Rationalizing the diverse reactivity of [1.1.1]propellane through sigma-pi-delocalization

Abstract

<p>[1.1.1]Propellane has gained increased attention due to its utility as a precursor to bicyclo[1.1.1]pentanes (BCPs) – motifs of high value in pharmaceutical and materials research – by addition of nucleophiles, radicals and electrophiles across its inter-bridgehead C–C bond. However, the origin of this broad reactivity profile is not well-understood. Here, we present a comprehensive computational study that attributes the omniphilicity of [1.1.1]propellane to a moldable, delocalized electron density, characterized by the mixing of the inter-bridgehead C–C bonding and antibonding orbitals. Reactions with anions and radicals are facilitated by stabilization of the adducts through sigma-pi-delocalization of electron density over the cage, while reactions with cations involve charge transfer that relieves Pauli repulsion inside the cage. These results provide a unified framework to rationalize propellane reactivity, opening up opportunities for the exploration of new chemistry of [1.1.1]propellane and related strained systems. </p>

Synthesis of All-carbon Disubstituted Bicyclo[1.1.1]pentanes by Iron-Catalyzed Kumada CrossCoupling

“…[19] In keeping with couplings carried out using 'smaller' aryl Grignards with amine additives, we therefore favour a reaction favour a reaction pathway involving single electron transfer from an Fe(I) species such as 4 [16c, 20] to the iodo-BCP (Figure 3b), generating a relatively stable bicyclopentyl radical 5. [21] Reaction with of this species with the LArFe(II) complex / ArMgBr liberates the cross-coupled product and results in catalyst turnover; however, the precise mechanism by which C-C bond formation occurs (e.g. in cage / out of cage) is not apparent.…”