Effect of Additives in the Hydroamination/Cyclization of Aminoalkenes Catalyzed by a Binaphtholate Yttrium Catalyst

Abstract

AbstractA series of various solvents and additives were tested in enantioselective hydroamination/cyclization reactions of aminoalkenes catalyzed by a binaphtholate yttrium catalyst. The functional group tolerance of the catalyst and the influence on the reaction rate and enantioselectivity was studied. Some weakly coordinating polar solvents, such as Et2O, MTBE, and chlorobenzene led to slightly increased reaction rates compared to the less polar solvent benzene, presumably due to a better stabilization of the polar transition state. Stronger binding solvents and additives, such as THF, DMAP, pyrrolidine, n‐propylamine, and 1‐phenylethylamine, decrease the reaction rate and diminish the enantioselectivity of the hydroamination product. Some additives, such as THF, Et2O, MTBE, chloro‐ and bromobenzene, as well as (+)‐sparteine resulted in slightly higher enantioselectivities in the cyclization of the model substrate C‐(1‐allylcyclohexyl)methylamine, although this observation was not generally true for other aminoalkene substrates. The reaction rates and enantioselectivities were depressed in the presence of (−)‐sparteine using the (R)‐binaphtholate‐ligated catalyst. In case of C‐(1‐allylcyclohexyl)methylamine, the enantioselectivity was switched from 76% ee favoring the (S)‐enantiomer of the hydroamination product when using (+)‐sparteine to 22% ee in favor of the (R)‐enantiomer when (−)‐sparteine was used. The rates of cyclization of aminoalkenes and the resulting enantioselectivities significantly depend on substrate concentration with the highest rate (13.6 h−1) and enantioselectivity (68% ee) observed in dilute conditions (0.05 M) compared to a concentrated solution (0.5 M, 5.0 h−1, 35% ee) for 2,2‐dimethylpent‐4‐enylamine. These observations indicate that the reaction mechanism is shifted in favor of a slower, less enantioselective catalytic cycle involving a higher coordinate species when higher substrate concentrations or stronger binding additives are present.magnified image