A Three-Pronged Approach to the Development of New Trifluoromethylation Reactions
Feb. 22, 2019 16:00 - 17:00
MI Bldg.LargeConferenceRoom 2F
Gavin Chit TSUI
Chinese University of Hong Kong
Fluorinated molecules continue to be of major interest for the applications in pharmaceuticals, agrochemicals and functional materials. To address the synthetic challenges in the area of trifluoromethylation reactions, a three-pronged approach is needed to solve the problems of efficiency, selectivity and CF3-source. We have recently developed a series of novel trifluoromethylation methods using the fluoroform-derived CuCF3 reagent. By employing common feedstocks such as terminal alkynes and simple alkenes, a variety of valuable CF3-containing building blocks including the trifluoromethylated alkynes, alkenes and β-trifluoromethyl alcohols can be synthesized in one step. These processes, namely trifluoromethylation, hydrotrifluoromethylation and hydroxytrifluoromethylation, allow the distinctive construction of C(sp)-CF3, C(sp2)-CF3 and C(sp3)-CF3 bonds, respectively. Furthermore, a three-component vicinal trifluoromethylation-allylation of arynes was realized where two carbon-carbon bonds (C-CF3 and C-allyl) are formed in one pot to provide the trifluoromethylated allylarenes. Even 1,2-bis(trifluoromethylation) of arynes was made possible for the first time. This reagent also enables the preparation of α-trifluoromethyl esters and ketones directly from α-diazo carbonyl compounds under mild conditions. Overall, the ultimate CF3 source in these versatile fluorinated molecules is the inexpensive industrial by-product fluoroform from Teflon manufacturing.
We have also investigated the synthesis of diverse trifluoromethylated heterocycles via domino strategies with copper. An interrupted click reaction, using CuI/phen as the catalyst and (trifluoromethyl)trimethylsilane (TMSCF3) as the nucleophilic CF3 source, has been developed to synthesize 5-trifluoromethyl 1,2,3-triazoles in one step from readily available terminal alkynes and azides. The reaction shows complete regioselectivity, broad substrate scope and good functional group tolerability. Moreover, domino 5-endo-dig cyclization/trifluoromethylation of α, β-alkynic tosylhydrazones and propargylic N-hydroxylamines allows convenient access to 4-(trifluoromethyl)pyrazoles and 4-trifluoromethyl-4-isoxazolines, respectively. These reactions are facilitated by the Cu(OTf) 2/TMSCF3/KF combination. By employing easily accessible 2-alkynylanilines and the low-cost fluoroform-derived CuCF3 reagent, both 2- and 3-(trifluoromethyl)indoles can be prepared in good yields with no ambiguity of the CF3 position. Analogous cyclization/trifluoromethylation of 2-alkynylphenols can afford 3-(trifluoromethyl)benzofurans and one of the compounds was identified as a promising antibacterial and antifungal agent. Applications of the above methods in the expedient synthesis of trifluoromethylated drug analogues including rufinamide, celecoxib, bazedoxifene, melatonin, papaverine and estrone have also been successfully demonstrated.