 <unk> <unk> of the <unk> <unk> reaction ( known as <unk> @-@ <unk> <unk> reactions ) have few instances ; there is one example of a <unk> <unk> @-@ <unk> <unk> reported ( shown below ) , and several <unk> @-@ <unk> @-@ <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 , <unk> <unk> . 
