A Ghost Trapped: Realization of the 1‐Titanacyclobuta‐2,3‐diene as the First Four‐Membered Group 4 Metallacycloallene

Abstract

Very recently the synthesis of the first four‐membered metallacycloallene of a group 4 metal, the very unusual 1‐titanacyclobuta‐2,3‐diene [rac‐(ebthi)Ti[‐C(SiMe3)=C=C(SiMe3)‐] (rac‐ebthi = rac‐1,2‐ethylene‐1,1′‐bis(η5‐tetrahydroindenyl)) was reported by Reiß and Beweries. This complex has its own story since 2004 when the five‐membered metallacycloallene was proposed, for which later many examples were described. Over the years several studies have described preparative and computational attempts to stabilize and describe such a complex, before the successful synthesis of the above mentioned smaller four‐membered metallacycloallene. Only by appropriate variation of the cyclopentadienyl ligand of the titanocene complex it was possible now to realize this highly strained compound.