| Autor: |
Baughman, Notashia N., Akhmedov, Novruz G., Petersen, Jeffrey L., Popp, Brian V. |
| Zdroj: |
Organometallics; January 2021, Vol. 40 Issue: 1 p23-37, 15p |
| Abstrakt: |
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 N-heterocyclic carbene (NHC) ligand IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine), were isolated and characterized. In situ1H NMR time course studies and subsequent Hammett analysis (σp) 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 CO2insertion as the primary mechanism for electron-rich benzyl–copper carboxylation. Kinetically sluggish carboxylation of the electron-poor trifluoromethyl-substituted benzyl–copper complex (benzyl = CH2Arp-CF3) was accelerated upon the addition of exogenous triphenylphosphine (PPh3). Conversely, the additive inhibited the reactions of the 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 metastable 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. |
| Databáze: |
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