Substituent and solvent effects on the reactions of organoboronic acids with fluoride

Article abstract of DOI:10.1246/bcsj.74.509

The introduction of an electron-withdrawing substituent or a substituent protonated to be positively charged and potentially involved in hydrogen bonding enhances the reactivities of phenylboronic acids with fluoride. The use of a 60% aqueous MeOH medium suppresses the formation of the negatively charged species, irrespective of whether their structures are trigonal or tetrahedral, but enhances the overall reactivity because of less solvation of fluoride.

Full text of DOI:10.1246/bcsj.74.509

Short Articles
Bull. Chem. Soc. Jpn., 74, 509–510 (2001) 509
titration. The addition of MeOH enhances the protonation of
fluoride 10 times (KHF = [HF]/[H+][F–] = 103.98 compared with
102.97), while depressing the formation of HF2– (negligible
Substituent and Solvent Effects on
the Reactions of Organoboronic
Acids with Fluoride
–
compared with KHF2 = [HF2 ]/[HF][F–] = 100.59). The protona-
tion constant of [PhB(OH)3]–, yielding [PhB(OH)2], in 60%
H
MeOH (K20H = 109.3) is 3 times higher than that in water (K20
=
108.8).3 The protonation constant of the amino group of
[ampB(OH)2], yielding [+HampB(OH)2] in 60% MeOH, was
Akio Yuchi,* Atsushi Tatebe, Shinpei Kani, and
H
105.0, which is 3 times lower than that in 50% MeOH (K20
=
Tony D. James† #
,
105.5).2 In any case, the neutral species are relatively stabilized
in 60% MeOH compared with those in water and in 50%
MeOH.
Department of Applied Chemistry, Nagoya Institute of
Technology, Gokiso, Showa, Nagoya 466-8555
The UV-spectral change accompanied by the acid-base reac-
tion of PB at 10–3 mol dm–3 was studied in both water (Fig. 1a)
and 60% MeOH (Fig. 1b). Upon deprotonation, both hypsoch-
romic and hypochromic effects were observed; the wavelength
for the strongest absorption shifted from 267 to 257 nm and the
molar absorption coefficient decreased from 520 to 280
mol–1 dm3 cm–1. Such a change, generally found along with
the loss of planarity, is ascribed to a change in coordination
number around boron, from three of [PhB(OH)2] to four of
[PhB(OH)3]–. The protonation constants obtained from the ab-
sorbance change (K20H = 108.6 at 1 mol dm–3 KCl in water and
109.4 at 0.1 mol dm–3 KCl in 60% MeOH) were comparable to
those given in the literature3 and by the pH titration described
above, respectively.
†School of Chemistry, The University of Birmingham,
Edgbaston, Birmingham, UK B15 2TT,
The United Kingdom
(Received October 13, 2000)
The introduction of an electron-withdrawing substituent
or a substituent protonated to be positively charged and po-
tentially involved in hydrogen bonding enhances the reactiv-
ities of phenylboronic acids with fluoride. The use of a 60%
aqueous MeOH medium suppresses the formation of the
negatively charged species, irrespective of whether their
structures are trigonal or tetrahedral, but enhances the overall
reactivity because of less solvation of fluoride.
New methods for quantifying fluoride in solution by the use
of organoboronic acids have recently been proposed; e. g., vol-
tammetry using ferrocenylboronic acid (FB)1 and fluorometry
using 2-[benzyl(methyl)aminomethyl]phenylboronic acid
(AMPB, [ampB(OH)2]),2 phenylboronic acid (PB,
[PhB(OH)2]),2 and naphthylboronic acid.3 The relevant reac-
tion equilibria have also been studied at a fixed pH, and the
equilibrium constants, conditional upon pH, have been ob-
tained.1,2 The intrinsic equilibrium constants of PB in water, on
the other hand, have been obtained by potentiometry at varying
pH values and have afforded a deeper insight into its effective
use.3 In this paper, we extend such studies to 3-nitrophenylbo-
ronic (NPB) and methylboronic (MB) acids to examine the
electronic effects on boron atoms and also to AMPB to assess
the effects of a substituent protonated to be positively charged
and potentially involved in hydrogen bonding.2 Since 60%
MeOH is to be used for a potentiometric study of AMPB, the
effects of MeOH on the reactivity of PB are also studied for
comparison.#
Since the acid-base equilibria of hydrogen fluoride and orga-
noboronic acids in water have been well characterized, the pro-
tonation constants in the literature were used for subsequent
analyses. The corresponding constants in 60% MeOH were
determined at an ionic strength of 0.1 mol dm–3 KNO3 by pH
Fig. 1. UV spectral changes by the acid-base reaction
of PB (a, b) and by the reaction of PB with F−(c, d)
in water (a, c) and in 60% MeOH (b, d). CPB
=
10−3 mol dm−3. −log [H+] = 6 11 (a, b), 3.0 (c, d).
# Present address: Department of Chemistry, University of Bath,
Bath BA2 7AY, The United Kingdom
CF/mol dm−3 = 0 (a, b), 0 10−2 (c, d).

510 Bull. Chem. Soc. Jpn., 74, No. 3 (2001)
© 2001 The Chemical Society of Japan
The reactions of PB with F– in water cause a spectral change
(Fig. 1c) similar to that produced by the acid-base reaction
(Fig. 1a and b). A gradual increase in coordination number
along with a reaction giving [PhBF3]– is suggested. On the
contrary, reactions in 60% MeOH show a simple reduction in
intensity without any shift in wavelength at a lower F– concen-
tration (Fig. 1d); this suggests the prevailing formation of a
neutral trigonal species of [PhBF2]. An increase in coordina-
tion number from [PhBF2] occurs discretely at a higher F– con-
centration. The chemistry of PhBXY (X, Y = F, Cl, Br, I) in in-
ert solvents has been well characterized by 11B NMR.4
The equilibrium constants obtained by potentiometry in wa-
ter and in 60% MeOH are summarized in Table 1. The reaction
of PB with F– in water yields five species, {(1,1), (2,1), (0,2),
(1,2), and (0,3)}:(m,n) indicates the species of [PhB(OH)mFn],
the electric charge of which is omitted for clarity.3 MB, which
has a lower Brønsted acidity (K20H = 1010.4), loses the reactivity
to F– (not included in Table). On the other hand, NPB, which
has a higher Brønsted acidity due to a lower electron density on
boron, showed a higher affinity to F– than PB. In this reaction
system, only the anionic tetrahedral species of (2,1), (1,2), and
(0,3) were formed, and neither neutral trigonal species of (1,1)
nor (0,2) was detected because of the higher Brønsted acidities
of these species than those of PB.
H,F
As for PB in 60% MeOH, K11 and K02H,F, which corre-
spond to the reactions keeping the electric neutrality, are in-
creased. Neither anionic tetrahedral species of (2,1) or (1,2)
was formed and K03F, which corresponds to the reaction from
neutral to minus charge, was not increased. The neutral species
were also stabilized in this reaction. The overall reactivity of
PB was enhanced in 60% MeOH compared with that in water,
because of the less solvation of F–.
In 60% MeOH, the reactivity of PB was further enhanced by
introducing a 2-(benzylmethylaminomethyl) group. The reac-
tion of AMPB with F– yields only the tetrahedral species of
(2,1), (1,2), and (0,3), which are, in contrast to the case of PB,
not anionic, but neutral, because of the protonation of an amino
group in an acidic medium. Such an enhancement in reactivity
is ascribed to an intramolecular electrostatic interaction be-
tween the negatively charged boronic and positively charged
ammonium moieties.
Table 1. Changes in Charge and Logarithmic Equilib-
rium Constants on the Reactions of Organoboronic
Acids with F− a)
Compound NPB
PBb)
Water Water 60% MeOH 60% MeOH
PB
AMPB
Medium
− to 0
0 to +
5.0c)
—
—
H
K20
K11
K02
7.0
—
—
8.8
3.9
2.1
9.3
—
—
H
H
Experimental
Phenylboronic, 3-nitrophenylboronic, and methylboronic ac-
ids(Aldrich) were used as received. 2-(Benzylmethylaminometh-
yl)phenylboronic acid was synthesized as described previously.2
The supporting electrolytes used were 1.0 and 0.1 mol dm–3 KNO3
for potentiometry, and 0.1 and 0.1 mol dm–3 KCl for spectropho-
tometry, in water and 60% MeOH, respectively, and the concentra-
tion scale was adopted for equilibrium analyses. Potentiometric
work was carried out as described previously.3
0 to −
+ to 0
F
K21
K12
K03
0.9
—
—
0.6
2.3
4.4
—
—
4.4
2.5
—
—
F
F
0 to 0
+ to +
H,F
K11
K02
—
—
4.5
4.4
4.7
6.7
—
—
H,F
References
− to −
0 to 0
H,F
1
C. Dusemund, K. R. A. S. Sandanayake, and S. Shinkai, J.
Chem. Soc., Chem. Commun., 1995, 333.
C. R. Cooper, N. Spencer, and T. D. James, Chem. Com-
mun., 1998, 1365.
K21
K12
K03
7.9
6.8
6.1
9.4
6.2
6.5
—
—
—
7.5
7.9
7.2
H,F
H,F
2
a) Kmn = [ArB(OH)mFn]/[ArB(OH)m+1Fn][H+],
H
3
A. Yuchi, J. Sakurai, A. Tatebe, H. Hattori, and H. Wada,
Kmn = [ArB(OH)mFn]/[ArB(OH)mFn−1][F−] and
F
Anal. Chim. Acta, 387, 189 (1999).
H,F
Kmn
= [ArB(OH)mFn]/[ArB(OH)m+1Fn−1][H+][F−] for NPB
and PB; Ar to be +Hamp for AMPB. All the constants have 0.1
error. b) Ref. 3. c) Protonation to the amino group.
4
S. S. Krishnamurthy, M. F. Lappert, and J. B. Pedley, J.
Chem. Soc., Dalton Trans., 1975, 1214.