Antiaromaticity to aromaticity: From boroles to 1,2-azaborinines by ring expansion with azides

Article abstract of DOI:10.1002/chem.201403101

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

Full text of DOI:10.1002/chem.201403101

DOI: 10.1002/chem.201403101
Communication
&
Boron Heterocycles
Antiaromaticity to Aromaticity: From Boroles to 1,2-Azaborinines
by Ring Expansion with Azides
Holger Braunschweig,* Christian Hçrl, Lisa Mailꢀnder, Krzysztof Radacki, and
Johannes Wahler[a]
tems,[7] organic semiconducting,[8] or hydrogen storage
Abstract: We have exploited the reactivity of antiaromatic
boroles, gaining access to aryl-substituted monocyclic 1,2-
materials.[9]
The first 1,2-azaborinine (2), a polycyclic derivative, was iso-
azaborinines. The observed ring-expansion reaction of in-
lated by Dewar and co-workers in the late 1950s.[10] Despite
herently electron-deficient boroles with organometallic
the increasing interest in azaborinines, only very few routes
and organic azides is demonstrated for representative ex-
have since been developed for this substance class, and fewer
amples. This substance class is expected to provide a new
still for monocyclic derivatives.[6b,11] Usually, the organic periph-
avenue into 1,2-azaborinine chemistry, especially in the
ery is established around a preformed B=N-containing precur-
area of functional organoboron materials. Our results are
sor. Examples are the hydroboration of secondary butenyl
based on NMR and UV/Vis spectroscopy as well as single-
amines followed by catalytic dehydrogenation of the obtained
crystal X-ray crystallography and provide a virtually quanti-
1,2-azaboriranes (the research groups of Dewar, White, Gou-
tative approach that also offers numerous points of
beau, and Gronowitz),[12] the ring expansion of 1,2-azaborole
variation.
anions by the insertion of carbenes (Ashe and co-workers)[13] or
ring-closing metathesis reactions combined with subsequent
oxidation of the precursors (the research groups of Ashe and
Liu).[14,5a,4] While much effort has been invested in the basic re-
Boron-containing heterocycles continue to gain importance as
building blocks for functional materials and related applica-
tions.[1] The doping of boron and/or nitrogen atoms into
graphene is one area of interest in materials science,[2] as these
elements create points of electron deficiency and excess, re-
spectively, altering the properties of the derived material. One
result of this doping into polyaromatic networks is the forma-
tion of rings in which one C=C bond has been replaced by an
isosteric and isoelectronic B=N bond. In molecular terms, such
rings are known as azaborinines, of the general formula
C4H6NB. The three possible isomers, with B and N atoms with
1,2-, 1,4- or 1,3 relationships, generally decrease in thermody-
namic stability in the listed order.[3] The unsubstituted parent
compound, 1,2-dihydro-1,2-azaborinine 1,[4] as well as relatively
simple derivatives of monocyclic 1,4- and 1,3-azaborinines
were isolated only recently.[5] These studies clearly point out
that the physical and chemical properties of these heterocycles
distinctly differ from those of benzene, related in part to the
polar nature of the B=N fragment.[6] The close relation of aza-
borinines to the ubiquitous aromatic aryl functionality makes
azaborinines very interesting for applications in biomedical sys-
search of relatively simple monocyclic 1,2-azaborinines, that is,
with a narrow variation of the functionalization pattern of the
carbon framework, few investigations towards derivatives with
larger substituents such as aryl groups have been reported
thus far (Figure 1).[15]
Figure 1. Examples of boron-containing heterocycles. Fc=ferrocenyl.
In 2012, we reported a new route to less accessible 1,4-aza-
borinines by a Rh-catalyzed cyclization reaction of alkynes with
iminoboranes.[16] We have recently extended this method to
selectively yield 1,2-azaborinines by simple variation of the
alkyne.[17]
[a] Prof. Dr. H. Braunschweig, Dr. C. Hçrl, L. Mailꢀnder, Dr. K. Radacki,
Dr. J. Wahler
The growing research interest in antiaromatic boroles,
a class of p-conjugated, five-membered boracycles (for exam-
ple, 1,2,3,4,5-pentaphenylborole 3), has led to remarkable
progress over the past six years.[18] Pioneering work by Eisch
and co-workers[19] has been extended mainly by the research
groups of Piers,[20] Yamaguchi[21] and Braunschweig,[22] who
have refined the unique electronic properties of the substance
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/
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403101.
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class. Reactivity studies on boroles have been based mainly in
the quenching of their antiaromatic character, accomplished
through HÀH[23] and SiÀH[24] bond activation, reversible binding
of CO[25] by Lewis acid/base adduct formation,[26, 22d,e] and ex-
tensive redox chemistry.[27] In addition, they are known to read-
ily undergo cycloaddition reactions with unsaturated organics
such as alkenes and alkynes.[19b,28] Following this approach, we
attempted to perform 1,3-dipolar cycloadditions of boroles
with azides as the dipolar reagents. The results presented
herein define an efficient method for the synthesis of highly
substituted monocyclic 1,2-azaborinines by ring expansion of
boroles with azides.
about 3 days. The reaction was driven to completion by heat-
ing the mixture to 608C for 5 h, resulting in the formation of
the final product 1-trimethylsilyl-2-mesityl-3,4,5,6-tetraphenyl-
1,2-azaborinine 6 (d(11B)=41.4 ppm, d(1HMe)=À0.02 ppm, EI-
MS: m/z=573 [M+], yield :73%), which was isolated as a color-
less solid (Scheme 1, Table 1). Further support for the constitu-
tion of 6 was obtained by single-crystal X-ray diffraction
(Figure 2). However, the quality of the data is insufficient for
The first candidate investigated was the readily available azi-
dotrimethylsilane, which provides a useful 1H NMR probe in
1
the Me3Si group (denoted as HMe below). The addition of
Me3SiN3 to a dark blue suspension of 3 in benzene resulted in
the spontaneous dissolution of all components and a color
change to dark red. Immediately thereafter, gas evolution was
observed concomitant with a color change via orange and
yellow (2 h) to colorless (22 h). The initially observed red color
indicates the presence of a short-lived species, possibly a Lewis
adduct, which quickly releases N2. A second transient inter-
Figure 2. Molecular structures of 6 and 10 in the solid state. Hydrogen
atoms are omitted for clarity. Ellipsoids are set at 50% probability. Owing to
disorder, the molecular structure of 6 can be used as a proof of connectivity
but not for discussion of bond parameters. Selected bond lengths [ꢂ] and
angles [8] of 10: B1ÀN1 1.434(3), N1ÀC1 1.395(3), C1ÀC2 1.378(3), C2ÀC3
1.439(3), C3ÀC4 1.374(3), B1ÀC4 1.522(3); N1-B1-C4 115.7(7), N1-B1-C5
120.4(2), C4-B1-C5 123.9(2), C1-N1-B1 122.8(2), C1-N1-C6 117.6(2), B1-N1-C6
119.4(2).
1
mediate was detected by in situ H and 11B NMR spectroscopic
measurements (d(11B): 48.0 ppm, d(1HMe): 0.11 ppm; solvent:
C6D6). While none of the intermediates could be identified, the
colorless final product was formed quantitatively, as judged by
1H and 11B NMR spectroscopy (d(11B)=39.8 ppm, d(1HMe)=
À0.06 ppm). Considering that the 11B resonance at d=
39.8 ppm is nearly identical to that of known 2-phenyl-1-trime-
thylsilyl-1,2-azaborinine (d=38.9 ppm),[29] we assigned the iso-
lated product (EI-MS: m/z=531 [M+]; yield: 86%) as 2,3,4,5,6-
pentaphenyl-1-trimethylsilyl-1,2-azaborinine (4; Scheme 1,
Table 1).[5d]
discussion of bond lengths and angles as a result of disorder.
In addition to monofunctional boroles, more complex bor-
oles can serve as precursors for the ring-expansion reaction,
such as 1,3-bis(2,3,4,5-tetraphenylborolyl)benzene (7). Multinu-
clear NMR spectroscopy indicated the formation of 1,3-bis-(1,2-
azaborinin-2-yl)benzene 8 (d(11B)=40.4 ppm, ESI-MS: m/z=985
[M+H+]) in the reaction of 7 with two equivalents of Me3SiN3,
isolated in 78% yield as a colorless solid (Scheme 2, Table 1). In
principal, different conformational isomers of 8 are possible;
In order to explore the scope of this reaction, we also tested
the reaction of 1-mesityl-2,3,4,5-tetraphenylborole (5) with
Me3SiN3. The reaction proceeds significantly slower at rt, with
the green color of 5 only slowly changing to yellow within
1
however, the 13C{1H} and H NMR spectra of 8 show only one
set of signals for a molecule with at least C2 symmetry at rt
(see the Supporting Information, Figures S2–6). Variable-tem-
1
perature NMR measurements monitoring the H NMR signal of
the Me3Si groups revealed that cooling a solution of 8 from rt
to À808C allowed the detection of different species in which
the Me3Si resonance splits into two signals (for further details
see the Supporting Information, Figure S7). Unfortunately, we
could not identify which isomers of 8 are formed.
Scheme 1. Ring-expansion of boroles with a range of azides.
Table 1. Overview of the 1,2-azaborinines prepared herein.
4
6
8
9
10
RB
Ph
Mes
Me3Si
41.4
322
14900
1,3-Ar[a]
Me3Si
40.4
335
28450
Ph
Ph
35.0
315
17500
Mes
4-iPr-Ph
37.2
314
15100
RN
Me3Si
39.8
332
d11B [ppm]
lmax [nm]
e [LmolÀ1 cmÀ1
]
15350
Scheme 2. Synthesis of bis(azaborinine) 8 derived from the bis(borole) pre-
cursor 7.
[a] For RB =1,3-Ar, see Scheme 2.
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Communication
We were also interested in whether the azide substituent
can be modified in order to broaden the accessible product
spectrum. As such, we chose the reaction of the common or-
ganic azide, azidobenzene, with 3; the reaction resulted in the
formation of hexaphenyl-1,2-azaborinine (9, d(11B)=35.0 ppm,
EI-MS: m/z=535 [M+]) in 89% yield (Scheme 1, Table 1). While
single crystals suitable for X-ray diffraction analysis were ob-
tained and analyzed (see the Supporting Information, Fig-
ure S8), it was not possible to locate the BÀN unit within the
azaborinine ring owing to disorder in the molecule.
In conclusion, the ring-expansion of boroles with azides pro-
vides an efficient synthetic protocol for highly aryl-substituted
1,2-azaborinines and the reaction permits the use of different
substituents at both heteroatoms. Even sterically challenging
groups such as mesityl are tolerated, albeit with increasing re-
action times and temperatures. The reaction mechanism in-
volves extrusion of N2 and several intermediates that will be
identified and analyzed in future mechanistic studies. As
shown in earlier work by Ashe et al., the Me3Si group of 4, 6,
and 8 can be regarded as a protecting group for the nitrogen
atom,[29] allowing access to more advanced derivatives by fur-
ther functionalization. With the growing number of boroles
available, along with the variety of established organic azides,
we expect this synthetic method to be very useful for the pro-
liferation of functionalized 1,2-azaborinine compounds. Com-
parison of the UV/Vis absorption properties of 4, 6, 8, 9, and
10 with less-substituted examples indicates that the electronic
properties are related not only to the groups at boron and ni-
trogen centers, but also to the substituents in the backbone of
the heterocycle.[13,29,17] In the future, we aspire to extend this
synthetic method to other combinations of boroles and azides,
as well as other 1,3-dipolar reagents, in order to access a pa-
lette of potent precursors for novel BN-conjugated materials.
To overcome this crystallographic issue we targeted 1,2-aza-
borinines with functionalized aromatic residues at the B and
N atom. The reaction of 5 with 1-azido-4-isopropylbenzene led
selectively to 1-(4-iso-propylphenyl)-2-mesityl-3,4,5,6-tetraphen-
yl-1,2-azaborinine (10, d(11B)=37.2 ppm, ESI-MS: m/z=620
[M+H+]), which was formed as a colorless solid in 71% isolat-
ed yield (Scheme 1, Table 1).
The 1,2-azaborinine structural motif of 10 was unequivocally
verified by single-crystal X-ray diffraction analysis. The solid-
state structure shows an essentially planar 1,2-azaborinine ring
with a maximum deviation of 0.02 ꢂ from the idealized C4BN
plane. The trigonal-planar coordination of both nitrogen and
boron atoms was confirmed by their angular sums of 359.88
and 360.08, respectively. The B–N (1.434(3) ꢂ), B–C (1.522(3) ꢂ),
N–C (1.395(3) ꢂ) and C–C (1.374(3) to 1.439(3) ꢂ) distances
within the 1,2-azaborinine ring compare well to previously re-
ported examples of the substance class, which reflects the de-
localized character of the aromatic p-system.[5b,a,30] The aryl
substituents in the periphery of the azaborinine are observed
in the expected propeller-like arrangement.
Acknowledgements:
Generous financial support from the Deutsche Forschungsge-
meinschaft is gratefully acknowledged.
The UV/Vis absorption spectra of 4, 6, 8, 9, and 10 are de-
picted in Figure 3. Their lowest-energy absorption maxima
show significant bathochromic shifts as compared to the
Keywords: azaborinines · azides · BN heterocycles · boroles ·
ring expansion
parent
1,2-dihydro-1,2-azaborinine
1
(lmax(e)=269 nm
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Received: April 15, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
&
Boron Heterocycles
H. Braunschweig
,* C. Hçrl, L. Mailꢀnder, K. Radacki,
J. Wahler
&& – &&
Monocyclic 1,2-azaborinines can be
synthesized through ring expansion of
boroles with azides (see figure). The
highly substituted heterocycles were
characterized by multinuclear NMR
measurements, single-crystal X-ray crys-
tallography and UV/Vis spectroscopy.
This class of molecules is expected to
add fresh impetus to the study of aza-
borinine chemistry, especially in the
area of functional organoboron
materials.
Antiaromaticity to Aromaticity: From
Boroles to 1,2-Azaborinines by Ring
Expansion with Azides
1,2-Azaborinines…
can be synthesized by a ring-
expansion reaction of boroles with
azides. Such BN heterocycles are
very interesting for applications in
material science and biochemistry
owing to their close resemblance to
benzene. For more details on the
synthesis and characterization of
these molecules, see the
Communication by H. Braunschweig
et al. on page && ff.
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ