Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO2(L)2] (1–4) with aryloxide-pyrazole ligands L = L1–L4 is described. Catalysts 1–4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively. This is in contrast to the structure found for the related aryloxide-oxazoline coordinated Mo(VI) dioxo complex 5. This was confirmed by the determination of the molecular structures of complexes 1–3 by X-ray diffraction analyses. Compounds 1–4 show high catalytic activities in the epoxidation of various olefins. Cyclooctene (S1) is converted to its epoxide with high activity, whereas the epoxidation of styrene (S2) is unselective. Internal olefins (S3 and S4) are also acceptable substrates, as well as the very challenging olefin 1-octene (S5). Catalyst loading can be reduced to 0.02 mol % and the catalyst can be recycled up to ten times without significant loss of activity. Supportive DFT calculations have been carried out in order to obtain deeper insights into the electronic situation around the Mo atom.

Planar chiral 5,11-disubstiuted dibenzo[a,e]cyclo-octatetraenes (dbCOTs) have been developed as the first useful chiral homologs to dbCOT-ligands for asymmetric applications. Methods enabling the preparation of such compounds on a gram-scale in enantiomerically pure form are described. Evaluated as ligands in rhodium(I)-catalyzed 1,4- and 1,2-arylation reactions, tertiary and quarternary stereogenic centers were formed with excellent yields and selectivities of up to >99 % ee. A catalytic asymmetric synthesis of a key cyclization precursor to (-)-penifulvin A highlights the system in an applied context.

Pure hydrocarbons with shape and conjugation properties that can be switched by external stimuli is an intriguing prospect in the design of new responsive materials and single-molecule electronics. Here, we develop an oligomeric [8]annulene-based material that combines a remarkably efficient topological switching upon redox changes with structural simplicity, stability, and straightforward synthesis: 5,12-alkyne linked dibenzo[a,e]cyclooctatetraenes (dbCOTs). Upon reduction, the structures accommodate a reversible reorganization from a pseudo-conjugated tub-shape to a conjugated aromatic system. This switching in oligomeric structures gives rise to multiple defined states that are deconvoluted by electrochemical, NMR, and optical methods. The combination of stable electromechanical responsivity and ability to relay electrons stepwise through an extended (pseudo-conjugated) π-system in partially reduced structures validate alkyne linked dbCOTs as a practical platform for developing new responsive materials and switches based on [8]annulene cores.

Chiral dienes are useful ligands in a number of asymmetric transition-metal-catalyzed reactions. Here, we evaluate the efficiency of 2,5-disubstituted 1,3 a ,4,6 a -tetrahydropentalenes as ligands to rhodium(I). 2,5-Dibenzyl and diphenyl tetrahydropentalenes were synthesized in two steps and resolved, either chromatographically, or through fractional crystallization of diastereomeric rhodium(I) salts. When evaluated in a 1,4-arylation reaction, the 2,5-dibenzyl ligand gave up to 99% ee. The use of a well-defined rhodium complex as catalyst, Cs 2 CO 3 as the base, and toluene/water as solvent was found to have a pronounced beneficial effect on the selectivity of the reaction. The homologous 2,5-diphenyl ligand on the other hand proved to be highly prone to racemization/loss of chirality during catalysis. Control experiments reveal that this rearrangement proceeds via a rhodium-mediated 1,3-hydride shift. Implications for ligand design and catalysis are discussed.

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