Assessing the pKa-Dependent Activity of Hydroxyl Hydrogen Bond Donors in the Organocatalyzed Cycloaddition of Carbon Dioxide to Epoxides: Experimental and Theoretical Study

The development of hydrogen bond donors (HBDs) as catalytic moieties in the cycloaddition of carbon dioxide to epoxides is an active field of research to access efficient, inexpensive and sustainable metal-free systems for the conversion of carbon dioxide to useful chemicals. Thus far, no systematic attempt to correlate the activity of a diverse selection of HBDs to their physico-chemical properties has been undertaken. In this work, we investigate factors influencing the catalytic activity of hydroxyl HBDs from different chemical families under ambient conditions by considering the HBDs Bronsted acidity (expressed as pK(a)), the number of hydroxyls and structural aspects. As an effect, this study highlights the crucial role of the hydroxyl protons' Bronsted acidity in determining the catalytic activity of the HBDs, identifies an ideal range for the hydroxyl HBDs proton acidity (9 a <11) and leads to a revaluation of phenol and to the discovery of a simple ascorbic acid derivative as efficient HBDs for the title cycloaddition reaction. Density functional theory (DFT) calculations show mild reactions barriers for the reaction catalysed by phenol and suggest the occurrence of aggregation between molecules of ascorbic acid as a further factor affecting catalytic activity.