Very Large Cyclic Compounds

Very large ring systems, containing more than 50 ring members, are becoming increasingly important in different topics in natural sciences. These so-called gigantocycles differ from smaller macrocycles in physical properties, special structural features and chemical behaviour. This article is meant to be the first summary of such ring systems and a synopsis of the most remarkable examples with their fascinating nano-scaled structures and ingenious synthesis. To restrict the scope of the article, only isolated and completely characterized, monodisperse compounds are presented. Furthermore, attention is mainly directed at organic gigantocycles. Some ultracycles with more than 100 ring members, mainly occurring in polymer chemistry and nature, will also be described.     

1,1,2,2-Tetraethynylethanes: Synthons for Tetraethynylethenes and Modules for Acetylenic Molecular Scaffolding

The synthesis of functionalized 1,1,2,2-tetraethynylethanes (= 3,4-diethynylhexa-1,5-diynes) as synthons for tetraethynylethenes (3,4-diethynylhex-3-ene-1,5-diynes) and as building blocks for three-dimensional acetylenic molecular scaffolding targeting the synthesis of the molecular carbon belts 3 and 4 is reported (Scheme 1). Reaction of diethyl oxalate and (trialkylsilyl)ethynyl Grignard reagents afforded the silyl-protected 3,4-diethynylhexa-1,5-diyne-3,4- diols 7 and 8 which were transformed in high yields into the cyclic carbonate 9 and the cyclic orthoesters 10–13 , respectively (Scheme 2). The solid-state structures of 9 and 10 were elucidated by X-ray crystallography. The alkyne protecting groups in 9, 10 , and 12 were smoothly removed to give the free tetraynes 14–16 as relatively stable oils in nearly quantitative yields (Scheme 3). Orthoesters 15 and 16 underwent Pd-catalyzed cross-coupling with iodobenzene to give the tetraphenyl derivatives 17 and 18 (Scheme 4). Thermal acid-catalyzed elimination of the orthoester moieties in 12 and 13 produced the silyl-protected tetraethynylethenes 19 and 20 and concluded a novel, simple three-step synthesis of these fully two-dimensionally conjugated π-chromophores (Scheme 5).     

Multivariate comparison of concentration profiles in materials analysis

Hierarchical cluster analysis (HCA) and principal components analysis (PCA) were applied to find groups between similar depth-profiles in thin-layers investigated by Rutherford backscattering spectrometry (RBS).HCA yields in one run an objective hierarchy of similarity for several profiles. Among the similarity coefficients examined the linear measure, the Euclidean distance and the exponential measure respond with different sensitivity to overall shifts in direction of the concentration axis, whereas the correlation measure relates to parallelism of the profiles.For agglomerative HCA with Euclidean distance, a lowest significant linkage level has been defined by use of Fisher'sF-test. For divisive HCA based also on Euclidean distance, the maximum of a separating function marks the most separating cluster step. The outcomes of both proposals agree for the data set investigated.PCA is useful for verifying the results of HCA and yields additional information about the data structure. In the actual example quite different positions of features (concentrations at definite depths) in the space of the two first principal components hint at peculiarities during the metallurgical coating process.