Manganese-catalyzed [2+2+2] cycloaddition reactions – defined precatalyst vs. in situ activation approaches

The applicability of the cyclopentadienyl oligoolefin manganese complex [Cp’Mn(CHT)] (Cp’ = methylcyclopentadienyl, CHT = η 6 -cycloheptatriene) ( Mn2 ) for the [2 + 2 + 2] cyclotrimerization of triynes to arenes under photochemical conditions is showcased. The reactivity of Mn2 , which is prepared from the commercially available manganese precursor methylcymantrene [Cp’Mn(CO) 3 ] ( Mn1 ) and cycloheptatriene, is demonstrated either as an isolated precatalyst or in an in situ protocol, combining the Mn2 synthesis with the catalytic [2 + 2 + 2] cyclization. The optimization of the reaction conditions as well as the scope and limitation for the cyclotrimerization reaction are outlined. Mechanistically, the reaction is investigated by light-on/light-off experiments and additive/poisoning studies, highlighting the necessity of continuous irradiation and indicating an inner-sphere mechanism.