Catalytic ring size effect

The creation of nanostructured surfaces is one thing, the study of electrochemical reactions on such nanostructures is another one. Especially in electrocatalysis, where size effects on reactivity are often discussed, there have been attempts to use the tip of an STM as a detector electrode for reaction products from, say, catalytically active metal nanoclusters [84]. Flowever, such ring-disk-type approaches are questionable,... 

Only two general methods have been developed for the synthesis of the macrocyclic annulenes.9 The first of these, developed by Sondheimer and co-workers, involves the oxidative coupling of a suitable terminal diacetylene to a macrocyclic polyacetylene of required ring size, using typically cupric acetate in pyridine. The cyclic compound is then transformed to a dehydroannulene, usually by prototropic rearrangement effected by potassium i-butoxide. Finally, partial catalytic hydrogenation of the triple bonds to double bonds leads to the annulene. 

Effect of Ring Size (in Number of C Atoms) on the Relative Rates of Catalytic Reactions... 

As discussed for biphenol-based phosphites, phosphites derived from ClMeO-biphenol should also be sterically more demanding and have an increased ring size as compared to catechol-based phosphites. Moreover, by comparing the catalytic results of biphenol- and ClMeO-biphenol-based ligands, conclusions on the influence of the substitution pattern and resulting electronic effects on the activity and selectivity in the hydroformylation can be drawn. Therefore, ClMeO-biphenol-based phosphites with PEG side chains consisting of one to three ethylene glycol units were included in the present study. 

A. Zanardo, R. A. Michelin, F. Pinna, G. Strukul, Epoxidation of olefins catalyzed by chelating diphosphine-platinum(II) complexes. Ring-size and ring-shape effects on the catalytic activity, Inorg. Chem. 28 (1989) 1648. 

Since the skeletal framework of the commercially important silicones is composed of siloxane (Si—O) bonds, their ring-ring and ring-polymer interconversions are not only of academic interest but of practical concern as well. Information has been selected electrically here and is meant to help understand the much larger body of information that cannot be included. Of first consideration are the key parameters that govern the ring-chain equilibria of siloxane systems concentration of Si—O skeletal components, functionality of the Si components, ring size, steric effects of Si substituents, temperature and catalytic conditions. These factors are seldom independent. 

Structure was determined by X-ray analysis. A detailed paper on the intramolecular acylation of long-chain >-(2-thienyl)alkanoic acids at the 5-position in acetonitrile solution in the presence of trifluoroacetic anhydride and catalytic amounts of H3PO4 has appeared. The ring-sizes of the resulting (2,5)thiophenophan-l-ones ranged from 12 to 21. Reliable rate data and effective molarities were obtained, for use in structure-reactivity correlations. This cyclization procedure was used for the preparation of a key intermediate in a five-step synthesis of //-muscone. ... 

Shinkai and coworkers [8] have prepared the water soluble hexasulfonate of calix[6]arene and have shown that it enhances the rate of the acid-catalyzed addition of water to A/ -benzyldihydronicotinamide. With the water soluble carboxy calixarenes now available by the procedure described in Section 2 it is of interest to compare these with the sulfonate and also to ascertain the effect of ring size on the catalysis. The observed rates of hydration, the values of the catalytic constant... 

The synthesis and characterization of a series of complexes 35 bearing six-and seven-membered NHC and general formula (NHC)Pd(dvtms) (dvtms = divin)dtetramethyldisiloxane) was recently reported [44]. In order to investigate the effect of increasing ring size on the catalytic activity, the complexes were... 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

---