C-F…H-X相互作用在有机反应中的影响

Although the debate on whether or not C-F bonds can function as H-bond acceptors lasted for tens of years, dating back to 1939 when Pauling pointed out in The Nature of the Chemical Bond that C-F bonds do not have significant power to act as proton acceptors in the formation of hydrogen bonds, more and more evidences support the existence of C-F…H-X interactions, and in particular, C-F…H-O and C-F…H-N interactions cannot be ignored.Because the sum of the van der Waals radii of hydrogen and fluorine atoms is reported to be around as 2.55 Å, C-F…H-X interactions may exist if the calculated distance of F…H is less than 2.50 Å. Strong C-F…H-X interactions may occur if the calculated distance is less than 2.30 Å and the F…H-X angle is greater than 120°.In 2011, we observed strong fluorine effects on the Strecker reaction of ketimines: while Schreiner's thiourea could catalyze the Strecker reaction of acetophenone derived ketimine using TMSCN, it was unable to mediate the corresponding reaction of analogy α-CF ₃ or α-CF ₂ H ketimines. Theoretical calculations revealed that the C-F…H-N interactions between the C-F bond of fluorinated ketimines and thiourea played the key role. This is the first report on the influence of such subtle interactions on organic reactions. Since then, reports from our and other groups revealed various types of C-F…H-X interactions that may be present in the reaction course, to strongly influence the reactivity and selectivity. Although successful examples are still limited, these achievements have suggested that C-F…H-X interactions may exist between the substrate and the catalyst; the substrate and the solvent; different reaction partners, or engender in the transition state with the reaction intermediate. Importantly, known examples demonstrate it possible to harness C-F…H-X interactions to tune reactivity and/or selectivity, which are useful for new reaction development, as well as for the design of new catalysts. To provide reference and inspiration for researchers engaged in organic synthesis, especially the organic fluorine chemistry, we summarize in this review the recent advances in the study of the influences of C-F…H-X interactions on organic reactions.