Study on the reactivity of oxabicyclic alkenes in ruthenium-catalyzed [2+2] cycloadditions.

The ruthenium-catalyzed [2+2] cycloadditions of various bicyclic alkenes with an alkyne have been investigated. The presence of the oxygen in the bridgehead of the bicyclic alkene significantly enhanced the rate of the ruthenium-catalyzed [2+2] cycloadditions. The presence of a C1-substituent on the oxanorbornadiene decreased the rate of the cycloaddition and electron-withdrawing C1-substituents were found to be more reactive than electron-donating C1-substituents in the Ru-catalyzed [2+2] cycloaddition. The nature of the substituent on the benzene ring of oxabenzonorbornadienes showed little effect on the rate of the cycloaddition.

Ruthenium(II)-catalyzed cyclization of azabenzonorbornadienes with alkynes.

The ruthenium-catalyzed cyclization of azabenzonorbornadienes with alkynes leads to an unanticipated dihydrobenzoindole framework. Depending on the structure of the alkyne and the Ru catalyst, either a dihydrobenzoindole and/or a [2+2] cycloaddition product could be formed. Cp*Ru(COD)Cl was found to be an active catalyst for the cyclization of an azabenzonorbornadiene with a propargylic alcohol to produce the dihydrobenz[g]indole as a single regio and stereoisomer in good yield. For other alkynes, selective formation of the dihydrobenz[g]indole is possible by using a cationic Ru catalyst, [Cp*Ru(CH3CN)3]PF6.