Ligand Architecture on Stereocontrol of Half‐Metallocene Benzylidene Complexes of Tantalum

This brief review discloses our studies on half‐metallocene benzylidene complexes of tantalum bearing 1,3‐butadiene, 2,3‐dimethyl‐1,3‐butadiene, o ‐xylylene, 1,4‐diaryl‐1,4‐diaza‐1,3‐butadiene, or 1‐aryl‐4‐phenyl‐1‐aza‐1,3‐butadiene as an auxiliary ligand. These auxiliary ligands can control the stereochemistry of the benzylidene moiety and induce intriguing reactivity. Furthermore, the butadiene and o ‐xylylene complexes catalyze ring opening metathesis polymerization of norbornene to respectively give polymers with cis ‐ and trans ‐CC bonds. Such selectivity can be attributed to control of the stereochemistry of the propagating alkylidene chain end by the auxiliary ligand: the butadiene complex favors the anti ‐rotamer while the o ‐xylylene complex favors the syn ‐one.