Nitro-substituted stilbeneboronate pinacol esters and their fluoro-adducts. Fluoride ion induced polarity enhancement of arylboronate esters

Article abstract of DOI:10.1021/jo070084v

(Chemical Equation Presented) A series of stilbeneboronate pinacol cyclic esters, containing none to three nitro groups, have been synthesized by various olefination reactions and characterized by X-ray single-crystal structure analysis. A stilbeneboronat

Full text of DOI:10.1021/jo070084v

Nitro-Substituted Stilbeneboronate Pinacol Esters and Their
Fluoro-Adducts. Fluoride Ion Induced Polarity Enhancement of
Arylboronate Esters
Alexander Oehlke,† Alexander A. Auer,‡ Ina Jahre,‡ Bernhard Walfort,§ Tobias Ru¨ffer,§
Petra Zoufala´,§ Heinrich Lang,§ and Stefan Spange*,†
Department of Polymer Chemistry, Department of Theoretical Chemistry, and Department of Inorganic
Chemistry, Institute of Chemistry, Chemnitz UniVersity of Technology, Strasse der Nationen 62,
09111 Chemnitz, Germany
stefan.spange@chemie.tu-chemnitz.de
ReceiVed January 15, 2007
A series of stilbeneboronate pinacol cyclic esters, containing none to three nitro groups, have been
synthesized by various olefination reactions and characterized by X-ray single-crystal structure analysis.
A stilbeneboronate ester bearing electron-acceptor groups experiences transition to a push-pull π-electron
system upon complexation with one fluoride ion at the boron atom. The UV-vis absorption maxima of
the presented nitro-substituted stilbeneboronate esters are red-shifted upon addition of fluoride ions,
indicating this binding event. The enhancement of the polarity of the investigated compounds and the
changes in the electronic system were investigated by UV-vis absorption spectroscopy and solvato-
chromism. Additionally, studies were performed by natural bond orbital (NBO) analysis and RI-CC2
calculations of the vertical excitation energies. The synergism of fluoride ion complexation and solvation
upon the UV-vis band shift is interpreted in terms of linear solvation energy relationships (LSERs)
using the Kamlet-Taft solvent parameter set. It is found that the UV-vis absorption of the fluoro-
boronates is strongly dependent on the solvents hydrogen-bond donating ability.
Introduction
compounds with a boron moiety not directly bound to the
chromophore have been prepared, i.e., colorimetric internal
charge-transfer (ICT) carbohydrate sensors.4 A large number
of chromophoric compounds containing sp2-hybridized boron
moieties directly bound to a conjugated π-electron system were
synthesized during the past decade.5-10 Derivatives of arylbo-
ronic acids, arylboronate esters, and triarylboranes belong to
these classes of compounds in which the vacant p-orbital of
Arylboronic acids and arylboronate esters, known as building
blocks for a variety of coupling reactions,1 i.e., Suzuki cross-
coupling,1a are also well-established compounds for the detection
of carbohydrates2 as a result of their ability to reversibly form
cyclic esters with appropriate diols.3 Several π-chromophoric
† Department of Polymer Chemistry.
‡ Department of Theoretical Chemistry.
§ Department of Inorganic Chemistry.
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10.1021/jo070084v CCC: $37.00 © 2007 American Chemical Society
Published on Web 05/18/2007
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J. Org. Chem. 2007, 72, 4328-4339

Polarity Enhancement of Arylboronate Esters
the boron atom participates in the aromatic system. Azo
compounds,5 Schiff bases,6 stilbenes,7 and related compounds8
were reported with directly bound boronic acid or boronate ester
moieties. Derivatives of triarylboranes that are essential com-
ponents of chromophoric π-systems (stilbenes, biphenyls,
acetylenes, thiophene derivatives)9,10 have been also well
established as new materials in the field of organic light-emitting
devices (OLED),9 optoelectronic, and two-photon materials.10
Another scope of application of boron-containing chromophoric
π-systems is the area of selective anion sensing.11 Intensive
research was focused on the field of non-boron-containing
fluoride ion sensors that is especially based on the strong
hydrogen-bond accepting ability of the fluoride ion.12 However,
fluoride ion sensing with organoboron compounds is still a
promising field of science, including arylboronic acids, aryl-
boronate esters, and arylboranes.13-16 The strong interaction of
three-coordinate boron compounds as “hard” Lewis acids and
fluoride ions as “hard” Lewis bases is one of the origins for the
high selectivity of boron-containing sensing materials toward
fluoride ions. Hitherto manifold detection methods were devel-
oped utilizing different fluoride-driven responses. These ap-
proachesusechangesofthefluorescence13 andthecolorimetric13d,14
behavior. Also fluoride sensors based on redox reactions9j,15 and
probes using phosphorescence16 were reported. Furthermore,
sensors based on boron-containing subphthalocyanines are
known.17 In addition, approaches related to fluoride-sensing exist
for the detection of cyanide ions.17a,18 Arylboronic acids are
mostly reported as boron-containing fluoride sensors, but they
show multiple equilibria due to the formation of various fluoro-
borate species.13a,b,g They also have the disposition to form cyclic
boroxines, which implies difficulties during syntheses and
analyses. These disadvantages can be circumvented by the
utilization of arylboronate esters13e,f,h,15c as protecting groups
such as pinacol cyclic esters.13h
The coordination of Lewis bases such as amines,19 F-, HO-,
and CN- at the planar boron atom alters the hybridization from
sp2 to sp3 and, hence, changes the electronic character. Up to
now, Hammett constant characteristics of the electronic effect
of boron-containing substituents are rarely reported for related
types of moieties including sp2 and sp3-hybridized boron
atoms.20 This is particularly true for fluoro-adducts of arylbo-
ronate esters. To display the difference between the electronic
effects of the two different states of hybridization, the Hammett
constants of the boronic acid moiety -B(OH)2 (σp ) 0.12; σm
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) -0.01)20 and its boronate form -B(OH)3 (σp ) -0.44; σm
-
) -0.48)20 can be taken into account. However, the latter form
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J. Org. Chem, Vol. 72, No. 12, 2007 4329

Oehlke et al.
with the appropriate diethylphosphonates to the stilbeneboronate
pinacol esters 3 and 4. Compound 5 was obtained by Knoev-
enagel condensation of 2 with 2,4-dinitrophenylacetic acid using
catalytic amounts of piperidine. Rozhkov and co-workers at last
reported the well-known reaction of 2,4,6-trinitrotoluene (TNT)
with aromatic aldehydes.22 A modified procedure was applied
for the synthesis of the substituted trinitrostilbene 6 by using
catalytic amounts of piperidine. Use of equimolar amounts of
piperidine should be avoided because of the formation of
piperidine-boron adducts or Meisenheimer-analogue intermedi-
ates. As both obstacles represent equilibria reactions, catalytic
amounts of the applied base are required for the respective
reaction. Compounds 3 and 6 were obtained in good yields,
while the yields of 4 and 5 are only 31% and 21%, respectively.
The exclusive trans-configuration of the central CdC double
FIGURE 1. An arylboronate ester unit and an electron acceptor (Ac)
are located at the same π-system. Change of hybridization of boron
upon complexation with a Lewis base (L) led to the formation of a CT
enhanced push-pull π-electron system.
bond of 3-6 in solution is shown by the coupling constant 3JHH
,
which is around 16.3 Hz for all stilbenes. H and 13C NMR
spectra are consistent with the molecular structure of 3-6. The
signal for the carbon atom directly bound to the boron atom is
not observed because of line broadening as a result of the
quadrupole moment of 11B. The chemical shifts observed for
11B at about δ ) 30 ppm are typical values for arylboronate
pinacol esters.6b,13h
1
FIGURE 2. Molecular structure of the investigated stilbeneboronate
esters 3-6.
including the pinacolboronate unit, approach zero, as can be
seen by σp ) 0.04 and σm ) 0.01 for the dimethoxyboronate
unit -B(OMe)2.20 These substituents can therefore be considered
as approximately neutral. The presence of electron-accepting
groups such as -NO2 and a boronate ester moiety at the same
chromophore can be considered as a pull π-electron system,
which is therefore changed to a push-pull π-electron system
upon complexation with fluoride. As a consequence of this
binding event an enhanced charge-transfer (CT) interaction
between the anionic arylboronate ester and the acceptor groups
(Figure 1) is observable.7c
In this paper we report on the influence of the polarity
enhancement of a boron moiety directly bound to a chro-
mophoric π-electron system. For this work we have synthesized
a series of stilbeneboronate pinacol cyclic esters bearing none,
one, two, or three nitro groups as electron acceptors to adjust
the acceptor strength (Figure 2).
It is suggested that the pull π-electron system will be
transformed to a push-pull π-system showing CT effects
noticeable by a corresponding bathochromic shift of its UV-
vis absorption maximum. The complex formation at the boron
atom will be performed with n-Bu4NF (TBAF) as fluoride ion
source. The solvatochromism of the arylboronate esters as well
as the fluoro-boronate adducts has been studied to show the
influence of synergism of solvent interactions and fluoride ion
complexation upon the chromophoric system. The geometries
of the four stilbeneboronate pinacol esters and the corresponding
fluoro-adducts were optimized with DFT methods at the B3-
LYP/TZVP level of theory. Their electronic structure has been
investigated by means of natural bond orbital (NBO) analysis
and the optical properties by CC2 response calculations.
X-ray Single-Crystal Structure Analysis. The molecular
structures of the (E)-stilbeneboronate pinacol esters 3-6 could
be determined by X-ray crystallography. The crystallographic
data and collection parameters as well as ORTEP plots of 3-6
are provided as Supporting Information. The presence of a
central trans CdC double bond in the crystalline state is
displayed for all four compounds. There are two independent
molecules in the asymmetric unit for 3 and 5. There is a trend
toward a slightly longer central CdC double bond (C7-C8)
with an increasing number of nitro groups bound at the stilbene
unit referring to a higher degree of quinoidal resonance
structures. Experimental bond lengths are consistent with those
obtained from DFT calculations (see below). Selected bond
lengths and torsions angles are compared with computed DFT
values in Table 3.
Fluoro-Boronate Adducts. The fluoro-boronate adducts were
generated by adding TBAF (1 M in THF or 0.1 M in THF) to
freshly prepared solutions of the stilbenes 3-6. The formation
of the respective fluoro-boronate species was monitored by 11
B
NMR spectroscopy. Upon gradual addition of TBAF to sto-
ichiometric amounts of the stilbenes the 11B signal at ca. 30
ppm disappears, and a new signal at ca. 5 ppm is observed,
which is typical for tetrahedral coordinated fluoro-pinacolbor-
onate species.9j,13h The selectivity of boron compounds toward
fluoride ions was reported in the literature for several
examples.13h,15a Small changes of the UV-vis absorption of 6
were observed by addition of n-Bu4NCl. Addition of bromide
or iodide ions, in form of their n-Bu4N+ salts, to 3-6 did not
change their UV-vis absorption. The formation of an anionic
fluoro-boronate species refers to an enhancement of the polarity
from an approximately neutral boron unit to an electron-donating
group (Figure 3). The interaction of the generated electron-
donating group through the polarizable stilbene π-system with
an electron-withdrawing group leads to the formation of a CT
enhanced push-pull π-electron system. The resulting changes
Results and Discussion
Synthesis of Substituted (E)-Stilbeneboronate Pinacol
Esters. As shown in Scheme 1, the synthesis of compounds
3-6 starts with the protection of 4-formylbenzeneboronic acid
(1) with pinacol in dry diethyl ether.9j,21 The resulting pinacol
ester 2 is converted via Horner-Wadsworth-Emmons reaction
(22) (a) Rozhkov, V.; Kuvshinov, A.; Gulevskaya, V.; Chervin, I.;
Shevelev, S. Synthesis 1999, 12, 2065-2070. (b) Splitter, J.; Calvin, M. J.
Org. Chem. 1955, 20, 1086-1115.
(23) Fischer, K.; Prause, S.; Spange, S.; Cichos, F.; von Borczyskowski,
C. J Polym. Sci., Part B: Polym. Phys. 2003, 41, 1210-1218.
(21) Koolmeister, T.; So¨dergren, M.; Scobie, M. Tetrahedron Lett. 2002,
43, 5965-5968.
4330 J. Org. Chem., Vol. 72, No. 12, 2007

Polarity Enhancement of Arylboronate Esters
SCHEME 1. Synthesis of Substituted Stilbeneboronate Esters 3 - 6a
of the electronic properties, which occur upon complexation
with one fluoride ion, can easily be observed by UV-vis
absorption spectroscopy (Figure 4). Corresponding UV-vis
absorption maxima are given in Table 1.
results were reported for stilbeneboronic acids by DiCesare and
Lakowicz.13b The calculated binding constants for the boronate
esters 4-6 are comparable to those determined for donor-
substituted triarylboranes.10c,13c,d,j,14b However, the values cal-
culated in this work are larger than previously reported ones
for the interaction of boronate esters with fluoride ions.13h,15c
The binding properties of all investigated nitrostilbenes cor-
respond to a detection limit for F- in the micromolar range under
experimental conditions applied. According to the absorption
coefficients of the fluoro-boronate species 4‚F-6‚F-, detection
limits in the submicromolar range are possible down to 2 ×
10-7 M, as estimated by the Lambert-Beer law. A comparable
detection limit was reported for a B(pin)-substituted dithieno-
phosphole system.13h
Study of Solvatochromism. The nitrostilbeneboronate esters
4-6 and their fluoro-adducts 4‚F--6‚F- show solvatochromic
effects. The extent of the solvatochromic shift ∆ν˜ for each
compound is a function of the solvent polarity, given as
subscript. The solvatochromic ranges observed for the fluoro-
boronate species (∆ν˜2,2,2-trifluoroethanol/HMPA ) 2715 cm-1 (4‚F-),
∆ν˜2,2,2-trifluoroethanol/THF ) 2860 cm-1 (5‚F-), ∆ν˜2,2,2-trifluoroethanol/THF
) 3410 cm-1 (6‚F-)) are larger compared to those of the
stilbeneboronate esters (∆ν˜diethylether/DMSO ) 1110 cm-1 (4),
With the increasing number of nitro groups bound at the
stilbene unit (increasing acceptor strength) the UV-vis absorp-
tion maximum measured in the nonpolar solvent cyclohexane
is shifted to longer wavelengths, as can be seen from Table 1.
Compounds 5 and 6 measured in dichloromethane show the
same UV-vis absorption maximum caused by small but slightly
different solvatochromic effects (see below). The generation of
the fluoro-boronate species was performed in dichloromethane
solutions of the investigated stilbenes 3-6, due to the insolubil-
ity of TBAF in nonpolar solvents such as cyclohexane and
n-hexane, respectively. A bathochromic shift of the UV-vis
absorption band can be observed upon addition of TBAF to
4-6. Compound 3 shows only minimal spectral changes upon
addition of TBAF. Its UV-vis absorption maximum is not
shifted to longer wavelengths, due to the absence of a push-
pull π-electron system. The fluoride ion induced red shift of
the UV-vis absorption increases from 4/4‚F- (∆λmax ) 27 nm;
∆ν˜max ) 1991 cm-1) to 5/5‚F- (∆λmax ) 47 nm; ∆ν˜max
)
3109 cm-1) and 6/6‚F- (∆λmax ) 66 nm; ∆ν˜max ) 4174 cm-1).
This is caused by increasing the strength of the push-pull
π-electron system with the growing number of nitro groups at
the acceptor unit of the stilbene chromophore (increased acceptor
strength).
∆ν˜methanol/TCE ) 1050 cm-1 (5), ∆ν˜HMPA/tetrachloromethane
)
2250 cm-1 (6); see Supporting Information). However, com-
pounds 3 and 3‚F- show no significant solvatochromism. To
quantify individual solvation effects the simplified Kamlet-
Taft equation (eq 1) was used, from which the coefficients of
the individual interaction can be determined using multiple
correlation analysis.25
Gradual addition of fluoride ions to 4-6 (illustrated in Figures
5-7) allows the determination of binding constants.14d,24 The
coordination of one fluoride ion to the respective stilbenebor-
onate esters is clearly seen by the titration curves (see Supporting
Information). UV-vis titration experiments in dichloromethane
at 25 °C yielded different fluoride binding constants for 4 (1.9-
(0.3) × 105 M-1), 5 (3.8(0.4) × 105 M-1), and 6 (5.3(0.8) ×
105 M-1), which corresponds to the growing acceptor strength
with the growing number of nitro groups. The larger the
electron-withdrawing character of the acceptor group, the larger
the electrophilicity of the stilbeneboronate pinacol esters. Related
ν˜max ) ν˜max,0 + aR + bâ + sπ*
(1)
The parameter ν˜max is the longest-wavelength UV-vis absorp-
tion maximum of a compound measured in a particular solvent,
ν˜max,0 is that of a nonpolar reference environment, R is the
(25) (a) Reichardt, C. Chem. ReV. 1994, 94, 2319-2358. (b) Reichardt,
C. SolVents and SolVents effects in Organic Chemistry, 3rd ed.; Wiley-
VCH: Weinheim, 2003.
(24) Connors, K. A. Binding Constants. The Measurement of Molecular
Complex Stability; John Wiley & Sons, Inc.: New York, 1987.
(26) Marcus, Y. Chem. Soc. ReV. 1993, 22, 409-416.
J. Org. Chem, Vol. 72, No. 12, 2007 4331

Oehlke et al.
FIGURE 3. Coordination of one fluoride ion onto the pinacolboronate
unit illustrated for 4/4‚F- as their DFT optimized geometries.
FIGURE 5. UV-vis absorption spectra of 4 (c ) 0.6 × 10-4 M),
measured in dichloromethane with different concentrations of TBAF
added.
FIGURE 4. Changes in UV-vis absorption spectra of 3-6 upon
complexation with one fluoride ion measured in dichloromethane.
TABLE 1. Comparison of Experimental and Calculated UV-vis
Absorption Maxima of 3-6 and Their Fluoro-Boronate Adducts
3‚F--6‚F-
a
b
c
d
exptl λmax
(nm)
exptl λmax
(nm)
calcd λmax
(nm)
calcd λmax
(nm)
3
317
352
363
371
318
318
355
382
366
413
366
432
275
311
296
398
313
449
333
501
3·F-
4
336
384
354
428
370
474
4·F-
5
5·F-
6
FIGURE 6. UV-vis absorption spectra of 5 (c ) 0.8 × 10-4 M),
measured in dichloromethane with different concentrations of TBAF
added.
6·F-
a Measured in cyclohexane. b Measured in dichloromethane. c Calculated
gas-phase values using eq 1 regarding to solvatochromism results of Table
2 (gas phase: R and â are presumed to be 0; π* ) -0.8).23 d Calculated
at the RI-CC2/SVP level of theory.
hydrogen-bond donating (HBD) ability, â is the hydrogen-bond
accepting (HBA) ability, and π* describes the dipolarity/
polarizability of the solvent. The parameters a, b, and s are
solvent-independent correlation coefficients, the sign of which
indicates the direction of the shift of the UV-vis absorption
maxima. The shift relates to the strength of the different
solvation of the ground and the excited state of the solute.
To determine these correlation coefficients, the UV-vis
absorption maxima of 4‚F--6‚F- were measured in solvents
of different polarity by adding TBAF to respective solutions of
4-6. The use of solvents is restricted owing to the lack of
solubility of TBAF in nonpolar solvents. The UV-vis absorp-
tion maxima of 4-6 are accessible in a wider range of solvents.
These values are available as Supporting Information. Values
of R, â, and π* were taken from ref 26. The results of the
Kamlet-Taft correlation are compiled in Table 2 for the square
analyses with the best fit and significance. A representative
collection of solvent-dependent UV-vis spectra of fluoro-
boronate species is illustrated for 6‚F- in Figure 8.
FIGURE 7. UV-vis absorption spectra of 6 (c ) 1.2 × 10-4 M),
measured in dichloromethane with different concentrations of TBAF
added.
A comparison between measured and calculated ν˜max values
(Table 2) of the UV-vis absorption maxima of 4‚F--6‚F- is
given in Figures 9-11. Clear trends are visible in the stil-
beneboronate series and the fluoro-boronate series except for
species 4‚F- measured in solvents with R ) 0. The Kamlet-
4332 J. Org. Chem., Vol. 72, No. 12, 2007

Polarity Enhancement of Arylboronate Esters
TABLE 2. Solvent-Independent Correlation Coefficients a, b, and
s of the Kamlet-Taft Parameters r, â, and π*, Solute Property of
the Reference System ν˜max,0, Correlation Coefficient (r), Standard
Deviation (SD), Number of Solvents (n) and Significance (f) for the
Solvatochromism of 4-6 and 4‚F--6‚F-
ν˜max ,0
a
b
s
r
SD
n
f
4
5
6
28.846 0.253
27.630 0.216
27.000 0.398
0
-1.136 0.922 0.124 27 <0.0001
0.363 -0.728 0.863 0.156 29 <0.0001
1.630
0
-1.139
0
0
0
0
0
0.903 0.306 27 <0.0001
0.990 0.075 11 <0.0001
4·F- a 26.022 1.114
4·F- b 26.285
0
0.988 0.062
8 <0.0001
5·F-
6·F-
23.860 1.761
22.535 2.237
0.629 0.961 0.246 17 <0.0001
1.780 0.985 0.185 17 <0.0001
a Measured in solvents with R > 0 and 4-butyrolactone. b Measured in
solvents with R ) 0.
FIGURE 10. Relationship between measured and calculated νmax
values for 5‚F-, measured in 17 solvents of different polarity.
FIGURE 8. Selected UV-vis absorption spectra of 6‚F- in six
different solvents.
FIGURE 11. Relationship between measured and calculated νmax
values for 6‚F-, measured in 17 solvents of different polarity.
polar excited state in comparison to the ground state. Further-
more, a small and steady influence of the parameter R can be
observed for stilbeneboronate esters 4-6, which does not depend
on the number of nitro groups within the experimental error.
The types of interactions of solvents with the solute are
dramatically changed upon complexation with one fluoride ion.
In comparison to 4-6 a large increase of the importance of the
HBD parameter R take place for the solvatochromism of the
fluoro-boronate species 4‚F--6‚F-. The Kamlet-Taft correla-
tion generally yields a strong influence of the HBD parameter
R, which increases with growing number of nitro groups from
4‚F- to 6‚F- (Table 2). The positive value of a refers to negative
solvatochromism and its origin can be interpreted as following.
Solvents with HBD ability form H-bonds to the fluorine atom
of the fluoro-boronate unit, and hence a decrease of electron
donating to the boron atom occurs. As a consequence of
subtraction of electron density at the whole fluoro-boronate unit
the strength of the push-pull π-electron system of the bound
chromophor is diminished and a blue shift of the UV-vis
absorption results that increases with growing HBD ability of
the solvent. On the other hand, solvents with HBD ability can
also form H-bonds to the nitro groups, but this will cause
positive solvatochromism with negative values of a in this case.
The influence of the parameter π* increases with growing
FIGURE 9. Relationship between measured and calculated νmax values
for 4‚F-, measured in 11 solvents with HBD ability (9) and eight
solvents without HBD ability ()). TCE ) 1,1,2,2-tetrachloroethane;
HMPA ) hexamethylphosphoric triamide.
Taft analysis for 4-6 describes an increasing influence of
parameter â with growing number of nitro groups. This is caused
by interactions of HBD solvents at the nitrostilbene unit and
not as assumed at the boron center, which would yield negative
values of b. The steric shielding of the boron atom by the pinacol
cyclic ester is obviously large enough to prevent Lewis acid/
Lewis base interactions with the solvent. The influence of
parameter π* decreases with growing number of nitro groups
down to s ) 0 for 6. The negative values of s indicate a more
J. Org. Chem, Vol. 72, No. 12, 2007 4333

Oehlke et al.
TABLE 3. Selected Calculated Bond Lengths (Å) and Torsion Angles (deg) from Geometry Optimizations of 3-6 and 3‚F--6‚F- and
Selected Bond Lengths (Å) and Torsion Angles (deg) for 3-6 from X-ray Crystallography (in Italics)a
3c
3‚F-
4
4‚F-
5c
5‚F-
6
6‚F-
C4-C7
C7-C8
C8-C9
C12-B1
B1-O2
O1-B1
1.464
1.470(3)
1.343
1.313(3)
1.464
1.480(3)
1.552
1.548(3)
1.376
1.370(2)
1.376
1.461
1.460
1.473(2)
1.345
1.316(2)
1.462
1.481(2)
1.555
1.564(2)
1.374
1.363(2)
1.374
1.447
1.460
1.470(3)
1.346
1.331(3)
1.460
1.463(3)
1.557
1.565(3)
1.373
1.360(3)
1.373
1.440
1.459
1.468(3)
1.345
1.325(3)
1.458
1.475(3)
1.558
1.559(3)
1.372
1.355(3)
1.373
1.429
1.348
1.456
1.633
1.480
1.482
1.356
1.445
1.637
1.475
1.483
1.364
1.436
1.636
1.474
1.475
1.367
1.431
1.637
1.474
1.475
1.365(3)
1.369(2)
1.362(3)
1.356(3)
C3-C4-C7-C8
C7-C8-C9-C14
15.7
7.9(3)
19.2
9.0
4.7
13.8
17.8(3)
16.1
1.4
1.3
25.8
-29.3(3)
12.8
19.4
5.9
38.2
-22.9(4)
4.6
24.7
5.4
-9.3(3)
20.9(3)
-2.9(4)
20.2(4)
a The numbering scheme corresponds to Figure S1 (see Supporting Information). Numbers in parantheses are estimated SDs in the last significant figures.
Bond lengths and torsion angles of only one of the two crystallographically independent, but chemical indentical, molecules are listed.
number of nitro groups starting with s ) 0 for compound 4‚F-.
The positive values of s indicate a more polar ground state in
comparison to the excited state. This is an opposite behavior in
comparison to that of 4-6.
of 0.013 Å for the bond lengths of all non-hydrogen atoms of
3 (0.012 Å for 4, 0.010 Å for 5, 0.016 Å for 6), which is in
agreement with the deviations that can be expected for the B3-
LYP level of theory using basis sets of triple-ú quality in
comparison to X-ray data. As a result of packing effects in the
solid state, the agreement for angles and dihedral angles is not
as good, especially in the case of disordered fragments in the
solid-state structure. Selected bond lengths and torsion angles
as obtained from the geometry optimizations are given in
Table 3 (for a complete list of calculated bond lengths between
non-hydrogen atoms, see Supporting Information).
For species 4‚F- it was found that the parameter R is of
importance concerning solvents with HBD ability (R > 0) as
discussed above. Solvents with no HBD ability (R ) 0) show
an influence solely of the parameter â. This phenomenon is
likely caused by a synergistic Lewis base interaction of such
solvents and fluoride ions with the Lewis acidic arylboronate
ester 4. Both types of interaction support each other and lead
to a bathochromic shift of its UV-vis absorption maximum.
This is strongly supported by the fact that the UV-vis
absorption maximum of 4 in the absence of fluoride ions does
not show any influence on â. Beside this observation the HBA
parameter â is negligible for 4‚F- in solvents with R > 0, 5‚F-,
and 6‚F-. This is also in contrast to the solvatochromism of
4-6, which show a strong dependence on â.
Some changes of the structure of the noncomplexed species
that occur with increasing number of nitro groups at the stilbene
unit should be noted. The quinoidal character of the (nitro/
dinitro/trinitro)styryl unit slightly increases from 3 to 6 (C7-
C8 is slightly lengthened, also shown by X-ray data; C4-C7
and C8-C9 are slightly shortened). Changes occurring at the
phenyl ring directly bound to the boron atom are minimal. The
B-C bond distance is slightly stretched with growing number
of nitro groups. This is due to increased electron-acceptor
strength, which decreases the π-electron donation from the
stilbene unit to the boron atom. The B-O distances of the
pinacolboronate unit slightly decrease in the same manner as a
result of by π-donation from oxygen due to the larger electron
deficit at the boron atom.
Structural Changes upon Complexation with F-. Adduct
formation of boron with one fluoride ion gives an anionic fluoro-
boronate species and forces the boron atom to change the state
of hybridization from sp2 to sp.3 Therefore, structural changes
are observed both at the pinacolboronate unit and the stilbene
chromophore. Figure 12 gives an overview about the alterations
of calculated bond lengths between non-hydrogen atoms oc-
curring upon coordination of fluoride at the boron atom.
Computational Study. To gain deeper insight into the
changes of the molecular and electronic structure of the
stilbeneboronate esters upon complexation with fluoride, the
stilbenes 3-6 and the corresponding fluoro-boronate species
3‚F--6‚F- have been investigated. For this purpose the
molecular structure, electronic density, and vertical excitation
energies have been calculated using post Hartree-Fock ab initio
and density functional theory methods.27
Structural Parameter of the Noncomplexed and Com-
plexed Species. As density functional theory calculations have
proven to yield accurate molecular geometries, the results from
quantum chemical geometry optimizations can be used to
supplement experimental data, especially in cases where X-ray
structural data is difficult to obtain. While experimental X-ray
structural data has only been obtained for 3-6, the equilibrium
geometries for the noncomplexed (3-6) and their complexed
species (3‚F--6‚F-) have been obtained through geometry
optimizations at the B3-LYP/TZVP level of theory.28,29
The comparison between bond lengths obtained by calculation
and X-ray diffraction yields root mean square (rms) deviations
The most important changes take place in the π-electron
system especially in the vicinity of the central CdC double
bond. In general, the structures become more quinoidal upon
complexation with fluoride. This can be seen by alternating
lengthening and shortening of bonds of the conjugated carbon
frame. So the length of the central CdC double bond increases
and the bond lengths of both C-C bonds neighboring the central
CdC double bond decrease. These changes are more pro-
nounced than changes at both phenyl rings, but clear trends are
(27) Jahre, I. Diplomarbeit, Chemnitz University of Technology, 2006.
(28) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(29) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571-
2577.
4334 J. Org. Chem., Vol. 72, No. 12, 2007

Polarity Enhancement of Arylboronate Esters
FIGURE 12. Alteration of calculated bond lengths (Å) of 3-6 upon complexation with one fluoride ion. Positive values refer to longer bond
lengths upon complexation with one fluoride ion.
TABLE 4. Charges from NBO Analysis for 3-6 and 3‚F--6‚F-
subunit
3
3·F-
4
4·F-
5
5·F-
6
6·F-
NO2-1
-0.27
-0.34
-0.25
-0.23
-0.31
-0.28
-0.23
-0.21
-0.19
+0.47
+0.01
+0.08
-0.26
+1.18
-0.30
-0.24
-0.22
+0.40
-0.04
+0.11
-0.21
+1.24
-0.60
-1.73
+0.59
NO2-2
NO2-3
Ph-1
CH-1
CH-2
Ph-2
-0.01
+0.01
+0.01
-0.32
+1.19
-0.09
-0.05
+0.04
-0.36
+1.24
-0.61
-1.73
+0.57
+0.21
0.00
+0.04
-0.30
+1.18
+0.15
-0.06
+0.07
-0.31
+1.24
-0.61
-1.73
+0.58
+0.36
+0.02
+0.06
-0.28
+0.29
-0.03
+0.09
-0.25
+1.24
-0.60
-1.73
+0.58
B
F
+1.18
O-(B)-O
CMe2-CMe2
-1.60
+0.73
-1.60
+0.73
-1.60
+0.74
-1.60
+0.74
visible. The changes are generally larger in the order from 3‚F-
to 6‚F- owing to the increasing push-pull character, i.e., the
alteration of the central CdC double bond length for 6‚F- is
around 4 times larger than that of 3‚F-. As can be seen by
different bond length changes, the extent of conjugation of the
nitro group at the para position is larger than that of the nitro
groups at the ortho positions due to steric hindrance. Further
changes can be observed at the pinacolboronate unit. The B-O
bond distance increases, due to the absence of partial BdO
double bond character, because the neighboring oxygen is not
able to donate an electron pair to a tetrahedral boron. The B-C
distance also increases as a result of the disruption of the
resonance of boron with the π-electron system. In general, a
decrease of the dihedral angles between the mean plane of both
phenyl rings and the ethenyl bridge (C3-C4-C7-C8 and C7-
C8-C9-C14) upon transition from 3-6 to 3‚F--6‚F- takes
place (Table 3), which also conforms to an increase of the donor
strength of the boronate pinacol ester unit upon complexation
with fluoride.
atomic NBO charges that have been summed up for different
subunits of the molecules.
In the compilation made in Table 4, which allows the
quantification of the charge density redistribution, a clear trend
is visible. While the electron-withdrawing influence of the nitro
groups is obvious, also the influence and distribution of the
additional charge from the fluoride ion can be rationalized. For
all compounds the fluoride ion donates about 0.4 electrons, and
the immediate effect on the boron atom is a uniform decrease
of electron density for all compounds, as can be expected from
the difference in electronegativity. The electron density on the
neighboring oxygen atoms (O-(B)-O) altogether and the
pinacol carbon scaffold (CMe2-CMe2) is also increased uni-
formly for all molecules by 10-15% of an electron owing to
the decreased π-donation from oxygen to the boron atom. This
is in agreement with the stretching of the B-O bond of ca.
0.1 Å for all investigated fluoro-boronate species. Smaller yet
much more important changes take place in the chromophoric
stilbene/nitrostilbene unit. For the nitro-substituted phenyl group
Change of Electronic Density upon Complexation with
F-. In order to assess the changes in the electronic structure
that the four species (3-6) undergo upon complexation with
one fluoride ion, a NBO analysis of the Kohn-Sham orbitals
has been carried out for the eight molecules.30 Table 4 lists the
(31) The electron density given in Figure 13 has been calculated at the
B3-LYP/TZVP level of theory. The canonical orbitals given in Figure 14
have been calculated at the HF-SCF/SVP level of theory. The figures were
plotted using the gOpenmol32,33 and the Molden34 program packages in
combination with the povray raytracing tool (for details, see: http://
www.povray.org).
(30) For a thorough investigation of the physical significance of Kohn-
Sham orbitals, see: (a) Chong, D. P.; Gritsenko, O. V.; Baerends, E. J. J.
Chem. Phys. 2002, 116, 1760-1772. (b) Bickelhaupt, F. M.; Baerends, E.
J. In ReViews in Computational Chemistry; Lipkowitz, K. B., Boyd, D. R.,
Eds.; Wiley-VCH: New York, 2000; Vol. 15, pp 1-86.
(32) Laaksonen, L. J. Mol. Graphics Modell. 1992, 10, 33-34.
(33) Bergmann, D. L.; Laaksonen, L.; Laaksonen, A. J. Mol. Graphics
Modell. 1997, 15, 301-306.
(34) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000,
14, 124-134.
J. Org. Chem, Vol. 72, No. 12, 2007 4335

Oehlke et al.
into the conjugated π-electron system induces a further charge
redistribution from the boronate ester unit, which can be
regarded as an electron donor toward the nitrophenyl group
acting as an electron acceptor. This is also supported by
alternating lengthening and shortening of the C-C bonds.
Resonance between the tetrahedral fluoro-boronate unit and the
π-electron system can be neglected, and thus the observed
effects are based only on the strong inductive effect of the
fluoro-boronate unit. This is especially pronounced for the 2,4,6-
trinitrostilbene system 6/6‚F- that exhibits the strongest shift
of the UV-vis absorption band upon complexation with one
fluoride ion.
FIGURE 13. Superposition of electron density isosurfaces for neutral
and complexed species shown for 4/4‚F-; red areas indicate regions
of increased electron density, blue areas indicate regions of decreased
electron density.31
(denoted as Ph-1) as well as for the nitro groups (NO2-1, NO2-
2, NO2-3) themselves, an increased electron density is observed.
For the bridging CH units (denoted as CH-1 and CH-2 according
to the neighboring phenyl group), an increase and decrease of
electron density is observed that can easily be rationalized by
means of quinoidal resonance structures. For the phenyl unit
neighboring the boron atom (denoted as Ph-2), an increase of
electron density is observed for 3/3‚F-, little change occurs for
4/4‚F-, and a decrease of electron density is found for 5/5‚F-
and 6/6‚F-. Overall it can be concluded that the charge injected
Figure 13 gives a more intuitive view on this phenomenon.
In order to be able to directly compare the different electron
densities, this plot shows the superposition of the electron
densities of 4 and a fictitious 4‚F- molecule that has been
constructed by adding a fluoride ion at the equilibrium distance
in 4‚F- to the equilibrium structure of 4.
This example nicely illustrates the points made in the
discussion of the NBO charge analysis. While for the second
phenyl unit (Ph-2) the electron density is slightly enhanced, the
FIGURE 14. Topology of relevant orbitals of 3-6 and 3‚F--6‚F-.31
4336 J. Org. Chem., Vol. 72, No. 12, 2007

Polarity Enhancement of Arylboronate Esters
charge density at the nitro group and the first phenyl group (Ph-
1) is significantly enhanced upon complexation with fluoride.
density upon excitation which appears to be increasing in the
order 3‚F- < 4‚F- < 5‚F- < 6‚F-. In the series of the fluoro-
boronate adducts an increasing localization of the HOMO on
the boronate ester fragment and the LUMO on the nitrophenyl
fragment is observed. This finding is also in agreement with
the experimental observation of the solvatochromic effects. The
charge-density shift upon excitation shows the same trend as
the charge-density redistribution upon complexation with fluo-
ride.
Investigation of the Electronic Excitation Energies. In
modern electronic structure theory time-dependent density
functional theory (TD-DFT) methods have become a standard
tool in the interpretation of optical spectra. However, while these
methods have proven very reliable for many systems, one of
the typical failures of TD-DFT is the correct description of
charge-transfer excitations.35 In such cases, ab initio methods
have to be applied in order to achieve reasonable results. As
recent implementations of the CC2 method36 applying density
fitting techniques allow such calculations for larger systems,
the vertical excitation energies have been calculated using the
RI-CC2 scheme as implemented by Ha¨ttig et al.37
Conclusion
A series of stilbeneboronate pinacol esters 3-6 bearing none
to three nitro groups was prepared by olefination reactions of 2
with different CH-active compounds. X-ray single-crystal
structures were obtained for stilbeneboronate pinacol cyclic
esters 3-6. Addition of TBAF to nitrosubstituted stilbenes leads
to the formation of fluoro-pinacolboronate species representing
push-pull π-electron systems. This enhancement of the polarity
was studied by UV-vis spectroscopy showing bathochromic
shifts of the UV-vis absorption maxima up to ∆λmax ) 66 nm
for 6/6‚F-. Nitro-substituted fluoro-boronate species also show
distinctive solvatochromic behavior with mainly a strong
dependence of the HBD solvent parameter R.
Computational study of the molecular structures gave a good
agreement with experimental X-ray single-crystal data for
geometries of 3-6. Structural information has also been
obtained by DFT methods (B3-LYP/TZVP) for fluoro-boronate
species 3‚F--6‚F- not characterized by X-ray diffraction.
Analysis of bond lengths and electron density indicates that
complexation with one fluoride ion at the boron atom induces
a charge redistribution from the arylboronate pinacol ester
moiety to the nitrophenyl unit. This is especially pronounced
for the fluoro-boronates that exhibit strong shifts of the UV-
vis absorption maximum upon complexation. The analysis of
the electronic excitations as obtained at the RI-CC2/SVP level
of theory indicate that the most important excitations in the long-
wavelength region are excitations in which the same bias on
the charge density is present. This effect again increases from
3 to 6. Using theoretical methods, deeper understanding of
electronic effects and molecular structure can be achieved. Thus,
appropriate computational methods can be utilized in the design
of sensor molecules. Promising candidates can be pre-screened
before synthesis and interesting structures can be suggested on
the basis of theoretical models. Over all it can be concluded
that acceptor-substituted stilbeneboronate pinacol esters intro-
duced in this paper are promising compounds for the colori-
metric detection of fluoride ions.
For the optical spectra of the compounds under investigation,
especially of 4‚F--6‚F-, solvent effects play an important role.
Thus, quantitative agreement of simulated spectra from the CC2
method with experimental results cannot be expected. However,
the calculation of the electronic excitations can still facilitate a
deeper understanding of the influence of electronic effects on
excitation energies and absorption intensities. In order to
estimate the residual error for these results and for comparison
to TD-DFT methods, additional calculations of the vertical
excitation energies have been carried out for 4‚F-. While the
experimental value in dichloromethane solution is λmax ) 382
nm and the corresponding gas-phase value according to solva-
tochromism results is λmax ) 384 nm, the calculated vertical
electronic excitation energy at the RI-CC2/TZVP level of theory
correlates to λmax ) 411 nm. Using the smaller, more cost-
efficient SVP basis set,29 the result changes marginally to λmax
) 398 nm. Thus, an agreement to the experimental data within
about 50 nm can be expected at the RI-CC2 level of theory. It
should be pointed out that the corresponding TD-DFT result
obtained at the B3-LYP/TZVP level is λmax ) 488 nm. The
calculated values obtained at the RI-CC2/SVP level of theory
for the most intensive lines in the long-wavelength region for
all of the investigated compounds are given in Table 1. The
agreement with the experiment is satisfactory, a systematical
error occurs most likely due to the lack of solvent effects as
mentioned before. In regard to solvatochromism results the RI-
CC2 calculated values of the fluoro-boronate species are larger
compared to the experiment due to the nonpolar nature of the
gas phase. It should be noted that the RI-CC2 results also predict
an increasing shift of the most intensive absorption line upon
complexation from 3/3‚F- to 6/6‚F- correctly, which is not the
case for the TD-DFT calculations. An analysis of the dominant
orbital contributions to the most intensive excitations yields
deeper insight into the electronic effects that dominate the
spectral features of these molecules. Generally, for the stil-
beneboronate esters and the fluoro-boronate species the most
intensive line in the long-wavelength region is dominated by
the HOMO-LUMO transition. The topology of the correspond-
ing orbitals is displayed in Figure 14. From a closer inspection
of the orbitals it can be concluded that the effect of complexation
with one fluoride ion is the more pronounced shift of charge
Computational Details
All geometries have been optimized using density functional
theory with the B3-LYP functional in combination with the TZVP
basis set.28,29 The geometries have been converged to a residual
gradient norm of 10-3 au or better. The energy and density matrix
were converged to at least are 10-6 au, and fine quadrature grids
have been used to calculate the exchange and correlation potential
(size 5).38 All minima have been characterized by normal-mode
analysis. The excitation energies have been calculated using the
RI-CC2 method as implemented by Ha¨ttig et al.37 For this purpose,
the first 10 roots of the electronic hessian have been calculated.
(35) (a) Dreuw, A.; Head-Gordon, M. Chem. ReV. 2005, 105, 4009-
4037. (b) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007-
4016.
(36) Christiansen, O.; Koch, H.; Jørgenson, P. Chem. Phys. Lett. 1995,
243, 409-418.
(37) (a) Ko¨hn, A.; Ha¨ttig, C. J. Chem. Phys. 2003, 119, 5021-5036.
(b) Ha¨ttig, C. J. Chem. Phys. 2003, 118, 7751-7761. (c) Ha¨ttig, C.;
Weigand, F. J. Chem. Phys. 2000, 113, 5154-5161.
(38) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165-169.
J. Org. Chem, Vol. 72, No. 12, 2007 4337

Oehlke et al.
Further calculations have been carried out using time dependent
density functional theory35 at the B3-LYP/TZVP level of theory.
For this purpose, the first 50 roots of the electronic hessian have
been calculated. All excitation energies given are low enough to
be compared to -ꢀHOMO, which lies in the order of 6 eV for the
noncomplexed species and 3 eV for the complexed species. It should
be noted that the computed excitation energies are vertical excitation
energies without zero-point energy corrections.
For all of the aforementioned calculations the TURBOMOLE39
program package (for a current version see http://www.turbo-
mole.de) has been used. For the interpretation of the electron
distribution, NBO analyses40 of the Kohn-Sham orbitals have been
carried out at the B3-LYP/DZP level of theory41 using the NBO
program as distributed with the NWChem package.42
7.42 (m, 2H), 7.50-7.63 (m, 4H), 7.83 (d, J ) 8.1 Hz, 2H). 13C
NMR (62.5 MHz, CDCl3): δ 24.9, 83.7, 125.8, 126.6, 127.8, 128.6,
128.7, 129.6, 135.1, 137.2, 140.0 (the signal for the carbon atom
directly bound to the boron atom is not observed due to the
quadrupolar moment of 11B). 11B NMR (80 MHz, CDCl3): δ 31.0
(br). Anal. Calcd for C20H23BO2 (306.19): C, 78.45; H, 7.57.
Found: C, 78.13; H, 7.69. Crystals of 3 suitable for X-ray
diffraction were grown by slow diffusion of perfluoromethylcy-
clohexane into its petrolether solution.
(E)-4-Nitro-4′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
stilbene (4). 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ben-
zaldehyde (0.400 g, 1.72 mmol) was added to a solution of diethyl-
(4-nitrobenzyl)phosphonate (0.471 g, 1.72 mmol) in anhydrous THF
(15 mL) under argon atmosphere. Potassium tert-butanolate
(0.194 g, 1.72 mmol) was added to the clear solution at room
temperature, and the mixture was stirred for 3 h at room temper-
ature. The reaction mixture was filtrated, neutralized with 1 M HCl,
and dried with MgSO4. After filtration and evaporation of the
solvent under reduced pressure, a yellow solid was obtained that
was recrystallized from ethylacetate to give 4. Yield: 0.186 g (31%).
Mp (EtOAc): 216 °C. IR (KBr, cm-1): ν 3100, 3070, 2979, 2872,
Experimental Section
X-ray Crystallography. Crystal data of 4 and 6 were collected
at room temperature using Mo KR radiation (λ ) 0.71073 Å).
Crystal data of 3 and 5 were collected at 100 K using Cu KR
radiation (λ ) 1.54184 Å). All structures were solved by direct
methods using SHELXS-97.43 All structures were refined by full-
matrix least-squares procedures on F2, using SHELXL-97.44 All
non-hydrogen atoms were refined anisotropically. The hydrogen
atom positions have been refined with a riding model. In the case
of 3 the atoms C33 and C34 have been refined disordered on two
positions (C33/C34 and C33a/C34a) with occupation factors of
0.525 and 0.475, respectively. In the case of 4 the atoms O3 and
O4 have been refined disordered on two positions (O3/O4 and O3a/
O4a) with occupation factors of 0.452 and 0.548, respectively. In
case of 6 the atoms C15-C20 have been refined disordered on
two positions (C15-C20 and C15a-C20a) with occupation factors
of 0.688 and 0.312, respectively. Complete details are available as
Supporting Information.
1
1632, 1595, 1515, 1358, 1340. H NMR (250 MHz, CDCl3): δ
1.36 (s, 12H), 7.19 (d, J ) 16.3 Hz, 1H), 7.28 (d, J ) 16.3 Hz,
1H), 7.55 (d, J ) 8.1 Hz, 2H), 7.64 (d, J ) 8.8 Hz, 2H), 7.84 (d,
J ) 8.1 Hz, 2H), 8.22 (d, J ) 8.8 Hz, 2H). 13C NMR (62.5 MHz,
CDCl3): δ 24.8, 83.9, 124.1, 126.2, 126.9, 127.1, 133.2, 135.3,
138.7, 143.7, 146.8 (the signal for the carbon atom directly bound
to boron atom is not observed due to the quadrupolar moment of
11B). 11B NMR (80 MHz, CDCl3): δ 30.5 (br). Anal. Calcd
for C20H22BNO4 (351.19): C, 68.40; H, 6.31; N, 3.99. Found: C,
68.45; H, 6.39; N, 4.03. Crystals of 4 suitable for X-ray diffraction
were grown by slow evaporation of an n-hexane/ethylacetate
solution.
(E)-2,4-Dinitro-4′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)stilbene (5). 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
benzaldehyde (0.400 g, 1.72 mmol) and 2,4-dinitrophenylacetic acid
(0.390 g, 1.72 mmol) were added to anhydrous 1-propanol (5 mL)
and heated to reflux for 30 min. To the clear yellowish solution
was added piperidine (17 µL), and the mixture was stirred under
reflux for 48 h. Upon slow cooling of the reaction mixture, a yellow
solid was formed that was recrystallized from n-hexane/ethylacetate
(1:1 v/v) and dried under vacuum to give 5 as yellow needles.
Yield: 0.142 g (21%). Mp (n-hexane/EtOAc): 176 °C. IR (KBr,
cm-1): ν 3115, 3065, 2983, 2871, 1609, 1531, 1362, 1345, 966.
1H NMR (250 MHz, CDCl3): δ 1.36 (s, 12H), 7.28 (d, J )
16.1 Hz, 1H), 7.56 (d, J ) 8.1 Hz, 2H), 7.68 (d, J ) 16.1 Hz, 1H),
7.85 (d, J ) 8.1 Hz, 2H), 7.98 (d, J ) 8.8 Hz, 1H), 8.43 (dd, J )
8.8 Hz, J ) 2.3 Hz, 1H), 8.82 (d, J ) 2.3 Hz, 1H). 13C NMR (62.5
MHz, CDCl3): δ 24.9, 84.0, 120.7, 122.1, 126.8, 127.1, 129.0,
135.3, 137.9, 138.0, 138.7, 146.3, 147.4 (the signal for the carbon
atom directly bound to boron atom is not observed due to the
quadrupolar moment of 11B). 11B NMR (80 MHz, CDCl3): δ 31.5
(br). Anal. Calcd for C20H21BN2O6 (396.19): C, 60.63; H, 5.34;
N, 7.07. Found: C, 60.62; H, 5.45; N, 7.26. Crystals of 5 suitable
for X-ray diffraction were grown by slow diffusion from n-pentane
into its ethylacetate solution.
(E)-4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)stilbene (3).
4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde (0.400 g,
1.72 mmol) was added to a solution of diethylbenzylphosphonate
(0.393 g, 1.72 mmol) in anhydrous DMF (5 mL) under argon
atmosphere. Potassium tert-butanolate (0.580 g, 5.17 mmol) was
added to the clear solution at 0 °C, and the mixture was stirred for
24 h and allowed to warm up to room temperature. The reaction
mixture was poured into water (20 mL) and neutralized with 1 M
HCl. The white precipitate formed was collected by filtration,
washed thoroughly with water, and dried under vacuum. The residue
was recrystallized from petrolether to give 3 as colorless needles.
Yield: 0.431 g (82%). Mp (petrolether): 123-125 °C. IR (KBr,
cm-1): ν 3079, 3025, 2981, 2932, 1604, 1398, 1359, 1327, 1147,
1
973. H NMR (250 MHz, CDCl3): δ 1.37 (s, 12H), 7.12 (d, J )
16.4 Hz, 1H), 7.21 (d, J ) 16.4 Hz, 1H), 7.25-7.32 (m, 1H), 7.33-
(39) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346-354.
(40) NBO, version 5.0; Glendening, E. D.; Badenhoop, J. K.; Reed, A.
E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F.
Theoretical Chemistry Institute, University of Wisconsin: Madison, WI,
2001.
(41) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823-2833.
(E)-2,4,6-Trinitro-4′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)stilbene (6). 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-
benzaldehyde (0.400 g, 1.72 mmol, 1.00 equiv) was added to a
solution of 2,4,6-trinitrotoluene (0.388 g, 1.71 mmol, 0.99 equiv)
in anhydrous toluene (2 mL). The yellowish solution was heated
to 80 °C for 30 min, and piperidine (10 µL) was added whereby
an orange color appeared instantaneously. The solution was heated
at reflux for 4 h, whereupon the solution was allowed to cool to
room temperature. The yellow solid formed was filtered, washed
with cold toluene, and dried under vacuum. The remaining solution
was reduced under vacuum. The resulting solid was filtered, washed
with cold toluene, and dried under vacuum. The combined solids
were recrystallized from n-hexane/ethylacetate (8:5 v/v) and dried
(42) Apra`, E.; Windus, T. L.; Straatsma, T. P.; Bylaska, E. J.; de Jong,
W.; Hirata, S.; Valiev, M.; Hackler, M.; Pollack, L.; Kowalski, K.; Harrison,
R.; Dupuis, M.; Smith, D. M. A; Nieplocha, J.; Tipparaju, V.; Krishnan,
M.; Auer, A. A.; Brown, E.; Cisneros, G.; Fann, G.; Fruchtl, H.; Garza, J.;
Hirao, K.; Kendall, R.; Nichols, J.; Tsemekhman, K.; Wolinski, K.; Anchell,
J.; Bernholdt, D.; Borowski, P.; Clark, T.; Clerc, D.; Dachsel, H.; Deegan,
M.; Dyall, K.; Elwood, D.; Glendening, E.; Gutowski, M.; Hess, A.; Jaffe,
J.; Johnson, B.; Ju, J.; Kobayashi, R.; Kutteh, R.; Lin, Z.; Littlefield, R.;
Long, X.; Meng, B.; Nakajima, T.; Niu, S.; Rosing, M.; Sandrone, G.; Stave,
M.; Taylor, H.; Thomas, G.; van Lenthe, J.; Wong, A.; Zhang, Z. NWChem,
A Computational Chemistry Package for Parallel Computers, 4.7; Pacific
Northwest National Laboratory: Richland, WA, 2005.
(43) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467-473.
(44) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, 1997.
4338 J. Org. Chem., Vol. 72, No. 12, 2007

Polarity Enhancement of Arylboronate Esters
under vacuum to give 6 as yellow needles. Yield: 0.596 g (79%).
Mp (n-hexane/EtOAc): 181 °C. IR (KBr, cm-1): ν 3105, 3078,
2983, 2938, 1605, 1543, 1358, 976. 1H NMR (250 MHz, CDCl3):
δ 1.36 (s, 12H), 6.76 (d, J ) 16.5 Hz, 1H), 7.39 (d, J ) 16.5 Hz,
1H), 7.47 (d, J ) 8.1 Hz, 2H), 7.83 (d, J ) 8.1 Hz, 2H), 8.87 (s,
2H). 13C NMR (62.5 MHz, CDCl3): δ 24.8, 84.0, 117.2, 122.2,
126.6, 133.7, 135.3, 137.2, 138.8, 145.9, 150.2 (the signal for the
carbon atom directly bound to boron atom is not observed due to
the quadrupolar moment of 11B). 11B NMR (80 MHz, CDCl3): δ
29 (br). Anal. Calcd for C20H20BN3O8 (441.183 g/mol): C, 54.45;
H, 4.57; N, 9.52. Found: C, 54.45; H, 4.52; N, 9.58. Crystals of 6
suitable for X-ray diffraction were obtained by recrystallization from
n-hexane/ethylacetate.
Acknowledgment. Financial support by the Fonds der
Chemischen Industrie, Frankfurt am Main, and support by the
Chemnitz University of Technology is gratefully acknowledged.
Supporting Information Available: Synthesis of 2, X-ray
crystallographic data and collection parameters for 3-6, ORTEP
plots of 3-6, CIF files of 3-6, complete list of calculated bond
lengths between non-hydrogen atoms, UV-vis spectroscopical data
and calculated binding isotherms, Cartesian coordinates for all
geometry-optimized species, and 1H and 13C NMR spectra for 3-6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO070084V
J. Org. Chem, Vol. 72, No. 12, 2007 4339