Episulfonium ion-mediated cyclic peptide and triazine synthesis

The product of an episulfonium ion-mediated cyclotrimerisation, previously reported as being a 15-membered ring trilactam, has now been shown to be a 1,3,5-triazine. Smaller medium-ring bilactams have, however, been synthesised from linear precursors using the sulfur-based methodology.     

Synthesis and characterization of Pd and Pt complexes of 1,3-bis(ferrocenylchalcogeno)propanes: crystal structures of FcSe(CH2)3SeFc and [M{FcE(CH2)3E'Fc}2](PF6)2 (M = Pd, Pt; E, E' = Se, Te; Fc = [Fe(eta5-C5H5)(eta5-C5H4)])

Reaction of a 1,3-bis(ferrocenylchalcogeno)propane, FcE(CH2)3E'Fc (L: E, E' = Se or Te; Fc = [Fe(eta5-C5H5)(eta5-C5H4)]), with a palladium(II) or platinum(II) precursor [M(NCMe)4](PF6)2 (M = Pd or Pt) in acetonitrile at room temperature led in good yield to the bis-chelate complexes [ML2](PF6)2. The structures of FcSe(CH2)3SeFc and all six complexes have been determined by X-ray crystallography. Electrochemical studies showed that electronic communication between ferrocenyl groups, absent in all three bis(ferrocenylchalcogeno)propanes, is established on complexation only for E = Se and E' = Se or Te, when the through-bond Fe...Fe distance is reduced to 13.17 A or less.     

Theoretical and molecular modelling studies on organic transition metal complexes,II. Calculation of the stability constants of bidentate ligands in relation to theIrving-Williams order

The bidentate chelates of the transition elements Mn to Zn with salicylaldoxime, 8-hydroxyquinoline and 8-mercaptoquinoline have been constructed using molecular graphics and the stability constants derived from the calculated reaction enthalpies using an extended CNDO/2 method. The results generally follow theIrving-Williams order except for the mercaptoquinoline where large entropy effects are known to be present experimentally. The influence of substituents on the stability constant of salicylaldoxime is reflected in the calculated value particularly for electron-withdrawing groups.

---

Theoretische und Molekülmodellierungs-Studien von organischen Übergangsmetallkomplexen, 2. Mitt.: Berechnung der Stabilitätskonstanten von zweizähnigen Liganden in Relation zurIrving-Williams-Ordnung

Zusammenfassung Die zweizähnigen Chelate der Übergangsmetalle Mn bis Zn mit Salicylaldoxim, 8-Hydroxychinolin und 8-Mercaptochinolin wurden unter Verwendung von molekulargraphischen Methoden und den Stabilitätskonstanten aus den mittels einer erweiterten CNDO/2-Methode ermittelten Reaktionsenthalpien konstruiert. Die Ergebnisse folgen generell derIrving-Williams-Ordnung, mit Ausnahme von 8-Mercaptochinolin, von dem bekannt ist, daß experimentell große Entropieeffekte auftreten. Der Einfluß von Substituenten auf die Stabilitätskonstanten der Salicylaldoximkomplexe spiegelt sich besonders für elektronenabziehende Gruppen in den berechneten Werten deutlich wider.

     

Calculations of the structure and electronic properties of extended polar hydrocarbons

The structure and nonlinear optical potential of a series of polar unsaturated hydrocarbons containing the electron-donating heptafulvene ring and electron-attracting pentafulvene ring have been explored in terms of their first hyperpolarizabilities using a semiempirical sum-over-states and time-dependent coupled Hartree-Fock approach. Both methods predict that the hyperpolarizabilities of these simple hydrocarbons are substantial and exceed those found for many extended conventional donor-acceptor systems. © 1997 John Wiley & Sons, Inc.     

Theoretical studies on the structure and electronic properties of aryl sulfides and sulfones

Molecular orbital calculations are reported on the structure and electronic properties of diphenyl sulfide using both semiempirical and ab initio methods. Neither the MNDO nor AM1 methods give satisfactory structures, but better results are obtained with the PM3 method. At the ab initio level, the 4-31G basis set with polarization functions on sulfur alone (4-31G/S*) gives comparable results to those obtained with the 6-31G** basis set. The corresponding bond lengths and angles at the sulfur atom of 4-aminophenyl-4′-nitrophenyl sulfide and related derivatives of diphenyl sulfone, diphenyl disulfide, and phenylthiosulfonate calculated at the 4-31 G/S* level show a good correlation with crystallographic data where available. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 141–147, 1998     

1,2,3-Selenadiazoles as precursors to cyclopentadienylcobalt dithiolenes and 1,4-dithiins: molecular structures of bis(cycloocteno)-1,4-dithiin and bis(cycloocteno)-1,4-diselenin

Cycloocteno-1,2,3-selenadiazole ( 1 ) and cyclopenta-dienyldicarbonylcobalt ( 2 ) react in the presence of an excess of elemental sulfur to yield the dithiolene cyclopentadienyl(1,2-cyclooctenedithiolato)cobalt ( 3 ). Available evidence indicates that this reaction involves the intermediacy of a cobalt–alkyne complex. Compound 3 is the first example of a cyclopentadienylcobalt dithiolene bearing aliphatic substituents. Analogous reactions performed using only catalytic quantities of 2 provide high-yield syntheses of bis(cycloocteno)-1,4-dithiin ( 6 ) and bis(cycloocteno)-1,4-diselenin ( 7 ), whose structures have been determined by X-ray diffraction.     

Antibacterial Activity of Biosynthesized Selenium Nanoparticles Using Extracts of Calendula officinalis against Potentially Clinical Bacterial Strains

The use of selenium nanoparticles (SeNPs) in the biomedical area has been increasing as an alternative to the growing bacterial resistance to antibiotics. In this research, SeNPs were synthesized by green synthesis using ascorbic acid (AsAc) as a reducing agent and methanolic extract of Calendula officinalis L. flowers as a stabilizer. Characterization of SeNPs was performed by UV-vis spectrophotometry, infrared spectrophotometry (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. SeNPs of 40–60 nm and spherical morphologies were obtained. The antibacterial activity of marigold extracts and fractions was evaluated by disk diffusion methodology. The evaluation of SeNPs at different incubation times was performed through the colony-forming unit (CFU) count, in both cases against Serratia marcescens, Enterobacter cloacae, and Alcaligenes faecalis bacteria. Partial antibacterial activity was observed with methanolic extracts of marigold leaves and flowers and total inhibition with SeNPs from 2 h for S. marcescens, 1 h for E. cloacae, and 30 min for A. faecalis. In addition, SeNPs were found to exhibit antioxidant activity. The results indicate that SeNPs present a potentiated effect of both antimicrobial and antioxidant activity compared to the individual use of marigold extracts or sodium selenite (Na₂SeO₃). Their application emerges as an alternative for the control of clinical pathogens.     

A theoretical analysis of the structure and electronic properties of 2-nitrophenylcyanate and 2-nitrophenylthiocyanate

Molecular orbital calculations are reported on the structure and electronic properties of both 2-nitrophenylcyanate and 2-nitrophenylthiocyanate using both the semiempirical AM1 and PM3 methods and the ab initio 3-21G and 6-31G** basis sets. The resulting structures are compared with crystallographic data where available. The marked difference in biological activity observed between the two molecules appears to be attributable to large differences in their electronic properties rather than to any geometric factors. © 1996 John Wiley & Sons, Inc.