Liquid triarylamines: The scope and limitations of piers-rubinsztajn conditions for obtaining triarylamine-siloxane hybrid materials

Article abstract of DOI:10.1021/jo2020906

New liquid triarylamine-siloxane hybrid materials are produced using the Piers-Rubinsztajn reaction. Under mild conditions, liquid analogues of conventional and commonly crystalline triarylamines are easily synthesized from readily available or accessible intermediates. Using a diverse selection of triarylamines, we explored the effects of siloxane group and substitution pattern on the physical properties of these materials, and we have demonstrated that relatively large molecular liquids with desirable electrochemical properties can be produced. The interactions between the strongly Lewis acidic catalyst used for this transformation, tris(pentafluorophenyl)borane (BCF), and the Lewis basic triarylamine substrates were studied. Through UV-vis-NIR and ¹⁹F NMR spectroscopy, we have proposed that the catalyst undergoes a reversible redox reaction with the substrates to produce a charge transfer complex. The formation of this charge transfer complex is sensitive to the oxidation potential of the triarylamine and can greatly affect the kinetics of the Piers-Rubinsztajn reaction.

Full text of DOI:10.1021/jo2020906

Article
pubs.acs.org/joc
Liquid Triarylamines: The Scope and Limitations of Piers−
Rubinsztajn Conditions for Obtaining Triarylamine−Siloxane Hybrid
Materials
Brett A. Kamino,† Bridget Mills,† Christopher Reali,† Michael J. Gretton,† Michael A. Brook,‡
and Timothy P. Bender*,†,§
†Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College St., Toronto, Ontario M5S 3E5,
Canada
‡Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St., Hamilton, Ontario L8S 4M1, Canada
§Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: New liquid triarylamine−siloxane hybrid materials are produced using the Piers−Rubinsztajn reaction. Under
mild conditions, liquid analogues of conventional and commonly crystalline triarylamines are easily synthesized from readily
available or accessible intermediates. Using a diverse selection of triarylamines, we explored the effects of siloxane group and
substitution pattern on the physical properties of these materials, and we have demonstrated that relatively large molecular
liquids with desirable electrochemical properties can be produced. The interactions between the strongly Lewis acidic catalyst
used for this transformation, tris(pentafluorophenyl)borane (BCF), and the Lewis basic triarylamine substrates were studied.
Through UV−vis−NIR and 19F NMR spectroscopy, we have proposed that the catalyst undergoes a reversible redox reaction
with the substrates to produce a charge transfer complex. The formation of this charge transfer complex is sensitive to the
oxidation potential of the triarylamine and can greatly affect the kinetics of the Piers−Rubinsztajn reaction.
with rough surfaces and the possibility of solvent-free (or
solvent-minimal) device processing. To our knowledge, there
are only two organic semiconducting materials that are free-
flowing liquids at room temperature and have been applied in
devices: tris(4-methoxyethoxyphenyl)amine (TMEPA)6 and N-
(2-ethylhexyl)carbazole.7 These have been successfully used as
charge-transporting materials in dye-sensitized solar cells
(DSSCs) and OLEDs, respectively. However, the further
exploration of this unique class of materials has been hampered
by the lack of a general synthetic strategy to produce them.
One strategy to produce new liquid organic semiconductors
is the modification of known solid organic semiconductors with
functionalities that can alter their physical properties while not
affecting their electronic and photophysical properties. Using
INTRODUCTION
■
The study of organic electronic devices such as organic light
emitting diodes (OLEDs),1 organic photovoltaics (OPVs),2
organic field effect transistors (OFETs),3 and photoreceptors4
has resulted in the synthesis and characterization of a multitude
of novel semiconducting materials specifically designed for
these devices. A considerable range of useful electronic and
photonic properties has been achieved through changes in the
molecular structure of the semiconductor. However, consider-
ing only the physical state of the semiconducting material, the
vast majority are designed to be amorphous, crystalline, or
semicrystalline solids with varying degrees of solubility in
organic solvents.5 Recently, there have been examples of
organic electronic devices incorporating organic semiconduc-
tors, which replace conventional materials with free-flowing
liquids.6,7 The incorporation of liquid semiconductors has a
number of potential advantages such as good interfacial contact
Received: October 13, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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this general strategy, we have shown that the incorporation of
simple silyl ethers into the structure of triarylamines results in
some derivatives that are liquids. The nature of the silyl ether
group as well as the overall symmetry and structure of the
triarylamine was found to dictate whether or not these well-
known organic semiconductors are liquids, waxes, or crystalline
solids.8
Building upon this idea, we have recently shown that the
incorporation of short, discrete disiloxane units around the
periphery of triarylamines is a potentially more versatile
strategy to produce liquid triarylamines (Figure 1).9 Central
Our original study was limited to performing this reaction on
simple triarylamines with a single nitrogen center. Those in the
field will recognize that simple triarylamines have a limited
range of electrochemical oxidation potentials and applications
in functional devices.20 We report herein our efforts to broaden
the scope of the Piers−Rubinsztajn process and its intersection
with triarylamine−siloxane hybrid materials with an aim at
producing a larger number of liquid triarylamines. We have
focused our attention on multinitrogen centered triarylamines
as well as molecules containing the related carbazole moeity.
Triarylamines containing multiple conjugated nitrogen centers
are known to possess a wider range of electrochemical
properties and can exhibit better stability upon oxidation
(better radical cation stability), leading to improved device
performance. Conversely, they also have the potential to be
significantly more Lewis basic than simple triarylamines
because of their multiple nitrogen centers. Therefore, these
substrates provide an opportunity to probe the scope and
limitation of the reaction of methoxy-functionalized triaryl-
amines with discrete silanes under Piers−Rubinsztajn con-
ditions. The limitation of most interest is whether the increase
in Lewis basicity on moving to triarylamines with multiple
centers will result in a significant shift in charge transfer
equilibrium between BCF and the triarylamine substrate, thus
inhibiting the reaction.
Figure 1. Previous work on the synthesis of siloxane-functionalized
triarylamines (reproduced with slight modifications with permission
(© 2011 American Chemical Society)).9
RESULTS AND DISCUSSION
■
to this synthetic strategy is the use of the Piers−Rubinsztajn
reaction,10 which uses tris(pentafluorophenyl)borane (BCF) as
a catalyst (1 mol %, Figure 1). The reaction is performed under
very mild and ambient conditions and requires low catalyst
loadings. The only byproduct of the reaction is methane, and
the substitution proceeds without metathesis or redistribution
of the siloxane component, which commonly occur for many
types of siloxane chemistries and limit the ability to construct
discrete structures. Rapid conversion was observed despite the
use of the strongly Lewis acidic catalyst, which on first
examination may have been assumed to complex with the Lewis
basic triarylamine substrate, preventing the reaction from
proceeding. An investigation using UV−vis spectroscopy into
the potential complexation showed a small amount of charge
transfer between BCF and the triarylamine indicated by the
presence of an absorption, which could be directly attributed to
the oxidized arylamine (radical cation). Despite the detectable
presence of this absorption, the relatively weak signal observed
coupled with the known high extinction coefficient for radical
cations of triarylamines suggests that the equilibrium between
the charge transfer/interaction state and the freely dissociated
state lies far toward dissociation.
Piers−Rubinsztajn conditions have previously been shown to
enable the reaction of silanes with a number of different
coupling partners including carbonyl groups,11 aryl hydroxyl/
alkoxy groups,12 alkyl hydroxyl groups,13 siloxy groups, and
silanols.14 Piers−Rubinsztajn conditions have been shown to
have a wide scope and have been used for the synthesis of
structured and functional materials such as siloxane den-
drimers,15 polysiloxanes and copolymers thereof,16 and surface
active siloxane ambiphiles.17 While this chemistry has good
functional group tolerance,18 there are cases where Lewis basic
substrates react poorly because of competitive catalyst binding
to these Lewis basic functionalities.19 The strength of the Lewis
basic functionality is thus an important factor in the reaction
kinetics and whether the reaction can proceed at all.
While there are many different triarylamine structures from
which to choose for this study, we began by choosing two of
the more common motifs: triarylamines with a phenylene
diamine core and those with a benzidine core (Scheme 1).
Several variants on these backbones were synthesized so as to
understand the effect of molecular symmetry and degree of
siloxane functionalization on the physical and electrochemical
properties of the resulting materials. Furthermore, the structural
changes in each variant are also accompanied by inherent
electronic variations affecting the Lewis basicity of the
molecule.
Triarylamines based on a phenylene diamine core (com-
pounds 2a−b) were synthesized in two steps (Scheme 1). 1,4-
Phenylene diamine was first reacted with 4-bromoanisole under
standard Buchwald−Hartwig amination conditions21 to yield an
intermediate triarylamine 1.22 This aryl-methoxy-substituted
triarylamine was then functionalized with discrete siloxane
groups (1,1,1,3,3-pentamethyldisiloxane (MMH) or
1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM)) by a reaction
with the corresponding silanes in the presence of a catalytic
amount of tris(pentafluorophenyl)borane (BCF, the so-called
Piers−Rubinsztajn conditions) under ambient and open air
conditions. In our previous work,9 we found that the
functionalization of methoxy-containing triarylamines with
MMH under these conditions proceeded rapidly. However, in
this case, complete conversion took approximately 7 h to
achieve. Gentle heating (50 °C) was found to speed up the
process considerably. Caution: As the temperature in the flask
increased, the reaction would proceed rapidly with the rapid
evolution of methane gas.
Triarylamines with a benzidine core (5a−h) were synthe-
sized in three steps (Scheme 1). Diarylamines were first
prepared by Buchwald−Hartwig coupling of the corresponding
aryl bromides with an excess of the appropriate aniline.
Diarylamines 3a,23 3b,24 and 3c25 have been previously
described in the literature, although their synthesis was
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Scheme 1. Synthesis of Siloxane-Functionalized Arylamines
achieved using other means with differing yields. In our case,
each diarylamine was easily purified by an acidic aqueous
extraction followed by recrystallization from nonpolar solvents.
They were then reacted with 4,4′-dibromobiphenyl to achieve
the methoxy-functionalized triarylamines (4a−e), of which 4a26
and 4d22 have been previously described in the literature.
Finally, the triarylamines were functionalized with discrete
siloxane chains by reaction with the corresponding silane and
catalytic amounts of BCF to give compounds 5a−h. Unlike the
reaction with the 1,4-phenylene-based materials (2a−b), the
introduction of the siloxane groups proceeded quickly at room
temperature, requiring up to a minute of induction time before
reacting rapidly.
used (in some cases we used 2 equiv and in others 5 equiv) and
the crude yields. NMR analysis of the crude reaction mixtures
does suggest that the reaction generates small amounts of
polymeric silicone species as evidenced by a corresponding and
1
characteristic CH3 resonance in the H NMR spectrum. The
generation of this byproduct is likely due to the reaction of
ambient water with silane in the reaction mixture moderated by
BCF.27 Column chromatography was found to be effective in
removing this byproduct. Clearly, conducting the P−R process
under anhydrous conditions would preclude the formation of
the silicone species. However, as column chromatography is
generally necessary to produce triarylamine suitable for study in
organic electronic devices, its use to remove the produced
silicones is balanced against the ease of operating under
ambient vs anhydrous conditions.
Generally speaking, all reactions proceeded to high
conversion, and we did not see any correlation between
triarylamine molecular structure and crude yields. We also did
not see a correlation between the equivalent amount of silane
All of the final triarylamine−siloxane hybrid materials were
isolated by column chromatography (SiO2, toluene/cyclo-
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hexane) as viscous liquids (compounds 2b, 5a−c, e−f,h) or as
crystalline solids (compounds 2a, 5d, and 5g), and their
structure and purity (and those of the intermediates) were
unambiguously confirmed by high resolution mass spectrom-
etry (HRMS) and NMR analysis. Yields for the complete
unoptimized process ranged significantly between 48 and 90%
after column chromatography. Overall, compounds with the
bulkier −OD(M)2 group gave higher isolated yields than those
with the less bulky −OMM group.
Each compound was characterized by differential scanning
calorimetry (DSC) to determine the effect of silicone
substitution and molecular symmetry on the melting point
and/or glass transition temperatures of the materials (Table 1).
had a significant effect on the glass transition temperature (Tg)
and the presence of a crystalline state. Using pentamethyldi-
siloxane (−OMM) as a siloxane, the most symmetrically
substituted arylamines (2b, 5d, 5g) were isolated as crystalline
solids and exhibited well-defined and sharp melting points by
DSC. Upon heating, compound 5d exhibited two exothermic
transitions at 52 and 58 °C, possibly indicating polymorphism.
For each compound that showed a melting transition on the
first DSC scan, a glass transition temperature well below room
temperature was detected on the second heating cycle. These
low glass transition temperatures suggested that if crystal-
lization could be inhibited, the materials would behave as
liquids at room temperature. This hypothesis was tested by
utilizing the bulkier and branched 1,1,1,3,5,5,5-heptamethyl-
trisiloxane (−OD(M)2) group in place of the linear −OMM
group. The resulting compounds (2b, 5f, 5h) were isolated as
free-flowing oils, and no detectable crystallization was observed
over several months of storage at ambient conditions. The
bulkier siloxane groups appear to prevent the π−π stacking
interactions that are found in the crystal structures of these
molecules.28 Additionally, we found that the presence of
asymmetry in the substitution pattern of the triarylamine along
the axis bisecting the two nitrogen atoms resulted in
compounds that were isolated as highly viscous oils at room
temperature (5a, 5b, 5c, 5e). For example, compounds 5a−c
containing two −OMM groups and various methyl group
substituents on the adjacent phenyl ring are liquids, whereas
their symmetric counterparts are not. Increasing the number of
methyl groups raised the glass transition temperature, whereas
replacing the linear −OMM siloxane with a branched
−OD(M)2 siloxane (5e) lowered the glass transition temper-
ature by 15 °C as compared to its structural counterpart (5b).
From these observations, along with those in our previous
study,9 we can make several conclusions about the effect of
siloxane substitution on the physical state of arylamines. We
find that the incorporation of larger siloxane chains lowers the
glass transition temperature of the material. Furthermore, as the
base molecule increases in size, the number of siloxane chains
needed to achieve a room temperature liquid arylamine−
silicone hybrid increases. A secondary consideration is the
addition of asymmetry to the molecule, which is shown to
decrease the chances of crystallization in the molecule.
Table 1. DSC and CV Results from Silicone-Hybridized
a
Triarylamines
Tm
(°C)
Tg
(°C)
Eox 1 (mV vs Eox 2 (mV vs
compound appearance
Ag/AgCl)
Ag/AgCl)
b
2a
crystalline
87
−49
443
436
749
725
673
661
699
682
701
704
616
366
902
906
994
942
935
892
942
921
907
929
772
696
2b
5a
oil
−47
6
oil
5b
5c
oil
11
14
oil
b
5d
5e
crystalline
oil
52, 58
88
−23
−4
−31
−1
−9
28
5f
oil
5g
5h
9
crystalline
oil
crystalline
crystalline
oil
150
131
11
15a
15b
15c
15d
20
c
−44
−50
1495
c
oil
1320
b
c
crystalline
oil
47
−48
1485
c
−48
1510
a
Tm, melting temperature; Tg, glass transition temperature; Eox 1, first
half wave oxidation potential; Eox 2, second half wave oxidation
b
c
potential. Detected on second heating cycle. Irreversible, peak
potential on first scan reported.
Consistent with our observations regarding the analogous
single nitrogen centered triarylamine−siloxane hybrids,9 the
effect of adding discrete silicone groups to the arylamine cores
Scheme 2. Synthesis of Spiro Core Triarylamine 9
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Scheme 3. Synthesis of Triarylamine 11
Scheme 4. Synthesis of Siloxane-Functionalized Carbazoles
The electrochemical behavior of this initial group of
triarylamine−siloxane hybrids was studied using solution cyclic
voltammetry in dichloromethane with 0.1 M tetrabutylammo-
nium perchlorate as a supporting electrolyte. A small amount of
decamethylferrocene was added to all solutions and used as an
internal standard.29 The results of the electrochemical analysis
are included in Table 1. All of the compounds underwent two
reversible 1-electron oxidation events, which is characteristic of
triarylamines containing two nitrogen centers. This demon-
strates that the siloxane functionalization of arylamines does
not have an undesirable impact on their electrochemical
behavior. Variations in the position of the oxidation potentials
can be explained by number and strength of electron-donating
silicone groups as well as degree of conjugation between each
arylamine redox center. There does not seem to be a consistent
difference in the electron-donating potential of the −OD(M2)
versus the −OMM group. Voltammograms for all compounds
are included in the Supporting Information (Figures S1−S12).
Encouraged by the success of this chemistry on phenylene
diamine and benzidine-based substrates, we further extended
this study to larger (higher molecular weight) multinitrogen
centered triarylamines. Two well-known triarylamine structures
were chosen as model compounds: one based on the spiro-
TAD core30 and one based on the TDATA motif,5 each
containing four nitrogen centers (Schemes 2 and 3,
respectively). The spiro-TAD derivative (9) was synthesized
using a slightly modified literature procedure,31 resulting in a
structure containing four aryl-methoxy groups for substitution
(Scheme 2). When reacted with MDHM under the Piers−
Rubinsztajn conditions, the substrate reacted rapidly and
cleanly to yield a molecular glass upon isolation, which
ultimately crystallized over several weeks. The TDATA
structure (11) was synthesized in two steps from tris(p-
bromophenyl)amine (Scheme 3) and isolated as a crystalline
solid. In this case, the reaction proceeded very slowly requiring
approximately 16 h for completion. Both structures showed
typical electrochemical behavior relative to their respective
classes, and their synthesis demonstrates the ability of Piers−
Rubinsztajn conditions to successfully install −OD(M)2 units
on large multinitrogen centered substrates.
Next, we chose to extend this chemistry to a related group of
arylamine semiconductors: carbazoles. To this end, a number of
N-phenylcarbazoles with methoxy-substituents in various
positions were synthesized and subjected to the Piers−
Rubinsztajn reaction conditions. Carbazoles, like triarylamines,
are common materials in organic electronics and have been
used as both p-type semiconductors and ambipolar host
materials.5,32 Four N-phenylcarbazole precursors were synthe-
sized. For the structures with methoxy groups present on the
carbazole ring itself (14a−b, Scheme 4), Suzuki coupling of 1-
bromo-2-nitro-4-methoxybenzene and an arylhalide followed
by a reductive ring-closing reaction promoted by triethylphos-
phite resulted in the desired carbazoles (13a−b).33 These
carbazoles were then N-arylated under Ullman coupling
conditions34 to yield the final methoxy-functionalized pre-
cursors (14a−b), of which 14b has been previously described.35
Those with methoxy groups on the phenyl ring (14c−d) were
prepared in a one-step procedure under Ullman coupling
conditions using carbazole and the appropriate halo-anisole.
Carbazoles 14c35 and 14d36 have been previously described.
Because of the planar molecular structure of compounds 14a−
d, they were further functionalized exclusively with −OD(M)2
groups under Piers−Rubinsztajn conditions in order to inhibit
any possible crystallization. In each case, the reaction proceeded
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extremely rapidly, and the resulting siloxane-functionalized
carbazoles (15a−d) were isolated in good yields. For all but
compound 15d, the resulting materials were isolated as clear
free-flowing liquids. Electrochemical analysis of the carbazoles
15a−d showed that each underwent an irreversible one-
electron oxidation typical for carbazoles.32 Interestingly, the
disubstitued 14b appears to undergo two oxidations, which are
poorly resolved under standard cyclic voltammetry conditions.
It is unclear at this time whether these events represent two
subsequent electrons being removed or that another chemical
change is occurring. Full electrochemical and calorimetry data
for these compounds is included in Table 1. The effect of
substitution pattern on the physical properties of the carbazoles
is in line with our previous observations in that the most
symmetric substitution patterns (15d) result in a derivative that
was crystalline, whereas the asymmetrically substituted
structures are isolated as liquids.
Effect of Substrate on Catalyst Efficacy. In our approach
to the synthesis of arylamine−siloxane hybrid materials, the use
of Piers−Rubinsztajn conditions is critical to the installation of
discrete siloxane groups while avoiding unwanted metathesis
and redistribution reactions of the silicones. The previously
studied simple triarylamines were found to react rapidly at
room temperature (often reacting as quickly as silane could be
added to the mixture). However, in this study, it was observed
that the efficacy of this reaction (as indicated by its time to
completion at room temperature) varied depending on the
structure of the triarylamine substrate. The rate of the reaction
for this series of compounds could be qualitatively observed by
the evolution of methane gas. Compared to the triarylamines
used in our previous study, the benzidine-based substrates
reacted somewhat more slowly, with reactions typically
finishing several minutes after complete silane addition.
Substrates 1 and 10 stood out as the only substrates requiring
heat to proceed rapidly; leaving the substrates to react at room
temperature necessitated 7 and 16 h of reaction time for
complete substitution, respectively. We also observed rapid
reaction in the formation of triarylamines 9 and 11 and also for
all carbazole derivatives (14a−d).
In our previous work, we found an interaction between the
strongly Lewis acidic tris(pentafluorophenyl)borane (BCF) and
the weakly Lewis basic tris(p-methoxyphenyl)amine did occur
but that the level of interaction was quite weak.9 This
interaction between catalyst and substrate was shown to result
in the formation of the triarylamine radical cation. However, we
did not directly prove the existence of the corresponding radical
anion of BCF. While such reactions are uncommon, several
examples of BCF acting as an oxidant are noted in the
literature.37 A complete understanding of the strength of BCF
as an oxidant has not been established owing to the instability
of the borane anion.38 Since the radical cation of an arylamine
can be easily detected by its characteristic UV−vis−NIR
absorption, we set out to see if the observed differences in
reactivity between triarylamines could be explained by differing
interactions between the precursor and the BCF catalyst.
Solutions of arylamines 1 and 4d were prepared in toluene
(our reaction solvent) along with varying molar equivalents of
BCF. Compounds 1 and 4d were chosen as representative
compounds because their oxidized cations have been previously
studied in the literature.23 Furthermore, the observed reaction
kinetics with these substrates varied greatly under the described
conditions. To study this interaction under simulated reaction
conditions, solutions of both BCF and substrate were prepared
under ambient conditions and allowed to stand in open air for
15 min. These solutions were studied by UV−vis−NIR
absorbance measurements, which found large changes in the
visible and near-infrared region of the spectra upon addition of
BCF (see Supporting Information, Figure S3). Compounds 1
and 4d both exhibited broad absorption bands in the NIR as
increasing amounts of BCF were added. These broad
absorption bands can be assigned to intervalence charge
transfer (IVCT) bands of the radical cations of the arylamines
(1•+ and 4d•+, respectively), which result from the partial
delocalization of the charge between two conjugated redox
centers. The UV−vis−NIR absorbance spectra of 1•+ and 4d•+
are well-studied as model charge transfer systems, and their
molar extinction coefficients in dichloromethane are known.22
A comparison between the published spectra of 1•+ and 4d•+
and the spectra generated by mixing the neutral triarylamines
with BCF reveals that they are practically the same. Small
differences in the λmax found in the literature and in our study
are likely due to differences in the solvents used for each
measurement. Such charge transfer bands are inherently
sensitive to solvent polarity.39 The presence of atmospheric
conditions can potentially complicate this experiment, as it is
well-known that BCF readily forms adducts with water under
the conditions of this study.27 To rule out any possibility of
water playing a role in the observed reaction, solutions were
prepared under glovebox conditions. Under these anhydrous
conditions, the exact same color changes were noted, and the
samples were spectroscopically identical.
On the basis of these spectra and the known molar extinction
coefficients of 1•+ and 4d•+ (albeit in different solvents) we can
estimate the amount of arylamine that is oxidized (see the
Supporting Information, Table S2). If an equimolar mixture of
1 and BCF is considered, the percentage of 1 oxidized is 5 mol
% (corresponding to 5 mol % BCF reduced). If the same
consideration is given to a mixture of 1 with catalytic amounts
of BCF (0.01 equiv to 1), the amount of 1 oxidized is 0.2 mol
% (corresponding to 20 mol % of BCF reduced). In a similar
manner for compound 4, 2 mol % is oxidized at equal molar
amounts (corresponding to 2 mol % BCF reduced), and 0.03
mol % is oxidized at catalytic amounts (corresponding to 3 mol
% BCF reduced). We can therefore conclude that while the
presence of 1•+ and 4d•+ is detectable, even by eye, the result is
not the quantitative reduction of BCF to BCF•−, rather it is
only a partial reduction. Furthermore, given the partial
reduction and the absence of the formation of a precipitate,40
we hypothesize that the system is in a dynamic equilibrium
between the charge transfer couple and the freely dissociated
species. Taking into consideration our previous observation of
the very slight oxidation of tris(p-methoxyphenyl)amine by
BCF,9 we propose that the extent of oxidation of the arylamine
by BCF is directly related to the oxidation potential of the
arylamine. That is, the equilibrium shifts toward formation of
the radical cation/radical anion pair for arylamines with lower
oxidation potentials. The oxidation potentials of the tris(p-
methoxyphenyl)amine, 4d, and 1 are 654,41 605, and 375 mV
vs Ag/AgCl, respectively. Additionally, these values are
proportional to the observed kinetic rates of the substrates
under these conditions. This result suggests that the nature of
interaction between triarylamines and BCF is a redox reaction
where the BCF is acting as an oxidant. Furthermore, we can
conclude that the redox reaction between certain triarylamines
and BCF results in partial sequestration of the catalyst and
retardation of the rate of the desired reaction.
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To further support this hypothesis, mixtures of 1 and 4d
along with 1 mol % equiv of BCF were dissolved in toluene-d8
relates to the observed kinetics of the Piers−Rubinsztajn
reaction on these substrates.
and studied by 19F and H NMR. In each case, the H spectra
obtained were significantly broadened because of the presence
of the paramagnetic arylamine radical cation (Supporting
Information, Figures S14−S22). 19F NMR spectra were difficult
to obtain with a reasonable signal-to-noise ratio because of the
highly dilute nature the BCF and the relatively low solubility of
arylamines 1 and 4d. The 19F NMR of the mixture of
compound 1 and 1 mol % BCF displayed a large number of
resonances, with several corresponding to the free BCF
(Supporting Information, Figure S15). Under the same
conditions, the mixture of compound 4d and 1 mol % BCF
resulted in a cleaner spectrum comprising mostly of free BCF
(but still with a poor signal-to-noise ratio; Supporting
Information, Figure S17). Because of the poor signal-to-noise
ratio obtained and the paramagnetic nature of the solution,
concrete comparisons between the amount of free BCF in
solution in the UV−vis−NIR experiment and the NMR
experiment are difficult to make.
1
1
Through the work presented in both our preliminary study
and this one, we have successfully increased the number of
known liquid organic semiconductors from 2 to 15. These
novel materials emulate the basic electrochemical properties of
their widely used solid counterparts while spanning a wide
range of oxidation potentials. Perhaps more importantly, we
have shown that this synthetic strategy is quite general, which
may also extend to other popular classes of organic semi-
conductors and possibly be used to produce other liquid
organic semiconductor types in the future. This work will
hopefully help further the study of organic electronic devices
incorporating liquid layers and allow for the synthesis of device-
specific liquid materials.
EXPERIMENTAL SECTION
■
Safety note: For reactions involving Piers−Rubinsztajn chemistry
(coupling of silanes using tris(pentafluorophenyl)borane), the reaction
can proceed extremely rapidly, evolving flammable gases with a noticeable
exotherm. In our opinion, care should be exercised in scaling this chemistry.
The careful dropwise addition of silane(s) at a moderate rate is
recommended, and if no reaction is detected after several minutes, the
addition of silane should be halted.
We therefore considered mixtures of arylamines 1 and 4d
and BCF in a 1:1 molar ratio (in C6D6; Supporting
Information, Figure S19−20). Under these conditions, all
three fluorine resonances of the free BCF are visible and make
up the majority of the detected signal, although moderately
broadened. This was particularly evident for compound 1, and
the difference in observed amount of free BCF between the two
concentrations supports the idea of a dynamic chemical
equilibrium between the free and bound state.
General Piers−Rubinsztajn Procedure (General P−R proce-
dure). The aryl-methoxy-functionalized substrate, tris-
(pentafluorophenyl)borane, and toluene were added to a generously
sized round-bottom flask with a magnetic stirrer under atmospheric
conditions. The flask should be at least four times the volume of the
reagents to prevent any material from spilling out upon reaction. To
maximize the reaction yield, the P−R reaction should be done at
anhydrous conditions using predried reagents and solvent. Silane was
added dropwise to this stirring solution, taking care to control the rate
of methane evolving. Upon completion, standard activity basic alumina
(∼0.5 g) was added, and the solution was allowed to stir for 20 min.
After this time, the solution was filtered to remove the added alumina
and reduced under vacuum for purification.
CONCLUSIONS
■
In summary, the scope and limitations of Piers−Rubinsztajn
conditions for the synthesis of silicone−arylamine hybrids has
been explored with an emphasis on obtaining liquid triaryl-
amines having multiple nitrogen centers. The catalytic
functionalization of a series of two nitrogen centered
triarylamines was successful, and this substitution was found
to have a significant effect on the physical properties of the
arylamines, resulting in a number of new liquid triarylamines.
We observed that by using the bulkier and branched −OD(M)2
group, normally crystalline bis-arylamines could be made into
free-flowing liquids by effectively lowering the glass transition
temperature to below room temperature and inhibiting
crystallization. The flexibility of this synthetic strategy was
further explored by demonstrating that this chemistry can be
used on larger multinitrogen centered arylamines as well as on
carbazole-based precursors. The siloxane-functionalized triaryl-
amines retained the electrochemical behavior of their parent
compounds. Furthermore, the interaction between the Lewis
acidic BCF (tris(pentafluorophenyl)borane) and the Lewis
basic arylamine was further studied. It is proposed that the BCF
catalyst can act as a one-electron oxidant, forming a radical
cation/radical anion pair that is in equilibrium with the freely
dissociated arylamine and BCF. This redox reaction sequesters
the catalyst and retards the rate of the desired reaction in some
cases. This mechanism is supported by UV−vis−NIR and 19F
NMR spectra. From studying these compounds, we hypothe-
size that the extent of the charge transfer interaction is
dependent on the triarylamine oxidation potential. More easily
oxidized triarylamines appear to achieve an equilibrium state
further favoring the charge transfer pair than those triaryl-
amines that are more electron-poor. This observation directly
Compound 1. 1,4-Phenylene diamine (1.000 g, 9.25 mmol),
sodium tert-butoxide (5.33 g, 55.5 mmol), and bis-
(dibenzylideneacetone)palladium (106 mg, 0.184 mmol) were added
to a round-bottom flask. This flask was sealed under an argon
atmosphere. Anhydrous toluene (50.0 mL), 4-bromoanisole (7.61 g,
40.7 mmol), and tri-tert-butylphosphine (29.9 mg, 0.148 mmol, added
as a stock solution in toluene) were added. This mixture was refluxed
under an inert atmosphere for 2 h. Upon cooling, acidic clay (10.0 g,
Montmorillonite K10) and acidic alumina (1.00 g, standard basic)
were added to the mixture. This slurry was filtered, washing with
toluene to yield a clear, pale yellow solution. This solution was
concentrated under vacuum and precipitated into methanol. 4.24 g of a
1
fine pale white powder was obtained (86% yield): H NMR (400
MHz, C6D6) δ 7.14 (d, J = 9.08 Hz, integration obscured by solvent
peak), 7.08 (s, 4H), 6.73 (d, J = 9.08 Hz, 8H), 3.30 (s, 12H); 13C
NMR (100 MHz, C6D6) δ 156.3, 143.8, 142.6, 126.4, 124.2, 115.5,
53.4; HRMS (ESI) [M+] calcd for C34H32N2O4 532.2356, found
532.2372.
Compound 2a. Using the general P−R procedure, 1 (500 mg,
0.939 mmol), anhydrous toluene (10.0 mL), tris(pentafluorophenyl)-
borane (5 mg, 0.01 mmol), and pentamethyldisiloxane (3.14 g, 21.3
mmol) were reacted. After 20 min, the solution was immersed in an oil
bath at 50 °C. The solution vigorously evolved methane and was
allowed to continue to stir for 20 min. The product was purified by
column chromatography over silica gel eluting with 2:1 cyclohexane/
toluene. 661 mg of a flakey crystalline solid was isolated (yield 66%):
1H NMR (400 MHz, C6D6) δ 7.09 (d, J = 8.98 Hz, 8H), 7.01 (s, 4H),
6.91 (d, J = 8.98 Hz, 8H), 0.22 (s, 24H), 0.11 (s, 36H); 13C NMR
(100 MHz, C6D6) δ 151.0, 143.8, 143.3, 126.2, 124.6, 121.3, 2.1, 0.2;
HRMS (ESI) [M + H] calcd for C50H81N2O8Si8 1061.4141, found
1061.4146.
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Article
Compound 2b. Using the general P−R procedure, compound 1
(0.500 g, 0.939 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (2.51 g,
11.3 mmol), tris(pentafluorophenyl)borane (5 mg, 0.01 mmol), and
anhydrous toluene (5.00 mL) were reacted. The product was isolated
by column chromatography over silica gel eluting with 2:1 cyclo-
hexane/toluene. A viscous yellow oil was isolated (1.034 g, 81% yield):
1H NMR (400 MHz, C6D6) δ 7.10 (d, J = 8.77 Hz, 8H), 7.07 (s, 4H),
6.97 (d, J = 8.77 Hz, 8H), 0.27 (s, 12H), 0.15 (s, 72H); 13C NMR
(100 MHz, C6D6) δ 150.2, 143.4, 143.1, 125.9, 124.1, 120.9, 1.7, −3.0;
HRMS (ESI) [M+] calcd for C58H104N2O12Si12 1356.4821, found
1356.4814.
solids were filtered out, washing with additional toluene. The mother
liquor was collected and concentrated under vacuum until the solution
began to become a viscous oil. This oil was precipitated into rapidly
stirring methanol (50 mL) to yield a fine white powder. This powder
was collected by filtration and washed with cold methanol. 2.635 g of
1
product was isolated (67% yield): H NMR (400 MHz, C6D6) δ 7.41
(d, J = 8.61 Hz, 4H), 7.19−7.13 (m, peak obscured by solvent peak),
7.10 (d, J = 7.41 Hz, 4H), 7.06 (d, J = 9.00 Hz, 4H), 6.85 (t, J = 7.04
Hz, 2H), 6.71 (d, J = 9.00 Hz, 4H), 3.29 (s, 6H).
Compound 4b. Using the same general procedure as for
compound 4a, compound 3b (3 g, 14.1 mmol), 4,4′-dibromobiphenyl
(2.09 g, 6.67 mmol), sodium tert-butoxide (1.93 g, 20.1 mmol),
palladium(II) acetate (15 mg, 0.0668 mmol), tri-tert-butylphosphine
(11 mg, 0.0544 mmol, added from stock solution in anhydrous
toluene), and anhydrous toluene (25.0 mL) were reacted for 2 h at
reflux. 7.06 g of isolated material was collected (87% yield): 1H NMR
(400 MHz, C6D6) δ 7.43, (d, J = 8.77 Hz, 4H), 7.11 (d, J = 8.77 Hz,
integration obscured by solvent), 7.15−7.08 (m, integration obscured
by solvent), 6.94 (d, J = 7.99 Hz, 4H) 6.72 (d, J = 8.96 Hz, 4H), 3.30
(s, 6H), 2.12 (s, 6H); 13C NMR (100 MHz, C6D6) δ 156.7, 147.9,
146.3, 141.5, 134.4, 132.1, 130.3, 127.6, 127.4, 124.4, 123.0, 115.2,
55.0, 20.8; HRMS (EI) [M+] calcd for C40H36N2O2 576.2777, found
576.2769.
Compound 3a.23 Aniline (5.98 g, 64.2 mmol), 4-bromoanisole
(10.0 g, 53.5 mmol), sodium tert-butoxide (7.71 g, 80.2 mmol),
bis(dibenzylideneacetone)palladium (154 mg, 0.268 mmol), tri-tert-
butylphosphine (43.0 mg, 0.212 mmol, as a 10 g/L solution in
toluene), and anhydrous toluene (75.0 mL) were added to a round-
bottom flask. The flask was refluxed for 2 h under argon gas. Once
cool, acid-washed clay (montmorillonite K10 from Sigma Aldrich, 10.0
g) and standard basic alumina (1.00 g) were added to the slurry and
stirred for 30 min. The slurry was filtered, and the clear light yellow
mother liquor was collected. This organic phase was washed with 10%
HCl solution three times and with brine once and then dried over
magnesium sulfate. The organic phase was concentrated under vacuum
and recrystallized from boiling heptanes to yield small silver needles.
Compound 4c. Using the same general procedure as for
compound 4a, compound 3c (6.41 g, 28.2 mmol), 4,4′-dibromobi-
phenyl (4.00 g, 12.8 mmol), sodium tert-butoxide (3.08 g, 32.0 mmol),
palladium(II) acetate (115 mg, 0.512 mmol), tri-tert-butylphosphine
(83 mg, 0.410 mmol, added from stock solution in anhydrous
toluene), and anhydrous toluene (25.0 mL) were reacted for 3 h at
reflux. The compound precipitated poorly in methanol and was
purified by column chromatography over silica gel eluting with 1:1
cyclohexane/toluene. A fine white powder was collected (5.21 g,
1
7.43 g of product was isolated (67% yield): H NMR (400 MHz,
C6D6) δ 7.23 (t, J = 7.99 Hz, 2H), 7.09 (d, J = 8.57 Hz, 2H), 6.95−
6.82 (m, 5H), 5.50 (s, broad, 1H), 3.81 (s, 3H).
Compound 3b.24 Using the same general procedure as for
compound 3a above, p-toluidine (6.88 g, 64.2 mmol), 4-bromoanisole
(10.0 g, 53.5 mmol), sodium t-butoxide (7.71 g, 80.2 mmol),
bis(dibenzylideneacetone)palladium (154 mg, 0.268 mmol), tri-tert-
butylphosphine (43 mg, 0.212 mmol), and anhydrous toluene (75.0
mL) were reacted at reflux for 2 h. Product recrystallized from
heptanes to yield light brown flakes. 7.26 g of pure product was
1
67%): H NMR (400 MHz, C6D6) δ 7.44 (d, J = 8.96 Hz, 4H), 7.22
(d, J = 8.77 Hz, 4H), 7.15 (d, integration and coupling obscured by
residual solvent peak), 7.11 (d, J = 2.14 Hz, 2H), 7.03 (dd, J1 = 8.18
Hz, J2 = 2.14 Hz, 2H), 6.94 (d, J = 8.18 Hz, 2H), 6.73 (d, J = 8.96 Hz,
4H), 3.30 (s, 6H), 2.03 (s, 6H), 1.93 (s, 6H). 13C NMR (100 MHz,
C6D6) δ 156.6, 148.0, 146.6, 141.7, 137.7, 134.3, 131.0, 130.9, 127.6,
127.3, 126.0, 122.8, 122.4, 115.2, 55.0, 19.8, 19.1; HRMS (ESI) [M+]
calcd for C42H40N2O2 604.3090, found 604.3100.
Compound 4d. Using the same general procedure as for
compound 4a, bis(4-methoxyphenyl)amine (3.82 g, 16.8 mmol),
4,4′-dibromobiphenyl (2.50 g, 8.01 mmol), sodium t-butoxide (1.92 g,
20.0 mmol), palladium(II) acetate (72 mg, 0.321 mmol), tri-tert-
butylphosphine (52 mg, 0.257 mmol, added from stock solution in
anhydrous toluene), and anhydrous toluene (20.0 mL) were reacted
for 3 h at reflux. The compound was precipitated into methanol and
recrystallized from EtOAc. Slightly yellow flakes were collected (3.33
g, 70%): 1H NMR (400 MHz, C6D6) δ 7.46 (d, J = 8.61 Hz, 4H), 7.17
(m, obscured by residual solvent), 7.12 (d, J = 9.00 Hz, 8H), 6.74 (d, J
= 9.00 H, 8H), 3.31 (s, 12H); 13C NMR (100 MHz, C6D6) δ 156.4,
148.2, 141.7, 134.0, 127.6, 126.8, 122.0, 115.17, 55.0.
1
isolated (64% yield): H NMR (400 MHz, CDCl3) δ 7.06−7.00 (m,
4H), 6.88−6.82 (m, 4H), 5.39 (s, broad, 1H), 3.80 (s, 3H), 2.28 (s,
3H).
Compound 3c.25 Using the same general procedure as for
compound 3a above, 3,4-dimethylaniline (3.56 g, 29.4 mmol), 4-
bromoanisole (5.00 g, 26.7 mmol), sodium t-butoxide (3.84 g, 40.0
mmol), bis(dibenzylideneacetone)palladium (154 mg, 0.268 mmol),
tri-tert-butylphosphine (43 mg, 0.212 mmol), and anhydrous toluene
(35.0 mL) were refluxed for 2 h. Product recrystallized from heptanes
to yield silver needles. 4.50 g of pure product was isolated (67% yield):
1H NMR (400 MHz, CDCl3) δ 7.05−6.95 (m, 3H), 6.83 Hz (d, J =
8.96 Hz, 2H), 6.76−6.64 (m, 2H), 5.33 (s, broad, 1H), 3.78 (s, 3H),
2.19 (s, 6H).
Compound 3d. Using the same general procedure as for
compound 3a above, 2-bromo-6-methoxynaphthalene (10.0 g, 42.2
mmol), para-anisidine (6.23 g, 50.6 mmol), sodium t-butoxide (5.95 g,
61.9 mmol), bis(dibenzylideneacetone)palladium (119 mg, 0.207
mmol), tri-tert-butylphosphine (33 mg, 0.163 mmol), and anhydrous
toluene (50.0 mL) were refluxed for 1 h. Product recrystallized from
heptanes to yield fine gray needles (5.54 g, 47% yield): 1H NMR (400
MHz, CDCl3) δ 7.53 (d, J = 9.00 Hz, 1H), 7.39 (d, J = 0.99 Hz, 1H),
7.18 (dd, J1 = 11.35 Hz, J2 = 2.74 Hz, integration obscured by residual
solvent peak), 7.02−6.91 (m, 4H), 6.79 (d, J = 8.61 Hz, 2H), 5.01 (s,
broad, 1H), 3.44 (s, 3H), 3.37 (s, 3H). NMR (100 MHz, C6D6) δ
156.81, 156.09, 141.91, 137.09, 131.20, 130.49, 128.60, 128.50, 122.48,
120.45, 119.88, 115.42, 111.25, 106.85, 55.48, 55.19; HRMS (ESI) [M
+ H] calcd for C18H18NO2 280.1338, found 280.1346.
Compound 4e. Using the same general procedure as for
compound 4a, 3d (2.43 g, 8.71 mmol), 4,4′-dibromobiphenyl (1.35
g, 4.34 mmol), sodium t-butoxide (1.29 g, 13.4 mmol), palladium(II)
acetate (13 mg, 0.0579 mmol), tri-tert-butylphosphine (9 mg, 0.046
mmol), and anhydrous toluene (15.0 mL) were refluxed for 1 h.
Compound was precipitated into methanol to yield a fine white
powder. The product was further purified by recrystallization from
1
toluene/EtOAc to yield small white crystals (2.66 g, 86% yield): H
NMR (400 MHz, C6D6) δ 7.56 (d, J = 1.96 Hz, 2H), 7.52 (d, J = 9.00
Hz, 2H), 7.47 (d, J = 8.61 Hz, 4H), 7.42 (dd, J1 = 9.00 Hz, J2 = 2.35
HZ, 2H), 7.27−7.22 (m, 6H), 7.17−7.09 (m, integration obscurd by
residual solvent peak), 6.94 (d, J = 2.35 Hz, 2H), 6.75 (d, J = 9.00 Hz,
4H), 3.40 (s, 6H), 3.32 (s, 6H); 13C NMR (100 MHz, C6D6) δ 157.8,
157.1, 148.2, 144.8, 144.8, 135.0, 131.9, 130.9, 129.3, 128.5, 128.1,
127.8, 125.6, 123.6, 121.1, 119.8, 115.7, 106.6, 55.4, 55.2; HRMS
(ESI) [M + H] calcd for C48H41N2O4 709.3066, found 709.3048.
Compound 5a. Using the general P−R procedure, 4a (0.500 g
0.937 mmol), tris(pentafluorophenyl)borane (5 mg, 0.00977 mmol),
Compound 4a.26 Compound 3a (3.00 g, 15.1 mmol), 4,4′-
dibromobiphenyl (2.24 g, 7.17 mmol), sodium tert-butoxide (2.07 g,
21.5 mmol), palladium(II) acetate (16 mg, 0.0713 mmol), tri-tert-
butylphosphine (16 mg, 0.0791 mmol, added as a stock solution in
toluene), and anhydrous toluene (25 mL) were added to a round-
bottom flask. This solution was refluxed for 1 h under an argon
atmosphere. Upon cooling, acid-washed clay (montmorillonite K10
from Sigma Aldrich, 2.00 g) and standard activity basic alumina (0.500
g) were added and allowed to stir into the mixture for 10 min. The
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and 5.00 mL of toluene were stirred, and pentamethyldisiloxane (1.35
g, 9.11 mmol) was added dropwise. The reaction proceeded vigorously
and was allowed to stir for an additional 20 min after bubbling had
ceased. The product was purified by column chromatography over
silica gel eluting with 5:2 cyclohexane/toluene. The product was
(400 MHz, C6D6) δ 7.43 (d, J = 8.77 Hz, 8H), 7.16 (multiplicity and
integration obscured by solvent peak), 7.09 (d, J = 8.77 Hz, 8H), 7.00
(d, J = 8.96 Hz, 4H), 0.28 (s, 12H), 0.16 (s, 72H); 13C NMR (100
MHz, C6D6) δ 150.6, 148.0, 142.8, 134.3, 127.6, 128.5, 122.7, 121.0,
1.7, −3.0; HRMS (ESI) [M+] calcd for C64H108N2O12Si12 1432.5134,
found 1432.5153.
1
isolated as a viscous pale yellow oil (0.391 g, 51% yield): H NMR
(400 MHz, C6D6) δ 7.38 (d, J = 8.77 Hz, 4H), 7.16−7.12 (m,
integration obscured by solvent), 7.12−7.02 (8H), 6.92 (d, J = 9.16
Hz, 4H), 6.84 (t, J = 7.21 Hz, 2H), 0.23 (s, 12H), 0.12 (s, 18H); 13C
NMR (100 MHz, C6D6) δ 151.5, 148.7, 147.6, 142.2, 135.0, 129.6,
127.7, 127.3, 123.9, 123.9, 122.5, 121.1, 1.8, −0.2; HRMS (ESI) [M+]
calcd for C46H56N2O4Si4 812.3317, found 812.3300.
Compound 5g. Using the general P−R procedure, 4e (0.500 g,
0.705 mmol), pentamethyldisiloxane (1.05 g, 7.06 mmol), tris-
(pentafluorophenyl)borane (4 mg, 0.00781 mmol), and toluene
(10.0 mL) were reacted. Compound was purified by column
chromatography eluting with 1:1 cyclohexane/toluene to yield a
1
slightly green powder (0.426 g, 49% yield): H NMR (400 MHz,
Compound 5b. Using the general P−R procedure, 4b (0.500 g,
0.867 mmol), pentamethyldisiloxane (1.29 g, 8.67 mmol), tris-
(pentafluorophenyl)borane (5 mg, 0.00977 mmol), and anhydrous
toluene (5.00 mL) were reacted at room temperature; the reaction
proceeded vigorously. The compound was purified by column
chromatography eluting with 5:2 cyclohexane/toluene. The product
C6D6) δ 7.54 (d, J = 2.14 Hz, 2H), 7.47 (d, J = 9.2 Hz, 2H), 7.44 (d, J
= 8.77 Hz, 4H), 7.42 (d, J = 2.34 Hz, 2H), 7.34 (dd, J1 = 8.96 Hz, J2 =
2.34 Hz, 2H), 7.26 (d, J = 8.96 Hz, 2H), 7.20 (d, J = 8.77 Hz, 4H),
7.16 (obscured by solvent peak), 7.11 (d, J = 8.77 Hz, 4H), 6.97 Hz
(d, J = 8.96 Hz, 4H), 0.27 (s, 12H), 0.24 (s, 12H), 0.14 (s, 12H), 0.13
(s, 12H); 13C NMR (100 MHz, C6D6) δ 152.5, 151.7, 148.1, 145.0,
142.7, 135.2, 132.1, 131.4, 129.3, 127.4, 125.8, 124.0, 122.8, 121.4,
121.4, 115.6, 2.2, 2.2, 0.2, 0.1; HRMS (ESI) [M + H] calcd for
C64H89N2O8Si8 1237.4773, found 1237.4715.
1
was isolated as a viscous and pale yellow oil (0.350 g, 48% yield): H
NMR (400 MHz, C6D6) δ 7.40 (d, J = 8.77 Hz, 4H), 7.18 (m,
obscured by solvent), 7.13−7.06 (m, 8H), 6.97−6.89 (m, 8H), 2.11 (s,
6H), 0.23 (s, 12H), 0.12 (s, 18H); 13C NMR (100 MHz, C6D6) δ
151.2, 147.8, 146.2, 142.5, 134.6, 132.2, 130.3, 127.6, 126.9, 124.7,
123.3, 121.0, 20.8, 1.8, −0.1; HRMS (ESI) [M+] calcd for
C48H60N2O4Si4 840.3630, found 840.3639.
Compound 5h. Using the general P−R procedure, 4e (400 mg,
0.564 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (748 mg, 3.36
mmol), tris(pentafluorophenyl)borane (3 mg, 0.00587 mmol), and
toluene (10.0 mL) were reacted. Compound purified by column
chromatography eluting with 1:1 cyclohexane/toluene to yield a
Compound 5c. Using the general P−R procedure, 4c (0.500 g,
0.827 mmol), pentamethyldisiloxane (0.490 g, 3.30 mmol), tris-
(pentafluorophenyl)borane (4 mg, 0.00781 mmol), and anhydrous
toluene (5.00 mL) were reacted. The product was purified by column
chromatography eluting with 5:2 cyclohexane/toluene. The product
1
viscous clear oil (803 mg, 93% yield): H NMR (400 MHz, C6D6) δ
7.53−7.47 (m, 6H), 7.43 (d, J = 8.61 Hz, 4H), 7.34 (dd, J1 = 9.00 Hz,
J2 = 1.96 Hz, 2H), 7.30 (d, J = 9.00 Hz, 2H), 7.22 (dd, J1= 9.00 Hz, J2
=2.35 Hz, 2H), 7.19 (d, J = 8.61 Hz, 4H), 7.07 (d, J = 9.00 Hz, 4H),
7.01 (d, J = 9.00 Hz, 4H), 0.32 (s, 6H), 0.29 (s, 6H), 0.17 (s, 72H);
13C NMR (100 MHz, C6D6) δ 152.0, 151.3, 148.0, 145.0, 142.8, 135.2,
132.0, 131.4, 129.2, 127.5, 125.7, 124.0, 122.6, 121.5, 121.1, 115.5,
2.12, 2.10, −2.6, −2.7; HRMS (ESI) [M + H] calcd for
C72H113N2O12Si12 1533.5519, found 1533.5583.
1
was isolated as a viscous and pale yellow oil (0.467, 65% yield): H
NMR (400 MHz, C6D6) 7.41 (d, J = 8.96 Hz, 4H), 7.2 (d, J = 8.57 Hz,
4H), 7.13 (d, 8.96 Hz, 4H), 7.08 (d, J = 2.14 Hz, 2H), 7.01 (dd, J1 =
5.65 Hz, J2 = 2.14 Hz, 2H), 6.97−6.91 (m, 6H), 2.02 (s, 6H), 1.92 (s,
6H), 0.23 (s, 12H), 0.12 (s, 18H); 13C NMR (100 MHz, C6D6) δ
151.1, 147.9, 146.5, 142.7, 137.7, 134.5, 131.2, 130.9, 127.6, 126.8,
126.3, 123.2, 122.7, 121.0, 19.8, 19.1, 1.9, −0.1; HRMS (ESI) [M+]
calcd for C50H64N2O4Si4 868.3943, found 868.3930.
Compound 6.31 2-Bromobiphenyl (5.00 g, 21.4 mmol) in
anhydrous THF (11.0 mL) was reacted with magnesium (0.567 g,
23.3 mmol) under inert gas at room temperature. Upon formation of
the Grignard reagent, 9-fluorenone (3.90 g, 21.6 mmol in 5.00 mL of
THF) was added, and the solution was refluxed for 4 h and allowed to
cool. Upon cooling, a yellow precipitate was formed and collected,
washing with cold methanol. This solid was stirred into a 5% HCl
solution (22.0 mL) for 2 h at room temperature and washed with
additional methanol. Finally, the solid was dissolved in acetic acid
(22.0 mL) and refluxed for 40 min. Upon cooling, large white crystals
of the pure product were obtained (3.73 g, 56% yield).
Compound 5d. Using the general P−R procedure, 4d (0.500 g,
0.821 mmol), pentamethyldisiloxane (0.975 g, 6.57 mmol), tris-
(pentafluorophenyl)borane (4 mg, 0.00781 mmol), and 10.0 mL of
anhydrous toluene were reacted. The product was purified by passing
the compound through a plug of silica gel eluting with 1:1 hexanes/
toluene. A clear oil was obtained, which slowly crystallized to soft
1
white crystals after several days (0.860 g, 92% yield): H NMR (400
MHz, C6D6) δ 7.40 (d, J = 8.77 Hz, 4H), 7.14 (partially obscured by
solvent peak), 7.08 (d, J = 8.96 Hz, 8H), 6.93 (d, J = 8.96 Hz, 8H),
0.23 (s, 24H), 0.12 (s, 36H); 13C NMR (100 MHz, C6D6) δ 151.0,
148.0, 142.6, 134.3, 127.6, 126.5, 122.8, 121.0, 1.8, −0.2; HRMS (ESI)
[M+] calcd for C56H84N2O8Si8 1136.4382, found 1136.4368.
Compound 7.31 Compound 6 (1.00 g, 3.16 mmol), FeCl3 (2 mg,
0.0123 mmol), and 6 mL of chloroform were mixed under an inert
atmosphere. Molecular bromine (2.02 g, 12.6 mmol) in 2.00 mL of
chloroform was added, and the reaction was allowed to proceed at
room temperature in the absence of light for 5 days. Upon completion,
the reaction was quenched with aqueous ammonium hydroxide and
recrystallized from chloroform/ethanol (50/50) to yield a pure white
crystalline product (1.10 g, 55% yield).
Compound 8. Palladium(II) acetate (5 mg, 0.0223 mmol), 10
(200 mg, 0.316 mmol), 3b (340 mg, 1.59 mmol), sodium tert-butoxide
(145 mg, 1.51 mmol), and 2.30 mL of anhydrous toluene were added
to a round-bottom flask under an atmosphere of argon. Once sealed,
tri-tert-butylphosphine (3.75 mg, 0.0185 mmol) was added as a
toluene stock solution. The mixture was refluxed for 5 h under argon
and allowed to cool. The solution was filtered through a short plug of
silica to yield a clear yellow liquid; this solution was concentrated and
precipitated into stirring methanol to yield the product as a fine white
powder (258 mg, 67% yield): 1H NMR (400 MHz, C6D6) δ 7.12 (dd,
J1 = 7.82 Hz, J2 = 2.93 Hz, 8H), 7.09−7.01 (m, 16H), 7.00 (dd, J1 =
8.22 Hz, J2 = 2.35 Hz, 4H), 6.95 (d, J = 8.22, 8H), 6.71 (d, J = 9.39 Hz,
8H), 3.25 (s, 12H), 2.05 (s, 12H); 13C NMR (100 MHz, C6D6) δ
156.7, 151.1, 148.2, 146.9, 141.94, 135.6, 131.7, 130.3, 127.2, 124.0,
123.5, 121.1, 119.2, 115.4, 66.8, 55.3, 21.1. MS (ESI) [M+] calcd for
C81H68N4O8 1160.5, found 1160.5
Compound 5e. Using the general P−R procedure, 4b (0.500 g,
0.867 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (1.930 g, 8.67
mmol), tris(pentafluorophenyl)borane (4 mg, 0.00781 mmol), and
anhydrous toluene (5 mL) were reacted at room temperature; the
reaction proceeded rapidly. The compound was purified by column
chromatography eluting with 5:2 cyclohexane/toluene. The final
1
product was isolated as a viscous clear oil (0.592 g, 69% yield): H
NMR (400 MHz, C6D6) δ 7.41 (d, J = 8.77 Hz, 4H), 7.18 (d, J = 8.96
Hz, integration obscured by solvent), 7.13−7.07 (m, 8H), 7.00 (d, J =
8.96 Hz, 4H), 6.92 (d, J = 8.77 Hz, 4H), 2.11 (s, 6H), 0.28 (s, 6H),
0.17 (s, 36H); 13C NMR (100 MHz, C6D6) δ 150.8, 147.8, 146.2,
142.6, 134.6, 132.2, 130.3, 127.7, 126.9, 124.6, 123.2, 121.0, 20.8, 1.7,
−3.0; HRMS (ESI) [M+] calcd for C52H72N2O6Si6 988.4006, found
988.3995.
Compound 5f. Using the general P−R procedure, 4d (0.500 g,
0.821 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (1.46 g, 6.57 mmol),
tris(pentafluorophenyl)borane (4 mg, 0.00781 mmol), and anhydrous
toluene (10.0 mL) were reacted. Compound was purified by column
chromatography eluting with 1:1 cyclohexane/toluene. The final
product was isolated as a pale yellow oil (1.06 g, 90% yield): 1H NMR
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Compound 9. Using the general P−R procedure, 11 (200 mg,
0.172 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (445 mg, 2.00
mmol), 2.00 mL of toluene, and tris(pentafluorophenyl)borane (1
mg, 0.00195 mmol) were reacted at room temperature. Compound
was purified by column chromatography on silica gel eluting with
hexanes/toluene. Product was isolated as a glass (294 mg, 86% yield):
1H NMR (400 MHz, C6D6) δ 7.18 (signal obscured by solvent), 7.09
(d, J = 2.35 Hz, 4H), 7.05−6.95 (m, 36H), 2.05 (s, 12H), 0.28 (s,
12H), 0.14 (s, 72H). 13C NMR (100 MHz, C6D6) δ 151.0, 150.9,
148.1, 146.6, 143.0, 136.4, 132.2, 130.4, 126.8, 124.3, 123.5, 121.2,
120.8, 118.8, 66.8, 21.2, 2.1, −2.6; MS (ESI) [M + 3H] calcd for
C106H145N4O11Si12 1984.8, found 1987.8.
remaining solids were taken up in toluene and washed with water and
brine. The organic toluene phase was dried over magnesium sulfate
and filtered through a short plug of silica gel, eluting with toluene. This
light yellow liquid was then concentrated under vacuum until it
resembled a viscous oil. Hexanes (5.00 mL) were added to this
resulting oil, and the solution was cooled in a refrigerator overnight.
Light-yellow crystals were collected and washed with cold hexanes and
then dried under vacuum (2.55 g collected, 65% yield). This product
was used without further purification: 1H NMR (400 MHz, CDCl3) δ
7.34−7.30 (m, 2H), 7.21 (d, J = 8.96 Hz, 2H), 7.13 (dd, J1 = 8.57 Hz,
J2 = 2.73 Hz, 1H), 6.96 (d, J = 8.96 Hz, 2H), 3.89 (s, 3H), 3.84 (s,
3H); 13C NMR (100 MHz, CDCl3) δ 159.5, 159.0, 132.9, 129.6,
129.3, 128.4, 118.8, 114.3, 109.0, 104.9, 56.1, 55.4; HRMS (ESI) [M+]
calcd for C14H13NO4 259.0845, found 259.0845.
Compound 10. Tris(4-bromophenyl)amine (0.500 g, 1.04 mmol),
compound 3c (0.786 g, 3.46 mmol), sodium t-butoxide (0.444 g, 4.63
mmol), palladium(II) acetate (5 mg, 0.00223 mmol), tri-tert-
butylphosphine (3 mg, 0.00148 mmol), and anhydrous toluene
(10.0 mL) were refluxed for 16 h. Upon cooling, the mixture was
filtered, and the resulting liquor was concentrated and precipitated into
Compound 13a.33 Compound 12a (2.20 g, 9.56 mmol) was
added to a stirred round-bottom flask, and the flask was flushed with
argon gas and sealed under a positive pressure of gas. Triethylphos-
phite (7.70 mL) was added via syringe, and the solution was heated at
160 °C for 16 h. The reaction was allowed to cool, and 10.0 mL of
methanol was added to the solution. This was allowed to rest in a
refrigerator overnight to yield white square crystals. These crystals
were collected by filtration and washed sparingly with cold methanol.
The collected crystals were then dried under vacuum (1.69 g was
1
methanol to yield a light yellow fine powder (742 mg, 78% yield): H
NMR (400 MHz, C6D6) δ 7.16−7.12 (m, integration obscured by
residual solvent), 7.10−7.06 (m, 9H), 7.02 (dd, J1 = 8.02 Hz, J2 = 2.54
Hz, 3H), 6.92 (d, J = 8.22 Hz, 3H), 6.71 (d, J = 9.00 Hz, 6H); 13C
NMR (100 MHz, C6D6) δ 156.6, 147.2, 144.3, 143.1, 142.3, 137.9,
131.1, 130.8, 127.2, 125.7, 125.5, 124.5, 122.0, 115.5, 55.4, 20.2, 19.4;
HRMS (ESI) [M + H] calcd for C63H61N4O3 921.4744, found
921.4765.
1
isolated, 56% yield): H NMR (400 MHz, (CD3)2SO) δ 11.10 (s,
broad, 1H), 7.98 (m, 2H), 7.41 (d, J = 7.82 Hz, 1H), 7.28 (td, J1 = 7.63
Hz, J2 = 1.17 Hz, 1H), 7.10 (td, J1 = 7.82 Hz, J2 = 1.17 Hz, 1H), 6.96
(d, J = 2.35, 1H), 6.76 (dd, J1 =8.61 Hz, J2 = 2.35 Hz, 1H), 3.84 (s,
3H); 13C NMR (100 MHz, (CD3)2SO) δ 158.4, 141.0, 139.7, 124.0,
122.6, 120.9, 119.2, 118.5, 116.1, 110.6, 107.6, 94.4, 55.2; HRMS
(ESI) [M + H] calcd for C13H12NO 198.0913, found 198.0906.
Compound 13b.33 Compound 12b (2.55 g, 9.81 mmol) and
triethylphosphite (8.90 mL) were heated at 160 °C for 16 h under
argon. The reaction was allowed to cool, and 10.0 mL of methanol was
added to the solution. This was allowed to rest in a refrigerator
overnight to yield small white needles. These crystals were collected by
filtration and washed sparingly with cold methanol. The resulting
Compound 11. Using the general P−R procedure, compound 10
(0.349 g, 0.379 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (0.871 g,
3.91 mmol), tris(pentafluorophenyl)borane (2 mg, 0.00391 mmol),
and 5.00 mL of toluene were reacted for 16 h under ambient
conditions. Product was purified by column chromatography eluting
with hexanes/toluene and isolated as a white crystalline solid (269 mg,
46% yield): 1H NMR (400 MHz, C6D6) δ 7.14−7.09 (m, 12H), 7.08−
7.04 (m, 9H), 7.02−6.95 (m, 9H), 6.91 (d, J = 8.22 Hz, 3H), 2.01 (s,
9H), 1.92 (s, 9H), 2.07 (s, 9H), 0.15 (s, 54H); 13C NMR (100 MHz,
C6D6) δ 150.7, 147.1, 144.2, 143.3, 143.1, 137.9, 131.1, 130.9, 126.7,
126.0, 125.4, 124.7, 122.3, 121.3, 20.15, 19.4, 2.1, −2.7; HRMS (ESI)
[M + H] calcd for C81H115N4O9Si9 1539.6587, found 1539.6546.
Compound 12a.33 In an inert atmosphere glovebox, 4-bromo-3-
nitroanisole (3.00 g, 12.9 mmol), phenylboronic acid (1.73 g, 14.2
mmol), cesium fluoride (3.93 g, 25.8 mmol), bis-
(dibenzylideneacetone)palladium (149 mg, 0.258 mmol), tri-tert-
butylphosphine (52 mg, 0.258 mmol, added as a 10 g/L stock
solution in anhydrous toluene), and anhydrous tetrahydrofuran (60.0
mL) were added to a stirred round-bottom flask. This flask was
allowed to stir at ambient temperature for 24 h. The resulting solution
was removed from the glovebox, and the solids were filtered, eluting
with THF. This light brown solution was dried under vacuum. The
remaining solids were taken up in toluene and washed with water and
brine. The organic toluene phase was dried over magnesium sulfate
and filtered through a short plug of silica gel, eluting with toluene. This
light-yellow liquid was concentrated under vacuum until it resembled a
viscous oil. Hexanes (5.00 mL) were added to this resulting oil to
precipitate light yellow crystals. These crystals were collected and
washed with cold hexanes and then dried under vacuum (2.44 g
collected, 85% yield): 1H NMR (400 MHz, CDCl3) δ 7.43−7.32 ppm
(m, 5H), 7.30−7.25 (m, 2H), 7.15 (dd, J1 = 8.6 Hz, J2 = 2.7 Hz, 1H),
3.90 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 159.2, 137.4, 132.9,
128.80, 128.76, 128.2, 128.0, 118.8, 109.2, 56.1; HRMS (EI) [M+]
calcd for C13H11NO3 229.0733, found 229.0739.
1
white crystals were then dried under vacuum (1.83 g, 82% yield): H
NMR (400 MHz, (CD3)2SO) δ 10.94 (s, broad, 1H), 7.83 (d, J = 8.38
Hz, 2H), 6.92 (d, J = 2.14 Hz, 2H), 6.72 (dd, J1 = 8.38 Hz, J2 = 2.14
Hz, 2H); 13C NMR (100 MHz, (CD3)2SO) δ 157.5, 141.0, 119.9,
116.4, 107.3, 94.6, 55.2; HRMS (ESI) [M + H] calcd for C14H14NO2
228.1019, found 228.1011
Compound 14a. Iodobenzene (1.241 g, 6.08 mmol), copper(I)
iodide (77 mg, 0.404 mmol), compound 13a (0.800 g, 4.06 mmol), L-
proline (94 mg, 0.816 mmol), and potassium carbonate (1.12 g, 8.10
mmol) were added to a stirred round-bottom flask. DMSO (7.00 mL)
was added to the flask, and the solution was sparged with argon gas for
30 min. The solution was heated to 180 °C for 1 h. After the solution
was cool, toluene was added (25.0 mL), and any solids were removed
by filtration. The light brown liquor was concentrated under high
vacuum to completely remove any solvent and excess iodobenzene.
The resulting solids were taken up in toluene and passed through a
short plug of silica, eluting with toluene. Solvent was removed under
vacuum, resulting in large flakey white crystals. These crystals were
recrystallized from boiling heptanes and dried under vacuum. 794 mg
was isolated (72% yield): 1H NMR (400 MHz, CDCl3) δ 8.04 (dt, J1 =
7.83 Hz, J2 = 1.17 Hz, 1H), 8.00 (d, J = 8.22 Hz, 1H), 7.64−7.53 (m,
4H), 7.47 (m, 1H), 7.36−7.28 (m, 2H), 7.25 (m, 1H), 6.88 (td, J1 =
10.96 Hz, J2 = 2.35 Hz, 2H), 3.83 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 159.1, 142.2, 141.1, 137.7, 129.90, 127.5, 127.1, 124.6,
123.5, 121.0, 112.0, 119.4, 117.2, 109.5, 108.5, 94.0, 55.6; HRMS
(ESI) [M + H] calcd for C19H16NO 274.1226, found 274.1227.
Compound 14b. Same general procedure as for compound 14a
was used. Iodobenzene (1.0 g, 4.90 mmol), compound 13b (0.750 g,
3.27 mmol), copper(I) iodide (63 mg, 0.331 mmol), L-proline (76 mg,
0.660 mmol), potassium carbonate (912 mg, 6.60 mmol), and 7.00 mL
of DMSO were reacted for 2 h. Product recrystallized from heptanes
Compound 12b.33 In an inert atmosphere glovebox, 4-bromo-3-
nitroanisole (3.47 g, 15.0 mmol), 4-methoxyphenylboronic acid (2.50
g, 16.5 mmol), cesium fluoride (7.52 g, 49.5 mmol), bis-
(dibenzylideneacetone)palladium (259 mg, 0.45 mmol), tri-tert-
butylphosphine (91 mg, 0.450 mmol, added as a 10 g/L stock
solution in anhydrous toluene), and anhydrous tetrahydrofuran (30.0
mL) were added to a stirred round-bottom flask. This flask was
allowed to stir at ambient temperature for 24 h. The resulting solution
was removed from the glovebox, and the solids were filtered, eluting
with THF. This light brown solution was dried under vacuum. The
1
to yield white flakes. 821 mg of product was isolated (82% yield): H
NMR (400 MHz, CDCl3) δ 7.89 (d, J = 8.57 Hz, 2H), 7.61 (t, J = 7.41
Hz, 2H), 7.54 (d, J = 8.38 Hz, 2H), 7.47 (t, J = 7.41, 1H), 6.87 (dd, J1
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Article
= 8.38 Hz, J2 = 2.34 Hz, 2H), 6.82 (d, J = 2.34 Hz, 2H), 3.82 (s, 6H);
13C NMR (100 MHz, CDCl3) δ 158.3, 142.3, 137.6, 130.0, 127.5,
127.1, 120.1, 117.4, 108.2, 94.3, 55.7; HRMS (ESI) [M + H] calcd
C20H18NO2 304.1332, found 304.1340.
J = 7.00 Hz, 2H), 7.21 (dq, J1 = 7.80 Hz, J2 = 1.00 Hz, 1H), 7.17 (t, J =
1.95 Hz, 1H), 7.06 (ddd, J1 = 7.60 Hz, J2 = 2.34 Hz, J3 = 0.80 Hz,1H);
13C NMR (100 MHz, CDCl3) δ 155.5, 140.8, 138.6, 130.3, 125.9,
123.3, 120.3, 120.2, 119.8, 119.0, 118.7, 109.9, 1.6, −3.3; HRMS (ESI)
[M+] calcd for C25H33NO3Si3 479.1768, found 479.1755.
Compound 14c. Same general procedure as for compound 14a
was used. 3-Iodoanisole (10.92 g, 46.6 mmol), carbazole, (6.00 g, 35.9
mmol), copper(I) iodide (684 mg, 3.59 mmol), L-proline (827 mg,
7.18 mmol), potassium carbonate (9.92 g, 71.8 mmol), and 60.0 mL of
DMSO were reacted for 3 h. Product recrystallized from heptanes and
Compound 15d. Using the general P−R procedure, compound
14d (0.500 g, 1.83 mmol), tris(pentafluorophenyl)borane (10 mg,
0.195 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (0.814 g, 3.66
mmol), and anhydrous toluene (5.00 mL) were reacted. Product
was purified by loading onto a silica plug, washing first with hexanes
and then washing with toluene. Product was isolated as a clear oil
which crystallized into soft white needles after 2 months of storage
(728 mg, 83% yield): 1H NMR (400 MHz, CDCl3) δ 8.16 (d, J = 7.43
Hz, 2H), 7.45−7.38 (m, 4H), 7.34 (d, J = 7.82, 2H), 7.27 (t, J = 7.43
Hz, integration obscured by solvent peak), 7.14 (d, J = 8.61 Hz, 2H),
0.28 (s, 3H), 0.15 (s, 18H); 13C NMR (100 MHz, CDCl3) δ 153.6,
141.3, 131.1, 128.4, 125.81, 123.1, 121.1, 120.2, 119.6, 109.7, 1.6, −3.2;
HRMS (ESI) [M+] calcd for C25H33NO3Si3 479.1768, found 479.1776.
1
isolated as white flakes (7.75 g, 79% yield): H NMR (400 MHz,
CDCl3) δ 8.14 (d, J = 7.75 Hz, 2H), 7.50 (t, J = 8.09 Hz, 1H), 7.45 (d,
J = 7.99 Hz, 2H), 7.41 (t, d, J1 = 6.63 Hz, J2 = 1.36 Hz, 2H), 7.28 (ddd,
J1 = 7.75 Hz, J2 = 6.63 Hz, J3 = 1.36 Hz, 2H), 7.16 (dq, J1 = 7.80 Hz, J2
= 0.97 Hz, 1H), 7.11 (t, J = 2.24 Hz, 1H), 7.01 (ddd, J1 = 8.38 Hz, J2 =
2.53 Hz, J3 = 0.97 Hz, 1H), 3.86 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 160.8, 140.8, 138.8, 130.5, 125.9, 123.3, 120.3, 119.9, 119.3,
113.2, 112.6, 109.9, 55.5; HRMS (ESI) [M + H] calcd for C19H15NO
274.1226, found 274.1220.
Compound 14d. Same general procedure as for compound 14a
was used. Carbazole (1.00 g, 5.98 mmol), 4-bromoanisole (1.34 g, 7.18
mmol), copper(I) iodide (114 mg, 0.599 mmol), L-proline (138 mg,
1.20 mmol), potassium carbonate (1.653 g, 12.0 mmol), and DMSO
(10.0 mL) were added to a round-bottom flask. This solution was
sparged with argon gas for 30 min and then heated to 180 °C for 16 h.
The reaction was diluted with toluene, and the solids were filtered out.
The mother liquor was passed through a plug of silica and then
concentrated under high vacuum and heat to remove the solvents,
residual 4-bromoanisole and residual carbazole. The resulting solids
were recrystallized twice from boiling heptanes to yield long thin white
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, cyclic voltammetry and NMR
data for all compounds, and H and 19F data for catalyst
1
interaction experiments. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
1
needles. 685 mg were collected (42% yield): H NMR (400 MHz,
Corresponding Author
CDCl3) δ 8.14 (d, J1 = 7.70 Hz 2H), 7.45 (d, 8.96 Hz, 2H), 7.40 (m,
2H), 7.32 (d, J = 8.38 Hz, 2H), 7.27 (t, J = 7.02 Hz, 2H), 7.11 (d, J =
8.96 Hz, 2H), 3.92 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 158.8,
141.4, 130.3, 128.6, 125.8, 123.1, 120.2, 119.6, 115.0, 109.7, 55.6;
HRMS (ESI) [M+] calcd for C19H16NO 273.1148, found 273.1146.
Compound 15a. Using the general P−R procedure, tris-
(pentafluorophenyl)borane (10 mg, 0.0195 mmol), compound 14a
(0.500 g, 1.82 mmol), and anhydrous toluene (5.00 mL) were added
to a open stirred vessel. 1,1,1,3,5,5,5-Heptamethyltrisiloxane (1.623 g,
7.32 mmol) was added dropwise to this solution and allowed to stir for
20 min. Upon evaporation, a colorless oil was obtained. This oil was
diluted with hexanes and loaded onto a short plug of silica gel. This
plug was washed with hexanes (100 mL) and then washed with
toluene. The toluene fraction was collected and concentrated to yield a
colorless oil (834 mg, 95% yield): 1H NMR (400 MHz, C6D6) δ 8.00
(m, 1H), 7.93 (d, J = 8.61 Hz, 1H), 7.28−7.10 (m, integration
obscured by solvent peak), 7.04 (t, J = 7.43 Hz, 1H), 0.30 (s, 3H), 0.12
(s, 18H); 13C NMR (100 MHz, C6D6) δ 154.0, 142.8, 141.9, 138.2,
123.0, 127.5, 127.5, 125.2, 124.2, 121.4, 120.5, 120.1, 118.7, 113.8,
110.0, 101.0, 1.7, −3.1; HRMS (ESI) [M + H] calcd for C25H34NO3Si3
480.1841, found 480.1863.
*E-mail: tim.bender@utoronto.ca.
ACKNOWLEDGMENTS
■
We would like to thank the National Research Council of
Canada (NSERC) for funding of this project through the
Discovery Grant Program, The Bert Wasmund Fellowship
(UofT) and the Hatch Scholarship for Sustainable Energy
(B.K.), the NSERC Undergraduate Student Research Awards
program (B.M.), and the Siltech Corporation (Toronto,
Ontario, Canada) for their generous donation of materials.
REFERENCES
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yield): H NMR (400 MHz, C6D6) δ 7.85 (d, J = 8.22 Hz, 2H), 7.21
(d, J = 8.22 Hz, 2H), 7.19−7.11 (m, integration obscured by solvent),
7.04 (t, J = 7.43 Hz), 0.30 (s, 6H), 0.12 (s, 36H); 13C NMR (100
MHz, C6D6) δ 153.2, 143.1, 138.2, 130.1, 127.6, 127.6, 120.7, 118.9,
113.7, 101.1, 1.7, −3.1; HRMS (ESI) [M+] calcd for C32H53NO6Si6
715.2488, found 715.2486.
Compound 15c. Using the general P−R procedure, compound
14c (4.00 g, 14.6 mmol), tris(pentafluorophenyl)borane (75 mg, 0.146
mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (6.51 g, 29.2 mmol), and
toluene (40.0 mL) were reacted. Product was purified by loading onto
a silica plug, washing first with hexanes and then washing with toluene.
1
Product was isolated as a clear oil (6.04 g, 86% yield): H NMR (400
MHz, CDCl3) δ 8.17 (d, J = 7.79 Hz, 2H), 7.50−7.40 (m, 5H), 7.00 (t,
1673
dx.doi.org/10.1021/jo2020906 | J. Org. Chem. 2012, 77, 1663−1674

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