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N‑Alkylated 1,4-Diazabicyclo[2.2.2]octane–Polyethylene Glycol Melt as Deep Eutectic Solvent for the Synthesis of Fisher Indoles and 1H‑Tetrazoles

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journal contribution
posted on 22.06.2017, 09:03 by Sarfaraz Ali Ghumro, Rima D. Alharthy, Mariya al-Rashida, Shakil Ahmed, Muhammad Imran Malik, Abdul Hameed
1,4-Diazabicyclo­[2.2.2]­octane (DABCO)-based ionic liquids (ILs) 2–4 were synthesized by the N-alkylation of DABCO using alkyl halides of varying chain lengths (C2, C5, and C7). The N-alkylated DABCO-ILs were mixed with polyethylene glycols (PEGs) of varying molar masses as hydrogen bond donors (HBDs) to prepare new deep eutectic solvents (DESs). These DABCO–PEG-based DESs were successfully employed for the synthesis of a variety of indoles 7a–7h (by Fischer indole synthesis) and 1H-tetrazoles 9a–9i (by click chemistry). For comparison, DESs of DABCO-ILs with different alcohols (as HBD) were also prepared and investigated for the synthesis of indoles. Although comparable yields were observed in DES-containing alcohols and PEGs, the use of PEG as HBD in DES (as an alternative to alcohols) provides a much safer, nonvolatile, and environmentally benign reaction medium for synthetic reactions. The first successful application of PEG-polymer-based DES as benign reaction media for organic syntheses offers exciting opportunities to be explored in the realm of green synthesis.