Novel Ruthenium Complex-Catalyzed Dimerization of 2,5-Norbornadiene to Pentacyclo[6.6.0.0^2,6.0^3,13.0^10,14]tetradeca-4,11-diene Involving Carbon−Carbon Bond Cleavage

Bicyclo[2.2.1]hepta-2,5-diene (2,5-norbornadiene) dimerizes in the presence of a catalytic amount of Ru(1-2:5-6-η-cyclooctadiene)(1-6-η-cyclooctatriene) (Ru(cod)(cot)) and an electron-deficient olefin such as N,N-dimethylacrylamide, dimethyl fumarate, or dimethyl maleate in toluene or tetrahydrofuran (THF) to give a new compound, pentacyclo[6.6.0.0^2,6.0^3,13.0^10,14]tetradeca-4,11-diene (PCTD), in high yield along with a small amount of a known endo−endo dimer, heptacyclo[6.6.0.0^2,6.0^3,13.0^4,11.0^5,9.0^10,14]tetradecane (HCTD), which is a major product in the reaction in DMSO. Ru(cod)(cot)-dimethyl fumarate in THF was the most efficient catalyst, and the yield of PCTD was 96% even at 40 °C. The structure of PCTD was determined by X-ray analysis of its derivative, [AgOTf(PCTD)]_n. PCTD was found to be derived via endo−endo dimerization of 2,5-norbornadiene. Formation of PCTD from two molecules of 2,5-norbornadiene involves the cleavage of two carbon−carbon bonds. Dimerization of 7-tert-butoxy-2,5-norbornadiene gave the corresponding exo- and endo-4,9-disubstituted PCTD derivatives. Ru(cod)(cot) reacts with dimethyl fumarate to give a novel complex, Ru(cot)(dmfm)₂ (dmfm = dimethyl fumarate), in high yield. The structure of the complex was determined by X-ray analysis. At 40 °C in toluene, Ru(cot)(dmfm)₂ itself catalyzes the dimerization of 2,5-norbornadiene to give PCTD in excellent yield in the absence of olefinic additives. The mechanisms of the formation of PCTD are discussed.