Kinetic disubstituted 1-pyrrolines

Racemic 2,5-disubstituted 1-pyrrolines were kinetically resolved effectively by hydrogenation with a chiral titanocene catalyst 26 at 50% conversion, which indicates a large difference in the reaction rate of the enantiomers (Table 21.19, entries 4 and 5), while 2,3- or 2,4-disubstituted 1-pyrrolines showed moderate selectivity in the kinetic resolution (entries 6 and 7) [118]. The enantioselectivity of the major product with cis-configuration was very high for all disubstituted pyrrolidines. The high selectivity obtained with 2,5-disubstituted pyrrolines can be explained by the interaction of the substituent at C5 with the tetrahydroinde-nyl moieties of the catalyst [Eq. (17)]. 

Titanium, Zirconium, Hafnium - The catalytic asymmetric hydrogenation of imines has been reported using a chiral titanocene catalyst. " An enantiopure titanocene catalyst has been used in the catalytic asymmetric hydrogaution of disubstituted enamines. Kinetic resolution of a racemic disubstituted pyrroline has been effected by asymmetric reduction with a chiral titanocene catalyst. Poly(methylhydrosiloxane) has been used as a stoichiometric... 

The chiral titanocene catalyst 34 is very effective for the kinetic resolution of racemic 2,5-disubstituted 1-pyrrolines. When hydrogenation of racemic 5-methyl-2-phenyl-1-pyrroline with (Y)-34 is interrupted at ca. 50% conversion, unreacted R substrate with 99% ee is obtainable with a (2S,5S)-cA-pyrrolidine derivative with 99% ee (Scheme 1.98) [353], As summarized in the table, some other racemic substrates can be resolved in >95% optical yield. 

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