Quantification of Sophisticated Equilibria in the Reaction Pool and Amplifying Catalytic Cycle of the Soai Reaction

The relative abundance of various species in the reaction pool of the Soai autoamplification reaction was estimated by large-scale density functional theory (DFT) computations involving calculations of the thermodynamic parameters in solution. Detailed conformational analysis of the macrocyclic tetrameric species formed by dimerization of the Zn-O-Zn-O square dimers and of their ZnPr2i adducts revealed the structural diversification of the homo- and heterochiral species. Homochiral tetramers are exclusively formed in a specific brandyglass conformation with almost orthogonal pyrimidinyl rings that is virtually unaffected by the formation of a ZnPr2i adduct. On the other hand, for heterochiral tetramers the stabilities of brandyglass and layer conformations are approximately equal. The three-dimensional (3D) cavity observed in the ZnPr2i adduct of the homochiral brandyglass tetramer forms an ideal chiral pocket for the coordination of the aldehyde followed by perfectly enantioselective alkylation yielding monomeric alcoholate of the same handedness as the tetrameric catalyst. Similar cavity in the heterochiral brandyglass tetramer is significantly less spacious. Moreover, the cavity practically disappears upon the coordination of ZnPr2i, hence the heterochiral tetramers are excluded from the flow of catalysis that leads to the realization of Frank's scheme for chiral amplification.