Inertness of the Aryl−F Bond toward Oxidative Addition to Osmium and Rhodium Complexes:  Thermodynamic or Kinetic Origin?

Abstract

Quantum calculations with the density functional theory (B3LYP) have been carried out to compare the reactivity of aryl-H and aryl-F bonds toward oxidative addition and to understand the high degree of inertness of the latter. The thermodynamic energy patterns for oxidative addition of 1,4-difluorobenzene toward two very different metal fragments have been examined. In one of them the final product of oxidative addition could be a 16-electron unsaturated complex of the type Os(H)(CO)(C 6 F 2 H 3 )(PH 3 ) 2 and/or Os(F)(CO)(C 6 FH 4 )-(PH 3 ) 2 . In the other system the final product of oxidative addition could be an 18-electron saturated complex CpRh(PH 3 )(H)(C 6 F 2 H 3 ) or CpRh(PH 3 )(F)(C 6 FH 4 ). These two systems are models for experimental complexes which prefer the C-H to the C-F oxidative addition. The calculations reveal that, for both systems, the C-F oxidative addition is thermodynamically preferred, especially in the 16-electron case. The activation energy has been determined in the case of Rh, and it is shown that the activation energy for C-F activation is considerably higher than that for C-H activation. This clearly shows that the inertness of the C-F bond has a kinetic origin.

The Hydroxide‐Promoted Catalytic Hydrodefluorination of Fluorocarbons by Ruthenium in Aqueous Media

“…Control experiments show that no reaction is observed in the absence of 1 or H 2 . The observation that 3 is a major product (1 h) that is further hydrogenated with increasing [4] at longer times and the presence of minor amounts of 5 is consistent with a fast initial HDF followed by hydrogenation of the aromatic ring to ultimately generate 4 (Scheme 2). Additionally, the hydrogenated analogue of 2 with the C À F bond intact is never observed.…”

“…[d] [%] Product [d] [Yield (%)] 1 [b] C 6 F 5 COOK > 99 Cy-COOK (92) [e] 2 F 3 CA C H T U N G T R E N N U N G (CH 2 ) 2 COOK 85 C 3 H 7 COOK (73) 3 [c] CH 2 F 2 26 CH 4 (22) 4 C 3 F 7 COOK trace -5 [c] CF 4 trace CH 4 (< 1)…”

“…Although significantly more forcing conditions are required (200 8C, 5 h, 20 mol% Ru), partial conversion of the fluoroalkyl substrates F 3 CA C H T U N G T R E N N U N G (CH 2 ) 2 COOK and CH 2 F 2 was also seen ( Table 2, entries 2 and 3). Unfortunately, only trace levels of conversion were observed for the perfluorinated alkyl substrates F 3 CA C H T U N G T R E N N U N G (CF 2 ) 2 COOK and CF 4 (Table 2, entries 4 and 5); however, 19 F NMR analysis indicated the presence of F À dissolved in the aqueous KOH solvent, which was not observed during control reactions in the absence of 6. While further study and optimization will be required, these initial results are suggestive that Ru-catalyzed HDF in basic media may be feasible for aliphatic perfluorinated compounds, which is comparatively rare in the literature.…”

“…Despite the inherent strength of the C À F bond, high bond polarity makes it susceptible to heterolytic cleavage; and it is now generally accepted that kinetic factors (rather than thermodynamic) are the primary origin of its stability. [4] Indeed, most reported systems for catalytic C À F defluorination exploit the polarity of the C À F bond by utilizing either highly electrophilic [5] or nucleophilic species to induce reactivity. One area of study to address this challenge, which has received increasing focus in the past decades, is the use transition metal catalysts to affect C À F bond conversion.…”

Palladium-Mediated Organofluorine Chemistry

“…This has been shown by quantum calculations on Os and Rh complexes, where the reluctance to C-F activation has a kinetic origin. 304 In any case, the thermodynamics can be made more favorable by controlling the fate of the F atom after cleavage and providing an additional reaction acting as a thermodynamic sink. 224 Thus, LiI has been used as additive to ease R-F activation by forming a [PdRIL 2 ] and LiF which has a larger lattice energy than LiI.…”