Ascorbic Acid as a Bifunctional Hydrogen Bond Donor for the Synthesis of Cyclic Carbonates from CO₂ under Ambient Conditions

Abstract

Readily available ascorbic acid was discovered as an environmentally benign hydrogen bond donor (HBD) for the synthesis of cyclic organic carbonates from CO2 and epoxides in the presence of nucleophilic co-catalysts. The ascorbic acid/TBAI (TBAI: tetrabutylammonium iodide) binary system could be applied for the cycloaddition of CO2 to various epoxides under ambient or mild conditions. DFT calculations and catalysis experiments revealed an intriguing bifunctional mechanism in the step of CO2 insertion involving different hydroxyl moieties (enediol, ethyldiol) of the ascorbic acid scaffold.

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

“…The upper reaction barrier for phenol (20.2 kcal/mol) is comparable to that calculated by Maeda et al for a calix [4]pyrrole/TBAI organocatalytic system (21.1 kcal/mol) [39] and about 6-10 kcal/mol lower than for other recently studied organocatalysts. [5] These observations suggest that, in case of analogous compounds with the same Brønsted acidity (3 and ascorbic acid, 4 and 5), also structural features, such as the availability of additional H-bonding moieties, might have an impact on catalytic activity. This difference can be explained by the availability of the additional hydrogen-bonding ethyldiol protons in the side chain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 of 5 that contribute to the stabilization of the reaction intermediates ( Figures S23 and S24 in the SI).…”

“…[1] Organocatalysts are often readily accessible renewable compounds such as aminoacids, [2] peptides, [3] sugars [4] and vitamins [5] or their derivatives. [1] Organocatalysts are often readily accessible renewable compounds such as aminoacids, [2] peptides, [3] sugars [4] and vitamins [5] or their derivatives.…”

“…Among several families of organocatalysts, HBDs occupy a prominent position for their ability to activate the reaction substrates by a well-defined array of H-bonds. [11] The cycloaddition reaction can take place under atmospheric conditions, or even from flue gas, [12] when applying several types of catalytic systems including organocatalysts, [5,13] metal-organic complexes [14] coordination compounds [15] and porous organic polymers [16] thus resulting in a potentially sustainable, low-energy intensive process for CO 2 fixation. [8] It leads to industrially relevant cyclic organic carbonates [9] that find increasing application as chemical intermediates [10] and green solvents.…”

“…[7c,17] In this context, the activity of organocatalysts in the cycloaddition of CO 2 to epoxides is generally lower than that of metal-based systems. [5,19] Active HBDs for the cycloaddition of CO 2 to epoxides include several classes of compounds that have been careful reviewed. [7e-g,18] HBDs are known to activate the epoxide substrate by the formation of H-bonds facilitating its ring-opening by the attack of the nucleophilic catalytic component.…”

“…[7c] Neverthe-less, given several advantageous aspects of organocatalysis in terms of costs and sustainability, [1] the development of more active organocatalytic systems for the conversion of CO 2 to cyclic carbonates is highly sought after. [7e-g] Among them, hydroxyl HBDs are likely to represent the broadest family of catalytically active species and include functionalized monoalcohols, [20] glycols, [21] polyalcohols, [22] fluorinated alcohols, [23] hydroxy-functionalized N-heterocycles, [24] silanediols, [25] boronic acids, [26] carboxylic acids, [27] ascorbic acid [5] and several compounds containing phenolic [28] or polyhydroxyphenolic moieties. [5,19] Active HBDs for the cycloaddition of CO 2 to epoxides include several classes of compounds that have been careful reviewed.…”