Nitrene

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The generic structure of a nitrene group Nitrene triplet.svg
The generic structure of a nitrene group

In chemistry, a nitrene or imene (R−:Ṅ·) is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and monovalent, [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]

Contents

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. [5]

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. [6] 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 rarely isolated. Instead, they are formed as reactive intermediates during a reaction. There are two common ways to generate nitrenes:

Since formation of the nitrene typically starts from a diamagnetic precursor, the direct chemical product is a singlet nitrene, which then relaxes to its ground state triplet state. As has been shown for phenylazide as a model system, the direct photoproduct of photochemical-induced N2 loss can either be the singlet or triplet nitrene. [7] [8] [9] By using a triplet sensitizer, the triplet nitrene can also be formed without initial formation of the singlet nitrene. [10]

Isolated Nitrenes

Although highly reactive, some nitrenes could be isolated and characterized recently.

In 2019, a triplet nitrene was isolated by Betley and Lancaster, stabilized by coordination to a copper center in a bulky ligand. [11] Later on, Schneider and coworkers characterized Pd and Pt triplet metallonitrenes, where the organic residue is replaced by a metal. [12] [13] [14] In 2024, the groups of Beckmann, Ye and Tan reported the isolation and characterization of organic triplet nitrenes, which are protected from chemical reactivity by an extremely bulky ligand. [15] [16]

Reactions

Nitrene reactions include:

NitreneAmidation2.png
A nitrene intermediate is suspected in this C–H insertion involving an oxime, acetic anhydride leading to an isoindole: [18]
NitreneOximeReaction.png
NitreneTransferReactionsCatalyzedbyGoldComplexes.png
In most cases, however, [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) is prepared separately as follows:
Preparation of PhINNs.png
Nitrene transfer takes place next:
Copper catalyzed aziridination.png
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. [24]
NitreneRingContraction.png
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. [25]

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, [26] another system has a carbon radical group. [27]

NitreneTriRadical.svg

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.


Related Research Articles

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

A non-Kekulé molecule is a conjugated hydrocarbon that cannot be assigned a classical Kekulé structure.

The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.

In organic chemistry, arynes and benzynes are a class of highly reactive chemical species derived from an aromatic ring by removal of two substituents. Arynes are examples of didehydroarenes, although 1,3- and 1,4-didehydroarenes are also known. Arynes are examples of alkynes under high strain.

Click chemistry is an approach to chemical synthesis that emphasizes efficiency, simplicity, selectivity, and modularity in chemical processes used to join molecular building blocks. It includes both the development and use of "click reactions", a set of simple, biocompatible chemical reactions that meet specific criteria like high yield, fast reaction rates, and minimal byproducts. It was first fully described by K. Barry Sharpless, Hartmuth C. Kolb, and M. G. Finn of The Scripps Research Institute in 2001. In this seminal paper, Sharpless argued that synthetic chemistry could emulate the way nature constructs complex molecules, using efficient reactions to join together simple, non-toxic building blocks.

The azide-alkyne Huisgen cycloaddition is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole. Rolf Huisgen was the first to understand the scope of this organic reaction. American chemist Karl Barry Sharpless has referred to copper-catalyzed version of this cycloaddition as "the cream of the crop" of click chemistry and "the premier example of a click reaction".

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

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<span class="mw-page-title-main">Aziridines</span> Functional group made of a carbon-carbon-nitrogen heterocycle

In organic chemistry, aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine and two methylene bridges. The parent compound is aziridine, with molecular formula C2H4NH. Several drugs feature aziridine rings, including mitomycin C, porfiromycin, and azinomycin B (carzinophilin).

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<span class="mw-page-title-main">Nitrenium ion</span> Class of reactive intermediate species based on nitrogen and isoeletronic with carbene

A nitrenium ion in organic chemistry is a reactive intermediate based on nitrogen with both an electron lone pair and a positive charge and with two substituents. Nitrenium ions are isoelectronic with carbenes, and can exist in either a singlet or a triplet state. The parent nitrenium ion, NH+2, is a ground state triplet species with a gap of 30 kcal/mol (130 kJ/mol) to the lowest energy singlet state. Conversely, most arylnitrenium ions are ground state singlets. Certain substituted arylnitrenium ions can be ground state triplets, however. Nitrenium ions can have microsecond or longer lifetimes in water.

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<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

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<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

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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. ↑ Vyas, Shubham; Winter, Arthur H.; Hadad, Christopher M. (2013), "Theory and Computation in the Study of Nitrenes and their Excited-State Photoprecursors", Nitrenes and Nitrenium Ions, John Wiley & Sons, Ltd, pp. 33–76, doi:10.1002/9781118560907.ch2, ISBN   978-1-118-56090-7 , retrieved 20 December 2024
  6. 1 2 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.
  7. ↑ Gritsan, N. P.; Platz, M. S. (1 September 2006). "Kinetics, Spectroscopy, and Computational Chemistry of Arylnitrenes". Chemical Reviews. 106 (9): 3844–3867. doi:10.1021/cr040055+. ISSN   0009-2665.
  8. ↑ Soto, Juan; Algarra, Manuel; Peláez, Daniel (23 February 2022). "Nitrene formation is the first step of the thermal and photochemical decomposition reactions of organic azides". Physical Chemistry Chemical Physics. 24 (8): 5109–5115. doi:10.1039/D1CP05785E. ISSN   1463-9084.
  9. ↑ Domenianni, Luis I.; Bauer, Markus; Schmidt-Räntsch, Till; Lindner, Jörg; Schneider, Sven; Vöhringer, Peter (2023). "Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands". Angewandte Chemie International Edition. 62 (42): e202309618. doi:10.1002/anie.202309618. ISSN   1521-3773.
  10. ↑ Murthy, Rajesh S.; Muthukrishnan, Sivaramakrishnan; Rajam, Sridhar; Mandel, Sarah M.; Ault, Bruce S.; Gudmundsdottir, Anna D. (25 January 2009). "Triplet-sensitized photolysis of alkoxycarbonyl azides in solution and matrices". Journal of Photochemistry and Photobiology A: Chemistry. 201 (2): 157–167. doi:10.1016/j.jphotochem.2008.10.015. ISSN   1010-6030.
  11. ↑ 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.
  12. ↑ Sun, Jian; Abbenseth, Josh; Verplancke, Hendrik; Diefenbach, Martin; de Bruin, Bas; Hunger, David; Würtele, Christian; van Slageren, Joris; Holthausen, Max C.; Schneider, Sven (November 2020). "A platinum(ii) metallonitrene with a triplet ground state". Nature Chemistry. 12 (11): 1054–1059. doi:10.1038/s41557-020-0522-4. ISSN   1755-4349.
  13. ↑ Schmidt-Räntsch, Till; Verplancke, Hendrik; Lienert, Jonas N.; Demeshko, Serhiy; Otte, Matthias; Van Trieste III, Gerard P.; Reid, Kaleb A.; Reibenspies, Joseph H.; Powers, David C.; Holthausen, Max C.; Schneider, Sven (2022). "Nitrogen Atom Transfer Catalysis by Metallonitrene C−H Insertion: Photocatalytic Amidation of Aldehydes". Angewandte Chemie International Edition. 61 (9): e202115626. doi:10.1002/anie.202115626. ISSN   1521-3773. PMC   9305406 . PMID   34905281.
  14. ↑ Schmidt-Räntsch, Till; Verplancke, Hendrik; Kehl, Annemarie; Sun, Jian; Bennati, Marina; Holthausen, Max C.; Schneider, Sven (23 September 2024). "C═C Dissociative Imination of Styrenes by a Photogenerated Metallonitrene". JACS Au. 4 (9): 3421–3426. doi:10.1021/jacsau.4c00571. PMC   11423323 . PMID   39328761.
  15. ↑ Janssen, Marvin; Frederichs, Thomas; Olaru, Marian; Lork, Enno; Hupf, Emanuel; Beckmann, Jens (19 July 2024). "Synthesis of a stable crystalline nitrene". Science. 385 (6706): 318–321. doi:10.1126/science.adp4963.
  16. ↑ Wang, Dongmin; Chen, Wang; Chen, Haonan; Chen, Yizhen; Ye, Shengfa; Tan, Gengwen (19 November 2024). "Isolation and characterization of a triplet nitrene". Nature Chemistry: 1–6. doi:10.1038/s41557-024-01669-9. ISSN   1755-4349.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ 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.
  21. ↑ 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.
  22. ↑ 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.
  23. ↑ 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.
  24. ↑ Yudin, Andrei K., ed. (2007). Aziridines and Epoxides in Organic Synthesis. p. 120. ISBN   978-3-527-31213-9.
  25. ↑ The quinazoline is prepared from the corresponding bromide and sodium azide. The azide is in equilibrium with the tetrazole 3.
  26. ↑ 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.
  27. ↑ 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.
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