N-heterocyclic carbene (NHC) catalysis has emerged as a powerful strategy in organic synthesis. In recent years a number of reviews have been published on NHC-catalyzed transformations involving Breslow intermediates, acyl azoliums, α,β-unsaturated acyl azoliums, homoenolate equivalents, and azolium enolates. However, the azolium dienolate intermediates generated by NHCs have been employed in asymmetric synthesis only very recently, especially in cycloadditions dealing with remote functionalization. This Minireview highlights all the developments and the new advances in NHC-catalyzed asymmetric cycloaddition reactions involving azolium dienolate intermediates.

The sigma (s)-hole effect has emerged as apromising tool to construct novel architectures endowed with new properties.Asimple yet effective strategy for the generation of monofluoromethyl radicals is acontinuing challenge within the synthetic community.F luoromethylphosphonium salts are easily available,air-and thermally stable,aswell as simple-tohandle.H erein, we report the ability of the s-hole effect to facilitate the visible-light-triggered photolysis of phosphonium iodide salts,acharge-transfer complex, selectively giving fluoromethyl radicals.T he usefulness and versatility of this new protocol are demonstrated through the mono-, di-, and trifluoromethylation of av ariety of alkenes.Scheme 1. The application of s-hole effect of pnictogen atom.

Chiral N-heterocyclic carbenes were found to be efficient catalysts for the formal [4+2] cycloaddition reaction of alkyl(aryl)ketenes and 3-alkylenyloxindoles to give the corresponding 3,4-dihydropyrano[2,3-b]indol-2-ones in excellent yields with good diastereo- and enantioselectivities.