Transition metal-type reactivity at trivalent P-centers: how structural distortions promote bond breaking and forming via two-electron redox reactions and/or phosphorus-ligand cooperativity.

An ongoing goal to use Main Group elements like phosphorus to promote transition metal-type reactivity has received significant attention across the chemistry community. Transition metal (TM) complexes are unique because their frontier orbitals are closed spaced with the proper symmetry to activate small molecules and then incorporate them, in the presence of additional substrates, into a two-electron catalytic cycle, affording higher value chemicals. By using multidentate ligands to structurally distort trivalent P-centers into a strained geometry, a frontier orbital environment similar to a TM can be unlocked. This Perspective will highlight specific examples where geometrically confined P(III)-species directly promote bond activation via two-electron processes like oxidative addition or phosphorus-ligand cooperativity. In some cases, a second substrate can be introduced, and catalytic transformations like (transfer) hydrogenation, hydroboration, hydrodefluorination, hydrosilylation, etc. that normally occur with transition metal complexes can be achieved. Related two-electron functionalization processes with P-heterocycles that hinge on manipulating strain will also be discussed.