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Demystifying the Soai Reaction
preprintsubmitted on 22.07.2019, 18:40 and posted on 23.07.2019, 15:45 by Soumitra Athavale, Adam Simon, Kendall N. Houk, Scott Denmark
The extraordinary Soai reaction has profoundly impacted chemists’ perspective of chiral symmetry breaking, absolute asymmetric synthesis and its role in the origin of biological homochirality. Herein, we describe the unprecedented observation of asymmetry amplifying autocatalysis in the alkylation of 5-(trimethylsilylethynyl)pyridine-3-carbaldehyde using diisopropylzinc. Kinetic studies with a “Trojan-horse” substrate and spectroscopic analysis of a series of zinc-alkoxides that incorporate specific structural mutations reveal a ‘pyridine-assisted cube escape’. The new cluster functions as a catalyst that activates the ‘floor-to-floor’ bound aldehyde and poises a coordinated diisopropylzinc moiety for alkyl group transfer. Transitionstate models leading to both the homochiral and heterochiral products were validated by density functional theory calculations. Moreover, experimental and computational analysis of the heterochiral complex provides a definitive explanation for the non-linear behavior of this system. Our deconstruction of the Soai system contributes substantially to understanding the mechanism of this transformation that has stood as a longstanding challenge in chemistry.