Direct synthetic route to functionalized 1,2-azaborinines

Article abstract of DOI:10.1002/anie.201309707

A new catalytic synthetic route to functionalized 1,2-azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di-tert- butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene-functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2-azaborete complex, which results in the isolation of the first azaborinine boronic ester. The isolation of an NH-containing BN isostere by elimination of isobutene from an N(tBu) group under thermolytic conditions has also been achieved. Theoretical studies give further insight into the formation of 1,2-azaborinines and the elimination of isobutene from the N(tBu) group. 2+2+2: A straightforward method to access the first examples of ferrocene- and pinacolatoborane-functionalized 1,2-azaborinine derivatives has been developed by the tandem [2+2]/[2+4] cycloaddition reactions of di-tert-butyliminoboranes and alkynes.

Full text of DOI:10.1002/anie.201309707

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Communications
DOI: 10.1002/anie.201309707
BN Heterocycles
Direct Synthetic Route to Functionalized 1,2-Azaborinines**
Holger Braunschweig,* K. Geetharani, J. Oscar C. Jimenez-Halla, and Marius Schꢀfer
Abstract: A new catalytic synthetic route to functionalized 1,2-
azaborinines has been developed by the [2+2]/[2+4] cyclo-
addition reactions of di-tert-butyliminoboranes and alkynes in
presence of a rhodium catalyst. The first examples of
ferrocene-functionalized azaborinines have been synthesized
using this strategy. Moreover, the regioselectivity of this
reaction can be controlled by the formation of an intermediate
rhodium 1,2-azaborete complex, which results in the isolation
of the first azaborinine boronic ester. The isolation of an NH-
containing BN isostere by elimination of isobutene from an
N(tBu) group under thermolytic conditions has also been
achieved. Theoretical studies give further insight into the
formation of 1,2-azaborinines and the elimination of isobutene
from the N(tBu) group.
we started to investigate the rhodium-mediated co-cyclization
of alkynes and iminoboranes, leading selectively to the first
monocyclic 1,4-azaborinine with rupture of the iminoborane
[14]
ꢀ
B N triple bond. Herein, we report results of a distinct
advancement of this previous procedure, giving facile access
to highly functionalized 1,2-azaborinines, and moreover, an
unusual NH-substituted 1,2-azaborinine obtained by ther-
mally induced isobutene elimination.
In our early work on BN heterocycles, we established the
rhodium-mediated synthesis of 1,4-di-tert-butyl-1,4-azabori-
nine by tandem [2+2]/[2+4] cycloaddition reactions of an
ꢀ
iminoborane tBuB NtBu (1) with the non-polar substrate
acetylene.[14] Encouraged by our preliminary results, and to
create a range of BN/CC isosteric aromatic structures, we
intended to use ethynylferrocene as a monosubstituted polar
alkyne. Interestingly, reaction of 1 with ethynylferrocene in
the presence of [{(iPr3P)2RhCl}2] (2) as a catalyst led, in
contrast to our previous results, exclusively to a different
structural isomer: the novel 1,2-di-tert-butyl-4,6-diferrocenyl-
1,2-azaborinine (3; Scheme 1).
B
oron- and nitrogen-containing heteroaromatic compounds
are receiving ever-growing attention, particularly since the
À
presence of a polar B N moiety provides distinctly different
electronic properties from their isoelectronic organic counter-
parts.[1] The formal exchange of a pair of carbon atoms in
benzene by boron and nitrogen atoms leads to azaborinines,
of which three structural isomers are possible: 1,2-, 1,3-, and
1,4-azaborinines. As early as 1958, Dewar reported the
synthesis of the first monocyclic azaborinine[2] and the
chemistry of monocyclic and ring-fused polycyclic derivatives
of azaborinine was further developed, particularly by
Dewar,[3] White,[4] Ashe,[5] Perepichka,[6] and Paetzold.[7]
Most notably by the work of Liu[8] and others,[9,10] interest in
this class of BN heterocycles has been very recently rekindled,
which is partly due to their potential for applications in
biomedical research and materials science.[11] Despite these
important developments, the synthesis of monocyclic azabor-
inines in particular remains a significant challenge.[5a,8b] The
usual procedure consists of ring-closing metathesis of bis-
(allyl)aminoboranes and subsequent dehydrogenation steps
providing 1,2-azaborinines in moderate yields overall.
Inspired by the facile metal-mediated cyclotrimerization of
Scheme 1. Synthesis of 1,2-di-tert-butyl-4,6-diferrocenyl-1,2-azaborinine
(3).
Orange crystals of 3 suitable for single-crystal X-ray
crystallography were obtained by recrystallization from
saturated benzene solutions. The molecular structure of 3
À
(Figure 1) features a C4BN ring in which the B1 N1 bond
length was found to be 1.479(3) ꢀ. The latter is shorter than
[15]
À
a typical B N single bond (1.61 ꢀ),
but longer than
alkynes to benzenes[12] and the well-known isoelectronic
a typical localized B N double bond (1.403(2) ꢀ),
and
[16]
=
[13]
ꢀ
ꢀ
relationship between C C and B N triply bonded species,
thus similar to the delocalized double bond in 1,2-azabor-
inines (1.446(2) ꢀ).[16a] The C4 B1 bond (1.523(4) ꢀ) is
À
slightly longer than that reported for 1,2-azaborinine
[*] Prof. Dr. H. Braunschweig, Dr. K. Geetharani,
Dr. J. O. C. Jimenez-Halla, M. Schꢀfer
[17]
À
(1.503 ꢀ), as is the C1 N1 bond (1.410(3) ꢀ vs. 1.370 ꢀ).
Institut fꢁr Anorganische Chemie
The azaborinine ring is twisted from planarity by 0.15 ꢀ
Julius-Maximilians-Universitꢀt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
(average displacement of the ring atoms), which is imposed by
À
À
steric congestion of the bulky N tBu and B tBu substituents.
Moreover, this result is remarkable given the high regiose-
lectivity of the reaction with regard to the ferrocenyl units,
which are found exclusively in the 4 and 6 positions.
[**] K.G. thanks the Alexander von Humboldt Foundation for a post-
doctoral fellowship. Generous financial support from the Deutsche
Forschungsgemeinschaft is gratefully acknowledged.
Consistent with the crystallographic results, the 11B NMR
spectrum of 3 displays a singlet at d = 47.1 ppm and the
1H NMR spectrum reveals the presence of aromatic protons
with signals at d = 7.30 and 6.90 ppm, a ferrocenyl unit at d =
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309707.
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Figure 2. Molecular structure of 4. Ellipsoids are set at 50% proba-
bility; hydrogen atoms and some ellipsoids are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: B1–N1 1.544(3), B1–C2
1.548(4), N1–C1 1.469(3), C1–C2 1.428(3), Rh1–B1 2.232(3), Rh1–N1
2.141(2), Rh1–C1 2.092(2), Rh1–C2 2.141(2); N1-B1-C2 87.11(19), C1-
N1-B1 88.39(18), N1-C1-C2 94.66(19), C1-C2-B1 89.7(2).
Figure 1. Molecular structure of 3. The two crystallographically inde-
pendent molecules in the asymmetric unit have nearly identical geo-
metries, only one of which is displayed. Ellipsoids are set at 50%
probability; hydrogen atoms and some ellipsoids are omitted for
clarity. Selected bond lengths [ꢂ] and angles [8]: B1–N1 1.479(3), B1–
C4 1.523(4), N1–C1 1.410(3), C1–C2 1.364(3), C2–C3 1.435(3), C3–C4
1.371(3); C4-B1-N1 114.3(2), B1-N1-C1 116.90(19).
been revealed.[24] To this end, compound 3 is the first example
of a 1,2-azaborinine bearing ferrocenyl units, a family of
compounds which invite exploitation in a range of applica-
tions.
4.79-3.99 ppm, and two tert-butyl groups at d = 1.44 and
1.36 ppm. The infrared spectrum of 3 features two distinct
bands at 1591 and 1054 cmÀ1, which are regions that are
It has been proposed that this metal-mediated [2+2+2]
cyclotrimerization reaction involves a [2+2] cycloaddition of
non-polar acetylene and iminoborane, resulting in a 1,2-
azaborete complex.[14] Therefore, we were interested in
exploring the regioselectivity of this reaction in more detail,
particularly using internal alkynes. As a result, reaction of
1 with 4,4,5,5-tetramethyl-2-(phenylethynyl)-1,3,2-dioxabor-
olane yielded 1,2-azaborete complex [h4-1,2-{B(tBu)N-
(tBu)C(Ph)C(Bpin)}RhCl(PiPr3)] 5, which was not isolated.
The subsequent intramolecular cyclization with acetylene led
to the formation of 1,2-di-tert-butyl-6-phenyl-5-pinacolato-
1,2-azaborinine (6), providing the first example of an
azaborinine boronic ester (Supporting Information,
Scheme S1).
Pure compound 6 was isolated in 49% yield using column
chromatography on silica gel. The 11B NMR spectrum of 6
shows resonances at d = 49.8 and 32.0 ppm, and the 1H NMR
spectrum gives rise to two tert-butyl signals (d = 1.29,
1.26 ppm), two ring proton signals (d = 7.66, 6.79 ppm), and
three resonances for the phenyl and pinacol substituents (d =
7.44, 7.12, 0.93 ppm). These spectroscopic inferences were
subsequently confirmed by crystallographic studies, as illus-
=
À
characteristic for the C C and B N stretching frequencies,
respectively. Note that the UV/Vis absorption spectrum of 3
in benzene shows a band at 472 nm, which is highly red-
shifted compared to the parent 1,2-azaborinine (269 nm).[10a]
To improve our understanding of the synthesis of 1,2-
azaborinine 3, we have performed the stoichiometric reaction
of 1 and 2 in the presence of ethynylferrocene, resulting in the
formation of [h4-1,2-{B(tBu)N(tBu)C(Fc)C(H)}RhCl(PiPr3)]
4. The 11B NMR signal at d = 23.5 ppm is indicative of a 1,2-
azaborete complex and the 1H NMR chemical shifts are
consistent with this. The assignment based on spectral data is
further supported by a single-crystal X-ray crystallographic
À
À
analysis (Figure 2). The longer B1 C2 (1.548(4) ꢀ) and N1
C1 bonds (1.469(3) ꢀ) of 4 compared to those of 3
(1.523(4) ꢀ, 1.410(3) ꢀ) result in a distortion of the corre-
sponding four-membered ring. The observed bonding in 4 is
similar to that of a previously reported Rh-1,2-azaborete
complex[14] and other rhodium complexes[18] of boron hetero-
cycles.[19]
À
Compound 3 is of particular importance owing to its
ferrocenyl units, and it warrants further discussion. Ferrocene
derivatives are widely used in several areas.[20] In particular,
boryl- and borate-functionalized ferrocenes have received
a great deal of attention in recent years owing to their
applications in homogeneous catalysis, anion sensing, and
organometallic polymer chemistry.[21,22] Moreover, ferroce-
nylboronic acids are versatile precursors for other ferrocene
derivatives and serve as interesting building blocks for
supramolecular materials.[23] More recently, the potential of
1,1’-diborylated ferrocene derivatives as a receptors for
neutral and anionic substrates, and as Lewis acid activators
for various organic and organometallic reactions, has also
trated in Figure 3. The B1 N1 bond length was found to be
1.4707(17) ꢀ, which is comparable to those of known non-p-
extended
parent
1,2-dihydro-1,2-azaborinines
(1.43-
1.45 ꢀ).[16,25] The endocyclic B1 C4 bond (1.5223(19) ꢀ) of
À
À
6 is significantly shorter than the exocyclic B2 C2 bond
(1.5489(18) ꢀ). The 1,2-azaborinine ring in 6 is twisted by
approximately 578 relative to the phenyl ring and 418 relative
to the Bpin ring.
To gain further insight into the mechanism, we performed
the reaction of 4 with [D1]ethynylferrocene under similar
conditions. 1,2-di-tert-butyl-3-deuterio-4,6-diferrocenyl-1,2-
azaborinine (7) was obtained exclusively, and no other
isomers were detected by NMR spectroscopy. The position
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Scheme 3. Synthesis of 2-tert-butyl-4,6-diferrocenyl-1-hydro-1,2-azabori-
nine (8).
10À4 Torr.[29] Therefore, we hope that the removal of a tert-
butyl group under thermolytic conditions reported herein will
generate interest in the synthesis of parent BN compounds.
To shed further light on the electronic structure of 3, 4,
and 8, and the reactions connecting them, quantum-chemical
calculations were performed with DFT methods at the M06-
2X/(LANL2TZ+f,6-311 + G(2d,p)) level of theory. Forma-
tion of 3 from the reaction of two equivalents of ethynylfer-
Figure 3. Molecular structure of 6. Ellipsoids are set at 50% proba-
bility; hydrogen atoms and some ellipsoids are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: B1–N1 1.4707(17), B1–C4
1.5223(19), C3–C4 1.3534(18), C2–C3 1.4392(18), C1–C2 1.3743(18),
N1–C1 1.3996(16); N1-B1-C4 113.42(11), C1-N1-B1 117.70(10), C1-C2-
C3 119.00(11).
of the deuterium was confirmed by 13C NMR, which gives
a triplet for the boron-bound carbon atom (d = 123.4 ppm,
1JCD = 22.9 Hz). The exclusive formation of 7 clearly shows
that the first step entails the formation of a four-membered
ꢀ
rocene and one equivalent of tBuB NtBu is highly exother-
mic/exergonic, suggesting an efficient catalytic reaction
(Supporting Information, Figure S1). Most remarkable is
the observation of isobutene elimination from the N-tert-butyl
substituent, which is exothermic (À12.1 kcalmolÀ1) and quite
exergonic (À27.5 kcalmolÀ1), whereas the elimination of
isobutene from the B-tert-butyl substituent is endothermic
(10.6 kcalmolÀ1) and only slightly exergonic (À3.9 kcal
molÀ1). This is in agreement with our experimental results
(that is, the selective formation of 8) and the origins of this
selectivity can be understood in terms of aromaticity. To
evaluate the aromatic character of 1,2-azaborinines 3 and 8
more quantitatively, we calculated the nucleus-independent
chemical shift values NICS(0) and NICS(1) (Supporting
Information, Table S1) at the same level of theory mentioned
above. The fact that 3 is much less aromatic than 8 is
presumably due to the non-planarity of the ring in 3, as close
proximity of the N-bound tert-butyl group and the adjacent
À
ring, followed by the cleavage of the B C bond (Scheme 2).
Scheme 2. Synthesis of 1,2-di-tert-butyl-1,2-azaborinine (7).
Compounds containing a tert-butyl group attached to
À
a nitrogen atom are known to undergo homolytic C N bond
cleavage under flash vacuum thermolysis (FVT) conditions,
which results in the formation of isobutene by H elimina-
tion.[26] In this context, our aim was to access parent BN
isosteres, which can be further functionalized at both the
nitrogen and boron positions.[5,27] It is noteworthy in this
À
ferrocenyl moiety result in steric repulsion, forcing the tBu N
fragment out of the ring plane. This is in agreement with the
crystallographically observed deviation from planarity.
In summary, we have developed a facile and catalytic
synthetic route to novel 1,2-azaborinines by means of
a [2+2+2] cycloaddition reaction, and the introduction of
a number of different functional groups to the azaborinine
framework was achieved. To the best of our knowledge,
compound 3 is the first example of an isolated ferrocene-
functionalized 1,2-azaborinine, a class of compound which has
potential for applications in materials science. Our method
also enabled the synthesis and isolation of a unique example
of an azaborinine boronic ester (6), which may be a suitable
substrate for Suzuki–Miyaura cross-coupling reactions. Fur-
thermore, we have shown that simple thermolysis provides an
access to free-NH-containing BN isosteres. An investigation
of the scope and mechanism of the reaction and further
exploration of the chemistry of this heterocycle is currently
underway.
À
regard that attempts to remove a tBu N group from an
azaborinine by Liu et al. were unsuccessful, and thus the TBS
(tert-butyldimethylsilyl) group was used as a viable N-
protecting group in 1,2-dihydro-1,2-azaborinine chemistry.[28]
With the 1,2-azaborinine 3 in hand, we were thus pleased to
discover that upon thermolysis (2008C for 3 h in Kugelrohr
distillation apparatus), compound 3 can be easily converted
into 2-tert-butyl-4,6-diferrocenyl-1-hydro-1,2-azaborinine (8),
accomplished with the elimination of isobutene (Scheme 3).
À
The formation of isobutene as well as a signal for the N H
proton was observed in the 1H NMR spectrum of 8. The
11B NMR signal moved markedly upfield from d = 47 ppm in
3 to d = 38 ppm in 8, also upfield of previously reported 11B
chemical shifts in 1,2-azaborinine compounds. Unfortunately,
several attempts to grow single crystals of 8 suitable for X-ray
diffraction failed. It is important to note that recently
Chrostowska et al. reported the elimination of isobutene
and hydrogen from N-tert-butylimines at 8008C under 3 ꢁ
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Keywords: boron · cyclization · heterocycles · isosterism ·
rhodium
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