Formal Diels–Alder dimerisation of cyclo-octatetraene induced by carbonylruthenium complexes

Abstract

Heating cyclo-octatetraene (cot) in heptane with [Ru₄H₄(CO)₁₂] yields a previously unknown dimer of cot formally due to Diels–Alder self-addition, and derivatives [Ru(CO)₃(C₁₆H₁₆)] and [Ru₂(CO)₅(C₁₆H₁₆)]. These complexes are also formed on reaction of [Ru₃(CO)₁₂] with cot, but no free dimer is produced. Evidence for the intermediacy of [Ru(CO)₃(η⁴-C₈H₈)] in these reactions is provided by its yielding both cot dimer complexes on treatment with cot. A complex [Fe(CO)₃(C₁₆H₁₆)] of a known cot dimer is obtained when [Fe(CO)₃(η⁴-C₈H₈)] is heated with cot. Reaction of the new dimer with [Fe₂(CO)₉] provides its tricarbonyliron complex, while [Ru₃(CO)₁₂] yields [Ru(CO)₃(C₁₆H₁₆)] and also [Ru₃H₂(CO)₉(C₁₆H₁₄)]. The complex [Ru₂(CO)₅(C₁₆H₁₆)], which undergoes a unique fluxional process, is readily carbonylated to give [Ru₂(CO)₆(C₁₆H₁₆)], a fluxional cyclo-octatriene-based species. Mechanisms of the cot dimerisations are discussed and a stepwise metal template path is favoured.