Metallomimetic C–F Activation Catalysis by Simple Phosphines

Abstract

Delivering metallomimetic reactivity from simple pblock 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.

Ligand Centered Reactivity of a Transition Metal Bound Geometrically Constrained Phosphine

“…[6][7][8] These studies are further supported by a recent report describing hydrodefluorination and aminodefluorination of a range of fluoroarenes using simple phosphines. [9] Phosphorus-based reactivity of metal-phosphine complexes can afford metallo-phosphorane (or metal-phosphoranide) species, which are believed to play a role as reaction intermediates in several catalytic processes (Figure 1). [10][11][12][13][14] In recent years, the effect of geometric constraint at phosphorus has been explored in order to enable the reactivity of phosphorus compounds in reactions considered exclusive to transition metals.…”

Surface-Assisted Selective Air Oxidation of Phosphines Adsorbed on Activated Carbon

“…Phosphines are ubiquitous in chemistry due to their ability to coordinate to metal centers. Most recently, it was discovered that simple phosphines can even be applied as catalysts . In contrast, phosphine oxides were regarded more as an unwanted impurity in phosphorus chemistry for a long time.…”

“…Most recently, it was discovered that simple phosphines can even be applied as catalysts. 18 In contrast, phosphine oxides were regarded more as an unwanted impurity in phosphorus chemistry for a long time. However, nowadays phosphine oxides enjoy growing interest.…”

Transition Metal-Mimicking Relay Catalysis by a Low-Valent Phosphorus Compound

“…In this study, we aimed to investigate the feasibility of a P­(I)/P­(III) redox cycle in NHC-stabilized phosphinidenes. While existing reports primarily concentrate on the P­(III)/P­(V) cycle, − no instance of a P­(I)/P­(III) redox cycle in catalysis has been identified to our knowledge. Developing such a cycle poses considerable challenges, adding to its scientific intrigue.…”