Rhodium Phosphine–Phosphite Catalysts in the Hydrogenation of Challenging N-(3,4-dihydronaphthalen-2-yl) Amide Derivatives

Abstract

The enantioselective catalytic hydrogenation of N-(3,4-dihydronaphthalen-2-yl) amides (1) with rhodium catalysts bearing phosphine-phosphite ligands 4 has been studied. A wide catalyst screening, facilitated by the modular structure of 4, has found a highly enantioselective catalyst for this reaction. This catalyst gives a 93% ee in the hydrogenation of 1a and also produces high enantioselectivities, ranging from 83 to 93% ee, in the hydrogenation of several OMe- and Br-substituted substrates. In contrast, the structurally related enol esters 2 are very reluctant to undergo hydrogenation. A coordination study of the representative enamide 1d has shown an unusual η(6)-arene coordination mode, over the typical O,C,C chelating mode for enamides, as the preferred one for this substrate in a Rh(I) complex. Deuteration reactions of 1c,d indicate a clean syn addition of deuterium to the double bond without an isotopic effect on the enantioselectivity.