No abstract

A series of novel chelating N-heterocyclic bis-carbene rhodium(I) complexes based on 1,2,4-triazoles, benzimidazoles, and imidazoles with varying bridge length and N substituents has been synthesized. In addition to their structural characterization, their catalytic activity in the hydrosilylation of 4-fluoroacetophenone with diphenylsilane has been examined. The results reveal that selectivities and turnover frequencies (TOFs) are influenced by bridge length, steric demand of the N substituents, and electronic properties of the heterocycle. † Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth.

The current status of rhodium-catalyzed hydrosilylation of ketones is reviewed, focusing on development milestones leading to state of the art chiral ligand systems and mechanistic understanding. Four ligand classes are discussed: phosphorus-, nitrogen-, mixed-donor ligand-as well as N-heterocyclic carbene-based ligand systems. Results relevant for a mechanistic understanding of the reaction are presented, starting from the initial investigations by Ojima, limitations of the first mechanistic picture leading to the Zheng-Chan mechanism, which was recently replaced by a silylene-based mechanism introduced by Hofmann and Gade.

The dioxygen reactivity of a cyclic iron(ii) tetra-NHC-complex (NHC: N-heterocyclic carbene) is investigated. Divergent oxidation behavior is observed depending on the choice of the solvent (acetonitrile or acetone). In the first case, exposure to molecular oxygen leads to an oxygen free Fe(iii) whereas in the latter case an oxide bridged Fe(iii) dimer is formed. In acetone, an Fe(iii)-superoxide can be trapped, isolated and characterized as intermediate at low temperatures. An Fe(iii)-O-Fe(iii) dimer is formed from the Fe(iii) superoxide in acetone upon warming and the molecular structure has been revealed by single crystal X-ray diffraction. It is shown that the oxidation of the Fe(ii) complex in both solvents is a reversible process. For the regeneration of the initial Fe(ii) complex both organic and inorganic reducing agents can be used.