Chelated boronate-imine and boronate-amine complexes as chiral dopants for nematic liquid crystals

Article abstract of DOI:10.1002/ejoc.201000406

Enantiomerically and diastereomerically pure chelated boronate-imine complexes S and a library of boronate-amine complexes 8 have been synthesized by taking advantage of a modular concept based on aromatic aldehydes, aminoethanols and boronic acids. The configuration of the amine complexes 8 featuring stable stereogenic boron and nitrogen centers are assigned based upon crystal, structure analyses of the representative compounds 8aaa and. 8acj and the comparison of the CD spectra of all complexes 8. They serve as colorless, stable dopants for nematic liquid crystals and pro-vide high helical twisting power. The interaction of boronates 8 with nematic compounds ZLI-1840 and 5-CB featuring a benzonitrile moiety is studied by ¹⁹F and ¹H NMR spectroscopy. A strong π-stacking between the arylboronate residue and the nematic compound as well as a hydrogen bond are indicated by the spectroscopic data. Based thereupon, a model is proposed that correlates the configuration of the dopant with the sign of the helix formed, by twisting the nematic phase.

Full text of DOI:10.1002/ejoc.201000406

FULL PAPER
DOI: 10.1002/ejoc.201000406
Chelated Boronate–Imine and Boronate–Amine Complexes as Chiral Dopants
for Nematic Liquid Crystals
Sebastian Schlecht,[a] Walter Frank,[b] and Manfred Braun*[a]
Keywords: Chirality / Crystal structures / NMR spectroscopy / Optical properties / Supramolecular chemistry
Enantiomerically and diastereomerically pure chelated bo-
ronate–imine complexes 5 and a library of boronate–amine
complexes 8 have been synthesized by taking advantage of
a modular concept based on aromatic aldehydes, amino-
ethanols and boronic acids. The configuration of the amine
complexes 8 featuring stable stereogenic boron and nitrogen
centers are assigned based upon crystal structure analyses of
vide high helical twisting power. The interaction of boronates
8 with nematic compounds ZLI-1840 and 5-CB featuring a
benzonitrile moiety is studied by 19F and 1H NMR spec-
troscopy. A strong π-stacking between the arylboronate resi-
due and the nematic compound as well as a hydrogen bond
are indicated by the spectroscopic data. Based thereupon, a
model is proposed that correlates the configuration of the
the representative compounds 8aaa and 8acj and the com- dopant with the sign of the helix formed by twisting the ne-
parison of the CD spectra of all complexes 8. They serve as
matic phase.
colorless, stable dopants for nematic liquid crystals and pro-
Introduction
Among the various liquid-crystalline compounds, the
cholesteric or chiral nematic phase meets particular interest
due to its manifold applications in displays, polarizers, cer-
tain polymers and paints and as color-effect materials.[1]
The cholesteric phase is essentially a twisted nematic phase
with a nematic arrangement in each individual layer,
wherein the molecules are aligned along a common direc-
tion, the so called director. From one layer to the next, the
director rotates about a certain angle, thus creating a helical
arrangement.[2] The most elegant way to produce a choles-
teric phase was discovered more than 80 years ago by the
French physicist G. Friedel[3] who found that the addition
of a small amount of a chiral, nonracemic compound, a
dopant, to a nematic liquid crystal converts it into a choles-
teric phase (Figure 1).[1d] Later, the phenomenon was care-
fully studied by the research groups of Buckingham, Stege-
meyer, and Baesseler.[4] As shown by Stegemeyer and Mai-
nusch,[4c] an enantiomerically pure compound that, itself,
does not form a mesophase is nevertheless able to function
as a dopant and thus introduce a helical structure in a ne-
matic phase. Chiral doping can even serve for the determi-
nation of the absolute configuration and the enantiomeric
purity of dopants.[2c]
Figure 1. Conversion of a nematic phase into a cholesteric phase
by the addition of a dopant.[1d]
A measure of the efficiency of a chiral dopant is the so-
called helical twisting power (HTP). It is defined for small
concentrations of the dopant according to Equation (1).
Therein, p is the pitch of the induced helix, and x is the
molar fraction of the dopant, dissolved in the nematic
phase. Given the HTP of an individual dopant, the appro-
priate ratio of the dopant to the nematic liquid-crystalline
compound provides a hight of the pitch that is in the range
of the wavelength of visible light. This property is desirable
for most applications. In general, one aims at a high HTP
value, so that the smallest amount possible of the valuable
dopant has to be used.
(1)
[a] Heinrich-Heine Universität Düsseldorf, Institut für Organische
Chemie und Makromolekulare Chemie,
Universitätsstr. 1, 40225 Düsseldorf, Germany
Fax: +49-211-8115079
In the beginning, readily available natural products were
used as dopants, mainly terpenes and steroids, and deriva-
tives thereof.[4c] Among the numerous synthetic compounds
used as dopants later, biphenyls,[5a] binaphthol-derived
esters[5b] and, in particular, TADDOLs[6] displayed remark-
E-mail: braunm@uni-duesseldorf.de
[b] Heinrich-Heine Universität Düsseldorf, Institut für
Anorganische Chemie und Strukturchemie,
40225 Düsseldorf, Germany
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S. Schlecht, W. Frank, M. Braun
FULL PAPER
ably high values of helical twisting power. Metal complexes vealed the relative configuration and allows to assign the
featuring a stereogenic metal center in addition to chiral (RC,RB) configuration to the complex 5a.[8] The boron–
ligands seem to open an entry to a large variety of chiral nitrogen distance of 1.588 Å confirms the existence of a co-
dopants due to the manifold combinations of different li- ordinative bond. The CD spectra and the accordance of the
gands and central atoms. This concept has been realized Cotton effects for all the complexes 5a–d demonstrate the
by using tris(dionato)metal complexes of chromium, cobalt, homochirality at the boron atom and makes it plausible to
rhodium and ruthenium[7a,7b] as well as binaphthol-derived consider the complexes 5b–d as the (RC,RB) stereoisomers
titanium complexes.[7c] The fact that bis(chelated) alk- as well.
oxy(imine)titanium complexes developed by us[7d–7f] and,
more recently, tetrapyrrol-derived zinc complexes[7g] gave
the so far highest HTP values (up to 740 µm–1 and
920 µm–1, respectively) may illustrate the efficiency of
metal-containing dopants. The disadvantages, which will
hamper their application, are not only the toxicity of the
transition metals but even more the fact that most of them
are colored, partly due to the ligand, partly due to the metal
ion according to its oxidation stage. We were interested
whether the environmentally benign and safe half-metal
boron will be a suitable central atom in chelate complexes.
Having recently observed[8] that boron functions as a con-
figurationally stable stereogenic center in boronate–imine
complexes, we show here that diastereomerically and enan-
and conditions: (i) methanol/THF (1:1), Na2SO4, reflux 12 h, 55%;
Scheme 1. Synthesis of boronate–imine complexes 5a–d. Reagents
tiomerically pure boronate–amine complexes are readily ac-
cessible. For the first time colorless, air- and moisture-stable
boronate–amine complexes are demonstrated to serve as
chiral dopants with significant helical twisting power.
(ii) molecular sieves (3 Å), toluene, reflux 12 h, 54% (5a), 44% (5b),
86% (5c), 92% (5d).
The bright yellow color of the boronate–imine complexes
5 is a clear disadvantage with respect to their application as
chiral dopants, in particular if they are intended to serve as
effect colors.[1e] Therefore, we replaced the imine by color-
less amine ligands and designed enantiomerically and dia-
Results and Discussion
The novel boronate–imine complexes[8] and boronate– stereomerically pure boronate–amine complexes. The syn-
amine complexes were obtained by modular syntheses, thesis shown in Scheme 2 uses the same type of building
combining a chiral 2-aminoethanol with 2-hydroxy-1-naph- blocks, but involves as an additional step the reduction of
thaldehyde or aromatic ortho-hydroxy aldehydes as the sec- the imine to the amine moiety. The amino alcohols (R)-6a–
ond component and a boronic acid as the third one. Based c with a stereogenic center adjacent to the amino group
on this concept, a variety of boronate complexes with a served as the starting materials. Aminotriphenylethanol
substantial diversity became available. (R)-2-Amino-1,2,2- 6a[12] and the amino alcohol 6b featuring geminal meta-di-
triphenylethanol (1), readily accessible from methyl mandel- fluorophenyl substituents are readily available from (R)-
ate,[9] was chosen as the starting material for the synthesis phenylglycine, whereas 2-amino-2-phenylethanol (6c) is a
of boronate–imine complexes 5. As illustrated in Scheme 1, commercial product. They were condensed with the alde-
the amino alcohol 1 was treated with 2-hydroxy-1-naph- hydes 2a–c to give the imines 7aa–7ca. In order to avoid
thaldehyde (2a) to give the imine 3 whose condensation any racemization that could occur due to an imine/enamine
with boronic acids 4a–d was easily accomplished by heating tautomerism, the reaction was run at low temperature
a solution in toluene in the presence of molecular sieves. (–15 °C for 72 h).[13] Upon reduction with sodium cyano-
Thereby, boron is expected to become a stereogenic center borohydride,[14] the corresponding amines were isolated as
in the imine complexes 5a–d, and the question of diastereo- their hydrochlorides, which turned out to decompose when
selectivity and configurational stability arises. The role of stored for a longer time. Therefore, they were converted im-
boron as a stereogenic center has been studied occasion- mediately by treatment with boronic acids 4a, 4b, and 4e–j
ally[8,10] in acyclic and cyclic complexes, and the barrier of in the presence of sodium hydrogen carbonate that served
inversion was determined in enantiomeric acyloxybor- to liberate in situ the amine from the hydrochloride. The
anes[10g–10i] and boronate–imine complexes.[8,10j] Remarka- yields of the products 8 over two steps varied considerably,
bly, all the complexes 5a–d were obtained in a completely but were fair in most cases. The novel boronate–amine com-
diastereoselective manner. The formation of a single dia- plexes were found to be air- and moisture-stable com-
stereomer is clearly demonstrated by NMR spectroscopy. pounds. Their 11B resonances again indicate the existence
In particular, the 11B NMR spectra of the individual com- of tetra-coordinated boron compounds[11] (see Exp. Sect.).
plexes display a single resonance (see Exp. Sect.) in the shift Aside from the boron atom, the amine nitrogen atom be-
range that is typical for tetracoordinated boron.[11] The comes a stereogenic center during the formation of the che-
crystal structure of the representative compound 5a re- late. Here again, the condensation was found to be com-
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Chiral Dopants for Nematic Liquid Crystals
Scheme 2. Synthesis of a library of boronate–amine complexes 8. Reagents and conditions: (a) methanol/THF (1:1), Na2SO4, –15 °C,
72 h, 93% (7aa), 88% (7ab), 30% (7ac), 69% (7bb), 61% (7ca); (b) (i) NaCNBH3, methanol, 25 °C, then dilute HCl, (ii) toluene, NaHCO3,
reflux, 4 h, 46% (8aaa), 65% (8aab), 33% (8aae), 41% (8aaf), 90% (8aag), 13% (8aah), 38% (8aai), 55% (8aaj), 63% (8aba), 45% (8abi),
54% (8abj), 50% (8acj), 64% (8bbj), 37% (8caa).
pletely diastereoselective. The 1H, 13C, and 11B NMR spec- the (SB,RC,SN) configuration. Nevertheless, both complexes
tra clearly show that a single diastereomer has formed, and are homochiral, and the alternative configuration at the ste-
even traces of epimers could not be detected.
reogenic boron atoms is a formal one, due to the change of
The question of the configurations at the stereogenic bo- priorities according to the Cahn–Ingold–Prelog rules.[15] At
ron and nitrogen atoms is answered by crystal structure a glance, the heterochirality at the boron atom in the imine
analyses of the representative compounds 8aaa and 8acj complex 5a on the one hand and the amine complexes 8aaa
shown in Figures 2 and 3. In both compounds, the junction and 8acj on the other hand might be surprising, in view of
of the fused five- and six-membered ring is cis, thus minim- the identical configuration of the chiral backbone. One
izing the strain. The nitrogen and boron atoms are found should be aware, however, that the amino alcohols (R)-1
to be pyramidal, and the (RB,RC,SN) configuration is as- and (R)-6a are regioisomers. In all the three complexes 5a,
signed to the complex 8aaa, whereas complex 8acj adopts 8aaa, and 8acj, the aryl substituent at the boron atom and
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S. Schlecht, W. Frank, M. Braun
FULL PAPER
the phenyl group at the stereogenic center of the amino
alcohol moiety are cis-configured. In the crystal of com-
pound 8aaa, one molecule of methanol is included due to
the medium used for the crystallization. As shown in Fig-
ure 2, the alcohol is bound to the nitrogen atom by a hydro-
gen bond, wherein the NH group is the hydrogen donor
rather than the OH group. This observation can be interpre-
ted as a consequence of the enhanced acidity of the amine
hydrogen atom due to the complexation with the boron
atom. In both boronate–amine complexes 8aaa and 8acj,
the heterocyclic six-membered ring adopts a chair-like con-
figuration, presumably to avoid steric congestion caused by
the accumulation of aryl residues. The configurations of the
other boronate–amine complexes were again assigned by
the accordance of their Cotton effects in the CD spectra.
Figure 3. Molecular structure of 8acj in the crystal. Displacement
ellipsoids are set at 30% probability, hydrogen atoms bonded to
carbon atoms are omitted for clarity. Bond lengths [Å] and angles
[°]: B1–O1 1.463(3), B1–O2 1.426(3), B1–N1 1.654(3), B1–C4
1.642(3), O1–C11 1.349(3), O2–C1 1.414(2), N1–C2 1.519(2), N1–
C3 1.501(3), C1–C2 1.593(3), C3–C10 1.498(3), C10–C11 1.394(3);
O1–B1–O2 117.99(17), O1–B1–N1 108.12(15), O1–B1–C4
106.66(15), O2–B1–N1 100.12(14), O2–B1–C4 108.64(16), N1–B1–
C4 115.65(15); N1–H1 0.85(2).
of 1/p vs. the molar fraction equals the HTP value. The sign
of the helicity was determined from the motion of the col-
ored interference rings during the rotation of the azimuth
of the polarizer in the microscope to higher values. If the
colored rings move outwards, this indicates a left-handed
helix, moving inwards a right-handed one.[7e,18] The HTP
values are shown in Table 1. It turned out that negative
HTP values (M) resulted in all measurements, indicating a
left-handed helix.
Figure 2. Diagram of the chosen asymmetric unit of the crystal
structure of 8aaa·CH3OH. Dashed lines indicate hydrogen bonds.
Displacement ellipsoids are set at 30% probability, hydrogen atoms
not involved in hydrogen bonding are omitted for clarity. Distances
[Å] and angles [°]: B1–O1 1.472(5), B1–O2 1.468(5), B1–N1
1.619(6), B1–C4 1.622(6), O1–C11 1.356(5), O2–C1 1.460(4), N1–
C2 1.522(4), N1–C3 1.512(5), C1–C2 1.583(6), C3–C10 1.496(5),
C10–C11 1.390(6); O1–B1–O2 110.2(3), O1–B1–N1 107.4(3), O1–
B1–C4 112.2(3), O2–B1–N1 99.4(3), O2–B1–C4 112.9(3), N1–B1–
C4 114.0(3); N1–H2 1.00(3), H2···O3 1.96(3), N1–H2···O3 166(3),
N1···O3 2.936(4); O3–C38 1.404(5), O3–H1 0.82, H1···O2i 2.03,
O3–H1···O2i 172, O3···O2i 2.842(4) (i: 1 – x, –0.5 + y, 1 – z).
Scheme 3. Nematic phases used for HTP measurements of the dop-
ants 5 and 8.
Except for a few cases where a low HTP value was ob-
tained in one of the nematic phases (Entries 3, 10, 18) the
The helical twisting power of the boronates 5 and 8 used Grandjean-Cano measurement was performed in ZLI-1840
as dopants was determined in two different host com- and 5-CB. As the induction of helicity by the dopant, which
pounds, the commercial nematic phases ZLI-1840 and 5- plays the role of the guest in the nematic host, is based on
CB, whose structures are shown in Scheme 3. ZLI-1840 is noncovalent interactions, it is not surprising that an individ-
a mixture of eight alkyl-substituted cyclohexylphenyl and ual dopant exhibits different twisting powers in different
cyclohexylbiphenyl cyanides. It is used as a component in nematic compounds, depending on their functional groups,
LC displays. 5-CB, on the other hand, is a mesophase that the number of their aromatic and/or cycloaliphatic rings
consists of a single compound, 4-(4Ј-pentyl-phenyl)benzoni- and the molecular size and extension. Here, we have chosen
trile.[16] The boronates 5 and 8 were dissolved in the nematic two nematic phases that have an identical functional group,
compounds at different concentrations in the range of 10–2 the benzonitrile, but differ in the number of aromatic rings,
to 10–3 mol fraction of the dopant. As expected, imine com- the incorporation of cyclohexyl rings and the extension of
plexes 5 gave yellowish mixtures, whereas the mixtures of the aliphatic side chain. As shown in Table 1, the HTP
amine complexes 8 with the nematic compounds were col- values for the two nematic compounds ZLI-1840 and 5-CB
orless. The HTP values were determined by the Grandjean- roughly parallel the different dopants. For an individual
Cano wedge method,[17] that permits to determine the hight dopant, both nematic phases are in the same order of mag-
of the pitch p. The 1/p values were measured at different nitude. In the series of dopants, the HTP values varied con-
concentrations of the dopant, and the gradient of the plot siderably, ranging from values below 10 up to over
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Chiral Dopants for Nematic Liquid Crystals
Table 1. HTP values of boronate–imine complexes 5 and boronate–
amine complexes 8 in nematic phases ZLI-1840 and 5-CB.
Various attempts have been made in order to understand,
on a supramolecular level, the interaction between the chi-
ral dopant and the nematic phase, and different models
have been developed, based on theoretical calculations and
spectroscopic information.[2b,2c,7b,7h,20] In order to investi-
gate the noncovalent interaction of the nematic compounds
ZLI-1840 and 5-CB with the new, colorless broronate–
amine complexes 8, we took advantage of their modular
synthesis that permits to introduce probes at different sites
of the molecules. As a tool, we used 19F NMR shifts of
boronate–amine complexes 8aaj, 8abj, 8acj and 8bbj and
the alterations in the shift values that occurred after the
nematic compounds ZLI-1840 and 5-CB had been added.
The results are shown in Table 2. First the effect of the sol-
vent was studied with boronate complex 8aaj that was mea-
sured in chloroform, toluene and [D12]cyclohexane (En-
tries 1, 4, 7), whereas compound 8abj was measured in tolu-
ene (Entry 9), and 8acj and 8bbj in [D12]cyclohexane exclu-
sively (Entries 12 and 14). As expected, compound 8aaj dis-
plays in all solvents separate signals for the three groups
of anisochronous fluorine atoms in ortho, meta, and para
position of the (pentafluorophenyl)boronate moiety. In the
Entry
Boronate complex
HTP [µm–1]
ZLI-1840
5-CB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
5a
–12
–19
–17
–5
5b
–10
[a]
5c
5d
–2
–6
8aaa
8aab
8aae
8aaf
8aag
8aah
8aai
8aaj
8aba
8abi
8abj
8acj
8bbj
8caa
–32
–58
–12
–42
–72
–22
–95
–61
–45
–96
–12
–30
–35
–5
–50
–46
–30
–43
–60
[a]
–40
–25
–67
–115
–11
–32
–44
[a]
[a] Not determined.
100 µm–1. Imine complexes gave only moderate HTP values absence of an additive, the chemical shifts were not influ-
(Entries 1–4), and dopants with aliphatic residues at the bo- enced significantly by the solvent. In order to obtain insight
ron atom (Entries 3 and 4) display only marginal twisting in the interaction of dopant and nematic phase, the shift
power. Therefore, aryl–boron residues were implemented in differences were determined that occurred upon addition of
all the amine complexes (Entries 5–18). It turned out that the additives ZLI-1840 and 5-CB.
the substitution pattern of the aryl group at the boron atom
Table 2. Influence of the additives ZLI-1840 and 5-CB on the 19F
had a strong influence on the HTP values. This becomes
resonances in the pentafluorophenyl ring of boronate–amine com-
plexes 8aaj, 8abj, 8acj and 8bbj.
in particular evident from the series of dopants 8aaa–8aaj
(Entries 5–12) featuring a naphthyl moiety. Neither the pure
phenyl (Entry 7) nor the 3-thienyl substitution (Entry 10)
gave a good performance as dopants. A p-phenyl substitu-
ent turned out to be favorable (Entries 5, 6, 8, 9, 11), as
well as the pentafluorophenyl moiety (Entry 12), and the
biphenyl-substituted boron complex 8aai gave a high HTP
value of 95 µm–1 in ZLI-1840. When changing the naphthyl
moiety, derived from 2-hydroxy-1-naphthaldehyde, against
a di-tert-butyl-substituted aromatic backbone (Entries 13–
15), again a high HTP value was obtained with the p-eth-
oxybiphenyl derivative 8abi (Entry 14), leading to the best
results in ZLI-1840 (–96 µm–1) and 5-CB (–115 µm–1). The
pentafluorophenyl residue at the boron atom gave only a
poor result (Entry 15). Further variation in the phenolic
backbone with the difluoro derivative 8acj (Entry 16) and
replacement of the geminal diphenyl group by the corre-
sponding (3,5-difluoro)phenyl derivative 8bbj (Entry 17) did
not have exceptional influence on the HTP values (En-
tries 16 and 17 vs. Entries 12 and 15). All the amine com-
plexes listed in Entries 5–17 feature the chiral aminotriaryl-
ethanol pattern in the backbone and include a geminal di-
arylhydroxymethyl unit. In contrast, dopant 8caa is missing
In Table 2, the shift differences ∆δ are given, defined as
this pattern. When comparing the marginal HTP values the difference of the shifts in the presence and in the ab-
of 8caa (Entry 18) with otherwise identical compound sence of the nematic compounds. Accordingly, a positive
8aaa (Entry 5), the importance of the geminal diaryl- value indicates a downfield, a negative value an upfield shift
hydroxymethyl group, well known from asymmetric synthe- that is induced by the additive. In the first series (Entries 1–
sis,[19] becomes evident also in the context of induced helic- 3), where chloroform was used as the solvent, the high-field
ity.
and low-field shift differences are relatively small, with a
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S. Schlecht, W. Frank, M. Braun
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maximum upfield shift of the p-fluorine substituents of der to prove this assumption, 1H NMR spectra of boronate
0.5 ppm. It seems to be plausible that the relatively polar 8aaj in [D12]cyclohexane were measured at different
solvent chloroform prevents the nematic compounds from amounts of the additive 5-CB. Figure 4 shows the relevant
forming stable aggregates with the boronate 8aaj. Accord- part of the NMR spectra displaying the signals of the NH
ingly, significantly higher shift differences are observed in groups, the tertiary benzylic proton and the diastereotopic
the hydrocarbons toluene (Entries 5 and 6) and cyclohexane NCH2 protons. The downfield shift of the NH signal at
(Entries 7 and 8). Thus, upfield shifts up to 2.0 ppm were increasing concentrations of the nitrile 5-CB clearly indi-
observed for the p-fluorine atoms in the boronate 8aaj in cates a hydrogen bond.[22] The model deduced thereof also
toluene, and downfield shifts up to 1.9 ppm for the o-fluor- explains for the complex 8aaj the upfield shift of the m- and
ine position in [D12]cyclohexane. The comparison of ZLI- p-fluorine signals. Due to the attachment of the benzonitrile
1840 and 5-CB gave very similar shift differences, indicating moiety to the amino group, they become influenced by the
that both nematic compounds interact in the same way with diamagnetic field originating from the first benzene ring in
the boronate 8aaj (Entry 5 vs. 6 and 10 vs. 11). The mea- 5-CB and ZLI-1840. The downfield shift of the o-fluorine
surements of the remaining complexes 8abj, 8acj, and 8bbj signals, on the other hand, is caused by the proximity to
were restricted to the solvents toluene (Entries 9–11) or the cyano group.
[D12]cyclohexane (Entries 12–15). Here again, remarkable
upfield and downfield shifts were determined. A clear and
constant tendency becomes obvious from the comparison
of the o- with the m- and p-fluorine shift differences: in all
experiments, the signals of the o-fluorine atoms are shifted
downfield, whereas the signals of the fluorine atoms in meta
and para position display an upfield shift. Two additional
resonances in the 19F NMR spectra have to be taken into
account in the boronate complexes 8acj and 8bbj due to the
m-difluoro substitution pattern in the phenolic part of 8acj
and the pair of diastereotopic fluorine atoms in the geminal
3,5-difluorophenyl group of 8bbj, respectively. It turned out
Scheme 4. (a) Hydrogen-bonded cyano group of the nematic com-
pounds at the boronate–amine complexes 8; (b) model of (M) helic-
ity of the nematic compound induced by dopants 8.
that in toluene or [D12]cyclohexane the addition of 5-CB
led to a small upfield shift in the case of 8bbj (0.4 and
0.5 ppm). In the phenolate moiety of compound 8acj, the
p-fluorine signal is shifted by 0.6 ppm, the o-fluorine signal
by 1.2 ppm upon addition of 5-CB. Both resonances are
The π-stacking and hydrogen-bond formation will not
shifted to higher field. The effect of the nematic compounds only occur in the nonpolar solvent cyclohexane but also in
to the geminal diaryl group as well as the phenolate moiety the mixture of dopants 8 in the pure nematic phase 5-CB
can be explained by a nonspecific effect of the co-solvent or ZLI-1840. As seen from Table 1, all the homochiral bo-
that leads to an upfield shift of all 19F resonances. In con- ronate–amine complexes induce a left-handed helix of the
trast, the nematic compounds ZLI-1840 and 5-CB influence nematic phase. That means that the configuration of dopant
the aryl residue at the boron atom in a very specific way, as clearly correlates with the (M) helix. Taking into account
it becomes evident from a discrimination between the o- the noncovalent interactions between the dopant and the
(downfield shifted signals) and the m- and p- fluorine atoms nematic compound 5-CB, we propose a model for the origin
(upfield shifted signals). One can conclude thereupon that of the (M) helicity, deduced from the structure of the dop-
a π–π interaction between the aromatic rings of the nematic ant. As illustrated in Scheme 4b, the arylboronic residue
compounds and the arylboronic moiety occurs. This is in prevents the nematic compound from forming a stack
accordance with the results of the measurement of the HTP towards the front side. There remain two directions of a
values, shown in Table 1: The strongest influence on the he- stacking on the rear side. A turn to the right, however, will
lical twisting power originates from the arylboronate moi- be disfavored by the aryl groups of the amino alcohol moi-
ety.
ety, in particular the one at the stereogenic amine carbon
Aside from π-stacking, another noncovalent interaction atom, and the cis-oriented one of the geminal phenyl
between the nematic phase and the boronate–amine com- groups. As a consequence, the stacking turns to the left,
plexes is taken into account. Both ZLI-1840 and 5-CB are “open” rear side of the complex. The fact that this part of
similar in their HTP values and in the effect on the fluorine the molecule does not influence the stacking significantly,
resonances on the arylboronic moiety. This could corre- is demonstrated by the HTP values of dopants with a naph-
spond to the cyano group, a common feature of both ne- thyl and a di-tert-butylphenyl moiety, which turn out to be
matic compounds. As demonstrated by the crystal structure rather similar (Table 2, Entry 5 vs. 13). Thus, stacking to
of boronate complex 8aaa, the amino group becomes a hy- the left side can be interpreted as the first steps of the left-
drogen-bond donor upon complexation with the boron handed helix. The plausible correlation between the config-
atom. When a benzonitrile is offered as an acceptor,[21]
hydrogen bond might form, as shown in Scheme 4a. In or- licity is illustrated by the mnemonic shown in Scheme 4b.
a
uration of the boronate–amine complexes and the (M) he-
3726
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 3721–3731

Chiral Dopants for Nematic Liquid Crystals
Figure 4. 1H NMR shifts of 8aaj in different solutions of 5-CB in [D12]cyclohexane. Assignment of signals (from left to right): NH,
benzylic H and diastereotopic NCH2.
2.0 g (58%), Rf = 0.6 (hexane/ethyl acetate, 2:1), m.p. 39 °C. 1H
Conclusions
NMR (500 MHz, CDCl3): δ = 1.52 (s, 3 H), 1.97 (s, 2 H), 5.04 (s,
1 H), 6.20 (s, 1 H), 6.84–7.50 (m, 11 H) ppm. 13C NMR (125 MHz,
We have synthesized a series of diastereomerically and
enantiomerically pure boronate–imine and boronate–amine CDCl3): δ = 61.9, 79.1, 102.3, 103.7, 109.3, 110.1, 128.3–128.7,
139.0, 147.3, 150.4, 161.8, 162.8, 163.8, 164.8 ppm. 19F NMR
(470 MHz, CDCl3): δ = –110.5, –108.8 ppm. C20H15F4NO·H2O
(378.34): calcd. C 63.32, H 4.52, N 3.69; found C 63.09, H 4.26, N
3.59.
complexes. Their configurations are determined by repre-
sentative crystal structure analyses. For the first time, it is
shown that boronate–amine complexes 8 are suitable, color-
less, air-stable dopants for nematic liquid-crystalline phases.
Based on NMR studies of the boronate–amine complexes General Procedure for the Preparation of Imines 7ac, 7ca and 7bb:
Amino alcohol
6 (5.0 mmol), the corresponding aldehyde 2
in the presence of nematic compounds ZLI-1840 and 5-CB,
an interpretation of their noncovalent interaction is given,
and a rationale is offered that correlates the configuration
of the dopant with the sense of helicity in the induced
cholesteric phase.
(5.5 mmol, 1.1 equiv.) and sodium sulfate (3.5 g, 25.0 mmol) were
suspended in a solution of dry methanol (50 mL) and dry tetra-
hydrofuran (50 mL). The mixture was cooled to –15 °C and stirred
for 72 h. After filtration, the solvent was removed in a rotary evap-
orator, and the residue was purified by column chromatography to
deliver imines 7ac and 7ca as yellow, solid compounds.
7ac: Yield 0.8 g (30%), Rf = 0.73 (chloroform/ethyl acetate, 10:1),
m.p. 123 °C. H NMR (500 MHz, CDCl3): δ = 2.7 (s, 1 H), 5.4 (s,
Experimental Section
1
General: Melting points (uncorrected) were determined with a
Büchi 540 melting point apparatus. Optical rotations were
measured with a Perkin-Elmer 341 polarimeter. [α]D values are
given in units of 10–1 degcm2 g–1. NMR spectra were recorded at
25 °C with a Varian VXR 200 or 300 or a Bruker DRX 500 spec-
trometer. Mass spectra (ESI) were measured with a Finnigan
LCQDECA spectrometer. All boronate complexes display the typi-
cal isotope pattern of the [M + H] peak in the ESI mass spectra.
CD spectra were measured with a Jasco J-600 spectropolarimeter.
Silica gel 60 F254 TLC plates (Merck) were used. Elemental analy-
ses were carried out with a Perkin-Elmer CHN-Analysator 263 at
the Institut für Pharmazeutische Chemie (Universität Düsseldorf).
All reactions involving organometallic compounds or metal com-
plexes were carried out under anhydrous nitrogen.
1 H), 6.56 (m, 1 H), 6.81 (m, 1 H), 7.0–7.3 (m, 13 H), 7.5 (d, J =
8.5 Hz, 2 H), 8.14 (s, 1 H), 12.9 (s, 1 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 78.9, 80.6, 107.6, 111.7, 126.4–129.6, 137.5, 143.5,
144.2, 165.7 ppm. C27H21F2NO2 (429.46): calcd. C 75.51, H 4.93,
N 3.26; found C 75.73, H 4.77, N 3.24.
7ca: Yield 3.2 g (61%), Rf = 0.1 (chloroform/ethyl acetate, 10:1),
m.p. 186 °C. 1H NMR (500 MHz, CDCl3): δ = 1.6 (s, 1 H), 3.9 (m,
J = 12, J = 8.7 Hz, 2 H), 4.6 (dd, J = 8.2, J = 8.7 Hz, 1 H), 6.8 (d,
J = 9.2 Hz, 1 H), 7.1 (t, J = 7.8 Hz, 1 H), 7.25–7.37 (m, 7 H), 7.4
(d, J = 9.2 Hz, 1 H), 7.7 (d, J = 8.4 Hz, 1 H), 8.8 (s, 1 H), 14.8 (s,
1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 67.4, 71.3, 107.1,
118.1, 122.8, 123.1, 126.4, 126.8–129.2, 133.3, 136.9, 137.8, 159.3,
173.4 ppm. C19H17NO2 (291.35): calcd. C 78.33, H 5.88, N 4.81;
found C 78.13, H 5.93, N 4.72.
Procedure for the Determination of the HTP Values: See ref.[7d,7e]
7bb: Yield 0.96 g (69%), Rf = 0.55 (hexane/ethyl acetate, 10:1), m.p.
92 °C. 1H NMR (500 MHz, CDCl3): δ = 1.19 (s, 9 H), 1.33 (s, 9
H), 3.19 (s, 1 H), 5.24 (s, 1 H), 6.49–7.32 (m, 13 H), 8.33 (s, 1 H),
12.51 (s, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 29.7, 31.7,
34.5, 35.5, 78.5, 80.0, 103.3, 110.5, 127.1–129.9, 137.5, 141.1, 148.9,
158.1, 162.0, 162.5, 164.5, 170.0 ppm. 19F NMR (470 MHz,
Imines 3, 7aa, 7ab and Boronate–Imine Complex 5a: These were
prepared as described previously.[13,9,8]
Amino Alcohol 6b: This was obtained as yellowish solid from 1-
bromo-3,5-difluorobenzene and methyl (R)-2-amino-2-phenylacet-
ate hydrochloride according to the procedure given in ref.[13], yield
Eur. J. Org. Chem. 2010, 3721–3731
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3727

S. Schlecht, W. Frank, M. Braun
FULL PAPER
CDCl3): δ = –109.7, –108.9 ppm. C35H35F4NO2 (577.66): calcd. C
72.77, H 6.11, N 2.42; found C 73.00, H 6.19, N 2.32.
(0.19 g, 3.0 mmol) were dissolved in absolute methanol (75 mL).
After the addition of hydrochloric acid (10%) (5 mL), the yellow
solution was stirred at room temperature for 1 h, while it turned
colorless gradually. Distilled water (50 mL) was added, and the
solution was extracted three times with chloroform. The combined
organic layers were dried with sodium sulfate, and the solvent was
removed in a rotary evaporator. Immediately, the crude colorless,
product thus obtained, the corresponding boronic acid 4a, 4b, 4e–
j (1.0 mmol) and sodium hydrogen carbonate (0.13 g, 1.5 mmol)
were suspended in dry toluene (100 mL) and refluxed for 4 h. After
the addition of distilled water, the layers were separated, and the
aqueous phase was extracted with chloroform. The combined or-
ganic layers were dried with sodium sulfate, and the solvent was
removed in a rotary evaporator. The residue was purified by col-
umn chromatography to deliver boronates 8 as colorless, solid ma-
terials. According to this procedure, the following compounds were
obtained.
General Procedure for the Preparation of Boronate–Imine Com-
plexes 5b, 5c and 5d: The imine 3 (0.44 g, 1.0 mmol), the corre-
sponding boronic acid (1.5 mmol) and molecular sieves (3 Å)
(1.0 g) were suspended in dry toluene (100 mL) and refluxed for
20 h. After filtration, the solvent was removed in a rotary evapora-
tor, and the residue was purified by column chromatography (chlo-
roform/ethyl acetate, 10:1) to deliver boronates 5b–d as yellow, solid
compounds. For the preparation of boronate 7c, the imine 5b was
treated with (iPrO)2B(nBu) in an analogous way, however, in the
absence of molecular sieves. According to this procedure, the fol-
lowing compounds were obtained.
5a: Yellow solid, yield 791 mg (55%), Rf = 0.61 (chloroform/ethyl
1
acetate, 10:1), m.p. 214 °C. H NMR (500 MHz, CDCl3): δ = 6.19
(s, 1 H), 6.4 (s, 2 H), 6.87 (t, J = 7.6 Hz, 2 H), 6.9 (d, J = 8.0 Hz,
2 H), 7.09 (d, J = 8.0 Hz, 2 H), 7.12 (m, 5 H), 7.17 (m, 3 H), 7.27
(m, 3 H), 7.30 (d, J = 9.0 Hz, 1 H), 7.32 (t, J = 7.6 Hz, 1 H), 7.39
(t, J = 8.0 Hz, 1 H), 7.45 (d, J = 8.1 Hz, 1 H), 7.74 (d, J = 8.0 Hz,
1 H), 7.98 (d, J = 9.1 Hz, 1 H), 8.0 (s, 1 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 82.8, 87.2, 113.5, 120.6, 121.4, 124.7, 126.7,
126.8–129.0, 127.7, 129.1, 129.4, 131.8, 133.1, 134.4, 138.7, 139.0,
139.6, 156.2, 163.6 ppm. 11B NMR (160 MHz, CDCl3): δ =
6.8 ppm. MS (ESI): m/z (%) = 563 (24), 564 (100), 565 (48) [M +
H]+.
8aaa: Yield 260 mg (46%), Rf = 0.82 (chloroform/ethyl acetate,
1
10:1), m.p. 142 °C. H NMR (500 MHz, CDCl3): δ = 4.1 (d, J =
15.3 Hz, 1 H), 4.23 (dd, J = 15.3, J = 4.4 Hz, 1 H), 4.4 (dd, J =
11, J = 4.4 Hz, 1 H), 5.17 (d, J = 11 Hz, 1 H), 6.66 (d, J = 6.9 Hz,
2 H), 6.85 (d, J = 8.9 Hz, 1 H), 7.03 (m, 3 H), 7.08 (m, 3 H), 7.16
(m, 4 H), 7.22 (m, 3 H), 7.24 (d, J = 8.02 Hz, 2 H), 7.26 (t, J =
7.0 Hz, 1 H), 7.49 (d, J = 7.4 Hz, 2 H), 7.56 (d, J = 8.9 Hz, 1 H),
7.67 (d, J = 7.3 Hz, 1 H), 7.72 (d, J = 8.1 Hz, 2 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 42.1, 72.3, 85.4, 103.6, 119.7, 121.9, 122.7,
126.1–129.1, 127.9, 128.2, 129.0, 130.1, 131.7, 133.8, 134.0, 143.8,
147.9, 153.7 ppm. 11B NMR (160 MHz, CDCl3): δ = 5.1 ppm. MS
(ESI): m/z (%) = 565 (30), 566 (100), 567 (48) [M + H]+.
5b: Yellow solid, yield 261 mg (44%), Rf = 0.83 (chloroform/ethyl
1
acetate, 10:1), m.p. 216 °C. H NMR (500 MHz, CDCl3): δ = 6.2
(s, 1 H), 6.4 (d, J = 6.1 Hz, 2 H), 6.85 (t, J = 7.0 Hz, 2 H), 7.12–
7.18 (m, 6 H), 7.18 (d, J = 8.2 Hz, 2 H), 7.28 (d, J = 7.0 Hz, 2 H),
7.30 (d, J = 9.1 Hz, 1 H), 7.31 (m, 3 H), 7.32 (m, 1 H), 7.4 (d, J =
7.4 Hz, 2 H), 7.49 (d, J = 7.8 Hz, 1 H), 7.72 (d, J = 8.3 Hz, 1 H),
7.74 (d, J = 8.1 Hz, 1 H), 8.0 (d, J = 9.1 Hz, 1 H), 8.05 (s, 1 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 82.9, 87.3, 113.5, 120.6,
121.4, 123.3, 124.8, 126.7–130.5, 127.8, 129.5, 131.8, 132.9, 135.1,
138.6, 138.9, 139.8, 156.4, 163.5 ppm. 11B NMR (160 MHz,
8aab: Yield 234 mg (65%), Rf = 0.81 (chloroform/ethyl acetate,
1
10:1), m.p. 143 °C. H NMR (500 MHz, CDCl3): δ = 4.09 (d, J =
15.4 Hz, 1 H), 4.19 (dd, J = 15.4, J = 4.0 Hz, 1 H), 4.44 (dd, J =
11.3, J = 4.0 Hz, 1 H), 5.18 (d, J = 11.3 Hz, 1 H), 6.69 (d, J =
7.0 Hz, 2 H), 6.85 (d, J = 9.0 Hz, 1 H), 7.02 (m, 3 H), 7.06 (m, 3
H), 7.10 (d, J = 7.0 Hz, 1 H), 7.12–7.20 (m, 6 H), 7.22 (t, J =
7.0 Hz, 1 H), 7.48 (d, J = 8.2 Hz, 2 H), 7.49 (d, J = 7.7 Hz, 2 H),
7.53 (d, J = 9.0 Hz, 1 H), 7.64 (d, J = 8.1 Hz, 1 H), 7.89 (d, J =
7.7 Hz, 2 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 42.2, 72.4,
85.5, 103.7, 119.7, 121.8, 122.8, 124.6, 126.7–129.2, 128.2, 128.8,
130.1, 131.6, 132.5, 133.8, 143.7, 147.8, 153.6 ppm. 11B NMR
(160 MHz, CDCl3): δ = 5.0 ppm. 19F NMR (470 MHz, CDCl3): δ
= –62.9 ppm. MS (ESI): m/z (%) = 599 (29), 600 (100), 601 (40)
[M + H]+.
CDCl3):
δ = δ =
6.1 ppm. 19F NMR (470 MHz, CDCl3):
–62.4 ppm. MS (ESI): m/z (%) = 597 (22), 598 (100), 599 (40) [M
+ H]+.
5c: Yellow solid, yield 495 mg (86%), Rf = 0.69 (chloroform/ethyl
1
acetate, 10:1), m.p. 197 °C. H NMR (500 MHz, CDCl3): δ = 0.7
(t, J = 7.1 Hz, 3 H), 1.18 (m, 6 H), 6.07 (s, 1 H), 7.0–7.3 (m, 15
H), 7.2 (d, J = 9.1 Hz, 1 H), 7.28 (t, J = 7.1 Hz, 1 H), 7.38 (t, J =
7.7 Hz, 1 H), 7.5 (d, J = 8.2 Hz, 1 H), 7.7 (d, J = 7.9 Hz, 1 H),
7.86 (s, 1 H), 7.9 (d, J = 9.0 Hz, 1 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 14.2, 26.1, 27.4, 82.1, 88.1, 114.0, 120.6, 121.8, 124.2,
126.8–130.1, 127.5, 128.6, 129.2, 132.0, 136.1, 138.5, 139.1, 139.3,
155.0, 163.4 ppm. 11B NMR (160 MHz, CDCl3): δ = 8.8 ppm. MS
(ESI): m/z (%) = 509 (28), 510 (100), 511 (40) [M + H]+.
8aae: 175 mg (33%), Rf = 0.73 (chloroform/ethyl acetate, 10:1),
m.p. 140 °C. 1H NMR (500 MHz, CDCl3): δ = 4.08 (d, J = 15.4 Hz,
1 H), 4.25 (dd, J = 15.4, J = 4.0 Hz, 1 H), 4.53 (dd, J = 11.4, J =
3.8 Hz, 1 H), 5.17 (d, J = 11.4 Hz, 1 H), 6.67 (br. s, 2 H), 6.87 (d,
J = 9.0 Hz, 1 H), 7.04 (m, 2 H), 7.08 (t, J = 7.6 Hz, 2 H), 7.14 (d,
J = 7.8 Hz, 1 H), 7.15 (m, 2 H), 7.18 (t, J = 8.0 Hz, 1 H), 7.24 (m,
3 H), 7.26 (m, 2 H), 7.39 (t, J = 8.0 Hz, 1 H), 7.51 (d, J = 7.3 Hz,
2 H), 7.54 (d, J = 9.0 Hz, 1 H), 7.66 (d, J = 7.8 Hz, 1 H), 7.69 (d,
J = 7.1 Hz, 1 H), 7.8 (d, J = 7.8 Hz, 2 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 42.1, 72.2, 85.3, 103.7, 119.8, 122.0, 122.5,
126.2–129.9, 128.0, 128.1, 128.8, 130.0, 131.8, 132.3, 134.2, 143.1,
144.0, 148.2, 153.9 ppm. 11B NMR (160 MHz, CDCl3): δ =
5.2 ppm. MS (ESI): m/z (%) = 531 (23), 532 (100), 533 (38) [M +
H]+.
5d: Yellow solid, yield 555 mg (92%), Rf = 0.53 (chloroform/ethyl
1
acetate, 10:1), m.p. 211 °C. H NMR (500 MHz, CDCl3): δ = 1.0
(s, 9 H), 6.15 (s, 1 H), 6.9 (d, J = 7.6 Hz, 2 H), 7.02 (m, 6 H), 7.1
(t, J = 6.7 Hz, 1 H), 7.17 (t, J = 7.0 Hz, 1 H), 7.26 (d, J = 9.0 Hz,
1 H), 7.29 (m, 6 H), 7.41 (t, J = 7.0 Hz, 1 H), 7.53 (d, J = 8.0 Hz,
1 H), 7.72 (d, J = 7.5 Hz, 1 H), 7.82 (s, 1 H), 7.92 (d, J = 9.0 Hz,
1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 27.6, 31.2, 82.6, 87.6,
105.2, 114.1, 120.7, 121.7, 124.4, 126.4–130.8, 127.7, 128.6, 129.2,
131.8, 135.9, 138.6, 139.2, 155.1, 162.6 ppm. 11B NMR (160 MHz,
CDCl3): δ = 2.0 ppm. MS (ESI): m/z (%) = 533 (24), 534 (100), 535
(39) [M + H]+.
8aaf: Yield 234 mg (41%), Rf = 0.65 (chloroform/ethyl acetate,
1
10:1), m.p. 176 °C. H NMR (500 MHz, CDCl3): δ = 4.14 (d, J =
15.7 Hz, 1 H), 4.29 (dd, J = 15.7, J = 4.0 Hz, 1 H), 4.96 (dd, J =
11.3, J = 4.0 Hz, 1 H), 5.21 (d, J = 11.4 Hz, 1 H), 6.69 (d, J =
6.9 Hz, 2 H), 6.87 (d, J = 8.9 Hz, 1 H), 7.06–7.14 (m, 6 H), 7.16–
General Procedure for the Preparation of Boronate–Amine Com-
plexes 8: Imines 7aa–7ca (1.0 mmol) and sodium cyanoborohydride
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Eur. J. Org. Chem. 2010, 3721–3731

Chiral Dopants for Nematic Liquid Crystals
7.24 (m, 7 H), 7.28 (t, J = 7.0 Hz, 1 H), 7.44 (d, J = 7.5 Hz, 2 H), 1.18 (s, 9 H), 3.41 (d, J = 13.6 Hz, 1 H), 4.12 (d, J = 13.6 Hz, 1
7.51 (d, J = 7.4 Hz, 2 H), 7.57 (d, J = 9.0 Hz, 1 H), 7.69 (d, J = H), 4.14 (s, 1 H), 5.12 (d, J = 11.6 Hz, 1 H), 6.53 (d, J = 2 Hz, 1
7.9 Hz, 1 H), 7.9 (d, J = 8.1 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 42.2, 72.6, 85.5, 103.7, 119.8, 121.8, 122.8, 126.2–
129.2, 128.3, 128.8, 130.1, 131.3, 131.6, 132.9, 133.5, 143.6, 147.6,
153.3 ppm. 11B NMR (160 MHz, CDCl3): δ = 4.5 ppm. MS (ESI):
m/z (%) = 556 (26), 557 (100), 558 (38) [M + H]+.
H), 6.65 (d, J = 7.3 Hz, 2 H), 7.05 (m, 4 H), 7.12 (m, 5 H), 7.14
(d, J = 2 Hz, 1 H), 7.22 (m, 2 H), 7.25 (d, J = 8.1 Hz, 2 H), 7.51
(d, J = 7.7 Hz, 2 H), 7.7 (d, J = 8.0 Hz, 2 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 29.4, 31.6, 34.1, 34.9, 45.4, 70.9, 85.0,
112.2, 121.7, 124.0, 126.4–129.3, 127.7, 133.8, 133.9, 138.2, 140.2,
144.2, 148.2, 151.3 ppm. 11B NMR (160 MHz, CDCl3): δ =
4.9 ppm. MS (ESI): m/z (%) = 627 (23), 628 (100), 629 (53) [M +
H]+.
8aag: Yield 543 mg (90%), Rf = 0.75 (chloroform/ethyl acetate,
1
10:1), m.p. 228 °C. H NMR (500 MHz, CDCl3): δ = 0.2 (s, 9 H),
4.09 (d, J = 15.3 Hz, 1 H), 4.29 (dd, J = 15.2, J = 4.0 Hz, 1 H),
4.48 (dd, J = 11.2, J = 4.0 Hz, 1 H), 5.17 (d, J = 11.3 Hz, 1 H),
6.66 (br. s, 2 H), 6.88 (d, J = 9.0 Hz, 1 H), 7.04 (m, 4 H), 7.09 (m,
4 H), 7.15 (d, J = 7.8 Hz, 1 H), 7.19 (t, J = 7.3 Hz, 1 H), 7.26 (m,
4 H), 7.47 (d, J = 7.6 Hz, 2 H), 7.52 (d, J = 7.4 Hz, 2 H), 7.54 (d,
J = 9.0 Hz, 1 H), 7.67 (d, J = 8.0 Hz, 1 H), 7.79 (d, J = 7.6 Hz, 2
H) ppm. 13C NMR (125 MHz, CDCl3): δ = –1.1, 42.1, 72.3, 85.3,
103.7, 119.7, 122.1, 122.5, 126.2–129.0, 128.1, 128.8, 129.9, 131.6,
131.8, 133.0, 134.2, 139.8, 144.0, 148.2, 153.9 ppm. 11B NMR
(160 MHz, CDCl3): δ = 5.0 ppm. MS (ESI): m/z (%) = 603 (25),
604 (100), 605 (46) [M + H]+.
8abi: 321 mg (45%), Rf = 0.67 (chloroform/ethyl acetate, 10:1), m.p.
1
187 °C. H NMR (500 MHz, CDCl3): δ = 1.07 (s, 9 H), 1.19 (s, 9
H), 1.36 (t, J = 7.0 Hz, 3 H), 3.40 (d, J = 14.0 Hz, 1 H), 3.99 (q, J
= 7.0 Hz, 2 H), 4.20 (d, J = 14.0 Hz, 1 H), 4.22 (m, 1 H), 5.13 (d,
J = 12.0 Hz, 1 H), 6.54 (d, J = 2.0 Hz, 1 H), 6.66 (d, J = 7.3 Hz,
2 H), 6.88 (d, J = 8.8 Hz, 2 H), 7.06 (m, 2 H), 7.09 (d, J = 7.5 Hz,
2 H), 7.12 (d, J = 2.0 Hz, 1 H), 7.14 (m, 2 H), 7.19 (t, J = 7.5 Hz,
1 H), 7.30 (m, 2 H), 7.40 (t, J = 7.7 Hz, 1 H), 7.47 (d, J = 8.8 Hz,
2 H), 7.50 (d, J = 8.1 Hz, 2 H), 7.54 (d, J = 7.4 Hz, 2 H), 7.71 (d,
J = 8.2 Hz, 1 H), 7.85 (d, J = 8.1 Hz, 2 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 14.9, 29.4, 31.6, 34.1, 34.9, 45.4, 63.5, 70.8,
85.0, 112.2, 114.7, 121.7, 123.9, 126.1, 126.4–132.4, 128.0, 132.9,
134.0, 138.1, 139.9, 140.1, 144.4, 148.5, 151.5, 158.3 ppm. 11B
NMR (160 MHz, CDCl3): δ = 5.4 ppm. MS (ESI): m/z (%) = 713
(20), 714 (100), 715 (51) [M + H]+.
8aah: Yield 70 mg (13%), Rf = 0.64 (chloroform/ethyl acetate,
1
10:1). H NMR (500 MHz, CDCl3): δ = 4.1 (d, J = 15.4 Hz, 1 H),
4.3 (dd, J = 15.4, J = 4.2 Hz, 1 H), 4.45 (dd, J = 11.6, J = 4.2 Hz,
1 H), 5.17 (d, J = 11.6 Hz, 1 H), 6.6 (br. s, 2 H), 6.9 (d, J = 9.0 Hz,
1 H), 7.1 (m, 4 H), 7.19 (m, 1 H), 7.2 (m, 4 H), 7.25 (m, 4 H), 7.26
(d, J = 4.5 Hz, 1 H), 7.33 (d, J = 4.7 Hz, 1 H), 7.53 (d, J = 7.8 Hz,
2 H), 7.56 (d, J = 9.0 Hz, 1 H), 7.61 (d, J = 2.1 Hz, 1 H), 7.68 (d,
J = 7.8 Hz, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 42.0,
71.9, 85.3, 103.8, 119.8, 122.2, 122.6, 125.3, 126.4–129.3, 128.2,
128.8, 129.1, 129.9, 131.1, 131.6, 133.6, 143.6, 147.5, 153.4 ppm.
11B NMR (160 MHz, CDCl3): δ = 4.4 ppm. MS (ESI): m/z (%) =
537 (24), 538 (100), 539 (43) [M + H]+.
8abj: Yield 369 mg (54%), Rf = 0.75 (chloroform/ethyl acetate,
10:1), m.p. 196 °C. 1H NMR (500 MHz, CDCl3): δ = 1.18 (s, 9 H),
1.29 (s, 9 H), 3.50 (d, J = 13.7 Hz, 1 H), 4.33 (dd, J = 13.7, J =
3.0 Hz, 1 H), 4.70 (d, J = 11.4 Hz, 1 H), 5.53 (q, J = 8.0 Hz, 1 H),
6.57 (d, J = 7.4 Hz, 2 H), 6.59 (d, J = 2.2 Hz, 1 H), 6.82 (d, J =
8.3 Hz, 2 H), 6.94 (t, J = 7.3 Hz, 2 H), 7.05 (t, J = 7.3 Hz, 2 H),
7.08 (d, J = 7.3 Hz, 3 H), 7.19 (d, J = 7.4 Hz, 3 H), 7.24 (d, J =
2.2 Hz, 1 H), 7.3 (t, J = 7.4 Hz, 1 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 29.5, 31.6, 34.2, 34.9, 46.4, 71.0, 85.4, 115.9, 122.4,
124.7, 126.8–129.4, 132.2, 139.1, 141.0, 141.9, 145.5, 151.1 ppm.
11B NMR (160 MHz, CDCl3): δ = 5.1 ppm. 19F NMR (470 MHz,
CDCl3): δ = –140.7 (d, J = 17 Hz, 2 F), –162.4 (t, J = 20 Hz, 1 F),
–167.7 (m, 2 F) ppm. MS (ESI): m/z (%) = 683 (24), 684 (100), 685
(44) [M + H]+.
8aai: Yield 250 mg (38%), Rf = 0.71 (chloroform/ethyl acetate,
1
10:1), m.p. 144 °C. H NMR (500 MHz, CDCl3): δ = 1.33 (t, J =
7.0 Hz, 3 H), 3.96 (q, J = 7.0 Hz, 2 H), 4.09 (d, J = 15.4 Hz, 1 H),
4.29 (dd, J = 15.4, J = 4.2 Hz, 1 H), 4.48 (dd, J = 11.2, J = 4.0 Hz,
1 H), 5.18 (d, J = 11.3 Hz, 1 H), 6.66 (br. s, 2 H), 6.86 (d, J =
8.7 Hz, 2 H), 6.88 (d, J = 9.1 Hz, 1 H), 7.0–7.3 (m, 12 H), 7.15 (d,
J = 7.7 Hz, 1 H), 7.18 (t, J = 7.8 Hz, 1 H), 7.44 (d, J = 8.7 Hz, 2
H), 7.47 (d, J = 7.9 Hz, 2 H), 7.52 (d, J = 7.7 Hz, 2 H), 7.54 (d, J
= 9.1 Hz, 1 H), 7.66 (d, J = 8.0 Hz, 1 H), 7.84 (d, J = 7.9 Hz, 2 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 14.9, 42.1, 63.4, 72.2, 85.3,
103.7, 114.7, 119.8, 122.0, 122.5, 126.2, 126.3–129.0, 127.9, 128.1,
128.8, 129.9, 131.8, 132.7, 133.9, 134.2, 140.4, 144.0, 148.1, 153.9,
158.3 ppm. 11B NMR (160 MHz, CDCl3): δ = 6.0 ppm. MS (ESI):
m/z (%) = 651 (25), 652 (100), 653 (50) [M + H]+.
8acj: Yield 304 mg (50%), Rf = 0.75 (chloroform/ethyl acetate,
1
10:1), m.p. 167 °C. H NMR (500 MHz, CDCl3): δ = 3.66 (d, J =
15.4 Hz, 1 H), 4.26 (dd, J = 15.4, J = 4.6 Hz, 1 H), 5.02 (d, J =
10.7 Hz, 1 H), 5.37 (m, 1 H), 6.28 (d, J = 8.0 Hz, 1 H), 6.63 (dd,
J = 9.0, J = 2.8 Hz, 1 H), 6.71 (d, J = 7.3 Hz, 2 H), 6.97 (d, J =
7.3 Hz, 2 H), 7.12 (t, J = 7.2 Hz, 1 H), 7.19 (d, J = 7.8 Hz, 2 H),
7.27 (t, J = 7.3 Hz, 1 H), 7.40 (t, J = 7.7 Hz, 2 H), 7.46 (d, J =
8.2 Hz, 2 H), 7.51 (t, J = 7.4 Hz, 1 H), 7.72 (d, J = 7.6 Hz, 2 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 45.5, 72.6, 86.2, 105.1,
108.1, 117.5, 126.2–132.4, 132.6, 139.2, 141.7, 146.0, 153.9 ppm.
11B NMR (160 MHz, CDCl3): δ = 4.8 ppm. 19F NMR (470 MHz,
CDCl3): δ = –124.5 (t, J = 8.4 Hz, 1 F), –130.7 (d, J = 10.7 Hz, 1
F), –138.0 (d, J = 19 Hz, 2 F), –158.2 (t, J = 20 Hz, 1 F), –164.9
(m, 2 F) ppm. MS (ESI): m/z (%) = 607 (23), 608 (100), 609 (35)
[M + H]+.
8aaj: Yield 343 mg (55%), Rf = 0.79 (chloroform/ethyl acetate,
1
10:1), m.p. 114 °C. H NMR (500 MHz, CDCl3): δ = 4.14 (d, J =
16.0 Hz, 1 H), 4.44 (dd, J = 15.9, J = 5.3 Hz, 1 H), 5.26 (d, J =
11.7 Hz, 1 H), 5.45 (m, 1 H), 6.68 (d, J = 7.4 Hz, 2 H), 6.98 (d, J
= 7.3 Hz, 2 H), 7.04 (d, J = 9.0 Hz, 1 H), 7.05 (d, J = 7.8 Hz, 2
H), 7.09 (d, J = 7.3 Hz, 1 H), 7.12 (d, J = 8.2 Hz, 1 H), 7.15 (m, 3
H), 7.21 (d, J = 7.8 Hz, 3 H), 7.25 (m, 2 H), 7.51 (d, J = 8.4 Hz, 2
H), 7.58 (d, J = 9.0 Hz, 1 H), 7.66 (d, J = 7.8 Hz, 1 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 42.8, 72.1, 86.2, 104.7, 119.8, 122.2,
123.1, 126.5–129.5, 128.5, 128.8, 130.0, 131.1, 132.6, 142.0, 146.5,
151.8 ppm. 11B NMR (160 MHz, CDCl3): δ = 4.3 ppm. 19F NMR
(470 MHz, CDCl3): δ = –135.7 (d, J = 23 Hz, 2 F), –155.5 (t, J =
21 Hz, 1 F), –162.1 (m, 2 F) ppm. MS (ESI): m/z (%) = 621 (27),
622 (100), 623 (38) [M + H]+.
8bbj: Yield 484 mg (64%), Rf = 0.78 (chloroform/ethyl acetate,
10:1), m.p. 153 °C. 1H NMR (500 MHz, CDCl3): δ = 1.19 (s, 9 H),
1.28 (s, 9 H), 3.54 (d, J = 13.6 Hz, 1 H), 4.35 (dd, J = 13.6, J =
3.2 Hz, 1 H), 4.67 (d, J = 10.6 Hz, 1 H), 5.54 (q, J = 7.4 Hz, 1 H),
6.34 (d, J = 8.9 Hz, 2 H), 6.52 (t, J = 8.6 Hz, 1 H), 6.55 (t, J =
8.4 Hz, 1 H), 6.64 (d, J = 2.3 Hz, 1 H), 6.69 (d, J = 7.3 Hz, 2 H),
6.67 (d, J = 8.7 Hz, 2 H), 7.26 (d, J = 2.4 Hz, 1 H), 7.31 (t, J =
8aba: Yield 380 mg (63%), Rf = 0.76 (chloroform/ethyl acetate,
10:1), m.p. 187 °C. 1H NMR (500 MHz, CDCl3): δ = 1.05 (s, 9 H), 7.3 Hz, 2 H), 7.39 (t, J = 7.4 Hz, 1 H) ppm. 13C NMR (125 MHz,
Eur. J. Org. Chem. 2010, 3721–3731
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3729

S. Schlecht, W. Frank, M. Braun
FULL PAPER
CDCl3): δ = 29.5, 31.5, 34.3, 34.9, 47.1, 71.3, 84.4, 103.1, 109.6,
111.1, 115.7, 122.4, 125.2, 128.4, 129.6, 130.3, 131.4, 139.3, 142.5,
145.0, 149.1, 150.8 ppm. 11B NMR (160 MHz, CDCl3): δ =
5.1 ppm. 19F NMR (470 MHz, CDCl3): δ = –109.4 (t, J = 8.0 Hz,
2 F), –109.5 (t, J = 8.0 Hz, 2 F), –135.5 (d, J = 24 Hz, 2 F), –156.8
(t, J = 21 Hz, 1 F), –162.9 (m, 2 F) ppm. MS (ESI): m/z (%) = 755
(23), 756 (100), 757 (43) [M + H]+.
cators given in Table 3 [8aaa·CH3OH: S = 0.78, (∆/σ)max = 0.000;
8acj: S = 1.05, (∆/σ)max = 0.000]. Anisotropic displacement param-
eters were refined for all non-hydrogen atoms. All atom coordinates
and isotropic displacement parameters were refined for the hydro-
gen atoms bonded to nitrogen atoms. The length of the O–H bond
of the methanol solvent molecule was restrained to 0.82 Å within
a standard deviation of 0.02. Idealized bond lengths and angles
were used for the CH3, CH2 and CH fragments; the riding model
was applied for their H atoms. Isotropic displacement parameters
of the H atoms were kept equal to 120% of the equivalent isotropic
displacement parameters of the parent secondary, tertiary, or “aro-
matic” carbon atoms. The refined Flack parameter [–0.11(10)]
clearly indicated the choice of the correct enantiomorph in the case
8caa: Yield 153 mg (37%), Rf = 0.4 (chloroform/ethyl acetate, 10:1),
m.p. 224 °C. 1H NMR (500 MHz, CDCl3): δ = 4.02 (d, J = 15.0 Hz,
1 H), 4.06 (m, 1 H), 4.1 (m, 2 H), 4.2 (d, J = 15.0 Hz, 1 H), 4.59
(dd, J = 10.0, J = 3.0 Hz, 1 H), 7.08 (d, J = 8.3 Hz, 2 H), 7.15 (d,
J = 6.6 Hz, 2 H), 7.21 (d, J = 8.9 Hz, 1 H), 7.26 (t, J = 7.0 Hz, 1
H), 7.28 (m, 4 H), 7.36 (t, J = 7.0 Hz, 1 H), 7.4 (d, J = 8.2 Hz, 2
H), 7.73 (d, J = 8.9 Hz, 1 H), 7.74 (d, J = 7.8 Hz, 1 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 41.1, 63.6, 68.8, 106.1, 119.8, 121.8,
122.9, 126.9, 127.8, 127.9–129.7, 128.5, 129.0, 130.4, 131.9, 133.6,
134.1, 154.2 ppm. 11B NMR (160 MHz, CDCl3): δ = 5.9 ppm. MS
(ESI): m/z (%) = 413 (24), 414 (100), 415 (39) [M + H]+.
of
the
chlorine-containing
compound.
CCDC-753266
(8aaa·CH3OH), and -753267 (8acj) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Crystal Structure Determinations of Compounds 8aaa·CH3OH and
8acj: Crystals suitable for X-ray study were selected by means of a Acknowledgments
polarisation microscope and investigated with a STOE IPDS by
using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å).
Unit-cell parameters were determined by least-squares refinements
on the positions of 8000 and 8000 reflections in the range 5.20° Ͻ
θ Ͻ18.35° and 5.20° Ͻ θ Ͻ 21.20°, respectively. As nonracemic
substances were used for the preparation of the crystals, with re-
spect to the monoclinic symmetries and the systematic extinctions
space group type No. 4 was uniquely determined in both cases. Lp
corrections were applied to the intensity data. The structures were
solved by direct methods,[23] and the positions of all hydrogen
atoms were found in subsequent ∆F maps. Refinements by full-
matrix least-squares calculations on F2[24] converged to the indi-
This work was supported by the Deutsche Forschungsgemeinschaft
(Br604/15-1,2). We thank Sabine Houben and Dr. Michael Sigloch
for the preparation of several compounds, and we are grateful to
Eleonore Hammes for technical assistance during X-ray data col-
lection. Special thanks go to Prof. Dr. Hans-Georg Kuball (Univer-
sity of Kaiserslautern) for clarifying and very valuable advice and
Dr. Jochen Brill (BASF AG) for helpful discussions.
[1] a) C. J. Booth in Handbook of Liquid Crystals (Eds.: D. Demus,
J. Goodby, G. W. Gray, H.-W. Spiess, V. Vill), Wiley-VCH,
Weinheim, 1998, vol. 2A, chapter IV, 1; b) H. Coles in Hand-
book of Liquid Crystals (Eds.: D. Demus, J. Goodby, G. W.
Gray, H.-W. Spiess, V. Vill), Wiley-VCH, Weinheim, 1998, vol.
2A, chapter IV, 2; c) S. M. Kelly in Handbook of Liquid Crys-
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Vill), Wiley-VCH, Weinheim, 1998, vol. 2B, chapter VI, 1; d)
F. Vögtle, Supramolekulare Chemie, Teubner, Stuttgart, 1992,
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[2] For reviews, see: a) G. Solladié, R. G. Zimmermann, Angew.
Chem. 1984, 96, 335; Angew. Chem. Int. Ed. Engl. 1984, 23,
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tals (Series on Partially Ordered Systems), Springer, Heidelberg
2001; c) S. Pieraccini, A. Ferrarini, G. P. Spada, Chirality 2008,
20, 749.
Table 3. Summary of crystal data, details of intensity measurements
and structure refinements of 8aaa·CH3OH, and 8acj.
8aaa·CH3OH
8acj
Empirical formula
Formula mass
Crystal system
Space group; no.
a [Å]
b [Å]
c [Å]
C38H33BClNO3
597.91
monoclinic
P21; 4
10.3282(10)
12.2347(8)
12.7875(12)
92.521(12)
1614.3(2)
2
C33H21BF7NO2
607.32
monoclinic
P21; 4
8.1899(5)
18.7018(9)
9.5680(6)
107.030(7)
1401.23(15)
2
β [°]
[3] G. Friedel, Ann. Phys. 1922, 18, 273.
V [Å3]
[4] a) A. D. Buckingham, G. P. Ceasar, M. B. Dunn, Chem. Phys.
Lett. 1969, 3, 540; b) H. Baessler, M. M. Labes, J. Chem. Phys.
1970, 52, 631; c) H. Stegemeyer, K. J. Mainusch, Chem. Phys.
Lett. 1970, 6, 5.
Z
F(000)
628
1.230
0.156
620
1.439
0.121
Dcalcd. [Mgm–3]
µ (Mo-Kα) [mm–1]
Crystal size [mm]
T [K]
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0.30ϫ0.30ϫ0.30 0.30ϫ0.18ϫ0.18
291(2) 291(2)
1.97 Ͻ θ Ͻ 25.00 2.18 Ͻ θ Ͻ 25.91
–12 Յ h Յ 12
–14 Յ k Յ 14
–15 Յ l Յ 15
21114
θ range [°]
hkl ranges
–10 Յ h Յ 10
–22 Յ k Յ 22
–11 Յ l Յ 11
19937
No. of reflns. measd.
No. of unique reflns.
No. of reflns. obsd.
[IϾ2σ(I)]
5667
5378
2653
4259
No. of param./restraints
402/2
401/1
R1 [IϾ2σ(I)][a]
0.0535
0.1131
0.28/–0.18
0.0345
0.0897
0.18/–0.18
wR2 (all data)[a]
Max./min. ∆ρ [eÅ–3]
[a] As defined in SHELXL97-2.
3730
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 3721–3731

Chiral Dopants for Nematic Liquid Crystals
K. Kürschner, M. Braun, A. Hahn, R. Fleischer, Eur. Pat.
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pylidene-1,1,4,4-tetra(2-naphthyl)]--threitol] in 5-CB (also
termed K15). Both the HTP value described as well as the sign
of the helix were reproduced; cf. ref.[6a]
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[19]
[20]
M. Braun, Angew. Chem. 1996, 108, 565; Angew. Chem. Int.
Ed. Engl. 1996, 35, 519.
a) W. J. A. Goossens, Mol. Cryst. Liq. Cryst. 1971, 12, 237; b)
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[21] Cf. D. J. Aschaffenburg, R. S. Moog, J. Phys. Chem. B 2009,
113, 12736 and references cited therein.
[22] In addition, the IR spectra also point to hydrogen bonding
between the dopant and the cyano group of the nematic com-
pound: A slight but significant shift of the CN stretching fre-
quency from 2224.7 (pure 5-CB) to 2227.7 cm–1 (mixture of 5-
CB and 8abi; molar ratio: 1:1) was observed. Similar blue shifts
in benzonitrile (approximately 5 cm–1) have been attributed to
the formation of hydrogen bonds with 2-propanol; see ref.[21]
[23] G. M. Sheldrick, SHELXS97, Program for the Solution of
Crystal Structures, University of Göttingen, Germany, 1990.
[24] G. M. Sheldrick, SHELXL97, Program for the Refinemant of
Crystal Structures, University of Göttingen, Germany, 1997.
Received: March 25, 2010
Published Online: May 21, 2010
[16] ZLI-1840 was purchased from Merck; 5-CB is available from
TCI.
Eur. J. Org. Chem. 2010, 3721–3731
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3731