Abstract
Developing the high-efficient and green synthetic method for chiral amino alcohols is an intriguing target. We have developed the Mg2+-doped Cu/ZnO/Al2O3 catalyst for hydrogenation of L-phenylalanine methyl ester to chiral L-phenylalaninol without racemization. The effect of different L-phenylalanine esters on this title reaction was studied, verifying that Cu/ZnO/Al2O3 is an excellent catalyst for the hydrogenation of amino acid esters to chiral amino alcohols. DFT calculation was used to study the adsorption of substrate on the catalyst, and showed that the substrate adsorbs on the surface active sites mainly by amino group (-NH2) absorbed on Al2O3, and carbonyl (C=O) and alkoxy (RO-) group oxygen absorbed on the boundary of Cu and Al2O3. This catalytic hydrogenation undergoes the formation of a hemiacetal intermediate and the cleavage of the C–O bond (rate-determining step) by reacting with dissociated H to obtain amino aldehyde and methanol ad-species. The former is further hydrogenated to amino alcohols, and the latter desorbs from the catalyst surface.
Highlights
Developing the high-efficient and green synthetic method for chiral amino alcohols is an intriguing target
It can be seen that 90.1% conversion and 95.1% selectivity of L-phenylalaninol were obtained when L-phenylalanine methyl ester (L-p-me) was used as the reaction substrate
Only 2.5% conversion was achieved when the amino group was removed from the L-p-me, which suggests that the reaction undergoes towards the other reaction pathway without the adsorption of amino group, resulting in the low reactivity and low selectivity (52%) to the desired product
Summary
To gain more insight into the role of the amino group of L-phenylalanine methyl ester (L-p-me) in the L-p-me hydrogenation over the Cu6/γ-Al2O3(100) catalyst, the energy barriers for the transferring of Had from Cu to C=O(Step III) and the cleaving of C–O bond in hemiacetal (Step IV, rate-determining step) were calculated for 3-phenylpropionic acid methyl ester (3-p-a-me) and L-p-me. In hydrogenation of the substrate without amino group, the energy barriers for step III and step IV (rate-limiting step) are significantly higher than these in the L-p-me hydrogenation, which is consistent with the experimental results that a high reaction temperature (>240 °C) is usually needed for the effective transformation of alkyl carboxylic esters to the corresponding alcohols over the Cu-based catalysts[31,37,38,39,40], and hydrogenation of 3-p-a-me at 110 °C was not effective over this CuZn0.3Mg0.1AlOx catalyst (Table 1). It is reasonable to conclude that the adsorption of amino group in the substrate plays a vital role in this catalytic hydrogenation cycle, which activates the substrate and stabilizes the adsorption species to start up whole catalytic hydrogenation reaction, and reduces the activation energy barrier for this title reaction
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