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.

Graphical abstractThe formation of a benzocymantrene derivative from the reaction of a substituted lithio-indene with (CO)5MnBr was investigated theoretically and experimentaly.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Treatment of an indenyllithium derivative with (CO)5MnBr affords new benzocymantrene. ► DFT-D2 investigations underline the key role of decarbonylation steps. ► Degradation of transient (η1-indenyl)Mn(CO)5 intermediates is also responsible for low yields.