Photocyclization of o-nitrodiarylamines

Photocyclization of o-nitrodiarylamines

J. Photochem. Photobiol. A: Chem., 71 (1993) 133-136 Photocyclization 133 of a-nitrodiarylamines Ryszard Gawinecki, Wojciech Boszczyk, Danuta Rasa...

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J. Photochem. Photobiol. A: Chem., 71 (1993) 133-136



of a-nitrodiarylamines

Ryszard Gawinecki, Wojciech Boszczyk, Danuta Rasata ofChemishy, Pedagogical IJniwmi~, PL-25420 Kielce (Poland)





(Received September 11, 1992; accepted November 26, 1992)

Abstract Carbazole and cu-carboline are the photocyclization products of o-nitrodiphenylamine and 3-nitro-2-phenylaminopyridine respectively. The proposed mechanism includes the transfer of the amine hydrogen to the nitro group, the abstraction of HNOz and electrophilic cyclization or cyclization to nitrohydrocarbazole followed by the elimination of HN02. o-Aminodiarylamines do not cyckc under similar conditions.

1. Introduction Arylation of aromatic compounds is a convenient preparative method for oligophenyls [l, 21. The reaction follows the scheme Ar-X+Y-Ar’-


where X and Y are hydrogen, halogen or a diazonium group [l]. In an internal process reaction sites need to be part of the same molecule_ The photoinduced formation of carbazoles from diphenylamines is a well-known example of such cyclization [3-121. It should be noted that similar compounds are also products of the pyrolysis of diarylamines [13-1.51. This process proceeds via dihydrocarbazole [13]. Moreover, diphenylamines undergo electrochemical oxidative ring closure in non-aqueous media to give dihydrocarbazole dication which then loses two protons [16]. The pulse radiolysis of diarylamines also affords carbazoles [17]. Dihydrocarbazole is a byproduct in this reaction [17]. The discovery of carbazole radical cation in the fragmentation products of diphenylamine 1181 confirms that similar processes can also be electron impact induced [19]. It has recently been observed [20, 211 that onitrodiarylamines lose HNO, to give carbazole radical cations on electron impact. This is the only known substitution of a nitro group by an aryl group (substitution reactions of the nitro group in nitroarenes are discussed in refs. 2 and 22). The present work shows that similar cyclization can also be photoinduced. The photolytic transformations of o-nitrodiphenylamine have not been studied. Recently, it


has been shown that its N-acyl derivatives photocyclize to phenazine-N-oxides [23]. 3-Nitro-2phenylaminopyridine irradiated in cyclohexane and tetrahydrofuran solutions does not lead to cyclization [24]. However, mass spectral investigations 120, 211 have shown that the elimination of HNO, from the molecular ions of such compounds is very effective. This prompted us to use o-nitrodiarylamines in photochemical experiments. The compounds chosen for study were o-nitrodiphenylamine (l), 3-nitro-2-phenylaminopyridine (2) Nphenyl-o-phenylenediamine (3) and 3-amino-2phenylaminopyridine (4). ; “Xi” 3 x=CH

;g 8 R=H R=H

.$ X=N

2. Experimental


o-Nitrodiphenylamine (l), 3-nitro-2-phenylaminopyridine (2) N-phenyl-o-phenylenediamine (3), 3-amino-2-phenylarninopyridine (4), carbazole (5) and a-carboline (6) were the same as used previously [21]. A methanolic solution (0.01 M) of the compound to be irradiated, i.e. 14, was placed in a Pyrex flask fitted with an adaptor enabling the deoxygenation of the sample with a slow stream of argon to be performed. The flask was placed in a water bath (15 “C) and exposed to the radiation of a Q-400 mercury lamp for 48 h. The solution was then concentrated in vucuo at the ambient temperature. High performance liquid chromatography

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R. Gawinecki et al. I Photqclization


(HPLC) was used to separate the reaction products (~01umn length, 300 mm; r$ =36 mm; packing, Si 60; d,=25 Frn; eluent, hexane; detector, UV,,). The isolated compounds were identified by the comparison of their UV-visible and mass spectra with those of original samples. Moreover, identification was performed by thin layer chromatography (DC Plastikfolien; Kieselgel 60; layer thickness, 0.2 mm; development by iodine vapour) Le. the Rr values were compared with those of carbazole and its aza analogue (see Table 1, Section 3). The UV-visible spectra were recorded at room temperature on a SPECORD M 40 (Carl Zeiss, Jena) spectrophotometer using solutions of approximately 0.04 mM in methanol (for spectroscopy). The parameters are collected in Table 2 (Section 3). The mass spectra were recorded as described previously [20, 211. Some data were taken from refs. 20 and 21. The electron-impact-induced formation of carbazole and carboline radical cations was confirmed by collision-activated dissociation mass-analysed ion kinetic energy (CAD MIKE) spectra [21].

3. Results Methanolic solutions of compounds l-4 were photon-radiated in order to determine if the expected cyclization to the respective carbazoles 5 and 6 takes place.

5 x=cti 6 X=N Scheme I.

Both 1 and 2 photocyclize in methanolic solution. The process requires the elimination of a molecule of HN02. However, cyclization was not confirmed for the o-aminodiarylamines 3 and 4. The Rf values and UV-visible spectral data of the species formed in the photolysis of 1 and 2 are collected in Tables 1 and 2 respectively. For the nitrodiarylamines 1 and 2 the reaction conversion does not exceed lo%, but cyclization is practically the only process, i.e. (aza)carbazoles are the sole products. As expected, due to possible electron transfer reactions [25], the photolysis in chloroform and carbon tetrachloride gives more complex mixtures.

of o-nitdia$amines


1. Rf values of 1, 2, 5 and 6


Eluent (EE-PE) 1:4







2 5 6

0.92 0.91 0.35

0.99 0.83 0.34

0.94 0.77 0.33

“EE, ethyl ether, PE, petroleum ether. TABLE 2. UV-visible 6 in methanol

spectral data of compounds 1,2, 5 and



1 2 5

425.5 (3.67), 278 (4.10)sh, 259 (4.19) 418.5 (3.78), 290 (4.08), 242 (4.21) 334.5 (3.57). 322.5 (3.64). 292.5 (4.28), 252 (4.42), 245 (4.49), 233.5 (4.68) 327 (3.72), 296.5 (4.32), 258.5 (4.19), 232 (4.39)


(nm) (log E_)

sh, shoulder.

4. Discussion 4.1. Mechanistic aspects The photoconversion of N-methyldiphenylamine proceeds via its triplet state, which is subsequently transformed into dihydrocarbazole (DHC) and then to carbazole, irrespective of whether or not H

and other \




resmance strwtures

‘Y Me

the sample is deoxygenated [ll]. The presence of oxygen is helpful in the last step since it can react with the hydrogen atoms of DHC to form water. DHC can also disproportionate to give carbazole and tetrahydrocarbazole [26]. N,2,4,6_Tetramethyldiphenylamine also cyclizes when irradiated [26], but the methylhydrocarbazole formed (see Scheme 2) cannot easily lose the CH, molecule. Thus it does not disproportionate to give the respective carbazole derivative. (Elimination of CH; from the molecular ion of 2-(2,4,6trimethylphenylamino)-3-nitropyridine is worthy of note [21]. Thus in the electron-impact-induced process the o-methyl group does not prevent the cyclization of diphenylarnines.) The ortho position in the nitrodiarylamines 1 and 2 is also occupied. However, their photocyclization proceeds easily, probably as a cyclization-elimination process, i.e. the nitrohydrocarbazoles formed lose the HNO, molecule to give

R Gowinecki et al. / Photocyclization

of o-nitrodiaylomines


Scheme 2.


Scheme 4. HO

X=CHor Scheme





5 and 6 respectively. Another possible mechanism, including amine hydrogen transfer to the nitro group, similar to that observed in some o-nitro[27-291,abstraction of the HNO, diarylmethanes molecule and electrophilic cyclization, is shown in Scheme 3. This process does not require the presence of oxygen or other oxidants (the effect of added base was not studied). Similar cyclization products are obtained from diphenylamine [18] and the o-nitrodiarylamines [20,21] 1 and 2 on electron impact. Their molecular ions lose H2 and HNO, respectively to give the carbazole radical cations. However, H’ lost by the molecular ions of 1 and 2 is the aromatic hydrogen atom [21]. NO; was also found among the primary fragments expelled from these molecular ions [21]. Scheme 4 shows the possible steps in the electronimpact-induced cyclization of o-nitrodiarylamines. The radical cations of 1 and 2 expel NO; [2O, 211 to give the cation I. Its cyclization is possible due to the electrophilic attack of the positively charged carbon on the ortho position of another ring. The NO; radical is not the only primary fragment expelled from the molecular ions of 1 and 2 [20, 211. Other species resulting from elimination of H‘, OH- and NO’ may also cyclize but, as shown by the low intensities of the respective peaks in

their mass spectra [21], such processes are of minor importance and will not be discussed here. 4.2. Efect of the Ieating group Although various diarylamines photocyclize to carbazoles, only selected ortho substituents have been tested as the leaving groups [13, 24, 30-331. Similar reactions of diary1 ethers and thioethers lead to dibenzofurans and thiophenes respectively [32, 34-361. The yields are usually high, so the procedures are often of synthetic value. The formation of carbazoles from diphenyiamines [4-13, 371, phenylaminopyridines and dipyridylamines [24] requires two hydrogen atoms to be eliminated. Another ortho substituent can also be abstracted. Thus vicinally substituted chlorophenylaminopyridines undergo photodehydrochlorination [32]. o-Fluorodiphenylamines can lose both Hz and HF, so carbazoles and their 1-fluoro derivatives are formed at the same time [31]. Similarly, two hydrogen atoms must be eliminated from diphenyl ether and thioether to give dibenzofuran and thiophene respectively [34]. o-Chlorophenoxybenzene, o-fluorophenoxybenzene, ochlorophenylthiobenzene and o-fluorophenylthiobenzene, as well as respectively substituted phenov and phenylthiopyridines undergo photodehydrohalogenation [32, 361. Of the two PhS groups attached to the positions 2 and 4 of 3chloropyridines, the latter is much more reactive [32]. In such a case 3-azadibenzothiophene was found to be the sole product of photocyclization [32]. Finally, the possibility of elimination of methanol from o-methoxydiphenyl ether under photolytic conditions should be mentioned [35]. All cyclization reactions may be assumed to proceed via the respective analogues of dihydrocarbazole. The elimination of H, takes place more smoothly in the presence of oxidizers such as iodine [34] due to the formation of hydrogen iodide. The use of a base, such as triethylamine, was found to be helpful in dehydrohalogenation reactions



R. Gawinecki et al. / Photocyciization

In conclusion, the species eliminated in the photocyclization of o-substituted diarylamines, diary1 ethers and thioethers are Hz, HF, HCl, MeOH and HNO,. The elimination of methane and ammonia is not possible from o-methyl and o-amino diarylamines, (a-Aminodiarylamines 3 and 4 do not lose NH3 on electron impact either [21].)

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of o-nitmdiqlamines

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