Bent N‐Heterocyclic Allenes From a Well‐Defined Titanium Vinylidene Complex

Abstract

ABSTRACT Allenes typically display a linear geometry. A notable exception occurs in allenes bearing two electron‐donating N‐heterocyclic capping groups, which can display a strongly bent C═C═C unit. These push‐push allenes, known as carbodicarbenes, are powerful carbon‐donor ligands with broad utility in chemistry. Computational studies have suggested that push‐pull allenes, featuring both an electron‐donating and withdrawing capping group, can also adopt a bent geometry. However, experimental confirmation has so far been lacking. Herein, we report allenes featuring an N‐heterocyclic capping group on one side, and either diarylmethylidene or fluorenylidene capping groups on the other. The synthesis of these allenes was accomplished via a titanium vinylidene complex. The latter could be isolated and analyzed by X‐ray diffraction, and it represents the first structurally characterized Ti vinylidene complex. The push‐pull allenes with fluorenylidene capping groups revealed markedly bent C═C═C units (α c‐c‐c < 140°), whereas the less polarized diarylmethylidene analogues were found to display a more linear geometry. The geometric differences correlate with a divergent reactivity upon thermal activation. Both types of N‐heterocyclic allenes can be used as carbon‐donor ligands for transition metal complexes.