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.
Simple quinine as an organocatalyst mediates the addition of various naphthols to halogenated quinones to afford non-C2 -symmetrical, axially chiral biaryl products, which are promising compounds as chiral ligands and organocatalysts. The rotational barrier required to have two distinct atropisomers has been evaluated in the products generated from the addition of naphthols to various quinones by means of DFT calculations and HPLC. The use of halogenated quinones as reagents was necessary to have configurationally stable enantiomeric products which can be obtained in good yield and stereoselectivity. These compounds have also been prepared in gram quantities and recrystallized to near enantiopurity.
An outline of the advantages, in terms of sustainability,
of Deep
Eutectic Solvents (DESs) is provided, by analyzing some of the most
popular DESs, obtained by the combination of choline chloride, as
a hydrogen bond acceptor, and six hydrogen bond donors. The analysis
is articulated into four main issues related to sustainability, which
are recurrently mentioned in the literature, but are often taken for
granted without any further critical elaboration, as the prominent
green features of DESs: their low toxicity, good biodegradability,
renewable sourcing, and low cost. This contribution is intended to
provide a more tangible, evidence-based evaluation of the actual green
credentials of the considered DESs, to reinforce or question their
supposed sustainability, also in mutual comparison with one another.
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