Mechanisms of the Ruthenium Sulfur Complex in Generation and Stabilization of Silicon and Aluminum Electrophiles for C(sp³)–F Bond Activation

Abstract

C−F bond transformation has attracted much attention in chemical synthesis. For aliphatic C(sp 3 )−F activation, strong main-group electrophiles are commonly employed to abstract the fluoride. Herein, we performed density functional calculations to gain insights into the hydrodefluorination of CF 3substituted aniline derivatives, CF 3 (C 6 H 4 )NMe 2 , by the Ru−S complex 1, [(PEt 3 )Ru(DmpS)] + (DmpS = 2,6-dimesitylphenyl thiolate) with HSiMe 2 Ph. The mechanism involves three main steps: (i) Si−H bond activation to generate Ru−H with the thiosilane ligand, (ii) fluoride abstraction by the sulfur-supported silylium ( + SiMe 2 Ph) to generate + CF 2 (C 6 H 4 )NMe 2 , and (iii) hydride transfer from the Ru−H to the carbocation to generate the first hydrodefluorination product, CF 2 H(C 6 H 4 )NMe 2 . To form the fully hydrodefluorinated product, the second and the third fluoride abstractions proceed with lower energy barriers than the first fluoride abstraction, which is the rate-determining step. As the Si−H activation generates Ru−H with the thiosilane ligand, the Si−S bond must be cleaved to generate the active silylium moiety for the fluoride abstraction. In contrast, the Al−H activation of HAl i Bu 2 leads to the formation of Ru−H with the alumenium ( + Al i Bu 2 ), stabilized by the donation from both σ(Ru−H) and sulfur lone pair. Upon fluoride abstraction, the donation from the sulfur lone pair to the alumenium remains while that from σ(Ru−H) is loosened and simply replaced by the donation from the fluoride. The fluoride abstraction by the alumenium has a higher interaction and also involves less structural change than that by the silylium. This corresponds with the higher activity for the C−F bond activation when HAl i Bu 2 is used. The insights into the difference in the structures and activities in the fluoride abstraction by the silicon and the aluminum electrophiles, generated and stabilized by the Ru−S complex 1, will assist with the development of catalysts for C−F bond activation.