Abstract:
The present invention relates to a process for producing 4-hydroxybutyraldehyde, characterized in that allyl alcohol dissolved in polar solvents is reacted with CO and H 2  in the presence of a catalytic system which is formed from a rhodium complex and a cyclobutane ligand which contains at least two trans-coordinated 3 5-dialkylphenylphosphinomethyl groups, with the exclusion of catalysts which contain an aliphatic, araliphatic or cycloaliphatic phosphine as ligand.

Description:
The present invention relates to a process for hydroformylating allyl alcohol in order to produce 4-hydroxybutyraldehyde. 
     BACKGROUND OF THE INVENTION 
     The hydroformylation of allyl alcohol is known and is utilized industrially (see e.g. U.S. Pat. Nos. 4,064,145; 4,215,077; 4,238,419; 4,678,857; 5,290,743). Allyl alcohol is reacted in these processes with CO/H 2  gas mixtures, giving 4-hydroxy-butyraldehyde (HBA). Following distillative removal of undesired by-products, HBA is hydrogenated in a known manner to give 1,4-butanediol (BDO). 
     A disadvantage of this mode of production is the formation of undesired by-products. In particular, as well as the desired linear product, the isomeric branched product 3-hydroxy-2-methylpropionaldehyde (HMPA) and other C 3  by-products such as n-propanol and propionaldehyde are formed. This adversely affects the economic viability of the process. 
     The present invention provides a process for producing 4-hydroxybutyraldehyde, characterized in that ally alcohol dissolved in polar solvents is reacted with CO and H 2  in the presence of a catalytic system which is formed from a rhodium complex and a cyclobutane ligand which contains at least two trans-coordinated , 3,5 -dialkylphenyl-phosphinomethyl groups, with the exclusion of catalysts which contain an aliphatic, araliphatic or cycloaliphatic phosphine as ligand. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Trans-1,2-(3,5-dialkylphenylphosphinomethyl)cyclobutanes which are used as ligands in the present invention have the formula [A] 
                                
in which
     R 1  is alkyl, preferably methyl, ethyl or propyl   R 2  is H or an alkoxy group,   R 3  and R 4 , independently of one another, are H, CH 2 OR 1 , CH 2 O-aralkyl, CH 2 OH, CH 2 —[P(3,5-R 1 ,4-R 2 -phenyl) 2 ] or   CH 2 O—(CH 2 —CH 2 —O) m —H where m is a number from 1 to 1000.   

     Preferably [A] is an all-trans-cyclobutane derivative. 
     An additional aspect of the invention is the use of novel catalysts of the formula [A] which permit different embodiments of the process according to the invention. 
     Usually, the hydroformylation of allyl alcohol with known catalysts is carried out in homogeneous phase. After the reaction, HBA, HMPA and other by-products are separated off from the catalyst by extraction with water. 
     The hydroformylation according to the invention in polar solvents allows lipophilic catalysts of the formula [A] to be separated off by extraction with hydrophobic solvents, and the catalyst to be returned to the first process stage. 
     Novel hydrophilic catalyst systems of the formula [A] with polyether groups can be used in membrane reactors and thus allow the process products to be separated off continuously after the hydroformylation. 
     The process according to the invention differs with respect to hitherto described processes for producing HBA by virtue of the exclusion of catalysts which contain ligands of the formulae PR′ 3 , where R′ is an aliphatic, araliphatic or cycloaliphatic radical. Catalysts of this type hydrogenate a considerable fraction of the allyl alcohol used to give undesired by-products (see U.S. Pat. No. 7,655,821 B1). 
     By contrast, in the process according to the invention, instead of catalysts which contain a cyclobutane ligand which has at least two trans-coordinated 3,5-dialkylphenylphosphino-methyl groups, it is also possible to use diphosphines of the DIOP series, such as e.g.2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis[bis(3,5-dimethylphenylphosphino]butane; (see WO2010/132087). 
     However, preference is given to catalysts of the cyclobutane series, which allow more favorable HBA:HMPA- proportions to be achieved. 
     The polar solvents used in the process according to the invention are not reactive toward the products of the hydroformylation. In contrast to the solvents used in known processes, (see e.g. WO2010/132087), they are soluble in significant proportions, and sometimes also completely, in water. 
     Polar solvents which may be mentioned are, for example, ethanol, propanol, n-butanol, isobutanol, 1,4-butanediol and polyethers of the formula CH 3 O—(—CH 2 —CH 2 —O) n —H with a molecular weight &gt;2000. 
     The hydroformylation takes place under reaction conditions known per se in the temperature range from 20 to 120° C. and in a pressure range from 2-20 bar. The optimum performance is ascertained by appropriate preliminary experiments depending on the predetermined equipment. 
     The molar ratio of CO:H 2  is ca. 1:1, but can vary considerably depending on the embodiment. 
     The reaction time is 0.5-4 hours. At the start of the reaction, the allyl alcohol concentration is 5-50%, preferably 10-25%, based on the solvent or solvent mixture. 
     HBA (and HMPA) are hydrogenated in a manner known per se to give the corresponding dihydroxy compounds, and fractional distillation of the crude product gives the desired 1,4-butanediol in pure form. Ni, Co, Ru, Pt, Pd, Cu, Zn and Cr catalysts can be used for the hydrogenation. Raney® Ni catalysts are particularly preferably used. The hydrogenation is generally carried out at temperatures of 60-200° C. and in the pressure range 15-70 bar. 
     The examples below illustrate the process according to the invention: 
     EXAMPLE 1 
     Preparation of a novel catalyst which is used in the process according to the invention:
     a) Preparation of a novel intermediate from known all-trans-1,2,3,4-(hydroxy-methyl)cyclobutane:   

     88 mg (0.5 mmol) of all-trans-1,2,3,4-(hydroxymethyl)cyclobutane were dissolved in 3 ml of anhydrous pyridine and, at 0° C., 251 mg (0.9 mmol) of trityl chloride were added with intense stirring. The reaction mixture was kept at 0° C. overnight with stirring. Then, it was added to 10 ml of water and extracted with ethyl acetate (3×5 ml), then dried with MgSO 4  and evaporated to dryness in a rotary evaporator. 
     The crude product was separated off by chromatography (silica gel). (Eluent: ethyl acetate:hexane 1:3→2:3→ethyl acetate:methano 195:5). 
     The main product obtained was 114 mg (34% of theory) of all-trans-1,2-(hydroxymethyl)-3,4-(trityloxymethyl)cyclo butane. 
     
       
                 
         
             
             
         
      
         
         b) In a manner known per se, this compound was tosylated and reacted with LiP(3,5-xylyl) 2 . 
       
    
     EXAMPLE 2 
     Hydroformylation corresponding to the process according to the invention: 
     In 4 ml of dried and degassed tert-butyl methyl ether, 16 mmol of all-trans-1,2,3,4-(3,5-xylylphosphinomethyl)cyclobutane are reacted under argon with [Rh(CO) 2 (acac)] (8 mmol). 
     The resulting solution is injected into an autoclave under argon and flushed with a CO:H 2 -1:1 mixture. Via a side arm, a solution of 1 ml of allyl alcohol in 15 ml of ethanol is then added and the reaction is carried out at a pressure of 40 bar and a temperature of 120° C. This gives 97% of theory of HBA+HMPA in a ratio of ca. 14:1.