Aryl-aryl coupling in a polycyclic aromatic hydrocarbon with embedded tetracoordinate boron centre

Article abstract of DOI:10.1039/c9ob00618d

The addition of 2 eq. of MesLi to the biphenylene-containing 9,10-dihydro-9,10-diboraanthracene (DBA) 2 results in the formation of the corresponding oxaboraphenanthrene 3 after column chromatography under ambient conditions. In the initial step, the anio

Full text of DOI:10.1039/c9ob00618d

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COMMUNICATION
Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
rsc.li/obc

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TOC: An oxaboraphenanthrene is generated from a 9,10-dihydro-9,10-diborDaOaIn: 1t0h.1r0a39c/eCn9OeB00618D
through boron extrusion/C–C coupling upon addition of mesityl lithium and
chromatographic workup.
Aryl–Aryl Coupling in a Polycyclic Aromatic Hydrocarbon with Embedded
Tetracoordinate Boron Centre
Sven Kirschner,a Songsong Bao,b Melina K. Fengel,a Michael Bolte,a Hans-Wolfram Lerner,a Matthias
Wagner*a
a.Institut für Anorganische Chemie, Goethe-Universität Frankfurt, Max-von-Laue Strasse 7, 60438
Frankfurt am Main (Germany). E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
b.State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210023, P.R. China.
Abstract
The addition of 2 eq. of MesLi to the biphenylene-containing 9,10-dihydro-9,10-diboraanthracene
(DBA) 2 results in the formation of the corresponding oxaboraphenanthrene 3 after column
chromatography under ambient conditions. In the initial step, the anionic B–Mes monoadduct of 2 is
generated, which eliminates a formal [Mes2B:]– ion with concomitant C–C-bond formation (room
temperature, 18 h). The resulting biphenylene-containing borafluorene 4 is still sufficiently Lewis
acidic to add the second equivalent of MesLi ([4Mes]–). Using pristine 9-mesityl-9-borafluorene as a
model system, we confirmed that both 4 and [4Mes]– should be capable of inserting an oxygen atom
to furnish the observed oxaboraphenanthrene scaffold. The biphenylene-containing
oxaboraphenanthrene 3 is a yellow compound with an absorption maximum at λmax = 450 nm. Similar
to its DBA analogue 2, 3 shows no photoluminescence.
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DOI: 10.1039/C9OB00618D
Introduction
Boron-containing polycyclic aromatic hydrocarbons (B-PAHs) constitute a rapidly developing class of
organoelement compounds with applications ranging from metal-free catalysis1 and sensor
technology2 to battery materials, organic field-effect transistors, and organic light-emitting devices.3
Amongst the currently available B-PAHs, the 9,10-dihydro-9,10-diboraanthracenes (DBAs) stand out
for their vicinally positioned B atoms (cf. the archetypal example 1; Scheme 1).4–7 We have recently
shown that key optoelectronic properties of DBAs critically depend on the electronic structures of
their π-conjugated hydrocarbon halves.8–11 In particular, the biphenylene derivative 2 is much easier
to reduce and a stronger Lewis acid than the pristine 1 (Scheme 1).12 We now disclose that 1 and 2
also show significantly different chemical reactivities: Even though both compounds are inert
towards air and moisture and can be purified by column chromatography,12 only 1 remains stable
also in the presence of excess MesLi, whereas 2 undergoes activation of two B–C-bonds, leading to
the formation of the oxaboraphenanthrene 3 after workup under ambient conditions (Scheme 1).
Compound 3 is a rare example of a π-extended member of the oxaboraphenanthrene substance
class.13
Scheme 1: Reaction of 2 with excess MesLi yields the oxaboraphenanthrene 3.
Comparable reactions are known for the [BPh4]– anion, which liberates biphenyl under irradiation
with UV light14 or in the presence of certain single-electron oxidants (e.g., Ce(IV), Fe(III), DDQ;
Scheme 2).15 More recently, an oxidative deborylation/C–C-coupling of borepins to furnish
phenanthrenes has been reported, too (e.g., by using FeCl3, MnO2).16 The famous Zweifel olefination
2

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and related protocols developed by Aggarwal et al. also involve organoboron species, however, the
DOI: 10.1039/C9OB00618D
C–C-coupling step requires the intermediate halogenation of one of the coupling partners.17–19
Scheme 2: Previously reported examples of deborylative C–C-coupling reactions.
Results and Discussion
As becomes obvious from Scheme 2 and a survey of the literature, intramolecular C–C-coupling
reactions of organoboranes seem to occur preferentially at tetracoordinate boron centres. To test
whether this is also likely true in the case of 2, we treated an authentic sample of the compound
with 2 eq. of donor-free MesLi20 in C6D6/THF (4:1; room temperature, sealed NMR tube). An
exothermic reaction took place, whereupon the mixture turned dark. 11B-NMR spectroscopy revealed
a broad signal at about –7.5 ppm accompanied by a small singlet resonance at –6.3 ppm. Both these
signals appear in a region characteristic of tetracoordinate boron nuclei (note that the 11B-NMR
resonance of the starting material 2 is generally broadened beyond detection).21 The corresponding
1H-NMR spectrum showed a series of broad, ill-defined humps that were not diagnostic in terms of
structure elucidation. However, two prominent signals at δ(1H) = 6.67 and 2.13 (integral ratio = 1:3)
testified to the formation of mesitylene. Workup of the reaction mixture in air gave back 2 as the
second unequivocally identifiable component after column chromatography; there was no indication
for the formation of 3. We therefore repeated the NMR experiment, but this time heated the sample
to 60 °C. After 1 h,22 the 11B-NMR spectrum still contained the previously observed singlet at –
6.3 ppm, together with a newly formed broad signal at 1.5 ppm. Most importantly, a doublet
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resonance had developed at –12.5 ppm (1JBH = 74.0 Hz). Workup at this stage furnished the expected
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DOI: 10.1039/C9OB00618D
oxaboraphenanthrene 3 (42%), while none of the starting material 2 was recovered.
So far, we have confirmed that the formation of 3 indeed starts off with a reaction between 2 and
MesLi.22 Yet, the actual mode of interaction still remains to be unveiled: Does it proceed via
deprotonation (cf. the formation of substantial amounts of mesitylene)23 or B–C-adduct formation
(cf. the 11B resonances in the region of tetracoordinate boron species)? To answer this question, we
switched from 2 to the more readily available model system 1.24 After a mixture of 1 and 2 eq. MesLi
in C6H6/THF (4:1) had been kept at 60 °C for 1 h, crystals of the salt Li(thf)4[1Mes]∙C6H6 were obtained
in a yield of 42%. X-ray crystallography revealed solvent-separated ion pairs. Each of the complex
anions consisted of one [Mes]– ligand bonded to an intact moiety of 1 (Scheme 3).
Scheme 3: Reactions of model systems performed in order to substantiate the putative intermediates in the formation
oxaboraphenanthrene. Note: Under the reducing conditions applied, it is also conceivable that 8a (partly) exists as its radical anion [8a]–.
Top right: Structures of Li(thf)4[1Mes]∙C6H6 and Li(thf)4[4Mes] in the solid state (C atoms: black, B atoms: green; H atoms, Li(thf)4, and C6H6
omitted for clarity). Selected bond lengths [Å] and angles [°]: [1Mes]–: B(sp3) atom: B–Cendo = 1.659(5)/1.659(5); B–Cexo = 1.703(4)/1.689(4);
Cendo–B–Cexo = 102.0(2)/115.3(3)/114.5(3)/100.6(2), Cendo–B–Cendo = 110.7(2), Cexo–B–Cexo = 114.4(2); B(sp2) atom: B–Cendo = 1.546(5)/1.541(5),
B–Cexo = 1.604(4); Cendo–B–Cexo = 118.0(3)/122.1(3), Cendo-B-Cendo = 119.8(3). [4Mes]–: B–Cendo = 1.651(9), 1.652(9), B–Cexo = 1.639(9),
1.673(10); Cendo–B–Cexo = 102.1(5)/120.1(5)/102.8(5)/121.1(5) Cendo-B-Cendo = 96.0(4), Cexo–B–Cexo = 114.4(5).
As to be expected, the B(sp3)–Cendo bond lengths are strongly elongated compared to the B(sp2)–Cendo
bond lengths (average values: 1.659 vs. 1.544 Å). The average length of the B–Cexo bonds about the
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tetracoordinate boron centre amounts to 1.696 Å and is thus larger by 0.047 Å than the average B–C
DOI: 10.1039/C9OB00618D
bond length in [Li(thf)2(H2O)2][BPh4] (1.649 Å).25 At the same time, the Cexo–B–Cexo bond angle in
[1Mes]– is significantly expanded to a value of 114.4(2)°. Despite the obvious steric congestion about
the tetracoordinate boron atom, the anion retains a static adduct structure in THF solution and
shows no indication of a dynamic association-dissociation equilibrium involving the added mesityl
ligand: 1) The 11B-NMR spectrum contains a sharp resonance at –7.9 ppm. 2) Two well-resolved sets
1
of mesityl resonances are visible in the H- (integral ratio = 1:2) and 13C{1H}-NMR spectrum. 3) The
two ortho-phenylene rings of the DBA scaffold are still related by symmetry, but the protons (and
also the carbon atoms) of each phenylene ring are magnetically inequivalent.
We have proven above that the sterically shielded 1 is still capable of adduct formation with one
mesityl ligand and thus propose that an analogous reaction is also the entry path for the 2→3
transformation (Scheme 1). Moreover, the structural data acquired for the model system [1Mes]–
indicate that it is the tetracoordinate boron atom that will be expelled in the course of the
intramolecular C–C-coupling reaction taking place with [2Mes]–.26 The resulting product should
possess a borafluorene structure with a boron-bonded mesityl substituent (cf. 4; Scheme 4). With the
aim to confirm that a borafluorene with two biphenylene halves can indeed exist, we next performed
the independent synthesis of an authentic sample of 4 using a recently established protocol.27
Scheme 4: Proposed mechanism of the transformation of 2 into oxaboraphenanthrene 3. Top right: Structure of 3 in the solid state (C
atoms: black, B atoms: green; H atoms omitted for clarity, only the Mes-Cipso atom is depicted). Selected bond lengths [Å] and angles [°]: B–
O = 1.371(9), B–Cendo = 1.55(1), B–Cexo = 1.57(1), C–O = 1.379(8), O–B–Cendo = 117.8(6), O–B–Cexo = 115.3(6), Cendo–B–Cexo = 126.8(6).
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To this end, the dibromobiphenyl derivative 5 (Scheme 3) was prepared in two steps from known
DOI: 10.1039/C9OB00618D
starting materials (cf. the Supporting Information). A twofold lithium/bromine exchange (n-BuLi, THF,
–78 °C), followed by the addition of 1 eq. of Mes2BF resulted in the formation of a dark slurry. Few
single crystals of [Li(thf)4][4Mes] could be isolated and structurally characterised by X-ray diffraction
(the low yield of the pure compound precluded NMR analysis as well as the originally intended
abstraction of the excess mesityl substituent to furnish 4).
Compared to the structure of [1Mes]–, the B(sp3)–Cendo bonds of [4Mes]– possess essentially the
same lengths (av. 1.652 Å). The B(sp3)–Cexo bond lengths are significantly smaller than the
corresponding values in [1Mes]– (av. 1.656 Å) but equal to those published for the mesityl adduct of
pristine 9-mesityl borafluorene ([Li(thf)4][6Mes]: 1.656 Å).27 The main scaffold of [4Mes]– shows a
marked helical twist with a dihedral angle of 26.6° between the two peripheral ortho-phenylene
rings.
The successful structure determination of [4Mes]– not only corroborates the structural proposal of 4
for the second intermediate of the reaction cascade, but further indicates that this compound can
readily add a mesityl ligand in the presence of excess MesLi. It therefore remains to be investigated,
which of the two species, the free Lewis acid or the borate anion, more readily inserts an oxygen
atom into one of its B–Cendo bonds.28 To clarify this point, we again resorted to a model system, i.e. 6
(ref. 29) and its mesityl adduct [6Mes]–.27
C6H6 solutions of 6 and [6Mes]– were exposed to atmospheres of dry air and the individual reaction
progress was monitored by 1H-NMR spectroscopy (C6D6; Scheme 3). In both experiments,
oxaboraphenanthrene 7 (ref. 30) formed in continuously increasing amounts, albeit at strikingly
different rates. Spectra recorded on reaction mixtures of the tetracoordinate species [6Mes]–
revealed the presence of 6 in addition to 7, which leads us to assume that an oxidative
decomplexation of [6Mes]– has to precede the desired ring expansion, thereby slowing down the
overall reaction progress. The formation of tetracoordinate mesityl-borafluorene adducts at this
stage of the reaction cascade is thus an unavoidable detour in the presence of excess MesLi – in stark
contrast to the boron extrusion/C–C-coupling step, for which borate-anion formation is a basic
prerequisite.
C–C-homocoupling on the [2Mes]– scaffold is formally accompanied by the reductive elimination of
the transient boryl anion [Mes2B:]–. In this context, it is important to note that the
oxaboraphenanthrene 3 was only detectable after workup, if 11B-NMR spectroscopy on the reaction
mixture had previously shown the prominent doublet resonance at –12.5 ppm (see above). The
1
doublet collapses into a singlet upon H decoupling and the coupling constant of 74.0 Hz indicates
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the presence of a direct B–H bond in the corresponding molecule. We therefore speculate that
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[Mes2B:]– stabilises itself through insertion of the carbene-type boron centre into an adjacent ortho-
CH3 group to furnish the benzodihydroborete-hydride adduct [8]– (Scheme 4). Precedence for such
reactivity is provided by the facile rearrangement of the sp2-sp3 diborane 9, which can be viewed as a
borane-stabilised diphenylboryl anion, to afford the corresponding diborylmethane-hydride adduct
9a (Scheme 5).31
Scheme 5: Rearrangement of the borane-stabilised diphenylboryl anion 9 to the diborylmethane-hydride adduct 9a via formal insertion of
the boryl anion into a C–H bond.31
Despite numerous attempts, we have neither succeeded in the isolation of [8]– nor in its selective
derivatisation through treatment with H2O2 or N-oxides. We therefore tried to prepare [8]– via
independent routes and thereby prove its molecular constitution. One seemingly obvious approach
was the reduction of Mes2BBr with excess KC8 or Na2K in various solvents (see the Supporting
Information for full details).32 Yet, all these experiments resulted in complex product mixtures largely
devoid of the desired species associated with the doublet at δ(11B) = –12.5. Instead, we rather
observed a triplet at –22.1 ppm, assignable to the dihydroborate [Mes2BH2]–.33 We next tried to
assemble the benzodihydroborete scaffold by heating a mixture of α,2-dibromomesitylene (10),
magnesium metal and MesB(OMe)2 in THF at reflux temperature for 24 h (at lower reaction
temperatures and/or shorter reaction times, significant amounts of MesB(OMe)2 remained
unconsumed). According to 11B-NMR spectroscopy, two products were formed, one corresponding to
1
a broad signal at +53.6 ppm, the other to a doublet at –15.0 ppm with JBH = 80.3 Hz. The latter
resonance possesses a chemical shift value and coupling constant very similar to that of the putative
benzodihydroborete-hydride adduct [8]– and may have been formed through reduction of a primary
benzodihydroborete 8a with excess magnesium through hydrogen abstraction via a radical pathway
(Scheme 3). In line with this interpretation, a 35% higher amount of consumed magnesium than
required for the formation of the Grignard reagent was determined by weighing the residual metal
turnings.
As mentioned in the introduction, the biphenylene-containing DBA 2 and the pristine DBA(Mes)2 (1)
differ strikingly in their optoelectronic properties. Therefore, we finally compared the biphenylene-
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containing oxaboraphenanthrene 3 with its parent congener 7 (Scheme 3). The key structural
DOI: 10.1039/C9OB00618D
parameters of 7 and the core region of 3 are essentially identical (cf. ref. 30, Scheme 4 and the
Supporting Information for more details). The UV/vis spectra of both compounds also show
qualitatively similar features. However, the longest-wavelength absorption of 3 (λmax = 450 nm) is
bathochromically shifted by 130 nm compared to λmax of 7 (320 nm; Figure 1). The corresponding
onset wavelengths (λonset) were determined by constructing tangents at the inflection points of the
bathochromic slopes of the lowest-energy absorption maxima of 3 (λonset = 462 nm) and 7 (λonset = 333
nm). From these values, the HOMO-LUMO energy differences of 3 (EGopt = 2.68 eV) and 7 (EGopt = 3.72
34
eV) were calculated using the formula EGopt = 1240/λonset
.
Most importantly, and in line with the
behaviour of 2, compound 3 shows no photoemission, whereas 7 fluoresces in the blue (λem = 348
nm) with a quantum yield of ΦPL = 25% (Figure 1).30
Figure 1: Normalised absorption (solid) and emission (dotted) spectra of 3 and 7 (ref. 30) in CH2Cl2. λem(7) = 268 nm. The emission spectrum
of 7 was scaled to fit the height of the longest-wavelength absorption.
Conclusion
The transformation of the biphenylene-containing DBA derivative 2 to the oxaboraphenanthrene 3 in
the presence of MesLi and after workup in air is initiated by the coordination of a mesityl ligand to
one of the boron atoms. Subsequent extrusion of a formal [Mes2B:]– anion, which likely stabilises
itself through boron insertion into one of the ortho-CH3 groups, affords a biphenylene containing 9-
borafluorene. During chromatographic purification under ambient atmosphere, oxygen insertion into
one of the B–Cendo bonds takes place to yield the final product 3. Apart from providing an access
route to a biphenylene-containing oxaboraphenanthrene, our findings are of general importance for
B-PAH synthesis: Even sterically protected boron centres are still prone to adduct formation with
bulky organic nucleophiles such as mesityl lithium. This might lead to solubility issues and yield
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losses, but also to initially unexpected side reactions such as reductive boron elimination
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accompanied by C–C-coupling.
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(a) A closely similar product distribution is also obtained upon stirring of the reaction
mixture at room temperature for 18 h. In the NMR monitoring experiments, a
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23
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(ref. 12)). Yet, also Li[1Mes] reacts further under forcing conditions (140 °C, 12 d, THF-
d8) and furnishes a product characterised by a doublet resonance at –14.6 ppm (1JBH
=
74.0 Hz) in the 11B-NMR spectrum.
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C9OB00618D
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