Liquid crystalline 2,6-disubstituted-trans-1,3-dioxadecalins
BACKGROUND OF THE INVENTION 
     The invention relates to liquid crystalline 2,6-disubstituted-trans-1,3-dioxadecalins, a method for the production of the compounds and their utilization in liquid crystal mixtures. 
     Liquid crystalline nematic substances are used as dielectrics in opto-electronic displays of various constructions /H. Kelker, R. Hatz: Handbook of Liquid Crystals, publishing firm Chemie 1980/. 
     In recent years, many new application purposes have been proposed, whereby displays of very specific properties are required. No pure substances are known which could even only approximately meet the technical requirements of the art. For this reason there are always used mixtures of liquid crystalline substances, which have the advantage that by suitable selection of the components of the mixture, it is possible to modify the properties within a great range and thus to adjust them to specific application purposes. 
     For mixtures intended for the guest-host effect, there are particularly advantageous mixtures of low optical double refraction (Δn). Mixtures for the Schadt-Helfrich luminosity impart to the displays advantageous properties with respect to the information&#39;s readability at shallow angles, if the mixtures have a low optical double refraction. Because the double refraction of a mixture is approximately the additive product of the values of the pure components, in order to attain mixtures of low double refraction, substances of low double refraction values are a necessary prerequisite. 
     The object of the invention is to find substances which impart a low optical double refraction to liquid crystalline nematic mixtures which are utilized in opto-electronic components. 
     SUMMARY OF THE INVENTION 
     It has been found that liquid crystalline 2,6-disubstituted trans-1,3-dioxadecalins of the general formula ##STR2## wherein X=H and Y=R 1   
     or 
     X=R 2  and Y=R 1 , R 3   
     wherein ##STR3## wherein n=1 to 10, are well suited as components of low double refraction for mixtures which are utilized in opto-electronic components. 
     According to the invention, liquid crystalline 2,6-disubstituted trans-1,3-dioxadecalins are produced by reaction of alkanols, substituted benzaldehydes, substituted cyclohexanecarbaldehydes or the acetals thereof with trans-2-hydroxymethylcyclohexanol or 4(e)-alkyl-1(e)hydroxy-2(e)-hydroxymethylcyclohexanes, which are obtained by reaction of metal enolates of the 4-alkylcyclohexanones with gaseous formaldehyde, equilibration of the 4-alkyl-2-hydroxymethyl-cyclohexanones and subsequent reduction with complex hydrides, in organic solvents in the presence of an acid catalyst, for example H 2  SO 4 , HCl, H 3  PO 4 , p-toluenesulfonic acid, AlCl 3 , BF 3 .Et 2  O according to the general scheme 1: ##STR4## 
     The up to now not known 4(e)-alkyl-1(e)-hydroxy-2-(e)-hydroxymethylcyclohexanes (II) are obtained in pure form by reduction of a mixture of the stereoisomer 4-alkyl-2-hydroxymethylcyclohexanones (III) with complex hydrides, preferably LiAlH 4 , and subsequent fractional crystallization from organic solvents, for example benzene, petroleum ether or n-hexane. ##STR5## 
     The 4-alkyl-2-hydroxymethylcyclohexanones (III) are synthesized according to scheme 2: ##STR6## 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The production of the substances according to the invention will be explained by an example. 
     Production of the 4-alkyl-1-trimethylsilyloxycyclohex-1-enes (IV) (variant B) 
     In a 500 ml three-necked flask, under exclusion of moisture, to a solution of 65 g (0.43 mol) NaI, 44 g (0.43 mol) triethylamine and 0.35 mol 4-alkyl-cyclohexanone in 350 ml absolute acetonitrile is dropwise added under ice-cooling and stirring 48 g (0.43 mol) trimethyl silane chloride (TMSCl) at room temperature. Subsequently follows stirring for 20 minutes at 25° C., the solvent is distilled off in vacuum, the solid residue is mixed with 200 ml petroleum ether and is filtered off from the insoluble residue. The latter is washed five times with 100 ml petroleum ether each. The petroleum ether solutions are reduced in vacuum and the silylenol ether (IV) is fractionally distilled in vacuum. 
     The following table shows the yields of the various silylenol ethers (IV). 
     
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 ##STR7##                     (IV)                                        
n      Yield (%)         B.P. °C./Torr                             
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4      91                72/0.08                                          
5      91                82/0.06                                          
6      96                92 . . . 93/0.06                                 
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     For the production of the 4-alkyl-2-hydroxymethylcyclohexanones (III), to a solution of 0.1 mol 4-alkyl-1-trimethylsilyloxycyclohex-1-ene (IV) and 10 mg α,α&#39;-dipyridyl in 50 ml absolute ether, under stirring and ice-cooling in an argon atmosphere is dropwise added 110 ml of a 1.0 molar solution of n-butyllithium in n-pentane at 20° to 25° C. within 1/4 hour. After stirring for 1 to 2 hours at this temperature the reaction mixture is cooled down to 0° C. and at this temperature is dropwise added 110 ml of a 0.5-molar ZnCl 2  solution in anhydrous ether. The reaction mixture is stirred for approximately 20 minutes at 0° to 10° C. Subsequently, under 5 minutes of vigorous stirring and cooling at -10° C. to 0° C., 4 g gaseous formaldehyde is conducted in an argon stream onto the surface of the reaction mixture. The formaldehyde is produced by depolymerization of α-polyoxymethylene in the argon stream at 175° C. For purification, the formaldehyde is conducted through a 6 m long, 1 cm wide glass tube before it reaches the reaction vessel. 
     The reaction mixture is immediately poured into a 500 ml solution containing 20% NH 4  Cl and is shaken until the precipitate is dissolved. 
     The organic phase is separated and the aqueous phase is ethered out twice. The combined organic phases are washed successively with NH 4  Cl solution and water, dried over Na 2  SO 4 , the solvent is distilled off at the rotary evaporator and the residue is twice fractionally distilled in vacuum, whereby the product equilibrates. The table indicates yields of various cyclohexanones (III). 
     
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 ##STR8##                     (III)                                       
n      Yield (%)         B.P. (°C./Torr)                           
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4      60                87-90/0.02                                       
5      43                117/0.06                                         
6      40                125/0.08                                         
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     For the production of the 4(e)-alkyl-1(e)-hydroxy-2(e)-hydroxymethylcyclohexanes (II) are prepared in a 500 ml three-necked flask 2.1 g (0.05 mol) LiAlH 4  in 250 ml absolute ether. 
     Under ice-cooling and vigorous stirring 0.05 mol 4-alkyl-2-hydroxymethylcyclohexanone (III) dissolved in 50 ml absolute ether is slowly dropwise added and subsequently stirred under reflux for 2 hours. Subsequently, the solution is carefully hydrolyzed with 25 ml H 2  O, the resulting precipitate is dissolved by adding 150 ml of 10% H 2  SO 4 . 
     The organic phase is separated, the aqueous phase is ethered out three times and the combined organic phases are washed with a solution containing 2% NaHCO 3  and H 2  O. 
     After drying with Na 2  SO 4  the solvent is distilled off in vacuum and the residue is dissolved in 50 ml n-hexane. The solution is cooled down to 0° C. to 20° C., the crystalline precipitate is suctioned off and purified by repeated recrystallizations from n-hexane. 
     The table indicates the yields of various cyclohexanes (II). 
     
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 ##STR9##                     (II)                                        
                                 Solidification                           
n      Yield (%) B.P. (°C./Torr)                                   
                                 Point                                    
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5      20        117/0.05        62-64 (°C.)                       
6      25        146/0.25        65-67                                    
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     For the production of the 2,6-disubstituted trans-1,3-dioxadecalins (I) 5 mmol alkanol or substituted benzaldehyde and 5 mmol 4(e)-alkyl-1(e)-hydroxy-2(e)-hydroxymethylcyclohexane (II) or trans-2-hydroxymethylcyclohexanol in 150 ml benzene or CHCl 3 , CH 2  Cl 2 , CCl 4  or toluene in the presence of 50 ml p-toluenesulfonic acid are heated in the water separator until the reaction is completed. After cooling of the reaction mixture follows washing with saturated NaHCO 3  solution and H 2  O and drying over Na 2  SO 4 . The solvent is distilled off in the rotary evaporator and the residue is recrystallized from methanol, ethanol or n-hexane until melting point and clarification point constancy is attained. The yields of (I) amount 70 to 95% of the theoretical. 
     In the tables 1-4 are indicated the transformation temperatures for the produced substances. 
     In the tables 
     K=crystalline solid 
     S A  =smectic A 
     N=nematic 
     I=isotropic liquid. 
     
                       TABLE 1                                                     
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 ##STR10##                                                                
Compound   R          K        N      I                                   
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1/1        CH.sub.3   . 137    (. 69) .                                   
1/2        C.sub.2 H.sub.5                                                
                      . 122    (. 80) .                                   
1/3        C.sub.3 H.sub.7                                                
                      . 111    (. 90) .                                   
1/4        C.sub.4 H.sub.9                                                
                      . 103    (. 91.5)                                   
                                      .                                   
1/5        C.sub.5 H.sub.11                                               
                      . 95     (. 84) .                                   
1/6        C.sub.6 H.sub.13                                               
                      . 112.5  (. 66) .                                   
1/7        OCH.sub.3  . 151.5  (. 118.5)                                  
                                      .                                   
1/8        OC.sub.2 H.sub.5                                               
                      . 112    .sup. . 135                                
                                      .                                   
1/9        OC.sub.3 H.sub.7                                               
                      . 113    (. 112.5)                                  
                                      .                                   
1/10       OC.sub.4 H.sub.9                                               
                      . 94     .sup. . 115                                
                                      .                                   
1/11       OC.sub.5 H.sub.11                                              
                      . 96     .sup. . 99                                 
                                      .                                   
1/12       OC.sub.6 H.sub.13                                              
                      . 98     .sup. . 99.5                               
                                      .                                   
1/13       OC.sub.7 H.sub.15                                              
                      . 96     (. 92) .                                   
1/14       OC.sub. 8 H.sub.17                                             
                      . 68     .sup. . 97.2                               
                                      .                                   
1/15       OC.sub.9 H.sub.19                                              
                      . 77     .sup. . 89.3                               
                                      .                                   
1/16       OC.sub.10 H.sub.21                                             
                      . 82     .sup. . 89                                 
                                      .                                   
1/17       CN         . 193    -- --  .                                   
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                       TABLE 2                                                     
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 ##STR11##                                                                
Compound R               K       N     I                                  
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2/1      C.sub.5 H.sub.11                                                 
                         . 87.5  (. 70)                                   
                                       .                                  
2/2      C.sub.6 H.sub.13                                                 
                         . 77.5  (. 56.8)                                 
                                       .                                  
2/3                                                                       
          ##STR12##      . 97    .sup. . 214                              
                                       .                                  
2/4      OC.sub.5 H.sub.11                                                
                         . 93    .sup. . 94.5                             
                                       .                                  
2/5      OC.sub.6 H.sub.13                                                
                         . 82    .sup. . 98                               
                                       .                                  
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                       TABLE 3                                                     
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 ##STR13##                                                                
Com-                                                                      
pound R.sup.1  R.sup.2      K    S.sub.A                                  
                                       N     I                            
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3/1   C.sub.5 H.sub.11                                                    
               OC.sub.3 H.sub.7                                           
                            . 77.5                                        
                                 -- -- (. 63.5)                           
                                             .                            
3/2   C.sub.5 H.sub.11                                                    
               C.sub.4 H.sub.9                                            
                            . 77 -- -- (. 74.0)                           
                                             .                            
3/3   C.sub.5 H.sub.11                                                    
               OC.sub.5 H.sub.11                                          
                            . 61 -- -- .sup. . 70                         
                                             .                            
3/4   C.sub.5 H.sub.11                                                    
               C.sub.4 H.sub.9                                            
                            . 61 (. 31)                                   
                                       -- -- .                            
3/5   C.sub.5 H.sub.11                                                    
               OOCCH.sub.3  . 90 -- -- (. 55.5)                           
                                             .                            
3/6   C.sub.5 H.sub.11                                                    
               OOCC.sub.2 H.sub.5                                         
                            . 86 -- -- (. 78)                             
                                             .                            
3/7   C.sub.5 H.sub.11                                                    
                OOCC.sub.5 H.sub.11                                       
                            . 79 -- -- .sup. . 79.2                       
                                             .                            
3/8   C.sub.5 H.sub.11                                                    
               CN           . 87 -- -- (. 85)                             
                                             .                            
3/9   C.sub.6 H.sub.13                                                    
               C.sub.4 H.sub.9                                            
                            . 53.5                                        
                                 (. 36)                                   
                                       -- -- .                            
3/10  C.sub.6 H.sub.13                                                    
               CN           . 87 -- -- (. 80)                             
                                             .                            
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                       TABLE 4                                                     
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 ##STR14##                                                                
Compound R.sup.1  R.sup.2  K     S.sub.A                                  
                                       N     I                            
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4/1      C.sub.5 H.sub.11                                                 
                  C.sub.6 H.sub.13                                        
                           . 58  . 83  . 94  .                            
4/2      C.sub.6 H.sub.13                                                 
                  C.sub.6 H.sub.13                                        
                           . 54  . 87.9                                   
                                       . 88.1                             
                                             .                            
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     The following examples are intended for the demonstration of the desired properties of the substances according to the invention in liquid crystalline mixtures. 
    
    
     EXAMPLE I 
     In some substances according to the invention the optical double refraction Δn for light of the wavelength 589 nm per 10° K. below the clarification point as well as the melting enthalpy ΔH have been measured. 
     
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Substance No.  Δn                                                   
                       ΔH/kJ · mol.sup.-1                  
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4/2            0.045   33.1                                               
3/3            0.056   34.9                                               
4/1             0.0335 40.8                                               
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     The measured double refractions are extremely small, liquid crystals predominantly have values Δn&gt;0.1 /H. Kelker, R. Katz, Handbook of Liquid Crystals, publishing firm Chemie 1980/. 
     EXAMPLE II 
     The influence of the substances according to the invention on electro-optical properties of mixtures can be seen from the following chart. 
     A mixture Mi 14, consisting of 
     
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4-n-propyl-cyclohexane carboxylic acid-4-cyanophenylester                 
                              34.5                                        
                              mol                                         
                              %                                           
4-n-butyl-cyclohexane carboxylic acid-4-cyanophenylester                  
                              31                                          
                              mol                                         
                              %                                           
4-n-pentyl-cyclohexane carboxylic acid-4-cyanophenylester                 
                              34.5                                        
                              mol                                         
                              %                                           
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     changed its properties as follows: 
     
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Composition                                                               
                              U =                                         
Mi 14 Subst.  mol         U.sub.o                                         
                              2U.sub.o                                    
                                   τ.sub.50                           
                                        τ.sub.50                      
                                             η.sup.d                  
mol % No.     %      C.P. V   V    E.sub.ms                               
                                        A.sub.ms                          
                                             μm                        
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100   --      --     72.0 1.3 2.6  580  200  33.0 24.0                    
90    4/1     10     73.0 1.4 2.8  750  350  46.5 21.7                    
90    4/2     10     73.5 1.4 2.8  680  150  46.7 20.0                    
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     wherein 
     C.P.--Clarification point in °C. 
     U o  --Threshold voltage at 20° C., 500 Hz 
     U--Operating voltage 
     τ E  50--Switching on time at 50% change of intensity 20° C. 500 Hz 
     τ A  50--Decay period at 50% change of intensity 20° C., 500 Hz 
     d--Thickness of layer 
     η--Viscosity at 20° C. in cP 
     EXAMPLE III 
     To a mixture of dioxane derivatives of the following composition ##STR15## 
     20 mol-% each substance has been added. 
     Thereby resulted the following changes of the clarification point: 
     
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added          Clarification point                                        
substance (No.)                                                           
               (°C.)                                               
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4/2            50                                                         
4/1            58                                                         
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