Synthesis of n-chloroquinolines and n-ethynylquinolines (n=2, 4, 8): homo and heterocoupling reactions

The n-(ethynyl)quinolines were satisfactorily prepared by heterocoupling reaction between the appropriate n-chloroquinoline and 2-methyl-3-butyn-2-ol, catalyzed by palladium, followed by treatment with a catalytic amount of powdered sodium hydroxide in toluene. The n-(ethynyl)quinolines were transformed in the corresponding conjugate 1,4-bisn′-(quinolyl)]buta-1,3-diynes by oxidative dimerization, catalyzed by cuprous chloride, with excellent yields. Moreover, the heterocoupling between n′-haloquinoline and n′-(ethynyl)quinoline (n′, 2′ or 3′), catalyzed by palladium, gives 2′,2′-bis(quinoline) or 1,2-di(3′-quinolyl)ethyne, respectively. The same coupling reaction with zerovalent nickel complexes, gives a mixture of 1,2,4- and 1,3,5-tri(n′-quinolyl)benzene.