Understanding the Reactivity of Group 6 Metal (M = Cr, W) Alkynyl Fischer Carbene Complexes with Multi‐Reactive Masked Dienes

Group 6 alkynyl Fischer carbene complexes behave as electrophiles with highly reactive masked dienes such as anthrone and 3‐hydroxy‐2‐pyrone, which also can react as bidentate nucleophiles. Both Cr0 and W0 alkynyl carbene complexes require tertiary amines to promote the formation of 1,4‐C‐addition products, but a marked dependence of the reactivity on the metal was observed. Although W0 complexes form exclusively the 1,4‐C‐adducts in the presence of a catalytic amount of base, Cr0 complexes lead to new anthrone‐derived bis‐carbene complexes, obtained by a sequential 1,4‐C‐ and 1,4‐O‐addition process with high diastereoselectivity and only in the presence of an excess of base. DFT computational studies show that the gap between the LUMO of the complex and the HOMO of the anthrone determines the differences in reactivity and that the stability of the intermediate metal allenyls formed after the initial 1,4‐addition is decisive for the outcome of the reaction. Group 6 alkynyl Fischer carbene complexes act as electrophiles with highly reactive masked dienes in the presence of tertiary amines to form different metal‐dependent products. DFT shows that the HOMOcomplex–LUMOdiene gap determines the reactivity of the Cr and W complexes, the stability of the intermediate metal allenyls formed in the initial 1,4‐addition being decisive for the reaction outcome.