Transition Metal-Catalyzed Intramolecular Cyclization of 1,5- And 1,6-Dienes via Direct Cleavage and Addition of the Carbon-Hydrogen Bond

Ruthenium- and rhodium-catalyzed intramolecular C-H/olefin coupling reactions of 1-(2-pyridyl)-, 1-(2-imidazolyl-, and 1-(2-oxazolyl)-1,5-dienes proceeded in a regiospecific manner to give 5-membered ring products. Their 1,6-diene analogues are also applicable to the cyclization reaction to give the corresponding 5- and 6-membered carbocycles. For the cyclization of the pyridine derivatives, [RhCl(PPh₃)₃] showed the highest catalytic activity with respect to efficiency and selectivity. When the imidazole derivatives were employed to the cyclization reaction, the combination of the rhodium(I) complex and the relatively sterically bulky phosphine (e.g., PCy₃ and PPFOMe) was a quite effective catalyst system. In the case of the oxazoles, the ruthenium complex, [Ru(H)₂(CO)(PPh₃)₃], showed the highest activity among the catalysts examined. These catalytic reactions usually proceed at 120°C. The [RhCl(PPh₃)₃]-catalyzed cyclization reaction of 2-[(1E)-4,4-dimethyl-1,5-hexadienyl]pyridine smoothly proceeded even at room temperature to give the corresponding cyclized product in 93% yield after 24 h. A hybrid catalyst system containing the rhodium(I) and both PPh₃ and P(o-tolyl)₃ had a slightly better catalytic activity compared with that of [RhCl(PPh₃)₃]. The intramolecular cyclization can be also applied to the C-H/CO/olefin coupling reactions. These carbonylation reactions gave the 5-membered ring ketones exclusively. The intramolecular C-H/olefin coupling reactions provide a new entry for constructing carbocyclic compounds from 1,n-dienes.