Ar-X Coupling Reactions of Cationic Aryl Copper(II) Complexes with Organic Substrates: What Role Do Radicals Play?

Copper(II) aryl species are proposed intermediates in copper catalysed C-X coupling reactions. Here, the formation and reactions of the cationic copper complexes [(phen)Cu(Ar)]⁺ (phen = 1,10-phenanthroline; Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄, p-MeCOC₆H₄) are explored in the gas-phase using electrospray ionisation (ESI) multistage mass spectrometry (MSⁿ) experiments in a linear ion trap mass spectrometer together with Density Functional Theory (DFT) calculations. A survey of three routes to the preparation of [(phen)Cu(Ph)]⁺ showed that transmetalation of [(phen)₃Cu][BPh₄]⁺ and desulfination of [(phen)Cu(O₂SPh)]⁺ both produced the desired complex [(phen)Cu(Ph)]⁺, but decarboxylation of [(phen)Cu(O₂CPh)]⁺ gave a higher yield with fewer competing side reactions, making it the preferred route to the other aryl complexes [(phen)Cu(Ar)]⁺. The aryl complexes [(phen)Cu(Ar)]⁺ undergo protonolysis reactions with formic acid to yield a formate complex [(phen)Cu(O₂CH)]⁺; react with N,N'-diisopropylcarbodiimide to form an adduct, likely by insertion to give [(phen)Cu((NiPr)₂CAr)]⁺; undergo C-S bond-forming reactions with dimethyl disulfide; and react with allyl iodide by Ar-I bond-forming reactions in preference to Ar-allyl bond cross-coupling reactions. The potential reaction mechanisms associated with the different reaction pathways were explored by DFT calculations.