Ring Contraction Reactions of a Non-Benzenoid Aromatic Cation and a Neutral Homoaromatic System into Benzene Derivatives

Aromaticity is one of the most intriguing concepts in organic chemistry. Simple and extended benzenoid aromatic systems have been very well established in undergraduate textbooks, and there are also mentions of non-benzenoid aromatic structures such as cyclopropenium, cyclopentadienide and cycloheptatrienylium (tropylium) ions. However, the structural relationship and the comparison of stabilization energy of such aromatic ions to benzene ring have been rarely studied and remained an underexplored area of advanced organic chemistry research. To contribute some insights into this topic, we focused on the chemical transformation, namely a ring contraction reaction, of the tropylium ion to benzene ring in this work. With an approach combining computational studies with experimental reactions, we also aim to turn this transformation into a synthetically useful tool. Indeed, this work led to the development of a new synthetic protocol, which involved an oxidative ring-contraction of tropylium ion, to formally introduce the phenyl ring onto a range of organic structures. Furthermore, the homoaromatic cycloheptatrienyl precursors of tropylium salts used in these reactions can also be rearranged to valuable benzhydryl or benzyl halides, enriching the synthetic utility of this ring-contraction protocol.

Tropylium Salt Promoted Hydroboration Reactions: Mechanistic Insights via Experimental and Computational Studies

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross coupling chemistry. This type of reaction has traditionally been mediated by transition metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to efficiently promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic paradigm, which is triggered by a hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of <i>in situ</i> counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

Tropylium Salt Promoted Hydroboration Reactions: Mechanistic Insights via Experimental and Computational Studies

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross coupling chemistry. This type of reaction has traditionally been mediated by transition metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to efficiently promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic paradigm, which is triggered by a hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of <i>in situ</i> counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

1,3-Dimethylbenzimidazolinium iodide and 1,3-dimethylbenzimidazoline in reduction processes of C=N group of imines

Interaction between 1,3-dimethylbenzimidazolinium iodide (as an analogue of 1,3-benzodithiolium and 1,3-benzothiolium and tropylium salts), aromatic Schiff bases, and sodium tetrahydroborate in tetrahydrofurane medium (in the presence of imidazole as a cation carrier, or without) at the ratio of starting reagents imine : 1,3-dimethylbenzimidazolinium iodide : sodium tetrahydroborate = 1:1:1 was studied. It was found out that, as basically distinct from the reaction of imines with analogues (1,3-benzodithiolium and 1,3-benzothiolium and tropylium salts) which, while reacting under similar conditions with Schiff bases, form accordingly the products of reductive heterylation – N-arylmethyl-4-(1,3-benzodithiol-2-il)aniline, or tropylation – N-arylmethyl-4-(7-cyclohepta-1,3,5-trienyl)aniline, whereas the reaction of imines with 1,3-dimethylbenz-imidazolinium iodide under similar conditions afforded N-arylmethylanilines in a high yield. This fact enables characterizing the 1,3-dimethylbenzimidazolinium iodide cation as more stable and less electrophilic due to an appreciable delocalization of a positive charge in the cation and, as a consequence, placing it as last (less reactive) in a series of known heteroanalogues (1,3-benzodithiolium > xanthilium > thioxanthilium > tropylium > N-methylacridinium > 1,3-dimethylbenzimidazolinium). The use of 1,3-dimethylbenzimidazoline as a donor of hydride-ion H (instead of sodium tetrahydroborate) also results in corresponding secondary aromatic amines as the reduction products of Schiff bases.

N-TROPYLATION OF ARYLAMINES

An efficient method for introduction of biogenic tropylium cycle into aromatic amines molecules is offered. Introduction is carried out in the presence of imidazole as a strong base. Interaction between tropylium salts (tetrafluoroborate or perchlorate) and aromatic amines with either nitro- or acetyl groups (meta-nitroaniline, para-nitroaniline, 2-methyl-4-nitroaniline and para-acetylaniline) as electron-acceptor substituents in the benzol ring results in stable products resulting from substitution of the hydrogen atom in the amino group of aromatic amines, namely: 4-nitro-N-(1'-cyclohepta-2',4',6'-trienil)aniline, 2-methyl-4-nitro-N-(1'-cyclohepta-2',4',6'-trienil)aniline, 3-nitro-N-(1'-cyclohepta-2',4',6'-trienil)aniline, 4-acetyl-N-(1'-cyclohepta-2',4',6'-trienil)aniline. The yields of the compounds obtained attain 60-87%. In this process, imidazole forms with the tropylium cation a complex which (1) serves as a carrier of tropylium ion to the nitrogen atom of aromatic amine, thus lightening the electrophilic substitution process at the hydrogen atom of the amino group; (2) prevents the dehydration process of resulting N-(1'-cyclohepta-2',4',6'-trienil) anilines. The latter phenomenon is an advantage in comparison with the method in which the dehydration process results in unstable N-aryl-8-azaheptafulvenes instead of stable N-tropylated anilines. The structure of the compounds obtained is confirmed by the method of mass spectrometry, NMR on protium nuclei and XRD analysis for 4-nitro-N-(1'-cyclohepta-2',4',6'-trienil)aniline. Antimicrobial activity is studied on conditionally pathogenic St. aureus 906, C. albicans ATCC 24433 and E. coli 1257 strains. The investigation results show all the synthesized compounds to exhibit antimicrobial activity. The compounds N-(3-nitrophenyl)cyclohepta-2,4,6-trienamine and N-(4-acylphenyl)cyclohepta-2,4,6-trienamine at concentration 125 microgram/ml are ascertained to exhibit inhibitory action on growth and development of the test strains, with this effect being less expressed for compounds N-(4-nitrophenyl)cyclohepta-2,4,6-trienamine and N-(2-methyl-4-nitrophenyl)cyclohepta-2,4,6-trienamine.