C−F Activation of Fluorinated Arenes using NHC-Stabilized Nickel(0) Complexes: Selectivity and Mechanistic Investigations

Abstract

The reaction of [Ni2((i)Pr2Im)4(COD)] 1a or [Ni((i)Pr2Im)2(eta(2)-C2H4)] 1b with different fluorinated arenes is reported. These reactions occur with a high chemo- and regioselectivity. In the case of polyfluorinated aromatics of the type C6F5X such as hexafluorobenzene (X = F) octafluorotoluene (X = CF3), trimethyl(pentafluorophenyl)silane (X = SiMe3), or decafluorobiphenyl (X = C6F5) the C-F activation regioselectively takes place at the C-F bond in the para position to the X group to afford the complexes trans-[Ni((i)Pr2Im)2(F)(C6F5)]2, trans-[Ni((i)Pr2Im)2(F)(4-(CF3)C6F4)] 3, trans-[Ni((i)Pr2Im)2(F)(4-(C6F5)C6F4)] 4, and trans-[Ni((i)Pr2Im)2(F)(4-(SiMe3)C6F4)] 5. Complex 5 was structurally characterized by X-ray diffraction. The reaction of 1a with partially fluorinated aromatic substrates C6H(x)F(y) leads to the products of a C-F activation trans-[Ni((i)Pr2Im)2(F)(2-C6FH4)] 7, trans-[Ni((i)Pr2Im)2(F)(3,5-C6F2H3)] 8, trans-[Ni((i)Pr2Im)2(F)(2,3-C6F2H3)] 9a and trans-[Ni((i)Pr2Im)2(F)(2,6-C6F2H3)] 9b, trans-[Ni((i)Pr2Im)2(F)(2,5-C6F2H3)] 10, and trans-[Ni((i)Pr2Im)2(F)(2,3,5,6-C6F4H)] 11. The reaction of 1a with octafluoronaphthalene yields exclusively trans-[Ni((i)Pr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a, the product of an insertion into the C-F bond in the 2-position, whereas for the reaction of 1b with octafluoronaphthalene the two isomers trans-[Ni((i)Pr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a and trans-[Ni((i)Pr2Im)2(F)(2,3,4,5,6,7,8-C10F7)] 6b are formed in a ratio of 11:1. The reaction of 1a or of 1b with pentafluoropyridine at low temperatures affords trans-[Ni((i)Pr2Im)2(F)(4-C5NF4)] 12a as the sole product, whereas the reaction of 1b performed at room temperature leads to the generation of trans-[Ni((i)Pr2Im)2(F)(4-C5NF4)] 12a and trans-[Ni((i)Pr2Im)2(F)(2-C5NF4)] 12b in a ratio of approximately 1:2. The detection of intermediates as well as kinetic studies gives some insight into the mechanistic details for the activation of an aromatic carbon-fluorine bond at the {Ni((i)Pr2Im)2} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni((i)Pr2Im)2(eta(2)-C6F6)] 13 and [Ni((i)Pr2Im)2(eta(2)-C10F8)] 14, have been detected in solution. They convert into the C-F activation products. Complex 14 was structurally characterized by X-ray diffraction. The rates for the loss of 14 at different temperatures for the C-F activation of the coordinated naphthalene are first order and the estimated activation enthalpy Delta H(double dagger) for this process was determined to be Delta H(double dagger) = 116 +/- 8 kJ mol(-1) (Delta S(double dagger) = 37 +/- 25 J K(-1) mol(-1)). Furthermore, density functional theory calculations on the reaction of 1a with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.

Consecutive C–F Bond Activation of Hexafluorobenzene and Decafluorobiphenyl

“…The nickel-carbon and nickel-chlorine distances are unexceptional and in a range found for other nickel-NHC complexes. [4,5] The tetrafluorophenyl bridge is almost perpendicularly orientated to the nickel coordination plane [best plane through Ni, C(1), C(19), C(10), Cl (1) …”

“…[2] We have shown that zero-valent nickel-NHC (N-heterocyclic carbene) complexes are attractive for these kinds of transformations for perfluorinated as well as partially fluorinated aromatic compounds. We have demonstrated that the compound [Ni 2 (iPr 2 Im) 4 (COD)] [1; iPr 2 Im = 1,3-bis(isopropyl)imidazolin-2-ylidene; Figure 1] acts as an excellent source for the very electron-rich [3] bis(carbene)-nickel(0) complex fragment {Ni(iPr 2 Im) 2 } in C-C, C-S [4] , and C-F [5] bond-activation reactions. selectivity.…”

Palladium-Mediated Organofluorine Chemistry

“…246 Oxidative addition of arylfluorides is preceded by arene coordination and this has been shown experimentally for Ni(0) compounds which coordinate octafluoronaphthalene, 305 and fluorobenzenes. 306,307 where a new P-F bond has been formed and a cyclohexyl group is transferred from the phosphorous to the metal (Scheme 102). The reaction rate follows the order Pt > Pd, whereas the opposite (Pd > Pt) is observed in oxidative addition reactions with other non-fluorinated aryl halides.…”

Palladium-Catalyzed Cross-Coupling Reactions of Perfluoro Organic Compounds

“…We speculate that the LiI-promoted C-F bond activation of 12 on Pd(PCy 3 ) 2 would take place to give the cis-oxidative addition product, and the rapid dissociation of a PCy 3 ligand then might occur due to the steric hindrance of the two bulkier PCy 3 ligands. On the basis of theoretical and experimental studies, Radius et al assumed that a related C-F bond activation of 12 on a Ni(NHC) 2 fragment, yielding trans-[Ni(NHC) 2 (C 6 F 5 )(F)], would proceed via the corresponding cis-oxidative addition product [61]. Transmetalation between A and 4 in the presence of LiI would take place to give a biarylpalladium(II) intermediate (B).…”

“…Radius et al reported a coupling reaction of octafluorotoluene (C 7 F 8 ) and decafluorobiphenyl (C 12 F 10 ) with arylboronic acid in the presence of a catalytic amount of NHC-nickel(0) catalyst (where NHC represents N-Heterocyclic carbene) [59]. This group also demonstrated the usefulness of a NHC-nickel(0) complex for the C-F bond activation of hexafluorobenzene (C 6 F 6 ), and the Ni(0)/NHC complex did indeed show catalytic activity toward the hydrodefluorination of C 6 F 6 [60][61][62]. However, an efficient catalytic transformation of C 6 F 6 involving a C-C bond formation is very rare.…”