Synthesis of a 2-benzocymantrenylpyridine and further mechanistic insights

The synthesis of a new benzocymantrene derivative containing an appended pyridyl moiety was attempted by reacting a lithioindenylpyridine with (CO) ₅MnBr. The reaction led to the formation of small amounts of the targeted benzocymantrene, which was subsequently characterized by structural X-ray diffraction analysis. Alternative treatment of the same ligand with (η¹-benzyl)pentacarbonylmanganese led to the formation of a five-membered manganacycle resulting from the C-H bond activation at the five-membered ring of the indenyl group. To rationalize the origin of the poor yield in the benzocymantrene derivative a complete study of the reaction profile was carried out by means of a state-of-the-art DFT-D2 method. The theory indicated that the main difficulty in producing the targeted benzocymantrene stems from the relatively high activation energy barrier necessary to convert a (η¹-indenyl)Mn(CO)₅ intermediate into a (η³-indenyl)Mn(CO)₄ transient species.