The Bridged Binding Mode as a New, Versatile Template for the Selective Activation of Carbon−Fluorine Bonds in Fluoroolefins: Activation of Trifluoroethylene

Abstract

We report the selective activation of carbon-fluorine bonds in trifluoroethylene using the diiridium complex [Ir(2)(CH(3))(CO)(2)(dppm)(2)][OTf] (1). Coordination of trifluoroethylene in a bridging position between the two metals in 1 results in facile fluoride ion loss in three different ways. Attack by strong fluorophiles such as Me(3)SiOTf and HOTf results in F(-) removal from one of the geminal fluorines to give the cis-difluorovinyl-bridged product [Ir(2)(CH(3))(OTf)(CO)(2)(μ-κ(1):η(2)-C(F)═CFH)(dppm)(2)][OTf]. A second activation can also be accomplished by addition of excess Me(3)SiOTf to give the fluorovinylidene-bridged product [Ir(2)(CH(3))(OTf)(CO)(2)(μ-C(2)FH)(dppm)(2)][OTf](2). Interestingly, activation of the trifluoroethylene-bridged precursor by water also occurs, yielding [Ir(2)(CH(3))(CO)(2)(κ(1)-C(H)═CF(2))(μ-OH)(dppm)(2)][OTf], in which the lone vicinal fluorine is removed, leaving a geminal arrangement of fluorines in the product. A [1,2]-fluoride shift can also be induced in the trifluoroethylene-bridged precursor upon the addition of CO to give the 2,2,2-trifluoroethylidene-bridged product [Ir(2)(CH(3))(CO)(3)(μ-C(H)CF(3))(dppm)(2)][CF(3)SO(3)]. Addition of hydrogen to the cis-difluorovinyl-bridged product results in the quantitative elimination of cis-difluoroethylene, while its reaction with CO yields a mixture of cis-difluoropropene and 2,3-difluoropropene by reductive elimination of the methyl and difluorovinyl groups with an accompanying isomerization in the case of the second product. Finally, protonation of the 2,2,2-trifluoroethylidene-bridged product liberates 1,1,1-trifluoroethane, in which one hydrogen (H(+)) is from the acid while the other hydrogen (H(-)) is derived from activation of the methyl group.

Facile Carbon–Fluorine Bond Activation and Subsequent Functionalisation of 1,1‐Difluoroethylene and Tetrafluoroethylene Promoted by Adjacent Metal Centres

“…The 31 P{ 1 H} NMR spectrum of 3 at À10 8C contains two resonances (characteristic of an AA'BB' spin system) at d = 13.0 and À7.8 ppm, showing mutual coupling (J PP = 24.7 Hz). This pattern is indicative of two chemical environments for the pairs of 31 P nuclei, and effectively rules out the bridged k 1 :h 2 binding mode for the fluorovinyl unit, which should give rise to four 31 P resonances, by virtue of both the "top/bottom" and "left/right" asymmetry that would arise with this bridging arrangement (for an example, see reference [16]). The two 1 H resonances observed for the methylene groups in the dppm ligand (at d = 4.51 and 3.54 ppm) also argue against a bridging C 2 F 3 group, which would be expected to give rise to four chemically inequivalent methylene protons.…”

“…(3), accompanied by the formation of trimethylsilylfluoride. In the original report on this product, [9d] we assumed a bridging k 1 :h 2 coordination mode for the trifluorovinyl group, as has been established for the cis-difluorovinyl group in [ [16] However, further NMR studies suggest that in 3 the trifluorovinyl group is terminally bound to one metal, as shown in Scheme 2.…”

“…(A) and subsequent functionalisation (from ref. [16]). Only the metals and the ligands involved in the reactions are shown.…”

“…Trifluoroethylene was shown to undergo a series of stereo-and regioselective C À F bond activations while bridging two iridium centres, the selectivity of which could be varied by changing the conditions. [16] This reactivity is summarised in Scheme 1, in which the trifluoroethylene-bridged precursor…”

“…CÀF bond activation in A could also be accomplished by water, providing a 2,2-difluorovinyl product, and was even possible under a CO atmosphere, resulting in isomerisation of the trifluoroethylene into a 2,2,2-trifluoroethylidene moiety, which gave 1,1,1-trifluoroethane on protonation. [16] Herein, we report facile C À F bond activation in tetrafluoroethylene and 1,1-difluoroethylene by a number of routes, along with methods for functionalising the resulting fluorocarbyl fragments and compare this chemistry to that of trifluoroethylene. [16] Although a preliminary report of some of this work has appeared, [9d] subsequent studies, reported herein, have allowed us to detect important differences in the reactivity of these three fluoroolefins, permitting a better understanding of the factors influencing the cooperative CÀF activation of fluoroolefins by adjacent metal centres.…”