Promotion of Organic Reactions by Non‐Benzenoid Carbocyclic Aromatic Ions

Abstract

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.

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

“…[69] In conclusion, aminocyclopropenium ILCs serve as welldefined model compounds to study self-assembly and nanosegregation, which are important in polyelectrolytes used for battery materials.T hese ILCs bridge the gap between low molecular weight organocatalysts and polymeric electrolytes, and thus,c ontribute to the general utility of 3-ring aromatic compounds. (9,14), guanidinium (12,13), and aminocyclopropenium (7, 10;* denotes decomposition). The values of 13 a and 13 b were taken from ref.…”

“…[2][3][4] Among the differently substituted cyclopropenium cations,t he aminocyclopropenium ions first described by Yoshida in 1971, [5] have received special attention. [6] Most recent work has focused on their use as phase transfer, Lewis acid or organocatalysts, [7][8][9][10][11][12][13][14][15][16] electrophotocatalysts, [17] ligands for catalytic metal complexes, [18][19][20] ionic liquids, [21][22][23][24][25] persistent radical cations, [26] redox active polymers for redox flow batteries, [27][28][29][30][31][32] fluorescent materials, [33][34][35] aromatic cations in hybrid halide perovskites, [36] biologically active compounds such as transfection agents, [37][38][39] and nanoparticles. [40,41] Surprisingly, the self-assembly of cyclopropenium compounds into liquid crystalline phases has not been reported.…”

Reactivity vs. Stability of Cyclopropenium Substituted Phosphonium Salts

“…Among three‐membered cycles, cyclopropenium cations belong to a category apart due to the delocalization of two π‐electrons over three 3p orbitals which confers higher stability . Beyond aromatic effects, the nature of cyclopropenyl substituents play a determinant role in the stability vs. reactivity of these two π‐electron Hückel prototypes . In comparison with cyclopropeniums bearing only carbon substituents, amino groups due to their strong π‐donating effect and the electronegativity of nitrogen induce additional stability which has the consequence of placing aminocyclopropeniums among the most stable carbocations .…”

Tropylium Ion Catalyzes Hydration Reactions of Alkynes

“…The non‐benzenoid aromatic tropylium ion has 6π electron and a positive charge fully delocalized on a conjugated planar seven‐carbon system . It therefore fulfills Hückel's rule of aromaticity and possesses a unique combination of stability and reactivity.…”