Michael Röper

Chelated Bisphosphites with a Calix[4]arene Backbone: New Ligands for Rhodium-Catalyzed Low-Pressure Hydroformylation with Controlled Regioselectivity

Extremely high regioselectivities were achieved in the rhodium-catalyzed low-pressure hydroformylation of 1-octene to n-nonanal with chelate bisphosphites that contain a p-tert-butylcalix[4]arene backbone (shown in the picture). It was possible to tailor the structures and ultimately the catalytic properties of these complexes using molecular modeling calculations.

Control of regioselectivity in the palladium catalyzed telomerization of isoprene with diethylamine; synthesis of a new head-to-head terpene amine

Abstract It has been shown that the regioselectivity of the palladium catalyzed telomerization of isoprene and diethylamine is drastically changed by the use of acidic co-catalysts. The tail-to-tail terpene amine is formed in the absence of acids, whereas the use of weak acids such as CO2 favours the formation of the head-to-tail and tail-to-head isomers. A variety of Bronsted and Lewis acids has been examined and it has been shown that a gradual shift towards increased formation of the tail-to-head and a new head-to-head isomer occurs with increasing acid concentration and strength. When strong acids such as BF3 · Et2O were used in combination with a basic ligand such as tricyclohexyl-phosphine, the head-to-head isomer was formed with a selectivity of up to 80%. A mechanistic model is proposed involving cationic palladium intermediates in an attempt to explain the observed selectivities.

Palladium catalyzed telomerization of isoprene with secondary amines and conversion of the resulting terpene amines to terpenols

Abstract Isoprene has been telomerized with a variety of symmetrical secondary amines using palladium-trialkylphosphite catalysts. The reactivity of the amines is mainly controlled by their stereochemical properties; thus dimethylamine reacted smoothly with isoprene, whereas diisopropylamine gave a sluggish reaction. The regioselectivity of isoprene coupling is not influenced by the nature of the secondary amines and the tail-to-tail terpene amines were formed in high selectivities. In a reaction sequence including oxidation, Meisenheimer rearrangement and reductive cleavage, the terpenol 2,7-dimethyl-1,7-octadien-3-ol has been prepared in high yields from the N,N-dimethyl terpene amine.

Structure-activity relationship for chelating phosphite ligands used in rhodium-catalyzed hydroformylations

Abstract A simplified kinetic model was developed for the rhodium-catalyzed low-pressure hydroformylation using chelating phosphite ligands. This allowed the determination of the relative rate constants for linear and branched aldehyde formation starting with terminal olefins using data from complex product mixtures. Structural data were obtained using X-ray crystallography and served as the basis for molecular modeling of rhodium complexes with such chelating ligands. The interaction of linear and branched alkyl groups with the ligand environment in such complexes was quantified using molecular modeling. The energy differences obtained with molecular modeling were plotted against the energy differences obtained from kinetic experiments and a linear correlation was found. This type of approach, i.e., development of a structure–activity relationship, can be used to quickly and efficiently investigate such catalytic systems.

Methanol homologation by cobalt-phosphine-iodine catalyst systems

Abstract Despite numerous research efforts, the mechanism of methanol homologation and especially the role of iodine promoters remain unclear. In an approach to improve the understanding of this reaction, the basic properties of cobalt-iodine-phosphine catalyst systems have been studied. In addition, well-defined cobalt complexes and clusters were used as catalysts, and experiments with labelled reagents have been carried out. Based on these data a reaction mechanism via oxidative addition of methyl iodide to a cobalt(I) center is proposed.

Chelatbisphosphite mit Calix[4]arenrückgrat: neue Liganden für die rhodiumkatalysierte Niederdruckhydroformylierung mit steuerbarer Regioselektivität

Erstaunlich hohe Regioselektivitaten werden bei der rhodiumkatalysierten Niederdruckhydroformylierung von 1-Octen zu n-Nonanal mit Chelatbisphosphiten erzielt, die ein p-tert-Butylcalix[4]aren-Ruckgrat aufweisen (siehe Bild). Mit Hilfe von Molecular-Modeling-Rechnungen konnten die Strukturen und damit die katalytischen Eigenschaften der Komplexe masgeschneidert werden.

Intermediates in cobalt-catalysed methanol homologation: labelling studies with deuterated methanol and methyl iodide

Abstract The cobalt-catalysed homologation of perdeuterated methanol with CO/H2 gives C2 products in which the CD3 group remains intact. GC/MS measurements showed that no H/D exchange unless the methanol conversion exceeded 50%. These results indicate that methylene species are unlikely to be as catalytic intermediates, and favour methyl species for such intermediates. In the presence of iodine promoters methyl iodide is a likely intermediate since it is much more readily consumed than methanol in carbonylation/hydrocarbonylation reactions. This was shown by treating a 5 1 mixture of CH3OH and CD3I with CO/H2 in the presence of Co2(CO)8; at short reaction times only the carbonylation/hydrocarbonylation products of methyl iodide could be detected by GC/MS. Methyl iodide can be formed from a variety of iodine compounds under homologation conditions, as was confirmed by separate experiments.

Preparation and properties of acetyliodotricarbonylcobaltate; a novel product from the reaction of methyl iodide with tetracarbonylcobaltate

Abstract The reaction of PPN[Co(CO) 4 ] with an excess of methyl iodide in THF at 0°C gives almost quantitatively PPN[Co(CO) 3 {C(O)CH 3 })I], which has been shown by X-ray crystallography and spectroscopic measurements to adopt a trigonal bipyramidal structure with three carbonyls in the equatorial plane; it reacts with hydrogen or with sulfuric acid to give acetaldehyde, and with methanol/pyridine to give methyl acetate. The new anion is of interest as a potential intermediate in the cobalt/iodine catalyzed carbonylation or hydrocarbonylation of methanol.

Alkyl acetates by stoichiometric reaction of alcohols with the methylene bridged complex [Fe2(μ-CH2)(CO)8]

Resume The stoichiometric reaction of the methylene-bridged complex [Fe 2 (μ-CH 2 )-(CO) 8 ] with methanol and ethanol gives methyl and ethyl acetate, respectively, in high yields. Labelling studies with CH 3 OD and CD 3 OD provide insight in the reaction steps involved.

Anchoring of hydridic clusters by acid–base reactions: new method for the preparation of highly active Fischer–Tropsch catalysts

Hydridic complexes such as HFeCo3(CO)12 can be effectively supported on basic supports such as silica modified by amino donor functions in an acid–base reaction to give highly active Fischer–Tropsch catalysts which give rise to an unusual product distribution.