Facile sonochemical heterocyclization of 2,5-dimethoxy tetrahydrofuran with primary amines using sulfonated MWCNTs as a recyclable catalyst in aqueous media

Figures & data

Scheme 1. Preparation of the MWCNTs-SO₃H catalyst.

Scheme 2. Reaction of 2,5-dimethoxy tetrahydrofuran and primary aromatic amines.

Table 1. Optimization of the catalyst amounts under ultrasonic irradiation (60 W) in water^a.

Entry	Amounts of catalyst (g)	Time (min)	Yield (%)
1	0.01	5	55
2	0.02	5	85
3	0.03	5	85

^aReaction condition: 2,5-dimethoxy tetrahydrofuran (1.2 mol), 4-methoxy aniline (1 mol), H₂O, ultrasound irradiation (60W), at room temperature, ultrasound time, 5 min.

Table 2. Optimization of ultrasonic power by using MWCNT-SO₃H (0.02 g) in water^a.

Entry	Power (W)	Time (min)	Yield (%)
1	60	5	85
2	65	5	96
3	70	5	70

^aReaction condition: 2,5-dimethoxy tetrahydrofuran (1.2 mol), 4-methoxy aniline (1 mol), MWCNT-SO₃H (0.02 g), H₂O, at room temperature, ultrasound time, 5 min.

Table 3. Optimization of solvents in the presence of MWCNT-SO₃H (0.02 g) under ultrasonic irradiation (65 W)^a.

Entry	Solvent	Time (min)	Yield^b (%)
1	EtOH	5	5
2	CH3CN	5	50
3	DMF	5	90
4	H2O	5	96

^aReaction condition: 2,5-dimethoxy tetrahydrofuran (1.2 mol), 4-methoxy aniline (1 mol), MWCNT-SO₃H (0.02 g), at room temperature, ultrasound irradiation (65 W), ultrasound time, 5 min.

^bIsolated yields.

Table 4. Synthesis of various N-aryl pyrrole using the catalytic amount of MWCNT-SO₃H as catalyst under ultrasonic irradiation in water^a.

Entry	Amine	Product	Time (min): Yield (%)^b
1			5:95
2			5.5:95
3			5:96
4			5.2:93
5			5:96
6			5:95
7			5.5:92
8			7:93
9			6:88
10			10:40
11			10:92
12			11:92
13^c			6(h):90
14^d			24(h):65
15^d			24(h):80

^aReaction condition: 2,5-dimethoxy tetrahydrofuran (1.2 mol) with different types of primary aromatic amines (1 mol), MWCNT-SO₃H (0.02 g), at room temperature, ultrasound irradiation (65 W), H₂O as solvent.

^bIsolated yields.

^cThe reaction of 2,5-hexandione and aniline catalyzed by 5 mg graphene oxide at room temperature (41).

^dThe reaction of 2,5-hexandione with substituted anilines at room temperature (42).

Table 5. Reaction of 2,5-dimethoxy tetrahydrofuran with several amines in different conditions^a.

Entry	Amounts of catalyst (g)	Time (min)	Product	Condition	Yield^b (%)
1	0.02	60	3c	Room temperature	–
2	0.02	40	3c	thermal (65°C)	–
3	0.04	55	3c	Thermal (65°C)	50
4	0.04	60	3a	thermal (67°C)	48
5	0.04	62	3h	Thermal (70°C)	60
6	0.02	7	3h	U.S	93
7	0.02	5	3a	U.S	95
8	0.02	5	3c	U.S	96

^aThe comparison conventional heating with ultrasound-assisted method.

^bIsolated yields.

Figure 1. Reusability of the catalyst in the reaction.

Scheme 3. Plausible reaction mechanism for the preparation of N-aryl pyrroles.

Chen, C.-Y.; Guo, X.-Y.; Lu, G.-Q.; Pedersen, C.-M.; Qiao, Y.; Hou, X.-L.; Wang, Y.-X. New Carbon Mat. 2017, 32(2), 160–167. doi: 10.1016/S1872-5805(17)60013-6

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