Cobalt catalyst with exclusive metal-centered chirality for asymmetric photocatalysis.

Abstract

For decades, progress in chiral transition metal catalysis has been closely linked to the design of tailor-made chiral ligands. Recently, an alternative to this conventional paradigm has emerged in which the overall chirality of the catalysts arises solely from a stereogenic metal center. However, the development of such chiral-at-metal catalysts based on earth-abundant metals is still a formidable challenge. Here, we report a reactive chiral-at-cobalt catalyst comprised entirely of achiral ligands, more than a century after Alfred Werner first introduced chiral cobalt complexes with exclusive metal-centered chirality. The cobalt center uniquely serves multiple functions: it is the sole stereocenter, redox center, catalytic site, and a chromophore. While the cobalt(III) complex is inert and bench-stable under ambient conditions, it can be photoactivated through an unexpected counterion-assisted mechanism, reducing inert cobalt(III) to catalytically active cobalt(II). This chiral-at-cobalt complex enables the visible-light-activated enantioselective conversion of isoxazoles into chiral 2H-azirines, achieving high enantiomeric excess of up to 97%.