Modification and unexpected reactivity of 2-borylbenzaldimines: Acylated and silylated derivatives as well as dimeric compounds

Article abstract of DOI:10.1021/jo401748d

Various novel N-alkyl and N-benzyl 2-borylbenzaldimines 3 were prepared by condensation of 2-(dimesitylboryl)benzaldehyde (1) with amines. Further functionalization of compound 3e was possible by deprotonation and subsequent regioselective reaction with e

Full text of DOI:10.1021/jo401748d

Article
pubs.acs.org/joc
Modification and Unexpected Reactivity of 2‑Borylbenzaldimines:
Acylated and Silylated Derivatives as Well as Dimeric Compounds
Benedikt Neue,† Atsushi Wakamiya,‡ Roland Frohlich,† Birgit Wibbeling,† Shigehiro Yamaguchi,*,§
̈
and Ernst-Ulrich Wurthwein*,†
̈
†Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
‡Institute for Chemical Research, Kyoto University Uji, Gokasho, Kyoto 611-0011, Japan
§Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: Various novel N-alkyl and N-benzyl 2-borylbenzaldimines 3 were prepared by condensation of 2-
(dimesitylboryl)benzaldehyde (1) with amines. Further functionalization of compound 3e was possible by deprotonation and
subsequent regioselective reaction with electrophiles to give compounds 4. Applying similar conditions to 3a led to the
unexpected formation of hitherto unknown dimeric compounds (5 and 6). All structural types were fully characterized, including
by X-ray diffraction (XRD). Furthermore, quantum chemical calculations on the SCS-MP2 and DFT levels gave insights into the
reaction mechanisms and the stereoselectivity. The B/N bonding situation in these molecules was analyzed using Wiberg bond
indices. Preliminary UV−vis and fluorescence measurements indicate that the substitution reaction leading to compounds 4 can
be utilized to tune the photophysical properties of these compounds.
herein the synthesis of novel N-alkyl and benzyl B/N
heterocyclic compounds 3 with a CH2R group in the position
α to nitrogen, allowing for deprotonation to give borylated 2-
azaallyl- and 2-azapentadienyllithium compounds.25 In the case
of 3e the lithiated intermediate was reacted regioselectively with
various electrophiles, yielding the modified B/N compounds 4,
which show tunable UV−vis and fluorescence properties and
photochemical reactivity. In case of 3a a base-promoted, unusual
dimerization reaction leading to the hitherto unknown bis-B/N
heterocyclic compound 6 with a tri- as well as a tetravalent boron
atom was observed. The experimental results are accompanied
by quantum chemical calculations to give insight into the
reaction mechanism and to analyze the B/N bonding (Wiberg
bond indices26) as well as the electronic structures (NBO27) of
these novel molecules.
INTRODUCTION
■
The incorporation of main-group elements into organic materials
has attracted tremendous interest in the past decade.1−4 Boron
and nitrogen are two key players used frequently in novel main-
group-element-containing aromatic systems, and many research
groups have shown the drastic influence of the combination of
these complementary Lewis acidic and basic elements on the
photophysical and electronic properties in comparison to all-
carbon-containing materials.5−18 In previous work we have
focused our attention on the synthesis, molecular structures, and
photophysical properties of 2-borylbenzaldimines.19 Now, we
investigate possible methods for the chemical modification of
such compounds by introducing further substituents via
deprotonation and electrophilic attack and so influencing the
photophysical properties of the B/N systems. This is in contrast
to most of the previously published syntheses, where the labile
boron functionality was introduced in the last step.
Our group is well experienced in the preparation and
application of anionic intermediates: e.g., by deprotonation of
aza- and diazapolyenes for subsequent electrocyclic reac-
tions20−23 and for effective functionalization.24 Thus, we report
Received: August 9, 2013
Published: November 12, 2013
© 2013 American Chemical Society
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important compounds reported in this paper see the Supporting
Information).
In the H NMR spectrum of the typical example 3b the
RESULTS AND DISCUSSION
■
Preparation and Characterization of 2-Borylbenzaldi-
mines 3a−g. 2-(Dimesitylboryl)benzaldehyde (1) was used as
a precursor for the synthesis of the N-benzyl- and N-alkyl-
substituted 2-borylbenzaldimines 3a−g. The synthesis of 1 was
described previously by Kawashima et al. and Gabbaı et al.
The condensation reaction of 1, which might be regarded as an
internally Lewis acid activated aldehyde, with alkyl- and
allylamines and various electron-poor and electron-rich benzyl-
amines (2; 1.02−2.00 equiv) gave colorless or yellow
borylbenzaldimines 3a−g in moderate to good yields (Scheme
1, Table 1). Compounds 3a−g show fluorescence in the solid
state and in solution (see Influence of the Substitution Pattern on
the Photophysical Properties).
1
benzylic CH2 protons give rise to two sets of doublets at 4.50 and
4.74 ppm (geminal coupling constant 16.0 Hz). Their
diastereotopicity is due to the bulky mesityl groups causing
restricted rotation about the CH2−N bond. The aromatic signals
are found between 6.20 and 7.86 ppm. Among these signals, two
broad singlets are attributed to the aromatic protons of the
mesityl groups. The iminic proton shows a resonance as a singlet
at 7.55 ppm. This signal is found at high field in comparison to
that of imines not coordinated to a Lewis acid. A sharp singlet at
6.79 ppm in the 11B NMR spectrum (see the Supporting
Information) is an indication of the presence of a tetravalent
boron atom. In the 13C NMR spectrum of 3b the carbon atoms of
the mesityl CH3 groups show resonances between 21.0 and 27.0
ppm. The CH2 group gives a signal at 54.3 ppm. Between 125.2
and 144.0 ppm the signals of the aromatic carbon atoms are
detected. At 167.8 ppm the iminic carbon atom shows a
resonance, which is shifted downfield from that of imines without
a coordinating boron moiety. We also studied the behavior of the
compound in solvents with different polarities (C6D6, CD2Cl2,
and DMSO-d6). According to the recorded spectra the
coordination of nitrogen to the boron center seems not to be
affected by solvent polarity. These observations indicate that the
N−B heterocyclic structure is stable in polar and coordinating
solvents, even in water containing DMSO. This increased
stability might be advantageous, for example, for the preparation
of electron-transporting materials. The NMR spectra of the other
compounds 3a,c−g are similar to those of 3b; all of the 11B NMR
signals are found in the range 5−7 ppm.
Deprotonation of 2-Borylbenzaldimines 3e−g and
Subsequent Reaction with Electrophiles. We were
interested in further functionalizing the photoactive compounds
3 in order to modify their electronic structure and properties.
Therefore, in a first series of experiments the B/N heterocycle 3e
was treated with different strong bases and subsequently reacted
with electrophiles. Use of lithium diisoproylamide (LDA; 1.02
equiv) as base in tetrahydrofuran (THF), temperatures of −78
°C, and slow warming after addition of an excess of the
electrophile (2.00 equiv) turned out to be the best conditions for
the reaction. After the addition of the base the light yellow
reaction mixture turned orange-red, indicating the formation of
an intermediate anionic species; after the addition of the
electrophile, the color changed back to light yellow. After
aqueous workup and purification by recycling gel permeation
chromatography (GPC) or recrystallization the acylated and
silylated products 4a−d were obtained (Scheme 2, Table 2)).
16,19,28
̈
Scheme 1. Synthesis of Compounds 3a−g
Table 1. Yields of 3a−g
compd
R
yield, %
3a
3b
3c
3d
3e
3f
Ph
90
37
52
62
70
56
58
4-Cl-Ph
4-OMe-Ph
2,4-OMe-Ph
H
tBu
3g
CHCH2
We were able to grow single crystals of 3a,d−g and to analyze
their molecular structures by X-ray diffraction. Figure 1
Scheme 2. Synthesis of Compounds 4
Figure 1. Molecular structure of compound 3e in the solid state
(Schakal plot).
depictsas a typical examplethe crystal structure of 3e (for
the structures of 3a,d,f,g see the Supporting Information). The
N1−B1 distance amounts to 1.645 Å, clearly indicating a
coordinative interaction. A value of 1.289 Å was found for the
length of the C7N1 double bond. The Wiberg bond indices
(natural bond orbital (NBO) analysis at the B3LYP/6-31G(d)
level of theory) for 3eas an exampleare 0.6459 and 1.5837
for the B1−N1 and C7N1 bonds, respectively, quantifying this
strong Lewis acid−base interaction (for NBO data of all
Similar to the case for 3a−g the compounds 4a−d also show
fluorescence (see Influence of the Substitution Pattern on the
Photophysical Properties). Interestingly, only one regioisomer
was formed by electrophilic substitution at the aza-bora ring
system (vide infra, Mechanistic Investigation/Quantum Chem-
ical Calculations). For the typical example 4a characteristic
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Wiberg bond indices, which amount to 0.608 (N−B) and 1.592
(CN).
Table 2. Yields for the Synthesis of Substituted 2-
Borylbenzaldimes 4
Deprotonation of compounds 3f (R = tBu) and 3g (R = CH
CH2) and subsequent treatment with electrophiles were also
investigated (Table 2). We observed a typical color change from
colorless to deep blue during the deprotonation, indicating again
the in situ formation of an anionic species. Addition of an
electrophile changed the color to light yellow. After workup
functionalized products were detected by NMR and MS control,
but due to competing side reactions only complex mixtures were
obtained.
a
starting material
R
electrophile (E+) product
yield, %
3e
3e
3e
3e
3f
H
tBuCOCl
PhCOCl
Me3SiCl
Ph3SiCl
4a
4b
4c
4d
b
72
65
56
58
H
H
H
tBu
tBuCOCl
tBuCOCl
3g
CHCH2
b
a
b
Reaction conditions: LDA; THF, −78 °C, 1 h. By NMR control the
reaction was found to proceed, but isolation/purification was not
Base-Promoted Dimerization of Compound 3a. In a
further series of experiments we investigated the reactivity of the
benzyl-substituted compound 3a toward strong bases (Scheme
3). Upon deprotonation using LDA the solution turned deep red,
and after aqueous workup and subsequent GPC the colorless
solid 6 was obtained in 74% yield. Other reaction conditions
turned out to be less successful (Table 3).
possible due to the formation of multiple side products (see text).
resonances of the iminic carbon and the carbonyl carbon were
found in the 13C NMR spectra at 175.3 and 210.5 ppm,
respectively. The boron NMR spectrum displays a sharp singlet
at 5.07 ppm, which is not significantly shifted in comparison to
the nonfunctionalized compound (3e; 5.00 ppm). Additional
structural proof for the α-keto imine 4a was gained by XRD
(Figure 2). The CN bond length amounts to 1.291 Å, whereas
Table 3. Reaction Conditions and Isolated Yields for the
Synthesis of Compound 6 from 3a as Starting Material
solvent/time, h/
electrophile
(E+)
temp, °C
base
LDA
product yield, %
THF/1/−78
THF/1/−78
THF/1/−78
THF/1/−78
THF/1/50
H2O
6
6
b
6
d
74
16
LDA
tBuCOCl
H2O
a
LHMDS
KOtBu
KOtBu
H2O
c
c
H2O
a
b
Lithium hexamethyldisilazide (LHMDS). Only starting material
c
d
could be obtained. Could not be purified/yield not determined.
A
complex mixture was obtained.
As the spectroscopic and crystallographic data indicate,
compound 6 unexpectedly adopts a dimeric structure.29 The
proton NMR spectrum of 6 shows the characteristic signal for the
CH2 group of the saturated B/N heterocycle as a singlet at 4.44
ppm (Figure 3). At 4.63 ppm the methine group shows a
resonance. The second methylene group splits into two doublets
(4.92/5.73 ppm). The 11B NMR shows two resonances at 6.21
Figure 2. Molecular structure of compound 4a in the solid state
(Schakal plot).
a length of 1.654 Å is measured for the N−B bond. Obviously the
functionalization at the B/N heterocyclic ring system has minor
effects on the structural properties. This is also reflected in the
ppm (sharp, BMes2) and 46.09 (broad singlet, BMes). The 13
C
Scheme 3. Formation of Compound 6 after Deprotonation of 3a and Reaction with Water: (Upper Line) Overall Conversion;
(Lower Line) Suggested Mechanism)
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Figure 3. 1H NMR spectrum of compound 6.
NMR resonance of the iminic carbon is found at 178.7 ppm.
Compound 6 is nonfluorescent.
Figure 4 displays the solid-state structure of 6. The B−N bond
length amounts to 1.673(2) Å for B(1)−N(1) and 1.408(2) Å for
Figure 5. Molecular structure of compound 5 in the solid state (Schakal
plot).
N and 1.639 Å for B→N in the unsaturated ring system and to
1.460 Å for the respective CH−NH bond in the open-chain
boron- and nitrogen-containing part.
Figure 4. Molecular structure of compound 6 in the solid state (Schakal
plot).
We suggest the electrophilic addition of 3a to the lithium
compound 3a−Li+, a subsequent 1,3-proton shift, addition of a
proton during the workup to give 5, and final mesitylene
elimination to form 6 as the mechanism for this unexpected
dimerization (Scheme 3, lower line).
In experiments where different electrophiles such as tBuCOCl,
benzyl bromide, methyl iodide, and silyl chlorides (as used in the
case of 3e) were added to the reaction mixture containing 3a−Li+,
these added electrophiles were not incorporated into the
respective products (Scheme 3); even then, shorter reaction
times, different solvents (hexanes, toluene, 1,4-dioxane), high
dilution, and excess electrophile (up to 10 equiv) were used. In all
cases the formation of compound 6 dominated and substituted
compounds could only be detected in traces (ESI MS). We also
tested compounds 3b−d under these conditions. In all cases the
reaction proceeded significantly more slowly, but we were not
able to isolate any further derivatives.
Mechanistic Investigation/Quantum Chemical Calcu-
lations. The mechanisms for the reaction of compound 3e to
give 4a and for the dimerization reaction of 3a leading to 5 and 6
were also investigated by quantum chemical calculations at the
SCS-MP2/6-311+G(d,p)/B3LYP/6-31G(d) level of theory
using the program package GAUSSIAN 0930 (see also ref 19).
In general, the relative energies obtained at this level of theory are
N(2)−B(2), respectively. In addition, the C−N bond distances
within both B/N heterocycles vary significantly: C(7)−N(1)
amounts to 1.310(2) Å, whereas the length of C(33)−N(2) is
measured to be 1.479(2) Å. Compound 6 has tetracoordinated
(B/N heterocycle, left) and tricoordinated boron centers (right)
(Figure 3).
We were also able to characterize the structure of the labile
compound 5 (Figure 5). 5 is suggested to be an intermediate of
the dimerization of 3a to give 6 by elimination of mesitylene
(compare Scheme 3). Although we were not able to obtain
satisfactory mass and NMR spectra for full characterization of
intermediate 5, we could identify characteristic peaks for 5 in the
proton NMR spectra, which allowed us to follow the progress of
the reaction from 3e over 5 to form 6. The recorded spectra
indicate the formation of 5 within a few minutes after addition of
the base to the solution of 3a. After 1 h compound 5 was
completely converted to 6. Furthermore, after immediate
quenching (10−15 min after the complete addition of base) of
the reaction mixture of 3a with water and recrystallization of the
crude product in the cold, we were able to obtain a few crystals of
5 suitable for XRD analysis (Figure 5). Possibly, the conversion
from 5 to 6 proceeds slowly in the solid state but quickly in
solution. Characteristic bond lengths amount to 1.297 Å for C
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Scheme 4. Proposed Mechanism for the Formation of the α-Keto Imine 4a by Deprotonation of 3e and Subsequent Addition of
a
Pivaloyl Chloride as Electrophile
a
For the 3D structure of 3e−Li+ see Figure 6. Relative energies (kcal/mol) are given for the anions (red, without lithium cation) and for the neutral
products (blue), respectively (SCS-MP2/6-311+G(d,p)//B3LYP/6-31G(d)).
in good agreement with those from M062x/6-31G(d)
optimizations.31
Deprotonation of 3e and Subsequent Reactions with
Electrophiles. In principle, the deprotonation reaction of 3e may
lead to the formation of two different ring systems, either to the
five-membered 2-azaallyllithium compound 3e−Li+ or to the six-
membered ring system 10−Li+ (Scheme 4). Surprisingly, from
the calculated total energies of the anions (without the lithium
counterion) the six-membered anion 10− is favored by 4.0 kcal/
mol over 3e−. Thus, the exclusive experimental finding of
derivatives 4 with a five-membered heterocycle is not in accord
with these calculated thermodynamic data. Obviously, the
cleavage of the N−B dative interaction by opening the five-
membered ring of 3 during the deprotonation is kinetically
disfavored under the reaction conditions (THF, −78 °C), thus
Figure 6. Calculated structure of the compound 3e−Li+·Me2O,
leaving the five-membered-ring system unchanged. This is
indicating the lithium mesityl interaction (dotted line, 2.23 Å). The
supported by quantum chemical calculations for the conversion
of 3e− to 10− involving the cleavage of the Lewis acid−base
Li−CH(N) distance amounts to 2.13 Å.
interaction in 3e−, which affords an activation energy of 20.2
kcal/mol. Similarly, the experimentally observed regioselective
formation of the α-keto imine 4a cannot be explained by
thermodynamic data. According to our calculations, the product
8, resulting from the electrophilic attack of the pivaloyl chloride
at the exoposition of the 2-azaallyl anion 3e−Li+, is significantly
lower in energy in comparison to the product resulting from
attack at the endocyclic position of the anion (7−). These data
suggest a significant influence of the lithium counterion on the
course of this reaction step in terms of kinetic control. Quantum
chemical calculations of the lithiated species 3e−Li+ show that the
lowest-energy structure is characterized by the close proximity of
the lithium cation to the endocyclic position position of the 2-
azaallyl anion and to the center of one of the two mesityl rings,
which is significantly bent toward the lithium cation (Figure 6
and the Supporting Information). In this way a type of pocket is
formed (template) which in the course of the reaction is filled by
the approaching electrophile. The NBO charges are significantly
higher at the endocyclic carbon atom of the 2-azaallyl subunit
(−0.381) in comparison to that at the exocyclic 2-azaallyl carbon
atom (−0.182). The HOMO of 3e−Li+ is characterized by a
significantly larger π-orbital coefficient at the benzylic 1-position
of the 1-phenyl-2-azaallyl subunit in comparison to that of the
terminal CH2− moiety at the 3-position (see the Supporting
Information). Thus, both the structural features and the
quantum chemical results suggest a strong directing influence
of the lithium cation toward a regioselective electrophilic attack
at the endocyclic position. This is in line with the experimental
outcome of the reaction. The final proton shift from 7 to give 4a
is a result of thermodynamic control, as seen in Scheme 4.
Base-Promoted Dimerization of Compound 3a. The
unexpected formation of the dimerization products 5 and 6
may be explained in a similar way (see Scheme 3). Similarly to
3e−Li+, the lithiated benzyl-substituted species 3a−Li+ may react
in a regioselective manner at the nucleophilic endo carbon atom
of the 2-azaallyl moiety. However, here the reaction takes place
exclusively with its neutral precursor molecule 3a, which acts as
an electrophile. In this case, the additional phenyl group (in
comparison to 3e) seems to contribute to the selectivity with
respect to the electrophile. We assume that 3a may adopt a
favorable dimeric aggregate in solution, consisting of the
deprotonated form 3a−Li+ and 3a itself, based again on the
template effect exerted by the lithium ion, which leads then to the
observed dimer 5. Quantum chemical calculations predict that
the base-catalyzed dimerization of 3a to give 5 is exothermic by
−14.6 kcal/mol (SCS-MP2; −7.0 kcal/mol at B3LYP/6-
31G(d)/D3).32 The formation of the isolated final product 6
from 5 is also highly exothermic (by −20.9 kcal/mol at SCS-
MP2, −25.4 kcal/mol at B3LYP/6-31G(d)/D3; Scheme 3).
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Figure 7. UV−vis (left) and fluorescence spectra (right) of 3e (blue) and 4a (green) (solvent dichloromethane).
Figure 8. Change in the solid state photophysical properties of compound 3e by functionalization to 4a (UV hand lamp, 360 nm).
Influence of the Substitution Pattern on the Photo-
physical Properties. After deprotonation of 3e and reaction
with electrophiles, changes in the photophysical properties of the
compounds were observed (Figure 7). For example, while the
UV−vis spectrum of 3e, dissolved in dichloromethane, displays
absorptions at 265 and 279 nm and a shoulder at 307 nm (Figure
7, left), the acylated compound 4a shows absorptions at 269 and
279 nm as well as a shoulder at 345 nm. In the fluorescence
spectra (Figure 7, right, CH2Cl2, ∼2.5 × 10−5 mol/L) compound
3e shows an emission maximum at 512 nm (Φf(CH2Cl2) = 0.11),
whereas 4a emits (weakly) at 561 nm (∼2.5 × 10−5 mol/L).
Thus, by this type of functionalization the emission maximum
was red-shifted by ∼50 nm. Similarly, in the solid state 3e shows a
blue fluorescence, while the acylated product 4a emits a green to
yellow fluorescence (irradiation at 360 nm, solid-state
fluorescence not quantified, Figure 8). These preliminary
photophysical studies underline the utility of these base-
promoted functionalization reactions. Introducing further
photophysically active or activating groups by such reactions
might broaden the library of B/N heterocyclic compounds in the
future.
Furthermore, compounds 3 show upon irradiation at 365 nm
the formation of a second species in low concentrations, which
could be observed by UV measurements. For example, the
colorless compound 3a changes its color upon irradiation at a
wavelength of 365 nm to deep purple (weak absorption at 534
nm using a small scan window and fast scan rate); this color
quickly fades after switching off the light source without heating.
Therefore, we were not able to detect the photoproduct by
NMR. This process could be repeated several times. In
accordance with the very similar behaviors of structurally closely
related compounds, we interpret this observation by photo-
chromic switching, as extensively investigated by Wang33 and co-
workers. This photochromic reaction is not observed for
compound 4a.
CONCLUSION
■
Herein we report the synthesis and functionalization of various 2-
borylbenzaldimines 3. The functionalization reaction was studied
using compound 3e as the starting material. Thus, deprotonation
with LDA and subsequent treatment with electrophiles in THF
gave regioselectively functionalized compounds 4, which show as
an interesting photophysical property a substantial red-shifted
emission. Compounds 3 display upon irradiation a reversible
formation of a second species in low concentration, which may
be interpreted in terms of photochromic switching, whereas 4a
does not show this photochemical phenomenon. In the case of
compound 3a dimer 6 was unexpectedly formed upon
deprotonation via the intermediate species 5. The unique
dimeric compound 6 contains two B/N heterocyclic systems
with tri- and tetravalent boron atoms. We have shown in this
study that the mild functionalization reaction reported here is
suitable even for sensitive compounds such as the B/N donor−
acceptor complexes 3, and it allows the modification of the
photophysical properties of this important class of B/N
heterocycles. We expect that this method might be a valuable
contribution for ongoing studies in this research field.
EXPERIMENTAL SECTION
Melting points are uncorrected. All signals in the H NMR and 13C
spectra were assigned on the basis of relative intensities, coupling
constants, and GCOSY, GHSQC, and GHMBC experiments. Mass
spectra (HRMS (ESI)) were recorded on a microTOF using electron
spray ionization.
General Procedure for the Synthesis of [1-(2-(Dimesityl-
boranyl)phenyl)meth-(E)-ylidene]amines 3. A Schlenk flask was
equipped with MS 4 Å, and a solution of compound 1, dissolved in dry
CH2Cl2, was transferred into the flask. Subsequently, the amine 2 was
added pure or dissolved in dry CH2Cl2. After it was stirred at room
temperature for 18 h, the mixture was filtered through a pad of Celite,
which was washed with CH2Cl2 (3 × 30 mL). The solvent was removed
in vacuo, and either the excess amine was removed by distillation or the
compound was purified by recycling GPC or recrystallization.
Benzyl[1-(2-(dimesitylboranyl)phenyl)meth-(E)-ylidene]-
amine (3a). A 0.75 g portion (2.12 mmol) of 1 was dissolved in 50 mL
■
1
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of dry CH2Cl2. Subsequently, 0.23 mL (2.12 mmol) of benzylamine was
added. The crude product was purified by washing with hexanes to give
0.84 g (1.90 mmol, 90%) of 3a as a colorless solid. To obtain single
crystals, the compound was recrystallized from CH2Cl2. Mp: 216 °C. IR
(vw), 1611 (vs), 1585 (vs), 1555 (m), 1506 (vs), 1468 (vs), 1458 (vs),
1449 (vs), 1439 (vs), 1420 (m), 1410 (m), 1379 (s), 1327 (m), 1285
(vs), 1267 (vs), 1240 (m), 1209 (vs), 1157 (vs), 1134 (vs), 1090 (vw),
1074 (vw), 1036 (vs), 1022 (vs). 1H NMR (CD2Cl2, 300.13 MHz): δ
1.40 (bs, 3H), 1.98 (bs, 3H), 2.04 (bs, 3H), 2.14 (bs, 3H), 2.17 (bs, 3H),
2.23 (bs, 3H), 3.68 (s, 3H), 3.81 (s, 3H), 4.63 (d, 2J = 16.2 Hz, 1H), 5.03
(d, 2J = 16.2 Hz, 1H), 6.43−6.43 (m, 1H), 6.50 (d, 3J = 2.4 Hz, 1H., 6.53
(bs, 1H), 6.64 (bs, 1H), 6.72 (d, 3J = 8.1 Hz, 1H), 6.76 (bs, 1H), 6.79
(bs, 1H), 7.16 (td, 3J = 7.5 Hz, 4J = 0.9 Hz, 1H), 7.29 (td, 3J = 7.2 Hz, 4J =
1.2 Hz, 1H), 7.52 (d, 3J = 7.5 Hz, 1H), 7.61 (d, 3J = 7.5 Hz, 1H), 8.00 (s,
1H) ppm. 13C NMR (CD2Cl2, 75.47 MHz): δ 20.7, 20.9, 22.4, 25.7,
26.2, 50.5, 55.5, 55.8, 98.8, 105.2, 115.2, 125.1, 126.1, 129.3, 129.6,
130.2, 130.5, 131.8, 132.9, 137.3, 159.5, 162.2, 167.1 ppm. 11B NMR
(CD2Cl2, 96.29 MHz): δ 5.34 ppm. HRMS (ESI): calcd for
C34H38BNNaO2 526.2888, found 526.2893. X-ray crystal structure
data of 3d can be found in the Supporting Information.
(neat): ν
̃
2914 cm−1 (w), 2357 (w), 2340 (w), 2226 (w), 2170 (w), 1607
(s), 1551 (w), 1443 (m), 1412 (m), 1375 (m), 1240 (m), 1219 (s), 1202
(m), 1175 (m), 1153 (m), 1115 (m), 1097 (m), 1070 (m), 1030 (m),
1022 (m). UV/vis (CH2Cl2): λabs (ν
M−1 cm−1), 336 (sh, 28490 cm−1, 2747 M−1.cm−1). Fluorescence
(CH2Cl2, 1.5 × 10−5 mol/L): λem 522 nm (ν 19157 cm−1), λexc 350 nm,
̃
, ε) 282 nm (35460 cm−1, 11319
̃
1
Φf(CH2Cl2) = 0.13. H NMR (C6D6, 300.13 MHz): δ 1.68 (bs, 3H),
2.00 (bs, 3H), 2.26 (s, 6H), 2.35 (bs, 3H), 2.51 (bs, 3H), 4.81−5.00 (m,
2H), 6.57−6.60 (m, 2H), 6.72 (bs, 1H), 6.86 (bs, 1H), 6.85−6.87 (m,
2H), 6.94−6.98 (m, 5H), 7.01−7.07 (m, 1H), 7.70 (s, 1H), 7.86 (d, 1H,
3J = 6.0 Hz) ppm. 13C NMR (C6D6, 75.48 MHz): δ 21.0, 23.0, 26.1, 26.2,
27.0, 55.12, 125.0, 125.8, 128.6, 129.3, 130.1, 130.4, 130.9, 131.1, 131.3,
132.3, 133.3, 135.1, 135.4, 136.9, 167.4 ppm. 11B NMR (C6D6, 96.29
MHz): δ 6.46 ppm. HRMS (ESI): calcd for C32H34BNH 444.2858,
found 444.2870. X-ray crystal structure data of 3a can be found in the
Supporting Information.
[1-(2-(Dimesitylboranyl)phenyl)meth-(E)-ylidene]methyl-
amine (3e). A 2.36 g portion (6.65 mmol) of compound 1 was
dissolved in 100 mL of dry CH2Cl2. Then, 0.80 mL (7.32 mmol) of
methylamine (40% in MeOH) was added with stirring. The crude
product was washed with pentane. Crystals could be obtained by
recrystallization from CH2Cl2 to give 2.17 g (5.90 mmol, 70%) of 3e as
[1-(2-(Dimesitylboranyl)phenyl)meth-(E)-ylidene](4-chloro-
benzyl)amine (3b). A 0.14 g portion (1.00 mmol) of 4-chlorobenzyl-
amine dissolved in 50 mL of dry CH2Cl2 was added to a solution of 0.35
g (1.00 mmol) of 1 in 50 mL of dry CH2Cl2. Compound 3b was purified
by washing with pentane to give 0.17 g(0.37 mmol, 37%) of 3b as a
colorless crystals. Mp: 230 °C. IR (neat): ν
̃
3057 cm−1 (vw), 3007 (w),
2990 (w), 2965 (m), 2914 (m), 2866 (w), 2729 (vw), 2361 (w), 2342
(w), 2334 (w), 2326 (w), 1638 (vw), 1614 (s), 1603 (m), 1551 (m),
1506 (w), 1468 (m), 1447 (s), 1406 (m), 1379 (m), 1371 (s), 1310 (w),
1283 (w), 1263 (w), 1229 (m), 1200 (w), 1186 (w), 1163 (m), 1138 (s),
1117 (w), 1099 (w), 1070 (w), 1045 (m), 1020 (m), 1013 (m). UV/vis
colorless solid. Mp: 229 °C. IR (neat): ν
̃
3053 cm−1 (w), 3021 (w), 2972
(m), 2955 (m), 2922 (m), 2866 (w), 1609 (s), 1551 (m), 1491 (m),
1468 (m), 1441 (s), 1408 (m), 1371 (s), 1321 (m), 1310 (m), 1294 (m),
1281 (m), 1265 (m), 1221 (s), 1188 (w), 1165 (m), 1138 (m), 1107
(m), 1096 (s), 1061 (w), 1028 (s), 1015 (vs). 1H NMR (C6D6, 400.13
MHz): δ 1.61 (bs, 3H), 1.94 (bs, 3H), 2.24 (s, 6H), 2.31 (bs, 3H), 2.49
(bs, 3H), 4.50 (d, 2J = 16.0 Hz, 1H), 4.74 (d, 2J = 16.0 Hz, 1H), 6.20−
6.23 (m, 2H), 6.72 (bs, 1H), 6.77 (bs, 1H), 6.87−6.91 (m, 3H), 6.93 (bs,
1H), 6.97−7.02 (m, 2H), 7.05 (td, 3J = 8.0 Hz, 4J = 3.0 Hz, 1H), 7.55 (s,
1H), 7.86 (d, 3J = 6.0 Hz, 1H) ppm. 13C NMR (C6D6, 100.61 MHz): δ
21.0, 22.8, 23.0, 26.0, 26.2, 27.0, 54.3, 125.2, 125.8, 128.4, 129.3, 130.1,
131.0, 131.1, 131.4, 131.5, 132.5, 133.4, 133.9, 134.6, 135.2, 136.7, 136.8,
141.0, 141.8, 144.0, 167.8 ppm. 11B NMR (C6D6, 96.29 MHz): δ 6.79
ppm. HRMS (ESI): calcd for C32H33BClNNa 500.2287, found
500.2265. Anal. Calcd for C32H33BClN (477.88): C, 80.43; H, 6.96;
N, 2.93. Found: C, 80.31; H, 6.97; N, 2.70.
(CH2Cl2): λabs 280 nm (ν
32467 cm−1, 4909 M−1 cm−1), 354 (sh, 28248 cm−1, 1834 M−1 cm−1).
Fluorescence (CH2Cl2, 2.5 × 10−5 mol/L): λem 512 nm (ν 19531 cm−1),
̃
35714 cm−1, ε = 9702 M−1 cm−1), 308 (sh,
̃
λexc 280 nm, Φf(CH2Cl2) = 0.11. 1H NMR (C6D6, 400.13 MHz): δ 1.72
(bs, 6H), 2.23 (bs, 12H), 2.83 (s, 3H), 6.75−6.94 (m, 5H), 6.97 (td, 3J =
7.2 Hz, 4J = 0.8 Hz, 1H), 7.07 (td, 3J = 7.2 Hz, 4J = 1.2 Hz, 1H), 7.18 (s,
1H), 7.82 (d, 3J = 8.0 Hz, 1H) ppm. 13C NMR (C6D6, 100.61 MHz): δ
21.0, 25.8, 40.5, 125.1, 125.2, 128.3, 130.3, 131.5, 132.0, 136.9, 141.3,
169.0 ppm. 11B NMR (C6D6, 96.29 MHz): δ 5.00 ppm. HRMS (ESI):
calcd for C26H30BNNa 390.2368, found 390.2360. Anal. Calcd for
C26H30BN (367.33): C, 85.01; H, 8.23; N 3.81. Found: C, 85.05; H,
8.38; N, 3.72. X-ray crystal structure data of 3e can be found in the
Supporting Information.
[1-(2-(Dimesitylboranyl)phenyl)meth-(E)-ylidene](2,2-di-
methyl-propyl)amine (3f). A 1.67 g portion (4.73 mmol) of
compound 1 was dissolved in 25 mL of dry CH2Cl2. Then, 0.55 mL
(4.82 mmol) of neopentylamine was added. The crude product was
washed with pentane. Slightly yellow crystals were obtained by
recrystallization from CH2Cl2. Yield: 1.13 g (2.67 mmol, 56%). Mp:
[1-(2-(Dimesitylboranyl)phenyl)meth-(E)-ylidene](4-meth-
oxybenzyl)amine (3c). A 0.35 g portion (1.00 mmol) of compound 1
was dissolved in 100 mL of dry CH2Cl2. Then, 0.13 mL (1.00 mmol) of
4-methoxybenzylamine was added. The product was purified by washing
with pentane to give 0.25 g (0.52 mmol, 52%) of 3c as a colorless solid.
Single crystals could be obtained by recrystallization from CH2Cl2. Mp:
115 °C. IR (neat): ν
̃
3024 cm−1 (vw), 2961 (s), 2918 (m), 2870 (w),
209 °C. IR (neat): ν
̃
3046 cm−1 (w), 3019 (m), 3005 (w), 2994 (w),
2359 (w), 2340 (w), 1609 (vs), 1551 (s), 1470 (vs), 1447 (vs), 1404
(m), 1379 (m), 1373 (m), 1339 (m), 1308 (w), 1281 (w), 1263 (m),
1234 (m), 1225 (m), 1204 (w), 1194 (m), 1163 (w), 1144 (m), 1105
(m), 1080 (s), 1059 (s), 1045 (s), 1030 (s), 1015 (vs). 1H NMR (C6D6,
300.13 MHz): δ 0.45 (s, 9H), 1.75 (bs, 3H), 1.94 (bs, 3H), 2.20 (bs,
3H), 2.22 (bs, 3H), 2.44 (bs, 3H), 3.40−3.45 (m, 1H), 3.87−3.92 (m,
1H), 6.65 (bs, 1H), 6.74 (bs, 1H), 6.89 (bs, 1H), 6.95 (bs, 1H), 6.99 (td,
3J = 7.2 Hz, 4J = 0.9 Hz, 1H), 7.10 (td, 3J = 7.2 Hz, 4J = 1.2 Hz, 1H), 7.29
2972 (m), 2955 (m), 2913 (m), 2866 (w), 2841 (w), 1609 (s), 1584
(w), 1551 (m), 1512 (s), 1491 (w), 1460 (m), 1441 (s), 1412 (m), 1371
(m), 1319 (m), 1302 (m), 1281 (m), 1256 (vs), 1217 (s), 1182 (m),
1175 (s), 1165 (m), 1138 (m), 1111 (m), 1094 (w), 1072 (w), 1038
(m), 1026 (s), 1015 (s). 1H NMR (C6D6, 400.13 MHz): δ 1.71 (bs, 3H),
2.03 (bs, 3H), 2.27 (bs, 6H), 2.37 (bs, 3H), 2.52 (bs, 3H), 3.21 (s, 3H),
4.744.88 (m, 2H), 6.53−6.61 (m, 4H), 6.73 (bs, 1H), 6.85 (bs, 1H),
6.86−6.90 (m, 1H), 6.95−6.96 (m, 1H), 6.99 (bs, 2H), 7.06 (td, 3J = 7.2
Hz, 4J = 1.2 Hz, 1H), 7.80 (s, 1H), 7.88 (d, 3J = 7.6 Hz, 1H) ppm. 13
C
(d, 3J = 7.5 Hz, 1H), 7.90 (d, 3J = 6.0 Hz, 1H), 8.03 (s, 1H) ppm. 13
C
NMR (C6D6, 100.61 MHz): δ 21.0, 22.9, 26.1, 26.2, 27.0, 54.6, 54.8,
114.8, 125.1, 125.7, 127.2 (C), 128.4, 130.1, 130.9, 131.1, 131.4, 131.7,
132.2, 135.1, 136.9, 137.0, 141.0, 141.8, 144.1, 160.2, 167.2 ppm. 11B
NMR (C6D6, 64.21 MHz): δ 5.05 ppm. HRMS (ESI): calcd for
C33H36BNONa 496.2788, found 496.2797. Anal. Calcd for C33H36BNO
(473.46): C, 83.71; H, 7.66; N, 2.96. Found: C, 83.60; H, 7.48; N, 2.71.
[1-(2-(Dimesitylboranyl)phenyl)meth-(E)-ylidene](2,4-
dimethoxybenzyl)amine (3d). A 0.35 g portion (1.00 mmol) of 1
was dissolved in 100 mL of dry CH2Cl2. Subsequently, 0.15 g (1.02
mmol) of 2,4-dimethoxybenzylamine was added. Recrystallization from
CH2Cl2 gave 0.31 g (0.62 mmol, 62%) of 3d as colorless crystals. Mp:
NMR (C6D6, 75.47 MHz): δ 21.0, 21.0, 23.1, 25.9, 27.6, 28.5, 33.4, 62.1,
125.0, 125.8, 127.7, 128.2, 128.4, 130.0, 130.2, 130.4, 131.3, 131.5, 132.5,
133.1, 135.1, 136.6, 136.7, 141.3, 142.5, 143.8, 168.7 ppm. 11B NMR
(C6D6, 96.29 MHz): δ 5.96 ppm. HRMS (ESI): calcd for C30H39BN
424.3170, found 424.3172. X-ray crystal structure data of 3f can be
found in the Supporting Information.
Allyl[1-(2-(dimesitylboranyl)phenyl)meth-(E)-ylidene]amine
(3g). A 0.38 g (1.07 mmol) portion of compound 1 was dissolved in 100
mL of dry CH2Cl2. Then, 0.10 mL (1.33 mmol) of allylamine was added.
To purify the crude product, the solid was washed with pentane.
Colorless crystals were obtained by recrystallization from diethyl ether.
256 °C. IR (neat): ν
̃
3019 cm−1 (vw), 2999 (w), 2974 (m), 2943 (m),
Yield: 0.25 g (0.62 mmol, 58%). Mp: 143 °C. IR (neat): ν
̃
3011 cm−1
2928 (m), 2882 (vw), 2866 (w), 2837 (vw), 2361 (w), 2342 (vw), 2330
(m), 2967 (s), 2959 (s), 2924 (s), 2884 (m), 2870 (m), 2862 (m), 1607
11753
dx.doi.org/10.1021/jo401748d | J. Org. Chem. 2013, 78, 11747−11755

The Journal of Organic Chemistry
Article
(vs), 1549 (vs), 1470 (vs), 1449 (vs), 1441 (vs), 1414 (s), 1377 (vs),
1321 (m), 1310 (m), 1298 (m), 1283 (w), 1263 (m), 1244 (m), 1234
(m), 1215 (s), 1163 (w), 1134 (vs), 1105 (m), 1088 (m), 1074 (m),
1047 (vs), 1022 (vs). 1H NMR (C6D6, 300.13 MHz): δ 1.67 (bs, 3H),
1.84 (bs, 3H), 2.22 (bs, 9H), 2.48 (bs, 3H), 4.01−4.22 (m, 2H), 4.60−
4.71 (m, 2H), 4.93−5.06 (m, 1H), 6.74 (bs, 2H), 6.94 (bs, 2H), 6.97 (td,
3J = 6.3 Hz, 4J = 0.9 Hz, 1H), 7.08 (td, 3J = 6.0 Hz, 4J = 1.2 Hz, 1H), 7.22
calcd for C33H34BNONa 494.2626, found 494.2602. X-ray crystal
structure data of 4b can be found in the Supporting Information.
(Z)-N-[(2-(Dimesitylboranyl)phenyl)(trimethylsilyl)methyl-
ene]methanamine (4c). A 0.48 g portion (1.31 mmol) of compound
3e was dissolved in 20 mL of dry THF and added slowly to a stirred
mixture of diisopropylamine (0.22 mL, 1.58 mmol) and nBuLi (0.98 mL,
1.58 mmol, 1.6 M in hexane) in 50 mL of dry THF. After 1 h at −78 °C,
the reactive intermediates were trapped with 0.33 mL (2.62 mmol) of
trimethylsilyl chloride. The crude product was purified by column
chromatography and recycling GPC (toluene) to give 0.32 g (0.73
mmol, 56%) of 4c as a yellow solid. Rf = 0.21, Al2O3, cyclohexane
(d, 3J = 7.2 Hz, 1H), 7.63 (s, 1H), 7.88 (d, 3J = 7.8 Hz, 1H) ppm. 13
C
NMR (C6D6, 75.47 MHz): δ 21.0, 22.7, 26.0, 27.1, 54.5, 120.7, 125.1,
125.7, 128.4, 130.1, 130.9, 131.4, 132.3, 133.2, 134.8, 137.1, 141.4, 143.7,
167.4 ppm. 11B NMR (C6D6, 96.29 MHz): δ 5.41 ppm. HRMS (ESI):
calcd for C28H33BN 394.2701, found 394.2708. Anal. Calcd for
C28H32BN (393.37): C, 85.49; H, 8.20; N 3.56. Found: C, 85.18; H,
8.23; N, 3.40. X-ray crystal structure data of 3g can be found in the
Supporting Information.
General Procedure for the Synthesis of Compounds 4.
Compounds 3 were dissolved in THF and added slowly to a stirred
solution of diisopropylamine and nBuLi at −78 °C. After 1 h the deeply
colored mixture was quenched with an excess of electrophile. After 15
min the reaction mixture was warmed to ambient temperature and
stirred for a further 1 h. Diethyl ether was added, and the organic phase
was washed three times with water and dried over MgSO4. The solvent
was removed in vacuo, and the crude products were purified by either
recrystallization or column chromatography.
(100%) Mp: 150 °C,. IR (neat): ν
̃
3059 cm−1 (w), 3017 (w), 2965 (m),
2949 (m), 2920 (s), 2855 (m), 1603 (m), 1591 (m), 1547 (w), 1526
(m), 1468 (s), 1447 (vs), 1414 (m), 1400 (m), 1379 (m), 1346 (w),
1308 (m), 1252 (vs), 1234 (m), 1209 (w), 1200 (w), 1184 (w), 1163
(m), 1152 (m), 1099 (w), 1074 (w), 1053 (m), 1030 (m), 1018 (m),
974 (w), 962 (w), 943 (w), 930 m), 845 (vs), 822 (vs), 804 (s), 785 (vs),
766 (s), 743 (vs), 718 (vs), 692 (s), 673 (m), 664 (w), 644 (s), 629 (m),
623 (m). 1H NMR (C6D6, 399.65 MHz): δ 0.08 (s, 9H), 1.7 (s, 3H),
2.23 (s, 3H), 3.23 (s, 3H), 6.77 (bs, 4H), 6.94−7.02 (m, 1H), 7.05−7.11
(m, 1H), 7.66 (d, 3J = 8.0 Hz, 1H), 7.95 (d, 3J = 8.0 Hz, 1H) ppm. 13
C
NMR (C6D6, 100.40 MHz): δ 0.2, 20.9, 30.1, 40.1, 124.5, 125.7,126.3,
128.4, 128.6, 129.3, 130.4, 130.9, 131.2, 141.6, 143.7, 190.2 ppm. 11B
NMR (C6D6, 128.15 MHz): δ 4.89 ppm. 29Si NMR (C6D6, 59.62 MHz):
δ −5.58 ppm. HRMS (ESI): calcd for C29H38BNSiNa 462.2764, found
462.2767. Anal. Calcd for C29H38BNSi (439.52): C, 79.25; H, 8.71; N
3.19. Found: C, 79.12; H, 8.77; N, 2.99.
(Z)-1-[2-(Dimesitylboranyl)phenyl]-3,3-dimethyl-1-(methyl-
imino)butan-2-one (4a). A 0.37 g portion (0.99 mmol) of compound
3e in 20 mL of dry THF was added slowly to a stirred mixture of
n
(Z)-N-[(2-(Dimesitylboranyl)phenyl)(triphenylsilyl)methyl-
ene]methanamine (4d). A 0.37 g portion (1.00 mmol) of compound
3e dissolved in 20 mL of dry THF was prepared and added slowly to a
diisopropylamine (0.14 mL, 1.02 mmol) and BuLi (0.64 mL, 1.02
mmol, 1.6 M in hexane). After 1 h at −78 °C, the reactive intermediates
were quenched with 0.22 mL (2.00 mmol) of trimethylacetyl chloride.
The crude product was purified by recrystallization from diethyl ether to
give 0.32 g (0.71 mmol, 72%) of 4a as colorless crystals. Mp: 174 °C. IR
n
stirred mixture of diisopropylamine (0.14 mL, 1.02 mmol) and BuLi
(0.64 mL, 1.02 mmol, 1.6 M in hexane) in 50 mL of dry THF. After 1 h at
−78 °C, the reactive intermediates were trapped with 0.59 mL (2.00
mmol) of triphenylsilyl chloride in 20 mL of dry THF. The crude
product was purified by recycling GPC (toluene) to give 0.36 g (0.58
(neat): ν
̃
2990 cm−1 (m), 2974 (m), 2955 (m), 2926 (m), 2872 (m),
1695 (s), 1603 (m), 1549(m), 1468 (s), 1445 (s), 1406 (m), 1398 (m),
1369 (m), 1362 (m), 1333 (s), 1310 (m), 1269 (m), 1233 (m), 1163
(m), 1148 (m), 1124 (m), 1036 (s), 1020 (m), 1001 (s). UV/vis
mmol, 58%) of 4d as a yellow solid. Mp: 229 °C,. IR (neat): ν
̃
3026 cm−1
̃
37175 cm−1, ε = 12485 M−1 cm−1), 279 nm
(w), 2974 (w), 2916 (w), 1526 (w), 1470 (w), 1447 (m), 1439 (m),
1414 (w), 1252 (s), 1150 (w), 1016 (w), 870 (w), 847 (vs), 820 (m),
785 (s), 766 (w), 743 (vs), 723 (vs), 700 (s), 692 (m), 681 (w), 667 (vs),
658 (w), 629 (w), 615 (w). 1H NMR (C6D6, 399.65 MHz): δ 2.11 (bs,
6H), 2.26 (bs, 12H), 3.23 (s, 3H), 6.71 (t, 3J = 7.2 Hz, 1H), 6.87 (bs,
4H), 6.96−7.03 (m, 1H), 7.05−7.15 (m, 9H), 7.25 (d, 3J = 8.0 Hz, 1H),
7.56−7.58 (m, 6H), 7.93 (d, 3J = 8.0 Hz, 1H) ppm. 13C NMR (C6D6,
100.40 MHz): δ 20.9, 42.2, 124.6, 128.1, 128.3, 128.9, 130.5, 130.9,
130.9, 131.6, 132.1, 133.8, 136.4, 145.5, 185.9 ppm. 11B NMR (C6D6,
128.15 MHz): δ 5.73 ppm. HRMS (ESI): calcd for C44H44BNSiNa
648.3236, found 648.3236. Anal. Calcd for C44H44BNSi (625.72): C,
84.46; H, 7.09; N 2.24. Found: C, 84.11; H, 6.93; N, 2.04.
(CH2Cl2): λabs 269 nm (ν
(35894 cm−1, 8602 M−1 cm−1), 345 nm (sh, 29002 cm−1, 2416 M−1
cm−1). Fluorescence (CH2Cl2, 2.5 × 10−5 mol/L): λem 561 nm (ν
17813
̃
cm−1), λexc 269 nm. 1H NMR (C6D6, 399.65 MHz): δ 0.88 (s, 9H), 1.96
(s, 6H), 2.21 (s, 9H), 2.44 (s, 3H), 2.91 (s, 3H), 6.64 (bs, 1H), 6.81−
6.86 (m, 2H), 6.91−6.94 (m, 2H), 7.03 (t, 3J = 7.2 Hz, 1H), 7.14−7.16
(m, 1H), 7.87 (d, 3J = 7.6 Hz, 1H) ppm. 13C NMR (C6D6, 100.40 MHz):
δ 20.8, 22.6, 25.6, 25.8, 26.2, 28.0, 30.0, 37.6, 44.4, 124.8, 125.3, 130.2,
130.6, 130.7, 130.8, 131.9, 132.4, 133.7, 134.8, 136.1, 141.1, 175.3, 210.5
ppm. 11B NMR (C6D6, 128.15 MHz): δ 5.07 ppm. HRMS (ESI): calcd
for C31H38BNONa 474.2939, found 474.2942. Anal. Calcd for
C31H38BNO (451.45): C, 82.47; H, 8.48; N 3.10. Found: C, 82.20; H,
8.57; N, 3.09. X-ray crystal structure data of 4a can be found in the
Supporting Information.
Dimer 5. Although compound 5 could neither be isolated in pure
form nor be fully characterized by spectroscopy, its structure in the solid
state could be determined unambiguously. See the Supporting
Information for details of the X-ray structure.
(Z)-2-[2-(Dimesitylboranyl)phenyl]-2-(methylimino)-1-
phenylethanone (4b). A 0.37 g portion (1.00 mmol) of compound 3e
in 20 mL of dry THF was added slowly to a stirred mixture of
Dimer 6. A 0.22 g portion (0.50 mmol) of compound 3a in 20 mL of
n
dry THF was added slowly to a stirred mixture of diisopropylamine
diisopropylamine (0.14 mL, 1.01 mmol) and BuLi (0.62 mL, 1.01
n
(0.07 mL, 0.51 mmol) and BuLi (0.32 mL, 0.51 mmol, 1.6 M in
mmol, 1.6 M in hexane) in 50 mL of dry THF. After 1 h at −78 °C, the
reactive intermediates were quenched with 0.24 mL (2.04 mmol) of
benzoyl chloride. The crude product was purified by column
chromatography and subsequent recrystallization from toluene to give
0.31 g (0.65 mmol, 65%) of 4b as colorless crystals. Rf = 0.33, Al2O3,
hexane). After 1 h at −78 °C, the reactive intermediates were quenched
with 10 mL of distilled H2O. The crude product was purified by
recycling GPC. Colorless single crystals were obtained by recrystalliza-
tion from CH2Cl2 to give 0.14 g (0.37 mol, 74%) of 6. Mp: 158 °C. IR
(neat): ν
̃
3026 cm−1 (w), 2974 (w), 2916 (w), 1526 (w), 1470 (w), 1447
cyclohexane/ethyl actetate (40/1). Mp: 215 °C,. IR (neat): ν
̃
2961 cm−1
(m), 1439 (m), 1414 (w), 1252 (s), 1150 (w), 1016 (w), 870 (w), 847
(vs), 820 (m), 785 (s), 766 (w), 743 (vs), 723 (vs), 700 (s), 692 (m),
681 (w), 667 (vs), 658 (w), 629 (w), 615 (w). 1H NMR (C6D6, 399.65
MHz): δ 1.83 (bs, 3H), 2.11 (bs, 3H), 2.18 (bs, 3H), 2.21 (bs, 15H),
2.31 (bs, 6H), 2.48 (bs, 6H), 4.44 (d, 2J = 16.4 Hz, 1H), 4.63 (d, 2J = 15.6
Hz, 1H), 4.92 (d, 2J = 16.4 Hz, 1H), 5.73 (d, 2J = 15.6 Hz, 1H), 6.02 (d, 3J
= 7.6 Hz, 2H), 6.25 (s, 1H), 6.63−7.14 (m, 21H), 7.49 (d, 3J = 8.0 Hz,
1H), 7.58 (d, 3J = 6.4 Hz, 1H), 7.89 (d, 3J = 7.6 Hz, 1H) ppm. 13C NMR
(C6D6, 100.40 MHz): δ 20.7, 20.9, 21.2, 21.3, 23.0, 23.4, 24.7, 25.7, 26.4,
27.9, 50.2, 54.8, 64.3, 123.1, 124.8, 125.7, 126.6, 126.8, 127.7, 127.8,
(s), 2930 (s), 2359 (w), 2342 (m), 1665 (vs), 1603 (s), 1595 (s), 1578
(s), 1516 (s), 1497 (s), 1470 (s), 1447 (s), 1441 (s), 1406 (s), 1371 (s),
1356 (s), 1342 (s), 1317 (s), 1240 (vs), 1179 (s), 1146 (s), 1107 (s),
1022 (s), 991 (s), 899 (s), 854 (s), 845 (s), 822 (s), 746 (vs), 735 (vs),
714 (s), 698 (vs), 681 (s). 1H NMR (C6D6, 300.13 MHz): δ 2.23 (bs,
18H), 2.98 (s, 6H), 6.73−6.78 (m, 1H), 6.85−6.90 (m, 5H), 6.94−7.08
(m, 3H), 7.10−7.14 (m, 1H), 7.78 (m, 2H), 7.92 (d, 3J = 7.5 Hz, 1H)
ppm. 13C NMR (C6D6, 75.48 MHz): δ 21.0, 27.2, 37.1, 125.1, 125.7,
128.3, 129.4, 129.6, 130.6, 131.8, 132.6, 134.3, 134.8, 135.5, 136.7, 173.8,
192.1 ppm. 11B NMR (C6D6, 100.61 MHz): δ 7.22 ppm. HRMS (ESI):
11754
dx.doi.org/10.1021/jo401748d | J. Org. Chem. 2013, 78, 11747−11755

The Journal of Organic Chemistry
Article
127.9, 128.4, 128.5, 128.6, 129.3, 129.4, 129.6, 130.4, 130.7, 131.5, 131.6,
132.0, 132.3, 135.3, 136.5, 137.7, 138.0, 138.6, 138.8, 140.5, 141.0, 141.2,
142.6, 142.9, 144.7, 146.6, 151.4, 178.7 ppm. 11B NMR (C6D6, 128.15
MHz): δ 6.21 ppm (s), 46.09 (bs). HRMS (ESI): calcd for
C55H56B2N2Na 789.4522, found 789.4513. Anal. Calcd for
C55H56B2N2 (766.67): C, 86.16; H, 7.36; N 3.65. Found: C, 85.87; H,
7.58; N, 3.20. X-ray crystal structure data of 6 can be found in the
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ASSOCIATED CONTENT
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S
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Text, tables, figures, and CIF files giving 1H, 11B, and 13C NMR
spectra for the new compounds, time-dependent spectra of NMR
experiments, graphics of the crystal structures showing thermal
ellipsoids with 50% probability, optimized Cartesian coordinates
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AUTHOR INFORMATION
Corresponding Authors
■
(29) Andrews, P. C.; Armstrong, D. R.; Clegg, W.; Craig, F. J.; Dunbar,
L.; Mulvey, R. E. Chem. Commun. 1997, 319−320. Pearson, W. H.;
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*S.Y.: e-mail, yamaguchi@chem.nagoya-u.ac.jp.
*E.-U.W.: e-mail, wurthwe@uni-muenster.de; fax, +49-251-83-
39772.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the IRTG 1143 Munster-Nagoya
(Deutsche Forschungsgemeinschaft (DFG, Bad Godesberg))
and the Fonds der Chemischen Industrie (Frankfurt).
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