Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2′-pyridyl)pyrrole

1H-pyrrolo[3,2-h]qinoline (PQ) and 2-(2′-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals. Our results indicate that compression up to 10 GPa (100 kbar) leads to considerable strengthening of the intermolecular hydrogen bond within the cyclic dimers. However, the intramolecular N–H∙∙∙N interaction is either weakly affected by pressure, as witnessed in PQ, or weakened due to compression-induced distortions of the molecule, as was found for PP. Therefore, we propose that the compression of these systems should facilitate double proton transfer within the cyclic dimers of PQ and PP, while intramolecular transfer should either remain unaffected (for PQ) or weakened (for PP).

Study on Kinetics of Thermal Degradation of Polymethyl Methacrylate/Cyclic Olefin Copolymer Blend

In this investigation, polymethyl methacrylate (PMMA) was mixed with cyclic olefin copolymer (COC) because of its hardness, strength, and transparency properties. The results of thermal analysis through TGA and DTG showed that the thermal properties of the alloy are improved using 40% cyclic olefin copolymer. Kinetics of thermal degradation (pyrolysis) of polymers have been studied and analyzed and thermal pyrolysis of polymethyl methacrylate and cyclic olefin copolymer thermoplastic polymer was conducted. The computation of kinetic analysis is measured along with the different methods used to study the kinetics. The activation energy (E) of degradation of studied materials was estimated using Ozawa Flynn and Wall (OFW), Starink and Kissinger’s methods, and evaluation of three kinetic parameters taken appropriate kinetic model in terms of percent change for both types of polymers have been proposed, and finally, simulated curves were compared with the experimental curves. Both mechanisms of degradation for COC and PMMA under nitrogen atmosphere will reflect intramolecular transfer and random scission of the main chain.

Photophysical properties of 4-(5-methylthiophen-2-yl)pyridinium–cyclic enolate betaine dyes tuned by control of twisted intramolecular transfer

In this paper, thienylpyridinium-cyclic enolate betaine (TPB) dyes were reported as unique skeletons of fluorescent donor–acceptor type molecules. TPB derivatives with five-membered cyclic enolates are coplanar because of the intramolecular...

Proton transfer in the benzimidazolone and benzimidazolthione tautomerism process catalyzed by polar protic solvents

The tautomeric equilibrium of benzimidazolone and benzimidazolthione have been studied by the density functional theory method using the CAM-B3LYP functional together with the 6-311G(d,p) basis set. Two separate mechanisms have been investigated: a direct intramolecular transfer using the polarizable continuum model and an indirect proton transfer assisted by a molecule of solvent (C6H12, H2O, CH3OH, and H2O2). In both cases, the results obtained indicate that ketone and thione are the most stable forms. However, the enhanced height of the activation barrier for the four-center mechanisms describing the tautomerism reaction as a direct intramolecular transfer implicates a relatively disadvantaged process. The participation of a polar protic solvent molecule allows the lowering of the activation energy barrier. Potential energy profiles of keto-enol and thio-enol tautomerism assisted by methanol and water are very different. The former one describes a concerted mechanism but the latter does not because it is associated with asynchronous processes that take place during the thio-enol tautomerism.

Highly Selective Gold-Catalyzed Cycloisomerization of Furanolabdanoid Dialkynes with Alkynyl Substituents in the Furan Ring

Aim and Objective: We studied the synthesis of new groups of functionalized isobenzofurans and isoindolines by selective gold catalyzed cycloisomerization of some new optically active furandialkynes derived from the natural furanolabdanoid lambertianic acid. Functionalized isobenzofurans and isoindolines are considered as privileged structures that are currently experiencing a renewed interest in medicinal chemistry. Materials and Methods: Using the AuCl3-catalyzed cycloisomerization of furanolabdanoid dialkynes we synthesized several new groups of biologically interested isobenzofurans and isoindolines with labdanoid substitution. The structure of all new compounds was proven by elemental and spectral analysis: IR, mass-, and NMR data. Results: The synthesis of 15,16-dialkynyl substituted furanolabdanoids was carried out. Au(III)-Catalyzed cycloisomerization of those compounds proceeded highly selectively with the formation of optically active 4- substituted 7-hydroxy-3(6)-propargylaminomethylisobenzofurans or 7-hydroxy-3(6)-propargyloxymethylisoindolines. The cycloisomerization of the 15-(prop-2-yn-1-yloxymethyl)-16-((N-p-toluenesulfonyl)-prop- 2-yn-1-ylaminomethyl)labdatriene proceeded in part on the 15 position and involved the previously unknown intramolecular transfer of N-(p-toluenesulfonyl)propargylaminomethyl substituent to the oxygen in the stage of formation of an aromatic ring. A feasible reaction mechanism for this transformation is presented. It was revealed that the conversion of labdanoid furan-15-ynes to 1,3-dihydroisobenzofurans depends on the nature of the substituent in the 16 position of the furanolabdanoid. Conclusions: The present study sheds light on the Au(III)-catalyzed cycloisomerization of new group of highly functionalized furandialkynes. Also, new groups of alkynyloxymethyl or alkynylaminomethyl substituted optically active isobenzofurans and isoindolines were obtained and fully characterized.

Mid-Chain Radical Migration in the Radical Polymerization of n-Butyl Acrylate

The occurrence of intramolecular transfer to polymer in the radical polymerization of acrylic monomers has been extensively documented in the literature. Whilst it has been largely assumed that intramolecular transfer to polymer leads to short chain branches, there has been some speculation over whether the mid-chain radical can migrate. Herein, by the matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS) of poly(n-butyl acrylate) synthesized by solution polymerization under a range of conditions, it is shown that this mid-chain radical migration does occur in the radical polymerization of acrylates conducted at high temperatures, as is evident from the shape of the molecular weight distribution. Using a mathematical model, an initial approximation of the rate at which migration occurs is made and the distribution of branching lengths formed in this scenario is explored. It is shown that the polymerizations carried out under a low monomer concentration and at high temperatures are particularly prone to radical migration reactions, which may affect the rheological properties of the polymer.

Palladium-catalyzed intramolecular transfer hydrogenation & cycloaddition of p-quinamine-tethered alkylidenecyclopropanes to synthesize perhydroindole scaffolds

A facile Pd-catalyzed intramolecular transfer hydrogenation and cycloaddition of p-quinamine-tethered alkylidenecyclopropanes has been developed, giving rigid tricyclic perhydroindole skeletons in moderate to good yields.