Simultaneous expansion of 9,10 boron-doped anthracene in longitudinal and lateral directions

Article abstract of DOI:10.1039/c8dt04820g

Doubly boron-doped anthracenes and pentacenes have been longitudinally and laterally expanded through annulation of thiophene or benzene rings. The obtained series of closely related compounds allowed an assessment of key structure-property relationships with a focus on optoelectronic characteristics. Most of the products are benchtop-stable blue emitters and capable of accepting two electrons in a reversible manner. The syntheses involved late-stage modifications through photocyclization or stepwise oxidative C-C coupling (DDQ/BF₃·Et₂O) as well as cyclocondensation of ortho-disilylated or -diborylated aryl building blocks.

Full text of DOI:10.1039/c8dt04820g

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Simultaneous expansion of 9,10 boron-doped
anthracene in longitudinal and lateral directions†
Cite this: DOI: 10.1039/c8dt04820g
Alexandra John,‡ Sven Kirschner, ‡ Melina K. Fengel, Michael Bolte,
Hans-Wolfram Lerner
and Matthias Wagner
*
Doubly boron-doped anthracenes and pentacenes have been longitudinally and laterally expanded
through annulation of thiophene or benzene rings. The obtained series of closely related compounds
allowed an assessment of key structure–property relationships with a focus on optoelectronic character-
istics. Most of the products are benchtop-stable blue emitters and capable of accepting two electrons in
a reversible manner. The syntheses involved late-stage modifications through photocyclization or step-
wise oxidative C–C coupling (DDQ/BF3·Et2O) as well as cyclocondensation of ortho-disilylated or
-diborylated aryl building blocks.
Received 6th December 2018,
Accepted 19th December 2018
DOI: 10.1039/c8dt04820g
rsc.li/dalton
electronic properties of longitudinally and laterally expanded
DBAs, the all-carbon congeners of which have recently been
Introduction
It is nowadays beyond discussion that the development of advertised as candidate materials for single-crystal field-effect
novel materials benefits greatly if “inorganic” p-block elements transistor (SCFET) devices.26
are combined with carbon-based matrices.1 In the specific
area of organic electronics, boron has been established as a
particularly valuable dopant of the ubiquitous polycyclic aro-
matic hydrocarbons (PAHs). Applications of B-containing
PAHs (B-PAHs) range from metal-free catalysis2–4 and sensor
technology5 to battery materials, organic field-effect transis-
tors, and organic light-emitting devices.6 Among the currently
available B-PAHs, the 9,10-dihydro-9,10-diboraanthracenes
(DBAs) stand out for their vicinally positioned, cooperating B
atoms.7–9 Three expedient synthesis routes are available for the
assembly of the DBA scaffold: (i) cyclocondensation of
1,2-(SiMe3)2-C6H4 with BBr3,10–12 (ii) cyclocondensation of
[1,2-(BH3)2-C6H4]2− upon H− abstraction with Me3SiCl,13,14 (iii)
cyclodimerization of 1-Li-2-B(OR)2-C6H4.15,16 Methods (i) and
(iii) have also been employed for the synthesis of
C-halogenated DBAs,12,15–18 which are viable starting materials
for Stille-type coupling reactions to introduce a broad variety
of dangling substituents (Chart 1).12,16,19,20
Additional protocols exist for the longitudinal21–24 or
lateral20,25 expansion of the DBA lead structure (Chart 1).
Herein we report on the synthesis, structural, and opto-
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
†Electronic supplementary information (ESI) available. CCDC 1883314–1883317
and 1883327–1883336. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c8dt04820g
Chart 1 Benchtop-stable 9,10-dihydro-9,10-diboraanthracene (DBA;
dashed frame) and representative derivatives of it obtained through peri-
pheral substitution or longitudinal/lateral expansion (Mes = mesityl).
‡These authors contributed equally to the publication.
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borinic acid derivative 2OH of 9,18-dibora-tetrabenz[a,c,h,j]-
anthracene in single-crystalline form. An X-ray analysis (cf. the
ESI†) of the colorless, non-fluorescent compound showed a
Results and discussion
Synthesis of longitudinally tetrafused DBAs
Our synthesis of a 9,18-dibora-tetrabenz[a,c,h,j]anthracene distorted molecular scaffold already with the small OH substitu-
(cf. 2; Scheme 1) is based on the 9,10-diborylated phenan- ent, which explains why the above-mentioned hydroboration
threne 1 bearing two solubilizing tert-butyl (t-Bu) groups. 1 is reactions had not been straightforward. Likely also for steric
accessible from 1,2-bis(4-t-Bu-C6H4)acetylene and B2pin2 reasons it proved impossible to synthesize the bromoborane 2Br
through a Cu-catalyzed diborylation reaction,27 followed by through the reaction of BBr3 with either the borinic acid 2OH or
photocyclization of the obtained (Z)-stilbene intermediate (cf. 9,10-(SiMe3)2-phenanthrene 3 (cf. the ESI† for more information
the ESI† for full details).28 Conversion of 1 into the corres- about the inaccessibility of 3,6-(t-Bu)2-3).12,16,31 Given that the
ponding trihydridoborate using LiAlH4 and subsequent lack of 2Br renders corresponding B-arylated derivatives inaccess-
hydride abstraction with Me3SiCl in the solvent SMe2 resulted ible, we moved on to the sterically less congested longitudinally
in a cyclocondensation reaction13 to form the diadduct expanded 6,13-dihydro-6,13-diborapentacenes (DBPs).
2H·2SMe2 together with BH3·SMe2 as the byproduct. We suc-
ceeded in the growth of single crystals of 2H·2SMe2, but failed
Synthesis of longitudinally tetrafused DBPs
to separate 2H·2SMe2 and BH3·SMe2 via fractional crystalliza-
Evidence that an annulation of four aryl rings to a DBP core is
tion on a preparative scale. Attempts to quantitatively remove
the BH3·SMe2 under a dynamic vacuum (10−3 mbar, rt, 4 h)
were also in vain, however, NMR spectroscopy indicated the
loss of both SMe2 ligands from 2H·2SMe2. Similar to the SMe2
diadduct of pristine DBA (DBA·2SMe2),7,11,29,30 2H·2SMe2 reacts
with the alkynes t-Bu-CuCH and 4-t-Bu-C6H4CuCH already at
room temperature. Contrary to the case of DBA·2SMe2, no well-
defined hydroboration products could be detected. The tar-
geted hydrolysis of the 2H·2SMe2/BH3·SMe2 mixture gave the
compatible even with sterically demanding mesityl (Mes) sub-
stituents at the B atoms was first obtained during repeated
emission measurements on the same sample of the known12
4H (Scheme 2): small differences between the individual fluo-
rescence spectra indicated a progressing photocyclization
between vicinal thienyl substituents already in the weak UV
beam of the spectrometer (cf. the related UV-photocyclization
of 1,2-bis(2-thienyl)ethylenes32). On an NMR scale, the tar-
Scheme 1 Synthesis of the tetrafused DBA 2H (obtained as diadduct
2H·2SMe2). Reagents and conditions: (i) LiAlH4 (3.0 eq.), Et2O, 0 °C, 14 h;
(ii) Me3SiCl (3.3 eq.), SMe2, 60 °C, 4 d (sealed ampoule); (iii) CuBr2
(10.0 eq.), EtOAc/MeOH/H2O, 130 °C, 4 h; (iv) Mg (4.0 eq.), 1,2-Br2C2H4
(0.2 eq.), Me3SiCl (8.0 eq.), THF, reflux temperature, 18 h; (v) BBr3
(3.5 eq.), n-hexane, 120 °C, 3 d (sealed ampoule).
Scheme 2 Synthesis of singly (5) and doubly (6) ring-closed derivates
of 4 via photocyclization (6H, 6Me) and oxidative cyclization reactions
(5Me 6Me). Reagents and conditions: (i) Irradiation with
, a medium
pressure Hg lamp (NMR scale), C6D6, rt, 6 h; (ii) DDQ (1.1 eq.), BF3·Et2O
(10 eq.), CH2Cl2, rt, 4.5 h; (iii) DDQ (2 eq.), BF3·Et2O (10 eq.), CH2Cl2,
50 °C, 16 h; (iv) DDQ (4 eq.), BF3·Et2O (20 eq.), CH2Cl2, 50 °C, 41 h.
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geted photoreaction of 4H in non-dried, non-degassed C6D6 standard protocol of DBA/DBP formation, i.e., the cyclocon-
using a medium pressure Hg lamp furnished no singly densation of vicinally disilylated arenes with BBr3.12,25,29,34
cyclized 5H but exclusively the DBP derivative 6H (quantitative The corresponding triphenylene 9 (Scheme 3) is available from
conversion, 56% yield; Scheme 2). However, 6H turned out to the photoassisted, Co-catalyzed [2 + 2 + 2] cycloaddition35 of
be photolabile, which precluded its photochemical synthesis 2,2′-(HCuC)2-4,4′-(t-Bu)2-biphenyl with Me3SiCuCSiMe3. After
on larger scales. This situation did not change with the DBA Si/B-exchange using BBr3 at 120 °C and mesitylation with
4Me as starting material (Scheme 2), even though its four reac- MesMgBr, 8 was isolated as a yellow solid.
tive thienyl α-positions are blocked with methyl groups – a
NMR spectroscopy
strategy successfully applied in comparable cases.23 Scholl-type
oxidative protocols are viable alternatives to photocyclization NMR spectra (1H, 11B{1H}, and 13C{1H}) were recorded in
reactions.33 In the case of 4Me, the combination of 2,3- CDCl3. As often encountered for triarylboranes, 11B signals
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and BF3·Et2O were generally broadened beyond detection.36 For each of the
performed best and provided selective access not only to the reported B-doped PAHs, the numbers and chemical shift
1
doubly ring-closed derivative 6Me (DDQ (4.0 eq.), BF3·Et2O (20 values of its H and 13C resonances agreed with the proposed
eq.), 50 °C, 41 h), but also to the singly ring-closed diboratetra- molecular structure. Diagnostic for
cene 5Me (DDQ (1.1 eq.), BF3·Et2O (10 eq.), rt, 4.5 h; Scheme 2). tion,21,23 the signals of protons pointing into the newly formed
To prepare the true laterally expanded homolog of the DBA bay regions experience significant downfield shifts (e.g. 4Me
a successful cycliza-
:
2 and to evaluate the influence of the four S atoms on the 6.59 ppm vs. 6Me: 7.30 ppm; see also: 7: 7.19 ppm vs. 8:
optoelectronic properties of 6-type PAHs, we next aimed at the 8.51 ppm). The chemical shift differences between the p- and
quadruply benzannulated DBP 8 (Scheme 3). To this end, the o-CH3 proton signals in Mes-substituted cyclic conjugated π
tetraphenylated DBA 7 was prepared from 2,3,6,7-Cl4-9,10- systems provide a measure of the overall magnetic anisotropy,
Mes2-DBA and 1-SnMe3-3-t-Bu-C6H4 via a Stille coupling reac- which strongly correlates with the ring current [Δδ = δ(p-CH3)
tion (cf., ref. 12). Different from its thienyl congener, however, − δ(o-CH3)].37 This approach yields the same qualitative results
7 neither underwent photocyclization nor oxidative C–C coup- as quantum-chemical calculations.38 9,10-Mes2-anthracene
ling with DDQ/BF3·Et2O. We therefore had to come back to the and 9,10-Mes2-DBA have Δδ values of 0.70 (ref. 39) and 0.33,25
respectively, which indicate a larger ring current on the central
part of the anthracene fragment than on the B2C4 heterocycle
and, in turn, a limited degree of BvC double-bond character.
A Δδ value of 0.37 measured for 6,13-Mes2-DBP25 points
toward a slightly increased π delocalization across the B
centers. This can be explained by the fact that, in DBA, both
B-attached benzene rings contain Clar’s sextets, whereas in
DBP the two sextets can alternatively be assigned to the peri-
pheral benzene rings. Similar arguments are applicable to the
DBP 8, the Δδ value of which is smaller by 0.13 than that of
6,13-Mes2-DBP: if Clar’s sextets are drawn in all t-Bu-substi-
tuted cycles of 8, one necessarily has to draw them also in the
B-attached benzene rings, which renders 8 more comparable
to DBAs than DBPs. More experimental evidence for a dimin-
ished aryl-to-B π-electron donation in 8 stems from cyclic vol-
tammetry (vide infra), which shows 8 to be a better electron
acceptor (E1/2 = −1.82 V) than 6,13-Mes2-DBP (E1/2 = −2.03 V).25
Moreover, the Δδ values of 7 (0.11) vs. 8 (0.24) and 4Me (0.24)
vs. 5Me (0.29) vs. 6Me (0.35) indicate that the degree of π deloca-
lization across the central B2C4-ring increases with an increas-
ing extension of the organic π system.
Crystal structures
The molecular structures of 2H·2SMe2, 2OH (cf. the ESI†), 6H,
and 8 have been confirmed by X-ray crystallography. The
adduct 2H·2SMe2 contains an inversion center in the solid
state (Fig. 1). Each tetracoordinated B atom deviates strongly
from a tetrahedral configuration and approaches a trigonal-
pyramidal geometry with the SMe2 ligand located at the apical
Scheme 3 Synthesis of 8. Reagents and conditions: (i) Irradiation with a
medium pressure Hg lamp (NMR scale), C6D6, rt, 6 h, or DDQ (4 eq.),
position (sum of C–B–C/C–B–H angles = 350°; sum of C–B–
S/H–B–S angles = 303°). The B–S bond length amounts to
BF3·Et2O (20 eq.), CH2Cl2, 50 °C, 41 h; (ii) BBr3 (3.5 eq.), n-hexane, 120 °C,
3 d (sealed ampoule); (iii) MesMgBr (2.05 eq. in THF), toluene, 0 °C, 18 h.
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The DBP 8 is formally derived from the DBA 2 through the
introduction of benzenoid spacers. The thus expanded gulf
regions can now host even bulky Mes substituents without sig-
nificant distortion of the centrosymmetric molecular backbone
(bay twist = 8.0(3)°, expanded gulf twist = 12.5(2)°, Mes//B2C4 =
70.6°; Fig. 2). The main scaffold of the thieno-fused DBP 6H
(point group Ci) is almost perfectly planar (bay twist = 0.9(8)°,
expanded gulf twist = 5.2(2)°, Mes//B2C4 = 86.8°; Fig. 2). In 8 as
well as 6H, the exocyclic and averaged endocyclic B–C bonds
are very much alike, which again underscores the small extent
of π delocalization via the B centers in the electronic ground
states (8: B–Cexo = 1.572(2) Å vs. B–Cendo = 1.566 Å; 6H: B–Cexo
1.585(7) Å vs. B–Cendo = 1.565 Å).
=
Electrochemical and optoelectronic properties
Cyclic voltammograms (CVs) were recorded in THF with
[n-Bu4N][PF6] as the supporting electrolyte and are referenced
against the ferrocene/ferrocenium (FcH/FcH+) couple. The
methylthieno-fused 6Me shows irreversible redox behavior with
a cathodic peak potential of Epc = −1.94 V. All other Mes-sub-
stituted compounds, including the non-methylated thieno-
fused 6H, show two reversible redox waves. The first half-wave
potentials E1/2 of 7 and the fully benzenoid 8 are identical
(−1.82 V; Table 1). In the cases of the S-containing species, the
E1/2 values become slightly more cathodic upon successive
cyclization: 4Me (−1.68 V) < 5Me (−1.78 V) < 6H (−1.83 V). The
similar electron affinities of ring-opened and respective ring-
closed B-PAHs are likely due to the fact that Clar’s sextets can
be drawn in all peripheral aryl rings, irrespective of whether
they are directly connected or not. The second reduction
Fig. 1 Molecular structures of tetrabenzanthracene (CCDC 1839293,
ref. 26), 2H·2SMe2, and 9,18-diphenyltetrabenzanthracene (CCDC
1151346, ref. 42) in the solid state (C atoms: black, H atoms: light grey, B
atoms: green, S atoms: yellow). C-Bonded H atoms and t-Bu groups
located at the C atoms marked with (*) are omitted for clarity. Selected
bond lengths [Å], and angles [°] of 2H·2SMe2: B–C = 1.600(3)/1.603(3),
B–H = 1.10(2), B–S = 2.118(2); C–B–C = 118.94(18), H–B–S = 98(1),
C–B–S = 98.1(1)/106.5(1), C–B–H = 116(1)/115(1).
2.118(2) Å. For comparison, DBA·2SMe2 possesses an average
sum of C–B–C/C–B–H angles of 342° and C–B–S/H–B–S angles
of 314°. Especially the short average B–S bond length of
2.031 Å in DBA·2SMe2 points toward a stronger adduct.7 The
C–C distance within the central B2C4 ring of 2H·2SMe2
(1.379(3) Å) lies between the length of the localized phenan-
threne CvC double bond (1.351 Å, average value of six crystal
structures)40 and that of the aromatic bond in tetrabenzanthra-
cene (1.415(2) Å; Fig. 1).26 Differences between 2H·2SMe2 and
DBA·2SMe2 are also evident for the mutual orientations of the
SMe2 ligands, which coordinate in anti- and syn-fashions,
respectively. The torsion angle spanned by the four C atoms in
the bay region of 2H·2SMe2 has an absolute value of 10.5(3)°
and the gulf region41 is even less distorted (1.7(2)°). Compared
to the parent tetrabenzanthracene (bay twist = 3.5(2)°, gulf
twist = 1.6(1)°; Fig. 1),26 2H·2SMe2 deviates only slightly stron-
Fig. 2 Molecular structures of 8 and 6H in the solid state (C atoms:
ger from planarity. The presence of two phenyl rings at the black, B atoms: green, S atoms: yellow). H atoms and the t-Bu groups
located at the carbon atoms marked with (*) are omitted for clarity. Only
the Mes-C-i atoms are shown for the Mes substituents. Selected bond
center of the gulf region of tetrabenzanthracene results in a
severe helical distortion (average bay twist = 15.6°, average gulf
lengths [Å] and angles [°]: 8: B–Cendo = 1.565(2)/1.566(2), B–Cexo
=
twist = 69.7°; Fig. 1),42 which explains why we have not been
able to replace the H atoms at the B sites of 2H by Br, let alone
Ph/Mes substituents.
1.572(2); Cendo–B–Cendo = 118.5(2), Cendo–B–Cexo = 123.2(2)/118.2(2).
6H: B–Cendo = 1.561(7)/1.569(6), B–Cexo = 1.585(7); Cendo–B–Cendo
119.1(4), Cendo–B–Cexo = 121.4(4)/119.3(4).
=
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Table 1 Electrochemical and photophysical data for 4Me, 5Me, 6Me, 6H,
7, 8, and 6,13-Mes2-DBP
c,d
c,d
e
λabs
λex
λem
ΦPL
Cmpd
Δδa
E1/2b [V]
[nm]
[nm]
[nm]
[%]
4Me
0.24
−1.68
−2.47
−1.78
−2.48
—/—
397f
397
360
370
538
19
20
26
5Me
6Me
0.29
0.35
451
427
451
425
507g
455
485
514
444
472
503
483
6H
0.33
−1.83
−2.47
440
415
392
371
304
430
405
385
350
407
386
329
303
359
24
7
8
0.11
0.24
−1.82
−2.73
−1.82
−2.44
304
350
8
438
463
495
56
6,13-Mes2-DBPh
0.37
−2.03
−2.75
303
412
434
461
491
47
a Δδ = δ(p-CH3) − δ(o-CH3). b THF, [n-Bu4N][PF6] (0.1 M). c In cyclo-
hexane. d Vibrational bands are also listed. e Quantum yields were
determined from degassed samples by using a calibrated integrating
sphere. f Bathochromic shoulder at approx. 470 nm. g Hypsochromic
shoulder at approx. 465 nm. h Values taken from ref. 25.
occurs at nearly the same electrode potential for any of the
investigated diboraacene-derivatives (8: −2.44 V, 4Me: −2.47 V,
5
Fig. 3 Absorption (a) and emission (b) spectra of the methylthienyl
derivatives 4Me, 5Me, and 6Me. All measurements were performed in
Me: −2.48 V, 6H: −2.47 V) with the exception of 7, for which
the injection of a second electron is somewhat harder to cyclohexane.
achieve (−2.73 V). As mentioned above for Mes-substituted
aryls, Δδ = δ(p-CH3) − δ(o-CH3) can be taken as a gauge to esti-
mate the ring current within the aryl ring: a higher Δδ is
associated with a larger ring current and, in the specific case
of mesitylated B-PAHs, a higher π-electron density at the B
centers. By these means, an increasing Δδ along a series of
closely related B-PAHs should lead to a gradual cathodic shift
of the reduction potentials E1/2. This rule of thumb indeed
holds for 4Me/5Me/6H as well as 8/6,13-Mes2-DBP (Table 1).
Only the pair 7/8 is an outlier, because both compounds have
identical redox potentials yet different Δδ values.
All B-PAHs discussed herein possess yellow to orange
colors. Electronic spectra were recorded in cyclohexane solu-
tions at room temperature. 4Me with four dangling methyl-
thienyl substituents shows a featureless absorption band at
397 nm with a bathochromic shoulder (λonset = 495 nm;
Fig. 3a). For the pristine 9,10-Mes2-DBA, which gives rise to a
qualitatively similar UV/vis spectrum, it has been firmly estab-
lished that the shoulder is due to an intramolecular charge
transfer from the electron-rich Mes substituents to the elec-
tron-poor DBA core.16,43 A comparable methylthienyl-to-DBA
transition likely takes place in the case of 4Me. The emission
band of 4Me has a maximum at λem = 538 nm and is also very
broad, due to rotation/libration of the methylthienyl substitu-
charge-transfer nature of the transition, explains the low fluo-
rescence quantum yield of ΦPL = 19%. The doubly ring-closed
6Me possesses a longest-wavelength absorption at λabs
=
451 nm with resolved vibrational fine structure; the corres-
ponding emission band at λem = 455 nm is the perfect mirror
image and ΦPL increases to a value of 26%. The absorption
spectrum of the singly ring-closed 5Me is similar to that of 6Me
(longest wavelength absorption: λabs = 451 nm), but the individ-
ual bands are somewhat broader. In contrast, the fluorescence
spectrum of 5Me resembles that of tetrathienylated 4Me, apart
from a hypsochromic shift of the emission band (λem = 507 nm,
ΦPL = 20%). Thus, the absorption and emission spectra of 5Me
are not mirror images, which testifies to structural reorganiz-
ations taking place in the excited state. Analogous general fea-
tures as in the case of 4Me vs. 6Me are also observable for 7 vs. 8
(Table 1; cf. the ESI† for plots of the spectra). As a distinct differ-
ence, the quantum efficiency increases from ΦPL = 8% to as
much as 56% upon going from 7 to 8.
Conclusions
ents (Fig. 3b). The intramolecular motions also open an Despite its sterically encumbered gulf regions, tetrabenzan-
efficient non-radiative decay channel, which, together with the thracene is a planar molecule. Already the introduction of two
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phenyl rings at the central (9,18) positions results in a severe
skeletal twist. Also the SMe2 diadduct of a 9,18-boron-doped
tetrabenzanthracene (2H·2SMe2) retains an essentially planar
scaffold. The B–H functionalities can still be transformed into
B–OH groups, whereas larger Br substituents are not compati-
ble with the confined space surrounding the B atoms. The
steric strain in the gulf regions can be alleviated by switching
from a diboraanthracene to a diborapentacene backbone. The
tetrabenzodiborapentacene 8 was prepared via cyclocondensa-
tion of an ortho-disilylated triphenylene with BBr3. While
several attempts failed to obtain 8 also through photo-
cyclization or intramolecular Scholl-type C–C coupling of the
2,3,6,7-tetraphenyl diboraanthracene 7, these methods faith-
fully worked for the synthesis of the tetrathienodiborapenta-
cene 6Me from 2,3,6,7-tetrathienyl diboraanthracene 4Me. The
Scholl protocol (DDQ/BF3·Et2O) is not only the method of
choice to synthesize 6Me on a preparative scale, but also allows
a selective monocyclization of 4Me to furnish the diboratetra-
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contrast, the fully planarized products 6Me and 8 give rise to
vibrationally resolved UV/vis and fluorescence spectra. Of par- 12 C. Reus, S. Weidlich, M. Bolte, H.-W. Lerner and
ticular interest is the partially planarized diboratetracene 5Me
because its absorption spectrum resembles that of 6Me
,
,
M. Wagner, J. Am. Chem. Soc., 2013, 135, 12892–12907.
13 Ö. Seven, Z.-W. Qu, H. Zhu, M. Bolte, H.-W. Lerner,
M. C. Holthausen and M. Wagner, Chem. – Eur. J., 2012, 18,
11284–11295.
whereas its emission spectrum is qualitatively similar to that
of 4Me
.
14 Ö. Seven, M. Bolte, H.-W. Lerner and M. Wagner,
Organometallics, 2014, 33, 1291–1299.
15 S. Luliński, J. Smętek, K. Durka and J. Serwatowski,
Eur. J. Org. Chem., 2013, 8315–8322.
Conflicts of interest
There are no conflicts of interest to declare.
16 S. Brend’amour, J. Gilmer, M. Bolte, H.-W. Lerner and
M. Wagner, Chem. – Eur. J., 2018, 24, 16910–16918.
17 A. Lorbach, C. Reus, M. Bolte, H.-W. Lerner and
M. Wagner, Adv. Synth. Catal., 2010, 352, 3443–3449.
18 C. Reus, N.-W. Liu, M. Bolte, H.-W. Lerner and M. Wagner,
J. Org. Chem., 2012, 77, 3518–3523.
Acknowledgements
We wish to thank Nico P. Thöbes, MSc., and Jasmin Busch,
MSc., for synthetic support in the early stages of this 19 C. Reus, F. Guo, A. John, M. Winhold, H.-W. Lerner,
project. M. W. is grateful to the Bundesministerium für
Wirtschaft und Energie for financial support through the
F. Jäkle and M. Wagner, Macromolecules, 2014, 47, 3727–
3735.
WIPANO grant number 03THW10F04. Donations of B2pin2 20 A. John, M. Bolte, H.-W. Lerner and M. Wagner, Angew.
(AllyChem Co. Ltd, Dalian City/China), n-BuLi (Albemarle Chem., Int. Ed., 2017, 56, 5588–5592.
Germany GmbH, Langelsheim), and Pd(PPh3)2Cl2 (Heraeus 21 V. M. Hertz, M. Bolte, H.-W. Lerner and M. Wagner, Angew.
Noble Metals, Hanau/Germany) are acknowledged.
Chem., Int. Ed., 2015, 54, 8800–8804.
22 V. M. Hertz, H.-W. Lerner and M. Wagner, Org. Lett., 2015,
17, 5240–5243.
23 V. M. Hertz, J. G. Massoth, M. Bolte, H.-W. Lerner and
M. Wagner, Chem. – Eur. J., 2016, 22, 13181–13188.
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