Nitrene

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File:Nitrene.png
The structure of a typical nitrene group.

In chemistry, a nitrene (R-N:) is the nitrogen analogue of a carbene. The nitrogen atom has only 6 electrons available and is therefore considered an electrophile. A nitrene is a reactive intermediate and is involved is many chemical reactions [1] [2].

Electron configuration

In the most simple nitrene linear imidogene (:N-H) two of the 6 available electrons form a covalent bond with hydrogen, two other create a free electron pair and the two remaining electrons occupy two degenerate p-orbitals. Consistent with Hund's rule the low energy form of imidogene is a triplet with one electron for each orbital and the high energy form is the singlet state with an electron pair in one filled orbital and one empty orbital.

As with carbenes, a strong correlation exists between the spin density on the nitrogen atom which can be calculated in silico and the zero-field splitting parameter D which can be derived experimentally from electron spin resonance [3]. Small nitrenes such as NH or CF3N have D values around 1.8 cm-1 with spin densities close to a maximum value of 2. At the lower end of the scale are molecules with low D (< 0.4) values and spin density of 1.2 to 1-4 such as 9-anthrylnitrene and 9-phenanthryl.

Formation

Nitrenes are very reactive and not isolated as such. They are formed as reactive intermediates in the reactions:

Reactions

Nitrenes reactions are:

Nitrene Amidation
A nitrene intermediate is suspected in this C-H insertion involving an oxime, acetic anhydride leading to an isoindole [5]:
Synthesis of Cyclic and Spiro-Fused Imines


Nitrene Transfer Reaction
In this particular reaction both the cis and trans (not depicted) stilbene result in the trans-aziridine suggesting a two-step reaction mechanism.
  • arylnitrene ring-expansion and ring-contraction. Aryl nitrenes show ring expansion to 7-membered ring cumulenes, ring opening reactions and nitrile formations many times in complex reaction paths. For instance the azide 2 in the scheme sketched below [3] trapped in an argon matrix at 20 K on photolysis expels nitrogen to the triplet nitrene 4 (observed experimentally with ESR and ultraviolet-visible spectroscopy) which is in equilibrium with the ring-expansion product 6.
Nitrene ring-expansion and ring-contration
The nitrene ultimately converts to the ring-opened nitrile 5 through the diradical intermediate 7. On the other extreme end of the temperature scale, FVT at 500 to 600 °C also yields the nitrile 5 in 65% yield [8].

References

  1. W. Lwowski, Ed. Nitrenes. (1970). Interscience. New York
  2. C. Wentrup. Reactive Intermediates. (1984). Wiley. New York
  3. 3.0 3.1 Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening in 2-QuinazolylnitrenesDavid Kvaskoff, Pawel Bednarek, Lisa George, Kerstin Waich, and Curt Wentrup J. Org. Chem.; 2006; 71(11) pp 4049 - 4058; (Article) doi:10.1021/jo052541i
  4. Intermolecular Amidation of Unactivated sp2 and sp3 C-H Bonds via Palladium-Catalyzed Cascade C-H Activation/Nitrene Insertion Hung-Yat Thu, Wing-Yiu Yu, and Chi-Ming Che J. Am. Chem. Soc.; 2006; 128(28) pp 9048 - 9049; (Communication) doi:10.1021/ja062856v
  5. Novel Intramolecular Reactivity of Oximes: Synthesis of Cyclic and Spiro-Fused Imines Cécile G. Savarin, Christiane Grisé, Jerry A. Murry, Robert A. Reamer, and David L. Hughes Org. Lett.; 2007; 9(6) pp 981 - 983; (Letter) {{DOI|10.1021/ol0630043}
  6. Nitrene Transfer Reactions Catalyzed by Gold Complexes Zigang Li, Xiangyu Ding, and Chuan He J. Org. Chem.; 2006; 71(16) pp 5876 - 5880; (Article) doi:10.1021/jo060016t
  7. Reactants cis-stilbene or trans-stilbene, nitrene precursor p-nitrosulfonamide or Nosylamine which is oxidized by iodosobenzene diacetate. The gold catalyst is based on a terpyridine tridentate ligand
  8. the quinazoline is prepared from the corresponding bromide and sodium azide. The azide is in equilibrium with the tetrazole 3.

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