NICKEL-CATALYZED [2-PI-PI+2-PI] (HOMO-DIELS-ALDER) AND [2-PI+2-PI] CYCLOADDITIONS OF BICYCLO[2.2.1]HEPTA-2,5-DIENES(2)

Active catalysts which promote the homo-Diels-Alder (HDA) cycloadditio n with a variety of electron-deficient olefins with bicyclo[2.2.1]hept a-2,5-diene (norbornadiene, NBD) have been developed. The nickel compl ex and the additives (e.g., Ligands, reducing agents) influence the ac tivity of the catalyst. Dienophiles which participate in this cycloadd ition include acyclic and cyclic enones, lactones, sulfones, and sulfo xides. The dienophile substituent, the catalyst, and the temperature a ffected the exo/endo selectivity in the HDA reaction. A diastereoselec tive reaction with an optically enriched vinyl sulfoxide led to the sy nthesis of an optically active deltacyclane. The regio- and stereosele ctivity of the HDA reaction between 2-substituted norbornadienes and e lectron-deficient dienophiles has also been studied. The substituents on the diene and dienophile as well as the ligands were found to exert a dramatic effect on the selectivity. With very reactive dienophiles, an alternative [2 pi+2 pi] cycloaddition was discovered for 2-substit uted norbornadienes. In some cases, the [2 pi+2 pi] cycloaddition can occur with high chemo- and regioselectivity and moderate levels of ste reoselectivity.