Systematic Evaluation of Substituted Cyclopentadienyl Ruthenium Complexes, [(η5‐C5MenH5−n)RuCl(cod)], for Catalytic Cycloadditions of Diynes

A series of η⁵‐cyclopentadienylruthenium complexes, (η⁵‐C₅Me_nH_5?n)RuCl(cod)] (cod=1,5‐cyclooctadiene), are evaluated as catalysts for the cycloaddition of 1,6‐diynes with alkynes. As a result, we unexpectedly found that the complex bearing the 1,2,4‐Me₃Cp ligand is the most efficient catalyst in terms of turnover number (TON) for the cycloaddition of a bulky diiododiyne with acetylene, recording the highest TON of 970 with a catalyst loading of 0.1 mol %. To obtain insight into this result, we evaluate the electron richness of all complexes by cyclic voltammetric analyses, which indicate that the electron density of the ruthenium center increases with an increase in methyl substitution on the Cp′ ligands. The initial rate (up to 10 % conversion) of the cycloaddition was then measured using ¹H NMR spectroscopy. The initial rate is found to decrease as the number of methyl substituents increases. According to these results, we assumed that the optimum catalytic performance exhibited by the 1,2,4‐trimethylcyclopentadienyl complex can be attributed to its robustness under the catalytic cycloaddition conditions. The steric and electronic effects of the Cp′ ligands are also investigated in terms of the regioselectivity of the cycloaddition of an unsymmetrical diyne and in terms of the chemoselectivity in the cycloaddition of a 1,6‐heptadiyne with norbornene.