Toward boronate ester mesogenic structures

Article abstract of DOI:10.1080/15421400390223158

This article describes efforts toward the development of a new core for calamitic mesogens based upon the introduction of an extended heteroaromatic boroncontaining ring. A series of tri-catenar mesogenic boronate ester derivatives of the 2-phenyl-1,3,2-benzodioxaborole has been synthesized and characterized. The flat central core appeared to be a suitable feature for these derivatives to support anisotropic alignment. Additionally, these derivatives should possess an inherent dipole in the core. However, thermal analysis (polarized optical microscopy and differential scanning calorimetry) did not reveal any mesophases.

Full text of DOI:10.1080/15421400390223158

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TOWARD BORONATE ESTER
MESOGENIC STRUCTURES
Maura Belloni a , M. Manickam a , Zehn-He Wang a
Jon A. Preece a
&
a School of Chemical Sciences , University of
Birmingham , Edgbaston, Birmingham, B15 2TT, UK
Published online: 18 Oct 2010.
To cite this article: Maura Belloni , M. Manickam , Zehn-He Wang & Jon A. Preece
(2003) TOWARD BORONATE ESTER MESOGENIC STRUCTURES, Molecular Crystals and
Liquid Crystals, 399:1, 93-114, DOI: 10.1080/15421400390223158
To link to this article: http://dx.doi.org/10.1080/15421400390223158
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Mol. Cryst. Liq. Cryst., Vol. 399, pp. 93–114, 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390223158
TOWARD BORONATE ESTER MESOGENIC
STRUCTURES
Maura Belloni, M. Manickam, Zehn-He Wang,
and Jon A. Preece*
School of Chemical Sciences, University of Birmingham,
Edgbaston, Birmingham B15 2TT, UK
This article describes efforts toward the development of a new core for calamitic
mesogens based upon the introduction of an extended heteroaromatic boron-
containing ring. A series of tri-catenar mesogenic boronate ester derivatives of
the 2-phenyl-1,3,2-benzodioxaborole has been synthesized and characterized.
The flat central core appeared to be a suitable feature for these derivatives to
support anisotropic alignment. Additionally, these derivatives should possess
an inherent dipole in the core. However, thermal analysis (polarized optical
microscopy and differential scanning calorimetry) did not reveal any
mesophases.
Keywords: boronate esters; 2-phenyl-1,3,2-benzodioxaborole; mesogenic
INTRODUCTION
The most important types of calamitic liquid crystals (LCs) commonly used
in display devices are 4-alkyl-40-cyanobiphenyls (nCBs) discovered by
George Gray in 1973 [1]. Since the discovery of nCBs, research in the field of
LCs has been driven by the development of new materials for display devices
[2]. The modification of the parent structural motif of nCBs by entire or
partial replacement of the phenyl rings, with heteroaromatic [3-8], cycloa-
liphatic [3,9,10], and cycloheteroaliphatic [5,6] moieties has been carried out
to investigate the structural features that favor liquid crystallinity. In the
1980s the insertion of the boron atom in nonaromatic cores of thermotropic
LCs was first achieved. Seto et al. [11–13] obtained 1,3,2-Dioxaborinane
liquid crystalline derivatives, which either had a wide nematic range
We would like to thank The University of Birmingham for the facilities, the EPSRC for fund-
ing M. Belloni’s Ph.D. studentship, and the Leverhulme Trust for funding M. Manickam’s post-
doctoral fellowship.
*Corresponding author. E-mail: j.a.preece@bham.ac.uk
93

94
M. Belloni et al.
or formed a chiral smectic C (SC*) phase exhibiting ferroelectric behavior.
Furthermore, other boron-containing LCs based on cholesterylphenyl-
boronic acids [14] were synthesized as potential sugar receptors.
The objective of the research presented in this article was to incorporate
the boron atom in the rigid core component of the pseudo-aromatic ring of
the 2-phenyl-1,3,2-benzodioxaborole derivatives (Figure 1). The 2-phenyl-
1,3,2-benzodioxaborole unit is much more planar [15] than the crystal
structures of the classical 4-alkyl-40-cyanobiphenyl LCs [16], in which there
is a significant deviation from planarity of the two phenyl rings. X-ray
studies of the 2-phenyl-1,3,2-benzodioxaborole have shown that the
molecule in the crystalline state is nearly flat [15]. The length of the BO
˚
(boron-oxygen) bonds (1.39 A) are intermediate between a single BO bond
˚
˚
(1.43 A) and a double BO bond (1.36 A), suggesting aromatic-like con-
jugation in the five-membered ring [15]. Further evidence for the aromatic
character [17] of the 2-phenyl-1,3,2-benzodioxaborole arises from IR [18]
and UV [19] experiments. The length of the 2-phenyl-1,3,2-benzodiox-
˚
˚
aborole is ꢀ 8.85 A, which is intermediate between the biphenyl (7.12 A)
˚
[20,21] and terphenyl (11.54 A) [22] cores. Additionally, the 2-phenyl-1,3,2-
FIGURE 1 Structure of the target boronate ester derivatives CxCxBCy 1–10.

Toward Boronate Ester Mesogenic Structures
95
benzodioxaborole structure should be inherently dipolar, unlike the
biphenyl and terphenyl, which have to be chemically modified by the
introduction of cyano groups, for example. Therefore, it was our principal
objective to design a core with structural similarities to the biphenyl and
terphenyl system while having electronic properties that would make it
attractive as a switching element in devices.
Thus, the research described in this article will focus on the synthesis of
the 2-phenyl-1,3,2-benzodioxaborole derivatives 1–10 (CxCxBCy series)
(Figure 1). These compounds are characterized by two features:
1. a boron atom incorporated into a pseudo-aromatic ring, and
2. a ‘‘Y’’ molecular shape distinctive of the polycatenar [23] and swallow-
tailed [24] LCs.
RESULTS AND DISCUSSION
The 2-(4-alkyloxyphenyl)-5,6-dialkyl-benzo[1,3,2]dioxaboroles 1–10 were
synthesized via the esterification of 4-alkyloxyphenylboronic acids 14–19
derivatives with 4,5-dialkylcatechol 11–13 derivatives (Scheme 1).
The two-step reaction to afford the desired boronic acids 14–19
(Scheme 2) required an initial standard O-alkylation [25–27] (with the
appropriate alkyl halide under basic conditions) followed by a standard
lithium/halogen exchange [28] (n-BuLi), followed by quenching with tri-
methyl borate and hydrolysis (aqueous HCl).
The 4-alkyloxyphenylboronic acids 14–19 were afforded as a mixture
with their anhydrides 26–31 (Scheme 3). The acids trimerize to the
anhydrides because of partial dehydration during isolation of the acids [29–
32]. The presence of the boronic acid/anhydride mixture is shown by a twin
1
set of signals in the H NMR (CDCl3) spectra of the isolated 4-alkylox-
yphenylboronic acid 14–19 [32].*
A five-step protocol was followed to yield the 4,5-dialkylcatechols 11–13
(Scheme 4).
*Throughout the synthesis of the 4-alkyloxyphenylboronic acids 14–19, clear evidence of
partial formation of the corresponding anhydrides 26–31 was observed. These species are
formed by trimerization of the acids with loss of three molecules of H2O, to generate the tri-
mers (triphenylboroxins). X-ray studies [31] of triphenylboroxin showed that the central unit,
constituted by the conjugated system of the B3O3 ring, is nearly planar. Additionally, also the
three-phenyl units were observed to be nearly coplanar with the central B3O3 ring, leading to a
wide, flat, and rigid central core, which correctly functionalized might be suitable for the de-
velopment of new discotic LCs. With this in mind, the anhydrides of the 4-alkyloxyphenylboro-
nic acids 14–19 were observed under POM. However, they did not display any mesomorphic
behavior, melting directly from the solid to the isotropic liquid.

96
M. Belloni et al.
SCHEME 1 Formation of the 2-phenyl-1,3,2-benzodioxaborole derivatives 1–10 by
esterification between 4,5-dialkylcatechols 11–13 and 4-alkyloxyphenylboronic
acids 14–19, with loss of two molecules of H2O.
Commercially available 1,2-methylenedioxy-benzene was chosen as a
convenient starting material because the bridging methylene unit (OCH2O)
protects both phenolic moieties, and can be removed in the final step. The
first step (Scheme 4) involved the acid-catalyzed bromomethylation [33] of
1,2-methylenedioxy-benzene at the 4 and 5 positions of the phenyl ring,
using paraformaldehyde and HBr in AcOH to afford 32. In the second step
32 was converted to the bis-phosphonium salt 33 with an excellent yield
[35,36]. The bis-phosphonium salt 33 was used in a Wittig [34,35] reaction
(EtOLi, EtOH [36,37]) with the appropriate aldehyde, to form the mixture
SCHEME 2 Reagents and conditions: (i) 1.0 eq CH3I or RBr, 10.0 eq K2CO3,
CH3CN, reflux, 24 h; (ii) 1.05 eq n-BuLi, ꢁ78ꢂC, anhydrous THF, N2 atmosphere,
45 min; 1.05 eq B(OMe)3, ꢁ78ꢂC, N2 atmosphere, 1 h; (iii) 10% HCl, rt, 1 h.

Toward Boronate Ester Mesogenic Structures
97
SCHEME 3 Dehydration and trimerization of boronic acids 14–19 to anhydrides
26–31, with loss of three molecules of H2O.
SCHEME 4 Reagents and conditions: (i) 4.7 eq (CH2O)n, 5.5 eq HBr/AcOH, rt, 3 h;
(ii) 2.0 eq PPh3, DMF, reflux, 3 h; (iii) 6.0 eq butyraldehyde, 1.0–2.5 eq EtOLi,
EtOH, reflux, 24 h; (iv) H2, Pd/C (10%), MeOH/CHCl3, rt, 48 h; (v) 3.0 eq AlBr3,
EtSH, 0ꢂC; then rt, 24 h, H2O for quenching.

98
M. Belloni et al.
of diastereoisomeric dienes 35a–35c. Although the dienes were obtained
as a mixture of the cis,cis, cis,trans, and trans,trans stereoisomers, which
were observed as three spots by TLC (hexane/EtOAc), separation of the
three isomers was not necessary for the synthetic purposes, as they were
hydrogenated (H2, Pd/C [38]) to the saturated derivatives 36a–36c in the
next step. The hydroxyl functionalities were generated using aluminium
tribromide (AlBr3) and ethyl mercaptan (EtSH), a system [39–41] that
cleaves the methylenedioxy group and reveals the catechol moieties in
compounds 11–13 (Scheme 1). The final step (Scheme 1) was the ester-
ification reaction to afford the boronate ester derivatives 1–10 (CxCxBCy)
with elimination of H2O. In order to obtain an efficient removal of H2O, the
esterification was performed using trimethyl orthoformate, CH(OMe)3
[42,43] as the solvent which acted as a dehydrating agent.
Compounds 1, 2, 3, 4, and 5 were crystalline at room temperature and
easily isolated by recrystallization. However, the other homologous 6–10
were waxy materials at room temperature, and purification by recrys-
tallization or column chromatography or solvent extraction was not suc-
cessful.* Thus, among the series, only compounds 1–5 were obtained in
an analytically pure form. However, we include compounds 6–10 in this
article to illustrate the range of materials that were prepared and high-
light the difficulties in their purification.
THERMAL ANALYSIS AND PHASE BEHAVIOR
Thermal behavior of these materials CxCxBCy (1–10) was investigated
using polarized optical microscopy (POM) and differential scanning
calorimetry (DSC). From the POM observations, compounds 1–4 melted
directly from solid to the isotropic liquid at 115, 92, 75, and 73ꢂC,
respectively. Compound 4 underwent two polymorphic transitions at 28ꢂC
*Attempts at purification methods carried out on CxCxBCy:
I. Chromatography on silica gel (eluent: 60% EtOAc in hexane) or preparative TLC on silica
plate (eluent: 30% EtOAc in CHCl3) were unsuccessful, due to hydrolysis of the products by
the acidic silica. In fact, generally boronate esters are easily hydrolyzed in acid conditions [19].
II. Chromatography on alumina gel (eluent: CH3CN) revealed that boronate esters were
decomposed. This result was confirmed by 1H-NMR data, which revealed that the different
bands collected from the preparative TLC on alumina were neither the boronate esters nor
the starting materials.
III. Continuous extraction was performed in CH3CN and hexane using a liquid-liquid
continuous extractor. The hexane (top layer) has been chosen as the extracting solvent, to
remove the impurities, while the products preferentially remained in CH3CN (bottom layer).
This method, performed for three hours under reflux, was partially successful. Only
compounds C6C6BC9 7 and C8C8BC5 9 were obtained with a satisfactory degree of purity
(judged by 1H-NMR).

Toward Boronate Ester Mesogenic Structures
99
and 43ꢂC before reaching the isotropic liquid state at 62ꢂC. On cooling it
passed directly to the solid state at 22ꢂC. The DSC analysis (Figure 2)
suggested that compound 5 did not form any mesophase. In fact, only one
peak on both the heating (41ꢂC) and cooling (9.4ꢂC) stages can be
observed. Therefore, it has been concluded that this derivative was not
liquid crystalline.
The POM observations on the other compounds 6–10 showed that they
did not melt directly to the isotropic liquid, but became fluid-like, main-
taining the same texture, which was not easily identifiable. The DSC phase
transitions are listed in Table 1 (DSC data).
As previously mentioned, compounds 6–10 were not pure, therefore the
interpretation of thermal analysis results cannot be conclusive. However,
the results obtained on the pure derivative 5, which did not exhibit
mesomorphic behavior, lead to the conclusion that the whole CxCxBCy
series of materials are not liquid crystalline.
CONCLUSIONS
A new series of mesogenic molecular structures CxCxBCy has been syn-
thesized with the purpose of extending general investigations upon the
molecular structures capable of displaying mesophase. In the CxCxBCy
series of boronate ester derivatives, the attractive molecular aspect is
represented by the planarity of the core and the inherent dipole due to the
presence of the boron atom adjacent to two oxygen atoms. Ten CxCxBCy
compounds were synthesized by esterification of a series of 4-alkylox-
yphenylboronic acids 14–19 (OCH3OC11H23) with 4,5-dialkylcatechol
11–13 in trimethyl orthoformate. These 2-phenyl-1,3,2-benzodioxaborole
derivatives 1–10 differ in the length of the flexible peripheral chains
attached to the central rigid core, so that the thermal properties could be
FIGURE 2 DSC trace of the pure compound 5 (C5C5BC9).

100
M. Belloni et al.
TABLE 1 Transition Temperatures (ꢂC) of 5–10 Recorded on Second Heating by
DSC Analysis (T Onset Reported Values)
Sample
Sol
X
–
Y
–
Z
–
I
C5C5BC9
5
C5C5BC11
ꢃ
–
–
–
–
41.0
(13.7)
18.2
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢁ16.1
(ꢁ10.8)
ꢁ27.7
(ꢁ3.6)
ꢁ10.2b
(0.5)
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢁ0.8
(0.1)
6.3
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
–
ꢃ
–
–
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
6
C6C6BC9
(11.3)
41.9
a
25.0
(6.9)
–
7
C8C8BC3
(0.08)
22.8
(0.2)
45.2
8
C8C8BC5
(ꢁ1.8)
ꢁ6.1
(0.03)
25.4
(1.6)
12.9
ꢁ9.7b
–
a
9
C8C8BC9
10
(0.07)
ꢁ10.4b
(0.2)
(13.7)
52.2
(0.03)
44.6
(0.1)
(14.0)
apure by 1H NMR.
bpeak values.
The enthalpy changes DH (kJ molꢁ1) for the transitions observed are reported in brackets.
Sol represents either the solid or waxy state. X, Y, and Z represent undefined intermediate
physical states.
investigated extensively over a range of homologues. Unfortunately, these
derivatives were difficult to purify and easily susceptible to hydrolysis and
degradation with time. Thermal analysis by POM and DSC revealed that
compounds CxCxBCy melted to an isotropic liquid and failed to display any
mesophases.*
The obvious question is why do these materials not exhibit mesophases,
since we designed them to have similar steric (planarity and length) and
electronic (16 p-electron) core requirements as the biphenyl and terphenyl
derivatives? The following thoughts may, retrospectively, shed some light
on this question. Firstly, the 2-phenyl-1,3,2-benzodioxaborole is a much
more planar species than the biphenyl or terphenyl species, which have a
torsional twist around the CC bonds joining the phenyl rings. There is no
such torsional twist around the BC bond in the 2-phenyl-1,3,2-benzodiox-
aborole. Secondly, the two phenylene rings in 2-phenyl-1,3,2-benzodiox-
aborole are probably electron deficient as a result of the electron-deficient
boron atom pulling electron density out of both phenylene rings, via
resonance effects in the ring the boron is directly attached to and inductive
*The rigid core might need to be extended with at least an extra ring, as well as introducing
a linking group such as an ester functionality, in order to build up flexibility in the molecular
structure and show mesogeneity.

Toward Boronate Ester Mesogenic Structures
101
effects on the other ring. Thus, taking these two points together there are
probably strong p-p interactions between adjacent 2-phenyl-1,3,2-benzo-
dioxaboroles, resulting in materials that upon heating are unable to display
anisotropic liquid phases.
The next obvious question to ask is how then might these materials be
modified to induce mesophase behavior? One avenue to pursue is the
replacement of the catechol moieties with 1,2-diamino moieties to afford
more stable boronate amides derivatives, and allow access to a wider range
of materials, as the purification process should be easier. However, it would
be more interesting to investigate further the ideas discussed in the pre-
vious paragraph about the stereoelectronics. Thus, one can envisage the
introduction of a Me substituent on the disubstituted phenylene ring ortho
to the CB bond, in order to break the planar nature of the 2-phenyl-1,3,2-
benzodioxaborole unit and hence reduce the strong tendency for aniso-
tropic alignment. Additionally, the R0 groups, instead of being weak
inductive þI groups, could be replaced with alkoxy groups which would
considerably increase the p-electron density on the tetrasubstituted aro-
matic ring, and hence reduce the p-p stacking interactions.
EXPERIMENTAL
General Procedures
Commercially available chemicals were purchased from Aldrich and used as
such without any purification unless otherwise stated. Anhydrous THF was
distilled under a N2 atmosphere over sodium/benzophenone. n-Butyl-
lithium (n-B uLi) was titrated using diisopropylamine, 4-phenylbenzylide-
nebenzylamine (as the indicator) in anhydrous THF, and 2-butanol (1 M in
xylene) as the titrating agent [44]. Thin-layer chromatography (TLC) was
carried out on aluminium plates coated with silica gel 60 F254 (Merck
5554). TLC plates were air-dried, revealed under UV lamp (254 nm) and
developed in iodine vapor if needed. Column chromatographic separations
were performed on silica gel 60 (Merck 9385, 230–400 mesh or ICN
EchoChrom 32-63, 60 A). 1H Nuclear Magnetic Resonance (1H NMR)
˚
spectra were recorded on a Bruker AC300 (300.13 MHz) spectrometer. 13
C
NMR spectra were recorded on a Bruker AC300 (75.5 MHz) spectrometer
using the PENDANT pulse sequences. 31P NMR spectra were recorded on a
Bruker AC300 (121.5 MHz) spectrometer using 1H decoupling. All chemical
shifts are quoted in d (ppm) to higher frequency from Me4Si, using deut-
erated chloroform (CDCl3), or acetone (acetone-d6) as the lock and the
residual solvent as the internal standard. The coupling constants J
(J ¼ 3Jvic, Jo ¼ 3J ortho, Jm ¼ 4J meta, JP-H ¼ 3J coupling phosphorus-
1
3
hydrogen, JP-C
,
2JP-C, and JP-C ¼ J coupling phosphorus-carbon) are

102
M. Belloni et al.
expressed in Hertz (Hz) with multiplicities abbreviated as follows:
s ¼ singlet, d ¼ doublet, dd ¼ double doublet, t ¼ triplet, m ¼ multiplet,
b ¼ broad. Electron impact mass spectrometry (EIMS) was performed on a
VG ProSpec and a Kratos Profile instrument. Liquid secondary ion mass
spectrometry (LSIMS) was performed on a VG ZabSpec instrument
equipped with a cesium ion source and using m-nitrobenzylalcohol
(NOBA) as a matrix. Electrospray mass spectrometry (ESMS) was per-
formed on a Micromass LCT time of flight (TOF) using methanol as the
running solvent. High resolution mass spectrometry (HRMS) was per-
formed on a Micromass ProSpec and Micromass ZabSpec instruments using
perfluorokerosene (PFK) as calibrant for the EI and Cesium Iodide or
Polyethylene glycol (PEG) for LSI. Elemental analysis was carried out on a
Carlo Erba EA 1110 (C H N S) instrument. POM experiments were carried
out using an Olympus BX40 optical microscope with crossed polarizers
equipped with Linkam LT350 hot stage. DSC data was recorded on a
Perkin-Elmer 7 Series thermal analysis system under a N2 or He purge,
using a cooling system of circling H2O or liquid N2, respectively. The
software used in connection with the DSC instrument was Pyris I. All
samples were heated and cooled in a double cycle, at the rate of 10.00ꢂC/
min. Melting points and boiling points were determined using an Electro-
thermal 9200 melting point apparatus and are uncorrected.
1-Bromo-4-alkyloxy-benzene derivatives (20–25)
and 4-alkyloxyphenylboronic acid (14–19)
The general procedure for the synthesis of compounds 20–25 and 14–19
follows a slightly modified preparation reported in the literature [27].
However, details regarding the synthetic procedure and the full char-
acterization of these compounds are thoroughly described in our previous
publication [45].
1,2-Methylenedioxy-4,5-dibromomethylbenzene
or 5,6-bis-bromomethyl-benzo[1,3]dioxole (32)
The preparation of compound 32 follows a literature procedure [46]. To a
stirred suspension of formaldehyde (28.3 g, 0.94 mol) in 1,2-methylene-
dioxy-benzene (26.6 mL, 0.20 mol), HBr/AcOH (45%, 1.1 mol, 200 mL) was
added at 0ꢂC. The mixture was stirred for further 30 min at 0ꢂC and then
allowed to warm to room temperature and stirred overnight. H2O was
added until the product precipitated. The precipitate was filtered and dried
in vacuo. The crude product was purified by silica gel column chromato-
graphy using CHCl3 as eluent. The chromatographed material was further
purified by recrystallization from hexane/toluene to yield the product as a

Toward Boronate Ester Mesogenic Structures
103
white solid [46] (43.9 g, 71%): m.p. 112.4–114.2ꢂC; nmax/cmꢁ1 1503, 1488,
1
1376, 1266, 1210, 1170, 1037, 931; H NMR (300 MHz, CDCl3) d (ppm):
4.58 (s, 4 H, BrCH2), 5.98 (s, 2 H, OCH2O), 6.81 (s, 2 H, ArH); 13C NMR
(75 MHz, CDCl3) d (ppm): 30.57 (2 C, BrCH2), 101.85 (1 C, OCH2O),
110.89 (2 C, ArCH), 130.59 (2 C, ArC), 148.33 (2 C, ArC); m/z (EIMS)
308 (average Mþ, 15%), 227 and 229 (M-Brþ, 75%), 148 (M-2Brþ, 100%).
Elemental analysis: C9H8Br2O2 (Mm: 307.969: calc. C, 35.10; H, 2.62; found
C, 35.14; H, 2.38).
1,2-Methylenedioxy-4,5-bis-(triphenyl)-methyl-benzene
bis-phosphonium dibromo salt (33)
The procedure for the synthesis of compound 33 is described in the lit-
erature [34,35]. To a stirred solution of 32 (44.0 g, 0.14 mol) in DMF
(300 mL), triphenylphosphine (75.3 g, 0.28 mol) was added. The reaction
was heated under reflux under a N2 atmosphere for 3 h and then cooled to
room temperature. The resulting precipitate was filtered, washed with
EtOAc, and then dried in vacuo at 280ꢂC for 24 h, affording the product as a
pale pink solid (114.0 g, 98%): m.p. 127ꢂC (decomposition); nmax/cmꢁ1
1
2797, 1483, 1435, 1263, 1110, 1037; H NMR (300 MHz, CDCl3) d (ppm):
3.91 (d, 4 H, JP-H 14 MHz, PCH2), 5.89 (s, 2 H, OCH2O), 6.36 (s, 2 H,
2
ArH), 7.52–7.96 (m, 30 H, ArH-Ph); 13C NMR (75 MHz, CDCl3) d (ppm):
28.53 (2 C, d, 1JP-C 49 MHz, PCH2) [48], 103.64 (2 C, OCH2O), 112.80 (2 C,
1
s, ArCH-3,6), 118.6 (6 C, d, JP-C 86–82 MHz, ArC) [48], 122.52 (2 C, t,
3
2JP-C 9.9 MHz, ArC) [49], 131.30 (12 C, t, JP-C 6.4 MHz, ArCH) [48],
2
135.44 (12 C, d, JP-C 4.7 MHz, ArCH) [48], 136.46 (6 C, s, ArCH), 149.46
(2 C, ArC); dP (121 MHz, CD3CN) 20.17 (2 P); m/z (LSIMS) 855 (MþNaþ,
10%), 753 (MBrþ, 100%), 671 (MBrPhþ, 65%), 595 (MBr2Phþ, 65%);
HRMS (LSIMS) C45H40Br2O2PBr calc. 753.1687; found 753.1682.
1,2-Methylenedioxy-4,5-di-(alk-1-enyl)benzene
or 5,6-di-alk-1-enyl-benzo[1,3]dioxole (35a–35c)
The conditions employed in order to perform the Wittig reaction were as
reported in the literature [34,35]. To a stirred solution of compound 33 (1.0
eq) in EtOH (100–300 mL) the appropriate aldehyde (6.0 eq) and EtOLi
(1.0–2.5 eq) were added under a N2 atmosphere. The reaction was stirred
at room temperature overnight. The reaction was then heated under reflux
under a N2 atmosphere for 8 h and then allowed to cool to room tem-
perature. The mixture was extracted with Et2O:EtOAc (150 mL:60 mL) and
washed with H2O (2630 mL) and brine (30 mL). The organic layer was
dried (MgSO4), filtered, and the solvent removed in vacuo. The product

104

105

106

107

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M. Belloni et al.
TABLE 5 Quantities of Reagents Used to Synthesize Compounds 1–10
Catechol
derivative
Boronic acid
derivative
Compound
Yield
Melting pointa
110ꢂC
1
Catechol:
165 mg, 1.5 mmol
Catechol:
14
210 mg, 1.5 mmol
30 mg,
30%
2
15
121 mg, 0.7 mmol
20 mg,
10%
92ꢂC
77 mg, 0.7 mmol
Catechol:
3
19
1.7 g, 6.0 mmol
2.0 g,
92%
75ꢂC
660 mg, 6.0 mmol
11
4
14
36 mg, 0.2 mmol
70 mg,
80%
73ꢂC
66 mg, 0.2 mmol
11
5
18
137 mg, 0.5 mmol
19
130 mg,
52%
62ꢂC
130 mg, 0.5 mmol
11
6
46 mg,
41%
27.5ꢂC
33.4ꢂC
52ꢂC
55 mg, 0.2 mmol
12
66 mg, 0.2 mmol
18
7
44 mg,
44%
66 mg, 0.2 mmol
13
63 mg, 0.2 mmol
15
8
20 mg,
21%
68 mg, 0.2 mmol
13
34 mg, 0.2 mmol
16
9
63 mg,
62%
29.6ꢂC
37.4ꢂC
67 mg, 0.2 mmol
13
67 mg, 0.2 mmol
38 mg, 0.2 mmol
18
53 mg, 0.2 mmol
10
48 mg,
43%
aMelting point determined by POM during the second heating cycle.
was purified by silica gel column chromatography, using petroleum ether
(60/80ꢂC fraction) as the eluent, to afford 35a–35c as pale yellow oils (70–
81%), as a mixture of the cis,cis, cis,trans, and trans,trans diastereoi-
somers. In Table 2 are listed the quantities of reagents used to synthesize
compounds 35a–35c and their analytical data.
1,2-Methylenedioxy-4,5-dialkyl-benzene
or 5,6-dialkyl-benzo[1,3]dioxole (36a–36c)
A general procedure to hydrogenate double bonds using H2 gas in the
presence of a metal catalyst was followed [38,48]. A catalytic amount of Pd/
C (10%) was added to a stirred solution of diastereoisomeric mixture of
dienes 35a–35c in CHCl3:MeOH (70 mL:70 mL). The reaction was stirred
under a H2 atmosphere for 48 h. The mixture was filtered to remove the Pd/
C powder and concentrated in vacuo and purified by silica gel column
chromatography, using hexane as the eluent, to yield 36a–36c as light
brown oils (71–81%). In Table 3 are listed the quantities of reagents used
to synthesize compounds 36a–36c and their analytical data.

109

110

111

112

Toward Boronate Ester Mesogenic Structures
113
4,5-Dialkyl-catechol or 5,6-dipentyl-benzene-1,2-diol (11–13)
A general procedure for deprotection of benzo[1,3]dioxole rings was fol-
lowed [39–41]. A stirred solution of compound 36a–36c (1.0 eq) in EtSH
(2–3 mL) was cooled to 0ꢂC. AlBr3 (2.5–3.0 eq) was added and the reaction
was stirred at room temperature overnight. The reaction was quenched
with H2O (10 mL). Extraction of the product was carried out using EtOAc
(20 mL), H2O (2620 mL), and brine (20 mL). The organic layer was dried
(MgSO4), filtered, and the solvent removed in vacuo. The crude product
was purified by silica gel column chromatography, using CHCl3 as eluent, to
afford 11–13 [49] as light brown oils (53–75%). In Table 4 are listed the
quantities of reagents used to synthesize compounds 1–10 and their ana-
lytical data.
4,5-Dialkyl-catechol-B-(4-alkyloxyphenyl)borane or
2-(4-alkyloxyphenyl)-5,6-dialkyl-benzo[1,3,2]dioxaborole (1–10)
The general procedure for the synthesis of compounds 1–10 is as follows:
to a stirred solution of catechol or 4,5-dialkyl-catechol 11–13 (1.5 eq) in
CH(OMe)3 (15 mL) was added the appropriate 4-alkyloxyphenylboronic
acid 14–19 (1.5 eq). The reaction was stirred at room temperature over-
night. The reaction mixture was then concentrated in vacuo. The crude
product was either recrystallized from toluene (1–3) or purified from the
mixture of reaction with CH3CN (4–5); otherwise the purification method
used consisted in a continuous extraction (3 h) using CH3CN (100 mL) and
hexane (40 mL) to yield a waxy yellow product (6–10). In Table 5 are listed
the quantities of reagents used to synthesize compounds 1–10 and in
Table 6 their analytical data are reported.
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