Experimental and Computational Studies of CO2 Addition Reactions Relevant to Copper-Catalyzed Boracarboxylation of Vinyl Arenes: Evidence for a Phosphine-Promoted Mechanism

An experimental and computational mechanistic investigation of the key carboxylation step in copper(I)-catalyzed boracarboxylation of vinyl arenes is presented here. Catalytically relevant intermediates, including a series of CuI-spiroboralactonate complexes, with electronically differentiated vinyl arenes and stabilized by the NHC ligand IPr (IPr = 1,3-Bis(2,6-di-isopropylphenyl)-4,5-dihydroimidazol-2-ylidine), were isolated and characterized. In situ 1H NMR timecourse studies and subsequent Hammett analysis (p) of carbon dioxide addition to (β-borylbenzyl)copper(I) complexes (benzyl = CH2Arp-X) revealed a linear correlation with a negative rho (ρ) value. Density functional theory (DFT) calculations support a direct CO2 insertion as the primary mechanism for electron-rich benzyl-copper carboxylation. Kinetically sluggish carboxylation of electron-poor trifluoromethyl-substituted benzyl-copper complex (benzyl = CH2Arp-CF3) was accelerated upon addition of exogenous PPh3. Conversely, the additive inhibited reactions of electron-rich tert-butyl-substituted benzyl-copper complex (benzyl = CH2Arp-tBu). These kinetic observations implied that a second carboxylation pathway was likely operative. DFT analysis demonstrated that prior binding of the electron-rich phosphine additive at (β-borylbenzyl)copper(I) yields a meta-stable intermediate that precedes an SE-carboxylation mechanism, which is kinetically favorable for electron-deficient benzyl-copper species and circumvents the kinetically challenging direct insertion mechanism. The mechanistic picture that emerges from this complementary experimental/computational study highlights the kinetic complexities and multiple pathways involved in copper-based carboxylation chemistry.