Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: New substituents and alternative substrates

Article abstract of DOI:10.1039/c9dt01555h

A series of new boranes capable of forming intramolecular N → B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations. A dimethylborane (BMe₂), haloborane derivatives (BBr₂, BF₂, BI₂) and mixed cyano/isocyano-borane (B(CN)(NC)) have been prepared by different techniques. Furthermore, 2′-alkynyl-substituted 2-phenylpyridines bearing terminal tert-butyl- and trimethylsilyl-groups are introduced as a new class of substrates for hydroboration. Successful hydroboration with either 9H-borabicyclo[3.3.1]-nonane (9H-BBN), dimesitylborane (Mes₂B-H), or Piers' borane ((C₆F₅)₂B-H, BPF-H) furnished new π-extended boranes capable of forming intramolecular six- or seven-membered N → B-heterocycles (tBuBBN, SiBPF), and, in the case of Mes₂BH, formation of a sterically crowded styrylborane (SiBMes₂) incapable of intramolecular N → B-coordination was observed. All the boranes listed above except BMe₂ have been structurally characterized, and a study of their electrochemical properties showed that the systematic variation of the substituents on boron allows for the incremental variation of the electron affinity of the phenylpyridine-model system over a total range of >0.7 eV between alkylboranes (BMe₂, BBN) and B(CN)(NC). B(CN)(NC) shows the strongest N → B-bond (≈175 kJ mol^-1), and highest electron-affinity observed so far, and is the first example of a borane bearing an isocyano-substituent on boron.

Full text of DOI:10.1039/c9dt01555h

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D. W. Schepper, J. Glöckler, Y. Sun and A. Orthaber, Dalton Trans., 2019, DOI: 10.1039/C9DT01555H.
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Expansion of the scope of alkylboryl-bridged N→B-Ladder
boranes: New Substituents and alternative substrates
Frank Pammer,*a† Jonas Schepper,a† Johannes Glöckler,b† Yu Sun,c Andreas Orthaberd.
Received 00th January 20xx,
Accepted 00th January 20xx
A series of new boranes capable of forming intramolecular N→B-heterocycles has been prepared and their properties have
been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations. A dimethylborane
(BMe2), haloborane derivatives (BBr2, BF2, BI2) and mixed cyano/isocyano-borane (B(CN)(NC)) have been prepared by
different techniques. Furthermore, 2’-alkynyl-substituted 2-phenylpyridines bearing terminal tert-butyl- and trimethylsilyl-
groups are introduced as a new class of substrates for hydroboration. Successful hydroboration with either 9H-
borabicyclo[3.3.1]-nonane (9H-BBN), dimesitylborane (Mes2B-H), or Piers’ borane ((C6F5)2B-H, BPF-H) furnished new π-
extended boranes capable of forming intramolecular six- or seven-membered N→B-heterocycles (tBuBBN, SiBPF), and, in
the case of Mes2BH, formation of a sterically crowded styrylborane (SiBMes2) incapable of intramolecular N→B-coordination
was observed. All boranes listed above except BMe2 have been structurally characterized, and a study of their
electrochemical properties showed that the systematic variation of the substituents on boron allows for the incremental
variation of the electron affinity of the phenylpyridine-model system over a total range of > 0.7 eV between alkylboranes
(BMe2, BBN) and B(CN)(NC). B(CN)(NC) shows the strongest N→B-bond (≈ 175 kJ/mol), and highest electron-affinity
observed so far, and is the first example of a borane bearing an isocyano-substituent on boron.
DOI: 10.1039/x0xx00000x
www.rsc.org/
N,N2-chelating ligands, i.e. ladder compounds.4 What renders
ladder boranes so interesting, is their potential use in organic
light emitting devices,4,5 and the recent finding, that ladder-
Introduction
Boron containing conjugated organic materials have attracted
the interest of synthetic chemists for years and new potential
applications for these compounds continuously emerge.1 The
tailoring of the electronic properties of π-conjugated organic
materials through incorporation of boron, has been widely
employed to tailor their optical and electronic properties. Either
through attachment of boryl-groups to conjugated systems,2,3
or in the form of tetracoordinate boron centers bearing N,O2-
polymers containing intramolecular N→B-coordinated bridging
units can serve as electron acceptors in organic photovoltaic
cells.6 This paper is concerned with ladder boranes featuring
N,C2-chelated ladder boranes, which have been investigated to
much lesser degree than other molecular architectures.
However, since being first introduced by Yamaguchi (A, Chart
1), N,C2-chelated ladder boranes may have been discussed as
potential electron-transporting (n-type) materials,7 and have
since been successfully employed as n-channel organic field
effect transistors,8 and as donors in organic photovoltaics that
provide an increased cell voltage (B, Chart 1).9
a. Dr. F. Pammer, M. Sc. Jonas Schepper
Institute of Organic Chemistry II and Advanced Materials
University of Ulm
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: frank.pammer@uni-ulm.de
b. M. Sc. Johannes Glöckler
Institut of Analytical and Bioanalytical Chemistry
University of Ulm
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: johannes.gloeckler@uni-ulm.de
c. Dr. Y. Sun
The most widely used methods to prepare N→B-ladder boranes
is direct electrophilic C-H-borylation with boron halides,10 which
can even serve to post-functionalize polymers,11 or the step-
wise metalation and electrophilic borylation of suitable precur-
sors.12 Other methods involve intramolecular nucleophilic
aromatic
cycloaddition,14
substitution,13
and unusual
1,3-dipolar
azide-alkyne
of
1,1-hydroboration
Fachbereich Chemie
benzoisoindoles.15 The latter method15 furnishes alkyl-boryl
bridged ladder boranes (C, Chart 1). This sub-class of ladder-
boranes is of particular interest, because they can be converted
into fully conjugated B,N-doped condensed arenes (D, Chart 1),
either thermally, photochemically,16 or within a working light
emitting device.5
Technische Universität Kaiserslautern
Erwin-Schrödinger-Strasse 54, D-67663 Kaiserslautern, Germany
E-mail: sun@chemie.uni-kl.de
d. A. Orthaber,
Department of Chemistry – Ångström laboratories
Uppsala University
BOX 523, 75120 Uppsala, Sweden
E-mail: andreas.orthaber@kemi.uu.se
†
Authors contributed equally
Supporting information for this article available on the www under
http//dx.doi.org/10.1002/.XXXXXXXXXX.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Journal Name
Ph Ph
B
H3C
N
CH3
Mes Mes
R
View Article Online
DOI: 10.1039/C9DT01555H
B
O
N
N
N
S
S
S
S
S
S
n
N
iPr
B
N
O
B
1
A
B
R
Mes Mes
N
OPV Donor, PCE = 4.95%
Dou, Xie, and Liu, 2016
9H-BBN

e = 1.4*10-4 cm2V-1s-1
Yamaguchi, 2006
60°C, 2d,
THF,
85%
R
R
BBN
R
I
Br
I
iPr
B
B
iPr
Br
B
B
N
N
T or h
BI3
N
N
N
N
C6H6,
RT, 48h,
81%
HBBr2•SMe2
D
S. Wang, 2015
R.T., 3h,
PhMe
56%
C
BBr2
BI2
Alkylboryl-bridged
R
Previously Reported
This work
Cl
Ph
Ph
iPr
Cl
iPr
iPr
R
B
B
B
9BBN, BH2, BPh2,
BCl2, B(C6F5)2,
9-Borafluorene
BF2, BBr2, BI2
B(CN)2, BMe2,
N
N
Ph2Zn
HBCl2•SMe2
R2B =
R2B =
PhMe,
60°C, 3h,
84%
R.T., 20h,
PhMe,
94%
E
BCl2
BPh2
Vinyl-boranes
R'
C6F5
Me
B
Me
BR2
R'
C6F5
iPr
iPr
B
9BBN,
B(C6F5)2,
BMes2
R2B =
N
N
N
H-BR2
(C6F5)2B-H
MeMgCl
THF, 2h,
-78°C-R.T.,
95%
85°C, 3d,
PhMe,
56%
R'' = tBu, SiMe3
F
G
BPF
BMe2
H
Chart 1. Previously reported alkylboryl-bridged boranes, and new boranes reported
herein. See Scheme 1 for details on compounds of type E.
iPr
H
F
B
F
iPr
B
N
AgPF6
-AgCl
H3B•THF
50°C, 3d,
THF,
N
In our group, we explored the scope of the preparation of
alkylboryl-bridged N→B-ladder-boranes by hydroboration of
suitable substrates. We could demonstrate, that this approach
allows to convert a variety of N-heterocyclic substrates in high
yield,17,18,19 serves to introduce a whole range of boryl-
substituents (E, Chart 1),20 and is also suitable for the post-
functionalization of polymers.21,22
DCM, R.T.
92%
87%
BH2
BF2
NC
tBu
iPr
NC
iPr
B
9H-BFlu
B
exc. Me3SiCN,
- 2 Me3SiCl
75°C, 4d,
PhMe,
59%
N
tBu
N
C6H6, 105 °C,
24 h
68 %
B(CN)(NC)
In this paper, we report our latest efforts to broaden the scope
of this preparative strategy (E, Chart 1), and report a series of
new vinylborane-containing N→B-ladders (G, Chart 1)
generated by hydroboration of alkynes (F, Chart 1).
BFlu
Scheme 1. Synthesis of alkylboryl-bridged boranes. BF2, BBr2, BI2, BMe2, and B(CN)(NC)
newly reported herein. All other compounds previously included in Ref. 20.
Firstly, we set out to complete the series of dihaloboranes.
Among these, the dibromoboryl-derivative BBr2 is directly
accessible in 56% yield by hydroboration of the ortho-styryl-
pyridine substrate (1) with commercially available Br2BH•SMe2.
Other derivatives can be prepared from the previously reported
BCl2. The diiodo-derivative was obtained in good yield (81%) by
treatment of BCl2 with an excess of boron triiodide (BI3), which
leads to its full conversion into BI2 within minutes. In a
serendipitous finding, BF2 was formed in near quantitative yield
(92%) upon treatment of BCl2 with one equivalent of silver(I)
hexafluorophosphate. Evidently, the borenium cation formed
upon abstraction of chloride, rapidly abstracts fluoride from the
PF6-counter anion. Importantly, this reagent proved much more
effective than the use of silver(I) fluoride, which provided only
67% yield, even when employed in excess. Furthermore,
reaction of BCl2 with MeMgCl gave BMe2 in excellent yield. The
Results and Discussion
Alkylboryl-bridged Ladder-Boranes
Synthesis and Structures: In order to expand our survey of the
effect of substituents on boron on the properties of the conju-
gated π-system in N→B-ladder boranes, we endeavored to
expand the series of previously known derivatives (Scheme 1).
1
above compounds were fully characterized by H, 13C, and 11B
NMR, and all except BMe2 could also be unambiguously
identified by single crystal X-ray crystallography (vide infra).
The chemical shifts in the 11B NMR range from +6.7 ppm (C6D6,
all data recorded in C6D6, unless stated otherwise) for BF2, and
2 | J. Name., 2012, 00, 1-3
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+4.9 ppm for BBr2 to -2.4 and -12.6 ppm for BMe2 and BI2, chemical shift in the expected range at δ = XXXX ppm.30
View ArStictliellO, ntlhinee
DOI: 10.1039/C9DT01555H
respectively. These values are in the expected range for tetra- isocyano-group could not be directly observed, due to the low
coordinate boron, and indicate, that N→B-coordination persists intrinsic receptivity this type of carbon atoms. Similarly, variable
in solution. The observed sequence of chemical shifts for the 1H NMR spectra down to -80°C show only a single set of signals
dihalo-boranes is also in agreement with previous reports on (see Figure S30 in the ESI), even though the diastereomeres of
base adducts of dihalo-alkylboranes (R3N→R’BI2: -17.5 ppm,23 B(CN)(NC) should be just as readily distinguishable, as those of
R3N→MeBBr2: +6.4 ppm,24 Et3N→EtBF2: +6.7 ppm25). BF2 and the presumed B(CN)Cl intermediate. It is therefore not
BMe2 can also be identified by the respective NMR signals of absolutely certain at this point, whether the B(CN)(NC)-
their diastereotopic substituents on boron. BF2 shows two structure is merely present in the solid state, or also persists in
broadened signals in the 19F NMR spectrum at -148.0 ppm and solution. The resonance in the 11B NMR spectrum observed at -
1
-162.1 ppm (in C6D6), while the H NMR signals of the BMe2- 13.0 ppm is in agreement with the N→B-coordination persisting
group in BMe2 are observed as broadened singlets at 0.61 and in solution.
0.15 ppm (C6D6).
Furthermore, quantum chemical calculations (vide infra) also
With this series of compounds, we have completed the full indicate that the coordinative bond to the pyridyl nitrogen (≈
series of homologous haloboranes. Significantly, to the best of 175 kJ/mol) is among the strongest encountered so far for any
our knowledge, this is only the second report of a diiodo- of the alkylboryl-bridged ladder boranes discussed herein. It is
alkylborane.23 The development of a preparative access to this therefore likely, that the coordination of the (iso)cyano-ligands
compound constitute a significant finding, since BI2 may serve is not labile.
as a versatile starting material for further organometallic Borate anions featuring isocyano- groups have been reported
transformations.
previously,28 as well as neutral nitrogen-base adducts of
A highly unusual find was made in an attempt to further extend isocyanoboranes.29 While anionic isocyanoborates can be
the spectrochemical series: Prolonged heating of BCl2 in isolated,30 and employed as anionic ligands to transition
benzene in the presence of an excess of Me3Si-CN26 resulted in metals,31,32 the neutral boranes rather exist as dynamic
the clean conversion of the starting material. Monitoring of the mixtures and isomerize to the isocyanide-coordination mode in
reaction by 1H NMR (see Figure S29 in the ESI) shows the the presence of suitable metal centers.33,34,35 To the best of our
stepwise consumption of BCl2, and the concurrent formation of knowledge, this is therefore the first example of a stable neutral
two intermediates, which we presume to be the isocyanoborane that can be isolated in substance.
diastereomeres of the mono-substitution product (B(CN)Cl). Single crystals of B(CN)(NC) for analysis by X-ray diffraction
The intermediates are eventually consumed, and a final single where obtained by layering the reaction mixture with hexane
product is formed that was deemed to be the dicyano derivative (Figure 1 D). B(CN)(NC) crystalizes in the monoclinic space group
BCN2. However, IR-spectroscopy and analysis of single crystals P121/c1 with four molecules in the unit cell, and adopts an
by X-ray diffraction revealed, that instead of the dicyano- N→B-coordinated structure in the solid state. Analogous to
borane, a mixed borane is formed that contains one cyano- and previously described ortho-styryl-azine derived ladder
one isocyano ligand. The compound will therefore in the boranes,17,18,19 the isopropyl groups adopts an axial orientation
following be referred to as B(CN)(NC). IR spectra show two in the central six-membered N→B-ring. Consequently, one
characteristic bands of comparable strengths at ṽ = 2211 and substituent on boron is oriented in a trans-axial position relative
2125 cm-1, that can be attributed to the stretching vibration of to the iPr-group, while the other adopts an equatorial
isocyano- (B-N≡C) and cyano-moiety (B-C≡N), respectively.27 orientation. The crystal structure unambiguously confirmed the
The cyano carbon atom could not be observed by 13C NMR at a presence of the isocyano-ligand, although the substituents on
Table 1. Space groups, characteristic bond lengths [Å] and angles [°] of the synthesized ladder compounds derived from their crystal structures, and structural parameters
calculated by DFT. Numbering according to Figure 2. CiPr: methin-C of axial iPr-group. Xax: axial substituent on boron.
DFT[b]
N→B / N1-C5-C6-C11
11B NMR[c]
N1-C5-
C6-C11
C12–B1
Space
Group
Borane
BF2
N1→B1
[Å]
C5–C6
[Å]
CiPr-C12-B1-Xax
[°]
C5-N1-B1-Xax
[°]
[°]
[Å]
[Å]
[°]
[ppm]
P-1
1.589(3)
18.1(2)
1.485(3)
1.591(3)
162.2(1)
86.6(2)
84.4(2)
75.0(2)
-86.7(3)
1.664
20.3
+6.7
[a]
BCl2
P21/c
P42/n
P121/c1
1.596(4)
1.588(3)
1.579(6)
1.592(2)
19.6(3)
18.5(3)
-22.4(6)
23.6(2)
1.483(4)
1.482(3)
1.473(6)
1.475(2)
1.592(4)
1.591(3)
1.598(6)
1.613(2)
166.2(1)
170.1(1)
168.0(3)
1.623
1.614
1.594
1.626
22.5
22.2
22.9
22.7
+9.4
+4.9
-12.6
-13.0
BBr2
BI2
167.9(7) (C)
169.3(7) (N)
85.4(7) (C)
81.7(7) (N)
B(CN)(NC) P121/c1
[a] Included for reference purposes. Data adopted from Ref. 20. [b] From iPrax-structures optimized by DFT at the M06-2X/6-31G(d,p) level of theory. [c] Recorded in
C6D6.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
the boron atom were systematically disordered. The crystallo- that give rise to clearly visible shoulder bands in tVhieewsAprteiccletOranlionef
graphic data for B(CN)(NC) is in best agreement with the BMe2 and BBr2, while a shoulder band inDthOeI: 1s0p.e10c3t9ru/Cm9DoTf0B15F525iHs
presence of one cyano- and one isocyano-ligand in either axial somewhat less pronounced. Simulation of UV-vis spectra by
or equatorial position, with a preference (60%) of the isocyano- time-dependent DFT (TDDFT) calculations showed that these
ligand to be located in an axial orientation.
underlying bands are most likely attributable to absorption of
The coordinative N→B-bond to the pyridine ring measures N→B-coordinated conformers, wherein the isopropyl-group
1.592(2) Å, and is therefore the shortest yet observed for this assumes an equatorial conformation, instead of the
type of ladder borane, except for the three newly reported energetically favorable axial conformation (vide infra).
haloboranes: BF2, BBr2, and BI2 crystallize in the space-groups The BI2 derivative shows a λmax (305 nm) in the same range as
P-1 (Z=2, triclinic), P42/n (Z=8, tetragonal), and P121/c1 (Z=4, the other halo-boranes. However, the absorption only gradually
monoclinic), respectively. Similar to B(CN)(NC) and previously fades towards longer wave-length and reaches the baseline at
described ladder boranes,17,18,19 the isopropyl groups in all three approximately 400 nm (see Figure 2B and Figure S4 in the ESI).
compounds also adopt an axial orientation in the central six- Initially, we attributed this latter observation to inadvertently
membered N→B-ring. The N→B-bond lengths range from decomposition of BI2. However, TD-DFT calculations indicate
d(N→B) = 1.589(3) Å for BF2, and d(N→B) = 1.588(3) Å for BBr2, that the red-shifted absorption onset may be attributed to very
to d(N→B) = 1.579(6) Å for BI2. The coordination is therefore weakly populated n-to-π*-transitions from lone-pairs on the
tighter than in BCl2 (d(N→B) = 1.596(4) Å)17 and BH2 (d(N→B) = iodide-ligands to π*-orbitals delocalized across the phenyl-
1609(2) Å).17 This sequence does not quite reflect the typically pyridine ring-system (vide infra).
observed order of Lewis acidity of haloboranes The influence of the substituents on boron was further
(BI3>BBr3>BCl3>BH3>BF3).36 Particularly the less tight coordina- elucidated through investigation of the electrochemical
tion in BCl2 and the short N→B-coordination in BF2 are reduction potentials by square-wave voltammetry (SWV) and
surprising. This observation may be attributable to
a
cyclic voltammetry (CV, all potentials reported in the following
combination of steric and electronic factors, since boron-halo- have been referenced vs. FcH/FcH+, peak potential Ep have been
gen-bonds to axial halogens are generally longer than to determined by SWV). All new boranes except BF2 exhibited only
equatorial halogens. The steric contributions are apparent from irreversible reduction waves in CV experiments (see Figure S1 in
the fact that effect is negligible for BF2 (Δd(B-F)ax/eq = 0.003 Å), the ESI for details). However, the results of the SWV
and substantially larger for BCl2, BBr2, and BI2 (Δd(B-X)ax/eq = measurements (Figure 2A), allowed for a reliable comparison of
0.033-0.028 Å).
these results with data for previously reported compounds. The
reduction peak of BMe2 appeared broadened, but agrees well
with the one of BBN. As expected, these diakylboranes
therefore exhibit very similar electron affinities. Among the new
boranes only BF2 underwent reversible electrochemical
reduction (see Figure S1 in the ESI), and also showed the lowest
red
electron affinity of all haloboranes with a peak current at Epeak
= -2.26 V (SWV) relative to the ferrocene/ferrocenium redox
couple. This value lies right between those of the previously
reported bisphenyl- and bis(pentafluorophenyl)-derivatives
red
red
BPh2 (Ep = -2.40 V, and BPF (Ep = -2.20 V), and therefore
closed a rather large gap in this series. Reduction of BBr2
red
occurred close to the potential as observed for BCl2 at Ep
=
-2.05 V. However, this compound was less “well-behaved” and
gave more broadened signals, presumably due to
decomposition of the reduced species. Similarly, no reliable
electrochemical data could be obtained for BI2. A surprising find
turned out to be B(CN)(NC), which exhibited the highest
electron-affinity in this series. Although this borane exhibited
only irreversible reduction (see Figure S1 in the ESI), a sharp
red
peak current was observed in SWV experiments at Epeak = -
Figure 1. A: Crystal structures of N→B-ladder boranes. A: BF2; B: BBr2; C: BI2; D:
B(CN)(NC). Structures shown at 55% probability level. Hydrogen atoms have been
omitted for clarity. Representative labels shown for BF2 in A.
1.95 V. This finding indicates that dicyanoboryl-ladder may
exhibit much higher electron affinities and higher redox-
stability than haloboranes.
Optical and electrochemical properties: Analyses of the
phenylpyridine-derived ladder boranes by UV-vis spectroscopy
showed comparable absorption spectra in all cases, with
longest-wavelength absorption maxima (λmax) between 304
(BMe2) and 316 nm (BBr2). Also present in the absorption
spectra are underlying absorption bands at longer wave-lengths
4 | J. Name., 2012, 00, 1-3
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Table 2. Schematic representation of possible borane
View Article Online
conformers and calculated relative energDieOsI:in10k.1J0/3m9/oCl9. DT01555H
N
R
B
R
B
H
N
R
N
R
H
R
B
H
R
open
iPrax
0
iPreq
+83
+12
BMe2
+91
0
+11
+10
+11
+8
+36
+33
+14
+51
BF2
+120
+135
+124
+177
+177
+176
+179
0
BCl2
0
BBr2
0
+23
+23
:= 0[a]
+40
BI2
B(CN)(NCax)[a]
B(CN)(NCeq)[a]
B(CN)2
[a]
B(NC)2
[a] Data for B(NC)2, B(CN)(NCax), and B(CN)(NCeq) given relative
to iPrax-conformer of B(CN)2.
In agreement with previous findings, iPrax conformers are
generally the most favorable (Table 2). However, the
corresponding iPreq conformers are only between 8 (BI2) and 13
kJ/mol (B(CN)(NCax)) higher in energy, and should therefore be
thermally populated. Open conformers are between 83 kJ/mol
(BMe2) and 135 kJ/mol (BBr2) higher in energy. Notably, BF2
shows the weakest N→B-coordination (ΔE = 91 kJ/mol) among
the haloboranes, while the N→B-bond is strongest in BBr2. The
strongest yet observed N→B-bonds were found for cyano-
substituted boranes, with decoordinated conformers being
between +176 kJ/mol and +179 kJ/mol higher in energy than
the iPrax conformer of B(CN)2 - the most stable isomer (:= 0
kJ/mol). The diisocyano-derivative B(NC)2 is the least stable one
(+40 kJ/mol for iPrax, +51 kJ/mol for iPreq), while the
Figure 2. A: Square-wave voltammetry scans of new N→B-ladder boranes (blue) and
previously reported boranes (black). Recorded in THF with 0.1M NBu4PF6 as supporting
electrolyte at a scan rate of 200 mV/s. * internal standard Ferrocene. B: UV-vis experimentally observed derivatives B(CN)(NCax) and
Absorption spectra of new boranes in THF-solution. BCl2 included for reference.
B(CN)(NCeq) lie between the two (+23 kJ/mol).
The optical properties of the boranes were also computed by
DFT-calculations: To compare the properties of the new
time-dependent DFT (TDDFT) calculations for iPrax, iPreq, and
compounds with previously reported ladder boranes, and to
elucidate the origin of their optical properties, the new boranes
were simulated by quantum chemical simulations. Geometry
open conformers (see Figure S4 in the ESI), and UV-vis
absorption spectra were simulated. Analyses of the involved
optical transitions revealed, that for all N→B-coordinated
optimizations at the M06-2X/6-31G(d,p) level of theory were
boranes, the lowest energy transitions are near exclusively
performed for three conformers of each borane: two N→B-
HOMO-to-LUMO in character (see Section 2 of the ESI for listed
data, and Orbital plots). While the LUMOs are typically π*-
orbitals delocalized across the phenyl-pyridine backbone, the
coordinated ones with the isopropyl-group placed either in an
equatorial (iPreq) or an axial (iPrax) position, as well as an open
(open) conformer without N→B-bond (Table 2). In the case of
exact nature of HOMO-orbitals varies: For BF2 the HOMO is a π-
the cyano-substituted borane the survey was expanded: iPreq-
orbital delocalized throughout the phenyl-pyridine, while the
and iPrax-, and open-conformers of the dicyano-borane
HOMO of BMe2 also shows σ-type contributions from electron
(B(CN)2), the diisocyanoborane (B(NC)2), and B(CN)(NC)-
rich B-CH3 bonds. In contrast, the HOMOs of BBr2 and
derivatives bearing the isocyano-substituent either in an axial
particularly BI2 are increasingly dominated by contribution from
(B(CN)(NCax)) or in an equatorial (B(CN)(NCeq)) orientation were
the lone-lairs on the halogen atoms. Open conformers show
simulated.
more complex lowest energy transitions, since the LUMO is
dominated by the pz-orbital on boron, which is not conjugated
to the π-system. As a consequence, optical transitions of open
conformers are expected to occur at much shorter wavelength
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than those of N→B-coordinated conformers. Furthermore, a
common feature observed for the N→B-ladder conformers of
BF2 and BBr2 and BMe2 is that the iPreq-conformers – which are
higher in energy, but can be thermally populated - exhibit
lowest energy transitions that are between 0.31 and 0.25 eV
lower in energy than those of the corresponding iPrax-
conformers. A qualitative comparison of the simulated UV-vis
spectra with the experimental data indicates that the longer
wavelength shoulder bands in the absorption spectra of these
compounds originate from a superimposition of the absorption
of both conformers.
View Article Online
R
R
R
B
DOI: 10.1039/C9DT01555H

R
iPr
iPr
R

B
1,3-R
-shift
R2B-H
N
N
N
B4
B3
2
Scheme 2. Generation of vinyl-borane-containing N→B-ladders and their conversion
into B,N-isosters.
Wang and co-workers recently showed that hydroboration of 2-
alkynyl-pyridines yields vinylboranes that form intramolecularly
N→B-coordinated five-membered rings via a formal trans-
hydroboration.37 Curiously, while hydroboration of 2-
ethynylpyridine38,37 results in exclusive borylation at the α-
position, in the above case borylation of the internal triple bond
favored the β-position. This latter selectivity would not allow for
the desired intramolecular N→B-coordination, as indicated in
Scheme 2. We therefore experimentally explored the hydro-
boration of two substrates equipped with more sterically
Furthermore, analysis of the expected absorption features of
BI2 showed, that this borane would be expected to show a much
red-shifted absorption. The lowest energy transition is
calculated at 3.17 eV (f = 0.003) for the iPrax -derivative (iPreq
:
3.33 eV, f = 0.004), and is therefore at least 0.7 eV lower in
energy than in any other borane investigated in this survey. The
reason for their weak population is that they represent formally
forbidden n-to-π*-transitions from lone-pairs on iodide atoms
to π*-type LUMO on the phenyl-pyridine system. These low
intensity transitions explain the only gradual fading of the
optical absorption, and therefore clarify, that the experimental
data indeed originated from the BI2, rather than from
inadvertent decomposition products.
The electronic situation in B(CN)(NC) is very similar to e.g. BF2
with the exception that for open conformers of this compounds
much lower energy longest-wavelengths absorptions would be
expected, that are in the same range as for the N→B-
coordinated conformers. However, since the N→B-bond in this
compound is the strongest of all included in this survey, we still
attribute the observed absorption features to the ladderized
conformers. iPreq conformers again show longer-wavelength
absorption, and optical transitions remain almost unaffected by
switching isocyano-substituent from an axial to an equatorial
position. The weak shoulder band in the experimental
absorption spectrum of B(CN)(NC) is therefore also assigned to
a superimposition of the absorption of iPreq and iPrax confor-
mers.
demanding tert-butylethynyl- (2a, R = Bu)39 and trimethyl-
t
silylethynyl-side chains (2b, R = SiMe3), to enforce the desired
α-borylation. While hydroboration of terminal alkynes readily
proceeds at ambient temperature, hydroboration of the
present substrates required more forcing conditions, simple
mixing of the reagents just resulted in adduct formation.40
Nevertheless, prolonged heating of 2a in the presence of 9H-
BBN resulted in conversion of the substrate alkyne and allowed
isolation of the α-vinyl-borane tBuBBN in 43% yield. The steric
pressure of the tert-butyl group enforced borylation in the
benzylic position, and even though the tBuBBN is still highly
strained, it is still capable of forming an intramolecular N→B-
bond in solution, as indicated by 11B NMR signals at 0.0 ppm in
C6D6 and -2.7 ppm in THF-d8. However, simulations of open and
closed structures indicate that the N→B-coordinated conformer
is only 45 kJ/mol lower in energy. The N→B-coordination is
therefore weaker than in any of the previously investigated
systems. For instance, the N→B-bond in the styrylpyridine
derived analogue41 was estimated to be about 69 kJ/mol
strong.17 After this exploratory experiment, we further
investigated the hydroboration of 2b (R = SiMe3). The Me3Si-
group was meant to serve as a protecting group that may
eventually be removed to access an unsubstituted borane. In
contrast to the previous results, hydroboration with 9H-BBN
resulted in unspecific decomposition of the substrate, and did
not allow the isolation of a product. Surprisingly, hydroboration
was possible with dimesitylborane (Mes2BH42), and the product
SiBMes2 was isolated in 32% yield. Notably, analysis by X-ray
crystallography showed that SiBMes2 is borylated at the β-
position, despite the resulting steric hindrance. While this result
can be explained by the well-known β-silicon effect,43,44,45 the
geminal borylation of the highly sterically encumbered borane
and the TMS groups is still surprising. Furthermore, due to the
syn-addition of Boron and hydrogen SiBMes2 cannot form an
intramolecular N→B-bond. In a final experiment, the borylation
was performed using Piers’ Borane ((C6F5)2BH, BPF-H46), which
again allowed the isolation of a well-defined product (SiBPF) in
40% yield. The reaction again results in silicon-directed
Summarizing the results discussed above, we have successfully
expanded the range of accessible N→B-ladder boranes across
the whole range of haloboranes, and also made a pseudo-halide
derivative (B(CN)(NC)) available, that shows the strongest
N→B-bond, and highest electron-affinity yet. These results are
fully consistent with our previous reports, and therefore
significantly expand the knowledge base to further explore this
chemistry.
Preparation and Properties of Vinylboranes
Synthesis and structure: A key drawback of the above strategy
was, that the resulting N→B-ladder-boranes cannot be
converted into fully conjugated B-N-isosters, unlike methylene-
boryl-bridged boranes (Chart 1, C).5 We therefore considered
the mono-hydroboration of alkynes as an alternative strategy to
generate the corresponding vinylboranes (B4, Scheme 2Figure
3). These might then be isomerized into the corresponding B,N-
isosters featuring tricoordinate boron (B3).
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borylation at the β-position. However, 1,2-hydroboration and Analysis of the solid-state structure of SiBPF confirmed the
View Article Online
formation of the BPF-analogue to SiBMes2, does not occur. formation of a seven-membered N→B-DhOeIt:e1r0o.1c0y3c9l/eC.9DThT0e15r5in5Hg
Instead, under the reaction conditions, the TMS-group migrates adopts a boat-conformation with the tetracoordinate boron
to the benzylic α-atom and thus furnishes the formal 1,1- center located at the ‘bow’. The N→B-coordination (SiBPF:
hydroboration product SiBPF. Notably, the highly Lewis-acidic d(N→B) = 1.640(2) Å) is less tight than in the BPF-containing
vinyl-B(C6F5)2-group can still coordinate onto the pyridyl- styrylpyridine-derivative that forms a six-membered ring (BPF:
nitrogen under formation of
a
seven-membered 1,2- d(N→B) = 1.627(2) Å17). Compared to SiBMes2, the carbon-
azaborepine-ring. Furthermore, the N→B-bond evidently boron bond to the vinyl carbon atom (d(B1-C12) = 1.623(3) Å) is
persists in solution, since SiBPF shows a signal in the 11B NMR at substantially longer, while the adjacent C=C-double bond
-4.7 ppm. According to DFT-calculations, the strength of the (d(B12-C13) = 1.346(3) Å) appears shortened. These observa-
N→B-bond is about 90 kJ/mol, and therefore ca. 30% weaker tions are in agreement with transition to a tetracoordinated
than the N→B-coordination in the analogous styrylpyridine- carbon atom, while the variation of the double-bond-length lies
derived BPF-containing phenyl-pyridine (≈132 kJ/mol17).
within the typical variation for this type of system.48
C6F5
C6F5
Me3Si
B
R = SiMe3
H-B(C6F5)2
N
C6D6, 85°C,
2 d
SiBPF, 40%
R
BMes2
Me3Si
R = SiMe3
Mes2B-H
N
N
C6D6, 85°C,
2 d
2a, R = tBu
SiBMes2, 32%
2b, R = TMS
R'
R'
9H-BBN
THF, 60°C
3 d, 43%
tBu
B
N
C6D6, 85°C,
2 d
R = tBu
tBuBBN, 43%
Scheme 3. Preparation of vinylboranes by hydroboration.
The three vinyl-boranes could be crystallized and were analyzed
by single-crystal X-ray crystallography. For tBuBBN the crystal
Figure 3. Crystal structures of vinyl-boranes. A: tBuBBN; B: SiBMes2; C1/C2: side- and
structure reveals an N→B-coordinated conformation in the top-down view of SiBPF. Structures shown at 55% probability level. Hydrogen atoms
have been omitted for clarity. Representative labels shown for tBuBBN in A.
solid state. With a bond-length of 1.662(4) Å, the coordinative
bond is weakened compared to its styrylpyridine-derived
Optical and electrochemical properties: The optical absorption
analogue (1.646(2) Å17). Still, coordination is tighter than in
properties differ somewhat from those of their styryl-pyridine-
more sterically encumbered boranes,17 or systems with less
derived analogous. SiBMes2 and SiBPF exhibit redshifted λmax
Lewis-basic N-heterocycles.14,17 The central N→B-heterocycle
values of 356 and 360 nm. However, their absorption falls of
assumes a half-chair conformation, with a biaryl-torsion of
sharply, resulting in gaps of 3.44 and 3.48 eV, respectively.
25.8(4)°. To allow for this torsion, the tert-butyl-vinyl-group has
to bend out of the ring-plane, which result in an angle between
the phenyl-ring-plane and the vinylic C=C-bond of 51.5(1)°. The
crystal structure of SiBMes2 confirms the β-borylation, and the
presence of a tricoordinate dimesityl-vinyl-borane. The steric
pressure of the TMS-group does not directly affect the boryl
group. The bond between boron and the vinylic carbon atom
(d(B1-C12) = 1.563(3) Å), and the adjacent C=C-double bond
These latter values are close to those of the previously reported
compounds BPh2 and BPF (BPh2: λmax = 306 nm, Egopt = 3.42 eV;
opt
BPF: λmax = 310 nm, Eg = 3.50 eV). tBuBBN (λmax = 339 nm
(shoulder band), Egopt = 3.09 eV) exhibits the lowest optical gap
in the series. Indeed, the optical gap of tBuBBN is almost 0.1 eV
lower than even that of its derivative BBN (BBN: λmax = 303 nm,
opt
Eg = 3.17 eV), owed to a distinct shoulder band centered on
339 nm. SiBPF is the only borane in the current and previous
surveys that shows fluorescence with an emission maximum at
(d(C12-C13) = 1.354(2) Å) are within the typical range for the
kind of system (e.g. nC5H11-C(BMes2)=CH-thienyl47: B-Cvinyl
=
420 nm. Curiously, SiBMes2 also turned out to be non-
fluorescent, even though structurally related vinyl-boranes
are.48b,37
The electrochemical properties of the three boranes were also
studied, to elucidate the impact of the extension of the
1.565(8) Å, C=C = 1.350(7) Å, C=C-CAr-CAr = 37.1(8)°). Still,
compared to this reference system, the additional steric
pressure from the TMS-groups leads to a stronger torsion
between the vinyl-group and the aryl-system (SiBMes2: C=C-CAr-
CAr = 53.4(2) Å).
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conjugated system, and the changes in molecular architecture.
For tBuBBN reversible electrochemical reduction was observed
with a peak potential at -2.50 V vs. FcH/FcH+. SiBMes2 shows a
reversible reduction at -2.37 V vs. FcH/FcH+, followed by a
second, irreversible reduction at -2.65 V. SiBPF exhibits the
highest electron affinity with a peak potential at -2.21 V, but
undergoes only quasi-reversible reduction. The reduction
potentials of tBuBBN and SiBPF are therefore close to those of
View Article Online
DOI: 10.1039/C9DT01555H
DFT calculations: To elucidate the origin of the optical
properties of these compounds, the structures were also
simulated by DFT, optical transitions were calculated by TD-DFT,
and the resulting UV-vis absorption spectra were simulated (see
Figure S5 in the ESI). These calculations showed that the
lowered optical gap in tBuBBN compared to BBN could be
attributed to a combination of the properties of the BBN-
moiety, and the extension of the π-system onto the vinyl group.
According to the calculations, the experimentally observed
longest wavelength absorption band is composed of two
comparatively weakly populated transitions (ES1: E = 3.69 eV /
336 nm, f = 0.052, 92% HOMO-to-LUMO; ES2: E = 4.03 eV / 308
nm, f = 0.038, 10% HOMO-2-to-LUMO + 81% HOMO-1-to-
LUMO). The LUMO is a pure π*-type orbital delocalized
throughout the phenylpyridine-ring system, with only minor
contributions from the vinylic side chain. In contrast, HOMO,
HOMO-1 and HOMO-2 show strong σ-type contributions of
boron-carbon-bonds in the BBN frame, as well as π-type
contributions of the vinylic bond in the side chain, while only
weak π-delocalization onto the phenyl-ring is present in the
HOMO and the HOMO-1. The σ-character of the occupied
orbitals and very limited spatial overlap with the LUMO explain
the weak population of the lowest energy transitions. Overall,
the extension of the conjugation into the sidechain both raises
the HOMO and somewhat lowers the LUMO of tBuBBN
(HOMODFT = -7.18 eV, LUMODFT = -1.36 eV) compared to BBN
(HOMODFT = -7.26 eV, LUMODFT = -1.31 eV). Notably, this is
another example for a 9BBN-containing ladder borane wherein
the σ-type contributions to the HOMO render the lowest energy
transition formally forbidden and thus only weakly
populated.17,18,19
p
their phenylpyridine-derived analogues (BBN: Ered = -2.25 V,
BPF: -2.20 V17). The reduction potential of SiBMes2 lies within
the typically range Mes2B-functionalized arenes,49 and also
agrees with the electron affinity of its diphenylboryl-analogue
p
(BPh2: Ered = -2.40 V17). Overall, these compounds offer
comparable electrochemical properties despite their more
extended π-systems.
For SiBMes2 the calculations predict the main absorption band
to be composed of two rather strongly populated transitions,
that are HOMO-1- and HOMO-to-LUMO in character (ES1: E =
4.02 eV / 308 nm, f = 0.129, 27% HOMO-1-to-LUMO + 39 %
HOMO-to-LUMO; ES2: E = 4.20 eV / 296 nm, f = 0.144, 47 %
HOMO-1-to-LUMO + 31 % HOMO-to-LUMO). The LUMO in this
system is a π*-orbital and shows strong contributions from the
empty pz-orbital on boron, but is also delocalized across the
vinyl-group and throughout the phenyl-pyridine π-system. The
HOMO and HOMO-1, however are mainly composed of the π-
Figure 4. A Square-wave voltammetry scans of vinyl-boranes tBuBBN, SiBMes2, and
SiBPF. Recorded in THF with 0.1M NBu4PF6 as supporting electrolyte at a scan rate of
200 mV/s. * internal standard Ferrocene. B Normalized UV-vis-absorption spectra of
tBuBBN, SiBMes2, and SiBPF (solid black line), and emission spectrum of SiBPF (dashed
black line) recorded in THF.
Table 3. Structural and electronic properties of vinyl-boranes.
C12–B1
N1-C5-
C6-C11
11B
DFT[c]
N→B
open vs.
opt[a]
red[b]
Space
Group
NMR[e]
Borane
N1→B1
C5–C6
CH=CB
[Å]
Eg
Ep
CHC-B
closed[d]
λmax
[kJ/mol]
[Å]
[°]
[Å]
[°]
[nm]
[nm]
401
[eV]
[V]
[Å]
[kJ/mol]
P121/c1
monoclinic
1.662(4)
25.7(4)
1.469(4) 1.621(4) 1.352(4) 339[f]
3.09
-2.50r
1.672
+45
-0.0
-2.7 (THF)
tBuBBN
P-1(2)
triclinic
P121/n
1.640(2)
-55.6(3)
26.0(2)
1.486(3) 1.623(3) 1.346(3) 321
1.482(3) 1.563(3) 1.354(2) 318
360
356
3.44
3.48
-2.21qr 1.637
+90
--
-4.7
SiBPF
-2.37r
-2.65i
--
78.4
(CD2Cl2)
SiBMes2
monoclinic
[a] Derived from absorption onset of UV-vis spectra in recorded in THF-solution. [b] Peak potentials derived by square wave voltammetry. i/r: (ir)-reversibility as observed
by cyclic voltammetry. [c] Optimized by DFT at the M06-2X/6-31G(d,p) level of theory. [d] Destabilization of non-coordinated open conformer over N→B-ladder
conformer. [e] Recorded in C6D6, unless stated otherwise. [f] Shoulder band. Maximum derived by deconvolution.
8 | J. Name., 2012, 00, 1-3
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orbitals of the electron rich mesityl-rings, with only partial Indeed, B(CN)(NC) shows the strongest N→B-bond (≈ 175
View Article Online
DOI: 10.1039/C9DT01555H
contributions from the vinyl-group and the phenyl-ring in the kJ/mol), and highest electron-affinity observed so far, while in
HOMO-1.
the sterically encumbered tBuBBN N→B-coordination is still
Lastly, for SiBPF the TD-DFT calculations predict a lowest energy predominant even though the N→B-bond is the weakest yet (≈
transition comparable in energy as those for SiBMes2, which is 45 kJ/mol).
in agreement with the experimental data. The optical With regard to potential applications of these compounds in
absorption is dominated by a single transition that is near organic electronics, we can also state that several of the
exclusively HOMO-to-LUMO in character (ES1: E = 4.12 eV / investigated boranes exhibit sufficient stability to be handled
301 nm, f = 0.071, 86% HOMO-to-LUMO, 5 % HOMO-to- and processed under ambient conditions. The BF2- and BCl2
LUMO+1). Both LUMO and LUMO+1 are π-orbitals delocalized derivatives are bench-stable as solids, and can be handled in
within the phenylpyridine-ring system, but with substantially solution over short periods of time, while BBr2 and BI2 are not
lower contributions from the phenyl-ring, while the HOMO stable under these conditions. The vinyl-boranes SiBMes2 and
represents a π-orbital delocalized across the phenyl-ring and SiBPF are insensitive against ambient conditions even in
the vinylic side chain. Notably, these calculations predict that an solution.
open conformer would exhibit a much more redshifted These results are fully consistent with our previous reports and
absorption (ES1: E = 3.35 eV / 370 nm, f = 0.032, 19 % HOMO-1- significantly expand both the scope and the knowledge base for
to-LUMO, 69 % HOMO-to-LUMO).
a further exploration of this chemistry.
The above results present a new class of substrates suitable for
conversion in N→B-ladder boranes by hydroboration. The
isolation and structural characterization of tBuBBN proves that
α-borylation can be sterically enforced, and N→B-ladder
formation is still possible. Steric pressure of a TMS-group is
evidently insufficient, however due to the dominating β-silicon
effect. In future works, the use of bulkier silyl-substituents,45
and indeed less bulky alkyl-groups,39,50 may be explored to
generate this kind of molecular structure.
Experimental Section
Materials and Instrumentation
All reactions and manipulations of sensitive compounds were
carried out under an atmosphere of pre-purified argon using
either Schlenk techniques or an inert-atmosphere glovebox
(MBraun Labmaster). Dichloromethane was purified using a
solvent purification system (MBraun; alumina
/ copper
columns). Hexane was dried by distillation from CaH2 under
argon atmosphere prior to use. Other commercially available
solvent and reagents (Merck, VWR, Acros, Alfa Aesar) were
either used as obtained or purified by standard procedures.51
1H-, and 13C-NMR spectra were recorded at 293 K on a Bruker
Avance DRX 400 (400 MHz) spectrometer or a Bruker Avance
Conclusions
In summary, we have prepared a series of new boranes capable
of forming N→B-heterocycles through intramolecular coordi-
nation. Through systematic variation of the substituents on
boron in an established 2-phenylpyridine-derived system we
completed the whole series of haloboranes (previously: BCl2,17
newly reported herein: BBr2, BF2, BI2), and also made a pseudo-
halide derivative (B(CN)(NC)) available. To the best of our
knowledge, very few isocyanoborates have been reported in
the literature, while B(CN)(NC) is the first example of a neutral
isocyano-borane. However, it is not quite certain at this point,
whether the B(CN)(NC) emergences only in the solid state, or is
also present in solution.
We have also introduced 2’-alkynyl-substituted 2-phenyl-
pyridines as a new class of substrates. Hydroboration of a tert-
butyl-terminated alkyne lead to the isolation of a new π-
extended ladder borane (tBuBBN), wherein the desired
regioselectivity is preserved, while in the hydroboration of
trimethylsilyl-terminated substrates, the regioselectivity is
reversed. Borylation either lead to the formation of an isomer
incapable of ladder formation (SiBMes2), or, after a strain
reducing 1,2-silyl-shift (SiBPF) to formation of an 1,2-
azaborepine by intramolecular N→B-coordination.
1
500 AMX (500 MHz). Solution H and 13C NMR spectra were
referenced internally to the solvent residual signals.52 Individual
signals are referred to as singlet (s), doublet (d), and multiplet
(m). High resolution mass spectrometry measurements were
performed on a Bruker SolariX FTMS using MALDI (Matrix
Assisted Laser Desorption Ionization) as ionization method with
trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
propenylidene]malononitrile (DCTB) as matrix. UV-visible
absorption spectra and photoluminescence spectra were
acquired on a Perkin Elmer Lambda 19 UV-vis/NIR spectrometer
and
a Perkin Elmer LS 55 fluorescence spectrometer,
respectively.
Deconvolution of UV-vis spectra was performed with OriginPro
9.0G. Melting points were measured on a Büchi M 565 melting
point apparatus with a heating rate of 1 °C/min. Infrared
spectroscopy was performed on a Perkin Elmer Spektrum BX FT-
IR System (Software: Spektrum v3.02 Version 3.02.01, Perkin
Elmer). Crystallography: X-ray diffraction intensities were
collected an Agilent Technologies SuperNova single-crystal X-
ray diffractometer at 150 K using Mo-Kα radiation, and the
structures were solved using direct methods (SIR92 or Shlexs-
2014), completed by subsequent difference Fourier syntheses,
and refined by full-matrix least-squares procedures. Semi-
empirical absorption corrections from equivalents (Multiscan)
Analyses of the new boranes by electrochemical methods and
UV-vis-spectroscopy were complemented by DFT calculations.
We showed that systematic variation of the substituents on
boron allows for the incremental variation of the electron
affinity of the phenylpyridine-model system over a range of >
0.7 eV between alkylboranes (BMe2, BBN) and B(CN)(NC).
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were carried out. CCDC 1899857 through 1899864 contain the and the residue was purified by column chromatography and
View Article Online
DOI: 10.1039/C9DT01555H
supplementary crystallographic data for BBr2, BF2, BI2, distillation. BrtBu was obtained as pale yellow oil in a yield of
B(CN)(NC), tBuBBN, SiBMes2, SiBPF, and 2b*HBMes2. These 12% (1.9 g, 8.1 mmol).
3
4
data can be obtained free of charge from the Cambridge 1H-NMR (CDCl3, 400 MHz): δ = 7.55 (ddd, JH,H = 8.0 Hz, JH,H
=
=
Crystallographic
Data
Centre
via 1.3 Hz, 5JH,H = 0.4 Hz, 1H, C1-H), 7.41 (ddd, 3JH,H = 7.7 Hz, 4JH,H
www.ccdc.cam.ac.uk/data_request/cif, and are also included at 1.8 Hz, 5JH,H = 0.4 Hz, 1H, C4-H) 7.23 – 7.17 (m, 1H, C3-H), 7.12 –
the end of this document. Computational Details: Quantum 7.08 (m, 1H, C2-H), 1.35 (s, 9H, (CH3)3) ppm. 13C-NMR (CDCl3,
chemical calculations were performed on the bwForCluster 101 MHz) : δ =133.14, 132.36, 128.69, 126.96, 126.12, 125.87,
JUSTUS at the University of Ulm, using release B.01 of the 103.73, 78.10, 30.97 (3C, (CH3)3), 28.40 ppm. MS (EI, 70 eV): m/z
Gaussian16 program package. Geometry optimizations were calcd for C12H13Br: 236.02 Da; found: 142.10 (100) Da [M -CH3 -
performed at the M06-2X/6-31G(d,p)-level. All converged Br]+, 236.05 (17) Da [M]+.
structures were found to be local energetic minima, as Synthesis of 2-(2-((trimethylsilyl)ethynyl)phenyl)pyridine (2b):
established by frequency calculations. Electronic and optical BrTMS (1.97 g, 7.8 mmol) was dissolved in 9 mL dry THF and the
properties were simulated using the M06-2X functional and solution was cooled to -78 °C. A 1.6 M solution of nBuLi (4.95
base set 6-311+G(d,p) in the gas phase Kohn-Sham frontier mL, 7.9 mmol) was added dropwise and the solution was stirred
orbitals were plotted using GaussView 5.0. Absorption spectra at -78 °C for 2 h followed by the addition of ZnCl2 (1.08 g, 7.92
were simulated using GaussSum 3.053 at a half-width at half mmol). The mixture was stirred at -78 °C for 45 min, after which
height of 0.62 eV (5000 cm-1). The above combination of the solution was allowed to reach room temperature. After the
functionals and base sets was chosen, because M06-2X addition of PdCl2(PPh3)2 (218 mg, 0.31 mmol) and 2-
functional reproduces non-covalent coordinative interactions in bromopyridine (1.53 g, 9.71 mmol) the solution was stirred at
organoboranes well. Also, the same level of theory was used in room temperature for 12 h. The reaction mixture was adsorbed
a previous comprehensive publication on the same basic on silica and purified by flash-gel silica column chromatography
system,17 and the results can therefore be compared directly.
using an eluent gradient of petroleum spirit/ethyl acetate 20/1
to 10/1 which furnished the product as yellow oil (1.19 g, 61%).
1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.72 (ddd, J = 4.9, 1.7, 1.0
Hz, 1H, C1-H), 7.99 (dt, J = 8.0, 1.0 Hz, 1H, C7-H), 7.76 (dd, J =
7.8, 1.3 Hz, 1H), 7.72 (td, J = 7.7, 1.8 Hz, 1H), 7.59 (dd, J = 7.7,
1.4 Hz, 1H), 7.43 (td, J = 7.6, 1.4 Hz, 1H), 7.34 (td, J = 7.6, 1.4 Hz,
1H), 7.29 – 7.24 (m, 1H), 0.16 (s, 9H, C14-H) ppm. 13C NMR (101
MHz, CDCl3, 25 °C): δ = 157.36 (C5), 149.34 (C1), 142.35, 135.75
(C6), 133.58, 129.73, 129.09, 128.34, 124.82, 122.41, 121.25,
104.54 (C12), 98.24 (C13), -0.17 (C14) ppm. HR-MS (ESI): m/z
calcd for C16H18NSi+ [M+H]+: 252.1203 Da; found: 252.1203 Da.
Synthesis of 2-(2-((tert-butyl)ethynyl)phenyl)pyridine (2a): A
Schlenk tube was charged with a solution of 1.3 g (5.5 mmol) of
1-bromo-2-(3,3-dimethylbut-1-in-1-yl)benzene (BrtBu) in dry
benzene and 3.5 g (0.3 mmol) of Pd(PPh3)4 and 2.8 g of (7.6
mmol) 2-(tri-n-butylstannyl)pyridine were added. The tube was
sealed, and the reaction mixture was stirred for 3 days at 110
°C. Subsequently, the solvent was removed under vacuum, and
the crude product was purified by column chromatography
(SiO2, hexanes/ethyl acetate 1:1, v/v), followed by distillation.
2a was obtained in 11% yield (142 mg, 0.6 mmol) as a pale
brown oil.
Synthetic Procedures
Synthesis of (2-o-bromophenyl)-1-trimethylsilylacetylene
(BrTMS): This compound was synthesized according to modified
literature procedure.54 To a solution of 1-bromo-2-iodobenzene
(2.93 g, 10.4 mmol), CuI (60 mg, 0.31 mmol) and PdCl2(PPh3)2
(218 mg, 0.31 mmol) in 15.6 mL dry NEt3 TMS-acetylene (1.22 g,
12.4 mmol) was added and the mixture was stirred at room
temperature for 19 h. After removal of the solvent under
reduced pressure the residue was dissolved in ethyl acetate and
the organic phase was washed with an aqueous NaHCO3 and a
NaCl-solution. The organic phase was dried over MgSO4
followed by removal of the solvent under reduced pressure. The
residue was purified by silica column chromatography using
petroleum spirit as eluent furnishing the product as yellow oil in
quantitative yield (2.64 g).
1H NMR (400 MHz, CDCl3): δ = 7.58 – 7.55 (m, 1H, C1-H), 7.49
(ddd, J = 7.7, 1.8, 0.4 Hz, 1H, C4-H), 7.26 – 7.22 (m, 1H, C3-H),
7.15 (ddd, J = 8.0, 7.5, 1.8 Hz, 1H, C2-H), 0.28 (s, 9H, C5-H) ppm.
13C NMR (101 MHz, CDCl3): δ = 133.74, 132.49, 129.71, 127.03,
125.90, 125.36, 103.15 (C6), 99.77 (C7), -0.01 (C5) ppm.
Synthesis of (1-tert-butyl)-(2-bromophenyl)acetylene (BrtBu):
This synthesis followed a modified literature procedure.55 A 500
mL Schlenk-flask was charged with 1-bromo-2-iodobenzene
(19.9 g, 70.3 mmol), and triethylamine (100 mL). The mixture
was degassed by bubbling argon inside the solution for 30 min
with vigourous stirring, and subsequently a suspension of 1.0 g
(1.4 mmol) of PdCl2(PPh3)2 and 0.5 g (2.8 mmol) of CuI in a few
mililiters of anhydrous THF was added. Cold (-20 °C) 3,3-
Dimethylbutyne (7.0 mL, 85.2 mmol) was added last, the
reaction vessel was sealed and stirred at ambient temperature
for 19 h. Subsequently, the reaction mixture was filtered over
Celite, solvent and triethylamine were removed under vacuum
b.p. = 45 °C at 3×10-3 mbar. 1H-NMR (CDCl3, 400 MHz): δ = 8.70
(ddd, 3JH,H = 4.8 Hz, 4JH,H = 1.9 Hz, 5JH,H = 1.0 Hz, 1H, C1-H), 7.97
– 7.95 (m, 1H, C4-H), 7.73 – 7.67 (m, 2H, C7-H, C3-H), 7.49 (ddd,
3JH,H = 7.5 Hz, 4JH,H = 1.5 Hz, 5JH,H = 0.6 Hz, C10-H), 7.38 – 7.34 (m,
1H, C8-H), 7.32 – 7.27 (m, 1H, C9-H), 7.24 – 7.21 (m, 1H, C2-H),
1.20 (s, 9H, (CH3)3) ppm. 1H-NMR (THF, 400 MHz): δ = 8.64 (ddd,
3JH,H = 4.8 Hz, 4JH,H = 1.9 Hz, 5JH,H = 1.0 Hz, 1H, C1-H), 8.10 – 8.07
(m, 1H, C4-H), 7.83 (ddd, 3JH,H = 7.8 Hz, 4JH,H = 1.5 Hz, 5JH,H = 0.5
3
Hz, 1H, C7-H), 7.75 – 7.70 (m, 1H, C3-H), 7.44 (ddd, JH,H = 7.5
Hz, 4JH,H = 1.5 Hz, 5JH,H = 0.6 Hz, C10-H), 7.37 – 7.33 (m, 1H, C8-
H), 7.30 – 7.26 (m, 1H, C9-H), 7.23 (ddd, 3JH,H = 7.5 Hz, 4JH,H = 4.8
5
Hz, JH,H = 1.1 Hz, 1H, C2-H), 1.24 (s, 9H, (CH3)3) ppm. 13C-NMR
(CDCl3, 101 MHz): δ = 157.94, 149.37, 142.01, 135.30, 132.99,
10 | J. Name., 2012, 00, 1-3
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129.51, 128.12, 127.93, 124.63, 122.07, 101.76, 78.65, 30.71 solution was concentrated and the product was crystallized at
View Article Online
(CH3), 28.14 ppm. 13C-NMR (THF, 101 MHz): δ = 157.36, 149.26, -30 °C yielding the product as colorless crDyOstIa: 1ls0.(120039m/Cg9,D3T20%15)5.5H
141.96, 134.75, 132.71, 129.67, 127.61, 127.46, 124.02, 121.78, 1H NMR (400 MHz, CD2Cl2): δ = 8.46 (ddd, J = 4.8, 1.7, 0.9 Hz, 1H,
121.65, 101.00, 79.01, 30.00 (CH3), 27.89 ppm. MS (EI, 70 eV): C1-H), 7.65 (br s, 1H, C12-H), 7.62 (td, J = 7.7, 1.9 Hz, 1H), 7.57
m/z calcd for C17H17N: 235.14 Da; found: m/z (%) = 220.10 (100) – 7.53 (m, 1H), 7.47 – 7.39 (m, 3H), 7.29 (dt, J = 7.9, 0.9 Hz, 1H),
Da [M -CH3]+, 235.05 (7) Da [M]+.
7.18 (ddd, J = 7.6, 4.9, 1.1 Hz, 1H), 6.74 (s, 4H, C17-H), 2.26 (s,
1
Synthesis of (Z)-2-(2-(1-(9-borabicyclo[3.3.1]nonan-9-yl)-3,3- 6H, C20-H), 2.11 (s, 12H, C19-H), -0.18 (s, 9H, C14-H) ppm. H
dimethylbut-1-en-1-yl)phenyl)pyridine (tBuBBN): Inside an NMR (400 MHz, C6D6): δ = 8.47 (ddd, J = 4.8, 1.6, 1.2 Hz, 1H, C1-
inert gas glovebox a NMR tube equipped with a Young valve was H), 8.06 (s, 1H, C7-H), 7.63 – 7.57 (m, 2H), 7.16 (m, J = 4.5 Hz,
charged with a solution of 36.1 mg (153 µmol) of 2a and 244 mg 2H), 7.13 – 7.07 (m, 2H), 6.72 (s, 4H, C17-H), 6.67 (m, 1H), 2.29
(97.5 µmol) of 9H-BBN in 0.5 mL of anhydrous THF-d8. The tube (s, 12H, C19-H), 2.16 (s, 6H, C20-H), 0.09 (s, 9H, C14-H) ppm.
was sealed and heated for 3 days at 60 °C, when control via 13C NMR (126 MHz, CD2Cl2): δ = 156.50, 149.65, 140.96, 140.42,
NMR indicated full conversion of the starting material. 138.66, 136.30, 129.84, 129.38, 128.57, 128.52, 128.13, 124.32,
Subsequently, the solvent was removed and the residue was 121.93, 23.52 (o-CH3), 21.29 (p-CH3), 1.54 (Si(CH3)3)ppm.
extracted with dry n-hexane. The extract was evaporated to (C19), 21.45 (C20), 1.70 (C14) ppm. 11B NMR (128 MHz, CD2Cl2):
dryness, and the resulting crude product was washed through a δ = 78.39 (br s) ppm.
1
short column of alumina with toluene under inert conditions. NMR data for adduct 2b*HBMes2 H NMR (400 MHz, C6D6): δ =
Evaporation of the solvent gave 26.8 mg (75.0 µmol, 49%) of 8.62 – 8.54 (m, 1H), 7.46 (dd, J = 6.7, 2.3 Hz, 1H), 7.28 (m, 2H),
tBuBBN as a pale yellow solid.
6.95 (t, J = 7.6 Hz, 1H), 6.89 (br s, 4H, Mes), 6.85 – 6.77 (m, 2H),
1H-NMR (C6D6, 400 MHz): δ = 8.48 (ddd, 3JH,H = 6.0 Hz, 4JH,H = 1.7 6.76 – 6.54 (m, 1H, HB-), 6.37 (t, J = 6.4 Hz, 1H), 2.30 (br s, 6H,
Hz, 5JH,H = 0.7 Hz, 1H, C1-H), 7.47 – 7.45 (m, 1H), 7.16 – 7.12 (m, Mes-CH3), 2.15 (br s, 12H, Mes-CH3), 0.09 (s, 9H, Si-CH3) ppm.
2H), 7.01 – 6.96 (m, 1H), 6.94 – 6.92 (m, 1H), 6.73 – 6.69 (m, 1H), 11B NMR (128 MHz, C6D6): δ = 3.85 (br s) ppm.
6.20 – 6.17 (m, 1H), 6.11 (s, 1H, C13-H), 3.05 – 2.96 (m, 1H, BBN- Synthesis of (E)-2-(2-(2-(bis(perfluorophenyl)boraneyl)-1-
H), 2.46 – 2.36 (m, 1H, BBN-H), 2.27 – 2.16 (m, 3H, BBN-H), 2.05 (trimethylsilyl)vinyl)phenyl)pyridine (SiBPF): Inside the
– 1.92 (m, 3H, BBN-H), 1.78 – 1.73 (m, 3H, BBN-H), 1.72 – 1.68 glovebox 2b (87 mg, 0.3 mmol) and Pier's Borane (109 mg, 0.3
(m, 1H, BBN-Bridgehead), 1.32 – 1.29 (m, 1H, BBN-H), 1.12 (s, mmol) were dissolved in 1.6 mL dry benzene and the solution
9H, (CH3)3), 0.79 – 0.74 (m, 1H, BBN-Bridgehead) ppm. 1H-NMR was stirred at 85 °C for 27 h. The solvent was removed under
(THF, 400 MHz): δ = 8.82 (ddd, 3JH,H = 6.0 Hz, 4JH,H = 1.8 Hz, 5JH,H reduced pressure and the residue was purified by silica column
= 0.7 Hz, 1H, C1-H), 8.07 – 8.03 (m, 1H, C3-H), 7.98 – 7.95 (m, chromatography using petroleum spirit/ethyl acetate 5/1 as
3
1H, C4-H), 7.70 – 7.68 (m, 1H, C7-H), 7.47 (ddd, JH,H = 7.4 Hz, eluent furnishing the product as colorless solid (83 mg, 40%).
4JH,H = 5.9 Hz, 5JH,H = 1.0 Hz, 1H, C2-H), 7.35 – 7.30 (m, 2H, C9-H, Crystals suitable for XRD-analysis were obtained by
C10-H), 7.24 – 7.18 (m, 1H, C8-H), 5.74 (s, 1H, BC-C-H), 2.55 – crystallization from a hexane solution at -30 °C.
2.46 (m, 1H, BBN-H), 2.38 – 2.29 (m, 1H, BBN-H), 2.08 – 1.95 (m, 1H NMR (400 MHz, C6D6) δ 7.91 – 7.85 (m, 1H, C1-H), 7.69 (dd, J
1H, BBN-H), 1.91 – 1.61 (m, 3H, BBN-H), 1.50 – 1.42 (m, 3H, BBN- = 5.0, 2.8 Hz, 1H, C7-H), 7.23 (dd, J = 7.8, 1.0 Hz, 1H), 6.88 – 6.82
H), 1.36 – 1.30 (m, 3H, BBN-H), 1.27 – 1.22 (m, 1H, BBN- (m, 1H), 6.82 – 6.76 (m, 2H), 6.74 – 6.65 (m, 2H), 6.27 (ddd, J =
Bridgehead H), 0.94 (s, 9H, (CH3)3), 0.36 – 0.32 (m, 1H, BBN- 7.6, 6.2, 1.5 Hz, 1H), 0.22 (s, 9H, C14-H) ppm. 13C NMR (101 MHz,
Bridgehead H) ppm. 13C-NMR (C6D6, 101 MHz): δ = 154.61, CD2Cl2): δ = 163.74, 158.22, 146.63, 146.60, 144.73, 144.11,
146.39, 144.73, 139.98, 138.32, 130.88, 129.95, 128.86, 125.15, 141.39, 134.37, 131.91, 131.88, 130.24, 129.77, 127.68, 125.54,
123.05, 121.52 (2C), 42.05 (br C-B), 35.90 (BBN), 34.88, 32.49 123.91, 0.07 (C14) ppm. 11B NMR (128 MHz, C6D6): δ = -4.67 (br
(BBN), 32.43 (3C, (CH3)3), 32.17 (BBN), 30.78 (BBN), 24.94 (BBN), s) ppm. 19F NMR (376 MHz, C6D6): δ = -129.79 (br s, 1F), -130.08
24.68 (BBN), 23.90 (br, BBN-Bridgehead C), 22.03 (br s, BBN- (dd, J = 23.9, 8.6 Hz, 1F), -132.58 (br s, 1F), -137.73 – -137.96 (m,
Bridgehead C) ppm. 13C-NMR (THF, 101 MHz): δ = 155.11, 1F), -157.28 (t, J = 21.1 Hz, 1F), -162.00 (t, J = 20.6 Hz, 1F), -
146.52, 145.55 (C1), 140.16 (C3), 139.54 (C3-H) , 131.57, 129.92 163.88 (br s, 1F), -164.64 (br s, 1F), -166.75 (dddd, J = 23.6, 20.8,
(C9), 128.83 (C10), 125.70 (C7), 125.57 (C8), 124.25 (C4), 123.02 8.7, 2.5 Hz, 1F), -169.05 – -169.26 (m, 1F) ppm.
(C2), 40.94 (br s, C-B), 35.77 (BBN), 34.93, 33.82 (BBN), 32.34 Synthesis of 2-(2-(1-(dibromoboryl)-2-methylpropyl)phenyl)-
(3C, (CH3)3), 32.08 (BBN), 30.84 (BBN), 24.76 (BBN), 24.51 (BBN), pyridine (BBr2): Inside an inert gas glovebox a Schlenk tube was
22.21 (br s, BBN-Bridgehead C), 20.43 (br s, BBN-Bridgehead C) charged with a solution of 1.0 g (4.8 mmol) 2-(ortho-
ppm. 11B-NMR (C6D6, 128 MHz): δ = -0.04 (br s) ppm. 11B-NMR styryl)pyridine in 50 mL of dry toluene. The Schlenk tube was
(THF, 128 MHz): δ = -2.68 (br s) ppm. MS (EI, 70 eV): m/z calcd taken out of the glove box, and 2.0 mL (18.2 mmol) HBBr2•SMe2
for C25H32BN: 357.26 Da; found: m/z (%) = 357.20 (22) [M]+.
were slowly added, and the reaction mixture was stirred at
Synthesis of (Z)-2-(2-(2-(dimesitylboraneyl)-2-(trimethylsilyl)- ambient temperature overnight. The resulting orange solution
vinyl)phenyl)pyridine (SiBMes2): Inside the glovebox was evaporated to dryness, the residue was dried at 120 °C
compound 2b (31 mg, 0.12 mmol) and HBMes2 (31 mg, 0.12 under oil pump vacuum for 3 h. Subsequently, the residue was
mmol) were dissolved in 1 mL anhydrous benzene and the extracted with hot toluene, and the combined extracts were
solution was heated to 85 °C for 48 h. After removal of the concentrated until crystallization of the product set in.
solvent under reduced pressure, the residue was suspended in Harvesting of the crystals yielded 1.0 g (24.0 mmol, 56%) of BBr2
hexane and the resulting precipitate was removed. The hexane as a colorless solid.
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3
1H-NMR (C6D6, 400 MHz): δ = 9.50 (psd, JH,H = 6.2, 1H, C1-H), After removal of the supernatant and drying under reduced
View Article Online
DOI: 10.1039/C9DT01555H
7.23 – 7.17 (m, 2H, C7-H, C10-H), 7.16 – 7.10 (m, 1H, C9-H), 7.05 pressure the product was obtained as pale yellow solid (105 mg,
– 7.01 (m, 1H, C8-H), 6.95 (psd, 3JH,H = 8.2, 1H, C4-H), 6.77 – 6.73 81%).
(m, 1H, C3-H), 6.29 – 6.26 (m, 1H, C2-H), 3.03 – 2.94 (m, 1H, 1H NMR (400 MHz, C6D6): δ = 9.70 (dd, J = 6.2, 1.0 Hz, 1H), 7.11
3
CH(CH3)2), 2.94 – 2.93 (br m, 1H, B-CH), 0.92 (d, JH,H = 6.9 Hz, (td, J = 6.8, 2.5 Hz, 3H), 7.02 – 6.96 (m, 1H), 6.78 (d, J = 8.1 Hz,
3H, CH3), 0.14 (d, JH,H = 6.8 Hz , 3H, CH3) ppm. 13C-NMR (C6D6, 1H), 6.57 (ddd, J = 9.2, 7.9, 1.6 Hz, 1H), 6.06 (ddd, J = 7.5, 6.4,
3
101 MHz): δ = 152.90 (C5), 145.68 (C1), 142.68 (C11), 142.30 1.4 Hz, 1H), 3.10 – 2.99 (m, 1H, CH(CH3)2), 2.91 (d, J = 2.3 Hz, 1H,
(C3), 133.85 (C10), 131.59 (C9), 130.33 (C6), 127.24 (C7), 126.50 B-CH), 0.85 (d, J = 6.9 Hz, 3H; CH3), 0.08 (d, J = 6.9 Hz, 3H, CH3)
(C8), 123.23 (C2), 122.40 (C4), 43.97 (br, C-B), 32.17 (CH(CH3)2), ppm. 11B NMR (128 MHz, C6D6): δ = -12.71 (br s) ppm. 13C NMR
23.69 (CH3), 17.28 (CH3) ppm. 11B-NMR (C6D6, 128 MHz): δ = (101 MHz, C6D6): δ = 152.26, 149.03, 142.59, 142.24, 133.50,
4.89 (br s) ppm. MS (EI, 70 eV): m/z calcd for C15H16BBr2N: 131.72, 130.39, 127.31, 126.60, 123.35, 122.40, 34.45
378.97Da; found: m/z (%) = 377.30 (1) [M]+.
(CH(CH3)2), 23.74 (CH3), 17.05 (CH3).
Synthesis of 2-(2-(1-(difluoroboryl)-2-methylpropyl)phenyl)- Synthesis of 2-(2-(1-(dimethylboryl)-2-methylpropyl)phenyl)-
pyridine (BF2): In an inert gas glove box 25.3 mg (0.1 mmol) pyridine (BMe2): Under inert gas conditions, 60.0 mg (0.2
silver hexafluorophosphate and 30.0 mg (0.1 mmol) BCl2 were mmol) of BCl2 were dissolved in 10 mL of dry THF, and cooled to
each dissolved in ca. 1 mL of dry dichloromethane. The solutions -78°C. A solution of Methylmagnesiumchloride (3M in THF, 0.2
were added together, and the resulting suspension was stirred mL, 0.5 mmol) was slowly added to give a yellow solution. The
at ambient temperature for 17h. Subsequently, the solvent was mixture was stirred for 2 h at -78 °C, and was then warmed
removed, the residue was washed with dry hexane, and ambient temperature, and quenched by addition of iso-
extracted with dry toluene. Removal of the solvent yielded 23.9 propanol. The solvent was removed, and the residue was taken
mg (0.1 mmol, 92%) BF2 as colorless solid.
up in toluene and the extract was washed through a short
When the procedure was performed analogously using 4.5 column of alumina. Removal of the solvent yielded 47.6 mg (0.2
equivalents of silver(I) fluoride (AgF) and stirring for 50h, BF2 mmol, 95%) of BMe2 as colorless solid.
was obtained in 67% yield.
1H-NMR (C6D6, 400 MHz): δ = 8.25 (dd, 3JH,H = 5.9, 4JH,H = 1.7, 1H,
3
1H-NMR (C6D6, 400 MHz): δ = 9.48 (psd, JH,H = 6.3, 1H, C1-H), C1-H), 7.31 (dd, 3JH,H = 7.5, 4JH,H = 1.4, 1H, C10-H), 7.27 (psd, 3JH,H
7.21 – 7.07 (m, 3H, C7-H, C10-H, C9-H) 7.02 – 6.98 (m, 1H, C8- = 7.8, 1H, C7-H), 7.23 – 7.19 (m, 1H, C9-H), 7.08 – 7.04 (m, 2H,
3
H), 6.90 (psd, JH,H = 8.2, 1H, C4-H), 6.68 – 6.64 (m, 1H, C3-H), C4-H , C8-H), 6.80 – 6.75 (m, 1H, C3-H), 6.33 – 6.29 (m, 1H, C2-
6.22 – 6.18 (m, 1H, C2-H), 3.03 – 2.94 (m, 1H, CH(CH3)2), 2.93 – H), 2.36 (heptd, 3JH,H = 6.9, 3JH,H = 3.6, 1H, CH(CH3)2), 1.98 (d, 3JH,H
3
2.90 (br m, 1H, B-CH), 0.90 (d, JH,H = 6.9 Hz, 3H, CH3), 0.13 (d, = 3.6, 1H, B-CH), 1.08 (d, 3JH,H = 6.9 Hz, 3H, CH-CH3), 0.61 (s, 3H,
3
3JH,H = 6.8 Hz, 3H, CH3) ppm. 1H-NMR (CD2Cl2, 400 MHz): δ = 8.79 B-CH3), 0.30 (d, JH,H = 6.9 Hz , 3H, CH-CH3), 0.15 (s, 3H, B-CH3)
3
(psd, JH,H = 5.9 Hz, 1H, C1-H), 8.20 – 8.16 (m, 1H, C3-H) 8.07 ppm. 13C-NMR (C6D6, 101 MHz): δ = 154.43, 147.17, 143.63,
3
3
(psd, JH,H = 8.5 Hz, 1H, C4-H), 7.80 (psd, JH,H = 5.9 Hz, 1H, C7- 138.86, 134.35, 131.95, 130.56, 126.74, 125.33, 122.71, 121.94,
H), 7.62 – 7.58 (m, 1H, C2-H), 7.45 – 7.41 (m, 1H, C9-H), 7.36 – 43.72 (br, C-B), 30.90 (CH(CH3)2), 24.68 (CH3), 18.96 (CH3), 13.10
7.32 (m, 2H, C8-H, C10-H), 1.88 (br m, 1H, CH(CH3)2), 1.71 – 1.62 (br, B-CH3), 9.49 (br s, B-CH3) ppm. 11B-NMR (C6D6, 128 MHz): δ
(m, 1H, B-CH), 0.83 (d, 3JH,H = 6.7 Hz, 3H, CH3), 0.52 (d, 3JH,H = 6.7 = -2.40 (br s) ppm.
Hz , 3H, CH3) ppm. 13C-NMR (C6D6, 101 MHz): δ = 152.90 (C5), Synthesis of 2-(2-(1-(dicyanoboryl)-2-methylpropyl)phenyl)-
145.68 (C1), 142.65 (C11), 142.30 (C3), 133.85 (C10), 131.59 pyridine (B(CN)(NC)): Inside the glovebox compound BCl2 (22
(C9), 130.33 (C6), 127.24 (C7), 126.50 (C8), 123.23 (C2), 122.40 mg, 0.08 mmol) and TMS-CN (17 mg, 0.17 mmol) were dissolved
(C4), 43.97 (br, C-B), 32.17 (CH(CH3)2), 23.69 (CH3), 17.28 (CH3) in 1 mL anhydrous benzene and the solution was heated to 105
ppm. 13C-NMR (CD2Cl2, 101 MHz): δ = 154.11 (d, 3JC,F = 2.5 Hz, °C for 24 h. After removal of the solvent under reduced
C5), 145.76 (d, 3JC,F = 7.2 Hz, C11), 143.05 (C3), 141.59 (d, 3JC,F
=
pressure, the residue was dissolved in DCM and precipitated in
12.5 Hz, C1), 133.95 (d, 4JH,H = 2.2 Hz, C10), 131.53 (C9), 129.79 hexane twice. After removal of the remaining solvent the
(C6), 127.28 (C8), 126.51 (C7), 124.12 (C2), 123.20 (d, 4JC,F = 2.5 product was obtained as brown solid (14 mg, 68%). Single
Hz, C4), 39.15 (br s, C-B), 30.92 (d, 2JC,F = 6.8 Hz, CH(CH3)2), 22.40 crystals suitable for X-Ray diffraction were obtained by layering
4
(d, JH,H = 2.7 Hz, CH3), 21.58 (CH3) ppm. 11B-NMR (C6D6, 128 a solution of B(CN)(NC) in benzene with hexane.
MHz): δ = 6.66 (br s) ppm. 11B-NMR (CD2Cl2, 128 MHz): δ = 6.08 1H NMR (400 MHz, CD2Cl2): δ = 9.02 (ddd, J = 6.0, 1.5, 0.6 Hz, 1H,
(br s) ppm. 19F-NMR (C6D6, 376 MHz): δ = -148.03 (br s, 1F), - C1-H), 8.31 – 8.26 (m, 1H), 8.18 – 8.15 (m, 1H), 7.84 (ddd, J =
162.06 (br s, 1F) ppm. 19F-NMR (CD2Cl2, 376 MHz): δ = -147.41 7.9, 1.4, 0.5 Hz, 1H), 7.71 (ddd, J = 7.5, 6.0, 1.4 Hz, 1H), 7.56 (td,
(br s, 1F), -162.04 (br s, 1F) ppm.
J = 7.5, 1.4 Hz, 1H), 7.48 – 7.44 (m, 1H), 7.40 (ddt, J = 7.5, 1.3,
Synthesis of 2-(2-(1-(diiodoboryl)-2-methylpropyl)phenyl)- 0.5 Hz, 1H), 2.19 (d, J = 4.5 Hz, 1H, B-CH), 2.16 – 2.07 (m, 1H,
pyridine (BI2): Inside the glovebox BCl2 (80 mg, 0.3 mmol) and CH(CH3)2), 0.95 (d, J = 6.8 Hz, 3H, CH3), 0.27 (d, J = 6.8 Hz, 3H,
BI3 (206 mg, 0.5 mmol) were dissolved in 1 mL dry benzene and CH3) ppm.
the solution was stirred at room temperature for 48 h. The 1H NMR (400 MHz, C6D6): δ = 8.46 (dd, J = 6.0, 0.7 Hz, 1H, C1-H),
suspension was concentrated under reduced pressure before 7.09 – 7.03 (m, 1H), 7.00 – 6.92 (m, 3H), 6.69 (d, J = 8.1 Hz, 1H),
hexane was added. After removal of the supernatant and 6.57 – 6.47 (m, 1H), 6.01 (ddd, J = 7.4, 6.1, 1.3 Hz, 1H), 2.31 –
washing of the precipitate for four times the pale-yellow solid 2.21 (m, 1H, CH(CH3)2), 2.14 (d, J = 4.1 Hz, 1H, B-CH), 0.81 (d, J =
was dissolved in DCM and precipitated once again in hexane. 6.8 Hz, 3H, CH3), 0.15 (d, J = 6.9 Hz, 3H, CH3) ppm. 11B NMR (128
12 | J. Name., 2012, 00, 1-3
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MHz, C6D6): δ = -13.02 (br s) ppm. 13C NMR (101 MHz, CD2Cl2): δ
= = 154.13, 145.91, 144.64, 141.47, 133.41, 132.67, 129.93,
127.92, 127.80, 125.23, 124.54, 38.17 (br, B-CH), 31.74
(CH(CH3)2), 23.27 (CH3), 19.35 (CH3) ppm. IR (KBr): ṽ = 2124 cm-1
(B-NC), 2211 cm-1 (B-CN).
View Article Online
DOI: 10.1039/C9DT01555H
(a) X. Long, J. Yao, F. Cheng, C. Dou and Y. Xia, Mater. Org.
6
7
Chem. Front., 2019, 3, 70-77. (b) Long, Z. Ding, C. Dou and L.
Wang, Mater. Chem. Front., 2017, 1, 852-858. (c) R. Zhao, C.
Dou, Z. Xie, J. Liu and L. Wang, Angew. Chem. Int. Ed., 2016,
55, 5313-5317. (d) X. Long, Y. Gao, H. Tian, C. Dou, D. Yan, Y.
Geng, J. Liu and L. Wang, Chem. Commun., 2017, 53, 1649-
1652. (e) X. Long, Z. Ding, C. Dou, J. Zhang, J. Liu and L. Wang,
Adv. Mater., 2016, 28, 6504-6508. (f) Dou, J. Liu and L. Wang,
Sci. China Chem., 2017, 60, 450-459.
Acknowledgements
The authors thank the German Chemical Industry Fund (FCI)
(Liebig scholarship for F. P.) for financial support. We thank
Bernhard Mueller for support with X-ray data collection.
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DOI: 10.1039/C9DT01555H