4 Scope of the biocatalytic one-pot reaction
Having obtained a system capable of high productivity, we chose a variety of aldehydes 1a -k , which were derived easily from biomass (e.g., primary, aromatic, and furan aldehydes) (Wu, Moteki, Gokhale, Flaherty, & Toste, 2016; Mahyari, Shaabani, & Bide, 2013; Boruah & Das, 2018; Lee et al., 2018) and mated with α, β-unsaturated carbonyl tandem reaction under optimized conditions. As shown in Table 2, the enzyme accepted a wide range of substrates. A high reaction rate was obtained using primary aldehydes whose corresponding intermediates may possess suitable sizes for adoption in the active center. The appropriate length branch-chain was vital in fastening the hydrophobic area in the active center of NerA(Peters, Frasson, Sievers, & Buller, 2019). Benzene (3f ) and furan (3h , 3i ) derivatives also achieved high yields (80%, 75%, and 70% respectively), other ERs like DBVPG and OPRI had only 3% and 4% yields after 24 hours (Reß, Hummel, Hanlon, Iding, & Gröger, 2015). Compared to 3f , the substrate with an electron-withdrawing group (3g ) showed a higher yield (85%). These results are probably because benzene has electron donors with conjugating effects, which is not conducive to hydrogen transfer. This can be further proven as furan, a 5-center, 6-electron conjugate system with a higher electron cloud density, caused a lower yield. We also noticed that although enantioselectivity is a general property of most reductases including NerA. However, e.e% of 3a -k are only around 10% (data not shown), since the intermediates can be flipped at the active center, and the keto-enol tautomerism of the ketone carbonyl in the products may be the other reason for racemization under an aqueous environment. Metal nanoparticles such as Ni and Pd were found to possess the ability to drive the cascade reactions containing Knoevenagel condensation and ene reduction as bifunctional catalysts similar to NerA (Javad Kalbasi, Mesgarsaravi, & Gharibi, 2019; H. Wang et al., 2019). Compared to our work, the reaction catalyzed by these metal nanoparticles must be separated into two sub-steps. Meanwhile, as an alternative to using metal catalysts, enzymes are coincident with the concept of green chemistry.