Metallomimetic C-F activation catalysis by simple phosphines

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 with no need for complex ligand architectures or external oxidising/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realised 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 pseudo-transmetallation 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 re-form the phosphine catalyst. This behaviour is analogous to many classical transition-metal catalysed reactions and so is a rare example of both functional and mechanistically metallomimetic behaviour 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.