In recent decades, N-heterocyclic carbenes have become established as a prevalent family of organocatalysts. N-Heterocyclic olefins, the alkylidene derivatives of N-heterocyclic carbenes, have recently also emerged as efficient promoters for CO2 fixation and polymerization reactions. Their extraordinarily strong Lewis/Brønsted basicity suggests great potential as a new class of organocatalysts for a broad range of reactions in synthetic chemistry.

The first three primary members of the non-benzenoid carbocyclic aromatic ion family, namely cyclopropenium, cyclopentadienide, and cycloheptatrienium (tropylium) ions, have planar cyclic structures with (4n+2)π electrons in fully conjugated systems. They fulfill Hückel's rule for aromaticity and hence possess extraordinary stability. Since the historic discovery of tropylium bromide in the late 19th Century, these non-benzenoid aromatic ions have attracted a lot of attention because of their unique combination of stability and reactivity. The charge on the aromatic ions makes them more prone to nucleophilic/electrophilic reactions than the neutral benzenoid counterparts. Within the last seven years, there has been a large number of investigations in utilizing aromatic ions to mediate organic reactions. This Review highlights these recent developments and discusses the potential of aromatic ions in promoting synthetically useful organic transformations.

N-Heterocyclic olefins (NHOs) have very recently emerged as efficient promoters for several chemical reactions due to their strong Brønsted/Lewis basicities. Here we report the novel application of NHOs as efficient phase-transfer organocatalysts for synthetically important alkylation reactions on a wide range of substrates, further demonstrating the great potential of NHOs in organic chemistry.

Tropylium salts were reported as organic-Lewis acids to efficiently catalyze acetalization reactions in batch and flow.

Tropylium ions possess an interesting combination of structural stability and chemical reactivity due to its Hückel aromaticity and its positively charged polyene nature, respectively. Herein we exploit the chemical versatility and unique structural properties of the tropylium ion to derive a family of novel push-pull organic dyes with strong absorption in the visible range via simple and practical synthetic protocols. These stable organic dyes are highly stimuli-responsive, as demonstrated by their sensitivity towards solvent, pH change, redox reaction, Lewis base and counterion, which marks them as potentially useful compounds for opto-electronic applications.