Solvent free enantioselective catalysis with chiral cobalt(iii) Werner complexes via ball milling

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.