Selective cyclotrimerization of enones and alkynes by a nickel and aluminum catalytic system

Enones 1 reacted with two molecules of alkynes 2 (R = R' in eq 1) in the presence of a nickel(0) and aluminum catalytic system to give cyclotrimerization adducts regioselectively. Aluminum phenoxide (MenAl(OPh)(3-n) (n = 0-3)) functions as a Lewis acid cocatalyst and activates 1. Stoichiometric experiments suggest that the cycloaddition of 1 and 2 proceeds via nickelacyclopentadiene intermediates, which are formed by the oxidative coupling of Ni(0) species with 2. In addition, the selective cyclotrimerization of 1 and two different alkynes 2 and 2' (R not equal R' in eq 1) was also accomplished in the presence of a binary metal catalytic system. The reaction occurs effectively when an alkyl- or aryl-substituted alkyne (alkyne B, 1 equiv vs 1) is added slowly to a mixture of 1 and a bulkier alkyne (alkyne A, 1 equiv vs 1) such as tert-butylacetylene (2f) or (trimethylsilyl)acetylene (2g).