Asymmetric Catalytic (2+1) Cycloaddition of Thioketones to Synthesize Tetrasubstituted Thiiranes

Herein, we report the first example of enantioselective (2+1) cycloaddition of thioketones with alpha-diazo pyrazoleamides for the direct synthesis of tetrasubstituted thiiranes. In the presence of chiral N,N'-dioxide/cobalt(II) complexes (2-5 mol%), excellent efficiency (up to 99% yield within 15 mins) and high stereoselectivity (up to > 19: 1 dr and 97% ee) are available. Elaborations of thiiranes via desulfuration have also been conducted to deliver tetrasubstituted olefins. Density functional theory calculations reveal that the reaction initiates from a doublet state cobalt(II) carbenoid, which is followed by a quartet cobalt(II)-bound thiocarbonyl ylide pathway. This work provides a route for the selective construction of tetrasubstituted thiiranes and olefins that are otherwise difficult to access.