Pd catalyzed couplings of “superactive esters” and terminal alkynes: Application to flavones and γ-benzopyranones construction

Triazine-wingtips accelerated NHC-Pd catalysed carbonylative Sonogashira cross-coupling reaction

The transmetalation as the rate-limiting step was effectively accelerated by newly designed N-heterocyclic carbenes with triazine wingtips (T-NHC). By using a ppm-level precatalyst T-NHC-Pd (8), the highly efficient coupling of aryl iodide, alkyne and carbon monoxide furnished a variety of ynone compounds. T-NHC-Pd (5), which deprotonated 4-methyl-phenylacetylene under mild conditions, converted into alkynyl-coordinated catalytic active species PdCl(T-NHC)(Py)(alkynyl). In the putative Pd/Pd catalytic cycle, both triazine-wingtips and NHCs are key players for establishing the carbonylative cross-couplings with high TON and TOF.

Regioselective Synthesis of Chromones via Cyclocarbonylative Sonogashira Coupling Catalyzed by Highly Active Bridged-Bis(N-Heterocyclic Carbene)Palladium(II) Complexes

The one-pot regioselective and catalytic synthesis of bioactive chromones and flavones was achieved via phosphine-free cyclocarbonylative Sonogashira coupling reactions of 2-iodophenols with aryl alkynes, alkyl alkynes, and dialkynes. The reactions are catalyzed by new dibromidobis(NHC)palladium(II) complexes. The new bridged N,N'-substituted benzimidazolium salts (L1, L2, and L3) and their palladium complexes C1, C2, and C3 were designed, prepared, and fully characterized using different physical and spectroscopic techniques. The molecular structures of complexes C1 and C3 were determined by single-crystal X-ray diffraction analysis. They showed a distorted square planar geometry, where the Pd(II) ion is bonded to the carbon atoms of two cis NHC carbene ligands and two cis bromido anions. These complexes displayed a high catalytic activity in cyclocarbonylative Sonogashira coupling reactions with low catalyst loadings. The regioselectivity of these reactions was controlled by using diethylamine as the base and DMF as the solvent.

Rhodium(III)-catalyzed one-pot synthesis of flavonoids from salicylaldehydes and sulfoxonium ylides

Rh(III)-catalyzed C–H activation of salicylaldehyde followed by an insertion reaction with sulfoxonium ylides and cyclization is applied to the synthesis of flavonoids. This one-pot strategy exhibits good functional group tolerance and gives flavones in moderate-to-good yields.

K2S2O8-Mediated Selective Trifluoromethylacylation and Trifluoromethylarylation of Alkenes under Transition-Metal-Free Conditions: Synthetic Scope and Mechanistic Studies

A practical and efficient method for selective intramolecular radical trifluoromethylacylation and -arylation of alkenes with inexpensive CF₃SO₂Na and K₂S₂O₈ in aqueous media has been developed, respectively, affording the highly chemoselective synthesis of CF₃-functionalized chroman-4-ones and chromanes in satisfactory yields. Control experiments and DFT calculations indicate that the CF₃SO₂Na/K₂S₂O₈ system is capable of trifluoromethylating the substrate of alkenes without a transition metal catalyst and the oxidation of CF₃SO₂Na to ·CF₃ by K₂S₂O₈ is involved in the rate-determining step.

A transition-metal-free fast track to flavones and 3-arylcoumarins

A highly regioselective and transition-metal free one-pot arylation of chromenones with arylboronic acids has been achieved employing K₂S₂O₈. The procedure consists of a sequence of some reactions including an arylation/decarboxylation cascade and proceeds well in aqueous media to afford biologically interesting flavones and 3-arylcoumarins. This method exhibited excellent selectivity and functional group tolerance under mild conditions. The reaction also showed perfect efficacy for the preparation of styryl coumarins.