Recent Progress on Direct Trifluoromethylthiolating Reagents and Methods

With a significantly high Hansch's hydrophobicity parameter (1.44), electron-withdrawing trifluoromethylthio group (CF 3 S-) has been considered as one of the most lipophilic substituents and privileged fragments that are able to improve drug molecules' pharmacokinetic and physicochemical properties such as lipophilicity and metabolic stability. It is well-known that incorporation of the trifluoromethylthio group into small molecules greatly enhances its ability to cross lipid membranes and in vivo absorpotion rate. In addition, the high electronegativity of the trifluoromethylthio group significantly improves the small molecule's stablity in acidic environments. Not surprisingly, the trifluoromethylthio group has been of special attention not only from the academia but also from pharmaceutical and agrochemical industry for their use in isostere-based drug design. Development of highly efficient methods for the introduction of the trifluoromethylthio group into small molecules, thereafter, has become a subject of recent focus in the field of organic chemistry. In the early 1960s, a few methods for the formation of trifluoromethylthioethers were reported, which typically involved halogen exchange of the trichloromethyl-substituted compounds and trifluoromethylation of thiolated substrates. However, the conditions of these methods were harsh and incompatible with many common functional groups. Since 2008, new reagents and methods that were able to efficiently incorporate the trifluoromethylthio group under mild conditions have emerged, that pave the way for the facile introduction of trifluoromethylthio group into site-specific positions of the target molecules. In this review, we will first briefly introduce the indirect strategies for trifluoromethylthiolation including halogen exchange and trifluoromethylation of thiolated substrates, and then focus on the direct trifluoromethylthiolation strategies including the transition metal-catalyzed trifluoromethylthiolation reactions, electrophilic trifluoromethylthiolation reactions with electrophilic trifluoromethylthiolating reagents and radical trifluoromethylthiolations. These methods represent the most straightforward and promising approaches for the incorporation of the trifluoromethylthio group into small molecules. At the end, we will discuss the remaining problems and challenges in this particular field.