VIDEO ABSTRACT.

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This article summarizes a series of recent and simple experiments to produce optically active substances from achiral precursors. These symmetry-breaking processes include either autocatalytic crystallization or asymmetric autocatalysis, and provide new insights into the origin of biomolecular homochirality. In addition, support from an extraterrestrial origin of chiral molecules has also come from recent findings.

The archetypal Cu(I)-catalyzed alkyne-azide click cycloaddition (CuAAC) has been explored thoroughly via density functional calculations, modeling copper nuclei with the LANL2DZ basis set and aqueous environments with CPCM solvation. All the mechanistic proposals, ranging from the intermediacy of copper acetylides to π-complexes and multinuclear clusters have been compared. The known features of the CuAAC reaction such as the observed second order kinetics for the Cu(I) species and the marked regioselectivity have been taken into account. The calculated energy barriers point to the intermediacy of copper(i) acetylides with two metal centers, in agreement with the observed kinetics, which exhibit barriers of 10.1 kcal mol(-1) and 13.7 kcal mol(-1) for the 1,4- and 1,5-regiochemistries, respectively, thus accounting for the marked regioselectivity of the copper catalyzed azide-alkyne cycloaddition. The copper acetylide versus π-complexes dilemma has also been experimentally addressed through the click reaction of benzyl azide and isotopically labeled phenylacetylene. The total proton/deuterium exchange in the afforded triazole demonstrates the formation of a copper acetylide intermediate during the transformation.

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