Dynamic Kinetic Resolution of Racemic Amines

Like the DKR of alcohols, the DKR of amines provides an efficient access to enan-tiopure compounds in theoretically 100% yield. In this case amides are the final products obtained after the combination of an adequate acyl donor and a racemi-zation agent [96, 99-101, 219]. Amine racemization usually requires higher temperatures and/or strongly basic media in comparison with alcohol racemization [96] and must be compatible with preserving the enzyme activity. 

The racemization strategies for the DKR of amines can occur mainly through two different pathways. On one hand, the amine racemization can proceed via metal-catalyzed formation of an achiral imine intermediate, which is subsequently hydrogenated in a nonselective fashion to obtain the racemic amine [220]. Alternatively racemization can be achieved by means of the reversible homolytic abstraction of the a-hydrogen atom of the amine, allowing the interconversion of the amine enantiomers via a carbon-centered a-amino radical intermediate. Owing to its high thermal stability, even at 90-100 °C, CAL-B has been the most common hydrolase in the DKR of amines. 

Recently, the DKR of arylamines has been successfully achieved with Pd/layered double hydroxide-dodecyl sulfate anion as racemization agent, a catalyst that was effectively reused more than 30 times without loss of activity and stereodiscrimina-tion [234]. Its high catalytic efficiency allows the reaction to be performed at 55 °C, opening the possibility of working with less thermostable enzymes. In this case, 4-chlorophenyl valerate was used as acyl donor. The search for a novel palladium catalyst 

9 ENANTIOSELECTIVE ACYLATION OF ALCOHOL AND AMINE REACTIONS IN ORGANIC SYNTHESIS 

DKR of aliphatic and benzylic amines using CAL-B, isopropyl acetate, and a dimeric ruthenium complex. 

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