Arylnitrenes cyclizations

For most substituted phenyl azides of interest " the rate constants of singlet nitrene decay and product formation (triplet nitrene and ketenimine) were foimd to be the same. With these arylnitrenes, cyclization to substituted benzazir-ines is the rate-determining step of the process of nitrene isomerization to ketenimine as in case of the parent phenylni-trene (Scheme 5.2). The only exception, o-fluorophenylnitrene, will be discussed in detail in Section 5.6.6.3.2. 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

As shown in the previous seetion (Figs 6 and 7), the magnitude of koBs decreases as the temperature decreases until a limiting value is reached. The pattern is similar to that observed with parent singlet phenylnitrene (Fig. 3). As before, the temperature-independent observed rate constants are associated with kisc- Plots of In (koBs —kjsc) were linear (Figs 10, 11) and these plots were used to deduce the Arrhenius parameters to cyclization of the substituted singlet arylnitrenes (Tables 2, 4 and 6). 

The intramolecular cyclization of the arylnitrenes derived from azido- or nitro-biphenyls to the adjacent aromatic ring has been well leviewed. - The reaction is a useful route to carbazoles, two recent examples of which are shown in Scheme IS. In Raphael s elegant proach to the indolocarbazole fam-... 

Examples of cyclization of arylnitrenes onto neighboring six-membered heterocycles, and of heterocyclic nitrenes onto aryl rings, are given in Scheme 50a and b.315-321 Numerous other such reactions have been reviewed by Kametani et al.9... 

This section considers cyclizations of arylnitrenes and carbenes onto another aromatic or heterocyclic ring of the type 290, where the two rings are joined by a bridging group Y. 

When aryl azides are heated above 130°C in inert aromatic or aliphatic solvents nitrogen is evolved and an arylnitrene generated that reacts with the solvent, dimerizes or, if suitably substituted, cyclizes. Thermal decomposition of aryl azides generates arylnitrenes in the singlet state which usually collapse rapidly to the triplet ground state before reaction unless certain structural features are present in the molecule. 

Intramolecular substitution by arylnitrenes has been known for a long time and the cyclization of o-azidobiphenyls to carbazoles is representative of an important series of nitrene reactions that yield heterocycles. Only fairly recently has an intermolecular counterpart of these reactions been observed. Originally when phenyl azide was thermolyzed in benzene only azobenzene (11%), aniline (18%), and tars were obtained, and no diphenylamine was found. Abramovitch considered that phenylnitrene is not sufficiently electrophilic to attack benzene, so he decomposed a series of aryl azides (bearing strongly electron-withdrawing substituents, NO2, CN, CF3) in solvents activated... 

Intramolecular cyclization reactions of arylnitrenes by aromatic substitution (singlet), C-H aliphatic insertion, and attack on ortho nitrogen functions are usually very much more facile than their intermolecular counterparts. 

Cyclization of o-azidobiphenyls to carbazoles is one of the oldest and best studied arylnitrene reactions. Carbazole formation (67) has finally been shown " to involve cyclization of a singlet nitrene (68), generated by photolysis or thermolysis of 2-azidobiphenyl, settling a controversy of some standing. ... 

After taking into account that isc is temperature independent, plots of In(feoBs- isc) were used to deduce the Arrhenius parameters for cyclization of the substituted singlet arylnitrenes (Figure 11.12, Tables 11.1 and 11.2). 

In the ultrafast LEP study of 80, the formation of two intermediates have been observed in acetonitrile at ambient temperature. One of the species, the S2 excited state of 80 (> ax 350 nm) has a lifetime within the instrument response (300 fs) due to its rapid convertion to the dissociative Si state. The second intermediate, singlet nitrene 82 ( ax 420 nm) has the shortest lifetime of any singlet arylnitrenes observed to date -1.8ps (k = 5.6 X 10 s ). Assuming that the pre-exponential factor for cyclization of 81 is 10 s" the activation energy to cyclization could be estimated as 1.7kcal/mol. 

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