Advances in Organocatalytic Asymmetric Reactions Involving Thioesters

Thioesters play a very important role in biosynthesis and organic synthesis. Due to smaller orbital overlap of the C(2p) and S(3p) orbitals, the alpha-proton acidity of thioesters is higher than that of the related oxoesters, making thioesters useful enolate precursors in nature as well as in the laboratory. Meanwhile, thioesters are also efficient acylation reagents which can be used for the construction of ester bond and amide bond. Organocatalysis, a biomimetic catalysis usually with metal-free small organic molecules, is an emerging research field that has been booming since the beginning of the 21st century. In the past decade, many important achievements have been made in the organocatalytic asymmetric reactions involving thioester substrates, which have greatly broadened the reaction types of organocatalytic reactions with ester substrates and realized some reactions that cannot be achieved by using their oxoester analogues. The advances in organocatalytic asymmetric reactions involving thioesters are summarized in this review. According to the types of thioester substrates, these advances are classified to two types. One type is the organocatalytic asymmetric reactions with enolizable thioesters such as trifluoroethyl thioesters, malonic acid half-thioesters (MAHTs), monothiomalonates (MTMs) and dithiomalonates (DTMs). For these reactions, noncovalent interactions between catalysts and thioesters, including hydrogen bonding and ion pair interaction, have been used to promote the reaction and to achieve the high enantioselectivity. Another type is the catalytic asymmetric reactions with alpha,beta-unsaturated thioesters. For the reaction of this type, various chiral organocatalysts, including chiral amines, ureas, NHC (N-heterocyclic carbene), isothiourea, amidine and others, not only activate the thioester substrates, but also control the enantioselectivity well through covalent and non-covalent bonds. Meanwhile, the mechanism of representative transformations will be briefly introduced and at last, the perspective in this area will be given.