Amide Reduction General Overview

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The reduction of amides to amines accounts for ~5% of the amine construction in the pharma industry. Given the ubiquitous nature of carboxylic acids/esters/acid chlorides and amine synthons, this would undoubtedly be higher if safer and greener methodology were available for amide reduction. Most imides can be reduced to amines with the reagents used for amide reduction. The greenest methods available have lowest utility – see later. The need to use high energy reactive reductants represents the strength of the amide bond and a thermodynamically unfavourable transformation. Typically most process groups would look to replace an amide bond reduction in a synthetic sequence.

Balanced Chemical Equation

Typical reagents used are:

Metal hydrides – usually Al based
Borane and derivatives
Catalytic hydrogenation
There are few reports of biocatalytic reduction of amides to amines

Green Criteria for amide reduction

  1. Reagents should be selected to minimise safety issues. Large molar excesses of reagents should be avoided if possible
  2. Catalytic transformations  are preferred over stoichiometric hydride reagents. If metal catalysed, base metals preferred over precious group metals.
  3. Process has no major safety issues and the generation of hazardous waste is minimised and controlled. Many reagents will generate hydrogen / and or other flammable gases.
  4. In situ generation of borane/ borane complexes removes the requirement to transport and store cylinders of boranes
  5. Solvents should be chosen to minimise any potential safety and environmental impact—see later.

General literature reviews on amide reduction reduction of amides

VenPure® NaBH4 for Amide/Lactam Reductions

Org. Process Res. Dev. 201216 1156-1184 Large-​Scale Carbonyl Reductions in the Pharmaceutical Industry

The Chemistry of Amides; Zabicky, J., Ed.Interscience Publishers: New York, 1970; pp 795-801.

Chem. Reviews 2014, 114, 5477-5510  - Review of Methods for the Catalytic Hydrogenation of Carboxamides 

Use of NaBH4 including amide reduction and in situ borane generation

Angew. Chem. Int. Ed. 2011, 50, 6004-6011 Selective Reduction of Carboxylic Acid Derivatives by Catalytic Hydrosilylation 

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