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 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]

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.

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:

NitreneAmidation2.png
A nitrene intermediate is suspected in this C–H insertion involving an oxime, acetic anhydride leading to an isoindole: [7]
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. [13]
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. [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]

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.

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

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.

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.

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 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">Curtius rearrangement</span> Chemical reaction

The Curtius rearrangement, first defined by Theodor Curtius in 1885, is the thermal decomposition of an acyl azide to an isocyanate with loss of nitrogen gas. The isocyanate then undergoes attack by a variety of nucleophiles such as water, alcohols and amines, to yield a primary amine, carbamate or urea derivative respectively. Several reviews have been published.

<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.

In organic chemistry, a cycloalkyne is the cyclic analog of an alkyne. A cycloalkyne consists of a closed ring of carbon atoms containing one or more triple bonds. Cycloalkynes have a general formula CnH2n−4. Because of the linear nature of the C−C≡C−C alkyne unit, cycloalkynes can be highly strained and can only exist when the number of carbon atoms in the ring is great enough to provide the flexibility necessary to accommodate this geometry. Large alkyne-containing carbocycles may be virtually unstrained, while the smallest constituents of this class of molecules may experience so much strain that they have yet to be observed experimentally. Cyclooctyne is the smallest cycloalkyne capable of being isolated and stored as a stable compound. Despite this, smaller cycloalkynes can be produced and trapped through reactions with other organic molecules or through complexation to transition metals.

Azirines are three-membered heterocyclic unsaturated compounds containing a nitrogen atom and related to the saturated analogue aziridine. They are highly reactive yet have been reported in a few natural products such as Dysidazirine. There are two isomers of azirine: 1H-Azirines with a carbon-carbon double bond are not stable and rearrange to the tautomeric 2H-azirine, a compound with a carbon-nitrogen double bond. 2H-Azirines can be considered strained imines and are isolable.

<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).

<span class="mw-page-title-main">Atomic carbon</span> Chemical compound

Atomic carbon, systematically named carbon and λ0-methane, is a colourless gaseous inorganic chemical with the chemical formula C. It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.

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.

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

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

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

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

Guy Bertrand, born on July 17, 1952, at Limoges is a chemistry professor at the University of California, San Diego.

Electrophilic amination is a chemical process involving the formation of a carbon–nitrogen bond through the reaction of a nucleophilic carbanion with an electrophilic source of nitrogen.

Trimethylenemethane cycloaddition is the formal (3+2) annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons in situ can be used to effect cycloaddition reactions with appropriate electron acceptors. Generally, electron-deficient pi bonds undergo cyclization with TMMs more easily than electron-rich pi bonds.

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring.

<span class="mw-page-title-main">Fluorenylidene</span> Chemical compound

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

An organic azide is an organic compound that contains an azide functional group. Because of the hazards associated with their use, few azides are used commercially although they exhibit interesting reactivity for researchers. Low molecular weight azides are considered especially hazardous and are avoided. In the research laboratory, azides are precursors to amines. They are also popular for their participation in the "click reaction" between an azide and an alkyne and in Staudinger ligation. These two reactions are generally quite reliable, lending themselves to combinatorial chemistry.

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. 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.
  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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