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-eta-cyclooctadiene)( 1-6-eta-cycloocta triene) (Ru(cod)(cot)) and an electron-deficient olefin such as N,N-dimethy lacrylamide, dimethyl fumarate, or dimethyl maleate in toluene or tetrahydr ofuran (THF) to give a new compound, pentacyclo[6.6.0.0(2,6).0(3,13).0(10,1 4)]tetra Il-diene (PCTD), in high yield along with a small amount of a know n 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(co d)(cot)-dimethyl fumarate in THF was the most efficient catalyst, and the y ield of PCTD was 96% even at 40 degrees C. The structure of PCTD was determ ined 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 t he 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 w as determined by X-ray analysis. At 40 degrees C in toluene, Ru(cot)(dmfm)n itself catalyzes the dimerization of 2,5-norbomadiene to give PCTD in exce llent yield in the absence of olefinic additives. The mechanisms of the for mation of PCTD are discussed.