C6-unsubstituted tetrahydropyrimidines (THPs) are compounds with a chiral carbon and strong aggregation-induced emission. The fluorescence properties of their racemates have been studied in detail, but those of their enantiomers have not. The solid-state fluorescence properties of the racemates and enantiomers of four chiral tetrahydropyrimidines (THPs 1–4) have been investigated by the steady-state and time-resolved fluorescence, single-crystal X-ray structures, and HOMOs and LUMOs of their seven racemic (three of them are polymorphs), four R- and three S-enantiomeric crystals. It was found that the R- and S-enantiomers of 1–4 can self-assemble as RS-paired, RS-, or RR/SS-overlapped mode in their racemates and as the same RR/SS-overlapped mode in their R- and S-enantiomers. Unexpectedly, the solid-state fluorescence quantum yields (SF) of racemic 1–4 could increase to 93, 48, 80, and 100%, respectively, via a suitable heteroenantiomeric self-assembly, but the SF values of their seven enantiomers are only 25–46%, owing to much larger nonradiative rate constants than those of their racemates. This means that heteroenantiomeric self-assembly can be used as a new efficient method enhancing SF values. The advantage of racemates is first reported and expected to encourage the development and application of racemates as a new kind of fluorescent materials.

A solvent-dependent sulfonylation of arylethynylene bromides with sodium arylsulfinates has been developed. The (E)-1,2-bis(arylsulfonyl)ethylenes were formed in DMSO, while the arylacetylenic sulfones were obtained in toluene. Utilizing simple and readily available starting materials, the sulfonylation products were generated with good selectivities and yields without the need for a metal catalyst or oxidant.

An aerobic oxidative carboamination of sp3C–H bonds with 2-(1H-pyrrol-1-yl)anilines has been developed. The oxidative carboamination processes utilized simple and readily available starting materials to produce pyrrolo[1,2-a]quinoxalines in good to moderate yields. The transformations also featured inexpensive metal catalysts (copper or iron) and a green oxidant (O2).

•The mechanism of the mechanofluorochromism of tetrahydropyrimidines is proposed.•The mechanism is based on the conversion between the RS- and RR/SS-packing modes.•Tetrahydropyrimidines have excellent writing-erasing characteristics.Mechanofluorochromic organic compounds have attracted increasing attention owing to their application in various fields. Here, dimethyl 1,3-bis(4-bromophenyl)-2-phenyl-1,2,3,6-tetrahydropyrimidine-4,5-dicarboxylate (Me-THP-2Br), a racemic C6-unsubstituted tetrahydropyrimidine with highly sensitive mechanofluorochromic characteristics based on a new mechanism is reported. Me-THP-2Br has excellent aggregation-induced emission (AIE) characteristics, that is, no emission in solution but becomes highly emissive upon aggregation. It can unusually form four stable solid forms, namely, three polymorphic crystals PC, BC and CC emitting purple, blue, cyan fluorescence respectively, and one amorphous state AS with 18–52% fluorescence quantum yields. We find that all these as-prepared solid forms can emit the same green fluorescence (490 nm) when they are ground into powder, and revert to purple fluorescence (425 nm) upon fuming/heating. The green and purple emissions can be switched reversibly by grinding-fuming/heating process. In addition, the ground powder of Me-THP-2Br can be used as an excellent rewritable material. More importantly, its sensitive mechanofluorochromism proves to originate mainly from conversion between the paired RS-packing and unpaired RR/SS-packing of enantiomers, which is first reported. Since racemic organic compounds can be easily obtained by chemical synthetic methods, more racemic organic fluorophores with sensitive mechanofluorochromism are expected to be developed.Racemic Me-THP-2Br can form three fluorescent crystalline polymorphs (PC, BC and CC) and an amorphous state (AS), which are packed in enantiomer-paired (RS-) or -unpaired (RR/SS-) mode, and exhibit sensitive reversible mechanofluorochromism (65 nm) and excellent rewritable characteristics via conversion between the RS- and RR/SS-packing modes.Download high-res image (391KB)Download full-size image

Organic fluorophores have attracted great interest owing to their wide applications. They usually contain an electron-conjugated system with an aromatic moiety and show high emission in dilute solutions but weaker or even no emission upon aggregation. Here, a simple one-pot, three-component reaction (3CR) (method I) for the synthesis of various di- and monosubstituted aminomaleimides (DAMIs and MAMIs) has been developed, and the reported 3CR (method II) has been found to be efficient only for the synthesis of MAMIs with R2 = alkyl. Twelve AMIs were designed and synthesized for investigation of the influence of structures on their optical properties in monomers and aggregates. It was found that alkyl MAMIs, alkyl DAMIs, and aryl AMIs/DAMIs show very different fluorescence efficiencies in different solvents, and only MAMIs with butyl and oleyl show high emissions in powders similar to those in nonpolar solutions. Single-crystal structures indicate that their fluorescence efficiencies in aggregates mainly correlate with molecular packing modes. The efficient synthesis method, the sensitive fluorescence on–off response to protic solvents or polar solvents, and the unusual high emissions of AMI without any aromatic moiety in both monomer and aggregates are expected to attract great interest in the fields of application and theory.

Two polymorphs of a tetrahydropyrimidine-based fluorophore with aggregation-induced emission characteristics show unusual sensitive and reversible thermo-stimulus solid-state fluorescence-switching, and can be used as a new type of fluorescence-ratiometric/on–off thermometer in different temperature ranges when excited at different red-edge excitation wavelengths.

Multicomponent reactions (MCRs) have become a powerful tool for drug discovery and development owing to their advantages of fast and efficient construction of a large library of products with complexity and diversity. However, conventional MCRs usually proceed in environmentally unfriendly organic solvents rather than in water, a green solvent used by nature for biological chemistry. Herein, a simple and efficient on-water urea-catalyzed chemoselective five-component reaction (5CR) has been developed for the synthesis of a series of novel octahydroquinazoline-5-ones (6), the derivatives of quinazolinones possessing diverse biological activities. The molecular structure of 6{1,1,12} has been confirmed by single-crystal X-ray diffraction. The 5CR can proceed at room temperature under normal atmospheric pressure in good yields and afford a large library of octahydroquinazoline-5-ones with various aromatic and aliphatic substituents at N-1, C-2, and N-3. In addition, a green method has been developed for the synthesis of enaminones, important intermediates in the 5CR and in synthetic chemistry.Keywords: enaminones; multicomponent reaction; octahydroquinazoline-5-ones; synthetic method; water solvent

Fluorescence enhancement and particle-size control of organic fluorophores are of general interest owing to their wide applications. We recently developed a convenient five-component reaction (5CR) for the synthesis of a novel class of racemates, C-6 unsubstituted tetrahydropyrimidines with strong aggregation-induced emission (AIE) properties. The copper-induced fluorescence enhancement and particle-size decrease of the racemate dimethyl 2-(4-hydroxy-3-methoxyphenyl)-1,3-diphenyl-1,2,3,6-tetrahydropyrimidine-4,5-dicarboxylate (THP-1), one of the 5CR products, were investigated in this paper. THP-1 shows useful AIE properties, which have attracted great attention recently, and the unusual size-independent emission (SIE) properties reported in our previous work. The fluorescence intensity of THP-1 increased upon adding Cu2+ and a good linear relationship between the fluorescence intensity at the peak position and the Cu2+ concentration was observed in the range of 0–80 μM of Cu2+. The fluorescence response to Cu2+ is highly selective over other common transition metal ions. Most importantly, the suspension particle sizes of THP-1 were found to gradually decrease upon adding Cu2+. This method for controlling organic particle-size is completely different from conventional ones. The investigation results of the influences of Cu2+ on the optical properties and particle sizes of THP-1 in ethanol–water suspensions as well as the molecular stacking mode in the single-crystal of THP-1 indicate that the copper-induced fluorescence enhancement and particle-size decrease are expected to result from the coordination-induced dissociation of intermolecular O–H⋯O bonds connecting non-emissive groups. In addition, THP-1 shows no cytotoxicity in EC109 cells at concentrations below 30 μM. These useful and interesting properties of THP-1 are expected to be very helpful for new probe design, theoretical research on the relationship between intermolecular hydrogen bonds and optical properties, as well as practical applications in organic particle-size control and chemical/biological fluorescence probes.

We previously reported the novel efficient proton/heat-promoted four-component reactions (4CRs) of but-2-ynedioates, two same/different primary amines, and aldehydes for the synthesis of tetra- and pentasubstituted polyfunctional dihydropyrroles. If aromatic and aliphatic amines were used as reagents, four different series of products should be obtained via the permutation and combination of aromatic and aliphatic primary amines. However, only three/two rather four different series of tetra-/pentasubstisuted dihydropyrroles could be prepared via the proton/heat-promoted 4CRs. Herein, Cu(OAc)2·H2O, a Lewis acid being stable in air and water, was found to be an efficient catalyst for the 4CR synthesis of all the four different series of tetra-/pentasubstisuted dihydropyrroles. The copper-catalyzed 4CR could produce target products at room temperature in good to excellent yields. Interestingly, benzaldehyde, in addition to being used as a useful reactant for the synthesis of pentasubstituted dihydropyrroles, was found to be an excellent additive for preventing the oxidation of aromatic amines with copper(II) and ensuring the sooth conduct of the 4CRs for the synthesis of tetrasubstituted dihydropyrroles with aryl R3. In addition, salicylic acid was found to be needed to increase the activities and yields of the copper-catalyzed 4CRs for the synthesis of petasubstituted diyhydropyrroles. On the basis of experimental results, the enamination/amidation/intramolecular cyclization mechanism was proposed and amidation is expected to be the rate-limited step in the copper-catalyzed 4CRs.Keywords: copper catalysis; dihydropyrroles; multicomponent reactions; nitrogen heterocycles; one-pot reaction; synthetic methods

Two polymorphs of a tetrahydropyrimidine-based fluorophore with aggregation-induced emission characteristics show unusual sensitive and reversible thermo-stimulus solid-state fluorescence-switching, and can be used as a new type of fluorescence-ratiometric/on–off thermometer in different temperature ranges when excited at different red-edge excitation wavelengths.