Acetamidocinnamic acids, catalytic asymmetric hydrogenation

Asymmetric hydrogen transfer from EtOH, /-PrOH or triethylammonium formate to Z-a-acetamidocinnamic acid Z-3.32 (R = Ph, Z = COMe, R = H) or to itaconic acid 7.7 (R = H) is highly enantioselective when the catalyst is a binap-Ru complex [873, 1333] (Figure 7.14). The relationship between the absolute configuration of the saturated add and the binap ligand is the same as in the catalytic hydrogenation. However, hydrogen transfer to other a,P-unsaturated acids, such as the precursor of naproxen (7.15), is less enantioselective [1333],... 

Of particular importance is the use of ionic liquids in asymmetric catalytic reactions. The first example - asymmetric hydrogenation of (Z)-o -acetamidocinnamic acid to (5)-phenylalanine (61)-was reported in 1995using [Rh(cod)(-)-diop][PF6] (diop = 4,5-bis[(diphenylphosphanyl)-methyl]-2,2-dimethyl-l,3-dioxolan-4,5-diol) in a biphasic mixture of [bmim] [SbFe] and f-PrOH (Scheme 17). °° At the end of the reaction, the product was obtained in isopropanol, and the ionic liquid containing the catalyst could be recycled. Only modest enantioselectivity was attained (64% ee), but this work opened the way to efficient asymmetric catalytic reactions in ionic liquids. 

A number of catalysts with the catalytic site in the dendrimer core has also been used in asymmetric hydrogenation. A rhodium complex with dendrimer diphos-phine with menthyl groups in its branches as a catalyst in acetamidocinnamic acid hydrogenation exhibited low enantioselectivity. For a catalyst where dendrimers were at the meta positions of phosphine, the reaction rate was substantially higher than that for a low molecular weight analogue and for a catalyst where dendrimers were at positions 2 and 5 of phosphine, the reaction rate was much lower [134-137],... 

Mechanism of Asymmetric Catalytic Hydrogenation of a-Acetamidocinnamic Acid Esters... 

---