 <unk> derivatives were first reported in the mid @-@ 1950s by <unk> and subsequently by <unk> and Horner and <unk> . Whereas oxygen and nitrogen typically act as <unk> due to their high <unk> , <unk> allow for <unk> transfer of both <unk> . This unusual reactivity is due to the presence of the highly strained three membered ring and the relatively weak N @-@ O bond . <unk> tend to attack at the <unk> nitrogen when the nitrogen <unk> is small ( R1 = H ) , and at the oxygen atom when the nitrogen <unk> has greater <unk> bulk . The unusual electronics of the <unk> system may be exploited to perform a number of oxygen and nitrogen transfer reactions including , but not limited to : α @-@ <unk> of <unk> , <unk> of <unk> , selective oxidation of sulfides and <unk> , <unk> of N @-@ <unk> and N @-@ <unk> . 
