Ultrasound-Mediated Green Synthesis of Imidazo[1,2-a]pyridines and Imidazo[2,1-b]thiazoles through C(sp³)–H Functionalization

Abstract

An ultrasound-assisted expedient protocol has been developed for the synthesis of imidazo[1,2-a]pyridines and imidazo[2,1-b]thiazoles by the C–H functionalization of ketones using a KI/tert-butyl hydroperoxide catalytic system. The reaction takes place in water, a green solvent, and does not require a metal catalyst; it also gives good yields of the products. The method offers numerous noteworthy characteristics, including mild reaction conditions, the absence of a base, broad functional-group compatibility, and excellent reaction yields. Moreover, it avoids the use of costly and air-sensitive chemicals, and can be conducted under ambient reaction conditions.

Key words

green chemistry - heterocycles - gram-scale synthesis - ultrasound - one-pot synthesis

Publication History

Received: 05 February 2024

Accepted: 07 February 2024

Article published online:
26 February 2024

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• 9 Imidazo[1,2-a]pyridines 3aa–hc; General Procedure A flask was charged with the appropriate aryl methyl ketone 1 (1.0 mmol), 2-aminopyridine 2 (1.0 mmol), KI (0.5 equiv), TBHP (1.0 equiv), and H₂O (3 mL) at r.t., and the mixture was exposed to ultrasound irradiation until the reaction was complete (TLC). The mixture was extracted with EtOAc, and the organic layer was dried (Na₂SO₄), filtered, and concentrated. The residue was purified by column chromatography (silica gel, EtOAc–hexane). 2-Phenylimidazo[1,2-a]pyridine (3aa)^10a White solid; yield:188 mg (97%); mp 135–136 °C. ¹H NMR (500 MHz, CDCl₃): δ = 8.11 (dt, J = 6.7, 1.1 Hz, 1 H), 7.99–7.93 (m, 2 H), 7.86 (s, 1 H), 7.66–7.60 (m, 1 H), 7.47–7.41 (m, 2 H), 7.37–7.30 (m, 1 H), 7.19–7.13 (m, 1 H), 6.77 (td, J = 6.8, 1.0 Hz, 1 H). ¹³C NMR (126 MHz, CDCl₃): δ = 145.97, 145.83, 133.90, 128.86, 128.11, 126.19, 125.71, 124.77, 117.73, 112.55, 108.24. 2-Phenylimidazo[2,1-b][1,3]benzothiazole (3ae)^10b Yellow solid; yield: 235 mg (94%); mp 106–107 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.96 (s, 1 H), 7.88 (d, J = 8.2 Hz, 2 H), 7.69 (d, J = 8.5 Hz, 1 H), 7.60 (d, J = 8.0 Hz, 1 H), 7.46–7.40 (m, 3 H), 7.32 (dt, J = 15.0, 7.7 Hz, 2 H). ¹³C NMR (126 MHz, CDCl₃): δ = 148.2, 147.8, 133.9, 132.3, 130.4, 128.8, 127.6, 126.3, 125.3, 124.9, 124.5, 112.7, 106.9.
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