Highly enantioselective construction of tertiary thioethers and alcohols via phosphine-catalyzed asymmetric gamma-addition reactions of 5H-thiazol-4-ones and 5H-oxazol-4-ones: scope and mechanistic understandings

Phosphine-catalyzed highly enantioselective gamma-additions of 5H-thiazol-4-ones and 5H-oxazol-4-ones to allenoates have been developed for the first time. With the employment of amino-acid derived bifunctional phosphines, a wide range of substituted 5H-thiazol-4-one and 5H-oxazol-4-one derivatives bearing heteroatom (S or O)-containing tertiary chiral centers were constructed in high yields and excellent enantioselectivities. The reported method provides facile access to enantioenriched tertiary thioethers/alcohols. The mechanism of the gamma-addition reaction was investigated by performing DFT calculations, and the hydrogen bonding interactions between the Bronsted acid moiety of the phosphine catalysts and the C=O unit of the donor molecules were shown to be crucial in asymmetric induction.