Composite Correlated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, SiMe₃, and SO₂Me substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH₂ with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline and their derivatives may be able to support ring opening metathesis polymerization and warrant further study.     

Aromatic stability energy studies on five-membered heterocyclic C4H4M (M = O, S, Se, Te, NH, PH, AsH and SbH): DFT calculations

Energetic, geometric and magnetic criteria were applied to examine the stability and/or aromatic character for the cyclic molecules C ₄ H ₄ M (M = O, S, Se, Te, NH, PH, AsH and SbH) at B3LYP/6-311++G^** and MP2/6-311++G^** levels of theory. The isodesmic reactions and nuclear independent chemical shifts (NICS) calculations were utilized to examine the molecules for energetic and magnetic criteria, respectively. The isodesmic reaction energies reveal that thiophene (C ₄ H ₄ S, ?23.269 kcal/mol) and pyrrole (C ₄ H ₄ NH, ?20.804 kcal/mol) have the greatest aromatic stabilization energies and tellurophene (C ₄ H ₄ Te, ?15.114 kcal/mol) and stibole (C ₄ H ₄ SbH, ?1.169 kcal/mol) have the lowest aromatic stabilization energies in their corresponding groups at MP2/6-311++G^**. The NICS calculations confirmed the results obtained through isodesmic reaction energies.     

Synthesis of a novel functionalized tricyclic pyrimidine-fused 1,5-benzodiazepine library

A series of novel tricyclic pyrimidine-fused 1,5-benzodiazepines (PFBZDs) was synthesized using an enaminone-based approach. The key step in the synthetic strategy involves the formation of the CCNMe₂ structure on vicinal carbonyl groups of the 1H-1,5-benzodiazepine-2,4(3H,5H)-dione (BZD). The synthesis of pyrimidine-fused 1,5-benzodiazepines was performed by a simple and efficient method in good to excellent yields under mild and green conditions. The β-enaminoamide intermediates were condensed with thiourea and guanidine derivatives to form the corresponding tricyclic PFBZDs. But reaction of aminoguanidine, thiosemicarbazide, 4-phenylthiosemicarbazide and ethane-1,2-diamine with β-enaminoamides didn't produce any desired product and led to recovery of the corresponding starting BZD.     

An Environmentally Benign Catalytic Method for Versatile Synthesis of 1,4-Dihydropyridines via Multicomponent Reactions

The synthesis of diverse 1,4‐dihydropyridines have been achieved via the multicomponent reactions of aldehydes, enaminones and amines. The reactions have been smoothly performed in water to provide all products with moderate to excellent yields by using lactic acid as a green catalyst.     

Light-Induced Reactions within Poly(4-vinyl pyridine)/Pyridine Gels: The 1,6-Polyazaacetylene Oligomers Formation

Cyclic 6-membered aromatic compounds such as benzene and azabenzenes (pyridine, pyridazine, and pyrazine) are known to be light-sensitive, affording, in particular, the Dewar benzene type of intermediates. Pyridine is known to provide the only Dewar pyridine intermediate that undergoes reversible ring-opening. We found that irradiation of photosensitive gels prepared from poly(4-vinyl pyridine) and pyridine at 254 or 312 nm leads to pyridine ring-opening and subsequent formation of 5-amino-2,4-pentadienals. We show that this light-induced process is only partially reversible, and that the photogenerated aminoaldehyde and aminoaldehyde-pending groups undergo self-condensation to produce cross-linked, conjugated oligomers that absorb light in the visible spectrum up to the near-infrared range. Such a sequence of chemical reactions results in the formation of gel with two distinct morphologies: spheres and fiber-like matrices. To gain deeper insight into this process, we prepared poly(4-vinyl pyridine) with low molecular weight (about 2000 g/mol) and monitored the respective changes in absorption, fluorescence, ¹H-NMR spectra, and electrical conductivity. The conductivity of the polymer gel upon irradiation changes from ionic to electronic, indicative of a conjugated molecular wire behavior. Quantum mechanical calculations confirmed the feasibility of the proposed polycondensation process. This new polyacetylene analog has potential in thermal energy-harvesting and sensor applications.     

Hydrogen bonding properties of oxygen and nitrogen acceptors in aromatic heterocycles

The directionality and relative strengths of hydrogen bonds to monocyclic aromatic heterocycles were investigated using crystal structure data and theoretical calculations. Surveys of the Cambridge Structural Database for hydrogen bonds between C^(sp3)(SINGLE BOND)O(SINGLE BOND)H and aromatic fragments containing one or more nitrogen and/or oxygen heteroatoms showed that hydrogen bonds to nitrogen atoms are much more abundant than to oxygen. Distinct preferred orientations were also revealed in these surveys. Theoretical calculations were performed on the interaction of methanol with pyridine, pyrimidine, pyrazine, pyridazine, oxazole, isoxazole, 1,2,4-oxadiazole, and furan as models for the heterocyclic fragments. The intermolecular potential surface was thoroughly scanned using a model potential that accurately described the electrostatic forces (derived from distributed multipole analysis) with empirical parameters for the repulsion and dispersion terms. Minima on this surface agreed well with the observed orientations in the data base and they were typically deeper for nitrogen than for oxygen acceptors, although the hydrogen bond strength and geometry was influenced by other heteroatoms in the ring. These results were confirmed by highly accurate intermolecular perturbation theory calculations, which also estimated the deviations from hydrogen bonding in the traditional nitrogen lone pair direction that could occur with negligible reduction in the interaction energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 2060–2074, 1997     

Zinc-assisted synthesis of imidazolium-tetrazolate bi-heterocyclic zwitterions with variable alkyl bridge length

Zinc halide-complexed alkyl-bridged bi-heterocycles were obtained as zwitterions from the Click reaction of a series of N-cyanoalkyl-N-alkylimidazolium halide ionic liquids with sodium azide. Reaction optimization showed fast reaction times and milder conditions compared to conventional Click syntheses from neutral nitriles.     

An efficient synthetic route to biologically relevant 2-phenylbenzothiazoles substituted on the benzothiazole ring

Certain 2-phenylbenzothiazoles containing substituents on the benzothiazole ring possess important biological properties, yet the majority of synthetic methods to 2-phenylbenzothiazoles described to date focus on their unsubstituted ring counterparts. Here we describe a new concise and efficient synthetic route to biologically relevant 2-phenylbenzothiazoles in high yield from the reaction of substituted 2-aminothiophenol disulfides and benzaldehydes, promoted by the inexpensive and non-toxic inorganic oxidant sodium metabisulfite in DMSO at 120 °C. Our new method is tolerant of a range of substituents on both the benzothiazole and phenyl ring, and affords efficient access to substituted 2-phenylbenzothiazoles without the need for column chromatography.     

Copper Complexes of Dicarboxy‐Functionalized Tetramethylcyclam. Crystal Structures of Complexes with the Axial Ligands Perchlorate,Chloride, and Iodide

6,13‐Dicarboxy‐1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane‐copper(II) perchlorate ( 2 a ) was synthesized via one‐step cyclocondensation from bis(N,N′‐dimethyl‐ ethylenediamine‐copper(II) perchlorate, formaldehyde and malonic acid. Anion exchange yielded the analogous copper(II) chloride ( 2 b ) and iodide ( 2 c ) complexes. Crystal structures of 2 a – 2 c show a square‐planar N₄‐co‐ordination of the copper(II) ion with RRSS configuration of the nitrogens and perchlorate, chloride or iodide ions weakly co‐ordinated axially. Relating to the macroring, the carboxy groups are in trans‐position. In 2 a , the packing is typical of parallel strands of complex subunits effected by dimerization of the carboxy groups, while in 2 b and 2 c formation of the supramolecular strands involve hydrogen bond interactions between carboxy groups and halide anions.