Contribution to the reactivity of N,N′-diaryl-1,4-diazabutadienes aryl-N=CH-CH=N-Aryl (aryl = 2,6-dimethylphenyl; 2,4,6-trimethylphenyl) towards boron trichloride

Article abstract of DOI:10.1002/ejic.200600728

The reaction of Aryl-N=CH-CH=N-Aryl (3a: Aryl = 2,6-Me₂C ₆H₃; 3b: Aryl = 2,4,6-Me₃C₆H ₂) with 2 equiv. BCl₃ in a toluene/hexane mixture at -50°C led to an inseparable mixture of

Full text of DOI:10.1002/ejic.200600728

FULL PAPER
DOI: 10.1002/ejic.200600728
Contribution to the Reactivity of N,NЈ-Diaryl-1,4-diazabutadienes Aryl–
N=CH–CH=N–Aryl (Aryl = 2,6-Dimethylphenyl; 2,4,6-Trimethylphenyl)
Towards Boron Trichloride
Lothar Weber,*[a] Jan Förster,[a] Hans-Georg Stammler,[a] and Beate Neumann[a]
Keywords: Diazabutadienes / Diazaboroles / Aminoboranes / Structures / Boron
The reaction of Aryl–N=CH–CH=N–Aryl (3a: Aryl = 2,6-
Me2C6H3; 3b: Aryl = 2,4,6-Me3C6H2) with 2 equiv. BCl3 in a
toluene/hexane mixture at –50 °C led to an inseparable mix-
ture of borolium salts [Aryl–Na=CH–CH=N(Aryl)BCl2(Na–
B)]+BCl4– (4a: Aryl = 2,6-Me2C6H3; 9a: Aryl = 2,4,6-Me3C6H2)
3a and 3b were slowly combined with 2 equiv. BCl3 in the
same solvents at –78 °C. The acyclic adducts Cl3BN(Aryl)=
CH–CH=N(Aryl)BCl3 (7a: Aryl = 2,6-Me2C6H3; 7b: Aryl =
2,4,6-Me3C6H2) were reduced with sodium amalgam to fur-
nish the aminoboranes Cl2BN(Aryl)CH=CH–N(Aryl)BCl2 (8:
Aryl = 2,6-Me2C6H3; 12: Aryl = 2,4,6-Me3C6H2). Stirring solu-
tions of 8 and 12 in the presence of CaH2 cleanly gave the
diazaboroles 1 and 2, respectively. The novel compounds
–
and [Aryl–Na=CH–C(Cl)=N(Aryl)BCl2(Na–B)]+BCl4 (4b: Aryl
= 2,6-Me2C6H3; 9b: Aryl = 2,4,6-Me3C6H2) and the bicyclic
species [HCa=N(Aryl)BCl2N(Aryl)–Cb=(Ca–Cb)]2 (5: Aryl =
2,6-Me2C6H3; 10: Aryl = 2,4,6-Me3C6H2). Sodium amalgam
reduction of borolium salts 4a,b and 9a,b afforded insepa-
rable mixtures of the diazaboroles 2,6-Me2C6H3NaCH=CR–
Nb(2,6-Me2C6H3)BCl(Na–B) (1: R = H; 6: R = Cl) and 2,4,6-
Me3C6H2NaCH=CR–Nb(2,4,6-Me3C6H2)BCl(Na–B) (2: R = H;
11: R = Cl). In order to obtain pure 1 and 2, diazabutadienes
1
were characterized by elemental analyses and H-, 11B-, 13C
NMR and mass spectra. The molecular structures of 1, 4a, 5
and 8 were elucidated by single X-ray diffraction analyses.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
The synthesis and reactivity of 1,3-dialkyl-2-halo-2,3-di-
hydro-1H-1,3,2-diazaboroles I as well as their molecular
structures have been thoroughly investigated and documen-
ted in the literature.[1–12] More recently, the electrochemical
and optophysical properties of such compounds have at-
tracted our interest. The electrochemical oxidation of 1,3-
di-tert-butyl-2,3-dihydro-1H-1,3,2-diazaboroles by cyclic
voltammetry displays clean but irreversible waves, and the
oxidation potentials Eox,1/2 vary strongly with the substitu-
tion pattern at the boron atom.[13] Diazaboroles that are
functionalized at the boron center by thienyl-, dithienyl- or
biphenyl groups are highly luminescent, emitting bright
blue light upon UV irradiation.[14,15] With respect to this,
we planned to extend the scope of our diazaboroles to rep-
resentatives with aryl groups at both ring nitrogen atoms.
In an early paper, we described the preparation of hetero-
cycle II by treatment of diacetyldianil with methylboron di-
bromide and subsequent reduction of the obtained borol-
ium salt with an excess of sodium amalgam.[3]
Later this route was followed to synthesize the 2-chloro-
1,3-dixylyl-2,3-dihydro-1H-1,3,2-diazaborole (1).[10] An al-
ternative route to 1 and to the corresponding 2-bromo de-
rivative was based on the reduction of the corresponding
diazabutadiene with lithium metal and the cyclocondensa-
tion of the dilithiated species with BX3 (X = Cl, Br).[10] In
this contribution we report on the reaction of N,NЈ-diaryl-
1,4-diazabutadienes with BCl3. It was found that the reac-
tivity of the N,NЈ-diaryl-1,4-diazabutadienes towards BCl3
is far less straightforward than that of N,NЈ-di-tert-butyl-
1,4-diazabutadiene.
Results and Discussion
For the synthesis of 1, it was obvious to follow the pro-
cedure that we previously elaborated.[10] In only one in-
stance did the combination of diazabutadiene 3a with two
molar equivalents of boron trichloride in a mixture of tolu-
ene and hexane at –50 °C and subsequent stirring for 15 h
[a] Fakultät für Chemie der Universität Bielefeld,
Universitätsstrasse 25, 33615 Bielefeld, Germany
Fax: +49-521-106-6146
E-mail: lothar.weber@uni-bielefeld.de
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Reactivity of N,NЈ-Diaryl-1,4-diazabutadienes Towards Boron Trichloride
FULL PAPER
at 20 °C lead to the precipitation of the burgundy-red borol- pure salt 4a gave a sample of pure 1 in one case, this syn-
ium salt 4a in about 70% yield. In all the other cases, how- thetic pathway was discarded because of its poor reprodu-
ever, a black solid precipitated, which was identified as an cibility (Scheme 1).
inseparable mixture of 4a and its analogue 4b with a chlo-
Slow combination (4–5 h) of a toluene solution of 3a
rine substituent at a ring carbon atom (about 30% yield). with a cold (–78 °C) solution of BCl3 in pentane in a molar
After filtration, the mother liquor was placed for 14 d at ratio of 1:2 gave a precipitate of the red-brown adduct 7a
+4 °C, and compound 5 separated as deep red crystals in (60% yield). A slurry of this material in n-pentane was
18% yield. When the supernatant solution was decanted treated with sodium amalgam at 20 °C. From the superna-
and concentrated to one-third of its volume, impure diaza- tant solution, the acyclic doubly borylated (E)-N,NЈ-diami-
borole 6 was isolated after storing this solution for 2 d at noethene 8 was isolated as colorless crystals in about 70%
–4 °C. Recrystallization from pentane led to a few crystals yield. Stirring a solution of 8 in pentane in the presence of
of pure compound 6, which is chlorinated at the C=C CaH2 led to cyclization and the formation of diazaborole 1
double bond.
(46% yield) (Scheme 2).
The usual reduction of the mixture of 4a and 4b with
The reaction of diazabutadiene 3b with 2 equiv. BCl3 at
sodium amalgam in hexane afforded an inseparable mixture –50 °C in a toluene/hexane mixture was performed analo-
of the diazaboroles 1 and 6. A 1/6 ratio of 10:1 was deter- gously. A 1:1 mixture of the borolium salts 9a (R = H) and
1
mined by H NMR spectroscopy. Although the reaction of 9b (R = Cl) precipitated as a black solid (about 20% yield).
Scheme 1.
Scheme 2.
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L. Weber, J. Förster, H.-G. Stammler, B. Neumann
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The mother liquor was concentrated and stored at +4 °C duced with sodium amalgam to furnish, near quantitatively,
for 14 d, after which product 10 separated as deep green the expected diazaborole 15 (Scheme 5).
crystals in 14% yield (Scheme 3).
Because of the pronounced thermolability of the 2-
In the 11B NMR spectrum of the mother liquor, a small chloro-1,3,2-diazaborole 15, this compound was converted
singlet at δ = 21.8 ppm indicates small amounts of a 1,3,2- immediately into the more stable 2-cyano-1,3,2-diazaborole
diazaborole. Its isolation, however, failed. The reduction of 16 by reaction with silver cyanide in acetonitrile (Scheme 5).
9a,b with sodium amalgam led to an inseparable mixture of
The 11B NMR spectra of the borolium salts show singlets
the diazaboroles 2 and 11. The generation of pure 2 first (δ = 7.7 ppm) in the typical region for tetracoordinate bo-
required the preparation of the violet adduct 7b from the ron atoms. For the tetracoordinate boron atoms in the binu-
reaction of 3b and 2 equiv. BCl3 at –78 °C for 4–5 h (70% clear compounds 5 and 10, singlets are observed at δ = 8.6
yield). The sodium amalgam reduction of 7b afforded the and 8.5 ppm, respectively. The 2-chloroboroles 1, 2, 6 and
acyclic aminoborane 12, which upon treatment with CaH2 11 give rise to 11B NMR signals at δ = 21.1–21.8 ppm. An
cyclized to 2 in 54% yield (Scheme 4).
influence of the Cl substituent at the C=C bond is not vis-
In order to suppress undesired side reactions at the car- ible. In the 11B NMR spectra of 2-cyanoborole 16, the 11B
bon atom of the heterocycle, diacetyl[bis(2,6-dimethylphen- resonance appears as a singlet at δ = 13.5 ppm. The 11B
ylimine)] 13 was subjected to reaction with 2 equiv. BCl3 in NMR resonances of the acyclic aminodichloroboranes 8
n-hexane. The green borolium salt 14 (yield 63%) was re- and 12 are at δ = 32.5 ppm. The signals for the ring protons
Scheme 3.
Scheme 4.
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Reactivity of N,NЈ-Diaryl-1,4-diazabutadienes Towards Boron Trichloride
FULL PAPER
Scheme 5.
in the diazaboroles 1, 2, 6 and 11 are observed in the range nitrogen atoms (interplanar angle between the heterocycle
from 5.51 to 5.93 ppm. The protons at the 1,6-diazahexatri- and arene ring φ = 82.2°). A C2 axis bisects the molecule
ene backbone of 5 and 10 are assigned as singlets at δ = along the B(1)–Cl(1) vector. The bond length B(1)–Cl(1)
6.85 ppm. In the (E)-configured diaminoethenes 7 and 12, [1.768(2) Å] is close to the sum of the covalent radii of bo-
the protons at the double bond give rise to singlets at δ =
6.40 ppm.
In the 13C{1H} NMR spectra of diazaboroles 1 and 2,
singlets at δ = 117.7 and 117.8 ppm, respectively, are as-
signed to the identical carbon atoms of the C=C group of
the rings. The corresponding carbon atoms in the chlori-
nated derivative 6 give rise to two singlets at δ(13C) = 113.7
and 126.8 ppm. The carbon atoms of the N2C4 backbone
of 5 and 10 are attributed to singlets at δ = 161.3 and
170.3 ppm, respectively.
Figure 1. Molecular structure of 1 in the crystal; H atoms have
X-ray Structural Analysis of 1
been omitted for clarity. Selected bond lengths [Å] and angles [°]:
B(1)–Cl(1) 1.768(2), B(1)–N(1) 1.419(1), N(1)–C(1) 1.401(2),
Single crystals of 1 were grown from the crude reaction
C(1)=C(1A) 1.349(2), N(1)–C(2) 1.433(2); N(1)–B(1)–N(1A)
mixture at +4 °C. The molecular structure of 1 (Figure 1,
106.9(1), N(1)–B(1)–Cl(1) 126.6(1), B(1)–N(1)–C(1) 107.2(1), B(1)–
Table 1) features a planar 1,3,2-diazaborole ring with two
N(1)–C(2) 130.3(1), C(1)–N(1)–C(2) 122.5(1), N(1)–C(1)–C(1A)
almost orthogonally oriented ortho-xylyl substituents at the 109.4(1).
Table 1. Crystal data and collection parameters.
1
4a
5
8
Empirical formula
Mr [mgmol–1]
Crystal dimensions [mm]
Crystal system
Space group
a [Å]
C18H20BClN2
310.62
0.30ϫ0.12ϫ0.06 0.24ϫ0.17ϫ0.04
monoclinic
C2/c
14.5730(4)
7.4850(2)
16.6840(4)
114.9510(16)
1650.02(7)
4
C18H20B2Cl6·0.5C7H8
C36H38B2Cl4N4
690.12
0.15ϫ0.15ϫ0.14
monoclinic
P21/n
10.3100(3)
14.8210(3)
11.4650(4)
93.6320(14)
1748.39(9)
2
1.311
0.371
C18H20B2Cl4N2
427.78
0.30ϫ0.30ϫ0.24
monoclinic
P21/n
7.54300(10)
14.4950(2)
10.1860(2)
109.3190(10)
1050.98(3)
2
544.75
monoclinic
C2/c
26.9130(6)
10.7970(2)
17.6110(3)
90.8770(12)
5116.80(17)
8
b [Å]
c [Å]
β [°]
V [Å3]
Z
ρcalcd [mgm–3]
1.250
0.229
1.414
0.685
1.352
0.568
µ [mm–1]
F(000)
Θ [°]
656
3–30
2232
3–27.5
720
3–27.5
440
3–30
Collected reflections
Unique reflections
R(int)
Refined parameters
Gof
22930
2417
0.043
141
49724
5848
0.052
293
21040
4002
0.040
262
28834
3061
0.030
120
1.035
1.020
1.017
1.045
RF [I Ͼ 2σ(I)]
0.0402
0.1125
0.281/–0.352
–
0.0311
0.0788
0.279/–0.248
disorder of toluene on an
inversion centre
616596
0.0354
0.0923
0.0261
0.0708
0.327/–0.310
–
2
wRF [all data]
∆ρmax/min [eÅ–3]
0.274/–0.265
disorder of hydrogens at C(17)
and C(18)
Remarks
CCDC number
616595
616597
616598
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L. Weber, J. Förster, H.-G. Stammler, B. Neumann
FULL PAPER
ron (0.81 Å) and chlorine (0.99 Å)[16] and compares well
with the corresponding value in solid B2Cl4 (1.75 Å).[17]
Atomic distances and valence angles within the diazaborole
ring are in good agreement with the corresponding data for
XylNa–CH=CH–Nb(Xyl)BI(Na–B)[10] and need no particu-
lar discussion at this point.
X-ray Structural Analysis of 4a
The carbon–carbon bond length in the cation
[1.477(2) Å] indicates a bond order of unity, whereas in di-
azaboroles, double bonds [1.362(8) Å in 1] are typical. In
comparison with 1 [106.9(4)°], the angle at the boron atom
is compressed to 95.8(1)°, whereas the endocyclic angles
B(1)–N(1)–C(1) [110.7(1)°], B(1)–N(1)–C(2) [111.5(1)°],
N(1)–C(1)–C(2) [110.7(1)°] and N(2)–C(2)–C(1) [110.6(1)°]
are slightly more obtuse than that in 1 [107.2(1), 109.4(1)°].
Red plates of 4a suitable for an X-ray diffraction study
(Table 1) were grown from the toluene/hexane mother
liquor after removal of solid 4a. The analysis features a
planar diazaborolium cation [sum of bond angles
539.3°(1)], in addition to a tetrachloroborate anion (Fig-
ure 2).
X-ray Structural Analysis of 5
The X-ray analysis (Figure 3, Table 1) discloses a mole-
cule in which two planar BN2C2 rings (sum of angles
539.7°) are fused by an elongated C(1)–C(1A) multiple
bond [1.398(3) Å] (C=Ccalcd. = 1.33 Å) with an inversion
center in the middle of the bond. The bond C(1)–N(1)
[1.387(2) Å] is between the double bond N(2)=C(2)
[1.300(2) Å] on the one hand and the single bonds N(1)–
C(3) [1.440(2) Å] and N(2)–C(11) [1.447(2) Å] on the other
hand. The distance C(1)–C(2) [1.437(2) Å] corresponds to a
single bond.
Figure 2. Molecular structure of 4a in the crystal; H atoms have
been omitted for clarity. Selected bond lengths [Å] and angles [°]:
B(1)–Cl(1) 1.813(2), B(1)–Cl(2) 1.791(2), B(2)–Cl(3) 1.872(2), B(2)–
Cl(4) 1.830(2), B(2)–Cl(5) 1.844(2), B(2)–Cl(6) 1.866(2), B(1)–N(1)
1.615(2), B(1)–N(2) 1.594(2), N(1)=C(1) 1.276(2), N(1)–C(3)
1.462(2), N(2)=C(2) 1.279(2), N(2)–C(11) 1.464(2), C(1)–C(2)
1.477(2); N(1)–B(1)–N(2) 95.8(1), B(1)–N(1)–C(1) 110.7(1), B(1)–
N(1)–C(3) 127.4(1), B(1)–N(2)–C(2) 111.5(1), B(1)–N(2)–C(11)
125.9(1), C(1)–N(1)–C(3) 121.2(1), C(2)–N(2)–C(11) 121.7(1),
N(1)–C(1)–C(2) 110.7(1), N(2)–C(2)–C(1) 110.6(1), Cl(1)–B(1)–
Cl(2) 113.3(1), B(2)–Cl(3–6) 107.4(1)–111.2(1).
There are no bonding contacts between the cation and
the anion. The bond lengths B(1)–Cl(1) [1.813(2) Å] and
B(1)–Cl(2) [1.791(2) Å] are slightly shorter than the B–Cl
Figure 3. Molecular structure of 5 in the crystal. Selected bond
lengths [Å] and angles [°]: B(1)–Cl(1) 1.840(2), B(1)–Cl(2) 1.878(2),
B(1)–N(1) 1.518(2), B(1)–N(2) 1.576(2), N(1)–C(1) 1.387(2), N(1)–
C(3) 1.440(2), N(2)=C(2) 1.300(2), N(2)–C(11) 1.447(2), C(1)–C(2)
1.437(2), C(1)=C(1A) 1.398(3); Cl(1)–B(1)–Cl(2) 106.6(1), N(1)–
B(1)–N(2) 98.5(1), B(1)–N(1)–C(1) 111.4(1), B(1)–N(2)–C(2)
110.2(1), B(1)–N(1)–C(3) 125.4(1), B(1)–N(2)–C(11) 129.1(1),
N(1)–C(1)–C(2) 107.0(1), N(1)–C(1)–C(1A) 128.7(2), N(2)–C(2)–
C(1) 112.6(1), C(2)–N(2)–C(11) 120.3(1).
–
bonds in BCl4 , which range from 1.830(2) to 1.872(2) Å.
The boron–nitrogen bond lengths [B(1)–N(1) 1.615(2) Å,
B(1)–N(2) 1.594(2) Å] slightly exceed the average value for
the B–N single bond (1.59 Å) found in aminoboranes[18]
and compare well with the corresponding bond lengths in
cation III [1.607(3) and 1.611(3) Å].[19]
In 1,3,2-diazaboroles B–N bond lengths range from 1.40
The boron–nitrogen bond B(1)–N(1) [1.518(2) Å] reflects
to 1.45 Å. The endocyclic C–N bond lengths [C(1)–N(1) a single bond between a tetracoordinate boron atom and
1.276(2) Å, C(2)–N(2) 1.279(2) Å] are characteristic for an sp2-hybridized nitrogen atom. It is markedly shorter
double bonds and compare well with those in the cyclic than the coordinative bond B(1)–N(2) [1.576(2) Å]. The B–
ketiminoborane IV [1.273(3)–1.277(2) Å].[20]
Cl bond lengths [1.840(2) and 1.878(2) Å] are longer than
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Reactivity of N,NЈ-Diaryl-1,4-diazabutadienes Towards Boron Trichloride
those in ring 4 [1.791(2) and 1.813(2) Å] and compare with
FULL PAPER
–
the B–Cl bond lengths in the BCl4 ion.
The shortening of bond N(1)–C(1) and the elongation of
bond C(1)=C(1A) are consistent with an extensive delocal-
ization of charge as indicated by the two limiting formulae
(Scheme 6).
Figure 4. Molecular structure of 8 in the crystal; H atoms have
been omitted for clarity. Selected bond lengths [Å] and angles [°]:
B(1)–Cl(1) 1.766(1), B(1)–Cl(2) 1.767(1), B(1)–N(1) 1.395(1), N(1)–
C(1) 1.422(1), N(1)–C(2) 1.451(1), C(1)=C(1A) 1.333(2); Cl(1)–
B(1)–Cl(2) 118.1(1), Cl(1)–B(1)–N(1) 120.2(8), Cl(2)–B(1)–N(1)
121.7(1), B(1)–N(1)–C(1) 123.2(1), B(1)–N(1)–C(2) 121.1(1), C(1)–
N(1)–C(2) 115.7(1), N(1)–C(1)–C(1A) 123.2(1).
Scheme 6.
The xylyl rings are oriented in a near perpendicular fash-
ion to the plane defined by the two heterocycles, with in-
terplanar angles of φ = 99.3° at N(1) and φ = 88.1° at N(2).
The angle B(1)–N(1)–N(2) [98.5(1)°] is markedly more
acute than those in 1,3,2-diazaboroles [cf. 106.9(1) in 1].
The angles B(1)–N(1)–C(1) [111.4(1)°] and B(1)–N(2)–C(2)
[110.2(1)°] compare with those in the chloroborolium cation
of 4a [110.7(1) and 111.5(1)°] and are slightly more obtuse
than that in diazaborole 1 [107.2(1)°]
In going from diazaboroles with various alkyl substitu-
ents at the ring atoms to xylyl and mesityl units, the genera-
tion of diazaboroles with one chlorine atom at the C=C
bond was unexpected. In the case of compound 15 in which
the double bond is substituted by two methyl groups, such
a behavior could not be observed. Here, a recent paper on
the reactivity of BCl3 towards glyoxal[bis(2,6-diisopro-
pylphenylimine)] by Mair et al. seems relevant.[21] He pos-
tulated the initial formation of a borolium salt V, the cation
X-ray Structural Analysis of 8
Single crystals of 8, suitable for an X-ray structural study, of which undergoes a twofold nucleophilic attack by chlo-
were grown from a toluene/hexane mixture at 4 °C. The ride ions to eventually yield the trichlorinated diazaborolid-
analysis shows the features of a planar 2,5-diaza-1,6-di- ine VII (Scheme 7). If, in our case, transient diazaborolid-
bora-hexa-1,3,5-triene with an inversion center in the mid- ines are dehydrochlorinated by a base, diazaboroles such as
dle of the C=C bond (Figure 4, Table 1). The planes of the 6 and 11 would be the products. It is conceivable that the
two xylyl substituents at the nitrogen atoms are perpendicu- carbon chlorination of diaryldiazabutadienes is favored by
larly oriented to the plane of the chain (φ = 85.8°). The bulky arene groups.
chlorine atoms at the boron are located in the plane of the
The initial step in the formation of 5 and 10 is most likely
molecule and feature single bonds with lengths of 1.766(1) the electrophilic attack of one immonium carbon atom of
and 1.767(1) Å. The B–N bond length of 1.395(1) Å is con- 4a or 9a at the C=C bond of 6 or 11 to give intermediate
sistent with a double bond, as is the central bond length VIII. Elimination of HCl leads to cation IX, which finally
C(1)–C(1A) of 1.333(2) Å. The distances N(1)–C(1) adds a chloride ion at the tricoordinate boron atom to yield
[1.422(1) Å] and N(1)–C(2) [1.451(1) Å] reflect single bonds. products 5 or 10 (Scheme 8).
Scheme 7.
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L. Weber, J. Förster, H.-G. Stammler, B. Neumann
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Scheme 8.
was warmed up to ambient temperature and stirring was continued
for 15 h. A microcrystalline solid was filtered off, washed with hex-
ane and dried in vacuo to give 11.0 g of a mixture of borolium salts
4a and 4b. The filtrate was stored for 14 d at +4 °C, and dark red
crystals of 5 (4.2 g, 18%) separated. In the supernatant liquid, the
presence of a 1,3,2-diazaborole was evidenced by 11B NMR spec-
troscopy (δ = 21.6 ppm). The filtrate was concentrated to 30% of
its volume and stored for 2 d at –4 °C, from which crystals of im-
pure 6 were formed. Recrystallization from pentane yielded a few
crystals of pure 6.
Conclusions
The extension of methodologies for the synthesis of 1,3-
dialkyl-2,3-dihydro-1H-1,3,2-diazaboroles to the construc-
tion of 1,3-diaryl-2,3-dihydro-1H-1,3,2-diazaboroles is not
straightforward. The direct combination of N,NЈ-diaryl-1,4-
diazabutadienes with 2 equiv. boron trichloride and the re-
duction of the obtained borolium salts by sodium amalgam
invariably afforded mixtures of singly and doubly chlori-
nated products. Thus, a detour to pure 2-chloro-1,3-diaryl-
2,3-dihydro-1H-1,3,2-diazaboroles was developed. Reaction
of the components at –78 °C gave a 1:2 adduct, the re-
duction of which furnished aminoboranes Cl2BN(Aryl)-
CH=CHN(Aryl)BCl2. Cyclization of these molecules to 2-
chloro-1,3,2-diazaboroles was effected by solid calcium hy-
dride.
In only one case did the reaction of 3a and BCl3 cleanly afford the
burgundy red borolium salt 4a in about 70% yield. A slurry of the
borolium salts 4a and 4b (11.0 g) in a mixture of toluene (150 mL)
and hexane (300 mL) was treated with sodium amalgam (300 g of
a 1% alloy) and vigorously stirred for 20 h. The solution was de-
canted, filtered, and the solvents evaporated to dryness to give 4.8 g
of an inseparable mixture of 1 and 6. If the solution, after having
been separated from mercury, was kept for 14 d at +4 °C, a few
crystals of pure 1[10] could be separated.
Experimental Section
1
1: H NMR: δ = 2.17 (s, 12 H, CH3), 5.85 (s, 2 H, HC=N), 6.99
(m, 6 H, aryl-H) ppm. 13C{1H} NMR: δ = 18.1 (s, CH3), 117.7 (s,
HC=N), 127.3 (s, p-aryl-C), 135.8 (s, o-aryl-C), 139.9 (s, i-aryl-C)
ppm. 11B{1H}NMR: δ = 21.1 (s) ppm. EI-MS: m/z (%) = 310 (100)
[M+].
General: All experiments were performed under dry, oxygen-free
dinitrogen by using standard Schlenk techniques. Solvents were
dried with appropriate drying agents and freshly distilled under N2
before use. The following compounds were prepared according to
literature procedures: 2,6-Me2C6H3N=CHCH=NC6H3Me2-2,6
(3a)[22], 2,4,6-Me3C6H2N=CHCH=NC6H2Me3-2,4,6 (3b)[23] and
2,6-Me2C6H3N=C(Me)C(Me)=NC6H3Me2-2,6 (13).[24,25] 1H, 11B
and 13C NMR spectra were recorded at ambient temperature in
C6D6 except for 5 and 10, which were investigated in CD2Cl2 with
a Bruker AM Avance DRX 500 instrument (1H: 500.13 MHz; 11B:
160.46 MHz; 13C: 125.75 MHz). The spectra were referenced versus
SiMe4 (1H, 13C) and BF3·OEt2 (11B) standards. Boron trichloride,
glyoxal, 2,6-dimethylaniline and 2,4,6-trimethylaniline were pur-
chased commercially.
1
6: H NMR: δ = 2.11 (s, 6 H, CH3), 2.16 (s, 6 H, CH3), 5.75 (s, 1
H, HC=N), 6.98 (m, 6 H, aryl-H) ppm. 13C{1H} NMR: δ = 18.02
(s, CH3), 18.05 (s, CH3), 113.7 (s, HC=N), 126.6 (s, ClC=N), 127.2
(s, p-aryl-C), 127.7 (s, p-aryl-C), 127.9 (s, m-aryl-C), 129.1 (s, m-
aryl-C), 135.5 (s, o-aryl-C), 136.7 (s, o-aryl-C), 138.9 (s, i-aryl-C),
138.5 (s, i-aryl-C) ppm. 11B{1H} NMR: δ = 21.6 (s) ppm. EI-MS:
m/z (%) = 344.1 (100) [M+], 309.1 (79) [M+ – Cl]. C18H19BCl2N2
(345.06): calcd. C 62.65, H 5.55, N 8.12; found C 62.67, H 5.44, N
8.15.
1
5: H NMR: δ = 2.24 (s, 12 H, CH3), 2.45 (s, 12 H, CH3), 6.85 (s,
Reaction of Glyoxal-bis(2,6-dimethylphenylimine) (3a) with Boron 2 H, HC=N), 7.15 (m, 12 H, aryl-H) ppm. 13C{1H} NMR: δ =
Trichloride: A solution of 3a (18.2 g, 69.0 mmol) in a mixture of
toluene (300 mL) and hexane (100 mL) was added over 4.5 h to a
chilled solution (–50 °C) of boron trichloride (16.4 g, 140.0 mmol)
in hexane (250 mL). After stirring at –50 °C for another hour, it
19.1 (s, CH3), 19.7 (s, CH3), 128.9 (s, p-aryl-C), 129.2 (s, p-aryl-C),
130.9 (s, m-aryl-C), 133.7 (s, o-aryl-C), 136.8 (s, o-aryl-C), 137.7 (s,
i-aryl-C), 139.1 (s, i-aryl-C), 161.3 (s, HC=N) ppm. 11B{1H} NMR:
δ = 8.6 (s) ppm. EI-MS: m/z (%) = 688.2 (2) [M+], 652.2 (100)
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Reactivity of N,NЈ-Diaryl-1,4-diazabutadienes Towards Boron Trichloride
FULL PAPER
[M+ – Cl], 618.2 (82) [M+ – 2 Cl]. C36H38B2Cl4N4 (690.12): calcd.
C 62.65, H 5.55, N 8.12; found C 62.71, H 5.38, N 8.09. Because
of the poor solubility, no reliable NMR spectra of the borolium
salts 4a,b were obtained.
H, NC(H)=C], 6.75 (s, 4 H, aryl-H) ppm. 13C{1H} NMR: δ = 17.3
(s, o-CH3), 20.9 (s, p-CH3), 120.9 (s, CH=CH), 130.1 (s, m-aryl-
C), 134.2 (s, o-aryl-C), 136.9 (s, p-aryl-C), 137.8 (s, i-aryl-C) ppm.
11B{1H} NMR: δ = 32.6(s) ppm. EI-MS: m/z (%) = 338 (100) [M+ –
BCl3]. C20H24B2Cl4N2 (455.85): calcd. C 52.69, H 5.33, N 6.14;
found C 52.63, H 5.35, N 6.13.
(E)-Cl2B(o-Xyl)N–CH=CH–N(o-Xyl)BCl2 (8): A solution of 3a
(5.29 g, 20.0 mmol) in toluene (100 mL) was added dropwise over
4–5 h to a chilled solution (–78 °C) of BCl3 (4.67 g, 40 mmol) in
pentane (75 mL). Immediately after the addition, the red-brown
precipitate of the adduct 7a was collected by filtration and dried at
10–3 bar. Reduction of the adduct was effected by treatment with
sodium amalgam (100 g of a 1% alloy) in pentane (150 mL) over
24 h. The supernatant liquid was separated and concentrated to
about 100 mL. Storage overnight at +4 °C afforded colorless crys-
Cyclization of 12 to 2: A mixture of 12 (0.50 g, 1.1 mmol), CaH2
(0.05 g) and hexane (50 mL) was stirred at 25 °C for 48 h and then
filtered, and the filtrate was concentrated to 10 mL. Crystallization
at –4 °C afforded pure 2 (0.20 g, 54%). 1H NMR: δ = 2.13 (s, 6 H,
p-aryl-CH3), 2.19 (s, 12 H, o-aryl-CH3), 5.92 [s, 2 H, NC(H)=C],
6.79 (m, 4 H, aryl-H) ppm. 13C{1H} NMR: δ = 18.0 (s, o-aryl-
CH3), 20.9 (s, p-aryl-CH3), 117.8 (s, N–CH), 129.2 (s, m-aryl-C),
135.3 (s, o-aryl-C), 136.4 (s, p-aryl-C), 137.4 (s, i-aryl-C) ppm.
11B{1H} NMR: δ = 21.9 (s) ppm. EI-MS: m/z (%) = 338.2 (100)
[M+]. C20H24BClN2 (338.68): calcd. C 70.93, H 7.14, N 8.27; found
C 70.68, H 7.20, N 8.16.
1
tals of 8 (5.13 g, 60%). H NMR: δ = 1.99 (s, 12 H, CH3), 6.40 (s,
2 H, NCH=), 6.92 (m, 6 H, aryl-H) ppm. 13C{1H} NMR: δ = 17.3
(s, CH3), 120.5 (s, NCH=), 128.3 (s, p-aryl-C), 129.3 (s, m-aryl-C),
134.6 (s, o-aryl-C), 139.3 (s, i-aryl-C) ppm. 11B{1H} NMR: δ = 32.6
(s) ppm. EI-MS: m/z (%) = 428.0 (100) [M+], 395.2 (6) [M+ – Cl],
310.1 (73) [M+ – BCl3]. C18H20B2Cl4N2 (427.79): calcd. C 50.54, H
4.71, N 6.55; found C 50.26, H 4.82, N 6.43. Because of the low
solubility of 7a, no useful NMR spectra were obtained.
Xyl–NaC(CH3)=C(CH3)–Nb(Xyl)BCN(Na–B) (16): A solution of
13 (6.80 g, 23.0 mmol) in hexane (250 mL) was added dropwise
over 4.5 h to a chilled solution (–50 °C) of BCl3 (5.39 g, 46.0 mmol)
in hexane (250 mL). Stirring was continued for 1 h at –50 °C and
15 h at ambient temperature. Green borolium salt 14 (7.6 g, 63%)
was filtered off and dried at 10–1 bar. The slurry of the salt in hex-
ane (350 mL) was reduced with sodium amalgam (150 g of a 1%
alloy, 20 h). Decanting of the organic phase was followed by evapo-
ration to dryness to afford crude 15 (4.5 g, 98.6%) as a light grey
powder. A sample of solid AgCN (3.2 g, 24.0 mmol) was added to
a solution of 15 in acetonitrile (350 mL), and the mixture was
stirred for 20 h at 20 °C. It was filtered, and the filtrate was concen-
trated, after which 16 (3.10 g, 41.0%) was obtained. 1H NMR: δ =
1.44 [s, 6 H, =C(CH3)N], 2.03 (s, 12 H, aryl-CH3), 6.99 (m, 6 H,
aryl-H) ppm. 13C{1H} NMR: δ = 9.8 [s, =C(CH3)N], 17.9 (s, aryl-
CH3), 121.2 [s, C(CH3)=C(CH3)], 127.6 (s, p-aryl-C), 128.2 (s,
BCN), 128.5 (s, m-aryl-C), 135.6 (s, o-aryl-C), 138.1 (s, i-aryl-C)
ppm. 11B{1H} NMR: δ = 13.5 (s) ppm. EI-MS: m/z (%) = 329.2
(100) [M+], 314.2 (12) [M+ – CH3]. C21H24BN3 (329.24): calcd. C
76.61, H 7.35, N 12.76; found C 76.59, H 7.42, N 12.65.
Cyclization of 8 to 1: A sample of 8 (0.50 g, 1.2 mmol) was dis-
solved in hexane (50 mL). CaH2 (0.05 g) was added, and the slurry
was stirred at ambient temperature for 48 h. It was filtered, and
the filtrate concentrated to 10 mL and stored at –4 °C, after which
crystalline 1 (0.175 g, 46%) separated.
Reaction of Glyoxal-bis(2,4,6-trimethylphenylimine) (3b) with Boron
Trichloride: Analogously to the reaction of 3a with BCl3, a solution
of 3b (14.55 g, 50.0 mmol) in a mixture of toluene (200 mL) and
hexane (50 mL) was combined with a chilled solution (–50 °C) of
BCl3 (11.8 g, 100.0 mmol) in hexane (250 mL) over a period of
4.5 h. Stirring was continued for 1 h at –50 °C and then for 15 h at
20 °C. The microcrystalline black precipitate was filtered off and
dried (5.6 g of a mixture of salts 9a and 9b). Storage of the mother
liquor for 2 weeks at +4 °C afforded dark green crystals of 10
(2.7 g, 14%). Sodium amalgam (200 g of a 1% alloy) was added to
a slurry of the borolium salts (5.6 g) in a mixture of toluene
(100 mL) and hexane (200 mL). The mixture was vigorously stirred
for 20 h. The organic phase was decanted, and the solvents were
removed in vacuo. An inseparable 4:1 mixture (2.4 g) of 2 and 11
as a white powder was obtained.
15: 1H NMR: δ = 1.57 [s, 6 H, =C(CH3)N], 2.15 (s, 12 H, aryl-
CH3), 7.02 (s, 6 H, aryl-H) ppm. 13C{1H} NMR: δ = 10.2 [s,
=C(CH3)N], 18.2 (s, aryl-CH3), 118.9 [s, =C(CH3)N], 127.2 (s, p-
aryl-C), 128.3 (s, m-aryl-C), 136.6 (s, o-aryl-C), 138.6 (s, i-aryl-C)
ppm. 11B{1H} = 21.3 (s) ppm. EI-MS: m/z = 338.2 [M+].
1
10: H NMR: δ = 2.17 (s, 18 H, CH3), 2.29 (s, 6 H, CH3), 2.36 (s,
X-ray Structural Analyses: Crystal data were measured on a Nonius
Kappa CCD-diffractometer and are given in Table 1. CCDC-
616595 (1), -616596 (4a), -616597 (5) and -616598 (8) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
12 H, CH3), 6.85 (s, 2 H, =CHN), 6.93 (s, 4 H, aryl-H), 6.98 (s, 4
H, aryl-H) ppm. 13C{1H} NMR: δ = 18.6 (s, CH3), 20.5 (s, CH3),
20.6 (s, CH3), 129.4, 130.1, 131.2, 133.0, 134.2, 135.1, 136.0, 136.3,
138.6, 139.0 (10 s, aryl-C), 161.2 (s, =CHN) ppm. 11B{1H} NMR:
δ = 8.5 (s) ppm. EI-MS: m/z (%) = 744.2 (2) [M+], 710.3 (100)
[M+ – Cl], 674.4 (66) [M+ – 2 Cl]. C40H46B2Cl4N4 (746.27): calcd.
C 64.32, H 6.21, N 7.51; found C 64.30, H 6.11, N 7.52.
2 and 11: 11B{1H} NMR: δ = 21.8 (s) ppm. This mixture was not
analyzed further. Because of the poor solubility, no reliable NMR
spectra of the borolium salts 9a,b were obtained.
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1
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Received: August 4, 2006
Published Online: October 24, 2006
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Eur. J. Inorg. Chem. 2006, 5048–5056