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Nitrenes slideshare Reactive intermediates

  1. TUMKUR UNVIVERSITY Ph.D COURSEWORK SEMINAR Presented by Divyarani .K Research scholar in chemistry Tumkur university , Tumakuru Under the guidance Dr.S.Sreenivasa Associate professor in chemistry, UCS Tumkur university Deputy adviser NAAC , Bengaluru
  2. CONTENTS  Prelude  Electronic states of nitrenes  Stability of nitrenes  Generation of nitrenes  Reaction of nitrenes  Conclusions  References
  3. NITRENES Prelude: What are Nitrenes? Nitrenes are uncharged, electron deficient monovalent nitrogen species They are isoelectronic with carbenes The nitrogen atom in nitrenes has a sextet of electrons Other names found in literature are imidogens, azacarbenes, azenes and imenes.
  4. ELECTRONIC STATES OF NITRENES In singlet state , there are two pairs of electrons, one pair in sp orbital and other pair in in a p orbital. one p orbital in singlet state remains vacant . In triplet state, one electron pair is in sp- orbital and two p orbitals remain half- filled having electrons in parallel spin. Triplet nitrene is more stable and represents the ground state electronic configuration of a nitrene. ESR Spectroscopy also confirmed that the triplet state of nitrenes is in the ground state
  5. ELECTRONIC STATES OF NITRENES: • The energy difference between the singlet and triplet nitrenes is usually much larger for nitrenes than that of carbenes i.e 145 KJ/Mole for nitrene 32-47 KJ/mole for carbene. • The energy difference is due to the electronegativity difference in carbon and nitrogen. • Nitrogen is more electronegative than carbon and therefore it holds its electrons closer to the nucleus which decreases energy and hence increases stability.
  6. STABILITY OF NITRENES: Nitrenes are very reactive species and cannot be isolated. However , a nitrene has been trapped by its reaction with carbon monoxide to yield an isocyanate. Nitrenes can also be trapped in presence of ethylene.
  7. GENERATION OF NITRENES (1) DECOMPOSITION OF AZIDES: Thermal or photolytic decomposition of azides give nitrenes:
  9. GENERATION OF NITRENES (2)OXIDATION OF PRIMARY AMINO GROUPS IN N-AMINO COMPOUNDS: The removal of two hydrogen atoms from an –NH2 group can potentially form a nitrene .
  11. REACTIONS OF NITRENES (1)FORMATION OF AZIRIDINES: Usually nitrene , generated from azides react with olefins to form aziridines. Again azides themselves combine with olefins to form triozolines, which may again be decomposed photochemically to form aziridines.
  12. REACTIONS OF NITRENES (1)FORMATION OF AZIRIDINES: Like carbenes , singlet nitrenes react stereospecifically and triplet nitrenes react stereoselectively through diradical formation.
  13. REACTIONS OF NITRENES (1)FORMATION OF AZIRIDINES: It is to be noted that when an azide is subjected to photolysis or thermolysis, singlet nitrene is formed and when the concentration of olefin is large , the reaction is mostly stereospecific but if the reaction medium is diluted with an inert solvent , the singlet nitrene is converted into more stable triplet state by collision with inert solvent molecule(dissipation of energy ) and the reaction switches over to stereoselective.
  14. REACTIONS OF NITRENES FORMATION OF AZIRIDINES: Aromatic hydrocarbons react with nitrenes to give aziridines first, which undergo ring expansion to give azepines.
  15. REACTIONS OF NITRENES INSERTION REACTION: Like carbenes , nitrenes also undergo insertion reactions. Singlet nitrenes insert in a concerted manner and the stereochemistry of the substrate should be retained in the product. Triplet nitrenes could give the same, but by a two step process. Two radicals are formed which combine to give the product.
  16. REACTIONS OF NITRENES Nitrene intermediates are found to be involved in many intramolecular insertion reactions.If the teritiary C-H bond is at an asymmetric carbon atom , then the configuration at the chiral Centre should be retained in a concerted manner.
  17. REACTIONS OF NITRENES An interesting insertion reaction is observed when o-azidobiphenyl is converted into carbazole in a good yield. Both alpha insertion and gamma insertion can occur in a single substrate molecule leading to acyclic and cyclic compounds.Alpha insertion initially gives an imide which on hydrolysis gives the corresponding carbonyl compounds.
  18. REACTIONS OF NITRENES Metal catalyzed C-H insertion Ph CH3 F3C [(cod)Ir(OMe)]2 2 mol% Benzene rt, 15h Ph NH F3C
  19. REARRANGEMENT REACTIONS OF NITRENES: Nitrenes can also be obtained as reactive intermediates in Hofmann Curtius Schmidt Lossen
  24. REFERENCES: Reaction mechanisms in organic chemistry – Subrata Sen Gupta Organic chemistry- Jonathan Clayden, Nick Greeves and Stuart Warren