Halogen⋠⋠⋠Halogen and Halogen⋠⋠⋠π Interactions Enabled Reversible Photo-oligomerization of Conjugated Dienones: Visible Light Triggered Single-Crystal-to-Single-Crystal Transformation.

The synthesis of reversible oligomer/polymers is fascinating both from the perspective of the fundamental understanding as well as their applications, ranging from biomedical to self-healing smart materials. On the other hand, the reactions that occur in single-crystal-to-single-crystal (SCSC) fashion offer great details of the structure, geometry and stereochemistry of the product. However, SCSC [2+2] oligomerization is rather difficult and rare. Further, till date there are no reports for a reversible [2+2] oligomerization in SCSC fashion. In this work, four halogen-substituted acrylic dienone molecules were deliberately designed and their ability to participate in [2+2] cycloaddition reaction in solid state was studied under visible light. Despite of having the required alignment of double bonds of dienes in all four crystal structures, they were found to exhibit variable reactivities given the differences in their weak intermolecular interactions such as halogen⋅⋅⋅halogen, halogen⋅⋅⋅π and C-H⋅⋅⋅O interactions. Notably, one of these materials exhibits reversible oligomerization in a SCSC manner.

Visible-Light-Activated Divergent Reactivity of Dienones: Dimerization in Neat Conditions and Regioselective E to Z Isomerization in the Solvent.

2,4-Dienones undergo visible-light-promoted, photocatalyst-free dimerization in neat conditions to provide cyclohexene derivatives stereoselectively through cascade rearrangement pathways, whereas regioselective E → Z isomerization of the more dienophilic double bond takes place exclusively in nitromethane. On the basis of intermediate isolation and computational DFT studies, the dimerization reaction is proposed to proceed via s-trans to s-cis isomerization/regioselective E → Z isomerization/Diels-Alder cycloaddition.

Copper(II)-Catalyzed Reactions of α-Keto Thioesters with Azides via C-C and C-S Bond Cleavages: Synthesis of N-Acylureas and Amides.

Cu(II)-catalyzed reaction of α-keto thioesters with trimethylsilyl azide (TMSN3) proceeds with the transformation of the thioester group into urea through C-C and C-S bond cleavages, constituting a practical and straightforward synthesis of N-acylureas. When diphenyl phosphoryl azide (DPPA) is used instead as the azide source in an aqueous environment, primary amides are formed via substitution of the thioester group. The reactions are proposed to proceed through Curtius rearrangement of the initially formed α-keto acyl azide to generate an acyl isocyanate intermediate, which reacts further with an additional amount of azide or water and rearranges to afford the corresponding products. To demonstrate the potentiality of the method, one-step syntheses of pivaloylurea and isovaleroylurea, displaying anticonvulsant activities, have been carried out.