Nitrene

In chemistry, a nitrene or imene (R−:Ṅ·) is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and univalent,[1] so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded electrons. It is therefore considered an electrophile due to the unsatisfied octet. A nitrene is a reactive intermediate and is involved in many chemical reactions.[2][3] The simplest nitrene, HN, is called imidogen, and that term is sometimes used as a synonym for the nitrene class.[4]

The generic structure of a nitrene group

Electron configuration

In the simplest case, the linear N–H molecule (imidogen) has its nitrogen atom sp hybridized, with two of its four non-bonded electrons as a lone pair in an sp orbital and the other two occupying a degenerate pair of p orbitals. The electron configuration is consistent with Hund's rule: the low energy form is a triplet with one electron in each of the p orbitals and the high energy form is the singlet with an electron pair filling one p orbital and the other p orbital vacant.

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.[5] 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-phenanthrylnitrene.

Formation

Because nitrenes are so reactive, they are not isolated. Instead, they are formed as reactive intermediates during a reaction. There are two common ways to generate nitrenes:

Reactions

Nitrene reactions include:

Nitrene amidation
A nitrene intermediate is suspected in this C–H insertion involving an oxime, acetic anhydride leading to an isoindole:[7]
Synthesis of cyclic and spiro-fused imines
Nitrene transfer reaction
In most cases, however, [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) is prepared separately as follows:
Preparation of PhINNs
Nitrene transfer takes place next:
Nitrene transfer reaction
In this particular reaction both the cis-stilbene illustrated and the trans form (not depicted) result in the same trans-aziridine product, suggesting a two-step reaction mechanism. The energy difference between triplet and singlet nitrenes can be very small in some cases, allowing interconversion at room temperature. Triplet nitrenes are thermodynamically more stable but react stepwise allowing free rotation and thus producing a mixture of stereochemistry.[13]
  • 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 below[5] 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-contraction
The nitrene ultimately converts to the ring-opened nitrile 5 through the diradical intermediate 7. In a high-temperature reaction, FVT at 500–600 °C also yields the nitrile 5 in 65% yield.[14]

Nitreno radicals

For several compounds containing both a nitrene group and a free radical group an ESR high-spin quartet has been recorded (matrix, cryogenic temperatures). One of these has an amine oxide radical group incorporated,[15] another system has a carbon radical group.[16]

Nitrene radical

In this system one of the nitrogen unpaired electrons is delocalized in the aromatic ring making the compound a σ–σ–π triradical. A carbene nitrogen radical (imidyl radical) resonance structure makes a contribution to the total electronic picture.

In 2019, an authentic triplet nitrene was isolated by Betley and Lancaster, stabilized by coordination to a copper center in a bulky ligand.[17]

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "nitrenes". doi:10.1351/goldbook.N04145
  2. Lwowski, W., ed. (1970). Nitrenes. New York: Interscience.
  3. Wentrup, C. (1984). Reactive Intermediates. New York: Wiley.
  4. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "imidogens". doi:10.1351/goldbook.I02951
  5. Kvaskoff, David; Bednarek, Paweł; George, Lisa; Waich, Kerstin; Wentrup, Curt (2006). "Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening in 2-Quinazolylnitrenes". J. Org. Chem. 71 (11): 4049–4058. doi:10.1021/jo052541i. PMID 16709043.
  6. Thu, Hung-Yat; Yu, Wing-Yiu; Che, Chi-Ming (2006). "Intermolecular Amidation of Unactivated sp2 and sp3 C–H Bonds via Palladium-Catalyzed Cascade C–H Activation/Nitrene Insertion". J. Am. Chem. Soc. 128 (28): 9048–9049. doi:10.1021/ja062856v. PMID 16834374.
  7. Savarin, Cécile G.; Grisé, Christiane; Murry, Jerry A.; Reamer, Robert A.; Hughes, David L. (2007). "Novel Intramolecular Reactivity of Oximes: Synthesis of Cyclic and Spiro-Fused Imines". Org. Lett. 9 (6): 981–983. doi:10.1021/ol0630043. PMID 17319674.
  8. Li, Zigang; Ding, Xiangyu; He, Chuan (2006). "Nitrene Transfer Reactions Catalyzed by Gold Complexes". J. Org. Chem. 71 (16): 5876–5880. doi:10.1021/jo060016t. PMID 16872166. S2CID 43641348.
  9. Evans, David A.; Faul, Margaret M.; Bilodeau, Mark T. (1994). "Development of the Copper-Catalyzed Olefin Aziridination Reaction". J. Am. Chem. Soc. 116 (7): 2742–2753. doi:10.1021/ja00086a007. S2CID 55554519.
  10. Brandt, Peter; Sodergren, Mikael J.; Andersson, Pher G.; Norrby, Per-Ola (2000). "Mechanistic Studies of Copper-Catalyzed Alkene Aziridination". J. Am. Chem. Soc. 122 (33): 8013–8020. doi:10.1021/ja993246g. S2CID 98310736.
  11. Watson, Iain D. G.; Yu, Lily; Yudi, Andrei K. (2006). "Advances in Nitrogen Transfer Reactions Involving Aziridines". Acc. Chem. Res. 39 (3): 194–206. doi:10.1021/ar050038m. PMID 16548508.
  12. 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.
  13. Yudin, Andrei K., ed. (2007). Aziridines and Epoxides in Organic Synthesis. p. 120. ISBN 978-3-527-31213-9.
  14. The quinazoline is prepared from the corresponding bromide and sodium azide. The azide is in equilibrium with the tetrazole 3.
  15. Lahti, Paul M.; Esat, Burak; Liao, Yi; Serwinski, Paul; Lan, Jiang; Walton, Richard (30 May 2001). "Heterospin organic molecules: nitrene–radical linkages". Polyhedron. 20 (11–14): 1647–1652. doi:10.1016/S0277-5387(01)00667-2.
  16. Sander, Wolfram; Grote, Dirk; Kossmann, Simone; Neese, Frank (2008). "2,3,5,6-Tetrafluorophenylnitren-4-yl: Electron Paramagnetic Resonance Spectroscopic Characterization of a Quartet-Ground-State Nitreno Radical". J. Am. Chem. Soc. 130 (13): 4396–4403. doi:10.1021/ja078171s. PMID 18327939.
  17. Carsch, K. M.; DiMucci, I. M.; Iovan, D. A.; Li, A.; Zheng, S.-L.; Titus, C. J.; Lee, S. J.; Irwin, K. D.; Nordlund, D.; Lancaster, K. M.; Betley, T. A. (2019). "Synthesis of a Copper-Supported Triplet Nitrene Complex Pertinent to Copper-Catalyzed Amination". Science. 365 (6458): 1138–1143. Bibcode:2019Sci...365.1138C. doi:10.1126/science.aax4423. PMC 7256962. PMID 31515388.
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