Hydroboration of Carbon Dioxide Using Ambiphilic Phosphine–Borane Catalysts: On the Role of the Formaldehyde Adduct

Ambiphilic phosphine–borane derivatives 1-B(OR) 2 -2-PR′ 2 –C 6 H 4 (R′ = Ph ( 1 ), i Pr ( 2 ); (OR) 2 = (OMe) 2 ( 1a, 2a ); catechol ( 1b, 2b ) pinacol ( 1c, 2c ), −OCH 2 C(CH 3 ) 2 CH 2 O– ( 1d )) were tested as catalysts for the hydroboration of CO 2 using HBcat or BH 3 ·SMe 2 to generate methoxyboranes. It was shown that the most active species were the catechol derivatives 1b and 2b . In the presence of HBcat, without CO 2, 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·CH 2 O and 2b·CH 2 O are postulated to be the active catalysts in the reduction of CO 2 rather than being simple resting states. Isotope labeling experiments and density functional theory (DFT) studies show that once the formaldehyde adduct is generated, the CH 2 O moiety remains on the ambiphilic system through catalysis. Species 2b·CH 2 O was shown to exhibit turnover frequencies for the CO 2 reduction using BH 3 ·SMe 2 up to 228 h –1 at ambient temperature and up to 873 h –1 at 70 °C, mirroring the catalytic activity of 1b .