Functionalized 2-salicyloylfurans and 2-benzoyl-8H-thieno[2,3-b]indoles were prepared under mild conditions by reaction of 3-halochromones with β-ketoamides and 1,3-dihydroindole-2-thiones, correspondently. The subsequent oxidative cyclization of the products resulted in formation of the corresponding furo[3,2-b]chromen-9-ones. These molecules could also be directly prepared from 3-halochromones using a one-pot protocol. The cyclization reactions reported herein are mechanistically surprising as they proceed via the oxygen and not via the (more nucleophilic) nitrogen atom of the β-ketoamide.

2 + 2 + 2] Cycloaddition reactions belong to the not even large class of reactions, which can be catalyzed or mediated by a significant number of different transition metals. Cobalt complexes belong to the catalysts that paved the way for the extensive use and profound mechanistic knowledge on this particular transformation. Beside the established role, cyclopentadienyl (Cp) cobalt complexes inherit in synthetic applications of the cyclotrimerization reaction, modification of the precatalysts opened up novel reactivities and versatility for such catalytic initiators. At the same time the development of in situ generated cobalt catalysts allowed the conversion of novel substrates as well as novel reaction modes to be realized. In this personal account recent developments will be presented and the possibilities of catalysts containing cobalt atoms in different oxidation states be discussed.

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The Co II -catalyzed [2+2+2] cycloaddition of functionalized diynes and phosphaalkynes resulting in the formation of an array of structurally interesting phosphinines is reported. A combination of 1,2-bis(diphenylphosphino)benzene (dppbenz) and CoI 2 turned out to be the most suitable and active catalyst system under microwave reaction conditions. Chemoselective cyclizations of phosphaalkynes in the presence of nitriles have been observed, and a number of carbonyl functionalities are tolerated as well. The reaction can successfully be conducted with a 1:1 ratio of phosphaalkyne/diyne substrate. Model reactions with benzonitrile and diyne demonstrated that Co II catalysts in the presence of bisphosphines prefer carbocyclization, while reduction to lower oxidation states promotes heterocyclization to the corresponding pyridine. A reaction mechanism comprising a Co II oxidation state for the cyclotrimerization process is proposed and rationalized based on control experiments and theoretical calculations.

Catalytic cyclotrimerization routes to symmetrical [9]helical indenofluorene were explored by using different transition–metal complexes and thermal conditions. Depending on the reaction conditions, the cyclotrimerizations were accompanied by dehydro‐Diels–Alder reaction giving rise to another type of aromatic compounds. Structures of both symmetrical [9]helical cyclotrimerization product as well as the dehydro‐Diels–Alder product were confirmed by single‐crystal X‐ray diffraction analyses. Limits of enantioselective cyclotrimerization were assessed as well. DFT calculations shed light on the reaction course and the origin of diminished enantioselectivity.