Terminal Acetylenic Iminium Salts – Synthesis and Reactivity

Construction of Covalent Organic Frameworks via Multicomponent Reactions

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.

Renewable Schiff-Base Ionic Liquids for Lignocellulosic Biomass Pretreatment

Growing interest in sustainable sources of chemicals and energy from renewable and reliable sources has stimulated the design and synthesis of renewable Schiff-base (iminium) ionic liquids (ILs) to replace fossil-derived ILs. In this study, we report on the synthesis of three unique iminium-acetate ILs from lignin-derived aldehyde for a sustainable “future” lignocellulosic biorefinery. The synthesized ILs contained only imines or imines along with amines in their structure; the ILs with only imines group exhibited better pretreatment efficacy, achieving >89% sugar release. Various analytical and computational tools were employed to understand the pretreatment efficacy of these ILs. This is the first study to demonstrate the ease of synthesis of these renewable ILs, and therefore, opens the door for a new class of “Schiff-base ILs” for further investigation that could also be designed to be task specific.

Photoinduced C-C bond cleavage for the synthesis of 2,4-disubstituted-1-naphthols from indenone derivatives and sulfoxonium ylide

The synthesis of 2,4-disubstituted-1-naphthols has been developed employing photomediated C-C bond cleavage (UV-LED 390 nm) of cyclopropane fused-indanones generated in situ from the reaction between indenones and trimethylsulfoxonium chloride under basic conditions at room temperature. Seventeen substrates were examined in this study. The results showed that indenone precursors containing aryl substituents could smoothly provide the desired products in up to 81% yield.

Iminium-substituted 2,3-dihydroisoxazoles: synthesis from acetylenic iminium salts and nitrones, and some transformations

Iminium-functionalized 2,3-dihydroisoxazoles (4-isoxazolines) were prepared in high yields by [3+2] cycloaddition of N-methyl-C-phenylnitrone and acetylenic iminium salts. Conversion of the very hygroscopic cycloadducts into neutral compounds was achieved by hydrolysis or hydride reduction of the iminium functional group. By a thermal rearrangement, the cycloadducts were converted into 2,3-dihydro-1H-imidazolium salts. With C,N-diphenylnitrone as the reaction partner, the expected cycloadduct, a [(2,3-dihydroisoxazol-4-yl)-1-(phenyl)methylene]-N,N-dimethylammonium triflate, could be prepared at −75°C and was characterized by NMR. This product was found to be thermally unstable above −30°C, being converted into an inseparable product mixture. Basic hydrolysis of this mixture afforded a structurally interesting acyclic enaminone/half-aminal, which is likely derived from a 2,3-dihydro-1,3-oxazole.

Reactions of 3-aryl-1-(trifluoromethyl)prop-2-yn-1-iminium salts with 1,3-dienes and styrenes

3-Aryl-1-(trifluoromethyl)prop-2-yn-1-iminium triflate salts represent a novel, highly reactive class of acetylenic iminium salts. Herein we present several reactions which are based on the electron-poor acetylenic bond and on the high electrophilicity of the CF₃-substituted iminium group. These salts were found to be highly reactive dienophiles in Diels–Alder reactions with cyclopentadiene, 2,3-dimethylbutadiene and even anthracene. At higher temperature, the cycloadducts undergo an intramolecular S_E(Ar) reaction leading to condensed carbocycles incorporating a 1-(trifluoromethyl)-1-(dimethylamine)indene ring system. With styrenes and some substituted styrenes, cascade reactions take place, which likely include cyclobutene and several cationic intermediates and mainly yield 2-(1-phenylvinyl)indenes. In a similar reaction cascade, a fulvene derivative was obtained with 1,4-diphenylbutadiene as the substrate.

Iminium-functionalized 1,2,3-triazoles by [3+2] cycloaddition reactions of internal acetylenic iminium triflates with organoazides

Propyne iminium triflates 1–6 react as dipolarophiles in thermal [3 + 2]-cycloadditions with sufficiently electron rich organoazides to form 1,4,5-trisubstituted 1,2,3-triazoles with iminium functionalization. The reactions require rather strong thermal activation, but can be accelerated by microwave irradiation. The regioselectivity of the cycloaddition at the internal acetylenic bond of 3-cyclopropylpropyne and 3-arylpropyne iminium ions (1–3 and 4, respectively) is very high, but is lowered in the presence of sterically demanding substituents at the opposite end of the iminium-substituted C,C triple bond. The iminium-functionalized triazoles can easily be transformed into neutral compounds; herein reported is the formation of triazolyl ketones 10 by hydrolysis and of tertiary triazolyldimethyl amines 12 by LiAlH4 reduction. When the reduction is performed with sodium boranate or sodium cyanoboranate, amine–borane complexes 15 and 16 are obtained.

Cycloaddition reactions of acetylenic iminium salts and diazoacetates leading to pyrazole iminium salts

Acetylenic iminium triflates with the general formula [R–C≡ C–C(Ar)=N+R2 TfO−] were found to be excellent dipolarophiles in [3+ 2] cycloaddition reactions with diazoacetates leading to (1H-pyrazol-3(5)-yl)methanaminium triflates in high yields. The terminal acetylenic iminium salt (propyne iminium salt) [HC≡C–C(Ph)=N+Me2 TfO−] reacted with an equimolar amount of methyl diazoacetate instantaneously at 20°C to form the expected pyrazole in almost quantitative yield. When a 2:1 stoichiometry was applied, subsequent Michael addition of the pyrazole at the alkyne occurred and the bis(iminium) ditriflate 4 was obtained in high yield. By hydride reduction or hydrolysis of the iminium group, some of the highly hygroscopic pyrazole iminium salts were converted into neutral, twofold functionalized, di- and tri-C-substitued 1H-pyrazoles.