[2+2+2]-Cyclotrimerization of 1-Cyclopropyl-1,6-diynes with Alkynes: Formation of Cyclopropylarenes

Cyclotrimerization of 1-cyclopropyl-1,6-diynes with various terminal alkynes was tested under catalytic conditions using rhodium and ruthenium catalysts. We observed that the regioselectivity of the reaction, that is, formation of 1,2- r 1,3-regioisomers, was opposite for the two metals. For the ruthenium complex [Cp*Ru(cod)Cl]-catalyzed reactions the yields were in many cases high with a strong preference for the formation of 1,3-substituted regioisomers. In the case of catalysis by the rhodium complex [RhCl(PPh3)(3)], 1,2-substituted products were generally preferred, albeit the selectivity was often modest. However, by changing the ligand environment around the central rhodium atom the regioselectivity as well as yields of the products were significantly improved. For example, by using a combination of the rhodium complex [Rh(cod)(2)BF4] and 1,4-bis(diphenylphosphino)butane the regioselectivity was changed from 1: 1 to 1: 12 in favor of the 1,2-regioisomer. This catalytic system was also applied for synthesis of a substituted 4-cyclopropyl-3-hydroisobenzofuran-1-one that could serve as a potential intermediate for preparation of antihypertensive agents.