Ruthenium(II)-catalyzed isomer-selective cyclization of 1,6-dienes leading to exo-methylenecyclopentanes:: Unprecedented cycloisomerization mechanism involving ruthenacyclopentane(hydrido) intermediate

In the presence of a catalytic amount of ruthenium(II) complexes, [RuCl2(cod)](n), RuCl2(cod)(MeCN)(2), [RuCl2(nbd)](n). [RuCl2(CO)(3)](2), and Cp*Ru(cod)Cl, 1,6-dienes were effectively converted into the corresponding exo-methylenecyclopentanes in good to excellent yields with good isomer purity in i-PrOH at 90 degreesC. The alcoholic solvent was essential for the present catalytic cyclization, and the efficiency increased in the following order: t-BuOH << EtOH less than or equal to i-PrOH. In contrast, a Ru(0) complex. (C6Me6)Ru(cod), catalyzed the cycloisomerization only in 1,2-dichloroethane. The unusual isomer-selectivity occurred when a 1,7-octadiene was subjected to cyclization to give a similar exo-methylenecyclopentane isomer as the major product. The identical isomer selectivity was observed for the cyclization of unsymmetrical 1,6-dienes having one terminal- and one internal-alkene termini. On the basis of the results from the studies using the known ruthenium hydrides and deuterium-labeling substrates, the novel mechanism via the Ru(II) <--> Ru(IV) system involving a ruthenacyclopentane(hydrido) intermediate was proposed, which better explains the particular regiochemistry of the present cyclization than other previous mechanisms.