Mechanochemical Assisted Chemoselective and Stereoselective Hydrogen-Bonding Catalyzed Addition of Dithiomalonates to Enones

Dithiomalonates proved to be active nucleophiles in the stereoselective additions to chalcones, dienones, and en-ynones affording the desired Michael adducts with moderate to good yield and enantioselectivities up to 98%. In contrast, the analogous dibenzyl malonate remained inactive. Bifunctional Cinchona squaramides secured the effective chirality transfer and the selectivity towards Michael adducts of various bisthiomalonates following the soft enolization approach. The thioester's nature impacted the reactivity and stability of the reactants or products. The reactions performed in solution led to the product, but it required prolonged time along with byproducts formation such as sulfa-Michael adducts, thus limiting the applicability of more reactive dithioesters. On the contrary, the reactions performed under solvent-free, ball milling conditions furnished adducts in substantially less time, with negligible or no byproduct generation. Therefore, the mechanochemical approach revealed to be an effective tool for supporting the hardly effective reactions under standard solution conditions. The conducted comprehensive KS-DFT studies are in line with the experimental observations shedding more light on the intricate active nucleophile formation at a molecular level and different chemical reaction pathways, as well as indicating the crucial transitions state governing the observed stereoselectivities. image

Automated summary

Dithiomalonates have been shown to be active nucleophiles in stereoselective additions, with Cinchona squaramides facilitating chirality transfer and mechanochemical approach proving to be effective.