Iridium‐Catalyzed Selective Cross‐Coupling of Ethylene Glycol and Methanol to Lactic Acid

Abstract

An atom‐economic approach that has an unprecedented high selectivity for the synthesis of lactic acid (LA) based on a catalytic dehydrogenative cross‐coupling by using inexpensive bulk ethylene glycol and methanol is described. This method relies on the synthesis and utilization of a novel iridium catalyst bearing three N‐heterocyclic carbenes derived from 1,3‐dimethylbenzimidazolium salts, and exhibits outstanding activity in the production of LA [turnover frequency (TOF) up to 3660 h−1] owing to an elegant metal–ligand cooperation.

Photocatalytic Biorefinery to Lactic Acid: A Carbon Nitride Framework with O Atoms Replacing the Graphitic N Linkers Shows Fast Migration/Separation of Charge

“…For example, lactic acid and its salts (e. g., calcium, sodium, and potassium) have been considered as safe food additives by the United States Food and Drug Administration [21,25,26] . The projected market demand for lactic acid is expected to reach 4×10 5 tonnes per annum by 2022, primarily because of its role in production of polylactic acid [22,24,27] . According to estimations, over 90 % of lactic acid is produced by carbohydrate fermentation [28,29] .…”

“…Lactic acid, as an important biomass-derived carboxylic acid, is a valuable chemical with extensive applications in food, pharmaceuticals, cosmetics, and detergents, as well as in polymers synthesis (e. g., polylactic acid and acrylic polymers). [21][22][23][24] For example, lactic acid and its salts (e. g., calcium, sodium, and potassium) have been considered as safe food additives by the United States Food and Drug Administration. [21,25,26] The projected market demand for lactic acid is expected to reach 4 × 10 5 tonnes per annum by 2022, primarily because of its role in production of polylactic acid.…”

Recent Advances in Benzimidazole Based NHC‐Metal Complex Catalysed Cross‐Coupling Reactions**

“…Tu and coworkers utilized a tris-NHC-Ir complex derived from 1,3-dimethylbenzimidazolium salts for the atom economic, highly selective dehydrogenative cross-coupling of ethylene glycol and methanol to produce lactic acid (Scheme 44). [78] The catalytic performance of Ir complexes were found to increase with the number of NHC and the π ring size due to the enhancement of the steric bulkiness and the electron density leading to facile oxidation of Ir(I) to Ir(III) with the simultaneous dehydrogenation of MeOH and ethylene glycol. Due to the metal ligand cooperativity the catalytic activity followed the order mono-NHC-Ir < bis-NHC-Ir < tris-NHC-Ir.…”

Homogeneous Reforming of Aqueous Ethylene Glycol to Glycolic Acid and Pure Hydrogen Catalyzed by Pincer‐Ruthenium Complexes Capable of Metal–Ligand Cooperation

“…Reforming of ethylene glycol to glycolic acid homogeneously catalyzed by an iridium complex (4 mol %) was reported recently, although the catalytic mechanism was not studied [17] . Very recently, homogeneous iridium‐catalyzed dehydrogenative cross‐coupling of ethylene glycol and methanol to lactic acid was also disclosed by Tu, Xu and co‐workers [18] . The scarcity of reports on catalytic reforming of aqueous ethylene glycol to glycolic acid, despite its significant promise, demonstrates the need for new catalytic systems as well as a deeper understanding of the reaction mechanism.…”