Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides

Article abstract of DOI:10.1002/anie.201500970

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinin

Full text of DOI:10.1002/anie.201500970

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500970
German Edition: DOI: 10.1002/ange.201500970
B,N Azo Dyes
Hot Paper
Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles
with Azides**
Holger Braunschweig,* Mehmet Ali Celik, Florian Hupp, Ivo Krummenacher, and
Lisa Mailꢀnder
Abstract: Herein, we present the results of our investigations
on the effect of ortho substitution of aryl azides on the ring-
expansion reaction of boroles, five-membered unsaturated
boron heterocycles. These studies led to the isolation of the first
1,2-azaborinine-substituted azo dyes, which are bright yellow
solids. One of the derivatives, (E)-2-mesityl-1-(mesityldia-
zenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be
unstable in solution and to transform through a Jacobsen-like
reaction into an indazole and 1-hydro-1,2-azaborinine. DFT
calculations were performed to shed light on possible mech-
anisms to rationalize the unexpected azo-azaborinine forma-
tion and to draw conclusions about the role played by the ortho
substituents in the reaction.
functionalization at both boron and nitrogen positions,[4] but
introduction of substituents such as aryl groups at the carbon
framework remains difficult. In addition to the report by
Taniguchi and Yamaguchi on a 3,6-diarylated 1,2-azabori-
nine,[5] our group has recently found new entries to derivatives
with substituents on the C4 backbone, a) by a rhodium-
catalyzed [2+2]/[2+4] cycloaddition reaction of di-tert-buty-
liminoborane with alkynes[6] and b) by a ring-expansion
reaction of free boroles with organic azides.[7] The synthesis
from boroles, five-membered unsaturated boron heterocy-
cles,[8] offers a facile approach to perarylated 1,2-azaborinines,
which are not available by other methods. As a result of their
pronounced anti-aromatic[9] and Lewis acidic character,
boroles readily bind to nucleophiles[10] and are susceptible
to ring-expansion reactions,[7,11] thereby alleviating their
unfavorable 4p-electron delocalization in the ring.
T
he concept of isosterism, first introduced by Irving Lang-
muir in 1919,[1] continues to be a valuable stimulus for
enhancing the chemical, biological, or physical properties of
a given compound without making significant changes to its
Experimental and theoretical insight into the mechanism
of borole ring expansion by organic azides was provided by
the group of Martin.[11f] Based on DFT calculations, it was
proposed that azide coordination by the a-nitrogen atom to
the empty pz orbital on boron constitutes the first step in this
=
=
chemical structure. In inorganic chemistry, C C and B N
bonds are two well-established examples of isosteric groups,
meaning that they contain the same number of atoms and the
same number and arrangement of electrons.[2] Whereas the
analogy manifests itself in their similar structures, the polar
À
process. Given that organic azides (R N3) have multiple
donor sites,[12] we wondered if the reaction outcome can be
altered by forcing adduct formation to occur at the terminal
nitrogen atom. Herein, we report how the reaction pathway in
the ring-expansion of boroles with organic azides is redirected
by systematic variation of the steric nature of the substrates.
The products formed in this new process are 1,2-azaborinine-
substituted azo derivatives, and thus hitherto unknown BN
analogues of diaryl azo dyes.
=
B N bond imparts different chemical and physical properties.
Thus, the prospect of modifying existing properties in various
organic architectures by partially replacing C2 with BN units
has led to burgeoning interest. As a result, many improved
and new synthetic methodologies, particularly in the prepa-
ration of BN isosteres of aromatic hydrocarbons, have been
developed.[2b,3] With regard to the simplest arene, benzene,
different routes to singly BN-substituted derivatives, com-
monly referred to as azaborinines, have been described.[2b]
However, despite major advances in their preparation,
monocyclic azaborinines constitute challenging synthetic
targets and available derivatives remain limited in their
scope of substitution. In the case of the 1,2-isomers, suitable
precursors exist, from the work of Liu et al., for the facile
To favor complexation at the less nucleophilic, terminal
nitrogen atom of the azide, blocking of the a-nitrogen site by
ortho-substituted aryl azides seemed to be a viable strategy.[7]
We have thus investigated the reactivity of 1,2,3,4,5-penta-
phenylborole (2) toward mesityl azide (1, Scheme 1). The
product (3) of this transformation was obtained as a colorless
solid in good yield (67%). Its 11B NMR signal (d(11B) =
35.9 ppm) and UV/Vis absorption maximum (lmax = 316 nm,
e = 19600 LmolÀ1 cmÀ1) are in agreement with ring expansion
and formation of an 1,2-azaborinine.[7,13] An X-ray analysis
further confirmed the proposed structure (Figure 1). Despite
the steric shielding of the a-nitrogen atom by the mesityl
substituent, the reaction still follows the established pathway
of 1,2-azaborinine formation.
[*] Prof. Dr. H. Braunschweig, Dr. M. A. Celik, F. Hupp,
Dr. I. Krummenacher, L. Mailꢀnder
Institut fꢁr Anorganische Chemie
Julius-Maximilians-Universitꢀt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
Homepage: http://www-anorganik.ak-braunschweig.chemie.uni-
wuerzburg.de/
To further increase the steric demand of the reaction
partners we have included 1-mesityl-2,3,4,5-tetraphenylbor-
ole (4), which bears a bulky mesityl substituent at the boron
center. Using borole 4 in the reaction with mesityl azide (1)
indeed leads to a different outcome (Scheme 1). In this case,
[**] Generous financial support from the Deutsche Forschungsge-
meinschaft (DFG) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500970.
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are versatile reagents that serve many roles in organic
synthesis[14] and are furthermore biologically active.[15] As
highlighted below, compound 5 and its more stable deriva-
tives resemble classical azo compounds in many of their
properties, such as their optical absorption characteristics and
electrochemical behavior.[16] The analogy reflects the isosteric
=
relationship of 1,2-azaborinines and arenes, in which a C C
=
bond is swapped with an isoelectronic B N bond.
Compound 5 is unstable in solution at room temperature,
decomposing into indazole 6 and 1-hydro-1,2-azaborinine 7
(Scheme 1). The formation of the two products, which is
supported by GC/MS (6: m/z 146 [M+]; 7: m/z 501 [M+]),
À
NMR and UV/Vis spectroscopy (Figure S1–4), involves C H
activation of one of the ortho methyl groups on the mesityl
residue of the azo substituent. This type of reaction can be
described as a Jacobsen-like indazole formation, which
usually proceeds from diazoesters of the general formula
Scheme 1. Two possible pathways for the reaction of mesityl azide with
boroles.
[17]
=
“Ar-N N-OR”.
Similar reactivity has been observed by
Erker et al. for a five-membered C2BNP heterocycle, in which
the terminal nitrogen atom of the mesityl azide is analogously
incorporated into the ring.[18] Furthermore, in an attempt to
characterize 5 in the solid-state by X-ray diffraction, analysis
of the crystallographic data revealed that the asymmetric unit
not only consists of 5 but also contains about 10% of the
reaction products 6 and 7 (Figure S9).
To suppress the subsequent transformation of the azo-
azaborinine motif, we replaced the methyl groups in ortho
position of the aryl azide with bromide substituents. Treat-
ment of 2-azido-1,3-dibromo-5-methylbenzene (8) and 2-
azido-1,3-dibromo-5-isopropylbenzene (9), respectively, with
4 afforded the stable 1,2-azaborinine-substituted azo dyes 10
and 11 in good yields (10: 74%, 11: 61%) after 5 days at room
temperature (Scheme 2). Both compounds were isolated as
Figure 1. X-ray crystal structures of 3 and 10. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids set at 50% probability. Owing to
substitutional disorder in the molecular structure of 3, the data for 3
can only serve as proof of connectivity and does not allow for
a discussion of bond parameters. Selected bond lengths [ꢂ] and
angles [8] of 10: N2–N3 1.241(2), N1–N2 1.435(2), B1–N1 1.443(3),
N1–C1 1.396(2), C1–C2 1.373(3), C2–C3 1.443(3), C3–C4 1.384(3),
B1–C4 1.525(3); N1-N2-N3 110.2(2), N1-B1-C4 114.3(2), N1-B1-C5
121.8(2), C4-B1-C5 123.4(2), C1-N1-B1 124.3(2), C1-N1-N2 112.6(2),
B1-N1-N2 122.7(2).
Scheme 2. Synthesis of azo dyes 10 and 11.
no evolution of nitrogen gas was observed and an intensely
yellow colored product was obtained in good yield (53%)
after stirring the reaction mixture for 3 days at room temper-
ature. The 11B NMR chemical shift does not change signifi-
cantly (d(11B) = 35.1 ppm) and appears in the range typical
for 1,2-azaborinine compounds.[7,13] Analysis by single-crystal
X-ray crystallography confirms the formation of an 1,2-
azaborinine scaffold (Figure S8), in which the formerly g-
nitrogen atom is incorporated into the six-membered BN-
containing heterocycle. Interestingly, the three-nitrogen-atom
N3 arrangement of the former azide unit remains intact. This
situation is in contrast both to the conventional de-dinitro-
genative ring expansion reported by our group[7] and a result
from the group of Martin et al. in which formal three-atom
azide incorporation into the five-membered borole ring was
observed, resulting in an eight-membered C4BN3 heterocy-
cle.[11f] Compound 5 is identified as an 1-(arylazo)-substituted
1,2-azaborinine, which can also be described as a triazene, as
bright yellow solids and exhibit similar 11B NMR chemical
shifts at d = 36.6 ppm (10) and d = 36.0 ppm (11), respectively.
In contrast to compound 5, the 1,2-azaborinine-substituted
azo dyes 10 and 11 proved to be stable, with no decomposition
observed, even upon heating at 808C.
X-ray diffraction analysis of single crystals of 10 and 11
confirmed the formation of azo-azaborinines (Figure 1; see
also Figure S10). As a representative example, the solid-state
structure of 10 is discussed. All bond lengths and angles within
the 1,2-azaborinine core compare well to other reported
examples.[4a,7,19] The initially linear N3 moiety becomes bent
(N1-N2-N3 110.2(2)8), adopting a trans configuration with
a single bond between N1 and N2 (1.435(2) ꢀ) and a double
bond between N2 and N3 (1.241(2) ꢀ).[20] In the case of 10
and 11, the aryl and azaborinine substituents of the azo bridge
adopt an angle of 83.4(6)8 and 80.9(5)8, respectively, to each
other, whereas for 5 an angle of only 48.5(1)8 is found
= À
it contains a RN N NR2 motif. These classes of compounds
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(Figure S11). This structural difference may account for the
instability of 5. The close interaction between the b-N atom
and an ortho-methyl proton of the mesityl ligand of the azide
À
facilitates C H bond activation and indazole formation. All
the azo compounds (5, 10, and 11) are in a trans configuration,
presumably favored by the steric constraint imposed by the
bulky substituents on B1 and N3, which also makes photo-
induced trans–cis isomerization unachievable.[21] Accordingly,
1
the corresponding H and 13C{1H} NMR spectra display only
one set of signals and show no evidence of the cis isomer.
The UV/Vis spectra of 3, 10, and 11 are depicted in
Figure 2. The absorption band of 10 and 11 at lmax = 286 nm
(10: e = 17707 LmolÀ1 cmÀ1, 11: e = 22350 LmolÀ1 cmÀ1),
which can be assigned to the 1,2-azaborinine chromophore,
is shifted to slightly smaller wavelengths compared to
compound 3 (lmax(e) = 314 nm (19600 LmolÀ1 cmÀ1)). Both
10 and 11 show broad absorption maxima in the range typical
Figure 2. UV/Vis absorption spectra for solutions of 3, 10, and 11 in
CH2Cl2.
for
aryl
azo
compounds
(10:
lmax(e) = 365 nm
between azide 8 and the model compound 1-mesityl-2,3,4,5-
tetramethylborole (R2). The calculations were performed
using the B3LYP[24] hybrid functional. The calculated reaction
profile is shown in Figure 3. The theoretical data show that
the construction of the azo-azaborinine P1 is initiated by
formation of the adduct I1 (Figure S13), in which borole R2
binds to the terminal N atom of the azide. The calculated free
energy of I1 is 21.7 kcalmolÀ1. All attempts to locate
a transition state for the formation of I1 have failed. A
systematic scan of the potential energy surface from 8 and R2
to I1 showed that it is a barrierless process and that the energy
is decreasing in this direction (Figure S16). After initial Lewis
acid–base complexation, the reaction proceeds through an
early transition state (TS1) that resembles the adduct I1 to
afford the azo-azaborinine P1. The calculated free energy of
activation with respect to I1 is 3.7 kcalmolÀ1 and the overall
activation barrier with respect to 8 and R2 is 25.4 kcalmolÀ1.
This relatively high free-energy barrier is in good agreement
(4804 LmolÀ1 cmÀ1), lmax(e) = 425 nm (2354 LmolÀ1 cmÀ1);
11: lmax = 366 nm (5988 LmolÀ1 cmÀ1), lmax(e) = 425 nm
(2910 LmolÀ1 cmÀ1)) in trans configuration, although their
corresponding molar extinction coefficients vary significantly
(for
example,
1,2-dimesityldiazene:
lmax(e) = 328 nm
(17500 LmolÀ1 cmÀ1), lmax(e)=455 nm (850 LmolÀ1 cmÀ1)).[16,21]
Electrochemical characterization by cyclic voltammetry is
also in line with the description of 10 and 11 as BN analogues
of aromatic azo compounds (Figure S6 and S7). The voltam-
metric data show an irreversible oxidation around + 1.0 V
versus Fc/Fc+ (Fc = [(h-C5H5)2Fe]) characteristic for 1,2-
azaborinine structures,[22] and an irreversible reduction peak
(10: À2.1 V, 11: À2.0 V, potentials vs. Fc/Fc+), which can be
ascribed to reduction of an azo functional group (cf.
azobenzene, E1/2 = À1.81 V vs. Fc/Fc+).[23]
To elucidate the mechanism of azo-azaborinine forma-
tion, we carried out DFT calculations for the reaction
Figure 3. Calculated Gibbs free energy profile (kcalmolÀ1) of azo-azaborinine formation together with two alternative pathways for the possible
1,3-dipolar cycloaddition reactions at B3LYP/def2-SVP. The energies are shown in parentheses.
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.
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adduct, the structure decomposed to form 8 and R2. We
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at the boron center, thus preventing conventional adduct
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À
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adduct (I2) compared to I1, its activation barrier (35.3 kcal
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dipolar cycloaddition reaction between these sterically more
hindered substrates does not lead to 1,2-azaborinines.
In conclusion, we have presented the synthesis of the first
azo dyes based on 1,2-azaborinines by ring expansion of 1-
mesityl-2,3,4,5-tetraphenylborole (4) with different ortho-
substituted phenyl azides. The corresponding azo-azabori-
nines are bright yellow solids and show UV/Vis absorption
bands typical of azo dye compounds. Theoretical mechanistic
studies suggest that the reaction proceeds through a new type
of borole ring expansion involving a key step consisting of the
addition of the terminal N atom of the aryl azide to the Lewis
acidic borole. The calculations showed that the steric
restraints imposed by the ortho substituents on the aryl
ligands of both the borole and azide molecule are the reasons
for the kinetically favored azo-azaborinine formation. Our
future efforts will include the investigation of the electronic
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compounds.
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Received: February 2, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
B,N Azo Dyes
Azo borealis: The first 1,2-azaborinine-
substituted azo dyes have been synthe-
sized using sterically demanding boroles
and organic azides. The ring-expansion
mechanism of the unexpected formation
of the bright yellow chromophores was
investigated by DFT calculations.
H. Braunschweig,* M. A. Celik, F. Hupp,
I. Krummenacher,
L. Mailꢁnder
&&&& — &&&&
Formation of BN Isosteres of Azo Dyes by
Ring Expansion of Boroles with Azides
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!