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 parameterD 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:
In most cases, however, [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) is prepared separately as follows:
Nitrene transfer takes place next:
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
argonmatrix 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.
The nitrene ultimately converts to the ring-opened
nitrile5 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]
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]
^
abKvaskoff, 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.
PMID16709043.
^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.
PMID16834374.
^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.
PMID17319674.