text:  <unk> derivatives were first reported in the mid @-@ 1950s by Emmons and subsequently by <unk> and Horner and Jürgens . Whereas oxygen and nitrogen typically act as nucleophiles due to their high electronegativity , <unk> allow for electrophilic transfer of both heteroatoms . 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 substituent is small ( R1 = H ) , and at the oxygen atom when the nitrogen substituent has greater steric 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 @-@ hydroxylation of <unk> , epoxidation of alkenes , selective oxidation of sulfides and selenides , amination of N @-@ nucleophiles and N @-@ <unk> . 
  