An efficient and practical synthesis of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazole using silica-supported sodium hydrogen sulfate as a heterogeneous catalyst

Figures & data

short-legendScheme 1. Synthesis of 1-aryl-1H, 3H-thiazolo[3,4-a]benzimidazoles.

short-legendScheme 2. Possible mechanism.

Table 1. Synthesis of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazole using various catalysts.^a

Entry	Catalyst	Time (h)	Yield^b
1	None	20	Trace
2	Amberlyst-15	1	79
3	PMA-SiO2	1	85
4	Mantmorillonite clay	1	76
5	KF-Al2O3	1	43
6	NaHSO4·SiO2	1	93
7	Bismuth (III) salts	1	80
8	Indion 190 resin	1	75
^aReaction conditions: o-phenylendiamine (1 mmol), 2-mercaptoacetic acid (1 mmol) and benzaldehyde (1 mmol), catalyst (200 mg), time: 1 h, acetonitrile (5 mL), temp: 75 °C.
^bIsolated yield.

Table 2. Effect of solvents on preparation of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazole.^a

Entry	Solvent	Temperature (°C)	Time (h)	Yield^b (%)
1	Methanol	55	4	40
2	Ethanol	75	4	53
3	DMF	120	4	45
4	Acetonitrile	75	1	93
5	Neat	100	24	17
6	Toluene	100	4	71
^aReaction conditions: o-phenylenediamine (1 mmol), benzaldehyde (1 mmol), 2-mercaptoacetic acid (1 mmol), NaHSO4·SiO2 (200 mg), solvent (5 mL).
^bIsolated yield.

Table 3. Optimization of the amount of NaHSO_4·SiO₂ for the synthesis of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazole.^a

Entry	Weight of catalyst (mg)	Time (h)	Yield^b (%)
1	50	1	40
2	100	1	60
3	150	1	82
4	200	1	93
5	250	1	85
^aReaction conditions: o-phenylenediamine (1 mmol), benzaldehyde (1 mmol), 2-mercaptoacetic acid (1 mmol), NaHSO4·SiO2 (X mg), acetonitrile (5 mL), temperature: 75 °C.
^bIsolated yield.

Table 4. Optimization of temperature for the synthesis of 1-aryl-1H, 3H-thiazolo[3,4-a]benzimidazole.^a

Entry	Temperature (°C)	Time (h)	Conversion^b (%)
1	30	1	40
2	50	1	65
3	75	1	93
^aReaction conditions: o-phenylenediamine (1 mmol), benzaldehyde (1 mmol), 2-mercaptoacetic acid (1 mmol), NaHSO4·SiO2 (200 mg), and acetonitrile (5 mL).
^bIsolated yield.

Table 5. Synthesis of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazole.^a

Products	R1	R2	R3	R4	Yield^b (%)	Physical constant observed (liter)
4a	H	H	Cl	Cl	89	133–134 (132–134)
4b	H	H	H	H	93	134–136 (134–135)
4c	H	H	H	NO2	90	149–151 (150–152)
4d	H	H	H	Cl	87	125–127 (124–126)
4e	H	H	H	OH	85	482 (481.5)
4f	H	H	H	OMe	80	132–133 (133–134)
4g	H	H	F	F	91	140–141 (141–142)
4h	5-Me	H	F	F	92	160–161 (159–161)
4I	6-Me	7-Me	F	F	93	179–180 (178–179)
4j	8-Me	H	F	F	90	147–149 (149–150)
4k	3-NO2	H	H	H	79	183–184 (182–183)
^aReaction conditions: o-phenylenediamine (1 mmol), aldehydes (1 mmol), 2-mercaptoacetic acid (1 mmol), NaHSO4·SiO2 (200 mg), acetonitrile (5 mL), temperature: 75 °C, time: 1 h.
^bIsolated yield.

Table 6. Reusability of the catalyst.^a

Cycle	Yield (%)
1	93
2	91
3	87
^aReaction condition: o-phenylenediamine (1 mmol), benzaldehydes (1 mmol), 2-mercaptoacetic acid (1 mmol), NaHSO4·SiO2 (200 mg), acetonitrile (5 mL), temperature: 75 °C, time: 1 h.