A tautomeric equilibrium between functionalized 2-formylphenylboronic acids and corresponding 1,3-dihydro-1,3-dihydroxybenzo[c][2,1]oxaboroles

Article abstract of DOI:10.1039/b611195e

Functionalized 2-formylphenylboronic acids undergo an unprecedented tautomeric rearrangement in solution to form corresponding 1,3-dihydro-1,3- dihydroxybenzo[c][2,1]oxaboroles. X-Ray analyses of selected examples revealed diverse solid-state molecular structures from a planar open form with a hydrogen-bonded carbonyl group (X = 3-F) through a twisted conformer showing a weak carbonyl-boron interaction (X = 3,5-Br₂) to a cyclic oxaborole derivative (X = 3-Br). Variable-temperature ¹H NMR spectroscopy was used to determine equilibrium constants as well as enthalpies and entropies of tautomerization in a mixed solvent [D₆]acetone-D₂O (95:5). A computational approach to the process by DFT (B3LYP) and MP2 methods has also been performed. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b611195e

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A tautomeric equilibrium between functionalized 2-formylphenylboronic
acids and corresponding 1,3-dihydro-1,3-dihydroxybenzo
[c][2,1]oxaboroleswz
´
Sergiusz Lulinski,* Izabela Madura, Janusz Serwatowski, Halina Szaty$owicz and
Janusz Zachara
Received (in Montpellier, France) 2nd August 2006, Accepted 9th November 2006
First published as an Advance Article on the web 6th December 2006
DOI: 10.1039/b611195e
Functionalized 2-formylphenylboronic acids undergo an unprecedented tautomeric rearrangement
in solution to form corresponding 1,3-dihydro-1,3-dihydroxybenzo[c][2,1]oxaboroles. X-Ray
analyses of selected examples revealed diverse solid-state molecular structures from a planar open
form with a hydrogen-bonded carbonyl group (X = 3-F) through a twisted conformer showing a
weak carbonyl–boron interaction (X = 3,5-Br2) to a cyclic oxaborole derivative (X = 3-Br).
1
Variable-temperature H NMR spectroscopy was used to determine equilibrium constants as well
as enthalpies and entropies of tautomerization in a mixed solvent [D6]acetone–D2O (95 : 5). A
computational approach to the process by DFT (B3LYP) and MP2 methods has also been
performed.
force of a cyclization. This observation represents a new aspect
Introduction
of boronic acid chemistry.
Recently, arylboronic acids have been commonly used in
organic synthesis, especially as coupling partners in synthesis
of biaryls.1 Furthermore, specific properties of the boronic
group have become the basis for other applications. For
instance, the formation of cyclic boronates from boronic acids
and 1,2- and 1,3-diols2 led to a development of various systems
containing the arylboronic acid functionality which can act as
fluorescent PET ‘‘on–off’’ monosaccharide receptors.3 Some
of them have been synthesized using simple formylphenyl-
boronic acids as precursors.4,5 Another potentially promising
but less recognized field is the use of boronic acids in supra-
molecular assembly due to hydrogen bonding interactions by
B(OH)2 groups.6 In contrast to numerous reports concerning
the preparation and synthetic application of arylboronic acids
their structure has been studied less extensively. Some ortho-
functionalized arylboronic acids and esters tend to form boron
heterocycles bearing a resemblance to naturally occurring
purines.7 Accordingly, 1,3-dihydro-1-hydroxybenzo[c][2,1]
oxaboroles can be regarded as half-esters of 2-(hydroxy-
methyl)phenylboronic acid or its derivatives. A few such
heterocycles have been structurally characterized.8 Now we
have found that related species can exist in tautomeric equili-
brium with functionalized 2-formylphenylboronic acids.
Hence, there is no need for water elimination as a driving
Results and discussion
NMR evidence for a ring–chain tautomerism in
2-formylphenylboronic acids
Surprisingly, we have found that 3-fluoro-2-formylphenyl-
boronic acid (1) reveals a specific behaviour in a [D6]acetone
solution as proved by the 1H NMR analysis. D2O (5 wt%) was
added to avoid a potential competing equilibration with the
corresponding anhydride. Furthermore, a peak assignment is
facilitated due to the exchange of hydroxyl proton resonances.
The 1H NMR spectrum of 1 shows two sets of signals
indicating the existence of two different structures (Fig. 1).
The first one is the structure with separate formyl and B(OH)2
groups as confirmed by the signal of the aldehyde proton at
10.33 ppm. The signal at 6.40 ppm of a comparable intensity
points unequivocally to a quaternization of the aldehyde
carbon. This can be rationalized assuming a cyclic structure
of 1,3-dihydro-1,3-dihydroxy-4-fluorobenzo[c][2,1]oxaborole
(1-II) to be formed to a significant extent as shown in Scheme
1. The structural difference between 1-I and 1-II is reflected by
different multiplets in the aromatic region. Accordingly, co-
existence of both tautomers can also be observed using 13C
NMR spectroscopy, where a number of resonances is dupli-
cated. Moreover, characteristic peaks at 189.6 and 96.1 ppm
can be assigned to the formyl and hemiacetal carbons, respec-
tively. We have synthesized a series of other functionalized 2-
formylphenylboronic acids 2–8 to check if the observed pro-
Warsaw University of Technology, Faculty of Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland. E-mail:
serek@ch.pw.edu.pl; Fax: +(4822) 6282741
1
cess has a general character. In all cases H NMR spectra of
w Dedicated to Professor Tadeusz M. Krygowski from the University
of Warsaw on the occasion of his 70th birthday.
their solutions in [D6]acetone exhibit resonances in the range
10.12–10.35 ppm (CHO) and 6.15–6.35 ppm (CHOH). Simi-
larly, in 13C NMR spectra the existence of two tautomeric
forms is manifested by two peaks in the range 190.9–
z Electronic supplementary information (ESI) available: 1H, 13C
NMR and IR spectra of functionalized 2-formylphenylboronic acids,
selected thermodynamic and theoretical calculation data. See DOI:
10.1039/b611195e
ꢀc
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Scheme 2
taining 5 wt% D2O) was used as a standard solvent. 1 has been
chosen as a model compound as it shows an optimal ratio of
tautomeric forms (i.e., close to 1). Initial kinetic experiments
using 1H NMR spectroscopy revealed that at room and higher
temperatures equilibration 1-I 2 1-II is rapid on the macro-
scopic time scale: after a rapid (ca. 3 min) cooling from 321 to
293 K an integral ratio of hemiacetal vs. aldehyde protons
(further denoted as Kobs) was equal within experimental error
to the value obtained when the sample was kept for a longer
time at 293 K. However, at 253 K a relaxation to the state of
equilibrium is much slower and needs ca. 1 h to go to
completion. Under these conditions Kobs increases to 1.8
reflecting a higher stability of 1-II at a lower temperature.
In addition, we have found that increasing total concentra-
tion of 1 decreases Kobs(293 K) values from the value of 1.09
for 0.02 M solution to 0.85 for 0.8 M solution; this effect was
much weaker at higher temperature (321 K). Hence, this ratio
cannot be simply taken as the thermodynamic equilibrium
constant Kcycl of the tautomerization process. Supported by
the results of X-ray analysis, we suggest that the dimerization
of 1-I can also occur to a significant extent in solution at
higher concentrations (Scheme 3). Assuming two competing
equilibrium processes to proceed, estimation of the dimeriza-
tion constant Kdim(293 K) yielded the value of 0.3.16 This
corresponds to an association degree a of 10% for a 0.5 M
solution. To eliminate concentration effects, Kcycl values have
been estimated from the spectra of dilute solutions (0.02 M,
D2O : boronic acid ratio D150 : 1). Under these conditions,
for 1 a = 0.5% at 293 K, and hence Kcycl D Kobs. Temperature
dependence of ln Kcycl for compounds 1–3 and 5–8 is shown in
Fig. 2. Further calculations show the tautomerization to be
slightly exothermic with DH values falling in the range –8.0(3)
to ꢁ17.7(4) kJ molꢁ1. These results are summarized in Table 1.
1
Fig. 1 The H NMR spectrum of 1 in [D6]acetone–D2O (95 : 5).
195.6 ppm (CHO) and 96.4–98.8 ppm (CHOH). The acids 1–8
have also been characterized by 11B NMR spectroscopy. The
spectra show one signal in the range 28–32 ppm (h1/2
=
200–250 Hz), which indicates that the formation of a cyclic
tautomer has little effect on the boron chemical shift; a
shoulder was observed only in the spectrum of 1. In most
cases, broad boron resonances of both tautomers overlap with
each other and hence cannot be observed separately.
A chain–ring tautomeric equilibrium in 1–8 shows analogy
with a formation of 3-hydroxyphthalide9 from 2-formylben-
zoic acid isoelectronic with 2-formylphenylboronic acid
(Scheme 2). Moreover, an ortho-CHO group greatly facilitates
hydrolysis of N,N-dialkylbenzamides resulting in 3-hydro-
xyphthalide.10 A cyclic hemiacetal is formed from 2-(hydro-
xymethyl)benzaldehyde, too.11 However, in both cases the
equilibrium is shifted towards cyclic tautomers, whereas for
a more efficient formation of an analogous boron heterocycle
the presence of fluorine or other substituent in a 3-position is
required.12 It should be stressed that reactions leading to
related 1,3-dihydro-1-hydroxybenzo[c][2,1]oxaboroles are in-
tramolecular esterifications that can be assumed to be more
entropy-favored due to elimination of water molecule.13,14
Another pertinent example to be noted here is the behavior
of another isoelectronic system, namely 2-nitrobenzaldehyde.
This compound undergoes a facile photoisomerization to give
2-nitrosobenzoic acid via intramolecular oxygen transfer from
a nitro to a formyl group (Scheme 2).15 However, the mechan-
ism of this reaction involving a formation of an analogous
cyclic intermediate (or a transition state) has not been proved.
Solution equilibrium studies
We were interested in more precise information about a nature
of tautomeric equilibrium in solution. Wet [D6]acetone (con-
Scheme 3 The competition between dimerization and cyclization
of 1.
Scheme 1 Tautomeric rearrangement of functionalized 2-formylphe-
nylboronic acids.
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of a bulky substituent plays a significant role. This view could
be supported by the behavior of 4-bromo-2-formylphenyl-
boronic acid 8, which cyclizes to a lesser extent (Kcycl = 0.19
at 303 K). However, the situation observed for 5 and 6
illustrates that other changes may also be important. In the
case of 5, the value of Kcycl is close to that found for the
monobromo analogue 3. In the case of 6, the effect of the
remote methoxy groups leads to the better stabilization of the
open form (Kcycl = 0.46 at 303 K). The last example demon-
strates clearly that in fact it is difficult to rationalize the
equilibrium shift in terms of one predominant factor. The
contribution from electronic effects is debatable as differences
in thermodynamic data are small and cannot be meaningfully
interpreted. We suppose that irrespective of specific steric
interactions a varying (depending on substituents) stabiliza-
tion by a solvent may be important for a delicate balance
between both tautomeric forms.
Fig. 2 Temperature dependence of the cyclization constant, Kcycl, in
[D6]acetone–D2O (95 : 5), for functionalized 2-formylphenylboronic
acids 1–3 and 5–8.
X-Ray crystallographic study
We have undertaken X-ray diffraction analyses to check if the
ring–chain tautomerism is revealed also in the solid state. In
crystals of 1 obtained from acetone molecules exist in form I.
The molecular structure of 1-I is shown in Fig. 3 and selected
bond lengths and angles are given in Table 2.
The tautomeric equilibrium in 1 was also investigated using
other solvents. In [D6]dmso–D2O (95 : 5) a comparable
behavior was observed with Kobs decreasing gradually from
the value of 1.45 to 0.77 over a temperature range of 298–
368 K. In contrast, with D2O as a solvent the form 1-I is
strongly preferred (Kobs = 0.19 at 293 K). This result points
that the equilibrium can be significantly influenced by solvent
effects. It should be noted that contrary to the behaviour in
[D6]acetone and [D6]dmso the signals are broadened which
suggests a quite rapid exchange between 1-I and 1-II on the 1H
NMR time-scale. This may suggest that water can be involved
in the mechanism of the tautomerization.
The entire molecule of 1-I is almost planar. The formyl
group is essentially coplanar with the phenyl ring, while the
B(OH)2 group is slightly twisted (by 10.3(1)1) relative to the
phenyl plane. One hydroxyl H atom [H(2)] is directed to the
formyl group forming an intramolecular hydrogen bond while
the other one [H(1)] is engaged in an intermolecular hydrogen
bond. In consequence, adjacent monomeric moieties of 1 are
connected by the pair of intermolecular O–Hꢂ ꢂ ꢂO hydrogen
bridges leading to the H-bonded centrosymmetric dimer with
eight-membered ring (Fig. 3) typical of many boronic acids.18
Detailed hydrogen bonding geometries are given in Table 3.
The intramolecular O–Hꢂ ꢂ ꢂO interaction introduces some
angular strain and causes significant deviation of selected
endocyclic bond angles from ideal value for sp2 hybridi-
zation [e.g., B(1)–C(1)–C(2) 128.03(12)1, C(1)–C(2)–C(7)
126.25(13)1]. A similar molecular structure was reported for
the parent 2-formylphenylboronic acid.19 The compound 5
crystallizes as a monohydrate with centrosymmetric dimers of
the form 5-I due to hydrogen-bonding interactions between
dihydroxyboryl groups. The molecular structure of the dimer
As can be seen in Table 1 the equilibrium is substituent-
dependent. Heavier halogen analogues 2–5 as well as 7 (X =
3-Me3Si) show a stronger tendency to form corresponding
oxaboroles in [D6]acetone–D2O than does 1. For the acid 4
bearing iodine in the 3-position we obtained the highest value
of Kcycl = 10,17 which suggests unequivocally that a proximity
Table 1 Selected thermodynamic data for the tautomeric cyclization
of functionalized 2-formylphenylboronic acids 1–8
c
c
Kcycl
@303 Kb
DHcycl
/
DScycl /
Compd
Solventa
kJ molꢁ1
J Kꢁ1 molꢁ1
1
1
1
2
3
4
5
6
7
8
a
[D6]acetone
[D6]dmso
D2O
[D6]acetone
[D6]acetone
[D6]acetone
[D6]acetone
[D6]acetone
[D6]acetone
[D6]acetone
0.94
1.45d
0.17
ꢁ10.7 ꢃ 0.2
—
—
ꢁ12.5 ꢃ 0.5
ꢁ13.7 ꢃ 0.3
—
ꢁ12.0 ꢃ 0.5
ꢁ8.7 ꢃ 0.7
ꢁ17.7 ꢃ 0.4
ꢁ8.0 ꢃ 0.3
ꢁ36.0 ꢃ 0.4
—
—
ꢁ28.3 ꢃ 1.7
ꢁ30.0 ꢃ 1.0
—
4.74
6.22
ca. 10e
4.94
0.46
ꢁ26.4 ꢃ 1.6
ꢁ35.4 ꢃ 2.1
ꢁ46.5 ꢃ 1.2
ꢁ40.1 ꢃ 1.0
c
4.21
0.19
Fig. 3 An ORTEP diagram of hydrogen bonded dimer of 1 with the
atom numbering scheme. The atoms labelled with a prime (0) are at
symmetry equivalent position (ꢁx, ꢁy, ꢁz). Displacement ellipsoids
are drawn at the 50% probability level.
Contains 5 wt% D2O. Measured at c = 0.02 mol dmꢁ3
.
Assumed
b
to be temperature-independent and estimated by least-squares method
d
using linear van’t Hoff equation. Measured at 298 K. See ref. 17.
e
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Table 2 X-Ray and calculated gas-phase geometries (B3LYP/6-311+G**, in brackets selected values calculated by MP2/6-31+G* method) of
open forms I of 1, 2, 5 and 2-formylphenylboronic acid (FPB)
FPB (X-ray)a
FPB (calc.)
Bond lengths (A)
1-I (X-ray)
1-I (calc.)
2-I (calc.)
5-I (X-ray)
O(1)–B(1)
O(2)–B(1)
C(1)–B(1)
O(3)–C(7)
C(1)–C(2)
X–C(3)
1.3440(19)
1.3494(19)
1.590(2)
1.2066(19)
1.4210(19)
1.3556(17)b
—
1.375
1.349
1.599
1.221
1.429
1.357b
—
1.376
1.348
1.604
1.221
1.432
1.769c
—
1.353(4)
1.353(4)
1.598(4)
1.208(4)
1.401(4)
1.895(3)d
1.891(3)d
1.351(3)
1.358(2)
1.585(2)
1.212(2)
1.424(2)
—
1.375
1.351
1.594
1.219
1.425
—
X–C(5)
—
—
Bond angles (1)
O(1)–B(1)–O(2)
O(1)–B(1)–C(1)
O(2)–B(1)–C(1)
O(3)–C(7)–C(2)
C(7)–C(2)–C(1)
C(2)–C(1)–B(1)
118.62(14)
115.86(13)
125.50(13)
127.51(14)
126.25(13)
128.03(12)
118.10
114.75
127.15
127.19
126.58
127.97
117.76
114.49
127.75
127.44
124.69
128.76
119.3(2)
118.1(2)
121.8(2)
122.3(3)
118.1(2)
121.9(3)
118.94(18)
116.24(14)
124.82(13)
128.14(15)
126.70(15)
128.14(15)
117.99
115.09
126.92
128.53
126.36
128.33
Torsion angles (1)
C(2)–C(1)–B(1)–O(2)
C(1)–C(2)–C(7)–O(3)
ꢁ10.02(27)
0.06 [13.86]
ꢁ0.36 [ꢁ24.03]
83.58(2)
7.96(2)
4.42(28)
ꢁ5.11(31)
ꢁ0.11 [ꢁ2.32]
ꢁ0.04 [4.21]
2.84(29)
ꢁ0.14 [ꢁ16.59]
Data taken from ref. 19. X = F. X = Cl. X = Br.
0.40 [34.09]
a
b
c
d
is shown in Fig. 4 and selected bond lengths and angles are
given in Table 2.
MeC6H4, R = Et, iPr.22 Strikingly, such interaction was not
observed in a closely related pinacol ester 2-(iPr2NCO)
PhB(1,2-O2C2Me4).23
Contrary to 1, the B(OH)2 group is oriented almost per-
pendicularly to the phenyl ring and the dihedral angle between
least-squares planes of both moieties is 79.37(14)1. The angle
C(1)–C(2)–C(7) of 118.1(2)1 is much smaller than that in 1
which supports the view that the buttressing effect20 of bro-
mine impacts destabilization of a planar structure and twisting
of the B(OH)2 group. An important feature of 5 is a weak
oxygen–boron interaction as indicated both by short
CQOꢂ ꢂ ꢂB contact [2.561(4) A] and significant displacement
of boron atom from the plane defined by C(1), O(1), O(2)
atoms [0.069(3) A] towards the carbonyl oxygen. In a related
2-acetylphenylboronic acid twisting of the B(OH)2 group with
respect to the aromatic ring (77.41) allowed for a comparable
CQOꢂ ꢂ ꢂB contact of 2.49 A21 with simultaneous perching of
boron from the basal CO2 plane (0.078 A). It should be
noticed, that a much stronger Bꢂ ꢂ ꢂO intramolecular coordina-
tion has been found, for instance in amide ortho-functionalized
catecholborates 2-(R2NCO)ArB(1,2-O2C6H4), Ar = Ph, 3-
Molecules of water interact with B(OH)2 groups both as a
hydrogen acceptor and donor. Consequently, twelve-mem-
bered H-bonded rings with an extended-chair conformation
are constructed. Adjacent dimeric moieties of 5 linked together
through water-bridges form infinite chains along the crystal-
lographic a-axis (Fig. 5(a)). Chains are then cross-linked into
wave-layers parallel to the (001) plane by CQOꢂ ꢂ ꢂH–OH
bonds formed between water molecules and carbonyl O atoms.
Phenyl rings are almost perpendicular to the layer. Such an
arrangement allows the p-CH groups and oxygen atoms of
water from adjacent layers to form weaker C–Hꢂ ꢂ ꢂO interlayer
bonds (Fig. 5(b)). Compound 5 thus adopts an interesting
hydrogen-bonded 3D network structure in which water mole-
cules serve as the tetrahedral connectors assembled with
dimeric boronic acid units.
A very similar compound, 3, crystallizes also as a stable
monohydrate, but contrary to 5, involves cyclic forms 3-II.
Table 3 Hydrogen bonding geometry (A, 1) for 1 and 5
1
d(Hꢂ ꢂ ꢂA)
1.92(2)
1.70(3)
d(Dꢂ ꢂ ꢂA)
2.7740(17)
2.5801(18)
+D–Hꢂ ꢂ ꢂA
175(3)
165(2)
O(1)–H(1)ꢂ ꢂ ꢂO(2)#1
O(2)–H(2)ꢂ ꢂ ꢂO(3)
5
O(1)–H(1)ꢂ ꢂ ꢂO(2)#1
O(2)–H(2)ꢂ ꢂ ꢂO(4)#2
O(4)–(4A)ꢂ ꢂ ꢂO(3)#3
O(4)–H(4B)ꢂ ꢂ ꢂO(1)
C(4)–H(4)ꢂ ꢂ ꢂO(4)#4
a
1.93
1.85
2.01(5)
2.02(6)
2.40
2.744(3)
2.652(4)
2.865(3)
2.800(4)
3.324(4)
175
167
161(5)
176(5)
172
Fig. 4 An ORTEP diagram of a hydrogen bonded dimer of 5 with the
atom numbering scheme. The atoms labelled with a prime (0) are at
symmetry equivalent position (ꢁx, ꢁy, ꢁz). Displacement ellipsoids
are drawn at the 50% probability level.
Symmetry transformations: #1 ꢁx, ꢁy, ꢁz; #2 ꢁ1 + x, y, z; #3 1 ꢁ x,
#4
1 ꢁ y, ꢁz; 1 ꢁ x, 1 ꢁ y, 1 ꢁ z.
ꢀc
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Fig. 5 (a) A view of the H-bonded layer structure of 5. The dashed lines represent O–Hꢂ ꢂ ꢂO hydrogen bonds. (b) Side view of the layer along a
axis showing interlayer C–Hꢂ ꢂ ꢂO connections (dotted lines). Hydrogen atoms are omitted excluding those involved in H-bond formation.
Nevertheless, the analysis proved difficult due to substantial
disorder observed in all measured crystals. The best approx-
imation of the structure was obtained during the solution and
refinement in the Cc space group. There are four independent
molecules per asymmetric unit as two pairs of S and R
stereoisomers. The geometry of the molecules is almost the
same. The molecular structure of one of them is presented in
Fig. 6. The molecule of 3-II is comprised of two almost planar
fused rings, similar to other benzoxaborole derivatives.8 The
sum of the valence angles around the B(1) center of 3601 for all
molecules shows the typical sp2 hybridization. The mean
values of C(1)–B(1), B(1)–O(1), B(1)–O(2) and C(7)–O(2)
bond lengths are equal to 1.55, 1.35, 1.37 and 1.45 A,
respectively, and fall in ranges observed for the aforemen-
tioned structures.
p21/b11 with the non-periodic direction being parallel to c. The
new aL and bL basis vectors of a layer are consistent with the
vectors of the reduced primitive cell (aL = bL = 7.185 A, gL E
901). The layer symmetry is broken through the stacking of the
layers (it is a possible explanation for Z0 = 4). A layer can
occupy either of two different positions (related by translation
by 0.5a or 0.5b vector), which are almost equivalent with
respect to the nearest layer. Thus, the disorder arises through
stacking faults and could be analyzed by an order–disorder
theory using local symmetry operations. The final refinement
converged to R1 = 0.0593 but the resultant geometry para-
meters show relatively high esds, thus the further presentation
of the detailed geometry analysis is not justified. Nonetheless,
the topology of the double-layer is interesting. Molecules of
water interact with both B(OH) and C(OH) hydroxy groups
and form four-connected, almost tetrahedral nodes of a
resulting H-bonded 2D-network. The Oꢂ ꢂ ꢂO contacts vary
from 2.65 to 2.77 A and from 2.77 to 2.83 A for BOꢂ ꢂ ꢂO
and COꢂ ꢂ ꢂO interactions, respectively. The H-bonded net
consists of fused rings formed by eight hydrogen bridges and
connected together through water molecules. Hence, taking
into account only water molecules, the resulting 2D-net can be
reduced to a simple 4-connected uninodal 4462 net.
We detected that the crystal structure contains layers of
molecules connected via hydrogen bonded water molecules
(Fig. 7). The adjacent layers are related by a c-glide plane. All
of them are parallel to the (001) plane. The symmetry of a
layer can be described by the 2-periodic layer-group symmetry
In addition to X-ray structural studies we have employed IR
spectroscopy to determine the basic structure of compounds
1–3 and 5–8 in the solid state using a characteristic band of
CQO group absorption as a probe. For compounds 1, 5, 6
and 8 strong bands are observed in the range 1647–1684 cmꢁ1
,
confirming the stability of corresponding open forms I in the
solid state. These values are significantly lower when com-
pared to the standard wavenumbers of CQO group bands for
simple benzaldehydes. This can be explained primarily by
intra- and/or intermolecular hydrogen-bonding interactions
decreasing the CQO bond order. For the remaining com-
pounds 2, 3 and 7, IR spectra exhibit only weak bands in the
Fig. 6 An ORTEP diagram of the molecular structure of 3 with the
atom numbering scheme. Displacement ellipsoids are drawn for oxy-
gen and bromine atoms at the 50% probability level.
ꢀc
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ꢀ
Fig. 7 (a) A view of the H-bonded double-layer structure of 3 ꢂ H2O. (b) Side view of the double-layer along the [110] direction. The dashed lines
represent Oꢂ ꢂ ꢂO contacts. Hydrogen atoms are omitted for clarity.
range 1670–1680 cmꢁ1, thus providing an evidence of stability
for single-bond torsional potentials in other conjugated sys-
tems.25 For perpendicularly twisted conformations of the form
I local minima are observed (Fig. 8) with relative electronic
energies dependent on the calculation method (Table 4) in the
range 15–23, 11–20 and 7–13 kJ molꢁ1 for 2-formylphenyl-
boronic acid, 1 and 2, respectively. Including ZPE correction,
relative energies of I (twisted form) decrease and fall in the
range 12–20, 8–16 and 3–10 kJ molꢁ1, respectively. These
minima could be rationalized in terms of a weak boron–
carbonyl interaction being plausible for such geometry due
to a partial overlap of a 2p orbital of the boron and a lone pair
of the carbonyl oxygen atom. However, it should be noted that
there is only a slight deviation of the boron atom from planar
sp2 hybridisation. Alternatively, the small energy gain could be
rationalized in terms of weak electrostatic attractive forces.
Calculated relative energies of cyclic forms II without and
with ZPE correction fall in the range 13–27, 10–23, ꢁ1 to 12
(Table 4) and 18–31, 14–27, 3–17 kJ molꢁ1 for 2-formylphe-
nylboronic acid, 1 and 2, respectively. The stability order of I
(twisted) and II forms changes when including ZPE correction:
electronic energies are in favour of form II, whereas with a
of corresponding cyclic structures of type II.
Theoretical calculations
Results of calculations of 2-formylphenylboronic acid and its
halogenated analogues 1 and 2 under gas-phase conditions
gain an extended insight into the nature of the observed
tautomerism. We have found equilibrium geometries for open
I, cyclic II and open twisted forms of the aforementioned
boronic acids (Table 2, for full data set see ESIz). The energies
were obtained by geometry optimization at all theoretical
levels, followed by the calculation of the vibrational frequen-
cies at the same level (no imaginary frequency). This allowed
correction for zero-point energies (ZPE) and conversion of the
calculated energies to 298 K.
An approximately planar conformation of the open tauto-
meric form I with an intramolecular hydrogen bond between
B(OH)2 and CHO groups was predicted as the most stable,
i.e., as a global minimum, irrespective of the calculation
method (with ZPE correction included). The MP2 relative
electronic energies (without ZPE correction) for compound 2-
II are in the range ꢁ1.16 to 2.49 kJ (Table 4, for 6-31+G* and
6-31G* basis sets, respectively), which suggests comparable
stabilities of forms I and II. In addition, in the case of 2-I, MP2
calculations point to twisting of the B(OH)2 group, from ca.
101 (aug-cc-pVDZ basis set) to 241 (the others), and the CHO
one to the phenyl ring.
Table 4 The calculated relative energies of tautomeric forms I and II
DE/kJ molꢁ1
MP2
aug-cc- MP2
MP2 B3LYP
pVDZ 6-31+G* 6-31G* 6-311+G**
Compound Form
I (planar)
The B3LYP calculated geometries of planar forms I are in
better agreement than MP2 ones with those yielded by X-ray
analyses (Table 2). A significant elongation (ca. 0.03 A) of one
of B–O bonds in the gas phase structures is due to the lack of
hydrogen-bonding interactions by an adjacent O–H group; for
analogy, in the case of hydrogen-bonded p-nitrophenol com-
plexes with bases, lengthening of a C–O bond indicates a
weaker hydrogen-bond.24
0.00
15.99
0.00
12.74
14.74
0.00
16.41
18.93
0.00
26.89
23.29
II
I (twisted) 19.09
I (planar) 0.00
II 11.05
I (twisted) 15.16
0.00
9.90
11.30
0.00
15.41
16.18
0.00
23.16
19.64
Relative electronic energies were calculated vs. the torsion
angle O(1)–B(1)–C(1)–C(2) at B3LYP/6-311+G** (Fig. 8)
and MP2/6-31G* (see ESIz) levels of theory: obtained curves
are qualitatively almost identical. However, the MP2 curve
runs below the B3LYP one, which is in accord to results found
I (planar) 0.00
0.00
ꢁ1.16
0.00
2.49
9.00
0.00
12.23
13.05
II
I (twisted)
1.37
8.65
7.16
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Experimental
General comments
All reactions were carried out under an argon atmosphere.
Solvents were stored over sodium wire before use. Butyl-
lithium (10 M solution in hexanes), triethyl borate (99%),
anhydrous trimethyl orthoformate (99.8%), anhydrous N,N-
dimethylformamide (99.8%) and chlorotrimethylsilane (99%)
were used as received. All bromobenzaldehydes or their acetals
1
were prepared according to the literature procedures. H and
13C NMR chemical shifts are given relative to TMS using
residual solvent resonances. 11B NMR chemical shifts are
given relative to Et2O ꢂ BF3. Due to a varying proportions of
1
tautomers, numbers of nuclei obtained by integration of H
NMR resonances in spectra of arylboronic acids (e.g., 1 H),
are not given. Accordingly, for a necessary view, 1H and
13C NMR spectra are provided in the ESI.z
Syntheses
Fig.
8 Dependence of calculated (B3LYP/6-311+G**) relative
energy, kJ molꢁ1, of open forms I on a O(1)BC(1)C(2) torsion angle
1-Bromo-3-chloro-2-(dimethoxymethyl)benzene. To a mix-
ture of 2-bromo-6-chlorobenzaldehyde (9.6 g, 50 mmol) (pre-
pared in 75% yield using the known procedure26 from 2-
bromo-6-chlorobenzene), trimethyl orthoformate (6.4 g, 60
mmol) and methanol (10 mL) a drop of conc. H2SO4 was
added. The mixture was heated for 1 h at reflux and neutra-
lized with sodium methoxide. Volatiles were removed under
reduced pressure and the residue was distilled in vacuo to give
the title compound (12.9 g, 97%), bp 95–100 1C (ca. 1 Torr) as
a colorless oil (Found: C, 40.6; H, 3.8. Calc. for C9H10BrClO2:
C, 40.7; H, 3.8%); dH (CDCl3; 400 MHz; Me4Si) 7.51 (1 H, dd,
J 8.0 and 1.0, Ph), 7.35 (1 H, dd, J 8.0 and 1.0, Ph), 7.09 (1 H t,
J 8.0, Ph), 5.84 (1 H, s, CH(OMe)2) and 3.48 (6 H, s,
CH(OMe)2); dC (CDCl3; 100.6 MHz; Me4Si) 134.9, 133.8,
132.5, 130.2, 130.1, 123.8, 105.5 and 55.8.
in 2-formylphenylboronic acid and its analogues 1 and 2.
ZPE correction twisted form of I is predicted to be more
stable. To point out some exceptions, B3LYP electronic
energy results for 2-formylphenylboronic acid and 1 predict
open twisted forms as more stable than cyclic tautomers
whereas MP2/6-31+G* (with ZPE) calculation of 2 suggests
a cyclic form II as more stable. All computational methods
predict relative energies of I (twisted) and II forms to be
decreased upon increasing van der Waals radius of a halogen
in a 3-position of the phenyl ring.
To summarize this point, the effect of a halogen increasing
the relative stability of a twisted form I and a cyclic form II
and the much stronger tendency of 2 to form a cyclic tautomer
has been predicted, in agreement with experimental observa-
tions. Hence, it seems that gas-phase ab initio calculations are
useful for prediction of substituent effects. We suppose that an
appropriate solvent plays a significant role in a stabilization of
a cyclic form II. However, it is clear that for a more precise
information (e.g., estimation of Kcycl) other effects (especially
specific hydrogen-bonding interactions with a polar solvent)
must be rigorously taken into account.
1,3-Dibromo-2-(dimethoxymethyl)benzene. A similar protec-
tion was performed with 2,6-dibromobenzaldehyde26 (26.4 g,
0.10 mol), to give the title compound (30.5 g, 98%), bp
110–113 1C (ca. 1 Torr) as a colorless oil (Found: C, 34.8;
H, 3.2. Calc. for C9H10Br2O2: C, 34.9; H, 3.25%); dH (CDCl3;
400 MHz; Me4Si) 7.56 (2 H; d, J 8.0, Ph), 7.01 (1 H, t, J 8.0,
Ph), 5.83 (1 H, s, CH(OMe)2) and 3.48 (6 H, s, CH(OMe)2); dC
(CDCl3; 100.6 MHz; Me4Si) 134.9, 133.4, 130.5, 123.8, 106.7
and 55.7.
3-Fluoro-2-formylphenylboronic acid 1. This compound was
received from Aldrich. Crystals suitable for X-ray analysis
were grown from water–acetone solution by slow evaporation
of the solvent, mp 127–129 1C (Found: C, 50.1; H, 3.65. Calc.
for C7H6BFO3: C, 50.1; H, 3.6%); nmax(KBr)/cmꢁ1 3332,
3080, 1676, 1560, 1352, 1236 and 796; dH [(CD3)2CO–D2O,
95 : 5; 400 MHz; Me4Si] 10.33 (d, J 1.0), 7.64 (td, J 8.0 and
5.0), 7.51 (d, J 7.5), 7.43 (td, J 8.0 and 4.5), 7.39 (d, J 8.0) 7.21
(ddd, J 11.0, 8.0 and 1.0), 7.16 (ddd, J 10.0, 8.0 and 1.0), and
6.43 ppm (s); dB [(CD3)2CO–D2O, 95 : 5; 128.3 MHz,
BF3 ꢂ Et2O] 28; dC [(CD3)2CO–D2O, 95 : 5; 100.6 MHz; Me4Si]
189.6 (d, J 7.5), 165.2 (d, J 256), 158.8 (d, J 251), 140.8 (d, J
13.0), 136.5 (d, J 9.0), 132.1 (d, J 6.0), 129.2 (d, J 4.0), 127.4
Conclusions
It has been demonstrated that substituted 2-formylphenyl-
boronic acids exhibit remarkable structural diversity mani-
fested especially by a chain–ring tautomerism. Such a behavior
in solution is easily monitored by H and 13C NMR spectro-
1
scopy and predictable by theoretical calculation. This is
important from the viewpoint of theoretical interest as well
as in practical terms of boronic acid analysis and purity
determination. Moreover, we believe that reported here and
future structural investigations of arylboronic acids will result
in novel applications, especially in the field of supramolecular
chemistry.
ꢀc
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150 | New J. Chem., 2007, 31, 144–154

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(d, J 6.0), 126.8 (d, J 4.0), 118.5 (d, J 20.0) and 116.6 (d, J
(dd, J 8.0 and 1.0), 7.88 (dd, J 8.0 and 1.0), 7.71 (dd, J 8.0 and
1.0), 7.31 (t, J 7.5), 7.15 (t, J 7.5), 6.06 (s).
20.5), 96.1.
3,5-Dibromo-2-formylphenylboronic acid 5. The formyl
group of 2,4,6-tribromobenzaldehyde26 (3.10 g, 9 mmol) was
protected using the method described for 1-bromo-3-chloro-2-
(dimethoxymethyl)benzene, to give crude 1,3,5-tribromo-2-
(dimethoxymethyl)benzene (3.10 g, 8.0 mmol). This com-
pound was consecutively treated with BuLi and B(OEt)3 as
described for 2, to give 5 ꢂ H2O as colorless crystals (1.80 g,
70%), mp 224–227 1C (Found: C, 25.9; H, 2.35. Calc. for
C7H5BBr2O3 ꢂ H2O: C, 25.8; H, 2.2%). nmax(KBr)/cmꢁ1 3398,
3252, 3079, 1666, 1532, 1383, 1342 and 860; dH
[(CD3)2CO–D2O, 95 : 5; 400 MHz; Me4Si] 10.24 (s), 7.87 (d,
J 1.5), 7.83 (d, J 1.5), 7.80 (d, J 1.5 Hz), 7.60 (d, J 1.5) and 6.19
(s); dB [(CD3)2CO–D2O, 95 : 5; 128.3 MHz; BF3 ꢂ Et2O] 31; dC
[(CD3)2CO–D2O, 95 : 5; 100.6 MHz; Me4Si] 192.9, 152.9,
137.2, 135.8, 134.7, 132.8, 129.2, 127.0, 123.9, 119.4 and 98.1.
3-Chloro-2-formylphenylboronic acid 2.
A solution of
1-bromo-3-chloro-2-(dimethoxymethyl)benzene (5.31 g, 20
mmol) in diethyl ether (10 mL) was added to a solution of
BuLi (20 mmol) in diethyl ether (20 mL) at ꢁ78 1C. After 15
min stirring B(OEt)3 (3.2 g, 22 mmol) was added at ꢁ78 1C.
The mixture was stirred for 30 min, then it was allowed to
warm to ca. ꢁ50 1C followed by hydrolysis with 2 M aq. HCl.
The organic layer was separated followed by evaporation of
the solvent under reduced pressure. To an oily residue water
(20 mL) and a drop of conc. aq. HCl were added. The mixture
was heated shortly to ca. 70 1C to cleave the acetal moiety
followed by removal of methanol in vacuo. The crude solid
product was filtered off and washed with water (3 ꢄ 5 mL) and
toluene (2 ꢄ 5 mL) to give 2 ꢂ H2O (3.03 g, 75%) as colorless
crystals, mp 84–88 1C (Found: C, 41.9; H, 4.1. Calc. for
C7H6BClO3 ꢂ H2O: C, 41.55; H, 4.0%);
n
max(KBr)/cmꢁ1
3-Bromo-2-formyl-5-methoxyphenylboronic acid 6. The for-
myl group of 2,6-dibromo-4-methoxybenzaldehyde27 (2.94 g,
10 mmol) was protected using the method described for 1-
bromo-3-chloro-2-(dimethoxymethyl)benzene, to give crude
1,3-dibromo-2-(dimethoxymethyl)-5-methoxybenzene (2.95 g,
8.5 mmol). This compound was consecutively treated with
BuLi and B(OEt)3 as described for 2, to give 6 ꢂ H2O as
colourless crystals (1.75 g, 78%), mp 145–148 1C (Found: C,
35.1; H, 3.6. Calc. for C8H8BBrO4 ꢂ H2O: C, 34.7; H, 3.65%);
3364, 3212, 3068, 1404, 1220 and 1080; dH [(CD3)2CO–D2O,
95 : 5; 400 MHz; Me4Si] 10.40 (d, J 0.5 Hz), 7.64 (dd, J 7.5 and
1.0), 7.56 (m), 7.45–7.36 (m) and 6.30 (s); dB [(CD3)2CO–D2O,
95 : 5; 128.3 MHz; BF3 ꢂ Et2O] 31; dC [(CD3)2CO–D2O, 95 : 5;
100.6 MHz; Me4Si] 190.9, 150.9, 136.4, 134.7, 134.4, 131.4,
130.5, 130.4, 129.6, 129.1, 128.5 and 96.4.
3-Bromo-2-formylphenylboronic acid 3. This compound was
obtained as described for 2 from 1,3-dibromo-2-(dimethoxy-
methyl)benzene (6.20 g, 20 mmol) as colorless crystals of the
monohydrate 3 ꢂ H2O (4.00 g, 81%), mp 98–101 1C (Found: C,
34.2; H, 3.3. Calc. for C7H6BBrO3 ꢂ H2O: C, 34.1; H, 3.3%);
n
max(KBr)/cmꢁ1 3414, 3262, 3085, 2987, 1647, 1592, 1540,
1348, 1263 and 809; dH [(CD3)2CO–D2O, 95 : 5; 400 MHz;
Me4Si] 10.12 (d, J 1.0), 7.21 (d, J 2.0), 7.16 (m), 6.97 (dd, J 2.5
and 1.0), 6.15 (s), 3.90 (s) and 3.82 (s); dB [(CD3)2CO–D2O, 95
: 5; 128.3 MHz; BF3 ꢂ Et2O] 30; dC [(CD3)2CO–D2O, 95 : 5;
100.6 MHz; Me4Si] 192.1, 164.7, 161.9, 145.9, 129.8, 128.6,
121.6, 118.6, 118.4, 117.6, 114.2, 98.1, 56.4 and 56.1.
n
max(KBr)/cmꢁ1 3348, 3208, 3076, 1400, 1216 and 1076; dH
[(CD3)2CO–D2O, 95 : 5; 400 MHz; Me4Si] 10.29 (s), 7.67 (dd, J
7.5 and 1.0), 7.63–7.60 (m), 7.49–7.46 (m), 7.30 (t, J 7.5) and
6.21 (s); dB[(CD3)2CO–D2O, 95 : 5; 128.3 MHz; BF3 ꢂ Et2O] 31;
dC [(CD3)2CO–D2O, 95 : 5; 100.6 MHz; Me4Si] 193.5, 153.4,
136.4, 135.1, 133.5, 131.5, 131.2, 129.9, 129.7, 126.2, 118.2 and
98.0 ppm.
2-Formyl-3-(trimethylsilyl)phenylboronic acid 7. A solution
of 1,3-dibromo-2-(dimethoxymethyl)benzene (6.20 g, 20
mmol) in diethyl ether (10 mL) was added to the solution of
BuLi (20 mmol) in diethyl ether (20 mL) at ꢁ78 1C. After
15 min stirring the solution of TMSCl (2.40 g, 22 mmol) in
THF (10 mL) was added at ꢁ78 1C. The mixture was stirred
for 30 min, then it was allowed to warm to rt. Solvents were
removed in vacuo. The residue was redissolved in THF–Et2O
(1 : 1, 30 mL) and cooled to ꢁ78 1C followed by addition of
BuLi (2 M, 10 mL) and B(OEt)3 (3.2 g, 22 mmol). Subsequent
workup was performed as described for 2, to give 7 as colorless
crystals (4.00 g, 81%), mp 72–75 1C (Found: C, 54.0; H, 6.8.
Calc. for C10H15BSiO3: C, 54.1; H, 6.8%); nmax(KBr)/cmꢁ1
3544, 3464, 3296, 2925, 1584, 1444, 1248, 956 and 840; dH
[(CD3)2CO–D2O, 95 : 5; 400 MHz; Me4Si] 10.35 (s), 7.68 (dd, J
7.5 and 1.0), 7.65 (d, J 7.5), 7.59 (dd, J 7.5 and 1.0), 7.52 (t, J
7.5), 7.30 (t, J 7.5), 6.32 (s), 0.33 (s) and 0.32 ppm (s); dB
3-Iodo-2-formylphenylboronic acid 4. The solution of 1,3-
dibromo-2-(dimethoxymethyl)benzene (6.20 g, 20 mmol) in
diethyl ether (10 mL) was added to the solution of BuLi (20
mmol) in diethyl ether (20 mL) at ꢁ78 1C. After 15 min
stirring the solution of B(OEt)3 (3.2 g, 22 mmol) was added at
ꢁ78 1C. The mixture was stirred for 30 min, then it was
carefully hydrolyzed with 2 M aq. HCl (0 1C, pH = 6ꢁ7). The
organic phase was separated and solvents were removed in
vacuo. A solution of N-butyldiethanolamine (3.22 g, 20 mmol)
in Et2O (20 mL) was added to the residue and the mixture was
stirred for 1 h at rt. Solvents were removed in vacuo and the
residue was redissolved in THF–Et2O (1 : 1, 30 mL) and
cooled to ꢁ78 1C followed by addition of BuLi (2 M, 10 mL)
and iodine (3.2 g, 22 mmol) in THF (10 mL). After hydrolysis
with 2 M aq. HCl, the organic phase was concentrated. The
residue was redissolved in Et2O (30 mL) and washed with
water (20 mL) and aq. Na2S2O3 (10 wt%, 20 mL). Subsequent
workup was performed as described for 2, to give 4 as a
crystalline material (1.75 g), containing ca. 25% of 3, dH
[(CD3)2CO–D2O, 95 : 5; 400 MHz; Me4Si] 10.05 (s), 7.95
[(CD3)2CO–D2O, 95
:
5; 128.3 MHz; BF3 ꢂ Et2O] 32;
dC [(CD3)2CO–D2O, 95 : 5; 100.6 MHz; Me4Si] 195.6, 160.7,
144.9, 142.2, 137.7, 136.1, 135.2, 134.5, 132.4, 131.5, 128.4,
98.8, 0.5 and ꢁ0.3.
4-Bromo-2-formylphenylboronic acid 8. This compound was
obtained as described for
2
from 1,4-dibromo-2-
ꢀc
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(dimethoxymethyl)benzene28 (3.10 g, 10 mmol) as colorless
crystals (1.85 g, 79%), mp 128–131 1C (Found: C, 36.6; H, 2.8.
Calc. for C7H6BBrO3: C, 36.7; H, 2.6%); nmax(KBr)/cmꢁ1
3374, 3218, 3079, 1684, 1532, 1338, 1189, 1090, 1342 and 724;
dH [(CD3)2CO–D2O, 95 : 5; 400 MHz; Me4Si] 10.26 (s), 8.03
(d, J 2.0), 7.79 (dd, J 8.0 and 2.0), 7.74 (d, t, J 8.0), 7.62 (m),
7.55 (dd, J 8.0 and 2.0) and 6.24 (s); dB [(CD3)2CO–D2O, 95 :
5; 128.3 MHz; BF3 ꢂ Et2O] 28; dC [(CD3)2CO–D2O, 95 : 5;
100.6 MHz; Me4Si] 194.3, 158.0, 142.8, 137.1, 136.4, 133.1,
132.6, 132.4, 126.8, 126.0, 124.2 and 97.8.
tion correction based on a well-defined crystal shape was
performed for compound 5. The structures were solved by
direct methods using the SHELXS-97 program,29 and refined by
the full-matrix least-squares method against F2 (SHELXL-97).30
In both cases all non-hydrogen atoms were anisotropically
refined. All hydrogen atoms in 5 and water hydrogens in 1
were located from difference maps and their positional and
isotropic thermal parameters were refined. The remaining
hydrogen atoms in 1 were introduced at geometrically idea-
lized coordinates with a fixed isotropic displacement para-
meter equal to 1.2 times the value of the equivalent isotropic
displacement parameter of the parent carbon. During the final
stages of refinement an extinction correction was applied for
intensities collected for 1. In both cases, the final Fourier-
difference maps have no significant chemical meaning.
Solution equilibrium studies
1
Samples for H NMR analyses were taken as dilute (0.02 M)
solutions of compounds 1–3 and 5–8 in wet [D6]acetone (5
wt% D2O). Prior to the acquisition, samples were thermo-
stated for a time sufficient to allow the equilibrium to be
reached (ca. 10 min). Spectra were recorded at four tempera-
tures in the range 298–321 K. Chemical shifts were determined
using internal solvent resonances and given relative to TMS.
DH and DS values were calculated using a linear van’t Hoff
plot, ln Kcycl = f(1/T). A summary of results is presented in
Table 1.
Several attempts of collecting X-ray data for crystals of
compound 3 obtained from separate batches revealed a sub-
stantial disorder in all measured crystals. After processing with
the Lorentz-polarization effects and numerical absorption
correction the solving (direct methods) and refinement (full-
matrix least squares) procedures were applied for all data sets.
The best solution and then the approximation of the structure
were obtained in the Cc space group with four independent
molecules of 3-II and four water molecules in the asymmetric
unit. Only the heaviest atoms (Br, O) were anisotropically
refined. It was determined that the molecules are arranged into
layers in the (001) plane. After the analysis of the residual
maxima on the Fourier maps the disorder (layer-stacking
faults) was modeled as a superposition of two identical layers
shifted by 0.5a (or 0.5b) vector. The positional and thermal
parameters of the major layer component were refined while
X-Ray diffraction
Details including crystal data, data collection and refinement
parameters are provided in Table 5. X-ray diffraction data for
single crystals of 1, 3 and 5 were collected at room temperature
on a Siemens P3 diffractometer with graphite-monochromated
Mo-Ka radiation. The measured intensities were processed
with the Lorentz and polarization effects. Numerical absorp-
Table 5 Crystal data, data collection and refinement parameters for compounds 1, 3 and 5
1
3 ꢂ H2O
5 ꢂ H2O
Empirical formula
Mr
Radiation (l/A)
T/K
Crystal system
Space group (no.)
Z (Z0)
a/A
C7H6BFO3
167.93
C7H6BBrO3 ꢂ H2O
C7H5BBr2O3 ꢂ H2O
246.85
Mo-Ka (0.710 73)
293(2) K
325.76
Mo-Ka (0.710 73)
293(2) K
Triclinic
Mo-Ka (0.710 73)
293(2) K
Triclinic
Monoclinic
Cc (9)
16 (4)
11.159(3)
11.116(2)
29.383(7)
90
94.473(18)
90
3633.6(14)
1.805
4.501
0.24031/0.43515
2.59-25.04
ꢀ
ꢀ
P1 (2)
2 (1)
P1 (2)
2 (1)
7.2136(9)
7.6191(8)
8.1010(11)
102.960(10)
106.797(10)
111.387(9)
368.75(8)
1.512
0.130
—
7.0559(16)
8.3747(18)
9.656(2)
89.986(17)
99.607(18)
108.329(17)
533.2(2)
2.029
7.584
0.5754/0.1491
2.1 to 25.0
b/A
c/A
a/1
b/1
g/1
V/A3
Dc/Mg mꢁ3
m/mmꢁ1
Max./min. transmission
y Range/1 (data collection)
Index ranges, hkl
Reflections collected
2.8-25.0
0 to 8, ꢁ9 to 8, ꢁ9 to 9
0 to 13, 0 to 13, ꢁ34 to 34
ꢁ8 to 8, ꢁ9 to 9, ꢁ11 to 11
1415
3296
4077
Unique reflections (Rint
)
1302 (0.0116)
1302/0/134
1.066
3296 (0.0000)
3296/278/319
1.040
1877 (0.0169)
1877/0/137
1.050
Data/restraints/parameters
Goodness-of-fit on F2 a
Reflections with I 4 2s(I)
Final R indices (I 4 2s(I))b
R Indices (all data)b
Drmax, min/e Aꢁ3
1143
2557
1609
R1 = 0.0327,wR2 = 0.0874
R1 = 0.0370, wR2 = 0.0907
+0.180 and ꢁ0.122
R1 = 0.0593,wR2 = 0.1481
R1 = 0.0809, wR2 = 0.1617
+ 0.764 and ꢁ0.782
R1 = 0.0253, wR2 = 0.0622
R1 = 0.0327, wR2 = 0.0652
+0.367 and ꢁ0.398
a
2
Goodness of fit S = {[w(Fo ꢁ Fc2)2]/(n ꢁ p)}1/2 where n is the number of reflections and p is the total number of parameters refined.
P
P
P
P
b
2
R1
=
JFo| ꢁ |FcJ/ |Fc|, wR2 = { [w(Fo ꢁ Fc2)2]/ [w(Fo2)2]}1/2
.
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152 | New J. Chem., 2007, 31, 144–154

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the minor one was introduced entirely as a rigid group
(including water oxygens) with a common isotropic thermal
parameter. The geometry of the rigid group was subsequently
corrected according to changes of the major group on succes-
sive stages of the refinement. The final occupation factor for
the minor component was refined to 0.114(3). The hydrogen
atoms of 3 were introduced at geometrically idealized coordi-
nates with a fixed isotropic displacement parameter. Water
hydrogen atoms were not included in the refinement.
ORTEP drawings were made using ORTEP3 for Windows.31
The geometrical parameters for structural analysis were calcu-
lated using the PLATON package.32
8 (a) V. L. Arcus, L. Main and B. K. Nicholson, J. Organomet.
Chem., 1993, 460, 139; (b) V. V. Zhdankin, P. J. III Persichini, L.
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Williams, J. Chem. Soc., Perkin Trans. 1, 2001, 3269; (d) A.
Sporzynski, M. Lewandowski, P. Rogowska and M. K. Cyranski,
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H. Nishiyama and K. Itoh, J. Am. Chem. Soc., 2005, 127, 9625.
9 J. Kagan, J. Org. Chem., 1967, 32, 4060.
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Chem., 1988, 53, 224.
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amount of a cyclic form (ca. 5%) was detected by a careful 1H
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16 Kdim and Kcycl were calculated by the least-squares method, using
the following equation derived from equilibria in Scheme 2
CCDC reference numbers 614639 (1), 614640 (5 ꢂ H2O) and
626865 (3 ꢂ H2O).
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b611195e
Theoretical calculations
Becke-style 3-parameter density functional method using the
Lee–Yang–Parr correlation functional33 with 6-311+G**
basis set (B3LYP/6-311+G**) and the second-order
Møller–Plesset perturbation method34 with 6-31G* basis set
(MP2/6-31G*), 6-31+G* one (MP2/6-311+G*) and the
Dunning basis set35 aug-cc-pVDZ (MP2/aug-cc-pVDZ) were
used to optimize the geometry of the molecules. DFT (B3LYP)
calculations generate good results for an equilibrium geometry
of a molecule.36 All calculations were performed using the
Gaussian0337 series of programs. Obtained geometries were
characterized as minima by calculating the vibrational fre-
quencies (no imaginary frequencies). The rotational barriers
for tautomeric forms I (Scheme 1, X = H, F, Cl, Y = H) were
computed at B3LYP/6-311+G** and MP2/6-31G* levels of
theory. For each fixed torsion angle O(1)–B–C(1)–C(2) all
remaining internal degrees of freedom were optimized for both
methods.
1
1
2Kdim
½1 ꢁ 11ꢅ:
Kc2ycl
¼
þ
Kobsd Kcycl
.
17 This compound was prepared with a substantial amount of the
acid 3 impurity, and therefore it was not characterized in detail.
18 (a) S. J. Rettig and J. Trotter, Can. J. Chem., 1977, 55, 3071; (b) H.
Feulner, G. Linti and H. Noth, Chem. Ber., 1990, 123, 1841; (c) M.
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M. Strongin, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
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A. Fox, A. E. Goeta, H. P. Goodwin, T. B. Marder and A. L.
Thompson, Dalton Trans., 2003, 4395; (f) S. Das, V. L. Alexeev, A.
C. Sharma, S. J. Geib and S. A. Asher, Tetrahedron Lett., 2003, 44,
7719; (g) B. Zarychta, J. Zaleski, A. Sporzynski, M. Da˛browski
and J. Serwatowski, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 2004, 60, o344; (h) P. Rodriguez-Cuamatzi, G. Var-
gas-Diaz, T. Maris, J. D. Wuest and H. Hopfl, Acta Crystallogr.,
Sect. E: Struct. Rep. Online, 2004, 60, o1316; (i) J.-M. Lo, S.-M.
Chen, M.-H. Chen, Y.-J. Chen, F.-L. Liao and T.-H. Lu, Acta
Crystallogr., Sect. E: Struct. Rep. Online, 2004, 60, o1851; (j) P. N.
Horton, M. B. Hursthouse, M. A. Beckett and M. P. Rugen-
Hankey, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2004, 60,
o2204; (k) Y. Yang, J. O. Escobedo, A. Wong, C. M. Schowalter,
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Acknowledgements
This work was supported by the Warsaw University of
Technology. The support by Aldrich Chemical Co., Inc.,
Milwaukee, WI, through continuous donation of chemicals
and equipment is gratefully acknowledged. The authors thank
the Interdisciplinary Centre for Mathematical and Computa-
tional Modelling (Warsaw, Poland) for computational
facilities.
19 W. H. Scouten, X.-C. Liu, N. Khangin, D. F. Mullica and E. L.
Sappenfield, J. Chem. Crystallogr., 1994, 24, 621.
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Chem., 1995, 493, 91.
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R. Probert, G. E. Smith and A. Whiting, J. Organomet. Chem.,
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