Microbial decyanation of 1-benzylpyrrolidine-2,5-dicarbonitrile. Mechanistic investigations

Various bacterial and fungal strains were screened for their ability to catalyse the regioselective hydrolysis of 1-benzylpyrrolidine-2,5-dicarbonitrile (1). Among the examined strains, Rhodococcus opacus sp-lma whole cells transformed both isomers of 1 into 1-benzyl-5-cyano-2-pyrrolidinone (2) and N-benzylacetamide (3). These reactions are difficult to achieve chemically and the synthesis of compound 2 did not compete with microbiological catalysis in terms of efficiency and respect for the guidelines of green chemistry. To distinguish between an oxidative or hydrolytic based-mechanism, the origin of the oxygen atom in 2 was investigated by using 18O2 and 18OH2 coupled with GC–MS analysis. These experiments confirmed that the oxygen atom in 2 came from water and not from molecular oxygen. The reaction is probably initiated by the dehydrogenation of 1 to generate the iminium ion, which could be trapped by a water molecule to form the cyanohydrin. The cyanohydrin intermediate would spontaneously break down to the γ-lactam product 2. Conversion of 1 to 2 by induced rat liver microsomes suggests the involvement of a Cyt P-450-type enzyme. A mechanism that accounts for the formation of 3 is also proposed.