Environment-friendly and solvent-free synthesis of symmetrical bis-imines under microwave irradiation

Figures & data

short-legendScheme 1. General reaction scheme for the synthesis of bis-imines 3 and 6.

short-legendScheme 2. Synthesis of bis-imines 3a from EDA 1a and aldehyde derivatives 2.

short-legendScheme 3. Synthesis of bis-imines 3b from p-PDA 1b and aldehyde derivatives 2.

short-legendScheme 4. Synthesis of bis-imines 6 from terephthaldehyde 4 and amine derivatives 5.

Table 1. Effect of catalyst p-toluenesulphonic acid loading with MW power, time, and temperature on the yields of bis-imines 3a, 3b, and 6.

Entry	Catalyst loading	MW power (watt)	Reaction temperature (°C)^a	Yield (%)^b of 3a (entry 4, Table 2)/time (min)	Yield (%)^b of 3b (entry 4, Table 3)/time (min)	Yield (%)^b of 6 (entry 2, Table 4)/time (min)
1	No catalyst	280	87	35/7	26/3	28/7
		450	93	46/7	35/3	42/7
		600	99	56 /7	45/3	53/7
		900	112	53 /7	45/3	52/7
2	3 mol%	280	87	48 /7	51/3	53/7
		450	93	71 /7	65/3	61/7
		600	99	78 /7	74/3	76/7
		900	112	78 /7	73/3	76/7
3	5 mol%	280	87	65 /7	57/3	63/7
		450	93	90 /7	79/3	75/7
		600	99	99 /7	99/3	90/7
		900	112	89 /7	91/3	90/7
4	7 mol%	280	87	89 /7	87/3	76/7
		450	93	98 /7	99/3	91/7
		600	99	95 /7	97/3	90/7
		900	112	78 /7	81/3	76/7
5	10 mol%	280	87	78 /7	78/3	76/7
		450	93	81 /7	84/3	81/7
		600	99	77 /7	74/3	79/7
		900	112	76 /7	69/3	68/7
^aTemperature was calculated with an error ±2°C.
^bIsolated yield.

Table 2. Synthesis of bis-imines 3a from EDA 1a and aldehyde derivatives 2.

Entry	R	MW power (watt)/time (min)	Yield (%)^a
1	Ph–	600/10	75^68
2	Ph–CH=CH–	600/13	99^69
3	p–OMe–C6H4	600/8	98^68
4	p–Cl–C6H4	600/7	99^70
5	o–Cl–C6H4	600/7	98^68
6	p–OH–C6H4	600/3	95^68
7	o–OH–C6H4	600/2	99^71
8	p–Me–C6H4	600/6	98^68
9	p–NO2–C6H4	600/9	83^72
10	m–NO2–C6H4	600/2	98^73
11	Me–	600/10	61
12	Me–(CH2)2–	600/10	63
13	Me–(CH2)3–	600/9.5	63^74
^aIsolated yield.

Table 3. Synthesis of bis-imines 3b from p-PDA 1b and aldehyde derivatives 2.

Entry	R	MW power (watt)/time (min)	Yield (%)^a
1	Ph–	450/5	80^75
2	Ph–CH=CH–	450/2.5	99^76
3	p–OMe–C6H4	450/3	96^77
4	p–Cl–C6H4	450/3	99^78
5	o–Cl–C6H4	450/3	98
6	p–OH–C6H4	450/3	92^78
7	o–OH–C6H4	450/3	93^79
8	p–Me–C6H4	450/6	98^80
9	p–NO2–C6H4	450/7	87^81
10	m–NO2–C6H4	450/3	98
11	Me–	600/10	69
12	Me–(CH2)2–	600/10	69
13	Me–(CH2)3–	600/10	72
^aIsolated yield.

Table 4. Synthesis of bis-imines 6 from terephthaldehyde 4 and amine derivatives 5.

Entry	R	MW power (watt) /time (min)	Yield (%)^a
1	Ph–	450/9	85
2	p–Cl–C6H4	450/7	90
3	o–Cl–C6H4	450/6.5	91
4	p–NO2–C6H4	450/8	78
5	m–NO2–C6H4	450/5	88
6	p–Me–C6H4	450/7	93
7	o–Me–C6H4	450/7	92
8	p–OMe–C6H4	450/8	82
9	Ph–CH2–	600/10	75^44
^aIsolated yield.

short-legendScheme 5. Plausible reaction mechanism for the formation of imine derivatives.