Abstract:
The invention relates to a solution of lithium aluminum hydride in 2-methyltetrahydrofuran or a solvent mixture containing 2-methyltetrahydrofuran, a method for producing said solution and use of the same.

Description:
This application is a §371 of PCT/EP2006/065919filed Sep. 1, 2006, which claims priority from German Patent Application No. DE 10 2005 041 773.6 filed Sep. 1, 2005 and German Patent Application No. DE 10 2006 028 021.0 filed Jun. 14, 2006. 
     FIELD OF THE INVENTION 
     The present invention provides a solution of lithium aluminium hydride, a process for the preparation of this solution and the use thereof. 
     BACKGROUND OF THE INVENTION 
     Lithium aluminium hydride (LiAlH 4 ) is a strong reducing agent which dissolves well in polar solvents such as diethyl ether (Et 2 O) or tetrahydrofuran (THF). In the solid, powdered state it is difficult to handle on account of its high reactivity towards air and water, as well as the risk of dust explosions. On an industrial scale, therefore, LiAlH 4  solutions in organic solvents are preferably used. LiAlH 4  can be prepared by reacting an aluminium halide, in particular aluminium chloride (AlCl 3 ), with lithium hydride (LiH) in an ethereal solvent. In the synthesis developed by Schlesinger et al. (J. Amer. Chem. Soc. 69, 1199 (1947)), Et 2 O is employed as the solvent: 
     
       
                 
         
             
             
         
      
     
     LiAlH 4  is very readily soluble in Et 2 O with a proportion of 28%, but this solvent requires complex safety measures owing to its low flash point and boiling point. It is therefore avoided as far as possible in industrial applications. 
     Commercially supplied LiAlH 4  solutions generally contain no Et 2 O but are supplied as a 10% solution in THF or a 15% solution in THF/toluene (Chemetall brochure “Industrial Use”). THF has a significantly higher boiling point than Et 2 O. Thus, the boiling point of THF is 66° C. but that of Et 2 O is only 35° C. 
     There are various methods of preparing solutions of LiAlH 4  in THF or solvent mixtures containing THF. One possibility consists in the reaction of sodium aluminium hydride with lithium chloride (LiCl) (WO-A-97/06097): 
     
       
                 
         
             
             
         
      
     
     Disadvantages of this process are the relatively long reaction times and poor yields. 
     On an industrial scale, THF solutions of LiAlH 4  are prepared by dissolving solid, solvent-free LiAlH 4  in THF or THF/co-solvent. In this case, the solid LiAlH 4  is first synthesised as a solution in diethyl ether, which is subsequently evaporated completely at elevated temperatures and under reduced pressure. However, this process also has serious disadvantages. Firstly, it is relatively expensive owing to its complexity. 
     Furthermore, the thermal loading of the LiAlH 4  during evaporation leads to partial decomposition as in diagram 3: 
     
       
                 
         
             
             
         
      
     
     Li 3 AlH 6  is insoluble and therefore cannot be used for reduction processes. The solutions prepared by dissolving solid LiAlH 4  are markedly cloudy owing to the content of elemental aluminium and Li 3 AlH 6 ; the solids content must be removed, e.g. by filtration or decantation. This is because residual contents of elemental aluminium not only impair the handling properties but also cause a clear deterioration in the storage stability, since they catalytically accelerate the decomposition process according to diagram 3. 
     In order to achieve a solution of LiAlH 4  in THF, the synthesis according to diagram 1 can also, in principle, be carried out directly in THF. However, this has the disadvantage that, on the one hand, the solubility of the AlCl 3  employed is only relatively low. Thus, at room temperature (RT), the solubility of AlCl 3  in THF is only 16%, whereas in Et 2 O it is 57%. On the other hand, the LiCl formed as a by-product is readily soluble in THF. Thus, the solubility of LiCl in THF is 4.8%, but in Et 2 O it is &lt;0.001%. In this way, only dilute solutions of LiAlH 4  in THF could be prepared, and these are at the same time contaminated with LiCl. 
     OBJECTS OF THE INVENTION 
     The object of the present invention is to overcome the disadvantages of the prior art. 
     The object is achieved by the features of the invention. Preferred embodiments are described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows the thermal stability of LiAlH 4  solutions containing 2-MeTHF. 
     
    
    
     DETAILED DESCRIPTION 
     In particular, the present invention is intended to provide a LiAlH 4  solution which is as concentrated as possible, safe to handle and obtainable inexpensively. 
     According to the invention, these objects are achieved by a solution of LiAlH 4  in 2-methyltetrahydrofuran (2-MeTHF) or a solvent mixture containing 2-methyltetrahydrofuran. Preferably according to the invention, the preparation of LiAlH 4  is also carried out in 2-MeTHF or a solvent mixture containing 2-MeTHF. 
     Surprisingly, it has been found that 2-MeTHF dissolves AlCl 3  very well but LiCl hardly at all: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 RT solubility (wt. %) of 
                 AlCl 3   
                 LiCl 
                 LiAlH 4   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 In diethyl ether 
                 57 
                 &lt;0.001 
                 28 
               
               
                   
                 In THF 
                 16 
                 4.8 
                 21 
               
               
                   
                 In 2-MeTHF 
                 45 
                 0.3 
                 17 
               
               
                   
                   
               
             
          
         
       
     
     Thus, the necessary conditions exist for preparing concentrated LiAlH 4  solutions: 
     
       
                 
         
             
             
         
      
     
     The process according to the invention for the preparation of the LiAlH 4  solution according to the invention is generally carried out as described below, without limiting the invention thereto: 
     Powdered LiH is suspended in 2-MeTHF or a hydrocarbon or a 2-MeTHF/hydrocarbon mixture, and a solution of AlCl 3  in 2-MeTHF or a 2-MeTHF/hydrocarbon mixture is added to this suspension. The particle size of the LiH is preferably &lt;30 μm. The reaction is strongly exothermic and the heat of reaction is dissipated by external cooling. 
     The reaction temperature is only limited in the lower range by the solidification temperature and in the upper range by the boiling point of the solvent or solvent mixture employed. In practice, the reaction is carried out at temperatures of between 0° C. and 110° C. At higher temperatures, product decomposition can be expected to start. The preferred reaction temperature is between 20 and 90° C. LiH is generally used in excess, preferably with a 5 to 20% excess. 
     After complete conversion, the LiCl formed as a by-product is separated off. This operation can take place according to the prior art by a solid/liquid separation step, e.g. by filtration, centrifugation or decantation. Work-up by filtration is preferred. The filtrates are generally clear or only a little cloudy, colourless or slightly yellow. In particular, they are not contaminated by elemental aluminium. Should the concentration of LiAlH 4  be too low, they can be concentrated by evaporation, preferably in vacuo. 
     Furthermore, it has surprisingly been found that the solubility of LiCl in the LiAlH 4  solution according to the invention exceeds the solubility in dilute LiAlH 4  solutions known from the pure binary system. This can be seen from the following table: 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                 Molar ratio 
                   
                   
               
               
                 LiAlH 4  conc. 
                 2-MeTHF: 
                 LiCl solubility 
               
             
          
           
               
                 (wt. %) 
                 LiAlH 4   
                 (wt. %) 
                 (mole %) 
                 Notes 
               
               
                   
               
             
          
           
               
                 0 
                 n.a. 
                 0.3 
                 n.a. 
                 only 2-MeTHF 
               
               
                 5 
                 8.3 
                 1.2 
                 22 
                 only 2-MeTHF 
               
               
                 8 
                 5.1 
                 1.6 
                 16 
                 only 2-MeTHF 
               
               
                 12 
                 3.0 
                 0.9 
                 7 
                 only 2-MeTHF 
               
               
                 11.4 
                 2.2 
                 0.14 
                 1.1 
                 2-MeTHF/ 
               
               
                   
                   
                   
                   
                 toluene 
               
               
                 14 
                 1.6 
                 0.08 
                 0.6 
                 2-MeTHF/ 
               
               
                   
                   
                   
                   
                 toluene 
               
               
                   
               
               
                 n.a. = not applicable 
               
             
          
         
       
     
     It is clear that, as the 2-MeTHF:LiAlH 4  ratio falls, the LiCl solubility decreases markedly. To achieve the lowest possible concentration of LiCl, a 2-MeTHF:LiAlH 4  ratio of 3.0 should preferably not be exceeded in the synthesis mixture, and particularly preferably one of 2.2. This can be achieved either by using the highest possible concentrations of AlCl 3  solutions in pure 2-MeTHF or by employing solvent mixtures. Surprisingly, it has been found that aluminium chloride also dissolves well in mixtures of 2-MeTHF and a hydrocarbon. Thus, for example, the solubility of AlCl 3  in a mixture of 76% 2-MeTHF and 24% toluene is about 45%. In this solution, there is a 2-MeTHF:AlCl 3  molar ratio of 1.5. 
     Instead of, or in a mixture with, toluene, other hydrocarbons can also be used, preferably aromatic hydrocarbons such as ethylbenzene, xylenes, cumene, or aliphatics such as cyclohexane, hexane, methylcyclohexane, heptane, individually or as a mixture of at least two of these solvents. 
     If it is desired to prepare a LiAlH 4  solution with the lowest possible LiCl content, the synthesis is preferably carried out in a mixture of 2-MeTHF and a hydrocarbon, particularly preferably a 2-MeTHF/toluene mixture. In a mixture of this type, the 2-MeTHF:AlCl 3  molar ratio should not fall below a value of 1.3. 
     In a particularly preferred embodiment of the process of the invention, LiH is suspended in a hydrocarbon, e.g. toluene, and a solution of AlCl 3  in a 2-MeTHF/hydrocarbon mixture, preferably in a 2-MeTHF/toluene mixture, is added. In this solution, the 2-MeTHF:AlCl 3  ratio is between 1.3 and 3.0, with a 2-MeTHF:AlCl 3  ratio of 1.5 to 2.0 being preferred. 
     In comparison with the water-miscible THF, which has a marked tendency to form peroxide, 2-MeTHF offers substantial advantages when used in organic synthesis. On the one hand, owing to its higher boiling point, it allows higher temperatures to be set without the need to increase the external pressure. Thus, the boiling point of 2-MeTHF is 78° C. compared with only 66° C. for THF. This is an advantage for the hydrogenation of low-reactivity functional groups such as e.g. organic chlorides. 
     Surprisingly, it has also been found that LiAlH 4  solutions containing 2-MeTHF exhibit particular thermal stability: in contrast to solutions in THF, they decompose endothermically. Even at relatively high temperatures, therefore, no so-called “runaway” scenario can be expected. This makes it possible to work safely even at high temperatures. 
     The thermal stability of LiAlH 4  solutions containing 2-MeTHF is illustrated in  FIG. 1 . This compares the results of Differential Scanning Calometry tests in a Radex apparatus on a 12% solution of LiAlH 4  in 2-MeTHF/toluene with that of a 15% solution of LiAlH 4  in THF/toluene. In each case, 2 g were weighed out and the rate of heating was 45 K/h.
     Curve 1: 12% solution of LiAlH 4  in 2-MeTHF/toluene: ΔT;   Curve 2: 12% solution of LiAlH 4  in 2-MeTHF/toluene: pressure;   Curve 3: 15% solution of LiAlH 4  in THF/toluene: ΔT;   Curve 4: 15% solution of LiAlH 4  in THF/toluene: pressure.   

     The user draws further advantages from the fact that 2-MeTHF and water are miscible only to a limited extent. Thus, the solubility of 2-MeTHF in water is 15.1% but the solubility of water in 2-MeTHF is only 5.3%. It is therefore possible to isolate an organic synthesis product together with 2-MeTHF by phase separation. Only relatively minor product and solvent losses can be expected through the aqueous phase, from which the solvent cannot generally be recovered. For this reason, 2-MeTHF is simple to recycle. 
     The invention provides in detail:
         a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the LiAlH 4  content is at least 11 wt. %;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the LiAlH 4  content is at least 14 wt. %;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the LiAlH 4  concentration is at least 5 wt. % and the molar ratio of 2-MeTHF:LiAlH 4  does not exceed the value of 3.0;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the LiAlH 4  concentration is at least 5 wt. % and the molar ratio of 2-MeTHF:LiAlH 4  does not exceed the value of 2.2;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the LiAlH 4  concentration is at least 10 wt. % and the molar ratio of 2-MeTHF:LiAlH 4  does not exceed the value of 2.2;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the solvent contains at least one hydrocarbon;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the hydrocarbon is an aromatic hydrocarbon;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the aromatic hydrocarbon is selected from toluene, ethylbenzene, xylene or cumene or a mixture of at least two of these hydrocarbons;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the aromatic hydrocarbon is toluene;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the hydrocarbon is an aliphatic hydrocarbon;   a solution of LiAlH 4  in 2-MeTHF or a solvent mixture which contains 2-MeTHF, wherein the aliphatic hydrocarbon is selected from cyclohexane, hexane, methylcyclohexane or heptane or a mixture of at least two of these hydrocarbons;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein AlCl 3  is reacted with LiH in 2-MeTHF or a solvent containing 2-MeTHF;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the LiH concentration in the feed is between 3 and 17 wt. %, preferably between 5 and 15 wt. %;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the LiH powder employed has a particle size of &lt;30 μm;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the reaction temperature is between 0 and 100° C., preferably between 20 and 90° C., particularly preferably between 30 and 80° C.;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the reaction is carried out in the presence of hydrocarbons;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the hydrocarbons are selected from toluene, ethylbenzene, xylene, cumene, cyclohexane, hexane, methylcyclohexane, heptane or a mixture of at least two of these hydrocarbons;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the hydrocarbon is toluene;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the molar ratio of 2-MeTHF:AlCl 3  is between 1 and 10, preferably between 1.3 and 5.0;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein the LiCl forming as a by-product is separated off;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein, to remove LiCl, the solution is concentrated to a molar ratio of 2-MeTHF:LiAlH 4  of no more than 3.5 and the precipitated LiCl is separated off by a solid/liquid separation step, preferably by filtration;   a process for the preparation of the LiAlH 4  solution according to the invention, wherein, to remove LiCl, the reaction solution is concentrated in vacuo, preferably under a pressure of between 1 and 500 mbar, and at a temperature of between 20 and 100° C.;   the use of the LiAlH 4  solution according to the invention as a reducing agent.       

     In an embodiment of the invention, the molar ratio of 2-MeTHF:AlCl 3  is between 1 and 10, preferably between 1.3 and 5.0. 
     The invention is explained below by examples, without being limited thereto: 
     EXAMPLE 1 
     Preparation of an 8% LiAlH 4  Solution in Pure 2-MeTHF 
     In an inerted, i.e. dried and filled with protective gas, e.g. nitrogen or argon, 1-liter reactor having a double-walled jacket, 41.7 g LiH powder are suspended in 40 g 2-MeTHF. The suspension is heated to 35° C. and a solution of 158.7 g AlCl 3  in 370 g 2-MeTHF is added dropwise within 3 hours, while stirring well. After a 1.5-hour post-reaction period at 35 to 40° C., the reaction suspension is emptied on to a G3 glass filter. 
     407 g of a slightly yellow, clear solution are obtained. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Analysis (mmol/g) 
               
             
          
           
               
                   
                 Li 
                 Al 
                 H −   
                 Cl −   
               
               
                   
                   
               
               
                   
                 2.50 
                 2.13 
                 9.33 
                 0.37 
               
               
                   
                   
               
             
          
         
       
     
     From this, the following is calculated:
     LiAlH 4  =8.1 wt. % (based on Al analysis)   LiCl =1.6 wt. % ({circumflex over (=)}17 mole %, based on Al)   Yield: 73% of theory.   

     The filter residue is washed with 2-MeTHF; a further 16% of theory is obtained in the form of the wash filtrates. 
     EXAMPLE 2 
     Preparation of a low LiCl Content LiAlH 4  Solution in 2-MeTHF by an Evaporation Process 
     117 g of the LiCl-containing wash filtrates from example 1 are concentrated by distillation at 40 to 60° C. and under a reduced pressure of 250 mbar. A total of 80.0 g solvent is distilled off. After removing approximately ⅔ of the quantity mentioned, colourless crystals begin to precipitate. 
     On completion of the distillation process, the suspension is cooled to room temperature and filtered through a G3 fritted glass filter until clear. 
     Final weight: 36.0 g 
     The analysis gave the following results: 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
                 Li 
                 Al 
                 H −   
                 Cl −   
                 LiAlH 4   
                 LiCl 
               
             
          
           
               
                 Substance sample 
                 (mmol/g) 
                 (wt. %) 
                 (mole %) 
               
               
                   
               
             
          
           
               
                 Before distillation 
                 0.77 
                 0.45 
                 1.87 
                 0.18 
                 1.7 
                 40 
               
               
                 After distillation 
                 2.11 
                 1.75 
                 7.03 
                 0.28 
                 6.6 
                 16 
               
               
                   
               
             
          
         
       
     
     By means of a further evaporation step to a LiAlH 4  concentration of 12.3%, the content of soluble LiCl is reduced to 7 mole %. 
     EXAMPLE 3 
     Preparation of an 11% LiAlH 4  Solution in 2-MeTHF/toluene 
     In an inerted 1-liter reactor, 59.9 g LiH powder are suspended in 140 g toluene and heated to 75° C. Into this suspension, 504 g of a 44% solution of AlCl 3  in a mixture of 2-MeTHF and toluene and in a 2MeTHF:AlCl 3  ratio of 1.5 are metered within 80 minutes. On completion of the addition, stirring is continued for a further 30 minutes at 80° C. and the suspension is then emptied on to a preheated filter. 
     253 g of a yellow solution with the following composition are obtained: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Analysis (mmol/g) 
               
             
          
           
               
                   
                 Li 
                 Al 
                 H −   
                 Cl −   
               
               
                   
                   
               
               
                   
                 3.02 
                 3.00 
                 12.4 
                 0.034 
               
               
                   
                   
               
             
          
         
       
     
     The LiAlH 4  concentration is 11.4% ({circumflex over (=)}98% of theory) and the LiCl concentration is 1.1 mole %, based on LiAlH 4 . 
     The filter residue is washed twice with toluene. In all the filtrates, 17.4 g LiAlH 4  content is obtained ({circumflex over (=)}91% of theory).