An enzymatic intramolecular asymmetric reductive amination has been developed for the synthesis of chiral 1,4-diazepanes. Several enantiocomplementary IREDs were identified for the synthesis of (R)- and (S)-5-chloro-2-(5-methyl-1,4-diazepan-1-yl)benzo[d]oxazole with high enan- tioselectivity. The catalytic efficiency of (R)-selective IRED from Leishmania major (IR1) and (S)-selective IRED from Micromonospora echinaurantiaca IR25) was 0.027 and 0.962 s(-1) mM(-1), respectively. To further improve the catalytic efficiency of IR1, its double mutant Y194F/D232H was identified by saturation mutagenesis and iterative combinatorial mutagenesis, which exhibited 61-fold in the catalytic efficiency relative to that of wild-type enzyme. The density functional calculations and molecular dynamics simulations provided some insights into the molecular basis for the improved activity of mutant Y194F/D232H. Furthermore, Y194F/D232H and IR25 were applied to access a range of different substituted 1,4-diazepanes with high enantiomeric excess (from 93 to >99%). This study offers an effective method for construction of chiral 1,4-diazepanes of pharmaceutical importance via imine reductase-catalyzed intramolecular reductive amination of the corresponding aminoketones.