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 (Me n Al(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 ≠ 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 ).