A Modular Synthetic Approach to Monocyclic 1,4-Azaborines

Article abstract of DOI:10.1002/anie.201602840

A simple and general method for the synthesis of a wide range of monocyclic 1,4-azaborines, including the first examples containing B heteroatoms is described. Post-heterocycle-formation olefin isomerization was employed as a key strategy. This new synthetic method provides fundamental insight into the resonance stabilization and photophysical properties of 1,4-azaborines.

Full text of DOI:10.1002/anie.201602840

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201602840
German Edition: DOI: 10.1002/ange.201602840
BN heterocycles
A Modular Synthetic Approach to Monocyclic 1,4-Azaborines
Xuguang Liu, Yuanzhe Zhang, Bo Li, Lev N. Zakharov, Monica Vasiliu, David A. Dixon,* and
Shih-Yuan Liu*
Abstract: A simple and general method for the synthesis of
a wide range of monocyclic 1,4-azaborines, including the first
examples containing B heteroatoms is described. Post-hetero-
cycle-formation olefin isomerization was employed as a key
strategy. This new synthetic method provides fundamental
insight into the resonance stabilization and photophysical
properties of 1,4-azaborines.
BN/CC isosterism, which is the replacement of a CC unit
with the isoelectronic and isostructural BN unit, has emerged
as a viable strategy to increase the chemical space of
compounds relevant to materials science and biomedical
research.[1] The application of BN/CC isosterism to the
benzene motif leads to three structural isomers: 1,2-dihy-
drido-1,2-azaborine, 1,3-dihydrido-1,3-azaborine, and 1,4-
dihydrido-1,4-azaborine (from hereon, abbreviated as 1,2-
azaborine, 1,3-azaborine, and 1,4-azaborine, respectively).
While significant advances have been made in the develop-
ment of 1,2-azaborine derivatives,[1,2] less progress has been
reported for 1,3-[3] and 1,4-azaborine[4] derivatives, which is
arguably due to a lack of general synthetic access to these
families of compounds. For 1,4-azaborines, most of the
pioneering work has centered on polycyclic dibenzofused
derivatives (Scheme 1), which can be prepared from stable
halogenated diarylamines.[5] Recently, we reported the syn-
thesis of monobenzofused 1,4-azaborines and described their
potential application in coordination chemistry and cataly-
sis.[6,7] However, the chemistry of the simplest monocyclic 1,4-
azaborine has remained virtually unexplored. Braunschweig
and co-workers reported the only synthesis and isolation of
a monocyclic 1,4-azaborine to date, which involves Rh-
mediated cyclization of N-tBu-B-tBu iminoborane with
acetylene.[8] Although elegantly simple, the reported synthetic
scope was limited to one example, namely, N-tBu-B-tBu-1,4-
azaborine (Scheme 1).
Scheme 1. Development of 1,4-azaborines.
In order to unleash the full potential of the chemistry of
1,4-azaborines as a new benzene isostere, a more versatile
synthesis needs to be developed that can access a wide range
of substituted monocyclic 1,4-azaborine derivatives. In this
work, we disclose a modular synthetic approach to the
preparation of N- and B-substituted monocyclic 1,4-azabor-
ines.
Scheme 2 illustrates our retrosynthetic analysis for the
assembly of the 1,4-azaborine ring. In analogy to the synthesis
of dibenzofused 1,4-azaborines,[4,5] a monocyclic 1,4-azabor-
ine could potentially be prepared through coupling of the
dilithium reagent produced from the dibromide A with
a boron electrophile (Scheme 2, route I). However, in con-
[*] Dr. X. Liu, Y. Zhang, Dr. B. Li, Prof. Dr. S.-Y. Liu
Department of Chemistry, Boston College
Chestnut Hill, MA 02467 (USA)
E-mail: shihyuan.liu@bc.edu
Dr. L. N. Zakharov
Department of Chemistry, University of Oregon (USA)
Dr. M. Vasiliu, Prof. Dr. D. A. Dixon
Department of Chemistry, The University of Alabama
Tuscaloosa, AL 35487 (USA)
E-mail: dadixon@as.ua.edu
Dr. X. Liu
Present address: College of Chemistry and Chemical Engineering
Tianjin University of Technology (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602840.
Scheme 2. Retrosynthetic analysis.
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trast to brominated diaryl anilines (precursors to dibenzo-
fused 1,4-azaborines), bis-Z-b-halo-enamine A[9] suffers from
potential stability issues and the need to precisely control the
olefin geometry, thus rendering route I synthetically very
challenging. We envisioned a strategy involving post-hetero-
cycle-formation olefin isomerization to circumvent the chal-
lenges associated with enamine precursor A. The design
involves readily accessible allylamine B as an intermediate,
which upon heterocycle formation and olefin isomerization[10]
should produce the targeted aromatic 1,4-azaborine hetero-
cycle (route II, Scheme 2). Allylamine B can in turn be easily
accessed from commercially available amines and 2,3-dibor-
omopropene.
To demonstrate the feasibility of our synthetic strategy,
methylamine (1a) was first coupled with 2,3-dibromopropene
to furnish 2a in 88% yield (Scheme 3). Subsequent metal–
halogen exchange[11] followed by quenching with iPr2NBCl2,
Scheme 4. Functionalization chemistry at the boron atom of 1,4-
azaborines.
readily exchanged with substituents that can serve as good
leaving groups for further functionalization [e.g., pivalate
(5a), OMe (5b), Cl (5c); Scheme 4].[3c,13]
We then used compounds 5b and 5c as precursors for the
synthesis of a variety of B-substituted 1,4-azaborines through
a simple nucleophilic substitution reaction. As can be seen
from Table 1, a variety of nucleophiles produce the corre-
sponding B-substituted 1,4-azaborines (entries 1–9). We were
able to obtain the crystal structures for 1,4-azaborines 6d, 6 f,
and 6g (see the Supporting Information for details). The
structure of the B-phenoxy-substituted 1,4-azaborine 6g is
À
noteworthy because the B O distances of phenoxy-substi-
Scheme 3. A three-step synthetic sequence for monocyclic 1,4-azabor-
ines.
tuted 1,2- and 1,3-azaborines have been successfully corre-
lated with the extent of intraring electron delocalization of
the BN heterocycles.[3c] It is believed that the longer the
À
exocyclic B O distance, the stronger the extent of the
intraring electron delocalization.
gave BN heterocycle 3a in 93% yield. Finally, isomerization
of the exocyclic double bonds of 3a in the presence of a RuII
catalyst[12] produced the desired target. Thus, the monocyclic
1,4-azaborine 4a can be prepared from commercially avail-
able materials in a straightforward three-step sequence with
an overall yield of 71%. In addition to methylamine (1a),
benzylamine (1b), allylamine (1c), aniline (1d), para-
methoxyaniline (1e), and para-trifluoromethylaniline (1 f)
are also suitable amines for our reaction sequence to furnish
corresponding diisopropylamino-substituted 1,4-azaborines
4b–f (see the Supporting Information for details; for 4c, an
E/Z mixture of N-prop-1-en-1yl) isomers was obtained). For
para-methoxyaniline, we were able to obtain a crystal struc-
ture for the non-aromatic BN hetercocyle 3e, which shows
a boat-like conformation in the solid state (Scheme 3). While
the boron atom in 3e is trigonal planar (sum of the bond
angles = 360.08), the intraring nitrogen atom is pyramidalized
(sum of the bond angles = 347.58).
Table 1: Versatile synthesis of B-substituted 1,4-azaborines.
Entry
Nucleophile (Nu)
EtMgBr
Product
Yield [%][a]
Yield [%][a]
From 5b
From 5c
1
2
3
4
5
6
7
8
9
6a
6b
6c
6d
6e
6 f
6g
6h
4a
70
68
77
83
79
74
67
67
75
73
71
72
72
80
77
=
CH2 CHMgBr
=
CH2 CHCH2MgBr
ÀꢀÀ
Ph
MgBr
PhMgBr
mesitylithium
PhOLi
NR[b]
60[c]
85
Having demonstrated an efficient route to a variety of
monocyclic 1,4-azaborine derivatives with different substitu-
ent on the nitrogen atom, we were interested in investigating
the chemistry at boron. We determined that the diisopropyl-
amino group at the boron atom in 1,4-azaborine 4a can be
Et3BLiH
iPr2NLi
[a] Yield of isolated product. [b] No reaction. [c] Conversion based on
11B NMR.
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À
The observed B O distance in 6g is 1.408(2) ꢀ (Table 1),
which is between those of 1,2-azaborine (1.392(1) ꢀ) and 1,3-
azaborine (1.422(3) ꢀ). Additionally, we predicted the reso-
nance stabilization energy (RSE) of the parent 1,4-azaborine
to be approximately 22 kcalmolÀ1. This value was derived
computationally (G3MP2)[14] through Equation (1), which
reveals that the RSE of the parent 1,4-azaborine is approx-
imately 10 kcalmolÀ1 less than that of benzene (RSE =
32.6 kcalmolÀ1). Compared to a similar analysis for the
parent 1,2- and 1,3-azaborine (RSE = 20 and 31 kcalmolÀ1,
respectively), the parent 1,4-azaborine exhibits an intermedi-
ate RSE.[15] Overall, our observations are consistent with the
following resonance stabilization trend: 1,3-azaborine > 1,4-
azaborine > 1,2-azaborine.[16]
Figure 1. Normalized absorption (solid lines) and emission (dotted
To investigate 1,4-azaborines in the context of potential
materials applications, we investigated 1,4-azaborine ana-
logues of donor/acceptor-substituted para-terphenyls.[17] We
are particularly interested in the photophysical effects result-
ing from donor/acceptor substitution[18] featuring 1,4-azabor-
ine as the linking core. Scheme 5 illustrates the synthesis of
donor/acceptor-substituted 1,4-azaborines 8a and 8b, which
exhibit opposite polarity [8a: (N)donor/(B)acceptor, 8b:
(N)acceptor/(B)donor]. Treatment of the corresponding B-
NiPr2 1,4-azaborine precursors 4e or 4 f with MeOH produces
the corresponding B-OMe-substituted 1,4-azaborines 7e and
7 f in quantitative yield. Subsequent displacement of the B-
OMe group with the corresponding aryl nucleophiles produ-
ces the desired target compounds 8a and 8b. Compounds 6d,
6e, 6 f, 8a, and 8b do not appear to decompose when stored as
solids in air over a 24 h period based on NMR. In solution,
traces) spectra for 8a (green) and 8b (red) and their carbonaceous
analogues C-8a (black) and C-8b (purple) in THF at 1ꢀ10À5 m.
Photoluminescene quantum yields were determined using the compa-
rative method (see the Supporting Information for details).
less than 5% decomposition was observed for compound 8a
over a 24 h period.
Figure 1 shows the normalized absorption and emission
spectra of 8a and 8b, along with those of their corresponding
carbonaceous counterparts C-8a and C-8b in THF solution. It
is evident that the carbonaceous terphenyls C-8a (black) and
C-8b (purple) exhibit very similar absorption and emission
profiles. On the other hand, significant differences in both the
absorption and emission spectra can be observed between 8a
(green) and 8b (red). Thus, in contrast to carbon-based
terphenyls, the specific orientation of the donor/acceptor
placement on 1,4-azaborines has a dramatic impact on the
resulting optical properties. The table in Figure 1 lists the
observed photophysical parameters for 8a, 8b, C-8a, and C-
8b. Particularly striking is the difference in the Stokes shift
between 8a (13037 cmÀ1) and 8b (4097 cmÀ1). The Stokes
shift exhibited by BN terphenyl 8b is uniquely small among
the terphenyl compounds shown in Figure 1. This suggests
a relatively small structural reorganization between the
ground state and the excited state.
To address whether the N substituent or the B substituent
is causing the large discrepancy of the observed Stokes shift
between 8a and 8b, we prepared the 1,4-azaborine-based
biphenyl compounds 9a and 9b and their corresponding
carbonaceous analogues C-9a and C-9b (Figure 2). Consis-
tent with our observations for the terphenyl series, 1,4-
azaborine biphenyl 9b, which has an electron-withdrawing
aryl substituent on the nitrogen atom, also exhibits the
smallest Stokes shift among the biphenyl series (see the
Supporting Information).
In summary, we have developed a general method for the
synthesis of a wide range of monocyclic 1,4-azaborines. From
Scheme 5. Synthesis of donor/acceptor-substituted 1,4-azaborine ana-
logues of p-terphenyl.
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Acknowledgements
Support for this work has been provided by the National
Science Foundation (CHE-1361618) and Boston College.
S.Y.L. thanks the Camille Dreyfus Teacher-Scholar Awards
Program for a Teacher-Scholar award and the Humboldt
Foundation for the Friedrich Wilhelm Bessel Research
Award. We thank Prof. Wenfang Sun at North Dakota State
University and Dr. Zhiqiang Liu at Boston College for helpful
discussions.
Keywords: aromaticity · azaborines · BN heterocycles ·
resonance stabilization · synthetic methods
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Received: March 22, 2016
Published online: && &&, &&&&
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Communications
BN heterocycles
X. Liu, Y. Zhang, B. Li, L. N. Zakharov,
M. Vasiliu, D. A. Dixon,*
S.-Y. Liu*
&&&& — &&&&
Now BN done: A simple and general
method for the synthesis of a wide range
of monocyclic 1,4-azaborines, including
the first examples containing B with
heteroatom substituents is described.
Post-heterocycle-formation olefin iso-
merization was employed as a key strat-
egy. This new synthetic method provides
fundamental insight into the resonance
stabilization and photophysical proper-
ties of 1,4-azaborines.
A Modular Synthetic Approach to
Monocyclic 1,4-Azaborines
6
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