Photocyclyzation of 2-azidobenzophenone

Abstract: Thermolysis of the 2-azidobenzophenone azide group in the course of 1,3-dipolar cyclization reacts with carbonyl group without intermediate for mation of nitrene. The reaction gives high yields of 3-phenylanthranil. Photolysis of 2-azidobenzophenone as well as other aromatic azide gives singlet nitrene as decomposition products. Small singlet-triplet gap w ikens triplet forbiddance partly and transition from the singlet into triplet state becames possibl e by means of intersystem crossing. Thus, two high reactivity intermediates form under photolysis of the azide. It determines the subsequent reactions. The singlet nitrene can insert into carb onyl group giving 3-phenylanthranil, and the triplete nitrene can dimerise to form azocompound o r abstract an hydrogen atom from solvent to form amines. The 2-azidobenzophenone photolysis in acetonitrile we have found high yields of 3phenylanthranil. The scheme becomes complicated by the 3-phenylanthranil secondary decomposition. No azocompound was found. We have pr oposed that there has place transition of nitrene into low reactive conjugated byradical havi ng electron density localized on nitrogen atom of nitren and oxygen atom of carbonyl group. The byrad ical gives 3-phenylanthranil by means of introsystem crossing into the singlet state.


photolysis, nitrene
Introduction: Thermal, catalytic, and photochemical lability of azide group gives us a possibility to use aromatic azides as reagents to form nitrogen -carbon and nitrogen -heteroatom bonds.
Today, catalysis by transitional metal salts is the most actively developed direction in heterocycles synthesis [1]. However, in the reactions with substrates that can easily coordinate such salts (with biopolymers, for instance) their introduction into reaction media turns into irreversible sorption. Biopolymers can loose their biological activity in such circumstances.
The shortcoming is absent in photochemical reactions, when we have an ability to choose proper wavelength of actinic irradiation. However, the formation of radicals and other intermediates in the excited state complicates the photolysis mechanism and decrease the yield of the desirable product. That is why a search of optimal ways to form the concrete product in the photochemical reactions is very important.
In this paper we described our preliminary results of the 2-asidobenzophenone photolysis in acetonitrile.

Experimental Procedure
General information 2-asidobenzophenone synthesis 2.66 g (0.0135 mole) of 2-asidobenzophenone, 15 ml of conc. HCl, and 5 ml of water were place into a glass with volume of 200 ml. The mixture was stirred up to formation of gray homogeneous suspention. After that the reaction mixture was cooled and solution of 1.05 g of NaNO 2 in 10 ml of water was added dropwise into the glass under constant stirring. The precipitate obtained was dissolved to give yellow-brown solution, that was hold in ace in 40 min. That time the solution became brown and precipitate was formed. The diazonium salt was filtered, and 2.69 g of NaN 3 was added into reaction media under constant stirring. After that the reaction media was hold in dark for 12 hours.
The solution decame transparent with dark oil on the bottom. Azide was extracted 3 times using 30 ml and 2 times using 10 ml of CH 2 Cl 2 . Extract was dried above calcined Na 2 SO 4 , after that the solvent was removed in vacuo. 2-asidobenzophenone was purified with preparative colomn chromatography using silica gel and heptane-CH 2 Cl 2 (1:1, v/v) as an eluent. After purification the azide became light-yellow. IR spectrum of the azide is shown on Fig. 1  became from light-yellowish into brown-red. After cooling the dark-brown precipitate was formed.
The raw material was recrystalized from boiled undecane. The absorption maxima in IR (Fig. 3) and u.v. spectrum (Fig. 4)   Taking into account literature data and using analogy with 2-azidobenzoic acid and its esters we proposed that nitrene formed under u.v. irradiation of 1 was able to insert into multiple bond of carbonyl group giving 2 (scheme 2).

Scheme 2. Photolysis of 2-azidobenzoic acid and its esters
On the other hand the photochemical formation of 3 is possible in analogy with photochemical cyclization of 2-azidobiphenyl to turn into carbazole that has place with yield close to quantitative one (scheme 3). there observes an increase of absorption of 2 with asobestic point preservation that evidence that there has place formation of 2 with yields close to quantitative ones. Prolonged irradiation disturbs the isobestic point preserving, however the increase of absorbance in the region of absorbance of 3 was not observed.
Photolysis of 2 in the experimental conditions used does not form 3.
High yields of 2 from the photolysis (Scheme 6) and the high quantum yield of intersystem crossing of nitrenes forced us to propose the triplet path of their cyclization. It is possible that the triplet nitrene formed isomerizes into the ortho-quinone type biradical product with possible electron density localization on the oxygen atom of carbonyl group and nitreneous nitrogen atom.
Due to electron density redistribution this particle is more stable than the triplet nitrene and being not reactive forms 2 after the second change of the spin state.