Enzyme/Metal-catalyzed Synthesis Of Chiralα-trifluoromethylated Amines

Optically pure trifluoromethylated amines are crucial intermediates for the synthesis of a series of fluorinated pharmaceuticals and agrochemicals, so the synthesis of chiral trifluoromethylated amines is becoming an important issue of domestic and foreign research. The development of generally suitable and acceptable methodology for synthesis of optically pure a-trifluoromethylated amines is of our great interest, and chemoenzymatic processes are usually environmentally benign, efficient, economical, and the precursors are often stable and easy to be isolated, which process a very broad application prospect.Among various enzymes tested on the kinetic resolution (KR) of trifluoromethylated amines, only CAL-B showed acylation activity. For the acylation of1,1,1-trifluoro-2-propylamine (4a), the best result of enantioselectivity factor E=31was obtained when using THF as the solvent, and the product amide with R-configuration was obtained. However, the enantiopreference was converted to S for the amines (4b-4e) with substituents larger than methyl group, and only when choosing toluene as the solvent, CAL-B showed catalytic activity towards these amines. The enantiopreference and inversion for different trifluoromethylated amines displayed by CAL-B were observed and explained by docking modes. In addition, the effects of small molecule amides on the lipase CAL-B were studied. We discovered that formamide illustrated enzymatic inhibitation, but addition of acetamide or benzamide could increase conversions and enantioselectivities toward acylation of trifluoromethylated amine4a (ee and E values were increased to96.8%and75.5, respectively).In order to improve the conversions while retaining the same ee values as that in the KR process, dynamic kinetic resolution (DKR) of trifluoromethylated amines was carried out. A series of commercially available metal catalysts were tested as the racemization reagents, and the best conversion of73%was obtained when using as the racemization catalyst. Thereafter, a one-pot sequential process KR/DKR/KR was designed, and significant increase in the overall yield (best conversion84%) by applying this procedure were achieved with similar enantiomeric excesses (90%～99%). The catalysts recycle and scale-up experiments were then demonstrated successfully.The methods mentioned aboved had to be started from racemic amines, which was difficult to be synthesized and purified. Therefore, a new method was developed for the synthesis of chiral trifluoromethylated amines directly from ketoximes. Under the appropriate hydrogen pressure and reaction temperature, trifluoromethylated ketoximes could be completely transformed to amines, then lipase CAL-B was added to the reaction mixture to realize the one-pot cascade reaction which avoided separation and purification of intermediates. Conversions of the one-pot reactions could be more than60%, and the amide products were obtained with high enantiomeric excess (90～99%). With regards to1,1,1-trifluoroacetone oxime, the amide product with R configuration was obtained through ketoxime one-pot reaction, and enantiomeric excess only reached70%. Combined with resolution of L(+)-tartaric acid, the optical purity of product ran up to97%with the total yield of61%. Moreover,(S)-2,2,2-trifluoro-1-propylamine with98%ee value was obtained by direct condensation of1,1,1-trifluoropropan-2-one and (R)-1-phenylethanamine, followed by isomerization and CAL-B catalyzed kinetic resolution. Afterwards, the application of (S)-3,3,3-trifluoro-l-phenyl-2-propylamine to the synthesis of optically pure inhibitor of phenylethanolamine N-methyltransferase was achieved, and enantiomeric excess reached up to98%.However, reduction of ketoximes required high hydrogen pressure (1.15-1.55MPa), which didn’t match the low hydrogen pressure (0.01MPa) needed by DKR process. So a new type of atomic layer deposition (ALD) nanocatalysts was designed which could dramatically decrease the hydrogen pressure and reaction temperature for the reduction of ketoximes, because metal particles were highly dispersed on porous supports with an average size of2-3nm in diameter. The nanocatalysts were applied to both hydrogenation of ketoximes and racemization of the following amines, and ALD Ni/Al2O3was chosen as the optimal catalyst for ketoxime one-pot reactions with conversion up to95%and high enantiomeric excess.A series of enzyme-metal biohybrids were prepared through metal oxidation or reduction of metallic ions catalyzed by lipase CAL-B. Content of lipase in hybrid catalysts was tested by Bradford’s assay, and the amount of metal was determined by Atomic Absorption Spectrophotometry (AAS). The EDX experiments revealed that the formed biohybrids included the corresponding metals. XPS analysis confirmed the generation of metallic ion in biohybrids CAL-B/M2+, and metal particles in hybrid catalysts CAL-B/MNPs. The morphology of hybrid catalysts was investigated using HRTEM, which indicated that biohybrids CAL-B/M2+agglomerated together, and in hybrid catalysts CAL-B/MNPs, metal nanoparticles with an average diameter no more than5nm were uniformly distributed or embedded in the protein net. The HRTEM image of metals also clearly showed their atomic lattice. MALDI-TOF-MS experiments illustrated that biohybrids CAL-B/MNPs were macromolecules which confirmed the combination with protein. Then, a method was developed that involved tagging of CAL-B with gold nanoparticles to allow for the indirect determination of the enzyme distribution. Finally, biohybrid CAL-B/PdNPs was used for the racemization of (S)-phenethylamine and enantiomeric excess was dropped to6%. Then, CAL-B/PdNPs was applied to the DKR of trifluoromethylated amines4a and4d, the catalytic activity and selectivity were similar to that of commercial lipase CAL-B combining with Pd/Al2O3. Recycling of biohybrid CAL-B/PdNPs was also carried out with satisfactory results.