TRANSITION-METAL-CATALYZED INTRAMOLECULAR CYCLIZATION OF 1,5-DIENES AND 1,6-DIENES VIA DIRECT CLEAVAGE AND ADDITION OF THE CARBON-HYDROGEN BOND

Ruthenium- and rhodium-catalyzed intramolecular C-H/olefin coupling re actions of 1-(2-pyridyl)-, 1-(2-imidazolyl)-, and 1-(2-oxazolyl)-1,5-d ienes proceeded in a regiospecific manner to give 5-membered ring prod ucts. 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 t he highest catalytic activity with respect to efficiency and selectivi ty When the imidazole derivatives were employed to the cyclization rea ction, the combination of the rhodium(I) complex and the relatively st erically 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 catalys ts examined. These catalytic reactions usually proceed at 120 degrees C. The [RhCl(PPh3)(3)]-catalyzed cyclization reaction of 2-[(1E)-4,4-d imethyl-1,5-hexadienyl]pyridine smoothly proceeded even at room temper ature to give the corresponding cyclized product in 93% yield after 24 h. A hybrid catalyst system containing the rhodium(I) and both PPh3 a nd P(o-tolyl)(3) had a slightly better catalytic activity compared wit h that of [RhCl(PPh3)(3)]. The intramolecular cyclization can be also applied to the C-H/CO/olefin coupling reactions. These carbonylation r eactions gave the 5-membered ring ketones exclusively. The intramolecu lar C-H/olefin coupling reactions provide a new entry for constructing carbocyclic compounds from 1,n-dienes.