Controlled monodefluorination and alkylation of C(sp3)-F bonds by lanthanide photocatalysts: importance of metal-ligand cooperativity

Abstract

The controlled functionalization of a single fluorine in a CF₃ group is difficult and rare. Photochemical C-F bond functionalization of the sp³-C-H bond in trifluorotoluene, PhCF₃, is achieved using catalysts made from earth-abundant lanthanides, (Cp^Me4)₂Ln(2-O-3,5- ^t Bu₂-C₆H₂)(1-C{N(CH)₂N(ⁱPr)}) (Ln = La, Ce, Nd and Sm, Cp^Me4 = C₅Me₄H). The Ce complex is the most effective at mediating hydrodefluorination and defluoroalkylative coupling of PhCF₃ with alkenes; addition of magnesium dialkyls enables catalytic C-F bond cleavage and C-C bond formation by all the complexes. Mechanistic experiments confirm the essential role of the Lewis acidic metal and support an inner-sphere mechanism of C-F activation. Computational studies agree that coordination of the C-F substrate is essential for C-F bond cleavage. The unexpected catalytic activity for all members is made possible by the light-absorbing ability of the redox non-innocent ligands. The results described herein underscore the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.

Fig. 1. Previous examples of photocatalytic C–X (X = halide) bond cleavage, and this work.

Scheme 1. Syntheses of the complexes examined for C–F bond functionalization in this work (upper) and the solid-state structure of 1-Ce, H omitted for clarity, alongside the ligand variations studied for 1-Ce (lower).

Fig. 2. The absorption spectra of 1-Ce, 1-CeMes, 1-La and proligand HL. The absorption spectrum for the tert-Bu substituted analogue 1-Ce^tBu, is also shown overlayed in the ESI. The computed spectrum of 1-Ce using TD-DFT is included in the ESI.

Fig. 3. Depictions of the TD-DFT-calculated SOMO (left) and LUMO+1 (right, upper) and LUMO (right, lower) orbitals of 1-Ce.

Fig. 4. Depictions of the TD-DFT-calculated HOMO (left) and LUMO+1 (right) for 1-La.

Fig. 5. Cyclic voltammograms of 1-Ce (purple), 2-Ce (dark blue), 1-La (light blue), 2-La (green) and HL (orange) in THF with 0.085 M [nBu₄N][BPh₄] supporting electrolyte. [Analyte] = ca. 5 mM; ν = 0.5 V s⁻¹.

Scheme 2. General reaction scheme for C–F activation and hydrodefluorination by Ln complexes 1-Ln to 3-Ln and cleavage by the Ce photocatalysts 1-Ce–6-Ce with PhCF₃ (X = the monoanionic ligand Cp^Me4, Cl, OAr, OTf, Bn) n = 1–3.

Scheme 3. Photocatalytic defluoroalkylation of trifluorobenzene with 1–6-Ce and Mg(Bn)₂(THF)₂.

Fig. 6. Computed C–F activation pathway for the reaction of 1-Ce with PhCF₃, including TS1. Those for 1-La and 7-Mg are in the ESI.

Scheme 4. Pathway for the abstraction of fluoride (rather than F˙) to generate Ce(iv)–F and ArCF₂˙.

Scheme 5. The C–F bond activation of 1-CF₃-2-(C₃H₅)C₆H₄ mediated by 1-Ce.

Scheme 6. The proposed pathways of oxidation of 1-Ln that lead to turnover or catalyst degradation for Ln that have no accessible +IV oxidation state, i.e. Ln = La, Nd, and Sm.

Fig. 7. Computed C–C coupling pathway at MgBn₂(THF)₂.

Fig. 8. Computed C–C coupling pathway at 7-Mg.

Scheme 7. Proposed mechanism of photocatalytic C–C coupling.