Further Studies on Nickel-Promoted or -Catalyzed Cyclization of 1,3-Diene and a Tethered Carbonyl Group

Abstract

A nickel-promoted intramolecular cyclization of 1,3-diene with the tethered carbonyl group was developed using the catalyst generated by reduction of Ni(acac)2 with DIBAL-H in the presence of PPh3. It was found that the addition of 1,3-CHD to the reaction mixture affected the regiochemistry of olefin on the side chain of the cyclized product. The reaction course of this cyclization can be accounted for by two possible mechanisms. In one mechanism, a nickel hydride complex plays a key role and the cyclization proceeds via π-allylnickel intermediate. In the other mechanism, a zero-valent nickel complex is the active species and the cyclization proceeds via nickelacycle intermediates. These mechanistic considerations led to finding two nickel(0)-catalyzed cyclizations of 1,3-diene and the tethered aldehyde, in which the five- to seven-membered ring products were produced in a regio- and stereoselective manner via π-allylnickel intermediate or via a transmetalation process of nickelacycle intermediates with iBu2-ALAC.