Cascade Reactions of Dialkynyl Fischer Carbene Complexes Involving Intramolecular Alkyne Insertions Oriented to the Synthesis of Functionalized Polycycles

The cascade reactions of alkynylchromium Fischer carbene complexes featuring an additional pendant triple bond are presented. The processes are triggered by [2 + 2], [3 + 2], and [4 + 2] cycloadditions. Depending on the substitution of the appended triple bond, and the nature of the triggering cycloaddition reaction, different polycyclic compounds can be obtained. Predictable results are obtained in most of the cases when the Fischer carbene complex bears a TMS-substituted alkyne. In these cases, naphtalene derivatives substituted with a TMS-ketene are obtained in sequences that involve alkyne exo insertion followed by CO migration. However, the reaction with a carbene complex featuring a terminal alkyne follows an alternative reaction pathway, in which the nucleophilic attack of the triple bond at the carbenic carbon forms a benzo[7]annulene. Mechanistic interpretations based on DFT calculations are provided.