Optically active esters of 5-ethyl- and 5-vinyl-1,3-dioxolane-4-carboxylic acid, their use as doping substances in liquid crystal mixtures and the liquid crystal mixtures containing the new esters

Optically active 1,3-dioxolane derivatives of the general formula (I) ##STR1## are suitable as doping substances in liquid crystal mixtures. The symbols in the general formula have the following meaning: PA1 R.sup.1 has the structure which is analogous to the moiety of the general formula which is on the right of X or is an alkyl or alkylene radical which can be substituted, R.sup.2, R.sup.3 are H or alkyl, which can also be substituted [R.sup.2, R.sup.3 can also be a cycloaliphatic compound together with C(2) of the dioxolane ring], PA1 R.sup.4 is vinyl or ethyl, with the proviso that at least one radical R.sup.4 is vinyl or ethyl, if R.sup.1 is analogous to the moiety of the formula (I) which is on the right of X, --A.sup.1, --A.sup.2, --A.sup.3 are an aromatic, heterocyclic or aliphatic ring system, PA1 --M.sup.1, --M.sup.2 are --CO--O, --O--CO, --CH.sub.2 CH.sub.2, --CH.dbd.CH, --CH.sub.2 O, --OCH.sub.2, --C.dbd.C and PA1 X is O, S or O--CO--O. The particular advantage of the compound of the general formula (I) is that it can induce a high twisting capacity in a cholesteric phase.

The German Patent Application P 3,713,273.3 relates to optically active 
1,3-dioxolane-4-carboxylic esters. They lead, even in small amounts, to 
short switching times in tilted smectic phases and to high electroclinic 
coefficients in orthogonally smectic phases. In particular, the pitch of 
the helix induced by the doping is so large that compensation by means of 
other doping substances is not necessary. These are compounds of the 
general formula 
##STR2## 
in which the symbols have the following meaning: 
R.sup.1 is 
##STR3## 
a straight-chain or branched alkyl radical having 1 to 16 carbon atoms or 
a straight-chain or branched alkenyl radical having 2 to 16 carbon atoms, 
it being possible for these radicals themselves to contain asymmetric 
carbon atoms, in which one or more non-adjacent --CH.sub.2 -- groups can 
be replaced by 
##STR4## 
and one or more H can be replaced by F, Cl, Br or CN, 
R.sup.2, R.sup.2', R.sup.3 and R.sup.3' are each H or an alkyl radical 
having 1 to 10 carbon atoms, in which one or more H can be replaced by F, 
or R.sup.2 and R.sup.3 or R.sup.2' and R.sup.3' together with the C(2) 
atom of the dioxolane ring form a cyclopentane, cyclohexane or 
cycloheptane ring, 
R.sup.4, R.sup.4' are H or an alkyl radical having 1 to 10 or an alkenyl 
radical having 2 to 10 carbon atoms, 
j and l are zero, 1 or 2, 
k and m are zero or 1, 
n is zero, 1 or 2, with the following proviso: if j and/or l are zero, k is 
zero; if n is zero, m is zero; the sum j+l+n is at least 1 and at most 3, 
--A.sup.1 and --A.sup.2 are 
##STR5## 
and X is O or S. 
In a preferred embodiment, the symbols in the general formula (I) have the 
following meaning: 
R.sup.1 is a straight-chain or branched alkyl or alkenyl radical having 4 
to 14 carbon atoms, which can contain an asymmetric carbon atom, or in 
which a --CH.sub.2 -- group can be replaced by 
##STR6## 
or in which one or more H can be replaced by F, 
R.sup.2, R.sup.3, R.sub.4, H or an alkyl radical having 1 to 5 carbon atoms 
or R.sup.2, R.sup.3 together with the C(2) atom of the dioxolane ring form 
a cyclopentane or cyclohexane ring, 
j and l are zero or 1, 
k, m, n are zero or 1, 
--M.sup.1, --M.sup.2 are 
##STR7## 
X is O or S. 
In a further preferred embodiment, 1,3-dioxolane-4-carboxylic esters of the 
general formula (IV) are used 
##STR8## 
in which the symbols have the following meanings: 
R.sup.5 is a straight-chain or branched alkyl or alkenyl radical having 6 
to 12 carbon atoms, which can contain an asymmetric carbon atom, 
##STR9## 
The values for spontaneous polarization (Ps) of the compounds described in 
the German patent application are in the range of about 8-14 nC/cm.sup.2 
at 25.degree. C. and 10 mol % of doping of the tilted smectic liquid 
crystal phase and in the range of about 80-140 nC/cm.sup.2 upon linear 
extrapolation to the pure compound. 
Surprisingly, it has now been found that the values for Ps in tilted 
smectic liquid crystal phases in the presence of the same host substances 
are 50 to 100% higher compared to doping by means of the compounds 
mentioned, if the doping substances used are 1,3-dioxolane-4-carboxylic 
esters of the general formula (V) 
##STR10## 
in which R.sup.1, R.sup.2, R.sup.3, R.sup.5, A.sup.1, A.sup.2, A.sup.3, 
A.sup.4, M.sup.1, M.sup.2, M.sup.3, X, j, k, l, m and n have the same 
meaning as in formula (I) and (IV) and in which R.sup.4 is an ethyl or 
vinyl radical, in which in formula (V) one or both radicals R.sup.4, if 
R.sup.1 is a dioxolane-4-carboxyl radical, is an ethyl or vinyl radical 
and the other can be a radical R.sup.4 having the meaning given in formula 
(I). R.sup.2 and R.sup.3 in formula (V) and (VI) are both preferably 
CH.sub.3 or together with the C(2) atom of the dioxolane ring form a 
cyclohexane ring. 
The new compounds of the formula (V), in particular (VI), preferably 
include the compounds mentioned by name in the examples. The compounds of 
the formula (V) or (VI) are prepared by reacting mesogenic phenols or 
thiophenols of the formula (II) 
EQU R.sup.1 --(--A.sup.1).sub.j (--M.sup.1).sub.k (--A.sup.2).sub.l 
(--M.sup.2).sub.m (--A.sup.3).sub.n XH (II) 
or of the formula (IIa) 
EQU R.sup.5 (--M.sup.3).sub.k)--A.sup.4 --XH (IIa) 
with derivatives of 5-vinyl-1,3-dioxolane-4-carboxylic acid or 
5-ethyl-1,3-dioxolane-4-carboxylic acid (III) 
##STR11## 
in which R.sup.4 is an ethyl or vinyl radical in the presence of 
condensating agents (cf. e.g. Mar., Advanced Organic Chemistry, 2nd 
Edition, McGraw-Hill, p. 363-365), and isolating the reaction product and 
purifying it, for example by recrystallization or chromatographic 
separation processes. 
The phenols to be used as well as processes for preparing 
5-vinyl-1,3-dioxolane-4-carboxylic acid and their derivatives (cf. 
Synthesis 1987 (9), 801-806) are known from the literature. The 
corresponding 5-ethyl compounds are obtained by known methods, for example 
by hydrogenation in the presence of a Pd/carbon catalyst. 
Liquid crystal mixtures according to the invention form liquid crystal 
phases and contain at least one optically active compound of the general 
formula (V) or (VI). 
The term "liquid crystal phase" is understood to mean nematic, cholesteric, 
orthogonally smectic or tilted smectic, in particular S.sub.A, S.sub.B and 
S.sub.C, phases. The liquid crystal mixtures consist of 2 to 20, 
preferably 2 to 15, components, among them at least one of the chiral 
compounds claimed according to the invention. 
The other components are preferably selected from the known compounds which 
have nematic, cholesteric and/or smectic, for example S.sub.A, phases, 
and/or tilted smectic phases; these include, for example, Schiff bases, 
biphenyls, terphenyls, phenylcyclohexanes, pyrimidines, cinnamic esters, 
cholesterol esters, various bridged polynuclear esters of p-alkylbenzoic 
acids which have polar end groups. In general, the commercially available 
liquid crystal mixtures are already present before the addition of the 
optically active compound(s) as mixtures of a wide range of components, of 
which at least one is mesogenic, that is to say, as a compound which in 
the form of a derivative or in a mixture with certain co-components has a 
liquid crystal phase which can be expected to form at least one 
enantiotropic (clearing temperature&gt;melting temperature) or monotropic 
(clearing temperature&lt;melting temperature) mesophase. In particular, the 
liquid crystal mixture contains, in addition to at least one of the 
optically active compounds claimed according to the invention, an ester 
compound having an S.sub.c phase, for example a phenyl alkoxybenzoate, or 
a biaromatic compound having a nitrogen-containing heterocycle, for 
example an alkylpyrimidinylalkoxybenzene. 
Of the compound(s) according to the invention, liquid crystal mixtures in 
general contain 0.05 to 70% by weight, in particular 0.1 to 50% by weight. 
The compounds according to the invention are in particular suitable as 
doping substances for tilted smectic liquid crystal phases, since they 
convert these phases into ferroelectric liquid crystal phases; the values 
for spontaneous polarization (Ps) at 25.degree. C. are in the range of 
about 29-38 nC/cm.sup.2 for 10 mol % of doping and in the range of about 
290-380 nC/cm.sup.2 upon linear extrapolation to the pure compound. The 
switching times of the new systems are in most cases considerably below 50 
.mu.s for 10 mol % of doping, at 25.degree. C. and a switching voltage of 
.+-.10 V/.mu.m. The compounds according to the invention can also be used 
for achieving the electroclinic effect in orthogonal smectic phases 
(S.sub.A, S.sub.B, S.sub.E).