Abstract Intermolecular cycloadditions of conjugated dienes and nitrene precursors usually produce aziridines. A generally useful method was lacking to directly provide the [4+ 1] cycloadducts, 3-pyrrolines. We have realized this transformation by using an uniquely active catalyst, copper (II) 1, 1, 1, 5, 5, 5-hexafluoroacetylacetonate ([Cu (hfacac) 2]). The method is applicable to a wide array of dienes with good yields. When 1, 4-disubsituted dienes are ...