Synthesis of Complex Boron-Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes under Mild Conditions

Article abstract of DOI:10.1021/jacs.9b12336

The reactions of organic azides with diaryl(dihalo)diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron-boron bond, while the reaction of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B-B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide ?-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.

Full text of DOI:10.1021/jacs.9b12336

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Article
Synthesis of Complex Boron-Nitrogen Heterocycles Comprising
Borylated Triazenes and Tetrazenes Under Mild Conditions
Dominic Prieschl, Guillaume Bélanger-Chabot, Xueying Guo, Maximilian Dietz,
Marcel Müller, Ivo Krummenacher, Zhenyang Lin, and Holger Braunschweig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12336 • Publication Date (Web): 12 Dec 2019
Downloaded from pubs.acs.org on December 12, 2019
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Synthesis of Complex Boron-Nitrogen Heterocycles
Comprising Borylated Triazenes and Tetrazenes Under Mild
Conditions
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Dominic Prieschl,+,1,2 Guillaume Bélanger-Chabot,+,1,2 Xueying Guo,+,3 Maximilian Dietz,1,2 Marcel
Müller,1,2 Ivo Krummenacher,1,2 Zhenyang Lin*,3 and Holger Braunschweig*1,2
1 Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
2 Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland,
97074 Würzburg, Germany
3 Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong,
China
Supporting Information Placeholder
salts makes them capable of alkylating DNA, which ties them
closely to the development of anti-cancer molecules.
ABSTRACT: The reactions of organic azides with
diaryl(dihalo)diboranes(4) were explored, resulting in the
observation of a number of surprising reactivity patterns. The
This work
reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4)
Pharmacologically-
relevant polynitrogen
resulted in the formation of five-membered rings comprising
diboryl-triazenes with retention of the boron-boron bond, while the
compounds
N
Ph
Ar
N
Ar
Ar
Ph
N
B
N
N
N
reaction of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4)
with phenyl azide yielded a six-membered ring bearing a diboryl-
triazene whereby the B-B bond was ruptured by the insertion of an
arylnitrene-like reactive intermediate. Both types of heterocycles
feature unprecedented connectivity patterns and are very rare
examples of boryl triazenes beyond the more common 1,2,3-
triazolatoborane. They are also the product of a unique type of aryl
migration from a boron center to the phenyl azide γ-nitrogen center.
Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with
azide groups, using trimethylsilyl azide as the transfer reagent,
yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as
side-products), invoking the intermediacy of the first N-boryl-
substituted iminoboranes, which are BN isosteres of
monoborylated alkynes. The synthetic results are complemented
with mechanistic proposals derived from quantum-chemical
calculations.
R
diboryl-substituted
triazenes
N
N
B
BX2
BF2
N
R''
N
Ar
R'
Ph
triazenes
N
N3
B
R
R
N
N
boryl-substituted
tetrazole analogue
or
boryl-substituted
BN triazole
N
N
B
N
N
R
Ar
triazoles
N
Ar
N
N
isostere
N
R
tetrazoles
Figure 1. Important organic polynitrogen motifs in pharmacology
and their related BN compounds uncovered during this study.
The reactivity of boron-containing species with organic azides or
of boron azides themselves has been generating renewed4-5 interest
lately (Scheme 1).6-16 An intriguing case is the development of ring-
expansion reactions on boroles using various aryl azides.17-23 These
reactions have provided several surprising reactivity patterns and
often highly exotic heterocyclic boron species. Interestingly, no
reactivity between azides and diboranes(4) has been reported to
date. This chemistry would be expected to yield other novel boron-
nitrogen derivatives, and, in particular, has the potential to yield
boron derivatives of pharmacologically-relevant polynitrogen
moieties (Figure 1). In the context of increased interest in
pharmacologically-active boron compounds,24 we saw the
development of diborane azide chemistry as an area of fundamental
research with significant medium-term applicability. Moreover,
diboranes(4) are highly relevant to catalytic coupling reactions.25
Establishing their chemistry with common functional groups like
azides is an important step towards a broad knowledge of these
catalytically-relevant compounds.
Introduction
Despite their potentially highly energetic nature, organic azides
provide a panoply of synthetic uses1 beyond their applications as
energetic materials (i.e. as primary explosives).1-2 They are useful
synthetic intermediates and are involved in a variety of chemical
transformations, including cycloadditions (e.g., "click" or Huisgen
reactions), reductions to the corresponding amine, dinitrogen
extrusions to yield nitrenes as reactive intermediates, etc. Azides
are important precursors in the synthesis of other
pharmacologically important bioisosteres, such as 1,2,3-triazoles
and tetrazoles (Figure 1). Closely related to azides are triazenes,
which have a similarly versatile chemistry in cycloaddition
reactions, as sources of diazonium salts and as protecting groups
for amines functionalities.3 Their behavior as masked diazonium
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Herein we present the reactivity of four diaryl(dihalo)diboranes(4)
which the boron-boron bond remains intact. 1a was isolated as an
1
2
3
4
5
6
7
8
with phenyl and/or trimethylsilyl azide. These diboranes(4) were
chosen as substrates as our previous work has shown them to be
quite reactive and often provide structurally interesting products,
most likely due to the combination of labile, accessible B-X bonds
and steric stabilization from the adjacent aryl substituents.26-29 We
demonstrate that these reactions yield unprecedented boron-
substituted triazenes, found as exotic five- and six-membered rings
and resulting from unexpected 1,4-aryl-migrations, and
tetrazaboroles, resulting from cycloaddition reactions on a novel
boryliminoborane.
orange solid in 50% yield and 1b as orange crystals in 55% yield.
While we could ascertain the connectivity of both isolated
compounds 1a and 1b by single-crystal X-ray crystallography (see
Supporting Information), only the data for 1a is of sufficient quality
to be discussed in detail. The crystal structure of 1a clearly
indicates the presence of a triazene with a long (1.360(3) Å) and a
short (1.274(4) Å) N-N bond, typical of other triazenes.30 The B-N
bond lengths also agree with our assignment of the compound as a
boryl triazene (covalent B-N bond) forming an internal B-N adduct
(dative B-N bond), with a shorter B-N bond length of 1.469(5) Å
and a longer one of 1.582(5) Å. The B-B bond length in 1a
(1.681(5) Å) is noticeably shorter than in other sp2-sp3 diboranes
such as Dur2BB(sIMes)Br2 (1.762(4) Å) (sIMes = 1,3-bis(2,4,6-
trimethylphenyl)-4,5-dihydroimidazol-2-ylidene),31 and thus
comparable to that of diboranes(4), such as the starting material
(1.668(5) Å in Dur2B2Br2).32 The B-Br bond lengths are
expectedly longer in 1a than those of Dur2B2Br2 (2.030(3) and
1.999(4) Å in 1a, 1.940(3) and 1.943(3) Å in Dur2B2Br2), but
significantly shorter than those of (Dur)2BB(sIMes)Br2 (2.083(3)
and 2.061(3) Å).
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Scheme 1. Representative examples of the reactivity of
boron azides and that of organic azides with borane
substrates.
R
R
T
R
B
N
R'
B
N
B
N3
R'
R'
Paetzold
R'
R
N
B
N
N
N
Scheme 2. Reaction of phenyl azide with 1,2-diaryl-1,2-
dihalodiboranes(4) (top) and crystal structure of 1a
(bottom).a
R''
R'
R
N
R'
N
BR''2
H
N
R
B
R'' N3
B
R''
R
N2
R
R''N3
R'
B
B
B
RB NR'
N R'
R
R
B
N
N
N3
N
N
R
R'
N
H
Paetzold
X
Ar
X
X
Ar
X
PhN3
B
B
Ar
Ar
Ar
R
B N
RN3
-N2
B
B
B
B
Ar
N
N
benzene
Ar
Ar
Ar
Ar
Ph
N
Schulz
1
NHC
H
BAr3
H
X = Br, Ar = Dur
BAr3
-N2
N3
B N
H
H
1a
NHC
X = Cl, Ar = Mes
1b
Rivard
Ar
B
Ar
Ar'N3
-N2
B
Ar'
N
Ph4
Ph4
Braunschweig/Martin
cycloadditions
Curran
NHC
NHC
N
N
E
B
N3
B N
H
H
H
H
E = N, C-R
1a
aSelected structural parameters for 1a. Bond distances (Å): B1-
B2 1.681(5), B1-N3 1.582(5), B2-N1 1.469(5), N1-N2 1.360(3),
N2-N3 1.274(4), B1-Br1 2.030(3), B1-Br2 1.999(4). Bond angles
(°): N1-N2-N3 111.9(2). The ellipsoids are depicted at the 50%
probability level. Ellipsoids for the peripheral aryl groups and
hydrogen atoms were omitted for clarity.
Results and Discussion
Reaction of symmetrical diboranes(4) with phenyl azide.
Contrary to examples of Lewis adducts between boranes and
covalent azides, which sometimes lead to dinitrogen extrusion and
nitrene-like insertion reactions (Scheme 1),11-12 the reaction of
either a stoichiometric amount or an excess of phenyl azide (PhN3)
with 1,2-dibromo-1,2-diduryldiborane(4) (Dur2B2Br2) or 1,2-
dichloro-1,2-dimesityldiborane(4) (Mes2B2Cl2) does not lead to
gas evolution, despite clear signs of reaction, such as the
appearance of new 11B NMR signals at higher field than those of
the starting materials (1a: 61.7 and −2.2 ppm; 1b: 63.4 and 5.9
ppm) and a color change from colorless to orange. The 11B NMR
chemical shifts at ca. 60 ppm are characteristic of arylboranes in a
tricoordinate environment, while the signals at ca. 0 ppm are
Compounds 1 are intriguing as they require a migration of an aryl
moiety onto the terminal nitrogen of PhN3 and a 1,2 migration of a
halide of the diborane fragment. Quantum-chemical calculations
using 1a as the model compound suggest a mechanism (Scheme 3)
in which the reaction proceeds via an initial adduct formation
between the α-nitrogen of PhN3 and a B(aryl)Br moiety (1_1). The
bonds with the sp3 boron center in 1_1 are weaker than those with
the sp2 boron center. This makes the migration of the duryl group
attached to the sp3 boron onto the γ-nitrogen of PhN3 feasible, with
a 27.3 kcal•mol−1 Gibbs free energy barrier, making this process the
rate-limiting one. While the rupture of a B-C bond in a 1,2 aryl
migration has been observed before,31 the migration onto a nitrogen
atom to give boryltriazene 1_2, as observed here, is to our
knowledge unprecedented. The resulting boryltriazene 1_2 then
indicative of
a tetracoordinate boron environment. X-ray
crystallography confirmed that, in both cases, the resulting
compound was a five-membered ring that is best described as a
triazene in which the 1 and 3 positions are each attached to one of
the two boron atoms of a sp2-sp3 diborane(5) (Scheme 2) and in
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undergoes a cis-trans isomerization with a lower overall barrier of
Reaction of an unsymmetrical diborane(4) with phenyl azide.
1
2
3
4
5
6
7
8
23.2 kcal•mol−1, yielding boryltriazene 1_5. The cis-trans
isomerization is a two-step process, in which N-N bond cleavage
accompanied by the formation of an ion pair (B2N three-
membered-ring anion and duryldiazonium) occurs first, followed
by an electrophilic attack by the duryldiazonium at the three-
membered-ring N atom in 1_4 to give the trans isomer 1_5.
Because of steric constraints, the inversion of the N-Dur nitrogen
fragment in 1_3 necessary to the cis-trans isomerization process is
severely hindered and must involve the rupture of the PhN-N bond,
yielding a duryldiazonium moiety. In the last step, 1_5 undergoes
an internal N-B adduct formation and a simultaneous 1,2 halide
migration with a Gibbs free energy barrier of 11.1 kcal•mol−1,
yielding 1_P (1a).
1,1-Di(9-anthryl)-2,2-difluorodiborane(4) (An2BBF2),26-29, 45 an
unsymmetrical diborane(4) that features reactivity patterns often
quite divergent from those of its symmetrical relatives, was chosen
as another promising substrate for reactions with organic azides. Its
reaction with PhN3 proceeded in a seemingly similar way as that
leading to 1, with a color change from red to orange. 11B NMR
spectroscopy, however, suggested the cleavage of the B-B bond,
with two relatively high-field shifted signals at 31.5 and 1.0 ppm.
This was confirmed by X-ray diffraction experiments, which
showed that the main product (2) contained not a five-, but rather a
six-membered ring featuring a diboryl-substituted triazene where
the B-B bond had indeed been broken by the insertion of a
phenylnitrene-like moiety (Scheme 4). This cleavage is reminiscent
of that of the Al-Al bond in a dialane by PhN3.46 2 was isolated as
a yellow solid in 51% yield. It should be noted that at least 2.2
equivalents of PhN3 are required for the reaction to reach
completion.
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Compounds 1 represent, to our knowledge, the first incursion into
the field of diborane(4)-azide chemistry, and are rare examples of
structurally characterized boryl-substituted triazenes beyond 1,2,3-
13, 33-40
triazolate derivatives.6-7,
Only two examples of five-
membered rings with the BBNNN sequence are currently known,
and these are derived from non-classical boron clusters.33-34 The
closest example of a non-triazole boryl-triazene adduct with a
borane group is the borole ring-expansion intermediate reported by
Martin et al.21 Closely related examples are those of triazenes
borylated at the terminal position,8, 10, 41 including examples of ring-
expansion reactivity of azides with boroles,19 and compounds in
which an azide group bridges two boryl moieties.42 Compounds 1
are also new sp2-sp3 diboranes, a class of diboranes that are unusual
examples of nucleophilic boron species.43-44
A plausible mechanism derived from quantum chemical
computations (Scheme 5) suggests that the first steps of the reaction
are essentially identical to that of the formation of 1 (Scheme 3),
albeit with a slightly lower Gibbs free energy barrier of 20.5
kcal•mol−1 (migration of the anthryl group onto the terminal azide
nitrogen atom). The triazene 2_2 yields the five-membered ring 2-
_4 via a cis-trans isomerization followed by ring formation via a
B-N Lewis acid-base addition,
Scheme 3. Gibbs free energy profile calculated for the reaction of Dur2B2Br2 with phenyl azide, leading to the formation of
the five-membered ring product 1_P (1a). The relative Gibbs free energies and electronic energies (in parentheses) are given
in kcal•mol−1.
G kcal mol-1

Br
Br
E kcal mol-1)
Br
Br

Br
Br
B
B
B
B
Dur
B
N
B
Dur
Dur
N
N
N
N
Ph
N
Ph
Ph
Br
Dur
Br
TS1_1
N
N
N
B
B
Dur
Dur
27.3
(9.0)
Dur
TS1_4
TS1_3
N
N
N
Ph
20.9
(4.0)
20.0
(3.5)
Br
1_1
Br
13.3
(-4.3)
B
B
Dur
N
Ph
0.0
4.1
(-13.9)
N
N
Dur2B2Br2
-0.1
(-20.5)
Dur
TS1_5
-2.3
(-20.3)
Br
Br
+
Ph-N3
B
B
Dur
-9.8
(-28.3)
Br
N
Br
Br
N
N
Br
B
B
Dur
N
B
B
N
Dur
Ph
Dur
N
Ph
Dur
N
-20.9
(-38.3)
N
1_4
Ph
Dur
N
Dur
Br
Br
Br
Dur =
1_2
Br
B
B
1_3
B
B
Dur
N
N
Dur
N
N
Ph
Ph
N
N
1_P
-44.3
(-65.3)
Dur
1_5
N-N bond reformation
Duryl-1,4-migration
N-N bond cleavage
Five-membered ring
formation
cis-trans isomerization
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1
2
3
4
5
6
7
8
but without halogen migration. Unlike the cis-trans isomerization
19.3° between the N1-B1-N4 and B2-N2-N3 planes. Like 1,
in the reaction of PhN3 with Dur2B2Br2 (1_3 to 1_5 shown in
Scheme 3), the isomerization here is a one-step process. This is
because the sterically much less hindered BF2 and NNN-An units,
when compared with BBrDur and NNN-Dur, respectively,
facilitate the B-N interaction, leading to the five-membered-ring
formation during the inversion process of the lone pair associated
with the anthryl bonded nitrogen. This is supported by a shorter
distance between the BF2 and the N-anthryl moieties in 2_3 than
that between BDurBr and NDur in 1_3. Moreover, the shorter
(Ph)N-N distance and longer N=N(Ar) distance in 2_3 than in 1_3
do not point towards the involvement of an anthryl diazonium
fragment in the formation of 2_P, in contrast to that of 1_P. A
second equivalent of PhN3 then forms an adduct with the B(An)
boron atom of the five-membered cyclic intermediate, yielding the
intermediate 2_5, from which the loss of dinitrogen and ring
insertion proceeds very exergonically (134.6 kcal•mol−1) via a
Gibbs free energy barrier of 24.2 kcal•mol−1, yielding the six-
membered heterocycle 2_P (2). The possibility of forming 2_5 in
this system and the apparent non-existence of the analogous PhN3-1
adduct is ascribed to a higher local steric bulk due to the duryl
groups in 1, leading to a predicted Gibbs free energy increase of
23.8 kcal•mol−1 upon going from 1_P + PhN3 to the PhN3-1 adduct
(1_P1) (see Supporting Information, Scheme S1), in stark contrast
to the practically thermoneutral formation of 2_5 from 2_4 and
PhN3 (6.4 kcal•mol−1). This is also in agreement with our
experimental results that showed no reaction between isolated 1b
and PhN3.
compound 2 is among the few known examples of structurally
characterized diboryl-substituted triazenes21, 29-30 and comprises a
very rare cyclic BNBNNN connectivity motif.47
Scheme 4. Reaction of phenyl azide with 1,1-di(9-anthryl)-
2,2-difluorodiborane(4) (top) and solid-state structure of 2
(bottom).a
Ph
9
An
An
F
F
exc. PhN3
benzene
An
Ph
N
F
B
N
B
N
F
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B
B
N
An
2
2
The structural features in 2 again point towards a boryl-substituted
triazene (N-N bond lengths of 1.281(2) and 1.331(2) Å) that forms
an adduct with a borane group, with B-Ntriazene bond lengths of
1.491(2) Å (B2-N2) and 1.603(2) Å (B1-N4). The compound
features B-N bonds of significantly different lengths between the
Ph-N group and its two boryl substituents (B2-N1 1.382(2) Å; B1-
N1 1.514(2) Å). The B2-N1 bond length of 1.382(2) Å suggests a
strong double bond character, significantly shorter than the B2-N2
(triazene) bond length of 1.491(2) Å. The BF2 moiety lies
significantly above the N1-B2-N2-N3-N4 plane, with an angle of
aSelected structural parameters for 2. Bond distances (Å): B1-N1
1.514(2), B2-N1 1.382(2), B1-N4 1.603(2), B2-N2 1.491(2), N2-
N3 1.331(2), N3-N4 1.281(2), B1-F1 1.375(2), B1-F2 1.392(5);
Bond angles (°): N4-B1-N1 106.2(1), N4-B1-F2 105.9(1), N4-B1-
F1 108.5(1). Torsion angles (°): N1-B2-N2-N3 −4.2(2), B2-N2-
N3-N4 8.7(2), N2-N3-N4-B1 5.0(2), N3-N4-B1-N1 −19.6(2), B1-
N1-B2-N2 23.2(2). Angle (°) between N1-B1-N4/B2-N2-N3
planes: 19.3. The ellipsoids for the peripheral aromatic groups and
the hydrogen atoms are omitted for clarity.
Scheme 5. Gibbs free energy profiles calculated for the reaction of diborane An2B2F2 with phenyl azide leading to the six-
membered ring product 2_P (2). The relative Gibbs free energies and electronic energies (in parentheses) are given in
kcal•mol−1.
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G kcal mol-1
E kcal mol-1
F
F
An
B
1
2
3
B
)
N
N
N
An
F
Ph
B
B
TS2_1
F
N
Ph
4
5
20.5
(4.0)
N
N
Ph
An
N
N
6
7
8
9
N
N
TS2_3
7.4
(-8.2)
F
An
3.9
(-11.2)
0.0
B
B
N
F
An
An2B2F2
F
F
N
Ph
+
An
B
B
An
N
Ph
Ph-N3
N
TS2_5
N
N
B
N
Ph
F
-11.7
(-44.1)
N
An
10
11
12
13
14
15
16
17
18
19
20
21
22
-17.9
(-35.8)
An
2_1
B
N
F
-18.8
(-35.5)
An
N
Ph
An
F
F
N
An
G = 24.2 kcal mol-1
F
B
B
TS2_4
B
B
N
F
N
N
-29.4
(-62.2)
N
-29.5
(-62.8)
Ph
Ph
N2
N
Ph-N3
-35.9
(-53.4)
N
An
N
2_2
Ph
An =
N
N
Ph
N
An
F
F
F
2_3
An
F
B
B
B
B
N
F
An
B
N
B
N
F
N
N
N
Ph
Ph
An
N
An
N
N
An
Ph
2_5
2_4
2_P
-164.1
(-187.1)
Anthryl-1,4-migration
Six-membered-ring formation
cis-trans Isomerization
Five-membered-ring formation
An
Br
Br
Reaction of An2B2Br2 with PhN3. The reaction of the related
diborane 1,2-di(9-anthryl)-1,2-dibromodiborane(4) (An2B2Br2)
was surprisingly much less straightforward. Indeed, the reactions
led to relatively complex mixtures from which no pure compound
could be isolated. Still, some interesting reaction products could be
observed by single-crystal X-ray crystallography (Scheme 6). One
of them is a peculiar triazenium analogue (3). In contrast to the
complete migration of the anthryl group onto the terminal azide
nitrogen atom observed for 1 and 2, in the case of 3 the terminal N2
moiety (γ- and β-nitrogen) of the azide group apparently effected a
Diels-Alder-like addition to the central C6 ring of the anthryl
moiety, yielding a zwitterionic compound. Unfortunately, the X-
ray diffraction data collected for compound 3 were of insufficient
quality for discussion of bond parameters.
Ph-N3
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
B
B
poorly defined mixtures
benzene
An
Br
An
Br
B
B
N
C
Ph
N
N
H
3
Quantum-chemical computations suggest that the energy of
activation for the Diels-Alder-type reaction is 3.4 kcal•mol−1 lower
than that for the migration of the anthryl group onto the terminal
nitrogen atom of the PhN3, a process analogous to the pathway
observed for 1 and 2 (Scheme 7, red). Compound 3 is also predicted
to be more thermodynamically stable than the putative intermediate
3_2a (Gibbs free energy difference of 7.2 kcal•mol−1). Although
the [4+2]
4
Scheme 6. Reaction of phenyl azide with 1,2-dianthryl-1,2-
dibromodiborane(4) (top) and solid-state structures of some
of the observed reaction products (middle and bottom).a
H
H
H
H
N
N
Br3B
BBr3
aSelected structural parameters for 4. Bond distances (Å): C1-C2
1.596(5), C3-N1 1.313(4), C4-N2 1.308(3), N1-B1 1.556(4), N2-
B2 1.553(4), B1-Br1 2.016(5), B1-Br2 1.991(4), B1-Br3 2.037(4),
B2-Br4 2.009(4), B2-Br5 2.010(4), B2-Br6 2.031(5). The
ellipsoids for the peripheral aryl groups and some of the hydrogen
atoms were omitted for clarity.
cycloaddition leads to the dearomatization of the central ring of the
relevant anthryl substituent, our results indicate that this process is
not particularly energy-intensive – significantly less, in fact, than
that predicted for the analogous process involving the
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Journal of the American Chemical Society
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dearomatization of the duryl moiety, with a Gibbs free energy
barrier of 21.5 kcal•mol−1for 3, 36.8 kcal•mol−1 for the system
leading to 1_2a (see Supporting Information, Scheme S2).
phenyl
azide,
5-(9-anthryl)-1,4-diphenyl-4,5-
1
2
3
dihydrotetrazaborole, were detected, providing support for the
parallel processes suggested by our quantum chemical
computations (see Supporting Information Scheme S3).
Compound 3 is nevertheless an intriguing finding, as it is, to the
best of our knowledge, the only reported case of an azide involved
in a Diels-Alder type cyclization. While triazenium cations are a
known class of compounds,48-50 and are involved in the
decomposition of triazenes to diazonium species under acidic
conditions, compound 3 is a rare example50-51 in which the central
nitrogen is tricoordinated.
4
5
6
7
8
9
Scheme 7. Gibbs free energy profiles calculated for the
reaction of An2B2Br2 with phenyl azide leading to the [4+2]
cycloaddition product 3_P (3). The relative Gibbs free
energies and electronic energies (in parentheses) are given
in kcal•mol−1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Reactivity of diboranes(4) with trimethylsilyl azide. To our
knowledge, no example of an azide-substituted diborane(4) has
ever been reported. To this end we reacted Mes2B2Cl2, 1,2-
dibromo-1,2-dimesityl-diborane(4) (Mes2B2Br2) and Dur2B2Br2,
respectively, with three or more equivalents of trimethylsilyl azide,
a common reagent for the introduction of azide functional groups
(Scheme 8). Early on, the reactions showed clear signs of
G kcal mol-1
Br

An
Br
Br
E kcal mol-1
)
Br
An

B
B
B
B
N
N
Ph
TS3_1a
N
N
Ph
N
An
Br
Br
24.9
(0.8)
N
B
B
An
dinitrogen evolution (bubbling and a signal at −71 ppm in the 14
N
TS3_1
N
NMR spectrum)52 that coincided with the elimination of Me3SiX
observed by 1H NMR spectroscopy. Two major 11B NMR signals
could be observed at ca. 50 and 35 ppm in each reaction (5a: 50.7
and 34.0 ppm; 5b: 52.7 and 34.8 ppm; 5c: 51.5 and 34.8 ppm).
After ca 48 h, the azide/halogen exchange was complete and the
signals at ca. 50 ppm were replaced by higher-field signals at ca.
44 ppm (6a: 43.3 and 32.9 ppm; 6b: 43.8 and 33.5 ppm). 1H NMR
spectroscopy strongly suggested that at least two products were
present at the end of each reaction. Mass spectrometry and single-
crystal X-ray crystallography indicated that the main compounds
present were the boryl-substituted tetrazaboroles 5 and 6, which
differ by the nature of X in the boryl group -B(X)Ar (where X can
be Cl, Br or N3). Because of the difficulties in controlling the
occurrence of the second halogen/azide substitution (from 5,
yielding 6), the isolation of pure (B(halide)Ar)-substituted
compounds 5 was not attempted. Instead, the diborane starting
materials were treated with 3-10 equivalents of TMSN3 in benzene
or dichloromethane, which allowed the isolation of both MesB(N3)-
and DurB(N3)-substituted tetrazaboroles (6). In the case of
Mes2B2Cl2, the reactions always yielded crystals that could be
identified as those of a dimer (8) of boryl-substituted tetrazaboroles
(5a and 6a) by X-ray crystallography. 11B NMR chemical shift
predictions suggest that the bridging -B(Ar)(X) moiety in 8 and
analogues should have a signal at ca. 5 ppm. We indeed observed
a weak signal in that region in most reactions leading to 5 and 6,
which would suggest that the dimer species is only formed in small
amounts in solution and that the monomeric forms of the boryl-
N
N
Ph
3_1'
21.5
(3.8)
12.6
(-4.9)
Br
N
An
Br
An
N
B
B
10.3
(-7.4)
N
B
Ph
Br
0.0
Br
3_2a
An
An2B2Br2
-0.2
B
B
Br
Br
An
(-17.5)
An
+
N
Ph
Ph-N3
B
-7.4
N
(-28.3)
N
N
3_1
Ph
N
N
An =
3_P
[4+2] Cycloaddition
Anthryl-1,4-migration
For the system leading to 2 (Scheme 5), we also considered an
alternative putative pathway with a [4+2] cycloaddition transition
state from an isomer of 2_1 in which the PhN3 is coordinated to the
BF2 moiety. However, this transition state lies 31.7 kcal•mol−1
above the energy reference point (An2BBF2 + PhN3) leading to
2_2a (See Supporting Information, Scheme S2), which represents
a much higher barrier than that of the 1,4-anthryl migration shown
in Scheme 5. The concerted migration of bromide in the formation
of 3 via TS3_1 has significantly lowered the energy barrier with
respect to that of the analogous putative reaction of An2BBF2
(Scheme S2) because of the weaker B-X bond in the bromide
analogue relative to that of the fluoride compound. After the
bromide migration, the sp3 boron center is coordinatively saturated,
making the bonded anthryl group more electron donating and
stabilizing 3_P as well as the transition state TS3_1 that leads to it
(Scheme 7).
tetrazaboroles
5 and 6 are dominant in solution. X-ray
crystallography experiments in conjunction with NMR
spectroscopy data and mass spectrometry suggest that 6b is
overwhelmingly a monomer in all phases, while 6a is most likely a
dimer to a significant extent only in the solid phase, as the monomer
was never observed by X-ray crystallography.
The difficulty in reproducibly obtaining compound 3 suggests that
it is likely to be kinetically unstable and susceptible to dinitrogen
extrusion, potentially yielding compounds such as the coupled
bis(anthrylimine) double BBr3 adduct (4) found in some iterations
of the reaction. The C-N bond lengths in 4 are consistent with their
assignment as imines (C3-N1 1.313(4) and C4-N2 1.308(3) Å)
(Scheme 6). This intermediate is the result of an apparent radical
reaction, as shown by the C1-C2 bond and as suggested by the
presence of imine-bound protons, likely abstracted from the
reaction medium. The formation of 4 therefore strongly points to
complex processes. Moreover, at least two kinetically-accessible
pathways leading to very different outcomes were identified in our
calculations (see Supporting Information, Scheme S3). Several
species with masses identical to the proposed products could be
detected by high resolution mass spectrometry. Indeed, compound
The isolation of pure 6a (or its dimer 8) is complicated by the
formation of a second species in significant amounts (ca. 50%), as
observed by 1H NMR spectroscopy, and pure material can only be
obtained by repeated washing/recrystallization operations. We
assigned this second species to the 1,3-diboryldiazadiboretidine 7,
which could also be obtained in pure form by repeated
washing/recrystallization cycles. 7 typically crystallizes first from
mixtures, which allowed us to structurally confirm our assignment
by X-ray crystallography (Scheme 8). In contrast to 6a, compound
6b can be isolated as a white crystalline solid in reasonable yields
(50%) and purity through a single recrystallization cycle.
The structural features of 6b are consistent with those of a
tetrazaborole moiety with a N-N double bond (N2-N3 bond
distance of 1.259(2) Å) and two N-N single bonds (N1-N2 1.409(2)
3
and or its isomer 3_2a, (9-anthryl)dibromoborane,
phenylimino(9-anthryl)borane and its cycloaddition product with
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Journal of the American Chemical Society
and N3-N4 1.401(2) Å). The endocyclic B-N bond lengths are
due to the fact that the silyl substituent makes the N3 moiety too
1
2
3
4
5
6
7
8
consistent with some B-N π-overlap (B-N distances of 1.442(2) and
1.424(2) Å, statistically different from one another). The exocyclic
-B(N3)Mes also features some degree of π-overlap with the ring
nitrogen, with a B2-N1 distance of 1.452(2) Å and a small C2-B2-
N1-B1 torsion angle of 3.3(3)°, almost placing the B(N3)Mes boryl
plane in the tetrazaborole plane. The structural parameters for the
tetrazaborole core in 6b and 8 are comparable to those of the
reported tetraazaboroles.53-54 The N-N single bond distances in 8
are both significantly shorter than in 6b (N1-N2 1.367(3) and N3-
N4 1.355(3) Å). In 8, the -B(X)Mes moiety is coordinatively
saturated by the dative bond with the second tetrazaborole ring,
thus preventing any π-interaction with the neighboring ring-
nitrogen atoms (endocyclic B-N distances B1-N1 1.431(4) and B1-
N4 1.439(5) Å, statistically identical; Exocyclic B-N distances of
B2-N1 1.558(5) and N2-B2' 1.604(3) Å). The endocyclic B-N bond
distances in 7 are all statistically identical and the diazadiboretidine
ring is essentially planar, as it is for bis(C6F5)-bis(tert-
butyl)diazadiboretidine55 but unlike the reported tetrakis(tert-
butyl)diazadiboretidine.56 The exocyclic B-N distances (N1-B3
1.417(3), N2-B4 1.413(3) Å) are significantly shorter than the
endocyclic ones (B1-N1 1.475(3), B1-N2 1.466(3), B2-N1
1.464(3), B2-N2 1.477(3) Å). The torsion angles C2-B4-N2-B1
and C1-B3-N1-B2 (-11.2(4) and 13.8(4)°) show that the B(N3)Mes
moieties are slightly tilted with respect to the diazadiboretidine
plane.
electron-rich, preventing the migration of the boron-bound mesityl
group, which is assumed to carry the negative charge during the
migration process. The dinitrogen elimination and simultaneous
insertion of the remaining nitrene-like nitrogen atom into the B-B
bond proceeds via a Gibbs free energy barrier of 25.1 kcal•mol−1,
making it accessible at room temperature, in qualitative agreement
with our observations. That insertion results in
a
chloromesitylboryl-substituted mesityl iminoborane (8_3), which
is thermodynamically unstable with respect to dimerization to the
diazadiboretidine (yielding 8_3-Di) or with respect to the
cycloaddition of trimethylsilyl azide, yielding 8_5. The
dimerization of the resulting tetrazaborole (yielding 8_P (8), is also
predicted to be exergonic by 10.9 kcal•mol−1, in qualitative
agreement with its observation in the solid state and with its
possible presence in smaller amounts in solution. Our mechanism
also shows that the diazadiboretidine 8_3-Di and the tetrazaborole
4_5 (5a) have similar and accessible Gibbs free energy barriers,
although 8_3-Di is thermodynamically more favorable, again in
qualitative agreement with the observation of significant amounts
of the diazadiboretidine in the formation of 5a, 5b and 6a/8.
Interestingly, the insertion of the nitrene-like nitrogen into the
boron-mesityl bond, instead of into the boron-boron bond as shown
in Scheme 9, is both kinetically and thermodynamically
unfavorable (see Supporting Information, Scheme S4).
The formation of tetrazaboroles54, 56-83 is a representative example
of the classical reactivity of iminoboranes, which are BN-isosteres
of alkynes. In this chemistry, extensively explored by Paetzold et
al. , iminoboranes can be obtained by thermolysis of boron azides
and can be treated (or trapped) with organic azides to yield
tetrazaboroles.5, 56, 65, 71-76, 78-81, 83 Although well established, the
tetrazaborole chemistry has yielded little structural data.53-54, 66
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Quantum-chemical computations on the formation of tetrazaborole
5a and its dimer 8 as a model (Scheme 9) indicate that the rate-
limiting step is the azide/chloride exchange yielding 8_2, with a
Gibbs free energy barrier of 27.1 kcal•mol−1. Our calculations
show that the transition state for the 1,4-mesityl migration from
Mes2B2Cl2+TMSN3 via 8_1 is too high to be located. This is likely
Scheme 8. Reaction of trimethylsilyl azide with diaryl(dihalo)diboranes(4) and solid-state structures of compounds 6b, 7 and
8.a
Ar
observed for Ar = Mes and X = Br
dimerization
Ar
B
X
X
by X-ray crystallography as a
Ar
X
Ar
X
TMSN3
Ar
B
N
N
B
7
mixture with
Ar
B N B
B
B
B B
N3
-N2
- TMSX
benzene
Ar
X
Ar
Ar
B
Ar
boryliminoborane
X
-TMSX
Mes
TMSN3
Mes
TMSN3
X
N3
Ar
B
Ar
Ar
B
TMSN3
- TMSX
B
N
N3
B
Ar
B
N
B
N
N
B
N
not isolated
N
N
N
Me3Si
Mes
N
B
Mes
N
Me3Si
N3
6a
6b
Ar=Mes
Ar=Dur
5a
5b
5c
Ar=Mes X=Cl
Ar=Mes X=Br
Ar=Dur X=Br
7
7
dimerization
SiMe3
Ar
N
N
N
B
X
N
B
N
6b
Ar
Ar
B
N
X
B
N
Ar
N
Me3Si
8
observed only by X-ray diffraction for
X=Cl/N3; Ar=Mes
8
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Page 8 of 13
aSelected structural parameters for 6b: Bond distances (Å): B1-N1 1.442(2), B2-N1 1.452(2), B1-N4 1.424(2), N1-N2 1.409(2), N2-N3
1.259(2), N3-N4 1.401(2), B1-C1 1.567(2), B2-C2 1.575(2). Torsion angles (°): C2-B2-N1-B1 3.3(3). Selected structural parameters for 7:
Bond distances (Å): B1-N1 1.475(3), B1-N2 1.466(3), B2-N1 1.464(3), B2-N2 1.477(3), N1-B3 1.417(3), N2-B4 1.413(3). Bond angles (°):
N2-B1-N1 94.6(2), N2-B2-N1 94.6(2), B1-N2-B2 85.3(2), B1-N1-B2 85.4(2). Torsion angles (°): C2-B4-N2-B1 −11.2(4), C1-B3-N1-B2
13.8(4). Angle (°) between B4-N2-B1 and B3-N1-B2 planes: 1.95°. Selected structural parameters for 8: Bond distances (Å): B1-N1
1.431(4), B2-N1 1.558(5), B1-N4 1.439(5), N1-N2 1.367(3), N2-N3 1.284(4), N3-N4 1.355(3), N2-B2' 1.604(3), B1-C1 1.560(4), B2-C2
1.619(5). The ellipsoids for the peripheral aryl groups were omitted for clarity. In the crystal structure of 7, the modeled minor azide/bromide
substitutional disorder (refined N3:Br ratio of ca. 97:3) is not depicted for clarity). In the crystal structure of 6b, the two-part disorder model
for the two orientations of the azide group is not depicted for clarity. In the crystal structure of 8, the whole molecule is generated by
symmetry and the modeled azide/chloride substitutional disorder at B2 (refined N3:Cl ratio of ca 0.77:0.23) is not depicted for clarity (see
Supporting Information).
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
As our proposed quantum-chemically derived mechanism
suggests, the reactivity we observe is consistent with the formation
of transient novel boryl-substituted iminoboranes (Schemes 8 and
9). This is further supported by the observation of MesB(X)-
diazadiboretidine 7,47, 55-56, 76, 84-92 which is the dimer of the putative
iminoborane 8_3, by X-ray crystallography. Most iminoboranes are
unstable with respect to dimerization, and diazadiboretidines are
therefore typical byproducts of their formation. Given the relatively
harsh conditions required for the formation of most iminoboranes,
the spontaneous formation of transient boryl-iminoboranes at room
temperature is remarkable, particularly in view of the rather
restricted choice of substituents at the imine nitrogen, which largely
consists of bulky alkyl and aryl groups.5, 56, 65, 71-76, 78-81, 83
rare reaction motif. Although a boryl-iminoborane may be invoked
in the formation of 2,4,6-trihalo-1,3,5-tris(dihaloboryl)borazine,93-
our transient boryl-iminoboranes display classical iminoborane
chemistry beyond trimerization (i.e. dimerization and
cycloaddition) in a relatively controllable fashion. Excluding
borylated tetrazoles,96-99 species 5 and 6 also belong to a rare group
of (di)borylated tetrazenes, the only examples of which have
recently been isolated from the fixation of dinitrogen by low-valent
boron compounds.100
95
The apparent spontaneous decomposition of transient diborane(4)
azides that yielded compounds 5-8 is somewhat surprising
considering the variety of stable boron azides.13-14, 38, 101-104 A likely
explanation lies in the observations made recently that the reaction
of organic azides or of azido-boranes with strongly Lewis acidic
boranes led to the destabilization of the azide moiety with
dinitrogen evolution and concomitant rearrangements, yielding
amine-boranes12 or boraimines (Scheme 1).11 As shown in our
proposed mechanism (Scheme 9), the adduct formation between
To the best of our knowledge, only one structurally characterized
borylated diazadiboretidine (a mono-borylated example) has ever
been reported.87 While a borylated tetrazaborole has been invoked
as the product of a side-reaction in the formation of iminoboranes
(the addition of the azidoborane starting material to the
iminoborane),72 the isolation of borylated tetrazaboroles remains a
Scheme 9. Gibbs free energy profiles calculated for the reaction of Mes2B2Cl2 with trimethylsilyl azide leading to the dimer
product 4_P. The relative Gibbs free energies and electronic energies (in parentheses) are given in kcal•mol−1.
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Journal of the American Chemical Society
Mes
Cl
Si
Mes
Cl
G kcal mol-1
1
2
3
4
5
6
7
8

B
B
E kcal mol-1
)
Mes

Mes
Cl
N
B
B
N
N
N
TS8_1
N
27.1
(7.7)
N
Cl
B
TS8_2
16.3
(15.8)
Mes
N
Mes
B
TMSCl
0.0
B
N
5.4
(-14.0)
Mes
Mes
B
N
9
N
N
N
Mes2B2Cl2
N
TMS
TMS
N
N2
Cl
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
Mes
Mes
Cl
Cl
B
TS8_3Di
TMSN3
B
B
B
-8.8
(-13.4)
Mes
B
Cl
Cl
N
Mes
Cl
Mes
B
Mes
N
Mes
TMS
B
N
Mes
N
TS8_4
-98.4
(-103.5)
Mes
Cl
N
Mes
TS8_3
B
TMSN3
N
B
N
N
-97.3
B
B
N
N
TMS N
N
TMS
N
(-99.0)
N
8_1
B
-101.3
(-105.0)
-103.6
(-100.0)
-104.2
(-110.2)
Mes
Cl
TS8_5
Mes
N
N
Mes
8_2
N
TMS
B
N
Mes
-124.0
(-149.2)
N
B
B
Cl
Mes
N
Cl
Mes =
-139.0
(-149.4)
8_3
B
Mes
N
N
8_4
Cl
-149.9
(-181.0)
Mes Cl
N
N
TMS
B
-153.2
(-156.7)
B
Mes
Cl
Mes
8_5
Mes
Mes
B
N
B
B
B
N
N
N
N
B
N
N
B
N
TMS
TMS
N
Mes
N
Mes
B
Mes
B
Cl
Mes
Mes
Cl
8_P
8_3-Di
Five-membered ring formation
Dimerization
Boryl-group migration
TMSN3 addition
Dimerization
TMSCl elimination
the B-azide group and the neighboring boron atom in a transient
azido-diborane(4) (TS8_2 provides kinetically favored
groups, which should render them relatively safe. Indeed, no
incident occurred throughout our studies. However, the presence of
smaller, more nitrogen-rich molecules as contaminants as well as
of adventitious hydrogen azide or even transition metal azides,
which could potentially initiate the violent decomposition of the
bulk of the isolated compounds, should always be considered a
possibility. Therefore, the handling or isolation of more than 1
mmol of azide derivatives is not recommended. Moreover, proper
safety equipment, including heavy leather gloves and jacket,
earplugs and faceshield should be worn at all time.
)
a
intramolecular destabilization pathway for the azide group,
qualitatively explaining the instability of the diborane(4) azides.
CONCLUSION
We have uncovered a variety of unusual reactivity types and
connectivity patterns for diboranes, all accessible in one step from
known precursors under mild conditions: the formation of five- and
six-membered rings bearing a triazene moiety and two boron
atoms, unusual intermediates and decomposition products of the
di(9-anthryl)dibromodiborane/PhN3 system and the direct
formation of putative boryl-substituted iminoborane intermediates,
which were trapped by trimethylsilyl azide as boryl-tetrazaboroles.
Beyond their academic interest, the compounds uncovered in this
study are to a large extent related to important moieties in
pharmacology, such as triazenes,3 tetrazoles105 and triazoles.106
With the growing interest of boron compounds in pharmaceutical
applications,24 our findings may represent useful synthetic
pathways to hard-to-reach exotic boron heterocycles. Most
compounds present the hope of further substitution, either by halide
salt elimination or by exploiting the potential of click reactions for
the azide-containing species, which holds great promise for the
generation of more complex boron-nitrogen compounds.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website:
Synthetic,
crystallographic
and
computational
details,
characterization data and further discussions. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-Mails: chzlin@ust.hk; h.braunschweig@uni-wuerzburg.de
Author Contributions
+These authors contributed equally.
DISCLAIMER
CAUTION! Reactions involving azides are potentially hazardous.
The compounds reported herein are large molecules bearing a
substantial proportion of "non-energetic" aryl and alkylsilyl
Notes
The authors declare no competing financial interests.
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Page 10 of 13
(21)
Couchman, S. A.; Thompson, T. K.; Wilson, D. J. D.; Dutton, J.
ACKNOWLEDGMENTS
L.; Martin, C. D., Investigating the ring expansion reaction of
1
2
3
4
5
6
7
8
pentaphenylborole and an azide. Chem. Commun. 2014, 50, 11724-11726.
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. G.B.-C. thanks the Alexander von
Humboldt Foundation for a postdoctoral fellowship. X.G. and Z.L.
thank the Research Grants Council of Hong Kong for financial
support (HKUST 16304017).
(22)
Lindl, F.; Lin, S.; Krummenacher, I.; Lenczyk, C.; Stoy, A.;
Müller, M.; Lin, Z.; Braunschweig, H., 1,2,3-Diazaborinine: A BN
Analogue of Pyridine Obtained by Ring Expansion of a Borole with an
Organic Azide. Angew. Chem. Int. Ed. 2019, 58, 338-342.
(23)
Yruegas, S.; Martinez, J. J.; Martin, C. D., Intermolecular
insertion reactions of azides into 9-borafluorenes to generate 9,10-B,N-
phenanthrenes. Chem. Commun. 2018, 54, 6808-6811.
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