Hydroboration of Carbon Dioxide Using Ambiphilic Phosphine–Borane Catalysts: On the Role of the Formaldehyde Adduct
Résumé
Ambiphilic phosphine–borane derivatives 1-B(OR)2-2-PR′2–C6H4 (R′ = Ph (1), iPr (2); (OR)2 = (OMe)2 (1a, 2a); catechol (1b, 2b) pinacol (1c, 2c), −OCH2C(CH3)2CH2O– (1d)) were tested as catalysts for the hydroboration of CO2 using HBcat or BH3·SMe2 to generate methoxyboranes. It was shown that the most active species were the catechol derivatives 1b and 2b. In the presence of HBcat, without CO2, ambiphilic species 1a, 1c, and 1d were shown to transform to 1b, whereas 2a and 2c were shown to transform to 2b. The formaldehyde adducts 1b·CH2O and 2b·CH2O are postulated to be the active catalysts in the reduction of CO2 rather than being simple resting states. Isotope labeling experiments and density functional theory (DFT) studies show that once the formaldehyde adduct is generated, the CH2O moiety remains on the ambiphilic system through catalysis. Species 2b·CH2O was shown to exhibit turnover frequencies for the CO2 reduction using BH3·SMe2 up to 228 h–1 at ambient temperature and up to 873 h–1 at 70 °C, mirroring the catalytic activity of 1b.