Theoretical Insights into Mechanisms for Copper(I)-CatalyzedC–P Coupling of Diarylphosphines with Aryl Halides: A CombinedSolvent and Ancillary Ligand Effect on the Identity of Active Catalystand Reaction Mechanism.

A theoreticalstudy on reaction mechanisms for copper-catalyzedUllmann-type C–P coupling of diphenylphosphine with aryl halidesis reported herein. The equilibria and consequent relative concentrationsof possible copper species in the reaction solution were evaluatedto determine probable active catalytic species in the presence ofseveral typical ancillary ligands in toluene and DMSO solvents. Subsequently,reactivity of these key copper species with aryl halides were studiedin the context of commonly proposed mechanisms for copper(I)-catalyzedUllmann reactions, including oxidative addition/reductive elimination,σ-bond metathesis, single electron transfer (SET), and halogenatom transfer (HAT) mechanisms. On the basis of these studies, wepropose that for phenanthroline and 1,2-ethylenediamine types of bidentateligands, the active catalyst should be the neutral form LCu(I)-PPh2in nonpolar toluene, while the Cu(PPh2)2–anion should be significant in highly polar DMSO.In contrast, for phosphine type ligands, the active catalytic speciesshould be the neutral LCu(I)-PPh2complexes in both tolueneand DMSO. More interestingly, for both neutral LCu(I)-PPh2and anionic Cu(PPh2)2–complexes,HAT mechanism is proposed to be kinetically the most favorable intoluene. However, in DMSO, the operative reaction mechanism shouldbe influenced by the ancillary ligand used. For phosphine ligand,HAT mechanism is still the most favored with the LCu(I)-PPh2as the active catalyst. For phen and diamine ligands, SET mechanismhas been shown to be the most favored for anionic Cu(PPh2)2–complex, and HAT mechanism is proposedfor neutral LCu(I)-PPh2complexes; both contribute to theactivation of aryl halides in reaction solution. Therefore, a combinedeffect of solvent polarity and ancillary ligand has been recognizedon determining the identity of the active catalytic species and theoperative reaction mechanism for Ullmann-type P-arylationreactions. These results and findings distinguish the reaction mechanismfor Ullmann P-arylation reactions from the othertype of Ullmann reactions, such as N- and O-arylation reactions. [ABSTRACT FROM AUTHOR]