Nitrenes

Nitrene group
singlet (left); Triplet (right)

Nitrenes are single-bond nitrogen compounds with an electron sextet . They are highly reactive electron deficiency compounds and in terms of properties and reactions are similar to carbenes, which are isoelectronically structured . Because of their reactivity, nitrenes only occur as intermediates. Nitrenes are also known as derivatives of the imidogen, as aminyls, azenes, azylenes, azacarbenes, or imenes.

Structure and structure

A distinction is made between singlet and triplet spin states , depending on whether the two free electrons are paired or unpaired. In an orbital, paired electrons lead to singlet states, unpaired electrons to triplet states. In contrast to carbene, there are four electrons that are not involved in the bond in nitrene. Two of them occupy the well-known non-binding orbital, as is also the case in saturated nitrogen compounds, for example in ammonia or amines . The other two electrons are in the triplet case in two p orbitals, or in the singlet case in a further sp ² hybrid orbital. In general, the ground state is the triplet state, as one would expect according to Hund's rules . However, the energy difference between the two forms is considerably greater than with carbene, so that singlet states are more difficult to realize in reactions. As in carbene, strong π-donor substituents can stabilize the singlet state. This generally also leads to a nucleophilic reactivity of the nitrene. By being embedded in low-temperature matrices, nitrenes are now also accessible for spectroscopic investigation.

Syntheses

Nitrenes are usually so reactive that they cannot be isolated, but are generated in situ .

From azides

The most common way to produce nitrenes is the thermolysis or photolysis of azides . As a rule, the azide is introduced as an N ₃ ^- ion, for example from sodium azide . The organic azide then breaks down into nitrene and one molecule of nitrogen . The reaction is analogous to the production of carbene from diazo compounds.

${\displaystyle \mathrm {RN_{3}\longrightarrow RN+N_{2}} }$

From isocyanates

This reaction is analogous to the carbene synthesis from ketenes . The nitrene is formed by splitting off carbon monoxide . Aromatic isocyanates can stabilize the nitrene through their π-electron system.

${\displaystyle \mathrm {RNCO\longrightarrow RN+CO} }$

By α-elimination

In contrast to carbene, this reaction path is of no historical significance. A good starting material for such a reaction is the arylsulfonylhydroxylamine EtO – CO – NH – OSO ₂ Ar. The arylsulfonyl group is a very good escaping group , after abstraction of the hydrogen from the amino group, the nitrene EtO-CO – N :: is released.

Reductive nitrene production

By reduction , nitrenes can be obtained from nitro compounds with z. B. triarylphosphines or trialkylphosphites and produce from nitroso compounds with zinc and acetic acid .

Reduction of a nitro compound with a triarylphosphine:

${\displaystyle \mathrm {RNO_{2}+2\ Ar_{3}P\longrightarrow RN+2\ OPAr_{3}} }$

Reduction of a nitrosoamine with zinc in acetic acid solution to a substituted aminonitrene:

${\displaystyle \mathrm {R_{2}NNO+Zn+2\ H^{+}\longrightarrow R_{2}NN+Zn^{2+}+H_{2}O} }$

Oxidative nitrene production

Certain nitrenes can be generated by oxidation from amino compounds , substituted hydrazines, or hydroxylamine derivatives.

An aminonitrene from a substituted hydrazine:

${\displaystyle \mathrm {R_{2}NNH_{2}+1/2\ O_{2}\longrightarrow R_{2}NN+H_{2}O} }$

An alkoxynitrene made from a substituted hydroxylamine:

${\displaystyle \mathrm {RONH_{2}+1/2\ O_{2}\longrightarrow RON+H_{2}O} }$

Reactions

Insertions

As with the carbenes, nitrenes easily insert into C – H σ bonds. The singlet form adds to the CH bond in an intermediate step, so that the H atom migrates to the nitrene N atom. A new N – C bond is formed while the configuration of the substrate is retained. A possible two-stage reaction with a triplet nitrene, in which the nitrene first abstracts an H atom from the substrate and then recombines with the remaining substrate radical, rarely occurs. The reaction route is possible, for example, with aryl nitrenes. Since nitrenes are also capable of rearrangements through 1,2-hydrogen shifts, substituted nitrenes which do not have an α-hydrogen atom are used for insertion reactions. The reaction is chemoselective , tertiary carbon atoms are the most reactive. Since during the insertion of the nitrene in the transition state a positive charge is built up on the C atom of the C – H group at which the insertion takes place, those C – H bonds are particularly activated which have a molecular fragment in the vicinity, the one can stabilize positive charge. These are primarily heteroatoms such as O, N, but also double bonds (allylic position) or aromatics (benzylic position). In general, the insertion tendency of nitrenes generated by α-elimination is lower than that of carbenes, since more complex processes are necessary here (deprotonation, leaving group, coordination to the catalyst) than with carbenes in which only N ₂ is present as a leaving group.

Cycloadditions

Of alkenes

Nitrenes react with alkenes to form three-membered heterocycles , so-called aziridines . The stereospecificity of the reaction depends on the spin state of the nitrene. Singlet states react with retention of the configuration. As long as pure alkene is used as the solvent, good yields of the stereochemically unique product are obtained. However, other solvents can deactivate the singlet nitrene to triplet nitrene, which means that a clear stereochemistry is no longer guaranteed. If the nitrene is produced from an azide precursor, a five-membered heterocycle, a triazoline, can be formed, since an azide can also react as a 1,3-dipole before it breaks down to form nitrene.

Of alkynes

Reactions in which one wants to add nitrenes synthesized from azides to triple bonds usually proceed with addition of the 1,3-dipolar azide to a triazole. In contrast, the reaction with primary amines (RNH ₂ ), which release an imidonitrene with the help of an oxidizing agent, is possible. However, the azirine formed is antiaromatic and rearranges itself to the aromatic compound, as the organic remainder of the nitrene migrates to a carbon atom of the azirine. Thus there are only two π electrons and the system is aromatic.

Of aromatics

These additions serve, among other things, to expand the ring of cyclic aromatics . Carbonyl-substituted nitrenes are mostly used because of their greater electrophilicity . First, the nitrene adds to a formal double bond of the aromatic. A 6-3 bike is created. A rearrangement occurs, whereby the bond between the two rings is broken. What remains is a cycle extended by an N group. For example, an azepine can be produced from a nitrene and benzene via an unstable intermediate stage .

Rearrangements

Like many electron-deficient compounds, nitrenes are also capable of rearrangement. Imines are formed by 1,2-shifts of substituents of the carbene. Hydrogen atoms migrate the fastest because of their low mass.

Addition to centers with free p or d electron pairs

Examples are addition to carbon monoxide to form isocyanates or addition to sulfoxides to form sulfoximines .

Dimerizations

Azo compounds can be formed from nitrenes . This reaction starts from an azide with elimination of nitrogen via the intermediate stage of nitrene.

literature

• Lienhard Hoesch: Nitrenes - building blocks of organic nitrogen chemistry. In: Chemistry in Our Time . 10. 1976, No. 2, pp. 54-61.