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

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.