A facile and efficient synthesis of tri- and tetrasubstituted imidazoles with potassium hydrogen sulfate and DB18C6 in an aqueous medium

Figures & data

Scheme 1.  Potassium ion being trapped by the six oxygen atoms in the 18-crown-6 core and generation of hydronium ion in an aqueous medium.

Table 1. Condensation of benzil, benzaldehyde, and ammonium acetate in the presence of crown ethers and salts in an aqueous medium.

Entry	Crown ether (mol%)	Salt (mol%)	Temperature (°C)	Time (h)	Yield (%) (isolated)
1	–	–	25	24	Nil
2	–	–	100	24	15
3	18C6 (15)	KHSO4 (15)	25	24	Trace
4	18C6 (15)	KHSO4 (15)	60	6	75
5	DB18C6 (15)	KHSO4 (15)	25	24	Trace
6	DB18C6 (15)	KHSO4 (15)	60	4	92
7	DB18C6 (15)	NaHSO4 (15)	60	6	55

Scheme 2.  Synthesis of 2,4,5-trisubstituted imidazoles in an aqueous medium using DB18C6 and KHSO₄ (15 mol%).

Table 2. Synthesis of 2,4,5-trisubstituted imidazoles in an aqueous medium using DB18C6 and KHSO₄ (15 mol%).

Scheme 3.  Synthesis of 1,2,4,5-tetrasubstituted imidazoles in an aqueous medium using DB18C6 and KHSO₄ (15 mol%).

Table 3. Synthesis of 1,2,4,5-tetrasubstituted imidazoles in an aqueous medium using DB18C6 and KHSO₄ (15 mol%).

Scheme 4.  Proposed mechanism for the DB18C6-KHSO₄ catalyzed synthesis of substituted imidazoles formation.