Metallomimetic C-F Activation Catalysis by Simple Phosphines

Abstract

Delivering metallomimetic reactivity from simple p-block compounds is highly desirable in the search to replace expensive, scarce precious metals by cheap and abundant elements in catalysis. This contribution demonstrates that metallomimetic catalysis, involving facile redox cycling between the P(III) and P(V) oxidation states, is possible using only simple, cheap, and readily available trialkylphosphines without the need to enforce unusual geometries at phosphorus or use external oxidizing/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realized with good to very good yields under mild conditions. Experimental and computational mechanistic studies show that the phosphines undergo oxidative addition of the fluoroaromatic substrate via a Meisenheimer-like transition state to form a fluorophosphorane. This undergoes a pseudotransmetalation step with a silane, via initial fluoride transfer from P to Si, to give experimentally observed phosphonium ions. Hydride transfer from a hydridosilicate counterion then leads to a hydridophosphorane, which undergoes reductive elimination of the product to reform the phosphine catalyst. This behavior is analogous to many classical transition-metal-catalyzed reactions and so is a rare example of both functional and mechanistically metallomimetic behavior in catalysis by a main-group element system. Crucially, the reagents used are cheap, readily available commercially, and easy to handle, making these reactions a realistic prospect in a wide range of academic and industrial settings.

Scheme 1. Comparison of Recent Aromatic Hydrodefluorination (HDF) Reactions Promoted or Catalysed by Main Group Compounds Through Redox-Neutral (A) or Redox-Cycling (B) Pathways and the Present Work (C)

Scheme 2. Results of Substrate Scope Studies

Ph₂SiH₂ was used for all HDF reactions and Ph₂Si(Cl)(NR₂) were used for aminodefluorination reactions. Isolated yield after flash column chromatography shown in parentheses. Reactions were stirred at 60 °C for 4 days, followed by the remaining period at 80 °C. Stoichiometric reaction between PⁱPr₃ and 1 in MeCN (other phosphines behave similarly).

Scheme 3. Preparation of Phosphonium Ions Proposed to be Intermediates in the Catalytic Reactions and their Reactivity Toward Silanes in the Presence, and Absence, of a Fluoride Source

Scheme 4. Proposed Catalytic Cycle and Computed Potential Energy Surface (PES) for R = Me

All energies at the PBE0/def2-TZVP//BP86/SV(P) level in MeCN. Relative Gibbs energies (in kJ mol^–1 at 298 K) shown outside brackets and relative enthalpies (in kJ mol^–1 at 298 K) shown in brackets. See Supporting Information for details of solvent and dispersion corrections applied.

Figure 1

Transition states for the addition of PMe₃ to 1 (TS₁₂) and elimination of 2 from phosphorane M6 (TS₆₁). Hydrogen is shown in white, carbon in gray, phosphorus in orange, nitrogen in blue, and fluorine in green. Selected distances (in Å) are shown.