Influence of the diol structure on the Lewis acidity of phenylboronates

Article abstract of DOI:10.1002/poc.3102

A series of cyclic esters of pentafluorophenylboronic acid have been obtained and their Lewis acidity evaluated experimentally by a modified Gutmann method. The results based on ³¹P NMR measurements were compared with those determined by quantum mechanical calculations at the DFT-VSXC/pcS-2 level of theory. The differences in Lewis acidity are discussed on the basis of electronic and geometric parameters. The calculations revealed that the complexes of investigated esters with Et₃PO have multiple conformers of a wide range of calculated ³¹P NMR shielding constants. Additionally, a correlation between the calculated O-B-O angle of esters and the experimental acceptor number was found. Copyright

Full text of DOI:10.1002/poc.3102

Research Article
Received: 23 August 2012,
Revised: 2 January 2013,
Accepted: 29 January 2013,
Published online in Wiley Online Library: 19 March 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3102
Influence of the diol structure on the Lewis
acidity of phenylboronates
a
a
a
Agnieszka Adamczyk-Woźniak , Michał Jakubczyk , Piotr Jankowski ,
a
a
Andrzej Sporzyński * and Piotr M. Urbański
A series of cyclic esters of pentafluorophenylboronic acid have been obtained and their Lewis acidity evaluated
3
1
experimentally by a modified Gutmann method. The results based on P NMR measurements were compared with
those determined by quantum mechanical calculations at the DFT-VSXC/pcS-2 level of theory. The differences in
Lewis acidity are discussed on the basis of electronic and geometric parameters. The calculations revealed that
3
1
the complexes of investigated esters with Et PO have multiple conformers of a wide range of calculated P NMR
3
shielding constants. Additionally, a correlation between the calculated O-B-O angle of esters and the experimental
acceptor number was found. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: acceptor number; boronic esters; Laplacian; NMR shielding constants calculation
number and position of electron-withdrawing substituents in
INTRODUCTION
[
9]
the aromatic ring has been investigated. The influence of a diol
structure is the subject of the present paper. A combination of
experimental and theoretical methods has been used to evaluate
acceptor number (AN) of several phenylboronic esters to get
insight into various factors that affect their Lewis acidity.
The formation of cyclic esters in the reaction of phenylboronic
acids with diols is the basis of their several beneficial applica-
tions, including sensing of biologically important species as
[
1]
probably the most spectacular one. The esterification reaction
[
2]
is useful in boronic unit protection, dopamine detection, affin-
[3]
[4]
ity chromatography, and solid-state synthesis of saccharides.
Other interesting applications of that process are the formation
EXPERIMENTAL SECTION
[
5]
[6]
of self-repairing polymers or glucose-sensitive materials.
Synthesis
Phenylboronic esters themselves are also very advantageous
species. They usually display higher stability at elevated temper-
atures as well as increased solubility in organic solvents in com-
parison with phenylboronic acids.
stronger Lewis acids than the parent acids. All those issues result
Pentafluorophenylboronic acid was synthesized according to a known
[19]
procedure. Neopentyl glycol, 1,2-benzenedimethanol, ethylene glycol,
and Et PO were purchased from Sigma-Aldrich. meso-Hydrobenzoin was
[
7,8]
3
Phenylboronates are also
purchased from Alfa Aesar. All reagents where used as received. The
[9]
synthesis of catechol esters was described previously. All solvents
[
9]
in wide applications of phenylboronates, including binding of
electron-rich molecules, e.g. in fluoride anion sensing
and auxiliary materials were purchased from POCH and used as received.
[
10,11]
or
2-(Pentafluorophenyl)-1,3,2-dioxaborolane (2): An equimolar amount of
anion complexation, improving properties of polymer electro-
ethylene glycol and pentafluorophenylboronic acid were dissolved in 100
[12–14]
3
3
lytes.
The advantage of the use of phenylboronates over
cm of toluene in a two-necked flask (150 cm ) supplied with a thermometer
and a Dean–Stark trap for azeotropic water removal. The solution was
refluxed for 6 h. The solvent was then removed under vacuum to give a
other anion complexing agents are: low expenses, lack of reac-
tive groups (like –OH), and low toxicity.
white slurry. Sublimation under vacuum gave the ester as a white solid. Yield:
The Lewis base complexation changes the geometry of the
boron center from planar to tetrahedral. Therefore, the Lewis
acidity of a trigonal organoboron compound depends on the
degree in which its geometry is close to the tetrahedral one. This
is related to the energy barrier of geometry conversion (planar to
tetrahedral). The dependence of Lewis acidity from the O-B-O
1
8
(
8%. (Found: C, 40.35; H, 1.76. Calc. for C
8
H
4
BF
5
O
2
: C, 40.39; H, 1.69%) H NMR
19
CDCl
À148.35, À161.27 ppm, B NMR (CDCl
4R,5S)-2-(Pentafluorophenyl)-4,5-diphenyl-1,3,2-dioxaborolane (3), 2-
2,3,4,5,6-pentafluorophenyl)-5,5-dimethyl-1,3,2-dioxaborinane (4), and
-(pentafluorophenyl)-3,5-dihydro-1H-2,4,3-benzodioxaborepine (5) were
3
, 400 MHz): 4.38 (s, 4H) ppm, F NMR (CDCl
3
, 376.3 MHz): À128.73,
11
3
, 64 MHz): 29.0 ppm.
(
(
3
[
15]
angle as well as the concept of THC index are well known. This
has been given as an explanation of stronger Lewis acidity of
[20]
obtained by a solid-phase esterification. The reactions were performed
[
16]
boronic esters in comparison with corresponding acids. Lewis
acidity of the boron center influences affinity towards electron-rich
* Correspondence to: A. Sporzyński, Faculty of Chemistry, Warsaw University of
Technology, Noakowskiego 3, 00–664 Warsaw, Poland.
E-mail: spor@ch.pw.edu.pl
[
17,18]
species and has been the subject of intense studies.
It also
stands for useful properties of phenylboronic esters; therefore,
quantitative evaluation of this property is of crucial importance.
In spite of that, no systematic study of the acidity of phenylboronic
esters has been reported until recently, when the influence of
a A. Adamczyk-Woźniak, M. Jakubczyk, P. Jankowski, A. Sporzyński, P. M.
Urbański
Faculty of Chemistry, Warsaw University of Technology, Warsaw, Poland
J. Phys. Org. Chem. 2013, 26 415–419
Copyright © 2013 John Wiley & Sons, Ltd.

A. ADAMCZYK-WOŹNIAK ET AL.
(
Fig. 5 and Fig. 6) were calculated perpendicular to the plane fixed by
O-B-O atoms (VSCC’s maximum).
RESULTS AND DISCUSSION
In order to investigate the influence of the diol structure on the
Lewis acidity of esters, pentafluorophenylboronates of five diols
have been studied (Fig. 1). The esters were selected to probe
the influence of different possible structural aspects: from the
previously reported flat and aromatic catechol (1), relatively
small and aliphatic ethylene glycol (2), one-side hindered meso-
hydrobenzoin (3), six-membered ring neopentyl glycol (4) to
seven-membered ring of 1,2-di(hydroxymethyl)benzene ester (5).
Figure 1. Pentafluorophenylboronates of catechol (1), ethylene glycol (2),
meso-hydrobenzoin (3), neopentyl glycol (4), and 1,2-di(hydroxymethyl)
benzene (5)
[
9]
ꢀ
in an ordinary mortar, which was heated to about 60 C. The substrates
were mixed in equimolar amounts and ground manually for about an
hour to give the products in satisfying to excellent yields. (3): yield: 99%,
Relative Lewis acidity of the studied compounds (1–5) has
1
(
Found: C, 61.57; H, 3.12. Calc. for C20
NMR (CDCl , 400 MHz): 6.00 (s, 2H), 6.95 (m, 4H), 7.09 (m, 6H) ppm,
NMR (CDCl
CDCl , 64 MHz): 29.8 ppm. (4): yield: 98%, (Found: C, 47.05; H, 3.43. Calc.
for C11
H), 3.81 (s, 4H) ppm, F NMR (CDCl
H
12BF
5
O
2
: C, 61.58; H, 3.10%)
H
F
[26,27]
been experimentally evaluated in terms of the AN,
is the affinity towards triethylphosphine oxide (Et
which
1
9
3
1
1
3
PO). The
3
, 376.3 MHz): À127.94, À147.69, À160.81 ppm, B NMR
ANs were calculated according to the following formula:
(
3
1
AN = [(d
the P NMR chemical shifts of Et
À d )/(d À d )]Â 100, where d and d(2) are
complex
(1)
(2)
(1)
(1)
H
10BF
5
O
2
: C, 47.19; H, 3.60%) H NMR (CDCl
3
, 400 MHz): 1.07 (m,
, 376.3 MHz): À131.36, À151.18,
, 64 MHz): 28.0 ppm. (5): yield: 84%, (Found:
31
19
PO in hexane (41.0 ppm) and
6
3
3
1
1
À161.42 ppm, B NMR (CDCl
C, 53.46; H, 2.59. Calc. for C14
00 MHz): 5.28 (s, 4H), 7.38 (m, 4H), F NMR (CDCl
3
in SbCl (86.1 ppm), respectively. All five compounds are solids at
5
1
H
8
BF
5
O
2
: C, 53.55; H, 2.57%) H NMR (CDCl
3
,
room temperature; thus, the AN measurement was conducted in
19
[9]
4
3
, 376.3 MHz):
3
, 64 MHz): 28.3 ppm.
benzene-d according to the previously described procedure.
6
11
À131.49, À151.74, À161.72 ppm, B NMR (CDCl
Among the investigated compounds, neopentyl glycol ester (4) is
31
the weakest Lewis acid with the experimental d( P) NMR of Et
3
PO
3
1
P NMR measurements
complex equal to 47.2 ppm.
3
Simultaneously, DFT computations were performed to find out
whether such calculations can serve as a prediction tool in de-
All samples were prepared in atmosphere of dry argon. To the 0.297 mol/dm
3
solution of Et
esters were added. The P NMR spectra were recorded on a Varian Unity
Plus 200 MHz spectrometer using BF *Et O in CDCl and 95% D PO in
O as external reference, respectively. The shift of the complex (dcomplex
was extrapolated from the concentration-shift dependence to infinite
excess of the ester over Et PO and used for calculation of AN.
3 6
PO in benzene-d (0.4 cm ), variable amounts of boronic
31
signing compounds of desired Lewis acidity. The Et PO itself
3
has four conformations (A–D, Fig. 2).
The molecular structures of complexes of esters with all the
3
2
3
3
4
D
2
)
four forms of Et PO were analyzed for possible geometries, and
3
3
each was submitted for geometry optimization on B3LYP/6-
311G(2df,p) level of theory. In all cases, multiple conformations
were found and characterized as true minima as the frequency
calculations showed no imaginary frequencies. In the majority
of cases, the minimum energy conformations of complexes cor-
Calculations
All starting geometries of the complexes were prepared manually
and submitted for full optimization at the B3LYP/6-311G(2df,p) level
of theory with the use of IEFPCM model of solvent influence (Integral
respond to the Et PO conformation B (Fig. 2). This conformer is
3
[
21]
unsymmetrical and has one ethyl group rotated upwards,
away from the oxygen atom. This geometry is in agreement with
Equation Formalism variant of the Polarizable Continuum Model)
to simulate the benzene solution. Frequencies were calculated for
all found conformations to ensure that the optimized structure is
not a transition state. All presented relative energies were corrected
for Zero Point Energy. The s ( P) were calculated using Gauge-
Independent Atomic Orbital method and VSXC
the Et
Et PO*B(C F )
PO geometry observed in the crystal structure of the
3
[
28]
complex, which is the only example of a crystal
3
6 5 3
3
1
structure of a Et PO complex with organoboron Lewis acid.
3
[
22]
pure DFT func-
The NMR calculations were performed for all the found com-
plex conformations on the VSXC/pcS-2 level of theory. The calcu-
lated shielding constant was converted to chemical shift using
the following constant and scaling factor dcalc. = (361.04 À scalc.) *
tional in pcS-2 basis set. Polarization consistent basis set family was
optimized for DFT by Jensen
[
23]
for calculating NMR shielding
constants. Those computations were carried out using Gaussian
[
24]
09.
The 2D and 3D Laplacian of the electron density was calcu-
31
0
.91. The calculated P NMR chemical shifts were averaged
lated (for optimized geometries of the isolated esters) using
[
25]
assuming a Boltzmann distribution of the conformers, which
was calculated using the ΔΔGconf. values displayed in Table S1
(Supplementary Data). The same procedure – conformational
analysis, NMR calculations, population analysis, and scaling – was
CheckDen
Gaussian checkpoint files. The valence shell charge concentrations
VSCCs) differ in size around the B-O and B-CAr bonds (shown in
Fig. 7). Thus, the 2D-Laplacian cross-section maps along these bonds
software from wave functions extracted from the
(
A
B
C
D
3
Figure 2. Conformations of Et PO. Relative energies in kJ/mol, calculated on B3LYP/6-311G(2df,p) level of theory
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 415–419

INFLUENCE OF THE DIOL STRUCTURE ON THE LEWIS ACIDITY OF PHENYLBORONATES
the experimentally determined relative Lewis acidity. The
weakest acid (4) displays a positive energy of complexation,
whereas the strongest acid (1) displays the most negative energy
of complexation. The calculated energies of complexation of
compounds 2, 3, and 5 are moderate and close to each other,
Figure 3. Catechol esters: phenylboronate (1a), 2-fluorophenylboronate
1b), 3-fluorophenylboronate (1c), 4-fluorophenylboronate (1d), 2,4-
difluorophenylboronate (1e), 2,6-difluorophenylboronate (1f), 3,4,5-
trifluorophenylboronate (1g), 2,4,6-trifluorophenylboronate (1h)
(
31
similar to the experimental d P NMR of the complex.
The diol structure can affect the Lewis acidity of
phenylboronates in different ways. The first factor concerns the
steric hindrance in the interaction with Lewis base. It is obvious
that this issue affects only those Lewis bases which are of consid-
applied for fluorinated catechol esters investigated previously
[9]
(
Fig. 3) (Table S2, Supplementary Data).
erable volume themselves (e.g. Et PO), but not small molecules
3
Figure 4 shows data points for the minimal energy conformers
like fluorine anion. The differences in the measured AN of the
investigated compounds can be explained only partially on the
basis of different bulkiness of the diol moieties. The influence
of the steric hindrance in the case of 1 and 4 is not surprising.
The flat and rigid catechol residue of 1 causes the smallest hin-
drance; thus, this ester is the strongest Lewis acid. The branchy
neopentyl residue in the six-membered ring of 4 causes a great
steric hindrance, resulting in the lowest Lewis acidity. In the rest
of the cases, the relative differences in measured AN are not so
self-evident. The glycol ester 2 bears only four hydrogen atoms
that can repulse the upcoming Lewis base, whereas the ester 3
is strongly hindered from one side, so the overall Lewis acidity
of 2 should be higher than that of 3. However, it is not the case,
the measured AN of 3 is about six points higher that of 2
(Table 1).
which display better correlation of the DFT NMR calculated and
2
experimental data (R = 0.91, RMSD = 1.03 ppm) in comparison
2
with the averaged ones (R = 0.88, RMSD = 1.57 ppm). One can
easily notice that the shifts for the lowest energy conformers of
compounds 4 and 5 are significantly deviated from the correla-
tion line. In the case of 4, the minimum energy conformer is
about 12 ppm above the line. In the case of 5, the difference is
smaller (about 4 ppm). Similar situation is observed, considering
the whole population of the conformers (Fig. S3, S4, Supplemen-
tary Information). The results show that the correlativity of the
experimental and computational NMR data is fair enough for
complexes of five-atom ring esters (1, 1a–h, 2, and 3) but rather
poor for those of six (4) or seven-member rings (5). The calcu-
lated energies of complexation (Table 1) qualitatively reflect
3
1
Figure 4. Measured P NMR d of ester–Et
the data for compounds 1, 1a–h, 2, and 3
3
PO complexes against DFT-GIAO-calculated minimal energy conformer values. The line shows correlation of
31
Table 1. Ester*Et
3
PO complex P NMR measured shift (d exp.), acceptor number (AN), calculated shift of minimum energy
conformer of complex, Boltzman population averaged calculated shift, calculated complexation energy
a
Cmpd
d exp./ppm
AN
Min. energy d calc./ppm
Averaged d calc./ppm
Complexation energy /kJ/mol
1
2
3
4
5
77.8
51.5
54.2
47.2
54.8
81.5
23.3
29.2
13.7
30.6
70.7
60.6
59.8
70.3
65.1
68.8
61.0
59.9
69.3
64.5
À32.87
À4.81
À9.57
3.71
À7.54
a
Calculated for the reaction between the minimum energy conformer of Et PO (A) and an ester leading to minimum energy
3
conformer of the complex. The isolated esters have only one stable form each.
J. Phys. Org. Chem. 2013, 26 415–419
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. ADAMCZYK-WOŹNIAK ET AL.
The other reason of acidity differences can be the electronic
one, changing electron density of the boron center. This can
be analyzed by the Laplacian of the electron density, which re-
veals the localization of the charge accumulation (and depletion)
that can shield the acidic center and, in result, lower the Lewis
acidity. The two-dimensional Laplacian maps in Fig. 5 and Fig. 6
were calculated for optimized geometries of isolated esters
along B-CAr bond and one of the B-O bonds, in both cases per-
pendicular to the plane fixed by the O-B-O atoms. The VSCCs
are of the same size for all cases along both planes. This shows
that the charge concentration in the close vicinity of boron cen-
ter does not depend on the structure of the diol moiety. Hence,
the differences in the AN of investigated compounds are not
caused by any electronic effect originating from the diol moiety.
An example of three-dimensional Laplacian is shown in Fig. 7.
The other factor is the previously mentioned geometrical one,
which seems to be the reason of acidity differences among the
investigated species besides the simple steric hindrance of the
boron center. The differences in the relative Lewis acidity of
the investigated compounds are easy to explain when one corre-
lates AN with the calculated O-B-O angle in the ester. Figure 8
Figure 7. An overview of the 3D Laplacian of the electron density of the
neopentyl pentafluorophenylboronate (4)
presents measured AN plotted against O-B-O angle from opti-
mized geometries of isolated esters. The AN rises rapidly with
the reduction of the O-B-O angle. For the glycol ester (2), it is
ꢀ
equal to 23.3 (113.8 ) and for the catechol ester (1) it is 81.5
ꢀ
ꢀ
(111.2 ). However, above 120 the AN rises with increasing angle.
This is due to the reduction of the two O-B-CAr angles upon in-
creasing the O-B-O angle, what makes the planar-to-tetrahedral
interconversion also easier. This effect is weaker, for upon
Figure 5. Laplacian of the electron density along B-O bond perpendicular to O-B-O plane. The horizontal line indicates equality of the VSCCs. The
length unit is in Å
Figure 6. Laplacian of the electron density along B-CAr. bond perpendicular to O-B-O plane. The horizontal lines indicate equality of the VSCCs. The
length unit is in Å
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 415–419

INFLUENCE OF THE DIOL STRUCTURE ON THE LEWIS ACIDITY OF PHENYLBORONATES
Mazzarello for their helpful discussions and for suggestions
concerning the manuscript.
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SUPPORTING INFORMATION
Supporting information may be found on the online version of
this article.
Acknowledgements
The calculations reported in this work were partially performed
at the Interdisciplinary Center for Mathematical and Computa-
tional Modeling of the University of Warsaw within grant No.
G36-20. Financial support within grant No. NN 204127938 is
kindly acknowledged. The authors would like to thank Luis F.
Pacios and Alberto Fernandez who shared the CheckDen soft-
ware and provided help and support to its use. Authors would
like to thank Professor Joanna Sadlej and Dr. Aneta Jezierska-
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