Synthesis of Symmetric/Unsymmetric DPPE Derivatives via the Radical Difunctionalization of Ethylene: A Theory-Driven Approach

Abstract

1,2-Bis(diphenylphosphino)ethane (DPPE) and its synthetic analogues are important structural motifs in organic synthesis, particularly as novel diphosphine ligands with a C2-alkyl-linker chain. Since DPPE is known to bind to many metal centers in a bidentate fashion to stabilize the corresponding metal complex via the chelation effect originating from its entropic advantage over monodentate ligands, it is often used in transition-metal-catalyzed transformations, and many unique reactions have been developed using this bidentate ligand. Symmetric DPPE derivatives (Ar12P−CH2−CH2−PAr12) are well-known and readily prepared, but electronically and sterically unsymmetric DPPE (Ar12P−CH2−CH2−PAr22; Ar1 ≠ Ar2) ligands have been less explored, mostly due to the difficulties associated with their preparation. In this study, we disclose a novel synthetic method for not only symmetric but also unsymmetric DPPE derivatives using two phosphine-centered radicals and gaseous ethylene, the latter of which is a useful and fundamental C2 unit that is ubiquitous in industrial processes. The thus obtained unsymmetric DPPE ligands can coordinate to several transition-metal salts in a bidentate fashion to form the corresponding complexes, one of which exhibits distinctly different characteristics than the corresponding symmetric DPPE–metal complex. A feasible radical reaction pathway toward DPPE was successfully designed using the artificial force induced method (AFIR) method, a DFT-based automated reaction path search tool.