Biocatalysis- Enoate (Ene) Reductases

Mechanism + Description

For synthesis, the principal enzyme class is the Enoate or Ene reductases. These enzymes can reduce activated alkenes in a stereospecific anti fashion by the delivery of hydride from a reduced FMN co-factor followed by protonation. The oxidized FMN co-factor is regenerated by NADPH, so the reaction needs to be driven by the NADPH - NADP+ cycle.

General comments

It has been known for some time that 'old yellow enzymes' in yeasts will reduce activated alkenes to give chiral products. Recently, OYE-type enzymes have been the subject of a great deal of study, and many ene reductases can now be used as isolated enzymes rather than as whole-cell biocatalysts. The alkene needs activation with a(n) electron-withdrawing group(s) to stabilize the anion formed after the hydride addition. The current range of substrates that can be reduced with ene reductases is limited, but is constantly being expanded as new enzymes are developed.

 

Key references

Toogood, H. S.; Scrutton, N. S. New Developments in 'Ene'-ReductaseCatalyzed Biological Hydrogenations. Curr. Opin. Chem. Biol. 2014, 19,107–115.

Brenna, E.; Crotti, M.; Gatti, F. G.; Manfredi, A.; Monti, D.; Parmeggiani, F.; Pugliese, A.; Zampieri, D. Rationalisation of the Stereochemical Outcome of Ene-Reductase-Mediated Bioreductionof α,β-DifunctionalisedAlkenes. J. Mol. Catal. B. 2014, 71, 67-72.

Debarge, S.; McDaid, P.; O'Neill, P.; Frahill, J.; Wong, J. W.; Carr, D.; Burrell, A.; Davies, S.; Karmilowicz, M.; Steflik, J. Evaluation of Several Routes to Advanced Pregabalin Intermediates: Synthesis and Enantioselective Enzymatic Reduction Using Ene-Reductases. Org. Process Res. Dev. 2014, 18 (1), 109–121.

Toogood, H. S.; Gardiner, J. M.; Scrutton, N. S. Biocatalytic Reductions and Chemical Versatility of the Old Yellow Enzyme Family of Flavoprotein Oxidoreductases. ChemCatChem. 2010, 2 (8), 892–914.

Bechold, M.; Brenna, E.; Femmer, C.; Gatti, F. G.; Panke, S.; Parmeggiani, F.; Sacchetti, A. Biotechnological Development of a Practical Synthesis of Ethyl (S)-2-Ethoxy-3-(p-methoxyphenyl)propanoate (EEHP): Over 100-Fold Productivity Increase from Yeast Whole Cells to Recombinant Isolated Enzymes. Org. Process Res. Dev. 2012, 16 (2), 269–276.

 

Relevant Scale-Up Examples with Scheme

ChemCatChem. 2015, 7 (8), 1302 – 1311.
Experimental
300 mg scale

Specific solvent issues with chiral hydrogenation of C=C bonds

Asymmetric hydrogenations have been carried out in a wide range of solvents – preferred are lower mol wt alcohols, simple esters, and greener ethers like 2-MeTHF.

Chlorinated solvents like dichloromethane( DCM), 1,2-dichloroethane and CHCl3 should be avoided if at all possible.

With very high S/C rations, a solvent's quality needs careful consideration as even very small amounts of certain impurities can inhibit the catalyst.

Likewise, care should be used when using recycled solvents since low levels of impurities and oxidation products that may not be apparent on the specification may inhibit the catalyst.

Water is not a common solvent for the asymmetric hydrogenation of alkenes, but there are reports of immobilized/ water-soluble catalysts.

Wan, K.; Davis, M. E. Asymmetric Hydrogenation in Water by a Rhodium Complex of Sulfonated2,2′bis(diphenylphosphino)-1,1′-binaphthyl (binap). Chem. Commun. 1993, 0, 1262-1264.

Kleman, P.; Barbaro, P.; Pizzano, A. Chiral Rh Phosphine–Phosphite Catalysts Immobilized on Ionic Resins for the EnantioselectiveHydrogenation of Olefins in Water. Green Chem. 2015, 17, 3826-3836.

© 2015 Green Chemistry Institute Pharmaceutical Roundtable. All rights reserved.