Abstract:
Prepare α, β-saturated geminal biscarbamates by contacting an α,β-unsaturated ether with a carbamate under reaction conditions, optionally in the presence of a catalyst.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of Ser. No. 682,643, filed Dec. 17, 1984, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the production of geminal dicarbamates. 
     Compounds having carbamate moieties are useful in that they can be converted to the corresponding isocyanates via known methods. For example, U.S. Pat. No. 4,388,246 discloses a method of thermally cleaving dicarbamates to form diisocyanates. 
     It has long been known that aldehydes but not simple ketones condense with carbamates directly to give geminal biscarbamates. See, e.g., J. Org. Chem., V. 10, pp. 145-8 (1941). It is also known to react aldehydes with unsaturated carbamates, e.g., H 2  C═CH--CH 2  --O--CONH 2 , to form geminal di(unsaturated) dicarbamates. U.S. Pat. Nos. 2,384,074; and 2,385,911. These synthetic techniques are of limited usefulness in that they can only be used to synthesize products having a hydrogen on the geminal carbon atom. 
     Joseph Hoch, in Vol. 210, Academie Sci. Compt. Rendu., pp. 560-2 (1935), teaches the preparation of isopropylidene-bis-(ethyl carbamate) in unspecified yield by reacting urethane (ethyl carbamate, H 2  NCO 2  C 2  H 5 ) with 2,2-diethoxypropane at from 105° C.-190° C., optionally in the presence of aniline chlorohydrate. Thus, this technique produced a geminal dicarbamate not having a hydrogen atom on the geminal carbon atom. However, it is disclosed that yield is reduced in the absence of aniline chlorohydrate. 
     It would be desirable to have a new, simple method for the high yield production of a large class of geminal dicarbamates, including compounds which do not have a hydrogen on the geminal carbon atom. Heretofore, such a method has not been disclosed. 
     SUMMARY OF THE INVENTION 
     The present invention is a new synthetic method for the production of α,β-saturated geminal dicarbamates in high yield. The process involves contacting an α,β-unsaturated ether with a carbamate, optionally in the presence of an acid catalyst, under reaction conditions such that an α,β-saturated geminal dicarbamate is formed. One equivalent of a geminal ether is produced as a coproduct. Surprisingly, the process produces α,β-saturated geminal dicarbamates in high yield using an unsaturated ether as one of the starting materials. The geminal dicarbamates are useful in that they can be thermally cracked to form isocyanates. The geminal diethers can be thermally cracked to form alcohols and unsaturated ethers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the present invention requires an α,β-unsaturated ether, and a carbamate. An acid catalyst and a solvent independently are optionally employed. 
     Ethers having α,β-olefinic unsaturation advantageously are employed in the process of the present invention. For the purposes of the present invention, the term &#34;α,β-unsaturated&#34; refers to olefinic unsaturation between the carbon atom bonded to the ether oxygen atom, i.e., the alpha carbon atom, and a carbon atom bonded thereto. Examples of typical α,β-unsaturated ethers include diisopropenyl ether, isopropenyl methyl ether, phenyl isopropenyl ether, α-methoxy styrene, α-ethoxy styrene and the like. Preferred unsaturated ethers are represented by the formula ##STR1## wherein each R, R a  and R b  is independently H, aliphatic or aromatic; and wherein R c  is aliphatic or aromatic. Preferably, R and R a  are independently hydrogen or alkyl, and R b  and R c  are alkyl. Most preferably, R and R a  are H or lower alkyl, and R b  and R c  are lower alkyl. For the purposes of the present invention, the term &#34;lower alkyl&#34; refers to alkyl moieties of from 1 to about 4 carbon atoms. For the purposes of the present invention, the terms &#34;aliphatic&#34; and &#34;aromatic&#34; include moieties having atoms such as halogen atoms, O, S, and N. 
     A carbamate advantageously is employed in the process of the present invention. Examples of typical carbamates include methyl carbamate, ethyl carbamate, propyl carbamate, n-butyl carbamate, sec-butyl carbamate, benzyl carbamate, phenyl carbamate and the like. Preferred carbamates are represented by the formula 
     
         H.sub.2 NCO.sub.2 R.sub.e                                  (III) 
    
     wherein R e  is aliphatic or aromatic. Preferably, R e  is alkyl. Most preferably, R e  is lower alkyl. In the process of the present invention, the carbamate and the unsaturated ether can be employed in any amount sufficient for the reaction to proceed to yield at least one geminal dicarbamate. Typically, from about 0.5 to about 2 moles of carbamate are employed per mole of unsaturated ether. Preferably, from about 0.9 to about 1.05 moles of carbamate are employed per mole of unsaturated ether. Most preferably, a stoichiometric ratio is employed. 
     An acid catalyst optionally can be employed in the process of the present invention, and preferably an acid catalyst is employed. Examples of typical acid catalysts include organic acids such as sulfonic or carboxylic acids, inorganic acids such as mineral acids, Lewis acids such as zinc chloride, aluminum chloride, or boron trifluoride, solid acids such as silica gels, acidic clays, aluminas, zeolites, and polymeric acids such as sulfonated polystyrene resins. Examples of preferred catalysts include sulfonic acids and sulfonated resins. Typically, a catalytic amount of the catalyst is employed. Preferably, from about 0.0001 to about 0.01 equivalents of catalyst are employed per mole of carbamate reactant; most preferably, from about 0.0005 to about 0.002 equivalents are employed. 
     A solvent optionally can be employed in the process of the present invention, and preferably a solvent is employed. The solvent can be any inert material in which the carbamate and unsaturated ether are soluble. Typical solvents include, for example, chlorinated and nonchlorinated hydrocarbons such as methylene chloride, toluene, xylenes, dichlorobenzenes, chlorinated ethanes, and aliphatic, aromatic, or mixed aromatic-aliphatic ethers. Methylene chloride is the preferred solvent. While the solvent can be employed in any amount which solubilizes the reactants, the solvent typically is employed in amounts ranging from about 0.1 to about 10 liters of solvent per kg of reactants. Preferably, from about 0.5 to about 2 liters of solvent are employed per kg of reactants. Mixtures of solvents can be employed. 
     The reaction can be conducted at any combination of temperature and pressure at which the reaction proceeds to form a geminal dicarbamate. The reaction typically is conducted at from about -30° C. to about 200° C.; preferably is conducted at from about -10° C. to about 100° C.; and most preferably is conducted at a temperature of from about 0° C. to about 25° C. Advantageously, the process temperature is below the boiling point of the reaction mixture, although the process can be conducted at reflux temperature. The process pressure can be sub- or superatmospheric. Preferably, and for the sake of convenience, the process is conducted at ambient pressure. 
     When the carbamate and α,β-unsaturated ether are combined under the conditions previously described herein, an α,β-saturated geminal dicarbamate is produced. Preferred α,β-saturated geminal dicarbamates are represented generally by the formula: ##STR2## wherein each R d  is independently aliphatic or aromatic, and wherein R, R a , and R b  are as defined hereinabove. Preferably, each R d  is independently alkyl. Most preferably, each R d  is independently lower alkyl. The most preferred geminal dicarbamate is 2,2-N,N-bis(methyl carbamato)propane. The process of the present invention is advantageous in that it can produce a wide variety of α,β-saturated geminal dicarbamates in unexpectedly high yields. Preferably, the yield of α,β-saturated geminal dicarbamate is at least about 90 percent. For the purposes of the present invention, the term &#34;yield&#34; refers to the numerical product of conversion of the carbamate and selectivity to the geminal dicarbamate. The α,β-saturated geminal dicarbamates can be recovered using known techniques such as crystallization or distillation. 
     The process of the present invention produces a geminal diether in addition to the geminal dicarbamate. Preferred geminal diethers are represented generally by the formula: ##STR3## wherein R, R a , R b  , and R c  are as defined hereinabove. Preferably, the geminal carbamate and geminal diether are produced in approximately equimolar amounts. 
     SPECIFIC EMBODIMENTS 
     The following examples are intended to illustrate the invention and should not be construed as limiting its scope. 
     EXAMPLE 1 
     Synthesis of 2,2-N,N-bis(ethyl carbamato)propane 
     A solution of ethyl carbamate (8.91 g, 100 mmoles) in 40 ml of methylene chloride is added to 15 mg of p-toluene sulfonic acid in a 100-ml round-bottom flask equipped with a stirring means. The mixture is cooled with ice water under a nitrogen atmosphere. Isopropenyl methyl ether (14 ml, density 0.765 g/ml, 131 mmoles) is added to the mixture over a period of about 20 minutes. The mixture is then allowed to stand for 20 minutes. The resulting colorless solution is washed twice using 10 ml of saturated NaHCO 3  solution each time, and then is washed twice using 10 ml of water each time. The washed solution is dried with magnesium sulfate, and is filtered and evaporated to remove the methylene chloride and 2,2-dimethoxy propane, leaving 10.56 g of white crystals having a melting point of 98.5° C. to 100° C. 
     The yield of crude product is 97 percent calculated as follows: 100 mmoles of ethyl carbamate+131 mmoles of isopropenyl methyl ether reacts to form 10.56 g of the named product (10.56 g÷218 g/mole=0.04844 mole=48.44 mmoles). The ethyl carbamate conversion is 100 percent. Since, theoretically, 2 moles of ethyl carbamate is required per mole of product, the yield is: (100 percent conversion)(2×48.44 mmoles)/(100 mmoles ethyl carbamate)=96.88≈97 percent. Recrystallization of the white crystals using toluene gives crystals having a melting point of 107° C. to 108° C. Spectral analyses (proton and carbon nuclear magnetic resonance, infrared, and mass spectroscopy) are consistent with the assigned structure. 
     EXAMPLE 2 
     Synthesis of 2,2-N,N-bis(methyl carbamato)propane 
     The procedure of Example 1 is repeated with the following exceptions: ethyl carbamate is replaced with methyl carbamate (6.66 g, 88.7 mmoles); 35 ml of methylene chloride is employed; 38 mg of p-toluene sulfonic acid is employed; and 10.7 ml of isopropenyl methyl ether is employed. The initial white crystals weigh 7.99 g and have a melting point of 103° C. to 106° C. The yield is 94 percent. The white crystals are recrystallized from toluene to give crystals having a melting point of 122° C. to 123° C. Spectral analyses are consistent with the assigned structure. 
     The preceding examples demonstrate the high yield production of geminal dicarbamates which do not have a hydrogen atom on the geminal carbon atom.