·
Amides are not reduced directly by NaBH4, but if they are first converted to an imimium salts that readily proceed the reduction to give an amine (Scheme 5).
 |
Scheme 5 |
Examples:
 |
J. L. Luche, J. Am. Chem. Soc. 1978, 100, 2226. |
 |
A. L. Gemal, J. L. Luche, J. Org. Chem. 1979, 44, 4187. |
 |
K. Takao, G. Watanabe, H. Yasuki, K. Tadno, Org. Lett. 2002, 4, 2941. |
 |
D. L. Comins, A. L. William, Org. Lett. 2001, 3, 3217. |
2.1.5.2 Lithium Borohydride (LiBH4)
Lithium borohydride is prepared by reaction of NaBH4 with lithium bromide. It is more powerful reducing agent than NaBH4, reducing not only aldehydes, ketones, but also epoxides, esters and lactones (Scheme 6). This is due to the fact that the Li+cation is stronger Lewis acid than Na+ cation. Therefore, Li+ ion can co-ordinate with carbonyl compound which increases the electrophilicity of carbonyl carbon and facilitates the hydride transfer.
 |
Scheme 6 |