INTRODUCTION:

Photochemical reactions are very appealing processes from both economical and environmental aspects. Indeed, light is a very cheap reagent, and it does not generate chemical waste ^1,2. These feature have, however, been hampered for decades by a lack of selectivity and sometimes quite unpredictable results. One area in organic photochemistry showing an ever increasing activity is the photodeprotection of functional groups ^3,4. Nitro compounds are important building blocks in organic synthesis⁵ and routinely serve as precursors to amines. Reduction of nitro aromatics to the corresponding amines is a synthetically important transformation ⁶. It has long been known that the reduction of nitro arenes can occur as a consequence of their direct light absorption ^7-9. Photolysis of nitro arenes in solvent having abstractable hydrogen atoms leads to reduction products, the relative proportion of which depends upon the nature of the solvent and the wavelength of light being used ¹⁰. It has been proved that only the lowest excited triplet state of the nitro group abstracts hydrogen atoms and this has been shown to be of the n-π* excited triplet state ¹¹.Irradiations in neutral media afforded mainly dibenzo[c,f][1,2] diazepin-11-one-5-oxides and in acidic medium, the major product formed was 3-(2′-nitro phenyl)-2,1-benzisoxazole.

Due to their medicinal importance^12-14 much attention has been paid to the dibenzodiazepine derivatives in recent years. The photochemical synthesis of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazoles (BBNB, 2 ) from 5,5′dibromo-2,2′-dinitrodiphenylmethanes (BNDPM, 1 ) in acidic ethanol has recently been reported ^15,16. It was observed that in all the cases, continued irradiation of BNDPM resulted in considerable decrease in the yield of benzisoxazoles, whereas the amount of N-oxides and acridones notably increased. It is obvious from this that the benzisoxazole derivatives undergo further photo reaction. This observation, therefore, necessitated an examination of the photo behavior of benziosoxazole derivatives.

MATERIAL AND METHODS:

All the chemical used were of AR grade and were used without further purification. All the melting points were determined by digital Auto Melting point apparatus Labronies. FTIR spectra were recorded on a Perkin-Elmer Precisely Spectrum 100 in the 4000–400 cm^-1 region. UV - visible spectra were obtained on a Perkin – Elmer Lamda 750 UV – vis spectrometer using dimethyl sulphoxide as solvent in the 200 – 800 nm regions. The ¹H-NMR spectral analysis were performed on a JEOL 300 JMTC – 300 / 54 spectrometer using tetramethyl silane as internal standard. Irradiation was carried out with UV light using Heber Scientific company photo reactor UV lamp.

Irradiation of a solution of 5-bromo-3(5′-bromo-2′-nitroiphenyl)-2,1-benzisoxazole (1g) in ethanol (100mL) containing concentrated sulphuric acid (2mL) for 15 hrs resulted in almost complete conversion of the starting material. The photolysed solution was neutralized with solid sodium bicarbonate, the precipitated sodium sulphate filtered off and the solvent distilled off from the filtrate under reduced pressure. The residue was chromatographed on a column of neutral alumina. Elution of the residue with petroleum ether afforded unchanged starting material as the first fraction. Elution with petroleum ether / benzene (1:1 v/v) yielded 2,9-dibromodibenzo[c,f][1,2]diazepin-11-one-5-oxide. Further elution with benzene gave 2,7-dibromo acridone. Further elution with benzene/chloroform (1:1 v/v) gave 2,7- dibromo-N-hydroxyacridone. Finally elution with chloroform afforded 2,9-dibromodibenzo [c,f][1,2]diazepin-11-one-5,6-dioxide.

RESULTS AND DISCUSSION:

Irradiation of a solution of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazole (BBNB, 2a, 1g ) in acidified ethanol ( 100 mL ethanol containing 2 mL conc. H₂SO₄)

for 15 hrs and work-up of the photolysate afforded 2,9-dibromodibenzo[c,f][1,2]diazepin-11-one-5-oxide (DBDAO, 3a; 38%); 2,7-dibromo acridone (DBA, 4a; 28%), traces of 2,9-dibromo dibenzo[c,f][1,2]diazepin-11-one-5,6-dioxide (DBDADO,5a;2%) and 2,7-dibromo-N-hydroxy acridone (DBNHA, 6a; 5%) (Scheme 1). The generality of this interesting photoreaction was verified by irradiating dichloro (2b) and dibromo (2c) derivatives of ( BBNB 2). All of them yielded the related N-oxide, acridone, N-hydroxy acridone and N,N-dioxide. The results of irradiation of BBNB in acidified ethanol are listed in Table 1.

Table . 1 The Results of Irradiation of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazole ( BBNB ) in Acidified Ethanol

Starting Material

Duration of irradiation

(hr)

Conversion

( % )

Yield (%) of product formed

The isolation of the N,N-dioxides (DBDADO, 5 ) in very small quantities indicated that these could be a probable intermediate in the photoreaction of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazole (BBNB, 2 ). It has already been shown that N,N-dioxide ( DBDADO, 5 ) undergoes further photochemical reaction in protic solvents to yield diazepinone-N-oxide(DBDAO, 3) and acridone(DBA, 4). In order to confirm the intermediate of DBDADO in the photoreactions of BBNB, irradiation of 2a was carried out in dry benzene. It was expected that further hydrogen abstraction by the photoexcited DBDADO would go up considerably. The larger yield of DBDADO in this irradiation thus confirmed that DBDADO was the initial product in the photoreactions of BBNB. Formation of above compounds has been further confirmed by spectral analysis.

Scheme 1. Photolysis of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazole

UV-visible spectra:

The UV absorption maximum of DBDAO lies around 230, 250 and 370 nm. The peak at 225 and 250 nm is due to π→π* transition of the benzene ring and the peak at 370 nm is due to n→π* transition of azo group. UV-visible spectrum of this compound is presented in Fig.1. The UV absorption maximum of DBA lies around 240 and 270 nm.

Fig. 1. UV-visible spectrum of DBDAO

The DBDADO show λ_max value of 257 and 323 nm. The UV absorption maximum of DBNHA lies around 230 and 251 nm. This is due to the π→π* transition of the benzene ring.

FTIR spectra:

Strong absorption around 1670 cm^-1 and 1320 cm^-1 in FT-IR spectra of DBDAO indicated the presence of a highly polarized C=O group and N→O linkage respectively. The absorption at 1600 cm^-1 assigned the presence of N=N stretching frequency. The FTIR spectrum of this compound is presented in Fig.2. The DBA showed absorption in the region 3400 cm^-1 and 3300 cm^-1 in their FTIR spectra. This is due to N-H symmetric and asymmetric stretching vibration. The absorption at 1680 cm^-1 indicated the presence of a C=O group. FTIR spectrum of DBDADO shows band centered around 1680 cm^-1, 1310 cm^-1 and 1598cm^-1 which has been assigned to the presence of C=O, N→O and N=N stretching vibration respectively. FTIR spectrum of DBNHA showed in addition to peaks around 3400 cm^-1 (O-H) and 1650 cm^-1 (C=O), peaks around 1300 cm^-1 which are probably due to the contribution of N→O absorption ¹⁷. The IR spectral data of the compounds are presented in Table 2.

Fig. 2. FTIR spectrum of DBDAO

Compound	Frequency of the peak (cm^-1)
Compound	Υ (C=O)	υ (N→O)	υ (N=N)	υ (O-H)	υ (N-H) (sym)	Υ (N-H) (asym)
DBDAO	1670	1320	1600	-	-	-
DBA	1680	-	-	-	3400	3300
DBDADO	1680	1310	1598	-	-	-
DBNHA	1650	1300	-	3400	-	-

Mechanism:

A possible mechanism is suggested to account for the photo excited nitro in 2 by attack at the 3-position of the benzisoxazole provides the intermediate ( 7 ) which then undergoes rearrangement to form the DBDADO ( Scheme 2 ).

Scheme 2. Formation of DBDADO

Alternatively an initial N-O bond cleavage could take place in 2 to yield the nitrene intermediate ( 8 ). The electron deficient nitrogen in (8) could then undergo intramolecular coupling with the proximal photoexcited nitro group and subsequent rearrangement to form 5. (Scheme 3). The electron deficient nitrogen in 8 may also undergo intramolecular coupling with the vacant 2-position of the phenyl substituent and if it happens, the product formed would be a 1-nitro acridone. However this is ruled out since 1-nitro acridone was not isolated in the photoreactions of 2.

Scheme 3. Formation of DBDADO

It has been reported that the irradiation of 3-phenyl-2,1-benzisoxazoles resulted in the rupture of the isoxazole ring and subsequent formation of azepine derivatives by ring expansion ^18-21. Since no azepine derivatives were isolated from the photolysis of BBNB, it is very likely that in these the photo excited nitro group has interacted with the 2-position of the benzisoxazole ring resulting in the formation of DBDADO.

CONCLUSION

The present study reports the photochemistry of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazoles which is obtained by the photolysis of 5,5′-dibromo-2,2′-dinitro diphenylmethanes. Irradiation of 5-bromo-3(5′-bromo-2′-nitrophenyl)-2,1-benzisoxazoles in acidified ethanol gave different products. The structures of the photoproducts are assigned on the basis of spectral analysis.

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Received on 01.11.2012 Modified on 16.11.2012

Asian J. Research Chem. 5(12): Dec., 2012; Page 1444-1447