4 dicyclohexylphosphino 2 diphenylphosphino methyl n methyl aminocarbonylpyrrolidine rr mccpm is used as catalyst the preparation of this catalyst is known from the prior art ep a 0 251 164 ep a 0 336 123 the catalyst may also be bound to the polymer e g by having the chiral ligand 2r 4r 4 dicyclohexylphosphino 2 diphenylphosphino methyl n methyl aminocarbonyl pyrrolidine bound to a polymer via the phenyl groups for example the use of such polymer bound ligands does not totally rule out the simultaneous use of non polymer bound ligands such polymer bound catalysts are particularly advantageous for simple purification of the product the catalyst is either used as a prefabricated oxygen free solution of rh cod cl 2 and ligand or prepared in situ from rh cod cl 2 and ligand in the presence of salbutamone without oxygen in a protective gas atmosphere or hydrogen atmosphere the hydrogenation is generally carried out without oxygen conveniently under inert gas preferably under a hydrogen atmosphere however it is not essential to the reaction that the hydrogen for the hydrogenation should be capable of being taken from the atmospheric gas above the reaction mixture the hydrogen may also be produced in solution in situ from suitable hydrogen sources such hydrogen sources include e g ammonium formate formic acid and other formates hydrazines in the presence of metal ions such as fe 2 fe 3 and other hydrogen sources known from the prior art the reaction time for the asymmetric hydrogenation to be completed is generally between 2 and 48 hours preferably between 4 and.