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JEE Advanced levelSingle correct optionDifficulty 3 of 3g3103

Singlet ethoxycarbonylnitrene, , is generated by photolysis of ethyl azidoformate in the presence of (E)-but-2-ene. The nitrene adds to the double bond to form an aziridine. Which description of the product is correct? (Treat the N-substituted ring nitrogen as not a stereocentre.)

  1. AA racemic mixture of the trans-2,3-dimethylaziridine, i.e. the (2R,3R) and (2S,3S) enantiomers
  2. BAn equimolar mixture of cis and trans aziridines
  3. CA single enantiomer of the trans-aziridine
  4. DThe meso cis-2,3-dimethylaziridine

Answer: A

A singlet nitrene adds in one concerted, stereospecific step, like a singlet carbene, so the trans geometry of the alkene is kept. The two ring carbons become stereocentres without a symmetry plane in the trans isomer. Attack on either face of the alkene is equally likely, so equal amounts of (2R,3R) and (2S,3S) form, giving a racemate and not a single enantiomer.
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