Green Chemistry Meets Asymmetric Organocatalysis: A Critical Overview on Catalysts Synthesis

Abstract

Can green chemistry be the right reading key to let organocatalyst design take a step forward towards sustainable catalysis? What if the intriguing chemistry promoted by more engineered organocatalysts was carried on by using renewable and naturally occurring molecular scaffolds, or at least synthetic catalysts more respectful towards the principles of green chemistry? Within the frame of these questions, this Review will tackle the most commonly occurring organic chiral catalysts from the perspective of their synthesis rather than their employment in chemical methodologies or processes. A classification of the catalyst scaffolds based on their E factor will be provided, and the global E factor (E G factor) will be proposed as a new green chemistry metric to consider, also, the synthetic route to the catalyst within a given organocatalytic process.

Turning Renewable Feedstocks into a Valuable and Efficient Chiral Phosphate Salt Catalyst

“…The E factor of this catalyst has been calculated, and the resulting value of 741 is considerably lower than those of axially chiral phosphates. [24] Some attempts of greener phosphorylation with Stawinski reagent [25] (see SI for further details) resulted in a lower conversion of the substrate 5 a and in problematic chromatographic purification (Scheme 4).…”

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α‐Halo Acyl Halide Synthon in One Pot

“…It is interesting to note that eQNU and similar Cinchona alkaloids derived organocatalysts have been recently classified according to their E‐factor amongst the greenest organocatalysts to prepare. [29] …”

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α‐Halo Acyl Halide Synthon in One Pot

“…To our delight, heterocycle 3l was always isolated in high yields and 99 % ee,h ighlighting complete recovering of eQNU after chromatography and aremarkable robustness of this organocatalyst toward repetitive oxidative conditions up to four runs.Itis interesting to note that eQNU and similar Cinchona alkaloids derived organocatalysts have been recently classified according to their E-factor amongst the greenest organocatalysts to prepare. [29] Despite the importance of chiral non racemic tetrahydroquinoxalines as bioactive compounds, [11] to the best of our knowledge,their synthesis via reduction of enantioenriched 3aryl-substituted dihydroquinoxalinones has not been reported. Indeed, the most recurrent approach involves metalcatalyzed enantioselective hydrogenation or transfer hydrogenation of quinoxalines.…”