Nitrene

  In chemistry, a nitrene (R-N:) is the nitrogen analogue of a carbene. The nitrogen atom has only 6 electrons available and is therefore considered an electrophile. A nitrene is a reactive intermediate and is involved is many chemical reactions ^{[1] [2]}.

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Electron configuration

In the most simple nitrene linear imidogene (:N-H) two of the 6 available electrons form a covalent bond with hydrogen, two other create a free electron pair and the two remaining electrons occupy two degenerate p-orbitals. Consistent with Hund's rule the low energy form of imidogene is a triplet with one electron for each orbital and the high energy form is the singlet state with an electron pair in one filled orbital and one empty orbital.

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 ^[3]. 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-phenanthryl.

Formation

Nitrenes are very reactive and not isolated as such. They are formed as reactive intermediates in the reactions:

• from thermolysis or photolysis of azides with expulsion of nitrogen gas, analogues to the formation of carbenes from diazo compounds.
• from isocyanates, with expulsion of carbon monoxide, analogues to carbene formation from ketenes

Reactions

Nitrenes reactions are:

• nitrene C-H insertion. A nitrene can easily insert into a carbon to hydrogen covalent bond yielding an amine or amide. A singlet nitrene reacts with retention of configuration. In one study ^[4] a nitrene, formed by oxidation of a carbamate with potassium persulfate, gives an insertion reaction into the palladium to nitrogen bond of the reaction product of palladium(II) acetate with 2-phenylpyridine to Methyl N-(2-pyridylphenyl)carbamate in a cascade reaction:

A nitrene intermediate is suspected in this C-H insertion involving an oxime, acetic anhydride leading to an isoindole ^[5]:

• nitrene cycloaddition. With alkenes, nitrenes react to aziridines, very often with nitrenoid precursors such as sulfonyliminoiodinanes (PhI=NT) but the reaction is known to work directly with the sulfonamide in presence of a gold catalyst ^{[6] [7]}:

In this particular reaction both the cis and trans (not depicted) stilbene result in the trans-aziridine suggesting a two-step reaction mechanism.

• 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 sketched below ^[3] 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.

The nitrene ultimately converts to the ring-opened nitrile 5 through the diradical intermediate 7. On the other extreme end of the temperature scale, FVT at 500 to 600 °C also yields the nitrile 5 in 65% yield ^[8].

References