Nitrenes, aromatic

It is known that aromatic azides are photodecomposed to give active nitrenes as the transient species, which react with the environmental binder polymers to crosslink them. However, the mechanism of these photocrosslinking polymers has not been studied in detail. L.S.Efros et al. have proposed that the rubber polymer is crosslinkes in such a way that the aromatic nitrene inserts into an unsaturated bond of the polymer to give an aziridine ring. The experimental evidence for this, however, has not been given (8). 

In the present experiment, we have studied the mechanism of photocrosslinking of 1,2-polybutadiene by aromatic azide, based on the reaction of aromatic nitrene with unsaturated hydrocarbon monomeric compounds. 

With the purpose of understanding the crosslinking mechanism of 1,2-polybutadiene with aromatic nitrene, we studied the reaction of phenylnitrene with unsaturated olefine monomers such as 3-methyl-1-butene and cyclohexene. These monomers are structually similar to a unit segment of 1,2-polybutadiene. 

The IR spectrum of the effusion gives a strong absorption peak at lT Ocm due to v(C=0) of carbonyl group. This absorption indicates that the aziridine was oxidized on the silicagel surface. Because of such a fact, silicagel can not be used for the analysis of the reaction products of aromatic nitrene with unsaturated olefines. The present experiment, therefore, was repeated by replacing silicagel with alumina. 

Organic azides (RNS) are isoelectronic with diazo-compounds, and like them are yellow in colour. On irradiation azides lose nitrogen to produce monovalent nitrogen species, nitrenes lRN . In the absence of an addend, aliphatic nitrenes generally undergoa shift of hydrogen to give an imine (5.34), whereas aromatic nitrenes can dimerize to yield an azo-compound (S.3SI. 

The photolysis of aryl azides in low-temperature matrices yields triplet (ground) state nitrenes which have been identified by and absorption spectroscopy. Dinitrenes and trinitrenes have also been reported in the solid-state photolysis of di- and triazides. Quantum yields of photolysis of some aromatic azides are listed in Table 21 and it appears that nitrenes are produced in solution, at room temperature, as well. The lifetimes of some aromatic nitrenes and the absolute rates of some of their reactions have been measured . Some interesting features of photolytic azide decompositions will now be briefly described. 

The most versatile reaction of this type is the insertion of aromatic nitrenes into aromatic rings to form tricyclic systems. Pyrolysis or photolysis of o-azidobiphenyl (165) afforded a good yield ofcarbazole (166) The reaction has been utilized for the synthesis of a number... 

Intramolecular insertion in substituted aromatic nitrenes has been used as a means of ring closure to pyrrolidines . 

Aromatic nitrenes are as selective in their insertion reactions as carbonyl nitrenes. In phenyl nitrene the insertion yields into primary, secondary and tertiary C—H bonds are in the approximate ratios 1 10 100 . Aromatic nitrenes insert into aromatic G—H bonds about as efficiently as into secondary G—H bonds of aliphatic hydrocarbons photolysis and thermolysis of 2-azidobiphenyI produces... 

In suitable solvents even alkyl nitrenes produce small amounts of primary amines . Carbethoxynitrene abstracts hydrogen from hydrocarbons (12%) and more efficiently from alcohols (90%). Aromatic nitrenes are in general less reactive than carbonyl nitrenes, they abstract hydrogen, provided the attack on unreacted azido groups... 

The directions of ring expansion in condensed aromatic nitrenes are relatively easy to explain. The nitrene always moves toward the carbon atom with the highest electron density [e.g., 2-naphthylnitrene inserts into the 1,2-bond, not the 2,3-bond see Eqs. (52-61) and Scheme 41], This is to be expected for the addition of an electrophilic singlet nitrene to a double bond, and it indicates that an azirine is either an intermediate or a transition state in the ring expansion (see also Section VIII,H,2). 

The aminonitrenes have much in common with the five-membered aromatic nitrenes, but are a class apart due to strong resonance stabilization of the 1,1-diazene form. General reviews on aminonitrenes are available.390-392... 

When one estimates the heats of formation of aromatic nitrenes and carbenes from those of NH and CH2 with the aid of group additivity, the nitrenes will automatically become 10—15 kcal/mol more stable than the isomeric carbenes. These estimates are of necessity for the triplet species we do not know the resonance energies in the singlets. Nor do we know the singlet-triplet splittings, which may be different in carbenes and nitrenes. We have, therefore, also performed semi-... 

A. Reiser, L. Leyshon, Correlation between negative charge on nitrogen and the reactivity of aromatic nitrenes, J. Am. Chem, Soc., 1970, 92, 7487. 

Scheme 6.4 Electronic/steric effects of substituents in aromatic nitrene.

Sulfinylamines upon thermolysis produce aromatic nitrenes (Scheme 6.17). 

Scheme 6.17 Thermolysis of sulfinylamines to give aromatic nitrenes.

A series of aromatic nitrenes substituted by anionic r-donating groups have been studied regarding their electronic structure, ground state and stability. The results obtained have provided data indicative of the behaviour of such peculiar nitrenes, thus giving the opportunity to further understand, predict and tune the electronic properties of this class of reactive intermediates. 

Reiser, A., Bowes, G., and Horne, R.J., Photolysis of aromatic azides. Part 1. — Electronic spectra of aromatic nitrenes and their parent azides, Trans. Faraday Soc., 62, 3162,1966. 

---