Boracyclophanes: Modulation of the σ/π character in boron-benzene interactions

Article abstract of DOI:10.1002/anie.201303830

Character sketch: Boracyclophanes, in which the benzene ring coordinates to the boron atom, were synthesized. X-ray crystallographic analysis and theoretical calculations revealed the intrinsic character of the boron-benzene interaction. Its σ/π character

Full text of DOI:10.1002/anie.201303830

Angewandte
Chemie
Communications
Structure Elucidation
Character sketch: Boracyclophanes, in
which the benzene ring coordinates to the
boron atom, were synthesized. X-ray
crystallographic analysis and theoretical
calculations revealed the intrinsic char-
acter of the boron–benzene interaction.
Its s/p character can be modulated by
changing the electron-donating character
of the benzene ring.
T. Kushida,
S. Yamaguchi*
&&&& — &&&&
Boracyclophanes: Modulation of the s/
p Character in Boron–Benzene
Interactions
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DOI: 10.1002/anie.201303830
Structure Elucidation
Boracyclophanes: Modulation of the s/p Character in Boron–Benzene
Interactions**
Tomokatsu Kushida and Shigehiro Yamaguchi*
Friedel–Crafts alkylation is a powerful methodology in
organic synthesis for the modification of aromatic com-
pounds. The reaction is categorized as an electrophilic
aromatic substitution, in which a carbocation–arene s com-
plex, namely the arenium ion, has been assumed to be an
intermediate. Characterizations of the arenium ions have long
been investigated both theoretically and experimentally.[1,2] It
is also known that silylium ions (silyl cations) interact with an
arene to form a silylarenium species. However, this inter-
mediate exhibits a greater p-complex character rather than
the s-complex character.[3] The character of the arenium
intermediates generated with p-block-element electrophiles
vary from a h1 p complex to a s complex, as shown in
Scheme 1. Synthesis of the boracyclophane 2 through an intermediate
3·Li.
Figure 1.
have a h1 p-complex character rather than a s-complex
character.
Recently, we synthesized the planarized triphenylboranes
1 (Scheme 1, R = H or Me) with three methylene bridges and
demonstrated their high stability toward air and moisture.
Figure 1. Structural variation of the interaction between the p-block
electrophiles (e.g. E+ =C+, Si+) and benzene.
Based on this result, we claimed that the structural constraint
is an effective strategy for the stabilization of the reactive
triarylboranes.[8] However, we also demonstrated that the
planarized triphenylboranes still possess a sufficient Lewis
acidity to react with anionic Lewis bases, such as a fluoride
ion, thus resulting in the formation of a bowl-shaped
tetracoordinate borate species. These results prompted us to
study the reactivity of the planarized triphenylboranes with
Lewis bases in more detail. We now employed a stronger
carbon nucleophile, tBuLi, as the Lewis base. We found that
the reaction produced a boracyclophane, in which a tricoordi-
nate boron moiety was fixed above an eliminated benzene
ring in close proximity. Notably, this molecular structure
provides an ideal scaffold to study the intrinsic character of
the interaction between a tricoordinate boron and a benzene
ring. We succeeded in modulating the character of the boron–
benzene interaction from a p- to s-complex character by
changing the electron-donating character of the benzene ring.
In our previous study, when we treated the planarized
triphenylborane 1 (R = H) with bases, such as potassium
hydride and lithium tetramethylpiperidide, we cleanly
obtained a highly fluorescent borataanthracene anion.[9] In
contrast, we now found that the reaction of 1 with 3 equiv-
alents of tBuLi followed by quenching with water gave a one
benzene ring-eliminated product, namely the boracyclophane
2, as a single product (Scheme 1). This compound was isolated
by preparative-scale gel permeation chromatography (GPC)
in 73% yield as colorless solids. The structure of 2 was
unambiguously characterized by NMR spectroscopy, mass
spectrometry, and X-ray crystallography.
The tricoordinate group 13 element species ER3 (E = B,
Al, Ga, or In), which are isoelectronic to the carbocation, are
also potent Lewis acids and would interact with arenes to
form zwitterionic arenium complexes.[4] The interaction of
BX3 (X = H, F, or Cl) with benzene has been investigated by
an ab initio study, in which the binding energy for the
h1 p complexes are estimated to be 3–6 kcalmolꢀ1.[5] In
contrast, experimental characterizations of the boron–arene
complexes are still limited to only a few examples, including
those with triarylboranes[6] or borenium cations.[7] In these
examples, constrained arrangements between the boron and
arene moieties in close proximity assist in forming the boron–
benzene interactions, which are relatively weak and tend to
[*] T. Kushida, Prof. Dr. S. Yamaguchi
Institute of Transformative Bio-Molecules (WPI-ITbM), Research
Center for Materials Science, and Graduate School of Science
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
[**] This work was partly supported by CREST, JST, and a Grant-in-Aid for
Scientific Research on Innovative Areas (Stimuli-responsive Chem-
ical Species, No. 24109007) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. T.K. thanks the JSPS
Research Fellowship for Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303830.
2
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The reaction of 1 with tBuLi cleanly proceeded, and any
product produced by the elimination of the other benzene
moieties was not observed in the crude reaction mixture, thus
indicating that the boron–benzene bond was selectively
cleaved. The yield of 2 was highly dependent on the amount
of tBuLi. When 1 equivalent of tBuLi was employed, 2 was
obtained only in 18% yield. In contrast, the use of 5 equiv-
alents of tBuLi gave 2 in 71%, which is comparable to that
obtained with 3 equivalents of tBuLi (Scheme 1). These
results can be rationalized by considering the equilibrium
between the tricoordinate borane 1 and tetracoordinate tert-
butylborate 3·Li. The use of excess tBuLi shifts the equilib-
rium to the borate side, from which the protonation of the
most electron-rich benzene ring selectively produces the
boracyclophane 2.
2.23 ꢁ is shorter than the sum of the van der Waals radii of the
boron and carbon atoms (3.62 ꢁ).[11] This structure indicates
that an interaction occurs between the boron atom and the
benzene ring to some extent. In addition, the 11B NMR
spectrum of 2 in CDCl3 showed a signal at d = 51.1 ppm,
which is upfield-shifted compared to that of the alkyldiphe-
nylboranes, such as Ph2BMe (d = 70.6 ppm),[12] and indicative
of the electronic perturbation from the eliminated benzene
ring to the boron atom. To gain detailed insights into the
electronic structure of the boracyclophane 2, we conducted
theoretical calculations.
The optimized structure of 2, calculated at the B3LYP/
6-31G(d) level of theory,[13] showed good agreement with the
X-ray structure, except that the B1···C1 distance was slightly
overestimated to be 2.35 ꢁ. While the HOMO is mainly
localized on the two benzene rings connected to the boron
atom, the LUMO essentially consists of a vertical orbital
interaction between the vacant p orbital of the boron atom
and the p* orbital of the eliminated benzene ring (Figure 3a).
The clean formation of the intermediate 3·Li was con-
firmed by monitoring the reaction mixture by NMR spec-
troscopy (see the Supporting Information). In the 11B NMR
spectrum of a 1:1 mixture of 1 and tBuLi in C6D6, only two
signals were observed. One is a broad signal of 1 at d =
45.9 ppm. The other one is a sharp signal at d = ꢀ20.1 ppm,
which is assignable to the borate intermediate 3·Li (see
Figure S4 in the Supporting Information). The 1H NMR
spectrum also showed the signals of 1 and a new set of signals
corresponding to 3·Li (see Figure S5 in the Supporting
Information). Upon the addition of water to the solution,
the signals of the boracyclophane 2 appeared in the 1H NMR
spectrum, in addition to those of the unreacted 1. Based on
the integration ratio of the 1H NMR signals, the binding
Figure 3. The Kohn–Sham plots of the a) LUMO and b) HOMOꢀ5 of
the boracyclophane 2 (B3LYP/6-31G(d) level, isovalue 0.04).
constant of 1 with tBuLi was estimated to be 4.0 ꢀ 10mꢀ1
,
which is rather low compared to that with a fluoride ion (7.0 ꢀ
105 mꢀ1).[8] Therefore, an excess amount of tBuLi was neces-
sary to completely shift the equilibrium to the product side.
The X-ray crystallographic analysis of the boracyclophane
2 revealed the characteristic structural features (Figure 2).[10]
The tricoordinate boron atom is slightly pyramidalized. The
sum of the three C-B-C angles is 349.18. The most notable
feature is that the boron atom is fixed above the eliminated
benzene moiety in close proximity. The B1···C1 distance of
This interaction is in stark contrast to an ordinary p–p* orbital
interaction in conventional arylboranes.[14] In contrast, the
HOMOꢀ5 indicated a bonding interaction between the
boron moiety and the p orbital of the benzene ring (Fig-
ure 3b). Therefore, we conducted the atoms in molecules
(AIM) analysis[15] of 2 using its crystal structure to evaluate
the bonding interaction between the B1 atom and the closest
C1 atom in the benzene ring. We found a bond path between
the B1 and C1 atoms with a bond critical point, in which the
electron density 1(r) was 0.037. The small positive value of the
2
5 1(r) value (0.046) suggested that the bonding interaction is
slightly ionic.
These results demonstrated that the vertical p-p orbital
overlapping between the boron atom and the benzene ring
results in the attractive interaction. However, this interaction
should be inherently weak because of the low Lewis acidity of
the boron moiety and low nucleophilicity of the benzene ring.
We envisioned that the introduction of an electron-donating
group onto the eliminated benzene ring would enhance the
orbital interaction. Based on this idea, we synthesized the
boracyclophane 4 with a dimethylamino group at the para-
position of the benzene ring.
The compound 4 was synthesized in four steps from the
monobrominated precursor 5 (Scheme 2). The planarized
triphenylborane 6 with a diphenylimino moiety was prepared
by the palladium-catalyzed coupling of 5 with benzophenone
imine.[16] Hydrolysis of 6 under acidic conditions gave the
amino derivative 7. The subsequent reductive methylation
Figure 2. ORTEP drawing of 2 (thermal ellipsoids shown at 50%
probability). Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]: B1–C7 1.595(2), B1–C14 1.596(2), B1–C20
1.627(2), C1–C2 1.411(2), C1–C6 1.402(2), C2–C3 1.393(2), C3–C4
1.380(3), C4–C5 1.388(3), C5–C6 1.395(2); C7-B1-C14 110.42(13),
C7-B1-C20 118.95(13), C14-B1-C20 119.87(14).
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notable features. First, the structure of 4 has a short contact
between the boron atom and the benzene ring. The B1-C1
distance is 1.842(3) ꢁ, which is shorter than that in 2 (2.23 ꢁ).
The boron atom is more pyramidalized compared to that in 2,
where the sum of the three angles, C5-B1-C5*, C5-B1-C12,
and C5*-B1-C12, is 338.78. In addition, the C1 atom adopts
a tetrahedral-like geometry. The H1-C1-C4 angle is 141.78.
These structural features indicated the strong coordination of
the benzene ring to the boron. The AIM analysis using the
crystal structure of 4 showed a bond path between the B1 and
C1 atoms. At the bond critical point, the 1(r) value was 0.085,
which is larger than that of 2, and indicative of the bonding
interaction enhancement.
The second notable feature is that 4 has a significant bond
alternation in the eliminated benzene ring. The compound 2
only has the small bond alternation in the eliminated benzene
ring (1.380(3)–1.411(2) ꢁ). In contrast, the C1-C2 and C3-C4
=
Scheme 2. Synthesis of 4. Reagents and conditions: a) HN CPh2
(1.2 equiv), [Pd(dba)2] (5 mol%), dppf (10 mol%), tBuONa
(1.3 equiv), toluene, reflux, 12 h; b) 4n aq. HCl, THF, RT, 18 h; c) 37%
aq. HCHO, NaBH4, 3m aq. H2SO4, THF, 08C then RT, 15 h; d) tBuLi
(3 equiv), benzene, RT, 1 h, then 1n aq. HCl, RT. dba=dibenzylidene-
acetone, dppf=1,1’-bis(diphenylphosphino)ferrocene, THF=tetra-
hydrofuran.
bond lengths in
4 are elongated to 1.4338(18) and
1.4202(19) ꢁ, respectively, while the C2-C3 bond is shortened
to 1.371(2) ꢁ. The C4-N1 bond length is also shortened to
1.353(3) ꢁ, which is indicative of a double bond character.
These data suggest that the benzene ring is significantly
deformed to a quinoidal structure like 4q in Figure 4.
with aqueous formaldehyde and NaBH4 in the presence of
H2SO4 successfully produced the dimethylamino-substituted
planarized triphenylborane 8 in good yield. Finally, the
treatment of 8 with tBuLi and subsequent quenching with
aqueous HCl produced the boracyclophane 4 in 77% yield.
The structure of the dimethylamino-substituted 4 was
successfully determined by X-ray crystallography.[10] The
crystal structure consists of two crystallographically inde-
pendent molecules, whose structural parameters are almost
comparable to each other. Therefore, the structural data for
one of these molecules are considered in this discussion, in
which the H1 atom was located on the difference Fourier
maps and isotropically refined (Figure 4). There are several
To confirm the zwitterionic and quinoidal characters in 4,
we investigated the solvent effect on the structure of 4 by
NMR analysis. In CDCl3, the 11B NMR spectrum of 4 showed
a signal at d = 23.0 ppm with the full width at half maximum
(FWHM) of 280 Hz. The chemical shift is highly upfield-
shifted and the signal is sharpened compared to those of 2
(d = 51.1 ppm; FWHM, 660 Hz). This observation was con-
sistent with the shortened B1-C1 distance in the crystal
structure of 4. Furthermore, the 11B NMR chemical shift of 4
was downfield-shifted to d = 24.5 ppm in C6D6, and upfield-
shifted to d = 17.7 ppm in [D6]acetone, thus indicating that the
interaction between the boron atom and the benzene ring is
strengthened as the solvent becomes more polar. In con-
junction with these changes, the chemical shift of the H1 atom
was also shifted upfield as the solvent polarity increased from
C6D6 (d = 6.38 ppm) to CDCl3 (d = 6.19 ppm) to [D6]acetonce
(d = 5.85 ppm). Importantly, these results imply that the
degree of the boron–benzene interaction can be modulated
by the environment around the molecule. The structural
optimization calculations (B3LYP/6-31G(d)) taking the sol-
vent polarity into consideration using the polarizable con-
tinuum model (PCM) also supported this consideration.
While the optimized structure in benzene showed the B1-C1
distance of 2.07 ꢁ, it was shortened to 1.94 ꢁ in acetone.[13]
The natural bond orbital (NBO) analysis at the B3LYP/6-
31G(d) level of theory[13] provided insights into the bonding
interaction between the boron atom and the eliminated
benzene ring. In the boracyclophane 2, we found six s-
bonding NBOs and three p-bonding NBOs in the benzene
ring, thus indicating that the benzene ring remains as
a “benzene”. No bonding NBO was found between the B1
and C1 atoms. However, the electronic perturbation from the
C1-C2 p-bonding orbital to the p orbital on the boron atom
was suggested. The charge-transfer stabilization energy was
estimated to be 33.86 kcalmolꢀ1. The occupancy (1.58) of the
C1-C2 p-bonding NBO was less than 2.0, while that of the
Figure 4. ORTEP drawing of 4 (thermal ellipsoids shown at 50%
probability) and a postulated canonical structure 4q. Hydrogen atoms
except H1 are omitted for clarity. Selected bond lengths [ꢀ] and angles
[8]: B1–C1 1.842(3), B1–C5 1.610(2), B1–C12 1.663(3), C1–C2
1.4338(18), C2–C3 1.371(2), C3–C4 1.4202(19), C4–N1 1.353(3); C5-
B1-C5* 108.96(17), C5-B1-C12 114.88(10), C2-C1-H1 114.1(5), C2-C1-
C2* 119.56(18).
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boron p orbital was 0.25 (Supporting Information). The
Wiberg bond index[17] was calculated to be 0.18 for the B1···C1
of 2. These results demonstrated that the boracyclophane 2
can be regarded as a p complex between the Lewis acidic
boron atom and the benzene ring with electron donation from
the p orbital of the benzene to the boron atom.
In contrast, in the dimethylamino-substituted 4, while the
eliminated benzene ring has six s-bonding NBOs, only two p-
bonding NBOs (C2-C3 and C2*-C3*) were found. These
NBOs also support the quinoidal structure of the benzene
ring as previously mentioned. Most importantly, the analysis
showed a s-bonding NBO between the B1 and C1 atoms, and
consists of the hybrid orbitals of the boron and carbon atoms
with a high p character (B: sp4.77, C: sp5.08). This s-bonding
NBO clearly demonstrated that the boracyclophane 4 has
a high s-complex character, in which the boron atom and the
benzene ring are strongly bound to each other. The natural
localized molecular orbital (NLMO) also suggested the s-
bonding character (see the Supporting Information). The
Wiberg bond index for the B1-C1 bond of 4 was 0.49, which
was much higher than that for 2. In addition, the deformation
of the benzene ring in 4 was supported by the nucleus
independent chemical shift (NICS).[18] While the NICS(0)
value of the benzene ring in 2 was calculated to be ꢀ6.66 ppm
at the B3LYP/6-31 + G(d) level of theory, that of 4 was
ꢀ3.12 ppm, which is indicative of the reduced aromaticity in
4.
Figure 5. ORTEP drawing of 3·K (thermal ellipsoids shown at 50%
probability). Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]: B1–C1 1.608(5), B1–C7 1.608(5), B1–C14
1.610(5), B1–C20 1.737(5); C1-B1-C7 108.4(3), C1-B1-C14 108.1(3),
C7-B1-C14 107.5(3).
defined by the distance between the B1 atom and the plane
that consists of three carbon atoms (C4, C10, and C17) on the
para-position of the benzene rings, is 1.36 ꢁ. Thus, the bowl
structure of the tert-butylborate 3·K is shallower than the
corresponding fluoroborate, whose bowl depth is 1.65 ꢁ in
the crystal structure.[8] Interestingly, the B1-C20 bond adopts
an eclipsed conformation, which likely results from avoiding
the steric repulsion with the peripheral methyl groups.
Consequently, the B1-C20 bond of 1.737(5) ꢁ is rather long.
In addition, although the boracyclohexadiene ring with the
CH2 moiety adopts a boat conformation, the two CMe2
moieties are not significantly deviated from the planes of
the remaining four carbon atoms in the boracyclohexadiene.
This is also likely due to steric repulsion with the tert-butyl
group and the methyl groups on the benzyl position.
Finally, we investigated the reduction of the boracyclo-
phane 2 to generate a radical anion species. As previously
mentioned, its LUMO consists of the vertical orbital inter-
action between the vacant p orbital of the boron atom and the
p* orbital of the benzene ring. We envisioned that the one-
electron reduction would produce a radical anion with a one-
[19–21]
ꢀ
electron B C bond.
The treatment of 2 with potassium
metal in THFand subsequent recrystallization in the presence
of [2.2.2]cryptand produced colorless crystals (Scheme 3).
In summary, we have investigated the reaction of the
planarized triphenylboranes 1 and 8 with tBuLi and obtained
2 and 4, respectively. Despite the structural constraint in the
planarized triphenylboranes, they underwent the formal
nucleophilic substitution reaction on the boron center
through a tetracoordinate borate intermediate. The structural
elucidation of the boracyclophanes 2 and 4 provided insights
into the interaction between the boron atom and the
eliminated benzene ring. Notably, the interaction mode, that
is, the degree of the s- and p-complex character, is dependent
on the electron-donating character of the benzene ring. Thus,
the introduction of the dimethylamino group to the benzene
ring strongly alters the interaction from a p-complex charac-
ter to a s-complex character. These structures can be
regarded as the snapshots for the activation of an aromatic
benzene ring with a Lewis acid. These findings should be
important for the coordination chemistry of the main group
compounds.
Scheme 3. Reduction of boracyclophane 2.
The NMR spectroscopy and X-ray crystallographic analysis
revealed that the product was not the expected radical anion,
but a potassium salt of the tert-butylborate 3·K. Although the
mechanism of the generation of 3·K is unclear at this stage,
one possibility is the formation of the radical anion and
subsequent elimination of a hydrogen radical. This result was
consistent with the cyclic voltammogram of 2, which showed
an irreversible reduction wave with the cathodic peak
potential Epc of ꢀ2.38 V vs. Fc/Fc+.
In the X-ray crystal structure of 3·K (Figure 5),[10] similar
to the previously reported corresponding fluoroborate,[8] the
borate 3ꢀ has a bowl-shaped structure. The bowl depth,
Received: May 4, 2013
Published online: && &&, &&&&
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Keywords: arenes · boron · cyclophanes · structure elucidation ·
X-ray diffraction
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