C―F?H―X相互作用在有机反应中的影响

Influence of C—F?H—X Interactions on Organic Reactions

氢键相互作用在分子间或者分子内均广泛存在,并且对有机反应的选择性和活性具有深远影响.有机化合物中的N和O原子与不同氢键给体H―X的氢键作用已得到了广泛研究,并被巧妙地用于设计开发新的催化剂和新反应,但电负性最大的氟元素所形成的C―F键与氢键给体之间的C―F?H―X相互作用对有机反应的影响却研究很少. 2011年,我们首次在酮亚胺的Strecker反应中观察到强烈的氟效应,并提出可能是源于C―F?H―X相互作用对反应的活性和选择性的影响.随后,这种弱相互作用在更多的有机反应中被观察到,并被认为对有机化学反应可能存在显著的影响,甚至还能通过理性设计来实现高反应活性和选择性.本综述将介绍底物与催化剂或溶剂之间,底物与底物之间,以及反应过渡态中形成的C―F?H―X氢键相互作用对有机反应的影响,旨在为从事有机合成和有机氟化学的研究人员提供参考和启示,从而有助于新反应和新型手性催化剂的设计开发.

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 ?, C―F…H―X interactions may exist if the calculated distance of F…H is less than 2.50 ?. Strong C―F…H―X interactions may occur if the calculated distance is less than 2.30 ? 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-CF3 or α-CF2H 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.