Cyclohexanones, 4-substituted reductive amination

Thus reaction of cyclohexanone, n-propylamine, and sodium cyanoborohydride in methanol at pH 6-8 at 25° for 24 hr. gives n-propyleyelohexylamine in 85 % yield. The reaction is general for ammonia and primary and secondary amines aromatic amines are somewhat sluggish. All aldehydes and relatively unhindered ketones can be reduc-tively aminated. Yields are improved by use of 3A molecular sieves to absorb the water generated in the reaction. Note that reductive amination of substituted pyruvic acids with ammonia leads to oi-amino acids. Thus alanine can be obtained from pyruvic acid in 50 % yield. A pH of 7 is optimum for. synthesis of a-amino acids. 

Reductive amination. LiBH4 is said to be the reagent of choice for reductive amin-ation of substituted cyclohexanones. 

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

T. Suzuki, M. Itoh, S. Ogawa, and Y. Takegami. A highly stereoselective reduction of substituted cyclohexanones using tri-isobutylaluminium-amine complexes. Bull. Chem. Soc. Japan. 1978, 51, 2664. 

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