Regioselectivity of metallacycle formation via intramolecular CH activation

Abstract

Carbometalation of alkynes by dimethyltitanocene affords vinyl complexes which serve as intermediates for the formation of titanacyclic products through subsequent hydrogen abstraction/methane elimination. Although multiple sites for hydrogen abstraction are present in the intermediate vinyl complexes, the hydrogen abstraction step is in most cases highly regioselective, leading to the formation of either titanacyclobutenes (via γ-H abstraction) or titanacyclopropanes (via β-H abstraction). Rotational isomerization, restricted in the case of larger vinyl group substituents, provides a simple and consistent explanation for the observed regioselectivities, and this effect is sufficiently pronounced to lead to preferential γ-aliphatic CH activation even in the presence of γ-aromatic hydrogens. The titanacyclopropanes may be alternatively formulated as allene complexes of titanocene. An independently prepared trimethylphosphine-ligated analog of 3-methyl-1,2-butadiene, representing the first titanocene allene complex to be reported, undergoes reductive coupling reactions with excess 3-methyl-1,2-butadiene and with alkynes.