IIB.3. Present Work
II.4. Experimental Section
II.4.1. General Information. All the reagents were commercial grade and purified according to the established procedures. Organic extracts were dried over anhydrous sodium sulfate. Solvents were removed in a rotary evaporator under reduced pressure.
Silica gel (60-120 mesh size) was used for the column chromatography. Reactions were monitored by TLC on silica gel 60 F254 (0.25mm). NMR spectra were recorded in CDCl3
with tetramethylsilane as the internal standard for 1H NMR (400 and 600 MHz) CDCl3
solvent as the internal standard for 13C NMR (100 and 150 MHz). MS spectra were recorded using ESI mode. Elemental analyses were carried out on a Perkin-Elmer 2400 elemental analyzer. IR spectra were recorded in KBr or neat. 2,3-Diarylquinoxalines were prepared from corresponding o-phenylenediamine and benzil derivatives. 2- Arylbenzothiazoles were prepared from corresponding N-phenylbenzothioamides using hypervalent iodine mediated oxidative cyclization following the reported procedure.26
II.4.2. Crystallographic Description
IIA.4.2.
CCDC number for compounds 1e: CCDC 956425. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datarequest/cif.
Crystallographic Description of 1e: Crystal dimension (mm): 0.44 x 0.32 x 0.24.
C28H20N2O2, Mr = 416.46. monoclinic, space group C 2/c; a = 28.766 (2) Å, b = 10.9176 (8) Å, c = 14.9704 (18) Å; = 90o, = 113.435 (9) o, = 90o, V = 4313.7 (7)Å3 ; Z = 8; ρcal= 1.283 g/cm3; (mm-1) = 0.081; F (000) = 1744.0; Reflection collected / unique
= 3556 / 1957; Refinement method = Full-matrix least-squares on F2; Final R indices [I>2σl ] R1 = 0.0558, wR2 = 0.1134, R indices (all data) R1 = 0.1256, wR2 = 0.1402;
goodness of fit = 1.020.
II.4.3. Synthesis of o-Aroylated 2,3-Diarylquinoxaline and 2- Arylbenzothiaole
IIA.4.3.1. General Procedure for the Synthesis of Phenyl(2-(3-phenylquinoxalin-2- yl)phenyl)methanone (1a) from 2,3-Diphenylquinoxaline (1) and Benzaldehyde (a):
In an oven-dried 25 mL round bottom flask containing 1 mL each of toluene and 1,2- dichloroethane (DCE), 2,3-diphenylquinoxaline (1) (0.141g, 0.5 mmol), benzaldehyde (0.064g, 0.6 mmol) and Pd(OAc)2 (0.006g, 0.025 mmol) was added sequentially. Then reaction mixture was kept in an oil bath preheated to 110 oC. TBHP (0.75 mmol) was added in four equal lots at an intervals of 1.5 h. The progress of the reaction was monitored by TLC after each addition. After completion of the reaction (8 h) the reaction mixture was cooled to room temperature and was admixed with water (5 mL). The product was extracted with ethyl acetate (3 x 10 mL) and the combined organic layer was washed with saturated sodium bicarbonate solution (5 mL), dried over anhydrous sodium sulfate (Na2SO4) and concentrated under reduced pressure. The crude product so obtained was purified by silica gel column chromatography (hexane / ethyl acetate, 9.2/0.8) to give pure phenyl(2-(3-phenylquinoxalin-2-yl)phenyl)methanone (1a) (0.147g, yield 76%). The identity and purity of the product was confirmed by spectroscopic analysis.
IIA.4.3.2. General Procedure for the Synthesis of Phenyl(2-(3-phenylquinoxalin-2- yl)phenyl)methanone (1a) from 2,3-Diphenylquinoxaline (1) and Toluene: In an oven-dried 25 mL round bottom flask containing 1.5 mL of toluene, 2,3- diphenylquinoxaline (1) (0.141g, 0.5 mmol) and Pd(OAc)2 (0.006g, 0.025 mmol) was added in sequence. The reaction mixture was the kept in an oil bath preheated to 110 oC.
TBHP (1.75 mmol) was added in four equal lots at an intervals of 2 h. The progress of the reaction was monitored by TLC after each addition. After completion of the reaction (10 h) the reaction mixture was cooled to room temperature and was admixed with water (5 mL). The product was extracted with ethyl acetate (3 x 10 mL). The combined organic layer was washed with saturated sodium bicarbonate solution (5 mL), dried over anhydrous sodium sulfate (Na2SO4) and concentrated under reduced pressure. The crude product so obtained was purified by silica gel column chromatography (hexane / ethyl acetate, 9.2:0.8) to give pure phenyl(2-(3-phenylquinoxalin-2-yl)phenyl)methanone (1a)
(0.137g, yield 71%). The identity and purity of the product was confirmed by spectroscopic analysis.
IIB.4.3. General Procedure for the Synthesis of (2-(Benzo[d]thiazol-2- yl)phenyl)(phenyl)methanone (1´a´) from 2-Phenylbenzo[d]thiazole (1´) and 2- Acetoxyacetophenone (a´): 2-Phenylbenzo[d]thiazole (1´) (0.053g, 0.25 mmol), 2-oxo- 2-phenylethyl acetate (a´) (0.059g, 0.33 mmol), Pd(OAc)2 (0.006g, 0.025 mmol), acetic acid (0.030g, 0.5 mmol) and mesitylene (1.5 mL) were placed in an oven-dried 25 mL round bottom flask. The reaction mixture was then placed in an oil bath preheated at 130
oC. After completion of the reaction (12 h) the reaction mixture was cooled to room temperature and the reaction mixture was taken up in with ethyl acetate (30 mL). The combined organic layer was washed with saturated sodium bicarbonate solution (2 x 5 mL), dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product so obtained was purified by silica gel column chromatography (hexane / ethyl acetate, 9.4:0.6) to give pure colorless gummy (2-(benzo[d]thiazol-2- yl)phenyl)(phenyl)methanone (1´a´) (0.046g, yield 58%). The identity and purity of the product were confirmed by spectroscopic analysis.
II.4.4. Mechanistic Investigations
IIA.4.4.1. o-Aroylation of 2,3-Diarylquinoxaline in the Presence of Radical Scavenger TEMPO: In an oven-dried 25 mL round bottom flask 2,3- diphenylquinoxaline (1) (0.141g, 0.5 mmol), benzaldehyde (0.064g, 0.6 mmol), Pd(OAc)2 (0.006g, 0.025 mmol) TBHP in decane (5-6 M) (100 µL, 0.5 mmol) TEMPO (0.078g, 0.5 mmol), toluene and dichloroethane (DCE) 1 mL each of were added. The flask was fitted to a condenser and the reaction mixture was stirred in a preheated oil bath at 110 oC for 8 h. The reaction after 8 h afforded the benzoyl-TEMPO adduct 2,2,6,6-tetramethylpiperidin-1-yl benzoate (1A) in 52% yield along with paltry yield (~10%) of the desired product (1a). This experiment supports the formation of benzoyl radical in the medium from benzaldehyde (a) induced radically by Pd(OAc)2/TBHP and also the radical nature of the mechanism. The same benzoyl-TEMPO adduct (1A) was observed when toluene used as an aroyl surrogate in lieu of benzaldehyde.
IIB.4.4.1. Determine the Extrusion of CO from the Reaction: For the detection of evolution of carbon monoxide (CO), a strip containing PdCl2 and PMA (phosphomolybdic acid) was suspended from the neck of the reaction flask as shown in the Fig. IIB.4.4.1. The initial yellow colour of the strip before the reaction (Fig.
IIB.4.4.1) turned blue after 3 hrs of the reaction progress (Fig. IIB.4.4.2). This colour change confirms the extrusion of CO from the reaction.
Fig. IIB.4.4.1 Fig. IIB.4.4.2
PdCl2-PMA test strip before reaction PdCl2-PMA test strip after reaction