Pd/Fe3o4 Catalyzed Hiyama Syntheses of Angular Phenothiazine and Phenoxazine Derivatives and their Antimicrobial Activities
Magnetically recoverable Pd/Fe3O4-catalyzed Hiyama cross-coupling reactions of 6-Bromo- 5H-benzo[a]phenothiazin-5-one, 6-Bromo-5H-naphtho[2,1-b]pyrido[2,3-e][1,4]oxazin-5-one and 11-Amino-6-bromo-9-thio-5H-naphtho[2,1-b]pyrimido[5,4-e][1,4]oxazin-5-one (as intermediates) with 2-(Dimethylsilyl)pyridine and Dimethyl(2-thienyl)silanol were investigated. The intermediates were prepared by the reactions of 2-Aminothiophenol, 2- Aminopyridin-3-ol and 4,5-Diamino-6-hydroxylpyrimidin-2-thiol respectively with 2,3- Dibromonaphthalene-1,4-dione in the presence of anhydrous sodium carbonate using benzene/DMF as solvent. Sequel upon the successful syntheses of the intermediates, each was subjected to the Hiyama reaction with 2-(Dimethylsilyl)pyridine and Dimethyl(2- thienyl)silanol, refluxed for between 5-8 h (monitored with TLC) at 90-100 oC using NaOH as activator and H2O/THF was used as solvent. The magnetic Fe3O4 supported Pd catalyst used was synthesized by sonicating the nano-ferrites (prepared by co-precipitation of Fe2+ and Fe3+) with dopamine in water for 2 h, followed by addition of Sodium Palladium Chloride (Na2PdCl2) and subsequent reduction with NaBH4. Characterization of the catalyst by TEM and XRD confirms a size range of 16-25 nm. The Hiyama reaction gave (i) 6- (Pyridin-2-yl)-5H-benzo[a]phenothiazin-5-one and (ii) 6-(Thiophen-2-yl)-5H-benzo[a]phenothiazin-5-one (iii) 6-(Pyridin-2-yl)-5H-naphtho[2,1-b]pyrido[2,3- e][1,4]oxazin-5-one and (iv) 6-(Thiophen-2-yl)-5H-naphtho[2,1-b]pyrido[2,3-e][1,4]oxazin- 5-one (v) 11-Amino-9-thio-6-(pyridin-2-yl)-5H-naphtho[2,1-b]pyrimido[5,4-e][1,4]oxazin-5- one and (vi) 11-Amino-9-thio-6-(thiophen-2-yl)-5H-naphtho[2,1-b]pyrimido[5,4- e][1,4]oxazin-5-one as derivatives in 45-63 % yields. The ease of recovery of the catalyst using an external magnet, efficient recycling and use of water as the solvent are additional eco-friendly attributes of this protocol. All the synthesized compounds were characterized using IR, NMR, UV-Visible spectroscopies and elemental analysis. In the IR spectra of the compounds, the characteristic C=O stretching vibration was observed between 1629-1753 cm-1. The observed decrease in the C=O absorption band for some of the compounds was attributed to ionic resonance effect. The highly deshielded carbonyl signal appeared between 169-184 ppm in the NMR spectra of the compounds. The synthesized compounds were screened for their in vitro antibacterial and antifungal activities on Bacillus subtilis, Bacillus cereus, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae as bacteria and Asperigellus niger and Candida albican as fungi respectively using agar-well-diffusion technique. Ciprofloxacin and Ketoconazole were used as clinical reference drugs. All the synthesized compounds showed significant activity against the tested organisms, with the derivatives 14 and 19 found to be more active antifungal than Ketoconazole. The synthesized compounds could also be used as dye due to their intense colour.
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