Boron-substituted 1,3-dihydro-1,3-azaborines: Synthesis, structure, and evaluation of aromaticity

Article abstract of DOI:10.1002/anie.201302660

Getting the family together: A general synthetic strategy based on nucleophilic substitution provided B-substituted 1,3-dihydro-1,3-azaborines (see scheme), BN isosteres of arenes with potential for application in biomedicine and materials science. Experi

Full text of DOI:10.1002/anie.201302660

Angewandte
Chemie
Communications
B,N Heterocycles
S. Xu, T. C. Mikulas, L. N. Zakharov,
D. A. Dixon, S.-Y. Liu*
&&&& — &&&&
Boron-Substituted 1,3-Dihydro-1,3-
azaborines: Synthesis, Structure, and
Evaluation of Aromaticity
Getting the family together: A general
synthetic strategy based on nucleophilic
substitution provided B-substituted 1,3-
dihydro-1,3-azaborines (see scheme), BN
isosteres of arenes with potential for
application in biomedicine and materials
science. Experimental structural analysis
and calculations suggest that the aroma-
ticity of the 1,3-dihydro-1,3-azaborine
heterocycle is intermediate between that
of benzene and that of 1,2-dihydro-1,2-
azaborine.
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DOI: 10.1002/anie.201302660
B,N Heterocycles
Boron-Substituted 1,3-Dihydro-1,3-azaborines: Synthesis, Structure,
and Evaluation of Aromaticity**
Senmiao Xu, Tanya C. Mikulas, Lev N. Zakharov, David A. Dixon, and Shih-Yuan Liu*
BN/CC isosterism has emerged as an attractive strategy to
expand the chemical space of compounds relevant to
biomedical research and materials science.[1] Among the
three BN isosteres of benzene (i.e., 1,2-dihydrido-1,2-azabor-
ine,[2] 1,3-dihydrido-1,3-azaborine, and 1,4-dihydrido-1,4-aza-
borine;[3] see Scheme 1), 1,3-dihydro-1,3-azaborine (abbrevi-
materials and biomedical applications. Late-stage functional-
ization strategies are arguably the most efficient approach to
generate an array of derivatives from an assembled B,N-
heterocyclic core. We envisioned that the boron position in
1,3-azaborines could be utilized as a straightforward point of
attachment for a variety of functional substituents at a late
stage. For 1,3-azaborines, however, a general method for
substitution at boron has yet to be developed. Herein, we
describe protocols for the synthesis of a diverse array of B-
substituted 1,3-azaborines from the B-NiPr2-substituted 1,3-
azaborine 1 as a universal precursor. Furthermore, we provide
evidence based on structural, magnetic, and energetic con-
siderations that 1,3-azaborines are more aromatic than the
corresponding 1,2-azaborines.
Recently, we disclosed that 1,3-azaborine 1 was inert
towards anionic and neutral nucleophiles.[5] On the other
hand, we showed that the treatment of 1 with acetic acid
readily
furnished
the
B-OAc-substituted
product
Scheme 1. The three BN isosteres of benzene.
(Scheme 2).[5,6] Encouraged by this preliminary finding, we
envisioned that 1,3-azaborine 1 could be converted into B-
substituted derivatives under acidic conditions. The treatment
of precursor 1 with HCl (2 equiv) produced the B-Cl-
substituted 1,3-azaborine 2 in 52% yield (Scheme 2). Full
and clean conversion of 1 into 2 was observed by 11B NMR
spectroscopy under the optimized reaction conditions. We
believe that the moderate yield of the isolated product is due
to loss of the product upon purification by silica-gel chroma-
tography. The relative stability of 1,3-azaborine 2 towards
silica gel contrasts starkly with the reactivity of the B-Cl
substituted 1,2-azaborine, which undergoes complete degra-
dation upon exposure to silica gel.
ated herein as 1,3-azaborine) is thermodynamically the least
stable isomer.[4] Not surprisingly, its development has lagged
behind that of the other azaborines in the series. The first
synthetic example of a 1,3-azaborine was reported in late
2011,[5] and the diversity of synthetically accessible 1,3-
azaborines remains very limited to date.
1,3-Azaborines exhibit electronic structures that are very
distinct from those of their 1,2-azaborine and carbonaceous
analogues.[6] New synthetic tools need to be developed to
exploit the unique properties of this family of heterocycles in
[*] Dr. S. Xu, Prof. S.-Y. Liu
Department of Chemistry and Biochemistry, University of Oregon
Eugene, Oregon 97403-1253 (USA)
E-mail: lsy@uoregon.edu
Dr. L. N. Zakharov
Center for Advanced Materials Characterization in Oregon
University of Oregon, Eugene, Oregon 97403-1253 (USA)
T. C. Mikulas, Prof. Dr. D. A. Dixon
Department of Chemistry, The University of Alabama
Tuscaloosa, AL, 35487-0336 (USA)
[**] Correspondence concerning X-ray crystallography should be
directed to L.N.Z. (lev@uoregon.edu). Correspondence concerning
electronic-structure calculations should be directed to D.A.D. This
research was supported by the National Institutes of Health
(National Institute of General Medical Sciences, Grant R01-
GM094541). Funding for the University of Oregon Chemistry
Research and Instrumentation Services has been furnished in part
by the NSF (CHE-0923589).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302660.
Scheme 2. Substitution reactions of precursor 1 under acidic condi-
tions. Py=pyridine, TMS=trimethylsilyl.
2
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When heterocycle 1 was treated with MeOH, no reaction
was observed. However, under acidic conditions, that is, when
1 was treated with a 1:1 mixture of MeOH and HCl, the B-
OMe-substituted 1,3-azaborine 3 was isolated in 70% yield.
Similarly, the treatment of 1 with HF·pyridine (1.0 equiv)
gave the fluoride-substituted 1,3-azaborine 4. When the
substitution reaction was carried out with 1.5 equivalents of
HF·pyridine, it was complete within 6 min, as determined by
11B NMR spectroscopy. To the best of our knowledge,
compound 4 is the first example of an isolated B-F-substituted
azaborine.[7] The B-CN-substituted 1,3-azaborine 5 was syn-
thesized by the exposure of 1 to a 1:1 mixture of MeOH and
TMSCN, presumably through reaction with HCN generated
in situ.[8] The B-CN connectivity (vs. B-NC) in 5 was
confirmed by the broad peak at 128.8 ppm (CN) in the
13C NMR spectrum, which is consistent with the data reported
for the B-CN-substituted 1,2-azaborine analogue.[9]
Despite the successful examples shown in Scheme 2, the
acid–promoted B-substitution has significant limitations. For
example, derivatives with carbon-based substituents contain-
Scheme 4. Optimization of a nucleophilic-substitution protocol.
À
ing C H bonds with high pKa values (e.g., alkyl, aryl, vinyl,
alkynyl) cannot be accessed. A method that would enable
nucleophilic substitution at boron under neutral or basic
conditions would significantly expand the diversity of 1,3-
azaborines. We determined previously that heterocycle 1 did
not react with a number of anionic nucleophiles owing to the
weak leaving-group ability of the diisopropylamino group.[5]
Thus, our strategy was to convert 1 into a 1,3-azaborine
intermediate I with a good leaving group for nucleophilic
substitution (Scheme 3).
features the bulky pivalate leaving group at boron, was
treated with nBuLi at room temperature, the desired B-nBu-
substituted compound was formed in 83% yield (Scheme 4,
middle). The overall yield for the two steps from starting
material 1 was 72% owing to the potential loss of material
associated with an additional isolation process. To improve
the overall efficiency of the substitution protocol starting
from 1,3-azaborine 1, we envisioned that the conversion of
1 into 7 and the subsequent nucleophilic substitution reaction
could be performed in one single pot with the only operation
between the two reactions being the removal of the diisopro-
pylamine by-product under vacuum. Gratifyingly, the two-
step one-pot process gave the B-nBu-substituted heterocycle
in 80% overall yield (Scheme 4, bottom): an improvement of
8% over that of the step-by-step procedure.
Having established an optimized general protocol, we
investigated the scope of the substitution reaction. The one-
pot displacement of the diisopropylamino group in 1 occurred
readily in the presence of alkyl (Table 1, entry 1), vinyl
(entry 2), aryl (entry 3), and alkynyl nucleophiles (entry 4).
The substitution reaction also proceeded readily with steri-
cally hindered aryl nucleophiles, such as mesityllithium
(Table 1, entry 5). Interestingly, without first converting the
diisopropylamino group to B-pivalate-substituted intermedi-
ate 7, phenol and tert-butyl alcohol failed to react with 1,3-
azaborine 1 even in the presence of HCl. On the other hand,
with the optimized one-pot substitution protocol, heteroatom
nucleophiles become suitable as reaction partners. The use of
phenoxide (Table 1, entry 6) and tert-butoxide nucleophiles
(entry 7) resulted in the formation of the corresponding
products 8 f and 8g in high yield. The diisopropylamino
substituent in 1 can be readily converted into another amino
functionality, such as the bis(trimethylsilyl)amino group when
KN(SiMe3)2 is employed as the nucleophile; thus, 8h was
obtained in 69% yield (Table 1, entry 8). Notably, as an
alternative to the use of the HF·pyridine reagent, the B-F-
Scheme 3. Development of a strategy for the nucleophilic substitution
of 1.
With this strategy in mind, we focused our initial attention
on developing the B-OAc-substituted 1,3-azaborine 6 as
a precursor for nucleophilic substitution reactions on the basis
of our prior success in using 6 and LiAlH4 to produce N-Me-
1,3-BN-toluene.[6] However, treatment of 6 with the stronger
base nBuLi gave only a trace amount of the desired
substitution product (Scheme 4, top). We realized that two
possible side reactions may compete with the desired
nucleophilic attack at boron in compound 6 (path a) when
a strong nucleophile/base is used, such as nBuLi: 1) attack at
the carbonyl carbon atom (path b) and 2) deprotonation
(path c). The introduction of a bulky carboxylate as a leaving
group with a quaternary a-carbon atom could potentially
mitigate the problem. Indeed, when 1,3-azaborine 7, which
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Table 1: Synthesis of B-substituted 1,3-azaborines through nucleophilic
substitution.
Entry
Nucleophile (Nu)
Product
Yield [%][a]
1
2
3
4
5
6
7
8
9
nBuLi
8a
8b
8c
8d
8e
8 f
8g
8h
4
80
76
94
89
87
99
99
69
81
96
=
CH2 CHMgBr
PhMgBr
ÀꢀÀ
Ph
MgBr
mesityllithium
PhOLi
tBuOK
KN(SiMe3)2
CsF
Et3N/MeOH[b]
3
Figure 1. ORTEP illustrations of 8 f and 9, with thermal ellipsoids
drawn at the 35% probability level. The numbers in brackets are
optimized gas-phase bond distances calculated at the B3LYP/DZVP2
level. TBS=tert-butyldimethylsilyl.
10
[a] Yield of the isolated product. [b] A vacuum was not applied after the
treatment of 1 with pivalic acid.
substituted 1,3-azaborine 4 was also formed by a nucleophilic
reaction with CsF (Table 1, entry 9). Finally, the one-pot
protocol was used to convert 1,3-azaborine 1 into the B-OMe
derivative 3 with MeOH/NEt3 (Table 1, entry 10).
Within the azaborine series (1,2-, 1,3-, and 1,4-azabor-
ines), the 1,3-isomer is the only family that cannot not be
satisfactorily represented by a Lewis structure without
invoking formal charges (Scheme 5). Because the chemistry
of 1,3-azaborines is just emerging, structural information on
a 1,2-azaborine. To enable a more direct comparison of
diphenyl ether structures, we prepared 1,2-azaborine 9[10] and
À
determined its corresponding B O distance, which is
À
1.392(1) ꢀ (Figure 1). The weaker B O p bonding in 1,3-
azaborines in comparison to that in 1,2-azaborines is con-
sistent with the electrostatic-potential-map calculations,[6] in
which a highly delocalized intraring p-electron system
renders the boron atom relatively less capable of accepting
p electrons from exocyclic substituents.
Another striking feature of the solid-state structure of 8 f
is the short B C(2) distance of 1.476(5) ꢀ.[13,14] However, the
À
À
crystallographically determined short B C(2) distance is not
predicted by gas-phase density functional theory (DFT)
calculations at the B3LYP/DZVP2[15] level (B C(2)
À
1.516 ꢀ).[16] The observed B C(2) distance for the B-diiso-
À
propylamino-substituted 1,3-azaborine 1 is 1.525(2) ꢀ, and
the corresponding distances in 8c and 8d are 1.508(3) and
1.498(2) ꢀ, respectively; the calculated DFT values are
consistent with these results (see the Supporting Informa-
tion). Thus, the observed B-substituent-dependent intraring
Scheme 5. Lewis structures for the azaborine series.
this family of heterocycles is very limited. Compound 1 is the
only reported example that has been characterized by single-
crystal X-ray analysis.[5] A larger structural database should
facilitate the development of a better understanding of the
bonding and electronic structure of this family of compounds.
With a library of 1,3-azaborines now readily accessible, we
characterized compounds 8c, 8d, and 8 f crystallographi-
cally.[10] 1,3-Azaborine 8 f is the 1,3-BN isostere of a diphenyl
ether (specifically, 1-methyl-3-phenoxybenzene). The
observed bonding between the boron and exocyclic oxygen
atoms in 8 f is revealing (Figure 1): The exocyclic oxygen atom
of 8 f is mostly sp2-hybridized, with a B-O-C bond angle of
123.8(2)8. The C-O-B-C(2) torsion angle of 170.4(2)8 and the
À
solid-state B C(2) bond distances range from 1.476 to
1.525 ꢀ (i.e., a difference of ca. 0.05 ꢀ), which is relatively
large and distinct from 1,2-azaborines and arenes.[17]
Overall, the observed intraring bond distances in 1,3-
azaborines 1, 8c, 8d, and 8 f are not consistent with a single
Lewis structure description. This result corroborates previous
theoretical predictions of a significantly electron delocalized
structure.[4]
To more quantitatively evaluate the aromatic character of
1,3-azaborines, we calculated the nucleus-independent chem-
ical-shift values NICS(0) and NICS(1)[18] for the B,N-hetero-
cyclic portion of compounds 1, 8c, 8d, and 8 f (Scheme 6).
Without exception, the NICS values for the illustrated 1,3-
azaborines are more negative than those of the corresponding
B-substituted 1,2-azaborines but less negative than those of
the corresponding arenes.[19] Thus, on the basis of the NICS
values, 1,3-azaborine exhibits aromaticity that is intermediate
between that of benzene and that of 1,2-azaborine.
À
B O bond distance of 1.422(3) ꢀ suggest some double-bond
À
character of the B O bond (sum of single-bond covalent
radii: 1.48 ꢀ).[11] However, the observed B O distance in 8 f is
longer (by 0.03 ꢀ) than that of a typical B-alkoxide-substi-
tuted 1,2-azaborine (B O 1.389(2) ꢀ) and thus indicates
À
[12]
À
À
weaker B O p bonding in a 1,3-azaborine relative to that in
4
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potential of BN/CC isosterism, and specifically the use of
1,3-azaborines, in these important areas of research.
Received: March 30, 2013
Published online: && &&, &&&&
Keywords: aromaticity · azaborines · boron · heterocycles ·
.
synthetic methods
Scheme 6. Calculated NICS(0) and NICS(1) values for 1,3-azaborines
1, 8c, 8d, 8 f and their corresponding 1,2-azaborine and carbonaceous
(arene) counterparts.
[1] For an overview, see: a) P. G. Campbell, A. J. V. Marwitz, S.-Y.
Liu, Angew. Chem. 2012, 124, 6178 – 6197; Angew. Chem. Int. Ed.
2012, 51, 6074 – 6092; b) M. J. D. Bosdet, W. E. Piers, Can. J.
Chem. 2009, 87, 8 – 29; c) Z. Liu, T. B. Marder, Angew. Chem.
2008, 120, 248 – 250; Angew. Chem. Int. Ed. 2008, 47, 242 – 244.
[2] For leading references on 1,2-azaborines, see: a) D. H. Knack,
J. L. Marshall, G. P. Harlow, A. Dudzik, M. Szaleniec, S.-Y. Liu,
J. Heider, Angew. Chem. 2013, 125, 2660 – 2662; Angew. Chem.
Int. Ed. 2013, 52, 2599 – 2601; b) A. J. V. Marwitz, A. N. Lamm,
L. N. Zakharov, M. Vasiliu, D. A. Dixon, S.-Y. Liu, Chem. Sci.
2012, 3, 825 – 829; c) A. N. Lamm, E. B. Garner, D. A. Dixon, S.-
Y. Liu, Angew. Chem. 2011, 123, 8307 – 8310; Angew. Chem. Int.
Ed. 2011, 50, 8157 – 8160; d) P. G. Campbell, E. R. Abbey, D.
Neiner, D. J. Grant, D. A. Dixon, S.-Y. Liu, J. Am. Chem. Soc.
2010, 132, 18048 – 18050; e) L. Liu, A. J. V. Marwitz, B. W.
Matthews, S.-Y. Liu, Angew. Chem. 2009, 121, 6949 – 6951;
Angew. Chem. Int. Ed. 2009, 48, 6817 – 6819; f) E. R. Abbey,
L. N. Zakharov, S.-Y. Liu, J. Am. Chem. Soc. 2008, 130, 7250 –
7252; g) T. Hatakeyama, S. Hashimoto, T. Oba, M. Nakamura, J.
Am. Chem. Soc. 2012, 134, 19600 – 19603; h) X.-Y. Wang, H.-R.
Lin, T. Lei, D.-C. Yang, F.-D. Zhuang, J.-Y. Wang, S.-C. Yuan, J.
Pei, Angew. Chem. 2013, 125, 3199 – 3202; Angew. Chem. Int. Ed.
2013, 52, 3117 – 3120.
Finally, we predicted the resonance stabilization energy
(RSE) of the parent 1,3-azaborine to be approximately
29 kcalmolÀ1. This value was derived computationally
(G3MP2)[20] from Equations (1) and (2), which reveal that
the RSE of the parent 1,3-azaborine is approximately
5 kcalmolÀ1 (average of 3.5 and 6.6 kcalmolÀ1) less than that
of benzene (RSE = 34.1 kcalmolÀ1). A similar analysis for the
parent 1,2-azaborine gave a lower RSE (21 kcalmolÀ1 [21] than
)
that found for the parent 1,3-azaborine. These results are
consistent with the structural analysis and NICS calculations.
[3] For leading references on 1,4-azaborines, see: a) H. Braunsch-
weig, A. Damme, J. O. Jimenez-Halla, B. Pfaffinger, K. Radacki,
J. Wolf, Angew. Chem. 2012, 124, 10177 – 10180; Angew. Chem.
Int. Ed. 2012, 51, 10034 – 10037; b) T. Agou, M. Sekine, J.
Kobayashi, T. Kawashima, Chem. Commun. 2009, 1894 – 1896;
c) M. Kranz, F. Hampel, T. Clark, J. Chem. Soc. Chem. Commun.
1992, 1247 – 1248.
[4] a) M. H. Matus, S.-Y. Liu, D. A. Dixon, J. Phys. Chem. A 2010,
114, 2644 – 2654; b) M. Kranz, T. Clark, J. Org. Chem. 1992, 57,
5492 – 5500.
[5] S. Xu, L. N. Zakharov, S.-Y. Liu, J. Am. Chem. Soc. 2011, 133,
20152 – 20155.
[6] A. Chrostowska, S. Xu, A. N. Lamm, A. Maziꢁre, C. D. Weber,
A. Dargelos, P. Baylꢁre, A. Graciaa, S.-Y. Liu, J. Am. Chem. Soc.
2012, 134, 10279 – 10285.
[7] The preparation of B-F-substituted 1,2-azaborines has thus far
been challenging.
Thus, in terms of their structure, magnetism, and reso-
nance energy stabilization, we conclude that 1,3-azaborines
should be considered more aromatic than 1,2-azaborines.[22] It
appears that the crucial factors governing the relative
aromaticity of 1,2- and 1,3-azaborines originate in the
extent of p-electron delocalization as well as the lack of
À
a B N bond in the 1,3-isomer. The greater potential for
À
delocalization of the p electrons in the region of the B N
bond of the 1,2-azaborine[23] may disrupt electron delocaliza-
tion and result in a less-pronounced aromatic character.
In summary, we have developed the first general method
for the synthesis of B-substituted 1,3-azaborines. Our method
enabled the synthesis and isolation of the first example of a B-
F-substituted azaborine. Structural analysis in the solid state
[8] M. Negru, D. Schollmeyer, H. Kunz, Angew. Chem. 2007, 119,
9500 – 9502; Angew. Chem. Int. Ed. 2007, 46, 9339 – 9341.
[9] A. J. V. Marwitz, S. P. McClintock, L. N. Zakharov, S.-Y. Liu,
Chem. Commun. 2010, 46, 779 – 781.
[10] Crystallographic data for 8 f: C11H12BNO, Mr = 185.03, crystal
size 0.40 ꢂ 0.19 ꢂ 0.01 mm3, orthorhombic, space group Pca21, a =
11.230(12), b = 11.861(13), c = 7.474(9) ꢀ, V= 995.5(19) ꢀ3, Z =
4, 1calc = 1.234 gcmÀ3, m = 0.077 mmÀ1, F(000) = 392, MoKa radi-
ation, l = 0.71073 ꢀ, T= 173(2) K, 2Vmax = 50.008, 7967 reflec-
tions measured (Rint = 0.0544), 1748 reflections observed, 175
refined parameters, R1 = 0.0485, wR2 = 0.1017, and GOF =
1.072 for reflections with I > 2s(I), R1 = 0.0660, wR2 = 0.1097,
and GOF = 1.072 for all data, max/min residual electron density
+ 0.130/À0.180 eꢀÀ3. CCDC 930426 (8 f), 930428 (8c), 930427
(8d), and 931041 (9) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
À
revealed a B-substituent-dependent B C(2) intraring bond
À
distance. The relatively long B O distance, the calculated
NICS values for 1,3-azaborines 1, 8c, 8d, and 8 f, and our
evaluation of the resonance stabilization energy suggest that
1,3-azaborines are highly aromatic. In view of the ubiquity
and importance of arenes in biomedical research and
materials science, the synthetic procedures presented herein
represent a significant advance towards harnessing the
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from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[17] A CCDC search (version 2011) of monosubstituted benzenes
À
(> 1900 data points) revealed that Cipso C bond lengths for
PhOR, PhNR2, and PhCR3 range from 1.376–1.384, 1.376–1.408,
and 1.381–1.400 ꢀ, respectively (i.e., an overall range of
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À
0.032 ꢀ). The B C distance in reported 1,2-azaborines ranges
from 1.486–1.518 ꢀ (i.e., an overall range of 0.032 ꢀ).
[18] Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, P. R.
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À
[13] A B C bond distance of 1.476 ꢀ is consistent with substantial
[19] See the Supporting Information for details.
[20] L. A. Curtiss, P. C. Redfern, K. Raghavachari, V. Rassolov, J. A.
Pople, J. Chem. Phys. 1999, 110, 4703 – 4709.
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[22] This conclusion is not inconsistent with the fact that 1,3-
azaborines are thermodynamically less stable than 1,2-azabor-
ines. The crucial factors that determine the relative thermody-
namic stabilities originate in the strength of the s bonds (see
Ref. [4]).
À
double-bond character. The sum of the B C single-bond
À
covalent radii is 1.60 ꢀ, and the sum of the B C double-bond
covalent radii is 1.45 ꢀ (see Ref. [13a]).
[14] For an excellent discussion on the structural changes that
À
accompany the stepwise population of a B C p bond, see: C. W.
Chiu, F. P. Gabbaꢃ, Angew. Chem. 2007, 119, 7002 – 7005; Angew.
Chem. Int. Ed. 2007, 46, 6878 – 6881.
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[16] This result may suggest a potential origin of the observed B C
[23] D. J. Grant, D. A. Dixon, J. Phys. Chem. A 2006, 110, 12955 –
12962.
distances in crystal-packing forces in the solid state.
6
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!