Further studies on Ni(0)-catalyzed cyclization of a branched 1,3-diene and tethered aldehyde via oxa-nickelacycle intermediate

The reactivity of oxa-nickelacycles 7, generated by the reaction of a branched 1,3-diene and tethered carbonyl group with a Ni(0) complex, was investigated in detail. It was found that oxa-nickelacycles 7 are relatively stable and that β-hydride elimination from 7 occurred at a high temperature, producing the cyclized dienes 51 and/or 52 in good yields. This Ni(0)-catalyzed cyclization via β-hydride elimination from oxa-nickelacycles tolerated various substituents on the diene moiety and could be applied to a five- to seven-membered ring construction. Next, transmetalation of oxa-nickelacycle 7 with various organometallic reagents was investigated. It was found that the tandem reaction, i.e. cyclization of 6 followed by transmetalation of the resulting oxa-nickelacycle 7, proceeded smoothly, giving 53 and/or 54 in good yields. In addition, the catalytic cycle in this transmetalation reaction was also established.