| dc.description.abstract | Enantioselective additions of malonate esters to nitroalkenes can be catalyzed by a variety of salts of chiral cobalt(III) trications [Co(1,2-diamine)3]3+ in the presence of nitrogen donor bases in acetone. Catalysts that feature enantiopure 1,2-diphenylethylenediamine are particularly effective, and the base can also be incorporated into one of the counter anions, for example a (substituted) nicotinate. This study shows that such additions can be carried out under solvent free conditions and with reduced reaction times using ball milling, further enhancing the "green" credentials of this large family of earth-abundant-metal catalysts. The effect of various reaction variables are probed (base, counter anions, loading, time, quantity of balls, etc.), and the optimized conditions applied to twelve nitroalkenes, affording products in average yields and ee values of 89% and 74%. The enantioselectivities appear slightly lower than for analogous reactions in solution (0 °C), and possible factors and remedies are discussed. These tricationic Co(III) species were also analyzed using Electrospray Ionization-Mass Spectrometry to determine if the anions “fly” with the cation for further mechanistic analysis. Only half showed anions “flying” with the cation: Λ-(S,S)-23+ 2Cl–, Λ/D-(S,S)-23+ 2Cl–BArf–, Λ-(S,S)-23+ Cl–BArf–Nic–, and Λ-(S,S)-23+ 3BF4–. While anionic impurities were found in several cases, the larger, bulkier anions seemed to become ionized themselves leaving no indication of intact cation-anion species post-ionization. Enhanced purification techniques are shown to improve the removal of unwanted anions. | |
