Biphenyl, 2-azido

Bei der photochemischen Zersetzung von 2-Azido-biphenyl bzw. 2-(2-Azido-phenyl)-l, 3-benzothiazol entstehen sowohl die Azo-Verbindungen als auch die durch Einschub des intermediaren Nitrens gebildeten tri- und tetracyclischen Systeme ... 

So erhalt man z.B. aus Azido-benzol und Benzol (X = H) ein Gemisch von (2-Amino-phenyl) -phenyl-amin (73%), 2-Amino- (8%) und 4-Amino-biphenyl (8%). Wahrend in ana-logen Fallen ahnliche Gemische entstehen, erhalt man unter gleichen Bedingungen aus Azidobenzol und Naphthalin 1-Anilino-naphthalin (78%) und aus 1-Azido-naphthalin und Benzol (l-Amino-4-phmyl-naphthalin (82%). 

Miura and Kobayashi studied the photochemistry of 4,4-biphenyl bisazide. They concluded that photolysis of this diazide leads to extrusion of a single molecule of nitrogen with the formation of a singlet state species, X , with a lifetime of 19 ns. Miura and Kobayashi did not specify the nature of X. This species absorbs at 380 nm and its spectrum and lifetime are similar to our results with singlet para-biphenylnitrene. Therefore we conclude that X is singlet 4 -azido-4-biphenylnitrene. 

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-... 

The o-azido biphenyl system has been examined systematically by Smith and co-workers . In thermolysis, carbazoles were obtained in high yields, i.e. 

Thermolysis of bis (o-azidobenzoyl) peroxide (233) at 80° in benzene in the absence of air led to a number of products in which the azido group was left intact o-azidobenzoic acid (234) (102%), o-azido-biphenyl (235) (18%), phenyl o-azidobenzoate (236) (20%), and a mixture of unidentified, apparently aliphatic, esters of o-azidobenzoic... 

Thermolysis of phenyl azide leads to loss of nitrogen with formation of phenyl nitrene. This conclusion is well supported by experiment. Walker and Waters [6] studied the pyrolysis of phenyl azide at 130°C and confirmed the observations of Smith and Hall [7] from 2-substituted azido biphenyls that thermolysis of aryl azides, lacking ortho-substituents, in inert solvents leads... 

Photolysis of 2-azido biphenyl produces carbazole in good yield [19]. This has prompted mechanistically inclined chemists to study the details of this process. 

Sundberg et al. [21] reinvestigated this system by flash photolysis and were able to reproduce the earlier rate constant quoted in cyclohexane. They were able to demonstrate however that this reaction was subject to a considerable solvent effect. As previously mentioned, it was found that photolysis of 2-azido biphenyl in the presence of amines produced 3H-azepines in addition to carbazole. Although secondary amines reduced the yield of carbazole, some heterocycle is still formed even in neat amine. Thus there are two pathways for carbazole formation, one which can be quenched with amines and one pathway which cannot. 

A simple method for the preparation of a-azido ketones and esters 500 in good yields by direct azidation of carbonyl derivatives 499 at the a-carbon using 4,4 -bis-(dichloroiodo)biphenyl and sodium azide has been reported (Scheme 3.198) [575]. The hypervalent iodine reagent, 4,4 -bis-(dichloroiodo)biphenyl, can be easily recycled from the reaction mixture. 

By a skillful use of sensitizers, quenchers, filters, and choice of temperature used for photolysis, the decomposition of 2-azido-2 -methyl-biphenyl can be controlled to give a predominance of either carbazole (via singlet nitrene) or phenanthridine (via triplet) ... 

In order to observe the structure of arylnitrene directly, several kinds of arylazide derivatives were prepared and the crystal structures before and after photo-irradiation were analyzed [46, 47]. Crystals of the prepared arylazide derivatives, however, were nonreactive or easily broken on exposure to the UV lamp, except l-azido-2-nitrobenzene, 1, and 2-azido-biphenyl, 2, as shown in Schane 8.5. 

In the previous section, it was found that most of the arylazide crystals are nonreac-tive or decomposed after photo-irradiation, because the structural changes were too large to keep the single crystal form. Only the 2-azido-biphenyl was still crystalline after photo-irradiation the structure of its product, 2-biphenylnitrene, was successfully analyzed by X-rays. However, it was very difficult to observe the precise nitrene structure, because the produced nitrene and the reactant 2-azidebiphenyl are superimposed in the crystal structure after photo-irradiation. 

Although a variety of arylazide crystals were prepared and were irradiated with the Hg lamp to observe the structure of arylnitrene, all the crystals except the 2-azido-biphenyl were decomposed or nonreactive after photo-irradiation. The acid-base complex crystals were made, introducing the carboxyl group to the arylazide molecule as a substituent. The metastable structures were observed directly by X-ray analysis. This is because the complex crystal has three merits to observe the metastable species described above. It will be an indispensable technique to make an acid-base complex if the reactivity of molecule will be examined in the crystalline state to observe the metastable intermediates by X-ray analysis. 

Biphenyls carrying (a) an azido group or (b) a nitro group in the 2-position (1/5) can be transformed to carbazoles 3 (a) by thermolysis, photolysis, or catalysis of Rh(II) [199] and (b) by deoxygenating with triethyl phosphite. Both functionahties thus produce a nitrene intermediate 2, which cychzes to carbazole 3 by intramolecular C-H insertion ... 

The chemistry of the unsubstituted ortho-biphenyl system is complicated by the fact that the key intermediate of this reaction, 67a, undergoes two cyclization processes at competitive rates, and that azepine formation is reversible. To simplify the chemistry of singlet nitrene and allow straightforward study of the isocarbazole formation, 2-azido-3,5-dichlorobiphenyl (65b) was synthesized (Scheme 11.41) and its photochemistry was studied using nano- and picosecond transient absorption spectroscopy. 

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