Visible-Light-Promoted Polycyclizations of Dienynes.

A variety of linear dienynes can deliver complex tetracyclic frameworks in the presence of an IrIII complex and visible light. Product formation involves the generation of four new C-C bonds and six contiguous stereocenters, which decorate two [3.1.0] bicyclic units tethered through their bridging quaternary carbon atoms. The internal alkyne acts as a formal dicarbenoid for the generation of two cyclopropanes in these radical cation cascades. This behavior has not been previously observed for organic reactive intermediates and can be extended to intermolecular reactions and diendiynes.

Silica-supported sulfonic acids as recyclable catalyst for esterification of levulinic acid with stoichiometric amounts of alcohols.

Converting biomass into value-added chemicals holds the key to sustainable long-term carbon resource management. In this context, levulinic acid, which is easily obtained from cellulose, is valuable since it can be transformed into a variety of industrially relevant fine chemicals. Here we present a simple protocol for the selective esterification of levulinic acid using solid acid catalysts. Silica supported sulfonic acid catalysts operate under mild conditions and give good conversion and selectivity with stoichiometric amounts of alcohols. The sulfonic acid groups are tethered to the support using organic tethers. These tethers may help in preventing the deactivation of the active sites in the presence of water.

Boosting catalyst activity in cis-selective semi-reduction of internal alkynes by tailoring the assembly of all-metal aromatic tri-palladium complexes.

Highly symmetric [Pd3]+ clusters that present delocalized metal-metal bonds can catalyse the selective semi-reduction of internal alkynes to cis-alkenes. Studies on factors governing the formation of all-metal aromatics enabled the design of an optimised catalytic system that delivers cis-alkenes with almost complete selectivity on a gram scale with very low catalyst loadings (0.03 mol%).

Acid-catalyzed formal cycloaddition of α,ß-unsaturated carbonyls with epoxides: dioxepines or acetals?

It has been recently reported that the reaction of α,ß-unsaturated carbonyl derivatives with epoxides in the presence of a homogeneous acid catalyst readily delivers the corresponding dioxepines via formal (4 + 3) cycloaddition. We report herein that the same apparent reactivity can be triggered via heterogeneous catalysis. Characterization of products by means of NMR correlation experiments and DFT modeling revealed, however, that products are the acetals of the unsaturated reagent rather than the desired heterocycles.