Lewis acid catalysis and Green oxidations: sequential tandem oxidation processes induced by Mn-hyperaccumulating plants

Abstract

Among the phytotechnologies used for the reclamation of degraded mining sites, phytoextraction aims to diminish the concentration of polluting elements in contaminated soils. However, the biomass resulting from the phytoextraction processes (highly enriched in polluting elements) is too often considered as a problematic waste. The manganese-enriched biomass derived from native Mn-hyperaccumulating plants of New Caledonia was presented here as a valuable source of metallic elements of high interest in chemical catalysis. The preparation of the catalyst Eco-Mn1 and reagent Eco-Mn2 derived from Grevillea exul exul and Grevillea exul rubiginosa was investigated. Their unusual polymetallic compositions allowed to explore new reactivity of low oxidative state of manganese-Mn(II) for Eco-Mn1 and Mn(IV) for Eco-Mn2. Eco-Mn1 was used as a Lewis acid to catalyze the acetalization/elimination of aldehydes into enol ethers with high yields; a new green and stereoselective synthesis of (-)-isopulegol via the carbonyl-ene cyclization of (+)-citronellal was also performed with Eco-Mn1. Eco-Mn2 was used as a mild oxidative reagent and controlled the oxidation of aliphatic alcohols into aldehydes with quantitative yields. Oxidative cleavage was interestingly noticed when Eco-Mn2 was used in the presence of a polyol. Eco-Mn2 allowed direct oxidative iodination of ketones without using iodine, which is strongly discouraged by new environmental legislations. Finally, the combination of the properties in the Eco-Mn catalysts and reagents gave them an unprecedented potential to perform sequential tandem oxidation processes through new green syntheses of p-cymene from (-)-isopulegol and (+)-citronellal; and a new green synthesis of functionalized pyridines by in situ oxidation of 1,4-dihydropyridines.

Eco‐Mn Ecocatalysts: Toolbox for Sustainable and Green Lewis Acid Catalysis and Oxidation Reactions

“…After acidification, Fe 2 O 3 is also converted to soluble hydrated iron chloride ions. The final mixture is composed of MnO 2 , Mn(II) and Fe(III) chlorides, which can all act as Lewis acids (Scheme ) …”

“…Eco-Mn LA catalyzes the carbonyl-ene reaction, with 95 % yield and 90 % selectivity in the desired (À )isopulegol, with only 2 mol% of Mn in a very short reaction time (0.5 h) (Scheme 8). [34] Alkyl enol ethers are of great synthetic interests. They are very reactive towards electrophilic reagents, in particular carbonyl groups, and are used for the formation of CÀ C bonds in organic synthesis.…”

“…Eco-Mn Ox catalysts [34] Tandem sequential oxidation can be described as a multi-step process in which the substrate is transformed by two mechanistically distinct processes, one of which is an oxidation through the action of species present at the beginning of the reaction. [85] Tandem processes reduce the number of operations, by allowing the preparation and in-situ reaction of reactive intermediates, thus avoiding their isolation and often preventing safety problems.…”

“…The final mixture is composed of MnO 2 , Mn(II) and Fe(III) chlorides, which can all act as Lewis acids (Scheme 4). [34] -Eco-Mn OX2 : Eco-Mn LA is oxidized by hydrogen peroxide in the presence of a large excess of sodium hydroxide. The precipitate formed is composed of a mixture of Mn oxides: Na 0.…”

Ullmann reaction through ecocatalysis: insights from bioresource and synthetic potential

“…Moreover, TEs are some of the 2 most harmful pollutants. Ecocatalysts constitute an entire new generation of Lewis acid catalysts 9-15 , very efficient green oxidative 16 , reductive 17 , and sustainable coupling agents in organic synthesis 18 . For instance, the devastating effects of mining in the southern province of the Democratic Republic of the Congo characterize one of the critical patterns implicated in global environmental change.…”