.
(10) Bally, I.; Ciornei, E.; Vasilescu, A.; Balaban, A. T. Tetrahedron
1973, 29, 3185.

5-(4-Substituted-phenyldiazenyl)-1,3,2λ4-oxazaborines
Organometallics, Vol. 25, No. 8, 2006 2027
Scheme 3
Scheme 4
analogous process can also be presumed in the reaction studied
by us (Scheme 3).
The reaction between enaminone 2 and 4-methoxybenzene-
diazonium tetraphenylborate in CD2Cl2 at room temperature was
followed in an NMR cell. After ca. 50 min, the starting
enaminone disappeared from the reaction mixture and the
intermediate Im was formed. The Im molecule contains an
acetyl group (δ(13CdO) 193.56), as was proven by means of
1H-13C HMBC. Its boron chemical shift, δ(11B) -2.4, corre-
sponds to a tetracoordinated boron atom.13 According to the
chemical shifts δ(15N) (Table 1), this intermediate could exist
in CD2Cl2 solution in its tautomeric hydrazone form Im-H
(Scheme 3). This conclusion is also supported by the presence
of two different NH protons, whose coupling constants 1J(1H-
15N) agree with the suggested structure of intermediate Im
(Table 1) as well. This intermediate could be isolated, but its
stability was not sufficient for purification and elemental
analysis. Within ca. 12 h the intermediate is transformed into
oxazaborine 8, both in solution and in the solid phase. The slow
transformation Im f 8 could also be monitored in the NMR
cell.
In the second step, the protodeboronation is repeated and
another benzene molecule is produced (Scheme 3). The reaction
mechanism suggested is supported also by the fact that 4-(meth-
ylamino)-3-((4-methylphenyl)diazenyl)pent-3-en-2-one (7) reacts
with the triphenylborane added and also gives 3,4,6-trimethyl-
5-((4-methylphenyl)diazenyl)-2,2-diphenyl-1,3,2λ4-oxazabo-
rine (10) and benzene (Scheme 2). The formation of benzene
was proved by the method of standard addition in an experiment
carried out directly in an NMR cell.
If the reaction of enaminones with diazonium tetraphenylbo-
rates is carried out in the presence of pyridine as a base instead
of sodium acetate (pyridine has comparable basicity, but it is
soluble in CH2Cl2), then oxazaborines are not formed. This is
obviously a consequence of the fact that the proton needed for
the protodeboronation is depleted by a far faster acid-base
reaction. In these cases the azo coupling reaction produces a
substance of type 7. On the other hand, oxazaborines are formed
even if no base is used. (Note: the presence of base is necessary
in the preparation of azo coupling products from â-enaminones
and diazonium salts, since the acid set free retards the reaction
until it finally stops.)14
Long-term heating (26 h) of 5-((4-methoxyphenyl)diazenyl)-
4,6-dimethyl-2,2-diphenyl-3H-1,3,2λ4-oxazaborine (8) in boiling
toluene results in its transformation into 6-acetyl-2-(4-meth-
oxyphenyl)-5-methyl-3,3-diphenyl-4H-1,2,4,3λ4-triazaborine (17)
(Scheme 4). The structure of this compound was first suggested
on the basis of the presence of an acetyl group in the NMR
spectra and was subsequently confirmed by means of X-ray
diffraction (Figure 2). The course of rearrangement of the
oxazaborine 8 into triazaborine 17 was monitored at a temper-
ature of 200 °C without solvent by the method of taking samples
and their analysis by means of 1H NMR (Figure 3). At enhanced
temperatures, oxazaborine 8 probably undergoes reversible B-O
(11) Kiviniemi, S.; Nissinen, M.; Alaviuhkola, T.; Rissanen, K.; Pursi-
ainen, J. J. Chem. Soc., Perkin Trans. 2 2001, 2364.
(12) Meisters, M.; Vandeberg, J. T.; Cassaretto, F. P.; Posvic, H.; Moore,
C. E. Anal. Chim. Acta 1970, 49, 481.
(13) Kennedy, J. D. in Multinuclear NMR; Mason, J., Ed.; Plenum
Press: New York, 1987; Chapter 8.
(14) Macha´cˇek, V.; Cˇ egan, A.; Halama, A.; Rozˇnˇavska´, O. Collect.
Czech. Chem. Commun. 1995, 60, 1367.

2028 Organometallics, Vol. 25, No. 8, 2006
PesˇkoVa´ et al.
Scheme 5
Experimental Section
NMR. The NMR spectra were measured at laboratory temper-
ature with a Bruker Avance 500 spectrometer equipped with a 5
mm broad-band probe with a gradient of magnetic field in the
direction of the z axis at frequencies of 500.13 MHz (1H), 125.77
MHz (13C), and 50.69 MHz (15N) and with a Bruker AMX 360
spectrometer at frequencies of 360.14 MHz (1H), 90.57 MHz (13C),
1
and 115.55 MHz (11B). The H NMR spectra were calibrated in
CDCl3 with hexamethyldisiloxane (δ 0.05) and in CD2Cl2 with the
central signal of the solvent multiplet (δ 5.32). The 13C NMR
spectra were calibrated with the central signal of the solvent
multiplet (δ 76.9 in CDCl3 and δ 54.0 in CD2Cl2). The carbon NMR
spectra were measured in the standard way and by means of the
APT pulse sequence (spectral width 26.455 kHz, acquisition time
1.238 s, zero filling to 64K, and line broadening 1 Hz prior to
Fourier transformation). The 15N NMR spectra were calibrated on
external neat 15N nitromethane placed in a coaxial capillary (δ 0.0).
The 11B NMR spectra were calibrated on external B(OCH3)3 placed
in a coaxial capillary (δ 18.1). To suppress the signals of 11B nuclei
from the 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 δ(15N) values
were measured with the help of techniques with inversion detection
(gradient selected 2D 1H-15N HMBC) processed in the magnitude
mode. The gradient ratios were 70:30:50.1. Experiments were
performed with an NH one-bond coupling of 90 Hz and NH long-
range coupling of 5 Hz, with 2k × 160 zero filled to 2k × 1k,
sine-bell squared in both dimensions. One-bond coupling constants
1H-15N of the compound Im-15N were measured by means of
Figure 2. ORTEP view of one molecule of the asymmetric unit
of compound 17 displaying thermal ellipsoids at 30% probability.
Selected bond distances (Å) for molecules A/B: B1-N1 )
1.567(4)/1.567(4), N1-C1 ) 1.300(4)/1.302(4), C1-C2 ) 1.436(4)/
1.441(4), C2-N2 ) 1.338(3)/1.336(3), N2-N3 ) 1.307(3)/
1.307(3), B1-N3 ) 1.588(4)/1.591(3). The six-membered rings
B1-N1-C1-C2-N2-N3, for the two independent molecules A
and B, adopt boat conformations 1,4B and B4,1, respectively, with
the following puckering parameters:9 æ ) -1.8(5)/179.8(4)°, Q )
0.341(3)/0.408(11) Å, and θ ) 70.5(4)/108.5(6)°.
1
gradient-selected 1D H-15N HMBC optimized for 90 Hz. The
spectral width was 10 kHz, with acquisition time 3.277 s and line
1
broadening of 3 Hz prior to Fourier transformation. H and 13C
NMR data of the compounds prepared are given in the Supporting
Information.
Crystallography. X-ray diffraction data for compounds 9-13
and 17 were collected at room temperature, 295 K, on a Nonius
Kappa CCD diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.7107 Å). The structures were solved by direct
methods (SIR97)16 and were refined (SHELXL-97)17 using full-
matrix least squares. ORTEP18 views are given in Figures 1 and 2
and in the Supporting Information.
Crystal data for 9: C25H26BN3O2, triclinic, space group P1h, a )
7.5283(2) Å, b ) 10.7694(4) Å, c ) 14.2237(5) Å, R ) 91.562(2)°,
â ) 101.592(1)°, γ ) 97.572(2)°, V ) 1118.10(6) Å3, Z ) 2,
Dc ) 1.222 g cm-3, intensity data collected with θ e 30°,
6454 independent reflections measured, 3324 observed reflections
(I > 2σ(I)), final R index 0.0539 (observed reflections), Rw ) 0.1481
(all reflections), S ) 0.993.
Figure 3. 1H NMR analysis of the thermal rearrangement of 8 in
CD2Cl2. For details see the Experimental Section.
bond splitting followed by irreversible formation of a B-N bond
with the â-nitrogen atom of the azo group (Scheme 4). The
thermal rearrangement of a heterocycle with an O-B-N
grouping into an N-B-N type is known15 (Scheme 5). The
formation of triazaborine is probably controlled thermodynami-
cally.
Crystal data for 17: C24H24BN3O2, monoclinic, space group P21,
a ) 10.2803(4) Å, b ) 12.4823(5) Å, c ) 17.2317(8) Å, â )
96.390(2)°, V ) 2197.5(2) Å3, Z ) 4, Dc ) 1.201 g cm-3, intensity
In analogy to oxazaborine 8, also oxazaborine 15 undergoes
a rearrangement, giving the triazaborine 18 (Scheme 4). The
oxazaborines substituted at the 3-nitrogen give mixtures of prod-
ucts under the same conditions. For the NMR parameters of
the triazaborines, see the Supporting Information and Table 1.
(16) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 1999, 32, 115.
(17) Sheldrick, G. M. SHELXL-97, Program for Refinement of Crystal
Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1996.
(18) Burnett, M. N.; Johnson, C. K. ORTEP-III; Report ORNL-6895;
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
(15) Dorokhov, V. A.; Lavrinovich, L. I.; Botchkareva, M. N.; Zolotarev,
B. M.; Mikhailov, B. M. IzV. Akad. Nauk SSSR, Ser. Khim. 1978, 253.

5-(4-Substituted-phenyldiazenyl)-1,3,2λ4-oxazaborines
Organometallics, Vol. 25, No. 8, 2006 2029
data collected with θ e 28°, 5416 independent reflections measured,
4026 reflections observed (I > 2σ(I)), final R index 0.0481
(observed reflections), Rw ) 0.1250, and S ) 1.022. The asymmetric
unit is built up by the two independent molecules A and B.
Complete crystallographic data (excluding structural factors) for
the six structures in this paper have been deposited at the Cambridge
Crystallographic Data Centre and allocated the deposition numbers
CCDC 292282-292287. Copies of the data can be obtained free
of charge via WWW.ccdc.cam.ac.uk/conts/retrieving.html or on
application to the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. (fax, +44(0)-1223-336033; e-mail, deposit@ccdc.cam.ac.uk).
General Considerations. Melting points were determined with
a Kofler hot stage microscope and were not corrected. The elemental
analyses were carried out with a FISONS EA 1108 automatic
analyzer.
Dichloromethane was predried by treatment with anhydrous
calcium chloride and subsequent distillation with phosphorus
pentoxide. The anhydrous sodium acetate was fused on a porcelain
dish and left to cool in a desiccator.
4-(Methylamino)pent-3-en-2-one19 (1), 4-aminopent-3-en-2-one20
(2), 4-((2,4-dimethoxyphenyl)amino)pent-3-en-2-one7 (3), and
3-(methylamino)-1-phenylbut-2-en-1-one21 (4) were prepared ac-
cording to the literature procedures.
diisopropyl ether (30 mL) and the enaminone 1 (5 mmol). The
reaction mixture was stirred by means of a magnetic stirrer, and
remelted sodium acetate (1.23 g, 15 mmol) was added, followed
by 4-methylbenzenediazonium tetrafluoroborate4 (5 mmol). The
mixture was stirred at room temperature for 28 h and then filtered.
The filter cake was washed with diisopropyl ether, and the filtrate
was evaporated in a vacuum evaporator. After recrystallization from
cyclohexane, 7 was obtained: yield 68%; mp 113-115 °C. Anal.
Found: C, 67.73; H, 7.69; N, 18.45. Calcd for C13H17N3O: C,
67.51; H, 7.41; N, 18.17.
The substituted benzenediazonium chlorides were prepared by
a known way.4 The diazonium chlorides were transformed into the
corresponding diazonium tetraphenylborates by adding a saturated
aqueous solution of an equimolar amount of sodium tetraphenylbo-
rate. Because of their low stability, the diazonium tetraphenylborates
had to be prepared immediately before use.
General Procedure for the Synthesis of 5-Aryldiazenyl-
1,3,2λ4-oxazaborines. Remelted sodium acetate (1.23 g, 15 mmol)
and the respective benzenediazonium tetraphenylborate (5 mmol)
were added to a solution of enaminone (5 mmol) in dichloromethane
(30 mL) with stirring. The reaction mixture was stirred at room
temperature for 24 h, whereupon the solids were collected by
suction on a sintered-glass filter and the filter cake was washed
with dichloromethane. The filtrate was evaporated under vacuum,
and the evaporation residue was either recrystallized or submitted
to column chromatography. The following compounds were
prepared by the procedure described.
5-((4-Methoxyphenyl)diazenyl)-4,6-dimethyl-2,2-diphenyl-3H-
1,3,2λ4-oxazaborine (8). The title compound was obtained in 58%
yield (1.56 g) as yellow crystals after recrystallization in cyclo-
hexane-toluene: mp 160-163 °C. Anal. Found: C, 72.28; H, 5.88;
N, 10.59. Calcd for C24H24BN3O2: C, 72.56; H, 6.09; N, 10.58.
5-((4-Methoxyphenyl)diazenyl)-3,4,6-trimethyl-2,2-diphenyl-
1,3,2λ4-oxazaborine (9). The title compound was obtained in 50%
yield (0.89 g) as yellow crystals after recrystallization in cyclo-
hexane-toluene: mp 185-187 °C. Anal. Found: C, 72.96; H, 6.41;
N, 10.03. Calcd for C25H26BN3O2: C, 73.00; H, 6.37; N, 10.22.
3,4,6-Trimethyl-5-((4-methylphenyl)diazenyl)-2,2-diphenyl-
1,3,2λ4-oxazaborine (10). Method A (from the Diazonium Salt).
This compound was obtained as a yellow crystalline solid after
column chromatography (silica gel, chloroform-ethyl acetate
4:1): recrystallization from cyclohexane-toluene; yield 60% (0.94
g); mp 179-182 °C. Anal. Found: C, 76.14; H, 6.71; N, 10.64.
Calcd for C25H26BN3O: C, 75.96; H, 6.63; N, 10.63.
Method B (from Triphenylborane). A flask equipped with a
calcium chloride drying tube and a dropping funnel was used to
dissolve 4-(methylamino)-3-((4-methylphenyl)diazenyl)pent-3-en-
2-one (7; 0.19 g, 0.82 mmol) in dichloromethane (10 mL) at room
temperature. The solution obtained was treated with a solution of
triphenylborane (0.2 g, 0.82 mmol) in dichloromethane (10 mL)
added from the dropping funnel. The solution was stirred for ca.
24 h, whereupon the solvent was evaporated under vacuum. The
crude evaporation residue was recrystallized from ethanol: yield
25% (0.08 g); mp 179-182.5 °C.
3-(2,4-Dimethoxyphenyl)-5-((4-methoxyphenyl)diazenyl)-4,6-
dimethyl-2,2-diphenyl-1,3,2λ4-oxazaborine (11). This compound
was obtained as a yellow crystalline solid after column chroma-
tography (silica gel, chloroform) and recrystallization from cyclo-
hexane: yield 32%; mp 151-154 °C. Anal. Found: C, 71.78; H,
5.85; N, 8.18. Calcd for C32H32BN3O4: C, 72.05; H, 6.05; N, 7.88.
5-((4-Methoxyphenyl)diazenyl)-3,4-dimethyl-2,2,6-triphenyl-
1,3,2λ4-oxazaborine (12). The title compound was obtained in 27%
yield (0.74 g) as orange crystals after recrystallization (cyclohex-
ane-toluene): mp 211-213 °C. Anal. Found: C, 76.02; H, 5.98;
N, 8.80. Calcd for C30H28BN3O2: C, 76.12; H, 5.96; N, 8.88.
3,4-Dimethyl-5-(4-((dimethylamino)phenyl)diazenyl)-2,2,6-tri-
phenyl-1,3,2λ4-oxazaborine (13). This compound was obtained as
3-(Methylamino)-1,4-diphenylbut-2-en-1-one (5). 1,4-Diphen-
ylbutane-1,3-dione (6.5 g, 28 mmol) and a 33% solution of
methylamine in absolute ethanol (50 mL) was refluxed for 10 h
and cooled, and the separated product was collected by suction.
After recrystallization from cyclohexane, 5 was obtained: yield
3.74 g (49%); mp 51-55 °C. Anal. Found: C, 81.26; H, 7.08; N,
5.74. Calcd for C17H17NO: C, 81.21; H, 6.82; N, 5.57.
1,4-Diphenylbutane-1,3-dione. A 500 mL three-necked flask
equipped with a dropping funnel, thermometer, and condenser with
calcium chloride drying tube was charged with dry ether (70 mL)
and sodium amide (11.7 g, 0.3 mol). The suspension was stirred
and, within 10 min, treated with acetophenone (18 g, 0.15 mol) in
dry ether (25 mL), added drop by drop. After 5 min, methyl
phenylacetate (45 g, 0.3 mol) was added. The reaction mixture was
stirred and refluxed on a water bath for another 2 h. The jellylike
reaction mixture was poured onto crushed ice (150 g) with
concentrated hydrochloric acid (30 mL). The ether phase was
separated and extracted first with sodium carbonate solution (3 ×
100 mL) and then with water (3 × 100 mL). The organic phase
was dried with anhydrous sodium sulfate, and the ether was distilled
off. The evaporation residue was dissolved in methanol (60 mL),
and a hot solution of copper(II) acetate hydrate (30 g, 0.15 mol) in
water (175 mL) was added into it through a filter. After the mixture
was cooled to room temperature, the copper(II) salt of the
â-diketone was collected by suction, washed with petroleum ether,
and dried. The salt was placed in a 500 mL Erlenmeyer flask
together with 30% sulfuric acid (250 mL) and ether (100 mL). The
mixture was stirred by means of a magnetic stirrer until the salt
dissolved. The ether was separated, and the aqueous phase was
extracted again with ether (3 × 50 mL). The combined ether phases
were shaken with sodium carbonate solution (3 × 50 mL) and dried
with anhydrous sodium sulfate. The ether was evaporated, and the
separated crystals were recrystallized from ethanol: yield 9.73 g
(24%) of â-diketone; mp 48-50 °C (lit.22 mp 50-51 °C).
4-Amino-4-phenylbut-3-en-2-one (6) was prepared by a known
procedure.3
4-(Methylamino)-3-((4-methylphenyl)diazenyl)pent-3-en-2-
one (7). A 100 mL Erlenmeyer flask was charged with dry
(19) Jirkovsky, I. Can. J. Chem. 1974, 52, 55.
(20) Kazitsina, L. A.; Kupletskaya, N. B.; Polstyanko, L. L.; Kikot, B.
S.; Kolesnik, J. A.; Terentev, A. P. Zh. Obshch. Khim. 1961, 31, 313.
(21) Ribeiro DaSilva, M. A. V.; Ribeiro DaSilva, M. D. M. C.; Paiva, J.
P. A.; Nogueira, I. M. C. S.; Daman, A. M.; Barkley, J. V.; Gardiny, M.
M.; Akello, M. J.; Pilcher, G. J. Chem. Soc., Perkin Trans. 2 1993, 1765.
(22) Chen, R.; Wu, H.; Zhang, Y. J. Chem. Res., Synop. 1999, 11, 666.

2030 Organometallics, Vol. 25, No. 8, 2006
PesˇkoVa´ et al.
an orange crystalline solid after column chromatography (silica gel,
chloroform-ethyl acetate 4:1) and recrystallization from toluene-
cyclohexane: yield 19% (0.48 g); mp 199.5-203 °C. Anal.
Found: C, 76.52; H, 6.49; N, 11.29. Calcd for C31H31BN4O: C,
76.55; H, 6.42; N, 11.52.
4-Benzyl-5-((4-methoxyphenyl)diazenyl)-3-methyl-2,2,6-tri-
phenyl-1,3,2λ4-oxazaborine (14). This compound was obtained as
a sand-colored crystalline solid after heating of the crude reaction
mixture in ethanol and recrystallization from a cyclohexane-toluene
mixture: yield 14%; mp 198-200 °C. Anal. Found: C, 78.72; H,
5.74; N, 7.63. Calcd for C36H32BN3O2: C, 78.69; H, 5.87; N, 7.65.
5-((4-Methoxyphenyl)diazenyl)-6-methyl-2,2,4-triphenyl-3H-
1,3,2λ4-oxazaborine (15). The title compound was obtained in 43%
yield as yellow crystals after column chromatography (silica gel/
CH2Cl2) and recrystallization (petroleum ether): mp 119-123 °C.
Anal. Found: C, 75.92; H, 5.95; N, 9.11. Calcd for C29H26BN3O2:
C, 75.83; H, 5.71; N, 9.15.
3,4-Dimethyl-2,2,6-triphenyl-1,3,2λ4-oxazaborine (16). This
compound was prepared from 3-(methylamino)-1-phenylbut-2-en-
1-one (4) and triphenylborane by the procedure B for compound
10. Recrystallization from ethanol gave a yellow crystalline
compound: yield 0.32 g (38%); mp 178-184 °C. Anal. Found:
C, 81.48; H, 6.71; N, 4.25. Calcd for C23H22BNO: C, 81.43; H,
6.54; N, 4.13.
6-Acetyl-2-(4-methoxyphenyl)-5-methyl-3,3-diphenyl-4H-1,-
2,4,3λ4-triazaborine (17). Method A. Oxazaborine 8 (1.04 g, 2.6
mmol) was heated in boiling toluene (20 mL) for 26 h. The toluene
was distilled off, and the evaporation residue was submitted to
chromatography (silica gel/CH2Cl2). The chromatography gave 31%
of the starting material and 52% of the product 17 as an orange
crystalline solid: mp 210.5-211 °C. Anal. Found: C, 72.70; H,
6.23; N, 10.52. Calcd for C24H24BN3O2: C, 72.56; H, 6.09; N,
10.58.
submitted to column chromatography (silica gel/CH2Cl2) to give
0.46 g (84%) of an orange substance: mp 213-215 °C.
6-Acetyl-2-(4-methoxyphenyl)-3,3,5-triphenyl-4H-1,2,4,3λ4-
triazaborine (18). Oxazaborine 15 (0.26 g, 0.57 mmol) was heated
in a vial at 200 °C for a period of 40 min. The mixture was
submitted to column chromatography (silica gel/CH2Cl2) to give
0.18 g (69%) of a dark yellow substance: mp 195-198 °C. Anal.
Found: C, 76.08; H, 5.99; N, 8.89. Calcd for C29H26BN3O2: C,
75.83; H, 5.71; N, 9.15.
Preparation of intermediate Im. The intermediate was prepared
immediately before the NMR spectral measurement: enaminone 2
(0.29 g, 2.97 mmol) and 4-methoxybenzenediazonium tetraphen-
ylborate (1.35 g, 2.97 mmol) were stirred with dichloromethane
(20 mL) for 55 min. The reaction mixture was filtered with suction,
and the solvent was evaporated under vacuum without heating. The
evaporation residue was used for the measurements without further
purification. For the spectral characteristics see Table 1 and the
text.
Preparation of the Intermediate Im-15N. This substance was
prepared in the same way as that adopted for Im, using the
diazonium salt prepared from sodium nitrite (95% 15N).
Acknowledgment. We thank the Czech Science Foundation
(Grant No. 203/03/0356) and the Ministry of Education, Youth
and Sports of the Czech Republic (Grant No. MSM0021627501)
for financial support.
Supporting Information Available: Text and figures giving
crystallographic data and ORTEP diagrams of compounds 10-13
and NMR data for all the prepared compounds; crystallographic
data are also available as CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
Method B. A 0.55 g portion (1.38 mmol) of oxazaborine 8 was
heated in a vial at 200 °C for a period of 60 min. The mixture was
OM051078X