•Cation–π-controlled preorientation of (E)-styrylthiazoles is performed.•Two neighboring molecules are arranged in head-to-tail manner.•Irradiation in both solution and solid phases produces synHT dimers in high yields.Cation–π-controlled preorientation of (E)-styrylthiazoles was performed in both solution and solid phases. Irradiation of (E)-styrylthiazoles in the presence of HCl produced synHT dimers in good selectivities, while little selectivity was observed without HCl. X-ray structures for the HCl salts of (E)-styrylthiazoles showed a head-to-tail arrangement through cation–π interactions between the two neighboring molecules. Irradiation of these HCl salts produced synHT dimers in excellent yields.

A series of molecular seesaw balances 1–5 have been developed to measure the relative strength of pyridinium−π (cation−π) interactions. The cycloaddition of 1-azaanthracene and o-quinodimethane under microwave irradiation afforded the efficient synthesis of 1 and 5. Introduction of substituents to the pyridine ring of balance 1 was achieved to produce 2–4 in good yields. Anion exchange of 1·MeI afforded 1·MeX with a variety of counteranions (X = Cl, Br, I, BF4, PF6, OAc). These balances adopt two distinct conformers, A and B, which are stabilized by a cation−π interaction and a π–π interaction, respectively. The conformer ratio was determined on the basis of the observed averaged 3J coupling constants for H1–C–C–H2 by comparison with the boundary JA and JB values, which were estimated by applying the Carplus–Altona equation to the dihedral angles of the optimized conformers A and B. The effects of the solvent, substituent and counteranion on the ΔG values were elucidated using these molecular balances. Thermodynamic parameters obtained from a van’t Hoff plot as well as the electrostatic potential maps for both conformers A and B of the molecular balances helped us to better understand the obtained results.

A molecular seesaw balance 1·MeI has been developed to measure pyridinium−π interactions. This balance adopts two distinct conformers, A and B, which are stabilized by a cation−π interaction and a π–π interaction, respectively. The conformer ratio was determined on the basis of the averaged 3J coupling constants for H1–C–C–H2 and the corresponding boundary J values for conformers A and B. The effects of the solvent and the counteranion on the ΔG values were investigated using this molecular balance. Thermodynamic parameters obtained from a van’t Hoff plot helped us to better understand the solvent and counteranion effects.

Exposure of the yellow anhydrate crystals of 4′-alkoxy-4-azachalcones to water vapor gave red hydrate crystals. On the other hand, dehydration of the hydrate phase by heating recovered the anhydrate yellow phase. A comparison of the crystal structures of the anhydrate and hydrate crystals confirmed that the molecular arrangement is significantly changed during the hydration–dehydration processes: the changes in the orientation mode from head-to-tail to head-to-head and the displacement of chloride ions and water molecules are involved in the crystal transformations. PXRD studies clarified the reversibility of the crystal transformations occurring in the hydration–dehydration processes.

4-Nitrostyrylpyridine hydrochloride forms porous crystals stabilized by cation–π and NO2⋯Cl− interactions that involve water clusters with a T5(2) motif in the channels. The crystals reversibly release and re-uptake water molecules. Solid-state 17O NMR spectroscopic analyses revealed the behaviour of the water molecules present in the channels.

Tetrabutylammonium salts are remarkably effective for increasing diastereoselectivities in [6π]-photocyclization reactions of acrylanilides. This TBA+-assisted photocyclization is applicable to a variety of acrylanilides to afford trans-dihydroquinolones. Using a d5-labeled substrate, it was elucidated that a tetrabutylammonium ion shields a zwitterionic intermediate from an intermolecular H-transfer, which enables preferential occurrence of the [1,5] H-shift to give trans products stereoselectively.

Irradiation of 1-aryl-4-pyridylbutadienes in the presence of 1 equiv of HCl produced syn and anti head-to-tail dimers, among a number of possible dimers, whereas irradiation in the absence of HCl gave a complex mixture. This indicated that the acid serves as a catalyst for the regio- and stereoselective [2+2] photodimerization of 1-aryl-4-pyridylbutadienes through cation–π interactions between the pyridinium and aromatic rings. The produced synHT dimers underwent Cope rearrangement to produce cyclooctadienes, and they were in equilibrium at a ratio of 85:15 in CDCl3.

Tetrabutylammonium salts serve as templates for the Norrish–Yang cyclization of 2-benzyloxy-acylbenzenes to give trans-dihydrobenzofuranols in high stereoselectivities. The dual cation−π interactions between an ammonium with a benzene ring and a carbonyl group play a key role in changing the conformation of the substrate, which was supported by ab initio calculations.

A supramolecular dual-synthon with 2R4(10) ring and columnar motifs is observed in crystal structures of all 2-, 3- and 4-methylstyrylpyridinium chlorides and 3-bromo-, 3-nitro- and 4-trifluoromethylstyrylpyridinium chlorides. The dual-synthon assembled through N–H⋯Cl− and C–H⋯Cl− hydrogen bonds and cation–π interactions forms both ladder-like and head-to-head or head-to-tail columnar motifs. The position of the methyl group at the benzene ring affects the size and shape of the channel structure. Infinite hydrogen bond networks involving chloride anions, water and HCl molecules are found in the channels of 3- and 4-methylstyrylpyridinium chlorides. Variations in electron-withdrawing groups such as Br, NO2 and CF3 have little effect on the crystal structures.

Irradiation of trans-4,4′-bispyridylethylene in the presence of 1 equiv of concd HCl produced a syn dimer with high selectivity, whereas irradiation in the presence of more than 2 equiv of concd HCl or in the absence of HCl gave a mixture of dimers and by-products with much lower selectivity. This indicated that a suitable amount of acid served as a catalyst for the [2+2] photodimerization of BPEs through cation–π interactions between the pyridinium and pyridine rings.

Exposure of 4-azachalcones to HCl gas produced the corresponding HCl salts with a head-to-tail stacked alignment, irradiation of which produced the corresponding syn-HT dimers with high regio- and stereoselectivities, thus showing the effectiveness of the cascade process in crystals.

Regio- and stereoselective [4 + 4] photodimerization reactions of 1- and 2-azaanthracenes were performed in both methanol solution and solid phases to give anti-HT dimers in high yields. In these reactions, intermolecular cation−π interactions between the pyridinium cation and the benzene ring play a key role in preorientation prior to the photodimerization reactions.

Water molecules assist the assembly of (E)-arylvinylpyridine hydrochlorides in a head-to-tail and face-to-face fashion by way of N–H⋯O hydrogen bonds in combination with cation–π interactions between the pyridinium and aromatic rings. Photolysis of the pyridinium salt hydrates provided synHT dimers in high yields.

Cation–π interactions have been widely exploited and utilised in the structural biology arena, their fundamental importance in supramolecular chemistry and the pivotal role they play in host guest chemistry has rapidly expanded. In terms of organic synthesis π–π, CH–π and cation–π interactions are often invoked providing hypotheses for observed selectivities and reaction outcomes although fundamental studies of these interactions are less well reported, especially in the organic synthesis arena. This article considers cation–π interactions in the field of asymmetric organocatalysis and provides a summary of cases where such interactions may play an important role. Importantly this article sets out to highlight where such interactions could be operating in order to highlight the potential wealth of investigations to be had in this area rather than categorically claiming such interactions are in operation. For asymmetric catalysis this is particularly important as the geometry of a transition state dictates the stereochemical outcome of the reaction, this article provides a perspective on such phenomena.

The cis–cis–trans cyclobutanes are prepared by photodimerization of (Z)-4-styrylpyridine. A catalytic amount of HCl plays a key role in enhancing the [2+2] photocyclization reactions between (Z)- and (E)-4-styrylpyridines to give r-cct and r-ctc cyclobutane dimers through a cation–π interaction.

Controlling molecular conformation is a significantly important issue in a wide variety of organic reactions because the ground state structure is significantly responsible for the transition one. As observed in enzymes and proteins, the cation–π interaction plays a key role in the formation of the tertiary structure and the biochemical processes. Therefore, the cation–π interaction would be a promising conformation-controlling tool not only in large molecules, but also in small molecules due to its stronger interaction force. This article describes the utility of the intramolecular cation–π interaction in various organic syntheses with evidence for the existence of the cation–π interactions.

Controlling molecular conformation is a significantly important issue in a wide variety of organic reactions because the ground state structure is significantly responsible for the transition one. As observed in enzymes and proteins, the cation–π interaction plays a key role in the formation of the tertiary structure and the biochemical processes. Therefore, the cation–π interaction would be a promising conformation-controlling tool not only in large molecules, but also in small molecules due to its stronger interaction force. This article describes the utility of the intramolecular cation–π interaction in various organic syntheses with evidence for the existence of the cation–π interactions.

Cation–π interactions have been widely exploited and utilised in the structural biology arena, their fundamental importance in supramolecular chemistry and the pivotal role they play in host guest chemistry has rapidly expanded. In terms of organic synthesis π–π, CH–π and cation–π interactions are often invoked providing hypotheses for observed selectivities and reaction outcomes although fundamental studies of these interactions are less well reported, especially in the organic synthesis arena. This article considers cation–π interactions in the field of asymmetric organocatalysis and provides a summary of cases where such interactions may play an important role. Importantly this article sets out to highlight where such interactions could be operating in order to highlight the potential wealth of investigations to be had in this area rather than categorically claiming such interactions are in operation. For asymmetric catalysis this is particularly important as the geometry of a transition state dictates the stereochemical outcome of the reaction, this article provides a perspective on such phenomena.

Exposure of 4-azachalcones to HCl gas produced the corresponding HCl salts with a head-to-tail stacked alignment, irradiation of which produced the corresponding syn-HT dimers with high regio- and stereoselectivities, thus showing the effectiveness of the cascade process in crystals.

Water molecules assist the assembly of (E)-arylvinylpyridine hydrochlorides in a head-to-tail and face-to-face fashion by way of N–H⋯O hydrogen bonds in combination with cation–π interactions between the pyridinium and aromatic rings. Photolysis of the pyridinium salt hydrates provided synHT dimers in high yields.