Direct kinetic measurements for the fast interconversion process between trigonal boronic acid and tetragonal boronate ion at low temperatures

Article abstract of DOI:10.1016/j.inoche.2005.05.011

The rate constant, k₁, for the following addition reaction between 4-isopropyltropolonate (ipt^-) and meta-nitrophenylboronic acid (m-NO₂PhB(OH)₂) in acetonitrile was measured at various temperatures by using a low-temperature stopped-flow spectrophotometer:m-NO₂PhB(OH)₂+ipt ^-?k_-1k₁m-NO₂PhB(OH) ₂(ipt)^-;k₁ = 7.9 × 10⁴M ^-1 s^-1 at 25°C, ΔH₁=34. 7±1.0kJmol^-1,ΔS₁=-34.8±3. 8Jmol^-1K^-1, which indicates that the interconversion between the trigonal boronic acid and the tetrahedral boronate ion is much faster than the chelate formations for the complexation of boronic acid with bidentate ligands, but much slower than the diffusion-controlled reactions.

Full text of DOI:10.1016/j.inoche.2005.05.011

Inorganic Chemistry Communications 8 (2005) 713–716
www.elsevier.com/locate/inoche
Direct kinetic measurements for the fast interconversion
process between trigonal boronic acid and tetragonal
boronate ion at low temperatures
a
Takeshi Matsumura , Satoshi Iwatsuki , Koji Ishihara
b
a,c,
*
a
Department of Chemistry, School of Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
b
Graduate School of Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
c
Materials Research Laboratory for Bioscience and Photonics, Graduate School of Science and Engineering, Waseda University, Okubo,
Shinjuku-ku, Tokyo 169-8555, Japan
Received 28 February 2005; accepted 10 May 2005
Available online 16 June 2005
Abstract
The rate constant, k1, for the following addition reaction between 4-isopropyltropolonate (iptꢀ) and meta-nitrophenylboronic
acid (m-NO2PhB(OH)2) in acetonitrile was measured at various temperatures by using a low-temperature stopped-flow spectropho-
tometer:
k1
m-NO2PhBðOHÞ þ iptꢀ ꢀ m-NO2PhBðOHÞ ðiptÞꢀ;
2
2
k
ꢀ1
k1 = 7.9 · 104Mꢀ1 sꢀ1 at 25 ꢀC, DHꢁ1 ¼ 34.7 ꢂ 1 .0 kJ molꢀ1; DSꢁ1 ¼ ꢀ34.8 ꢂ 3.8 J molꢀ1 Kꢀ1, which indicates that the interconver-
sion between the trigonal boronic acid and the tetrahedral boronate ion is much faster than the chelate formations for the complex-
ation of boronic acid with bidentate ligands, but much slower than the diffusion-controlled reactions.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: m-Nitrophenylboronic acid; 4-Isopropyltropolonate; Interconversion; Kinetics
OH
OH
B
Boronic acids (RB(OH)2) are weak acids, of which
pKas are ranging from 7 to 11 depending on the substi-
tuent, R [1–5]. Lewis acid–base reaction of boronic
acid is expressed as Eq. (1), in which structural change
from trigonal to tetrahedral occurs. We have been
interested in the reactivity of boronic acid including
boric acid (R = OH in Eq. (1)), especially in the rate
of the structural change. Anderson et al. [6] performed
kinetic studies on polyborate formation by the temper-
ature jump technique and predicted the second-order
–
–
B
+
OH
OH
R
OH
R
OH
ð1Þ
Waton et al. [7] reported much faster rate constant
(1.07 · 107 Mꢀ1 sꢀ1) for the same reaction which was
obtained by the relaxation method with thymol blue as
indicator. Pizer and Tihal [8] stated that the addition of
OHꢀ to phenylboronic acid (PhB(OH)2) is much slower
than diffusion controlled. In these relaxation studies,
pH indicators were used to monitor the reactions indi-
rectly. Recently, we have tried to measure the rate
rate constant (ꢃ1010
M
ꢀ1 sꢀ1) for the forward reaction
of boric acid
*
Corresponding author. Tel.: +81 3 5286 3241; fax: +81 3 3203 2735
E-mail address: ishi3719@waseda.jp (K. Ishihara).
1387-7003/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.05.011

714
T. Matsumura et al. / Inorganic Chemistry Communications 8 (2005) 713–716
constant for the addition reaction of isoquinoline to boric
acid (Eq. (2)) in acetonitrile without the aid of pH indica-
tor. The UV–vis spectral change of isoquinoline due to
complexation with boric acid was followed as a function
of time. We have estimated the forward rate constant to
be in the order of 104 Mꢀ1 sꢀ1 at ꢀ35 ꢀC for the reaction
in Eq. (2) in acetonitrile, but the precise determination
could not be done because of interference by water in
the acetonitrile solution [5]
(UNISOKU Scientific Instruments, Osaka). Rate con-
stants for the complex formation of m-NO2PhB(OH)2
with Naipt in acetonitrile were measured by monitoring
the absorbance change at 380 or 400 nm with time
under the pseudo first-order conditions that the total con-
centration of boronic acid (CB) was in large excess over
the ligand (CL), at ꢀ30 to ꢀ0 ꢀC and I = 0.10 moldmꢀ3
(TBAP). All the reactions were observed as a two-step
reaction consisting of two single exponentials. The condi-
tional pseudo first-order rate constants, kobs1 and kobs2
,
OH
OH
B
were determined by applying a nonlinear least-squares
fitting of the equation [10]. The 11B{1H} NMR spectra
were recorded on a Bruker Avance 400 spectrometer.
The chemical-shift was referenced to boric acid
(B(OH)3, dB = 18 ppm to Et2O Æ BF3 adduct).
−
B
+
OH
+
N
HO
OH
HO
N
ð2Þ
In the present study, we studied kinetically the reac-
Under the pseudo first-order conditions, CB ꢄ CL, a
two-step reaction was observed for the reaction of
m-NO2PhB(OH)2 with iptꢀ in acetonitrile as shown in
Figs. S1 and S2, namely, the very fast first-order reaction
was followed by the slow first-order reaction. Both the
observed pseudo first-order rate constants, kobs1 and
kobs2, are linearly dependent on CB as shown in Fig. 1,
so they are expressed as Eqs. (4) and (5), respectively,
where kf1 and kf2 are the second-order rate constants [10]
tion of meta-nitrophenylboronic acid (m-NO2PhB(OH)2)
with 4-isopropyltropolonate ion (iptꢀ) (Eq. (3)) in aceto-
nitrile by using a low temperature stopped-flow spectro-
photometer. Fully deprotonated bidentate iptꢀ was
used as ligand, because it acts basically as monodentate
ligand in acetonitrile and gives rise to very large change
in UV–vis spectrum when it reacts with Lewis acid. We
have succeeded to determine directly the rate constant
and the activation parameters for the interconversion
step in the following equation
kobs1 ¼ kf1CB;
kobs2 ¼ kf2CB.
ð4Þ
ð5Þ
The 11B{1H} NMR spectra in Fig. 2 which were acquired
under the conditions, CB > CL, show that the products of
the slower reaction are the equimolar mixture of the
chelate complex (B, m-NO2PhB(ipt)(OH)) and meta-
HO
B
OH
B
O
k
k
1
O
+
−
−
O
O
1
−
OH
nitrophenylboronate ion (C, m-NO2PhBðOHÞꢀ). The
3
OH
UV–vis spectra of iptꢀ at various concentrations of
boronic acid were measured in acetonitrile, and analyzed
successfully into the spectra of the individual species as
shown in Fig. S3. So, the observed rapid and slow reac-
tions correspond to steps 1 and 2, respectively, as shown
in Scheme 1. Therefore, the rate equations for Scheme 1
are expressed in the following equation:
NO
2
NO
2
O
O
O
O
=
−
−
ipt
ð3Þ
ꢀ d½iptꢀꢅ=dt ¼ kobs1½iptꢀꢅ ꢀ k ½ðIÞꢅ;
Sodium 4-isopropyltropolonate (Naipt) was prepared
by neutralization of 4-isopropyltropolone (Hipt) which
was purified as described previously [9] with equimolar
sodium hydroxide. m-NO2PhB(OH)2 (Aldrich) was
recrystallized once from water. Dehydrated acetonitrile
(Kanto Chemical Co. Inc., Tokyo) was used as received.
Tetra-n-butylammonium perchlorate (TBAP, Kanto)
was recrystallized twice from ethyl acetate and n-hexane,
and dried in vacuo.
All the sample solutions were prepared in a dry box.
Water contents of sample solutions were measured by
an aquacounter (AQ-200, Hiranuma Sangyo. Co. Ltd.,
Mito, Japan). Purchased dehydrated acetonitrile con-
tained 9.7 · 10ꢀ4 M of water. Kinetic measurements were
performed with a rapid-scan/stopped-flow spectropho-
tometer RSP601S with a low temperature mixing unit
ꢀ1
d½ðIÞꢅ=dt ¼ kobs1½iptꢀꢅ ꢀ ðkobs2 þ kꢀ1Þ½ðIÞꢅ þ k ½Bꢅ½Cꢅ;
ꢀ2
d½Bꢅ=dt ¼ d½Cꢅ=dt ¼ kobs2½ðIÞꢅ ꢀ k ½Bꢅ½Cꢅ.
ð6Þ
ꢀ2
In Scheme 1, the oxygen atom bearing negative
charge in iptꢀ ion attacks the boron center of the
boronic acid to form the unchelated complex (I) in
the first step. Complex (I) reacts further when OHꢀ
acceptor such as boronic acid exists in the reaction
solution; the free boronic acid accepts OHꢀ from com-
plex (I) and becomes boronate ion, concurrently the
chelate ring clusure occurs to complex (I) to form che-
late complex (B) in the second step. The reaction of
complex (I) with the boronic acid would proceed by
SN2 mechanism with Walden inversion, i.e., coordina-
tion (chelation) of the uncoordinated oxygen atom in

T. Matsumura et al. / Inorganic Chemistry Communications 8 (2005) 713–716
715
50
40
30
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
CB / 10−3 Μ
(a)
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Fig. 2. 11B{1H} NMR spectra of the mixture of m-NO2PhB(OH)2 and
(CH3)4NOH (TMA-OH) (CB/CTMA-OH ꢃ 3) (i), the mixture of
m-NO2PhB(OH)2 and Hipt (CB/CHipt
ꢀ
ꢃ
2) (ii), the mixture of
m-NO2PhB(OH)2 and iptꢀ ðCB=Cipt ꢃ 4Þ (iii) in CD3CN at 21 ꢀC.
kobs1 ¼ k1½m-NO2PhBðOHÞ ꢅ;
ð7Þ
ð8Þ
kobs2 ¼ k2½m-NO2PhBðOHÞ2ꢅ.
2
Comparing Eqs. (5) and (6) with Eqs. (7) and (8), respec-
tively, the equations, kf1 = k1 and kf2 = k2 are obtained,
since CB ꢄ CL (CB ꢃ [m-NO2PhB(OH)2]). The rate con-
stants, k1 = 7.9 · 104 Mꢀ1 sꢀ1 and k2 = 100 Mꢀ1 sꢀ1 at
25 ꢀC, were calculated from the corresponding activa-
tion parameters obtained in this study (Table S1).
We obtained previously the forward rate constants,
kf = K1k2 = 144 Mꢀ1 sꢀ1 (I = 0.10 M) and 196 Mꢀ1 sꢀ1
(I = 1.0 M) at 25 ꢀC for the reaction in Eq. (9) in water
[9], which is comparable with that in acetonitrile
(171 2 kg molꢀ1 sꢀ1 at I = 0.10 molkgꢀ1 and 25 ꢀC)
[11]. This similarity suggests that the second-order rate
constants are essentially in the same order in both sol-
vents. The k1 value (7.9 · 104 Mꢀ1 sꢀ1 at 25 ꢀC) in aceto-
nitrile obtained in this study is sufficiently greater than
the values in water obtained so far for the bidentate reac-
tion systems (the highest value, 1.34 · 103 Mꢀ1 sꢀ1 at 25
ꢀC, are obtained for the B(OH)3 and H2ipt+ system [9]),
though it is substantially smaller than the value for Eq.
(1), which clearly indicates that the chelate ring-closure
should be rate determining in the bidentate reaction
systems
0
1
2
3
4
−3 M
(b)
C
B / 10
Fig. 1. Dependences of kobs1 (a) and kobs2 (b) on CB for the reaction of
m-NO2PhB(OH)2 with iptꢀ in acetonitrile. At I = 0.10 M,
CL = 1.0 · 10ꢀ4 M, and T/ꢀC = 0 (circle), ꢀ5 (diamond), ꢀ10 (square),
ꢀ15 (triangle), ꢀ20 (inverted triangle), ꢀ25 (circle), and ꢀ30
(diamond). Data points represented by solid marks were obtained in
the presence of excess water ðCH ¼ 7.64 ꢆ 10ꢀ3 MÞ.
2O
the mono-ligated iptꢀ to the boron center of complex
(I) and dissociation of one of two OH groups in com-
plex (I) occur concertedly with the assistance of free
boronic acid.
The 1H NMR spectra (Fig. S4) were also measured at
various CB=Cipt , which show that step 1 in Scheme 1 is
in fast equilibrium.
ꢀ
Eqs. (7) and (8) are derived from the reactions in
Scheme 1 under the conditions, CB ꢄ CL, since both
the reverse reactions (kꢀ1 and kꢀ2) contribute negligibly
to kobs1 and kobs2, respectively (Fig. 1). Negligible con-
tribution of the reverse reaction (kꢀ2) is also supported
by the exact single exponential curve observed for the
slower reaction
k2
K1
O
O
B(OH)3 + HO
O
B(OH)3(HO O)
(HO)2B
+ H2O
k−2
ð9Þ
The rate-determining step in step 2 in Scheme 1
should not be chelate ring closure but dissociation of
OH group in (I), because step 2 is an intermolecular

716
T. Matsumura et al. / Inorganic Chemistry Communications 8 (2005) 713–716
step 1
k 1
HO
B
OH
B
O
O
+
–O
–
k –1
O
OH
OH
ipt–
NO2
NO2
(a)
(I)
HO
step 2
k 2
OH
B
O
B
O
HO
–
B
+
B
+
O–
k –2
OH
O
OH
OH
OH
OH
NO2
NO2
NO2
NO2
(I)
(b)
(c)
O
O–
O
=
–O
ipt–
Scheme 1.
reaction and Lewis acid–base reaction of boronic acid
(Eq. (1)) is fast. The k2 value at 25 ꢀC (102 Mꢀ1 sꢀ1
suggests that the ligand substitution on the tetrahedral
boron occurs considerably slowly unlike the previous
results obtained by relaxation methods [1,2,8].
References
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Acknowledgment
Financial Support from the 21COE program ‘‘Practi-
cal Nano-Chemistry’’ from MEXT, Japan, and Waseda
University Grant for Special Research Projects are
gratefully acknowledged.
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[10] At = A + aexp(ꢀkobs1t) + bexp(ꢀkobs2t), where At and A are
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1
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Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.inoche.2005.05.011.
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