Facile Arylation of Four-Coordinate Boron Halides by Borenium Cation Mediated Boro-desilylation and -destannylation

Article abstract of DOI:10.1021/acs.organomet.5b00857

The addition of AlCl₃ to four-coordinate boranes of the general formula (C-N-chelate)BCl₂ results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C-N-chelate)BCl]⁺. The lat

Full text of DOI:10.1021/acs.organomet.5b00857

Article
pubs.acs.org/Organometallics
Facile Arylation of Four-Coordinate Boron Halides by Borenium
Cation Mediated Boro-desilylation and -destannylation
Daniel L. Crossley, Jessica Cid, Liam D. Curless, Michael L. Turner, and Michael J. Ingleson*
School of Chemistry, University of Manchester, Manchester M13 9PL, United Kingdom
S
* Supporting Information
ABSTRACT: The addition of AlCl3 to four-coordinate
boranes of the general formula (C−N-chelate)BCl2 results in
halide abstraction and formation of three-coordinate borenium
cations of the general formula [(C−N-chelate)BCl]+. The
latter react with both arylstannanes and arylsilanes by boro-
destannylation and -desilylation, respectively, to form arylated
boranes. Catalytic quantities of AlCl3 were sufficient to effect
high-yielding arylation of (C−N-chelate)BCl2. Boro-destannylation is more rapid than boro-desilylation and leads to double
arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the
nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation
derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less
nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.
philes are highly sensitive to protic species (ROH), and the
synthesis of organozinc reagents often results in mixtures
containing ionic species (termed zincates) and coordinated
etherate solvent, which can complicate transmetalation.6
Alternative nucleophiles are required that are readily synthe-
sized, are well-defined, can be handled in air, and enable the
boron-containing products to be easily isolated, preferably
without column chromatography. Arylsilanes and arylstannanes
meet these criteria; however, while three-coordinate boranes
(e.g., ArBBr2) undergo transmetalation with arylsilanes and
arylstannanes, four-coordinate boranes do not due to the Lewis
acidity at boron being effectively quenched by the dative bond.4
We hypothesized that conversion of (C−N-chelate)BX2 into
borenium cations,7 [(C−N-chelate)BX]+, using a halophilic
Lewis acid (e.g., AlCl3) will enable transmetalation using
arylstannanes and arylsilanes. The process is potentially
catalytic in the halophile, as the byproduct from transmetalation
will react as a functional equivalent of [R3Si]+ or [R3Sn]+,
abstracting halide to generate further equivalents of borenium
cations for subsequent transmetalation (Scheme 2). Related,
albeit stoichiometric in halophile, approaches have been
INTRODUCTION
■
Four-coordinate boron compounds containing a chelating π-
conjugated C/N donor and two exocyclic aromatic moieties,
termed (C−N-chelate)BAr2 (e.g., 1-BAr2 right Scheme 1), have
Scheme 1. Electrophilic C−H Borylation Followed by
Previously Reported High-Yielding Arylation Methodologies
been extensively studied for application in optoelectronic
devices.1,2 Changing the exocyclic aryl groups in 1-BAr2
significantly modulates the key optoelectronic properties
including the frontier orbital energies and the photolumines-
cence quantum yield.2,3 Therefore, efficient and versatile routes
to libraries of these compounds are important to optimize the
materials properties and deliver improved device performance.
A particularly attractive approach is the arylation of (C−N-
chelate)BX2 (e.g., 1-BX2, X = Cl or Br) to form a wide range of
(C−N-chelate)BAr2 compounds, as the starting compounds are
readily accessed by electrophilic C−H borylation (Scheme
1).3,4
Scheme 2. Borenium Cation Mediated Transmetalation
Installation of aromatic moieties at three-coordinate boron
species is generally achieved by reaction with either arylithium
or aryl Grignard reagents.5 However, reaction of these reagents
with Lewis base adducts of boranes often gives the desired
product in poor yield.4 Instead functionalization of borane-
Lewis adducts such as (2-phenylpyridyl)BBr2 (1-BBr2, Scheme
1) requires organozinc or organoaluminum reagents to achieve
high-yielding transmetalation.3,4 Unfortunately these nucleo-
Received: October 12, 2015
© XXXX American Chemical Society
A
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reported for activating chloro-boron subphthalocyanine and
F2B-dipyrromethenes toward substitution of B−X with
chalcogen-based nucleophiles.8 In contrast, the use of borenium
cations in boro-desilylation has extremely limited precedence,9
while their use in boro-destannylation has not been reported to
date to the best of our knowledge. Herein is reported catalytic
(in AlCl3 activator) borenium cation mediated borylation as a
simple method to functionalize (C−N-chelate)BCl2 species
based on benzothiadiazole (BT) and pyridyl with aryl and
heteroaryl groups.
reaction with [AlCl4]−, to provide access to additional
equivalents of borenium cations.
The installation on boron of heteroaryl substituents using 5-
Bu3Sn-2-Me-thiophene, 3 (prepared by lithiation of 2-
methylthiophene and quenching with Bu3SnCl) was also
explored. Mixing 2-BCl2 with 2.2 equiv of 3 gave no reaction,
but addition of catalytic AlCl3 (ca. 5 mol %) resulted in rapid
arylation at 20 °C (complete within 10 min), and facile
isolation simply by filtration through silica allowed 2-B(MeT)2
to be isolated in 67% yield. It is noteworthy that arylation using
3 is considerably more rapid at 20 °C than reactions with
PhSnBu3, consistent with the enhanced nucleophilicity of the
thienylstannane. Furthermore, the use of 3 indicates that
transmetalation occurs via direct boro-destannylation, as the α
C-Me in 3 precludes an alternative mechanism involving C−H
borylation followed by proto-destannylation, as determined by
RESULTS AND DISCUSSION
■
Our initial attempts to access new 2-BAr2 compounds used an
isolated organozinc reagent synthesized from ZnBr2 and p-
tolylMgBr in THF, but this led to low yields of the desired
arylated product. The low conversion was attributed to the
“Zn(p-tolyl)2” formed under these conditions actually being the
zincate [Mg(THF)4(μ-Br)2(Zn(p-tol)2)2]n.10 Due to the
significant challenge presented in forming etherate-free arylzinc
reagents,10 ArylSiMe3 and ArylSnBu3 nucleophiles were
investigated for expanding the exocyclic boron substituents.
Mixing 2-BCl2 (readily formed from the unborylated
precursor 2 (F8-BT-F8) and BCl3)3 with 2 equiv of PhSnBu3
in CH2Cl2 at room temperature led to no reaction until
catalytic (ca. 5 mol %) AlCl3 was added to the reaction mixture.
Compound 2-BCl2 then slowly transformed into diphenylated
2-BPh2 at 20 °C (Scheme 3). Heating of the reaction resulted
Jakle and co-workers for the borylation of a stannylated
̈
ferrocene.12
Borenium cation mediated transmetalation with organo-
stannanes is effective for tetraarylation of [4-(BCl)2]2+. The
diborenium cation [4-(BCl)2]2+ (Scheme 4) is produced by
Scheme 4. Tetraarylation of [4-(BCl)2]2+ with PhSnBu3
Scheme 3. AlCl3-Catalyzed Transmetalation from
Arylstannanes to 2-BCl2, with Isolated Yields in Parentheses
double borylative fusion of the unborylated precursor 4 (BT-
F8-BT)3 as previously reported. With a slight excess of
PhSnBu3 (4.2 equiv) [4-(BCl)2]2+ forms the previously
characterized tetra-arylated product 4-(BPh2)2 as the major
boron-containing complex after 72 h at 20 °C or 24 h at 60 °C
(by multinuclear NMR spectroscopy) in 1,2-Cl2C6H4. Trans-
metalation with ZnPh2 to form 4-(BPh2)2 required prior
conversion of [4-(BCl)2]2+ to neutral 4-(BCl2)2 by addition of
NMe4Cl for acceptable conversion.4 In contrast, the boro-
destannylation methodology requires the borenium for trans-
metalation; therefore it proceeds directly from [4-(BCl)2]2+.
The thiophene analogue of 2-BCl2, 5-BCl2 (Scheme 5), can
be readily prepared from the unborylated precursor 5 as
previously reported.4 Again while arylation with etherate-free
diaryl zinc reagents proceeds with high fidelity, the addition of
[Mg(THF)4(μ-Br)2(Zn(p-tol)2)2]n to 2-BCl2 led to an
extremely low conversion to 5-B(p-tolyl)2 (isolated in only
13% yield). Analogous to the fluorene congener 2-BCl2, the
addition of stoichiometric AlCl3 to 5-BCl2 resulted in halide
abstraction and formation of the borenium cation [5-BCl]-
[AlCl4] (based on the significant downfield chemical shift of
aryl 1H NMR resonances and the observation of [AlCl4]− in the
27Al NMR spectrum), indicating the feasibility of borenium-
mediated transmetalation with organostannanes. The addition
of stannane 3 to 5-BCl2 again resulted in no reaction until
addition of catalytic AlCl3 (ca. 5 mol %), at which point double
arylation proceeded rapidly (complete within 10 min at 20 °C)
to form 5-B(MeT)2. This product could be isolated by column
chromatography in 51% yield (Scheme 5). It should be noted
that both 2-B(MeT)2 and 5-B(MeT)2 undergo slow proto-
deboronation of the exocyclic thienyl groups on standing in wet
solvents but are stable in the solid state under ambient
in a more rapid reaction and good conversion to 2-BPh2 (89%
isolated yield after 16 h at 60 °C in CH2Cl2 in a sealed tube).
The addition of AlCl3 results in chloride abstraction from 2-
BCl2 and borenium cation formation (indicated by downfield
1
shifts in the H NMR spectrum and formation of [AlCl4]− in
the 27Al NMR spectrum), consistent with previous studies on
related compounds.3 The borenium cation [2-BCl]+ is then
sufficiently electrophilic to boro-destannylate PhSnBu3. An
alternative mechanism where AlCl3 and PhSnBu3 react to form
Al-Ph species (which have been previously reported to
transmetalate to four-coordinate boron halides)4 is precluded
based on previous work where the combination of these
reagents (in the absence of 2-BCl2) in haloalkane solvents
(such as CH2Cl2) leads to solvent activation via C−Cl···AlCl3
interactions (Friedel−Crafts-type reactivity) and carbodestan-
nylation to form R3C-Ph.11 Friedel−Crafts products are not
observed in the reaction with 2-BCl2, which is attributed to
AlCl3 reacting rapidly to form the borenium cation, thus
disfavoring solvent activation. The ability to form 2-BPh2 in
high conversion using catalytic AlCl3 confirmed that the
electrophilic [Bu3Sn]+ (or a functional equivalent thereof)
byproduct can react with further 2-BCl2, directly or via initial
B
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Scheme 5. Transmetalation to 5-BCl2 with Isolated Yield in
Parentheses
Scheme 6. Transmetalation Outcomes Using Varying
Arylsilanes and 2-BCl2
disfavoring boro-desilylation of 7. Analogous trends have been
previously observed when comparing the Lewis acidity of
[PhBCl(amine)]+ and [Cl2B(amine)]+ borocations.15 Com-
pound 2-BCl(MeT) then can be further arylated using other
organometallic reagents; for example reaction with Zn(C6F5)2
gave the mixed arylated complex 2-B(MeT)(C6F5) (in 81%
isolated yield). This provides a simple route to mixed arylated
compounds, (C−N-chelate)BAr1(Ar2). It is notable that
current routes to unsymmetrically substituted borane deriva-
tives are challenging and require multiple steps and
purifications. This is due to the formation of Ar1Ar2BX (for
reaction with lithiated C−N-precursors), often leading to
mixtures generally necessitating purification by fractional
distillation.16
atmosphere for at least three months. An alternative synthesis
of 5-B(MeT)2 by electrophilic C−H borylation was explored
based on our previous success using PhBCl2 to form 5-
B(Ph(Cl)) directly from 5.4 However, (5-(2-methylthiophe-
ne))2BCl ((MeT)2BCl) does not react with 5 (Scheme 5,
right), presumably due to the reduced Lewis acidity at boron
(relative to BCl3 and PhBCl2). Furthermore, (MeT)BCl213 also
fails to borylate 5. Thus, C−H borylation using BCl3 followed
by transmetalation is necessary to access this compound.
The boro-destannylation reaction was extended to 2-Bu3Sn-
9,9-dioctylfluorene (6), synthesized by standard procedures.
The reaction of 5-BCl2 with 2.2 equiv of 6 and catalytic AlCl3
(ca. 5 mol %) proceeded at room temperature, but required 18
h for formation of 5-(F8)2 in high conversion. The longer
reaction time compared to transmetalation with 3 is attributable
to the variation in arene nucleophilicity. Attempts to selectively
form the monoarylated product by addition of 1 equiv of 6 to
5-BCl2 (with catalytic AlCl3) led to a mixture of 5-BCl2/5-
BCl(F8) and 5-B(F8)2. 5-(F8)2 also can be synthesized from 5
in a two-step, one-pot reaction without the use of a glovebox in
88% yield. Compound 5-BCl2 is prepared by reaction of 5 with
BCl3, followed by degassing (removing excess BCl3 and the
HCl byproduct from C−H borylation) and subsequent
addition of catalytic AlCl3 and 2.2 equiv of 6 (both weighed
and handled under ambient atmosphere). The product, 5-
(F8)2, is then simply isolated by filtering through silica.
The use of arylsilanes in place of arylstannanes is preferable
from a toxicity perspective. However, reacting PhSiMe3 and 2-
BCl2 with a range of AlCl3 loadings and reaction conditions (at
20 and 60 °C) consistently resulted in minimal transmetalation.
It is well documented that silicon−boron exchange only
proceeds with highly electrophilic boranes, in contrast with
tin−boron exchange.14 This suggests that the borenium cation
[2-BCl]+ is insufficiently electrophilic to effect boro-desilylation
of PhSiMe3. A more nucleophilic silane, 2-Me-5-Me3Si-
thiophene, 7, was therefore utilized. Compound 2-BCl2 was
combined with an excess (2.2 equiv) of 7, resulting in no
reaction. Addition of AlCl3 (ca. 5 mol %) to the reaction
mixture initiated transmetalation, leading to only one trans-
metalation per boron, producing 2-BCl(MeT) (Scheme 6),
even after long reaction times. As the borenium cation [2-
B(MeT)]+ formed after the first transmetalation and
subsequent halide abstraction contains a thienyl π donor, its
Lewis acidity is presumably reduced relative to [2-BCl]+,
̧
(C−N-chelate)BAr2 compounds based on 2-arylpyridyls and
derivatives have been more extensively studied than the
benzothiadiazole systems for a range of optoelectronic
applications.2,6a,17 Therefore, the borenium cation mediated
boro-destannylation/boro-desilylation reactions of these spe-
cies were explored. 2-Phenylpyridine, 1, was readily borylated
by a modification of a literature method4 using BCl3, 2,4,6-tri-
tBu-pyridine (TBP), and AlCl3 to form 1-BCl2. Compound 1-
BCl2 was stable to ambient conditions and could be readily
isolated in air simply by sequential washing with H2O/MeOH
and pentane. In contrast BT derivatives (e.g., 2-BCl2) are
sensitive to water and column chromatography. The enhanced
stability of 1-BCl2 is attributed to a stronger N→B dative bond
in the pyridyl congener. The addition of an equivalent of AlCl3
to 1-BCl2 led to formation of the borenium salt [1-
BCl][AlCl4], as indicated by a signal at +39.0 ppm in the 11B
NMR spectrum and further confirmed by X-ray diffraction
studies (crystallized by cooling a saturated CH2Cl2 solution to 4
°C, Figure 1).
The solid-state structure of [1-BCl][AlCl4] reveals a
planarized tricyclic structure and a trigonal planar environment
at boron (∑ = 359.8°). Although two [AlCl4]− anions are
proximal, the four Al−Cl (two participating in Al−Cl−B
bridges and two not) distances are all identical (within 3σ),
suggesting that these close contacts are principally due to
electrostatic forces and packing effects. The ability of the
borenium cation [1-BCl]+ to mediate boro-destannylation was
investigated. Addition of 2.2 equiv of PhSnBu3 to 1-BCl2
resulted in no reaction until addition of ca. 5 mol % of AlCl3,
which resulted in rapid boro-destannylation at 20 °C to form 1-
BPh2. This compound has been previously synthesized by
Murakami and co-workers via 1-BBr2 and AlPh3.4 The
synthesis of 1-BPh2 in one pot in two steps from 2-
C
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Scheme 7. Boro-destannylation and Boro-desilylation
Reactions Using 1-BCl2 Activated by AlCl3 (Isolated Yields
a
in Parentheses)
Figure 1. Structure of [1-BCl][AlCl4] showing the two closest
[AlCl4]− anions in the extended structure. Thermal ellipsoids at the
50% probability level. Selected bond lengths and angles: B−N =
1.525(9); B−C1 = 1.522(9); B−Cl1 = 1.708(7); B−Cl2 = 3.223 Å;
B−Cl3(A) = 3.737 Å; N−B−C1 = 105.0(5)°.
phenylpyridine via electrophilic C−H borylation and sub-
sequent AlCl3-catalyzed boro-destannylation can be performed
without use of a glovebox in high conversion (72% isolated
yield).
The rapid room-temperature double boro-destannylation
observed on combination of 1-BCl2, catalytic AlCl3, and
PhSnBu3 is in contrast to the BT congener 2-BCl2 (which
requires heating to 60 °C). This suggests an enhanced
electrophilicity of the boron center in [1-BY]+ (Y = Cl and
Ph) relative to that in [2-BY]+. This was confirmed by the
observation that addition of 2.2 equiv of PhSiMe3 to 1-BCl2 in
the presence of catalytic (ca. 5 mol %) AlCl3 rapidly led to
monoarylation (<10 min) and complete double arylation of
boron within 10 h at 20 °C to form 1-BPh2. Thus, with 1-BCl2
double transmetalation is possible using the less toxic arylsilane
reagent. This methodology can also be performed without the
aid of a glovebox with no significant loss in yield, and the
doubly arylated products can be isolated simply by filtration
through a short plug of silica followed by drying in vacuo. The
electronically deactivated silane (meta-Br-C6H4)SiMe3 was also
a viable reagent for transmetalation to boron; however, at 20 °C
this led only to a single arylation of 1-BCl2 (using ca. 5 mol %
AlCl3), with no further arylation proceeding at 20 °C (Scheme
7). Double arylation of 1-BCl2 can be realized with (meta-Br-
C6H4)SiMe3 by heating 1-BCl2/catalytic AlCl3 in 1,2-Cl2C6H4.
The change in solvent is essential, as in this case heating a
mixture of AlCl3, CH2Cl2, and an arylsilane for prolonged
periods of time led to Friedel−Crafts alkyation reactions.11
Analogous conditions enabled the synthesis of the spiro
complex 1-B(biphenyl) (Scheme 7, bottom) in good yield
(82%) from the commercially available 9,9-dimethyl-9H-9-
silafluorene. Spiro complexes such as 1-B(biphenyl) have been
extensively explored as electron transport materials in electro-
luminescent devices.18 It is notable that attempts to make the
analogous spiro compound from 2-BCl2 using catalytic AlCl3
failed with no reaction observed at 20 or 60 °C, again indicating
the lower electrophilicity of the [2-BCl]+ borenium cation
relative to [1-BCl]+.
a
Inset bottom right: structure of 1-B(biphenyl), thermal ellipsoids at
50% probability.
Figure 2. Hydride ion affinity of [1-BCl]+ and [8-BCl]+ relative to
BEt3 (at the M06-2X/6311G(d,p) (PCM DCM) level).
structure of [1-BCl]+ was in excellent agreement with the solid-
state structure of [1-BCl][AlCl4]. Using a previously reported
approach the hydride ion affinity (HIA, eq 1)15 relative to BEt3
was assessed and found to be 6.7 kcal mol−1 greater for [1-
BCl]+ compared to [8-BCl]+. This indicates a greater Lewis
acidity for the pyridyl congener toward soft nucleophiles (such
as π systems) consistent with the relative reactivity observed.
The nitrogen sites in BT are weakly basic relative to that in
pyridyl; however examination of the calculated structure of [8-
BCl]+ indicates a greater N→B π donation than in [1-BCl]+
(B−N in [8-BCl]+ = 1.474 Å; B−N in [1-BCl]+ = 1.514 Å).
Furthermore, the N−S distances in [8-BCl]+ are different with
a longer S−N bond involving the nitrogen bound to boron
(N1−S = 1.69 vs N2−S = 1.59 Å, Scheme 8). Natural bond
orbital analysis also indicates a significant positive charge on
sulfur (+1.073e) and a greater negative charge on N1 in [8-
BCl]+ (−0.757e for N1, −0.511e for N2) relative to that of the
nitrogen in [1-BCl]+ (−0.588e). This indicates a significant
contribution from a resonance form where sulfur is formally in
the +4 oxidation state for [8-BCl]+ (Scheme 8, right).
Presumably this effect combined with the preference for the
ΔHHIA
[X2BL]+ + [HBEt3]− ⎯⎯⎯⎯⎯→ X2HBL + BEt3
(1)
The greater reactivity of [1-BCl]+ relative to [2-BCl]+
suggested an enhanced electrophilicity at boron; to assess if
this was due to the change in the aromatic moiety (i.e., thienyl/
fluorenyl vs phenyl), calculations comparing [1-BCl]+ with the
model BT analogue [8-BCl]+ were performed at the M06-2X/
6311G(d,p) (PCM DCM) level (Figure 2). The optimized
D
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Scheme 8. Resonance Structures of [8-BCl]+
aryl silanes represents a simple route to (C−N-chelate)B(aryl)2
species, which are useful for optoelectronic applications. The
methodology proceeds with a range of arylstannanes and
arylsilanes without the requirement for a glovebox or isolation
of the (C−N-chelate)BCl2. Single and double arylation of each
boron center can be selected by appropriate choice of reagents,
thus enabling facile access to unsymmetrically substituted four-
coordinate boron compounds that are challenging to access via
other methodologies.
five-membered boracycle to pyramidalize relative to the six-
membered boracycle (N−B−C angles of 104.8° in [1-BCl]+
and 115.7° in [8-BCl]+) leads to the observed Lewis acidity and
reactivity trend.
The significant electrophilicity of [1-BCl]+ suggested it may
be sufficiently reactive to directly borylate C−H bonds of
activated arenes.7c,d This would remove the requirement for
preinstallation of R3E− groups on the desired aryl moiety. The
addition of 1.1 equiv of 2-methylthiophene and TBP (to
sequester the proton) to [1-BCl][AlCl4] (generated in situ)
resulted in full consumption of [1-BCl][AlCl4], to form two
new resonances in the 11B NMR spectrum. However,
multinuclear NMR spectroscopy showed that ca. 0.6 equiv of
2-methylthiophene and 0.5 equiv of TBP and 1-BCl2 were
present in the reaction mixture. The observations are consistent
with the second new boron resonance being [1-B(MeT)]+.
Minor variations in starting stoichiometry (between 1-BCl2 and
AlCl3) led to the new 11B NMR resonance varying between 18
and 40 ppm. This is attributed to a fast exchange between
differing quantities of [1-B(MeT)][AlCl4] and 1-B(MeT)Cl
(Scheme 9). [1-B(MeT)]+ does not react with further 2-
EXPERIMENTAL SECTION
■
Unless otherwise stated, all manipulations were carried out using
standard Schlenk techniques under argon or in an MBraun UniLab
glovebox, under an atmosphere of argon (<0.1 ppm of O2/H2O).
Unless otherwise indicated, solvents were distilled from appropriate
drying agents: tetrahydrofuran (potassium); toluene (potassium); n-
hexane (NaK); and dichloromethane (CaH2). Tetrahydrofuran and
dichloromethane were stored over activated 3 Å molecular sieves,
while toluene and n-hexane were stored over potassium mirrors. 2, 2-
BCl2, 4, 5, 2-methyl-5-tributylstannylthiophene, trimethyl(5-methyl-
thiophen-2-yl)silane, tributyl(9,9-dioctyl-9H-fluoren-2-yl)stannane,
and [Mg(THF)4(μ-Br)2(Zn(p-tol)2)2]n were prepared according to
previously published procedures.3,10 All other compounds were
purchased from commercial sources and used as received. NMR
spectra were recorded on Bruker AvanceIII-400 or Bruker Ascend-400
spectrometers. Chemical shifts are reported as dimensionless δ values
and are referenced relative to residual protio-impurities in the NMR
solvents for 1H and 13C{1H}, respectively, while 11B and 19F{1H} shifts
are referenced relative to external BF3-etherate and hexafluorobenzene,
respectively. Coupling constants J are given in hertz (Hz) as positive
values regardless of their real individual signs. The multiplicities of the
signals are indicated as “s”, “d”, “t”, “pent”, “sept”, or “m” for singlet,
doublet, triplet, pentet, septet, or multiplet, respectively. Carbon atoms
directly bonded to boron are not always observed in the 13C{1H}
NMR spectra due to quadrupolar relaxation leading to considerable
signal broadening. In a number of compounds individual carbon
resonances are not observed for all inequivalent protons (particularly
in the octyl chains) due to resonance coincidence. High-resolution
mass spectra (HRMS) were recorded on a Waters QTOF mass
spectrometer. Microanalysis was performed by Stephen Boyer at the
London Metropolitan University microanalytical service. For the
arylated compounds accurate combustion data were not obtainable
with consistently low %C content observed. This is attributed to boron
carbide formation and persisted even when V2O5 was used as an
oxidant. For these compounds NMR spectra are included in the SI to
support compound purity,
Scheme 9. C−H Borylation of 2-Me-thiophene with [1-BCl]+
Synthesis of 2-B(MeT)2. BCl3, 1 M in DCM (0.3 mL, 0.3 mmol),
was added to a solution of 2 (95 mg, 0.10 mmol) in DCM (3 mL), and
the solution was stirred overnight under the dynamic flow of nitrogen.
The solvent was then removed under reduced pressure. The resulting
residue was dissolved in DCM (3 mL), and AlCl3 (1 mg) was added to
the solution. 2-Methyl-5-tributylstannylthiophene (90 mg, 0.22 mmol)
was added to the reaction mixture, which was then stirred overnight.
The solvent was then removed under reduced pressure, and the
purification was performed under ambient atmosphere using non-
purified solvents thereon. The residue was dissolved in hexane and was
passed through (using hexane initially and then 10% DCM/90%
hexane as eluent) a short plug of base-treated silica gel (pretreated
with 5% NEt3/hexane), and only the purple-colored solution was
retained. The solvent was removed to afford a purple residue. Yield: 78
mg, 67%.
methylthiophene (presumably due to insufficient Lewis acidity)
and is less chlorophilic than [1-BCl]+, resulting in the
consumption of 0.5 equiv of the latter by rapid halide transfer
from the expected initial product 1-B(MeT)Cl.19 The addition
of a second equivalent of AlCl3 to this reaction mixture led to
consumption of all 1-BCl2 and full conversion to [1-B(MeT)]+
(45 ppm in 11B NMR spectrum, Scheme 9). With only a single
C−H borylation of 2-methyl thiophene possible using the 2-
phenylpyridyl-chelated borenium cation double arylation at
boron requires addition of an organometallic nucleophile, e.g.,
an arylsilane or arylstannane reagent. Alternatively, conversion
of [1-B(MeT)]+ to form 1-BCl(MeT) is achieved by addition
of a halide source to form 1-BCl(MeT).
1H NMR (400 MHz, CD2Cl2): δ = 8.50 (d, J = 7.7 Hz, 1 H), 8.15
(s, 1 H), 8.09 (d, J = 7.6 Hz, 1 H), 8.04−7.96 (m, 3 H), 7.93−7.87 (m,
1 H), 7.82 (d, J = 2.3 Hz, 1 H), 7.76−7.69 (m, 1 H), 7.48−7.27 (m, 6
H), 6.82 (d, J = 3.3 Hz, 2 H), 6.69 (d, J = 3.1 Hz, 2 H), 2.44 (s, 6 H),
2.20−2.01 (m, 8 H), 1.27−1.04 (m, 40 H), 0.89−0.67 (m, 20 H) ppm.
13C NMR (101 MHz, CD2Cl2): δ = 155.1 (br), 154.4, 152.0, 151.9,
150.8 (br), 150.1, 148.0, 142.5, 142.5, 142.2, 141.4, 140.9, 134.7, 133.2,
CONCLUSIONS
■
The catalytic (in AlCl3) borenium cation mediated arylation of
four-coordinate boron compounds using aryl stannanes and
E
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131.3, 131.2, 130.0, 128.7, 128.4, 128.2, 127.9, 127.5, 127.3, 126.3,
126.1, 125.2, 124.3, 123.6, 123.5, 120.9, 120.6, 120.5, 117.0, 55.8, 55.5,
41.1, 40.7, 32.4, 32.4, 30.6, 30.6, 29.8, 29.8, 29.8, 24.5, 24.4, 23.2, 15.6,
14.4. ppm. 11B NMR (128 MHz, CD2Cl2): δ = −2 (v br). HRMS
then stirred overnight. The solvent was then removed under reduced
pressure, and the purification was performed under ambient
atmosphere using nonpurified solvents thereon. The residue was
dissolved in hexane and was passed through a short plug of base-
treated silica gel (5% NEt3/hexane), and only the dark blue colored
solution was retained. The solvent was removed to afford a purple
residue. Yield: 66 mg, 88%.
+
(APCI): calcd for C74H94BN2S3 (M + H) 1117.6667, found
1117.6664.
Synthesis of 2-BPh2. BCl3, 1 M in DCM (0.1 mL, 0.1 mmol), was
added to a solution of 2 (50 mg, 0.055 mmol) in DCM (3 mL), and
the solution was stirred overnight under the dynamic flow of nitrogen.
The solvent was then removed under reduced pressure. The resulting
residue was dissolved in DCM (3 mL), and AlCl3 (1 mg) was added to
the solution. Tributylphenylstannane (40 mg, 0.121 mmol) was added
to the solution, and the reaction mixture was stirred and heated
overnight at 60 °C. The solvent was then removed under reduced
pressure, and the purification was performed under ambient
atmosphere using nonpurified solvents thereon. The residue was
dissolved in hexane and was passed through (using hexane initially and
then 10% DCM/90% hexane as eluent) a short plug of base-treated
silica gel (pretreated with 5% NEt3/hexane), and only the purple-
colored solution was retained. The solvent was removed to afford a
purple residue. Yield: 53 mg, 89%. The spectra agree with that
previously reported.3
Synthesis of 4-(BPh2)2. BCl3, 1 M solution in DCM (0.30 mL, 0.3
mmol), was added to a bright yellow solution of 4 (50 mg, 0.076
mmol) and 2,4,6-tritbutylpyridine (38 mg, 0.154 mmol) in DCM (3
mL). The solution rapidly changed color to a dark red. AlCl3 (20 mg,
0.15 mmol) was then added to the reaction mixture. After rotating for
16 h, an additional portion of AlCl3 (20 mg, 0.15 mmol) was added to
the reaction mixture. The solution was rotated for a further 16 h,
whereupon the solution turned dark green. The DCM was removed
under reduced pressure, and the reaction mixture was dissolved in o-
DCB (4 mL). Tributylphenylstannane (0.15 mL, 0.456 mmol) was
added to the reaction mixture, which was then stirred at 20 °C for 48 h
and heated at 40 °C for 16 h. NMe4Cl (50 mg, 0.456 mmol) was
added to the reaction mixture, and after 1 h the solvent was removed
under reduced pressure. The purification was performed under
ambient atmosphere using nonpurified solvents thereon. The residue
was purified via column chromatography on base-treated silica gel (5%
NEt3/hexane) [eluent chloroform/hexane (2:8)] to afford a purple
residue. Yield: 24 mg, 32%. The spectra agree with that previously
reported.3
1H NMR (400 MHz, CDCl3): δ = 7.88 (d, J = 3.7 Hz, 1 H), 7.84 (d,
J = 7.6 Hz, 1 H), 7.66 (dd, J = 1.3, 6.2 Hz, 2 H), 7.61−7.50 (m, 5 H),
7.37−7.22 (m, 7 H), 7.04 (dd, J = 0.9, 7.6 Hz, 2 H), 6.86 (d, J = 3.7
Hz, 1 H), 2.89 (t, J = 7.6 Hz, 2 H), 2.81 (t, J = 7.7 Hz, 2 H), 2.07−1.85
(m, 8 H), 1.81−1.64 (m, 4 H), 1.42−0.98 (m, 60 H), 0.97−0.83 (m,
18 H), 0.68 (dt, J = 6.0, 13.8 Hz, 8 H). 13C NMR (101 MHz, CDCl3):
δ = 162.0 (br), 153.3 (br), 151.9, 150.9, 149.9, 149.0, 147.9, 147.6,
141.7, 139.3, 135.5, 132.0, 130.9, 130.2, 128.2, 127.4, 126.5, 126.4,
125.5, 125.2, 123.9, 122.8, 121.6, 119.3, 118.9, 54.9, 40.5, 31.9, 31.9,
31.8, 31.6, 30.7, 30.3, 30.3, 29.6, 29.5, 29.5, 29.4, 29.4, 29.3, 29.3, 29.2,
24.1, 24.1, 22.7, 22.7, 14.2, 14.2. 11B NMR (128 MHz, CDCl3): no 11
B
NMR peak was observed at 20 °C. HRMS (APCI): calcd for
+
C88H122BN2S3 (M + H) 1313.8858, found 1313.8862.
Synthesis of 2-B(MeT)(C6F5). AlCl3 (1 mg) was added to a
solution of 2-BCl2 (50 mg, 0.5 mmol) and trimethyl(5-methylthio-
phen-2-yl)silane (20 μL, 0.1 mmol) in DCM (0.7 mL). After inverting
for 14 h at room temperature NMR investigation showed only one
arylation had occurred. The reaction mixture was then evaporated to
dryness, and the residue was dissolved in DCM (0.7 mL). Zn(C6F5)2
(24 mg, 0.6 mmol) was added to the reaction mixture. After stirring
for 3 h the reaction mixture was filtered through a plug of base-treated
silica gel (5% NEt3/hexane). The reaction mixture was then purified
via column chromatography on base-treated silica gel (5% NEt3/
hexane) [eluent DCM/hexane (1:9)] to afford a dark purple residue.
Yield: 48 mg, 81%.
1H NMR (400 MHz, CD2Cl2): δ = 8.51 (d, J = 7.7 Hz, 1 H), 8.19−
8.03 (m, 3 H), 8.01 (s, 1 H), 7.98 (d, J = 7.9 Hz, 1 H), 7.90 (d, J = 7.8
Hz, 1 H), 7.85−7.77 (m, 1 H), 7.74 (dd, J = 3.4, 5.1 Hz, 1 H), 7.47−
7.27 (m, 6 H), 6.78 (d, J = 3.2 Hz, 1 H), 6.70−6.64 (m, 1 H), 2.44 (s,
3 H), 2.20−1.94 (m, 8 H), 1.26−1.01 (m, 42 H), 0.86−0.58 (m, 20
H). 13C NMR (101 MHz, CD2Cl2): δ = 154.4, 152.0, 151.9, 150.7,
148.1, 142.9, 142.7, 142.5, 141.2, 140.9, 134.6, 133.4, 131.3, 130.9,
130.1, 128.5, 128.5, 128.2, 128.1, 127.5, 127.4, 126.6, 125.8, 125.5,
124.4, 123.6, 123.6, 120.8, 120.6, 120.5, 117.2, 55.9, 55.6, 41.1, 40.9,
40.8, 32.4, 32.4, 32.3, 30.6, 30.6, 30.6, 29.8, 29.8, 24.6, 24.5, 24.5, 23.2,
23.1, 15.6, 14.4, 14.4. 19F NMR (376 MHz, CD2Cl2): δ = −131.7 (dd,
J = 9.0, 24.8 Hz, 2 F), −158.6 (t, J = 20.7 Hz, 1 F), −164.0 (m, 2 F).
11B NMR (128 MHz, CD2Cl2): δ ≈ −3 ppm. HRMS (APCI): calcd
Synthesis of 5-B(MeT)2. BCl3, 1 M in DCM (0.2 mL, 0.20 mmol),
was added to a solution of 5 (95 mg, 0.18 mmol) in DCM (3 mL), and
the solution was stirred overnight under the dynamic flow of nitrogen.
The solvent was then removed under reduced pressure. The resulting
residue was dissolved in DCM (3 mL), and AlCl3 (1 mg) was added to
the solution. 2-Methyl-5-tributylstannylthiophene (154 mg, 0.40
mmol) was added to the reaction mixture, which was then stirred
overnight. The solvent was then removed under reduced pressure, and
the purification was performed under ambient atmosphere using
nonpurified solvents thereon. The residue was purified via column
chromatography on base-treated silica gel (5% NEt3/hexane) [eluent
DCM/hexane (1:9)] to afford a dark blue residue. Yield: 67 mg, 51%.
1H NMR (400 MHz, CD2Cl2): δ = 7.71 (d, J = 3.7 Hz, 1 H), 7.58
(d, J = 7.6 Hz, 1 H), 7.35 (d, J = 7.6 Hz, 1 H), 6.94−6.83 (m, 1 H),
6.82−6.77 (m, 1 H), 6.76 (d, J = 3.2 Hz, 2 H), 6.71−6.64 (m, 2 H),
2.87 (q, J = 7.1 Hz, 4 H), 2.45 (s, 6 H), 1.83−1.68 (m, 4 H), 1.51−
1.22 (m, 20 H), 1.00−0.82 (m, 6 H). 13C NMR (101 MHz, CD2Cl2):
δ = 158.7 (br), 153.8 (br), 151.9, 149.4, 148.8, 147.0, 142.3, 135.8,
131.6, 131.3, 130.9, 128.1, 128.1, 126.4, 126.1, 124.5, 124.4, 122.8,
32.5, 32.5, 32.2, 31.0, 30.8, 29.9, 29.9, 29.9, 29.8, 29.8, 23.3, 15.5, 14.5.
11B NMR (128 MHz, CD2Cl2): δ = −2 ppm (v br). HRMS (APCI):
+
for C75H89BN2S2 (M + H) 1187.6475, found 1187.6471.
Synthesis of 1-BCl2. BCl3, 1 M in DCM (4.0 mL, 4 mmol), 2,4,6-
tri-tert-butylpyridine (0.8 g, 3.2 mmol), and 2-phenylpyridine (0.5 g,
3.2 mmol) were dissolved in DCM (40 mL). AlCl3 (0.854 mg, 6.4
mmol) was added to the reaction mixture, whereupon a color change
from colorless to yellow was observed. After stirring for 4 h the
reaction mixture was degassed under vacuum and NMe4Cl (0.351 g,
3.2 mmol) was added, whereupon the reaction mixture changed color
from yellow to colorless. The reaction mixture was evaporated to
dryness and washed with water (3 × 100 mL) and hexane (100 mL).
The resulting white powder was dried under reduced pressure. Yield:
0.584 g, 77%.
1H NMR (400 MHz, CDCl3): δ = 8.82 (d, J = 5.8 Hz, 1 H), 8.18
(dt, J = 1.5, 7.8 Hz, 1 H), 7.98−7.91 (m, 1 H), 7.86 (d, J = 7.3 Hz, 1
H), 7.77 (d, J = 7.8 Hz, 1 H), 7.62−7.53 (m, 2 H), 7.47−7.39 (m, 1
H). Carbon NMR data are not reported due to the extremely low
solubility of the product in a range of common organic solvents. 11B
NMR (128 MHz, CDCl3): δ = 7 (v br) ppm. Anal. Calcd for
C11H8BNCl2: C, 56.01; H, 3.43; N, 5.94. Found: C, 56.09; H, 3.32; N,
5.88.
Synthesis of [1-BCl][AlCl4]. AlCl3 (57 mg, 0.42 mmol) was added
to a suspension of 1-BCl2 (100 mg, 0.42 mmol) in DCM (10 mL).
This was stirred overnight, whereupon all the 1-BCl2 had dissolved
and the reaction mixture had changed color from colorless to yellow.
The reaction mixture was then concentrated to form a saturated
+
calcd for C35H44BN2S4 (M − C5H5S) 631.2486, found 631.2477.
Synthesis of 5-B(F8)2. BCl3, 1 M in DCM (0.1 mL, 0.1 mmol),
was added to a solution of 5 (30 mg, 0.057 mmol) in DCM (3 mL),
and the solution was stirred overnight under the dynamic flow of
nitrogen. The solvent was then removed under reduced pressure. The
resulting residue was dissolved in DCM (3 mL), and AlCl3 (1 mg) was
added to the solution. Tributyl(9,9-dioctyl-9H-fluoren-2-yl)stannane
(85 mg, 0.125 mmol) was added to the reaction mixture, which was
F
DOI: 10.1021/acs.organomet.5b00857
Organometallics XXXX, XXX, XXX−XXX

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Article
solution (∼4 mL), which was then filtered via cannula, and the
solution transferred to a 10 mL Young’s ampule. The sample was then
held at 2 °C for 16 h, whereupon amber-colored crystals formed. The
crystals were isolated via filtration. Yield: 93 mg, 60%.
(v br) ppm. HRMS (APCI): calcd for C23H17BN+ (M + H) 318.1447,
found 318.1449.
ASSOCIATED CONTENT
* Supporting Information
■
1H NMR (400 MHz,CD2Cl2): δ = 8.76 (d, J = 5.6 Hz, 1 H), 8.62−
8.50 (m, 1 H), 8.09 (d, J = 8.1 Hz, 1 H), 7.94 (d, J = 7.1 Hz, 1 H),
7.90−7.79 (m, 2 H), 7.75 (t, J = 8.1 Hz, 1 H), 7.69−7.59 (m, 1 H).
13C NMR (101 MHz, CDCl3): δ = 158.8, 153.0, 144.6, 140.4, 137.2,
135.8, 135.0, 127.0, 124.8, 121.3. 27Al (104 MHz, CD2Cl2): δ = 104
(br) ppm. 11B NMR (128 MHz, CD2Cl2): δ = 39 (v br). Anal. Calcd
for C11H8BNAlCl5: C, 35.78; H, 2.18; N, 3.79. Found: C, 35.84; H,
2.32; N, 3.82
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00857.
Full experimental procedures, compound characteriza-
tion data, copies of NMR spectra, and coordinates for all
calculations (PDF)
Synthesis of 1-BPh2 via Tributylphenylstannane. AlCl3 (2
mg) was added to a suspension of 1-BCl2 (31 mg, 0.13 mmol) and
tributylphenylstannane (106 mg, 0.286 mmol) in DCM (4 mL). 1-
BCl2 dissolved almost instantly upon the addition of AlCl3. The
reaction mixture was stirred overnight, and the solution was passed
through a short plug of silica gel. The purification was performed
under ambient atmosphere using nonpurified solvents thereon. The
reaction mixture was evaporated to dryness under reduced pressure,
and the resulting white residue was washed with hexane to yield the
desired product as a white crystalline solid. Yield: 30 mg, 72%.
Synthesis of 1-BPh2 via Trimethylphenylsilane. In a J.Young’s
NMR tube AlCl3 (1 mg) was added to a suspension of 1-BCl2 (23 mg,
0.10 mmol) and trimethylphenylsilane (33 mg, 0.21 mmol) in DCM
(0.7 mL). The reaction mixture was inverted for 10 h, whereupon
NMR investigation showed the reaction had gone to completion. The
purification was performed under ambient atmosphere using non-
purified solvents thereon The reaction mixture was then evaporated to
dryness and dissolved in DCM (5 mL), and the solution was passed
through a short plug of silica gel. The resulting solution was
evaporated to dryness under reduced pressure to yield the desired
product as a white crystalline solid. Yield: 29 mg, 92%.
Crystallographic data (CCDC 1429268) (CIF)
Crystallographic data (CCDC 1429269) (CIF)
Structure model (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Michael.ingleson@manchester.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research leading to these results has received funding from
Cambridge Display Technology (CDT/EPSRC Case Award to
D.L.C.), the EPSRC (EP/K03099X/1), and the European
Research Council (FP/2007-2013/ERC Grant Agreement
305868). M.J.I. acknowledges the Royal Society (for the
award of a University Research Fellowship), and M.L.T. thanks
InnovateUK for financial support of the Knowledge Centre for
Material Chemistry. The authors would also like to acknowl-
edge the use of the EPSRC UK National Service for
Computational Chemistry Software (NSCCS) at Imperial
College London in carrying out this work. Dr. Martin
Humphries at CDT is also thanked for useful discussions.
The spectra agree with that previously reported.4
Synthesis of 1-B(m-BrPh)2. AlCl3 (15 mg, 0.11 mmol) was added
to a suspension of 1-BCl2 (31 mg, 0.13 mmol) and trimethyl(3-
bromophenyl)silane (55 μL, 0.29 mmol) in o-DCB (0.7 mL). The
reaction mixture was heated at 60 °C for 16 h. NMe4Cl (15 mg, 0.14
mmol) was added to the reaction mixture, and the solvent was
removed under reduced pressure. The resulting residue was purified
via column chromatography on silica gel [eluent DCM/hexane (5:5)]
to afford the desired product as a white crystalline solid. Yield: 38 mg,
62%.
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1H NMR (400 MHz, CDCl3): δ = 8.43 (d, J = 5.7 Hz, 1 H), 8.13−
8.00 (m, 2 H), 7.88 (d, J = 7.6 Hz, 1 H), 7.69 (d, J = 7.3 Hz, 1 H), 7.48
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2 H), 7.16 (d, J = 7.5 Hz, 2 H), 7.09 (t, J = 7.6 Hz, 2 H). 13C NMR
(101 MHz, CDCl3): δ = 161.3 (br), 158.4, 153.2 (br), 143.8, 141.1,
135.8, 135.2, 131.6, 130.6, 129.3, 128.9, 126.5, 122.6, 122.3, 121.9,
118.5. 11B NMR (128 MHz, CDCl3): δ = 3 (v br) ppm. HRMS
(APCI): calcd for C23H17BBr2N+ (M + H) 477.9795, found 477.9796.
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silafluorene (30 mg, 0.14 mmol) in o-DCB (0.7 mL). The reaction
mixture was then heated overnight at 60 °C. The solvent was then
removed under reduced pressure, and the purification was performed
under ambient atmosphere using nonpurified solvents thereon. The
resulting residue was dissolved in hexane and was filtered through a
short plug of silica gel; hexane (100 mL) and then DCM (100 mL)
were then passed through the silica gel, and the DCM fraction was
collected and evaporated to dryness under reduced pressure to give the
desired product as a white crystalline solid. Yield: 33 mg, 82%.
1H NMR (400 MHz, CDCl3): δ = 8.10−8.05 (m, 1 H), 8.02−7.93
(m, 2 H), 7.85 (td, J = 1.1, 5.6 Hz, 1 H), 7.83−7.75 (m, J = 7.6 Hz, 2
H), 7.44−7.36 (m, 3 H), 7.31 (dt, J = 1.2, 7.5 Hz, 2 H), 7.14 (ddd, J =
1.2, 5.8, 7.2 Hz, 1 H), 7.05 (dt, J = 1.0, 7.2 Hz, 2 H), 6.93−6.83 (m, J =
7.1 Hz, 2 H). 13C NMR (101 MHz, CDCl3): δ = 159.3 (br), 158.8,
154.1 (br), 150.8, 143.1, 140.5, 137.5, 131.0, 130.7, 130.1, 127.2, 126.4,
126.2, 121.8, 121.4, 119.3, 117.8. 11B NMR (128 MHz, CDCl3): δ = 2
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(14) For recent examples exemplifying the disparate reactivity of
arylsilanes and arylstannanes toward boranes see: (a) Li, H.; Jakle, F.
̈
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̈
53, 9761−9765.
(15) Clark, E. R.; Del Grosso, A.; Ingleson, M. J. Chem. - Eur. J. 2013,
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(16) For an early example of the synthesis an unsymmetric
Ar1Ar2BCl see: Paetzold, P. I.; Habereder, P. P.; Muellbauer, R. J.
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(17) (a) Rao, Y. L.; Amarne, H.; Zhao, S. B.; McCormick, T. M.;
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(18) A patent search in Sept 2015 revealed seven separate patents
filed on the use of spiro materials related to 1-B(biphenyl) in
electroluminescent devices, the earliest being: Hasegawa, M.; Morii,
K.; Kuerya, T.; Akatsuka, I.; Jpn. Kokai Tokkyo Koho, 2013,
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(19) Cade, I.; Ingleson, M. J. Chem. - Eur. J. 2014, 20, 12874.
H
DOI: 10.1021/acs.organomet.5b00857
Organometallics XXXX, XXX, XXX−XXX