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Lithium aminoborohydride is a versatile reducing agent. In the scheme below, I don't understand how does unhindered LAB afford an alcohol in the reduction of amides while a hindered LAB affords an amine. Why are two different products obtained and by what mechanisms do these reductions take place?

enter image description here

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Very interesting question and not very obvious. As can be seen from the picture, LAB's reduce only tertiary amides. It is stated in the publication(1) about LAB reductions:

LAB reagents do not reduce primary and secondary amides even in refluxing THF.

In the same publication it is provided reason for different outcomes of reductions. They say that:

We found that the outcome of these reductions depends on the sterics of the amide as well as of the LAB reagent

Which means that if LAB and/or amide is sterically hindered it can't interact with nitrogen like in compound 5

In lower part of the picture is described reaction where more hindered LAB leads to amine product 9

enter image description here

(1)Pasumansky, L., Goralski, C. T., & Singaram, B. (2006). Lithium aminoborohydrides: powerful, selective, air-stable reducing agents. Organic process research & development, 10(5), 959-970.

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