 Nitrogen analogues of the <unk> <unk> reaction ( known as imino @-@ <unk> <unk> reactions ) have few instances ; there is one example of a generalized imino @-@ <unk> <unk> reported ( shown below ) , and several iso @-@ imino @-@ <unk> reactions in the literature . Even these tend to suffer from poor <unk> , poor yields , or narrow scope . The difficulty stems from the relative over @-@ stabilization of the <unk> cation by electron donation , impeding <unk> . 
