Azamacrocycles show high affinity for the complexation of a wide range of transition metals. By additional reversibly coordinated ligands, these metal complexes have shown to be interesting tools for molecular recognition, especially for biological systems. Herein, the role of metal complexes of 1,4,8,11-tetraaza-cyclotetradecane ([14]aneN4 or cyclam), 1,4,7,10-tetraaza-cyclododecane ([12]aneN4 or cyclen), 1,5,9-triaza-cyclododecane ([12]aneN3), 1,4,7-triaza-cyclononane ([9]aneN3 or TACN) is reviewed.

The thermal cyclisation of enediynes to benzene-1,4-diyl diradicals (Bergman cyclisation) is affected by geometrical and electronic conditions. While the effect of ring strain or conformational constrains on the cyclisation temperature has been investigated in detail, electronic contributions have been less studied. Often geometrical and electronic contributions cannot be clearly distinguished. In most cases metal ion chelation does involve both. In this review we have summarised clear-cut observations of electronic substituents effects on the thermal enediyne reactivity. The effects of substituents in the vinylic and terminal alkyne position, the influence of benzo-fusion and hetarene fusion, as well as the changes induced by heteroatoms in the enediyne skeleton, are within the scope of this review. With the exception of more complex heterocyclic hetarene-fused enediynes the experimental data of electronic substituent effects on the thermal Bergman cyclisation of enediynes follow theoretical predictions. A review with 57 references.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

No abstract

Organic-inorganic ultraviolet (UV) active hybrid materials have been prepared by a sol-gel process from benzophenone derivatives and tetraethylorthosilicate. The silica particles are spherical in shape and have a narrow size distribution which remains unchanged up to organic chromophore concentrations of 0.2 mmol g(-1). At higher concentrations the spheres become less regular and fuse. A dependence of the material absorption properties on the particle size (at the same organic chromophore concentration) and on the concentration of surface grafted chromophores was noted. The most effective UV filter materials were found in a combination of silica incorporated chromophores and surface grafted chromophores at an overall low chromophore concentration. A comparison of the photostability of chromophores at standardized UV irradiation revealed an increase in stability for silica incorporated and surface immobilized benzophenone compared to benzophenone in a homogeneous solution.