An overview on generation and general properties of N-heterocyclic carbenes: Applications of 1,2,4-triazolium carbenes as metal free organocatalysts

N-Heterocyclic carbenes (NHCs) have been widely explored in the areas of catalysis, organometallic and medicinal chemistry. In particular, NHCs catalyze the formation of C-C and C-heteroatom bounds with high regioselectivity and stereoselectivity through umpolung and non-umpolung processes for unconventional access to create a vast scope of heterocycle molecules, including beta-lactones, tetrahydroquinolines and pyrroloquinoles. Here we summarized the generation and general properties of NHC and important NHC-catalyzed reactions, which typically involve species such as Breslow intermediates, homoenolates, enolates and alpha, beta-unsaturated acyl azoliums, among others. Examples of aerobic oxidative/oxygenative carbene catalysis, specifically highlighting NHC-catalyzed reactions involving oxygen and strategies employed for activation of oxygen in NHC-catalytic processes are discussed in detail.The catalytic use of NHCs has been one of the most powerful tools in the chemist's arsenal, with applications to commercially important protocols. Given this context, the relevance of N-heterocyclic carbene chemistry is described in this review.