Process for preparing 4,4&#39;-dimethyl-1,1&#39;-binaphthyl
The present invention relates to a process for preparing 4,4&#39;-dimethyl-1,1&#39;-binaphthyl by electrolytic dimerization of 1-methylnaphthalene. 
     4,4&#39;-dimethyl-1,1&#39;-binaphthyl is an intermediate for polyamides, polyesters, optical brighteners and dyes. 
     The preparation of 4,4&#39;-dimethyl-1,1&#39;-binaphthyl by a chemical route can be carried out in principle by reductive coupling of 1-bromo-4-methylnaphthalene by the method of Kumada-Tamao (Org. Synth., Vol. 58, 127 (1978)). For this purpose, 1-bromo-4-methylnaphthalene is converted into the corresponding Grignard compound and subsequently coupled with 1-bromo-4-methylnaphthalene using a nickel-containing catalyst, for example Ni(PPh 3 ) 2  Cl 2 . The process gives as production waste a mixture of magnesium bromide and nickel salts which have to be separated and worked up. 
     An oxidative dehydrodimerization by a chemical route is described by McKillop et al. [J. Am. Chem. Soc. 102 (21) 6504-12]. 
     Although the reaction proceeds very selectively, it has the disadvantage of the use of Tl(COOCF 3 ) 3  as reactant in equimolar (based on product) amount. Likewise disadvantageous is the use of trifluoroacetic acid (yield 85%) as solvent or tetrachloromethane (3 l/mol) boron trifluoride etherate (1 l/mol) (yield 93%). 
     A further oxidative dimerization process is carried out using oxygen over platinum/carbon or palladium/carbon in trifluoroacetic acid. 
     An electrochemical method is described by Fichter and Herszbein. (Helvetica Chimica Acta 11, 1265.) 
     The electrolyte used in this publication is a mixture of acetone and sulfuric acid, the anode is the reaction vessel made of preoxidized lead, the cathode is a tin stirrer. The electrolysis solution contains about 10% by weight of 1-methylnaphthalene, with a yield of 10.5% by weight of 4,4&#39;-dimethyl-1,1&#39;-binaphthyl being achieved. Owing to the apparatus required and its unsatisfactory efficiency, this method is not suitable for industrial applications. 
     There was therefore a great need for a process which, starting out from readily available starting materials, makes 4,4&#39;-dimethyl-1,1&#39;-binaphthyl available in high yield and purity. 
     This object is achieved by a process for preparing 4,4&#39;-dimethyl-1,1&#39;-binaphthyl, which comprises electrochemically oxidatively dimerizing 1-methylnaphthalene in the presence of acetonitrile/water/conducting salt mixtures which additionally contain at least one further component which is immiscible or only partially miscible with water. 
     The electrochemical process proceeds according to the following reaction equation: ##STR1## 
     The reaction proceeds highly selectively when carried out according to the invention, the yield is about 50% of theory. 
     The electrolyte systems used for the process of the invention can be formed in various ways. Fundamentally, however, acetonitrile, water, a conducting salt and at least one further component which is immiscible or only partially miscible with water always participate in the makeup of such electrolyte systems. 
     The water-immiscible component can be (a) an aliphatic or cycloaliphatic hydrocarbon such as, for example, pentane, hexane, cyclohexane, methylcyclohexane, heptane, octane, isooctane, decane, dodecane or decalin or mixtures (distillation fractions) of such compounds, or (b) an aromatic hydrocarbon such as, for example, benzene, toluene, o-, m- or p-xylene, mesitylene, naphthalene or tetralin, or (c) a halogenated hydrocarbon such as, for example, methylene chloride or chlorobenzene, or (d) a ketone having from about 5 to 10 carbon atoms such as, for example, diethyl ketone, methyl t-butyl ketone or acetophenone. 
     This component should have a boiling point above the respective reaction temperature. 
     Use is preferably made of aliphatic, cycloaliphatic or aromatic hydrocarbons having from 6 to 10 carbon atoms; particular preference is given to heptane and toluene. The water-immiscible component of the electrolyte system can also be a mixture of two or more of the above-mentioned compounds. 
     Suitable conducting salts for the process of the invention are the alkali metal, alkaline earth metal, ammonium, monoalkylammonium, dialkylammonium, trialkylammonium or tetraalkylammonium salts of acids whose complex anions are derived from hexavalent sulfur, from pentavalent phosphorus or from trivalent boron. Mixtures of a plurality of the specified conducting salts can likewise be used. 
     Use can be made, for example, of the salts of the following anions: hydrogensulfate, methylsulfate, ethylsulfate, methanesulfonate, ethanesulfonate, propanesulfohate, butanesulfonate, octanesulfonate, benzenesulfonate, toluenesulfonate, 2-chlorobenzenesulfonate, p-chlorobenzenesulfonate, 2,4-dichlorobenzenesulfonate, naphthalene-1-sulfonate, naphthalene-2-sulfonate; methanephosphonate, ethanephosphonate, propanephosphonate, butanephosphonate, hexafluorophosphate; tetrafluoroborate; further suitable salts are those of alkanesulfonic acids or alkanephosphonic acids whose alkyl radical is polyfluorinated or perfluorinated, such as, for example, trifluoromethanesulfonate, 1,1,2,3,3,3-hexafluoropropanesulfonate, nonafluorobutanesulfonate, heptadecafluorooctanesulfonate, trifluoromethanephosphonate, nonafluorobutanephosphonate. 
     It has been found to be useful in many cases to use sodium or tetraalkylammonium salts (containing alkyl radicals having from 1 to 4 carbon atoms) of alkanesulfonic acids (containing alkyl radicals having from 1 to 8 carbon atoms or containing polyfluorinated or perfluorinated alkyl radicals having from 1 to 4 carbon atoms), of arylsulfonic acids (with aryl=phenyl, naphthyl; alkyl-substituted or chloro-substituted phenyl), of tetrafluoroboric acid and hexafluorophosphoric acid, as conducting salts. 
     Preferred conducting salts are sodium or tetraalkylammonium tetrafluoroborate. 
     The mixing ratio of acetonitrile to water in the electrolytes used is in the range from 100:1 to 1:1; it has been found to be favorable to work in the range from 20:1 to 2:1. The amount of the component which is immiscible or only partially miscible with water is, based on the total weight of the electrolyte, in the range from 10 to 90%; preference is given to the range from 30 to 80%. 
     The amount of the conducting salt is, based on the total weight of the electrolyte, in the range from 0.5 to 15%; preference is given to the range from 1 to 7%. 
     The mixing ratios of the components of the electrolyte are preferably set in such a way that there is formed a two-phase system in which water and conducting salt are either present in the acetonitrile phase or exist largely separated from the organic phase as independent phase, with different proportions of the conducting salt being in principle to be found in the two phases. However, a single-phase reaction procedure is also possible. 
     The process is carried out in an undivided electrolysis cell. For relatively large electrolyses, preference is given to using flow-through cells having a stack of electrodes arranged in a bipolar manner. Suitable anode materials are graphite, vitreous carbon, platinum or stainless steel; stainless steel anodes are preferred. The cathode material is not critical. All customary metals such as, for example, steel, stainless steel, nickel, titanium, copper, platinum and also graphite or vitreous carbon can be used; stainless steel is preferred. 
     The process of the invention is carried out at current densities in the range from 10 to 250 mA/cm 2 , preferably in the range from 25 to 150 mA/cm 2  and particularly preferably in the range from 40 to 100 mA/cm 2 . 
     The electrolysis is generally carried out at temperatures between 0° and 80° C.; a preferred temperature range is between 50° and 65° C. 
     The target compound is only sparingly soluble in most electrolyte systems at room temperature and, after the reaction is complete, can be obtained in coarse crystalline form by cooling the reaction mixture and can subsequently be isolated by filtration. 
     Particular advantages of the process of the invention are that expensive intermediates such as 1-bromo-4-methylnaphthalene and problematical wastes containing heavy metals are avoided and the isolation of the product and also the recycling of unreacted raw material, solvents and conducting salts are possible by simple means. 
     The process gives the product in improved current and material yields and achieves industrially utilizable conversions. 
     The process can also be carried out in a flow-through cell. 
    
    
     The following examples illustrate the invention without limiting it thereto. 
     EXAMPLE 
     1. 
     Starting material: 170 g of 1-methylnaphthalene (1.2 mol) 
     Electrolyte: 1500 ml of acetonitrile/1500 ml of n-heptane 
     Additives: 75 ml of methanol/75 ml of water 
     Conducting salt: 15 g of NaBF 4   
     Reaction Temperature: 50° C. 
     Cell: Undivided laboratory flow-through cell U7 
     Anode: Graphite 
     Cathode: Stainless steel 
     Electrode area: 200 cm 2   
     Current density: 50 mA/cm 2   
     Transmitted charge: 64 Ah (200% of theory) 
     Conversion: 59% 
     Yield: 40% 
     2. 
     Starting material: 200 g of 1-methylnaphthalene (1.4 mol) 
     Electrolyte: 1500 ml of acetonitrile/1500 ml of n-heptane 
     Additives: 100 ml of water 
     Conducting salt: 30 g of NaBF 4   
     Reaction Temperature: 55° C. 
     Cell: Undivided laboratory flow-through cell U7 
     Anode: Stainless steel 
     Cathode: Stainless steel 
     Electrode area: 200 cm 2   
     Current density: 50 mA/cm 2   
     Transmitted charge: 63 Ah (166% of theory) 
     Conversion: 75% 
     Yield: 51% 
     3. 
     Starting material: 300 g of 1-methylnaphthalene (2.11 mol) 
     Electrolyte: 2000 ml of acetonitrile/500 ml of toluene (single-phase) 
     Additives: 200 ml of water 
     Conducting salt: 25 g of NaBF 4   
     Reaction Temperature: 60° C. 
     Cell: Undivided laboratory flow-through cell U7 
     Anode: Graphite 
     Cathode: Stainless steel 
     Electrode area: 200 cm 2   
     Current density: 50 mA/cm 2   
     Transmitted charge: 56.5 Ah (100% of theory) 
     Conversion: 34% 
     Yield: 21% 
     4. 
     Starting material: 56.8 g of 1-methylnaphthalene (0.4 mol) 
     Electrolyte: 250 ml of acetonitrile/250 ml of heptane 
     Additives: 12 ml of water/12 ml of methanol 
     Conducting salt: 5 g of NaBF 4   
     Reaction Temperature: 20° C. 
     Cell: Pot cell, 500 ml, undivided 
     Anode: Graphite EH 
     Cathode: Stainless steel mesh 
     Electrode area: 50 cm 2   
     Current density: 50 mA/cm 2   
     Transmitted charge: 10.7 Ah (100% of theory) 
     Conversion: 32% 
     5. 
     Starting material: 28.6 g of 1-methylnaphthalene (0.2 mol) 
     Electrolyte: 250 ml of acetonitrile/250 ml of heptane 
     Additives: 12 ml of water 
     Conducting salt: 5 g of NaBF 4   
     Reaction Temperature: 52° C. 
     Cell: Pot cell, 500 ml, undivided 
     Anode: Stainless steel mesh 
     Cathode: Stainless steel mesh 
     Electrode area: 50 cm 2   
     Current density: 50 mA/cm 2   
     Transmitted charge: 10.7 Ah (200% of theory) 
     Conversion: 83% 
     Workup: 
     After cooling, the crystalline solid is filtered off from the reaction mixture and recrystallized from acetone. 
     Melting point: 148° C. (uncorrected)