A new class of low-loading catalysts for a highly enantioselective, metal-free imine reduction of wide general applicability

A new class of chiral Lewis bases for the enantioselective HSiCl3-mediated reduction of imines was developed. Through extensive catalyst structure optimization, an extremely active species was identified that was able to promote the reduction of a large variety of functionalized substrates in high yields with enantioselectivities typically above 90% with catalyst loadings as low as 0.1-1 mol%. The simple experimental procedure, the low cost of the reagents, the mild reaction conditions, and the straightforward isolation of the product make the methodology attractive for large-scale applications. Its synthetic potentiality was demonstrated by the preparation of advanced intermediates of important active pharmaceutical ingredients used in the treatment of Alzheimer's and Parkinson's diseases, hyperparathyroidism, neuropathic pains, and neurological disorders. The reaction was also successfully performed in flow reactors, which thus demonstrated the possibility to realize in continuo processes to yield multigram quantities of product.