Nano-Fe₃O₄-encapsulated silica particles bearing sulfonic acid groups as a magnetically separable catalyst for green synthesis of 1,1-diacetates

Figures & data

Table 1. Optimization of the model reaction.^a

Entry	Catalyst	Catalyst loading (g)^b	Condition	Time (min)	Yield (%)
1	No	–	Neat	25	–
2	Bulk Fe3O4	0.05	Neat	25	Trace
3	Nano-Fe3O4	0.05	Neat	25	42
4	Fe3O4@SiO2	0.05	Neat	25	36
6	Fe3O4@SiO2–SO3H	0.05	EtOH	75	45
7	Fe3O4@SiO2-SO3H	0.05	H2O	75	Trace
8	Fe3O4@SiO2–SO3H	0.05	CH3CN	75	72
9	Fe3O4@SiO2-SO3H	0.05	Toluene	75	32
10	Fe3O4@SiO2–SO3H	0.05	Neat	25	95
11	Fe3O4@SiO2-SO3H	0.075	Neat	25	95
12	Fe3O4@SiO2–SO3H	0.025	Neat	25	76

^a Reaction condition: 4-nitrobenzaldehde(1 mmol), acetic anhydride (5 mmol), catalyst (0.05 g), and room temperature.

^b The number of H⁺ site (0.32 mmol/g) was determined by acid-base titration.

Table 2. Fe₃O₄@SiO₂–SO₃H-catalyzed synthesis of various acylals derivatives.

Entry	R1	Product	R2	Time (min)	Yield (%)^a	Mp.°C (Lit.)
1	C6H5	1a	H	20	95	40.42 (42–43) (15)
2	2-Cl-C6H4	2a	H	45	90	50–52 (51–52) (30)
3	4-Cl-C6H4	3a	H	20	90	78.5–79.5 (78.9–80.1) (31)
4	2,4-Cl2C6H3	4a	H	35	97	98–99.5 (99–100) (30)
5	3-NO2-C6H4	5a	H	35	93	65–66 (65–66) (32)
6	4-NO2-C6H4	6a	H	25	95	124–126 (124–126) (32)
7	4-Br-C6H4	7a	H	25	95	91–92.5 (92–95) (33)
8	4-F-C6H4	8a	H	20	92	49–51 (50–51) (34)
9	2-MeO-C6H4	9a	H	20	90	66–68 (68–70) (35)
10	4-MeO-C6H4	10a	H	15	95	61–63 (62–63) (36)
11	4-Me-C6H4	11a	H	20	89	77–79 (80–81) (15)
12	2-OH-C6H4^b	12a	H	25	91	Oil (103–104) (36)
13	3-MeO-4-OH-C6H3^b^,^c	13a	H	30	98	77–79 (79–80) (37)
14	2-Naphthyl	14a	H	30	85	100–101 (100–101) (32)
15	C6H5–CH=CH–	15a	H	55	49	84–86 (84–86) (26)
16	Thiophen	16a	H	55	86	65–67 (65–67) (26)
17	4-(Me)2N-C6H5	–	H	120	N.R	–
18	C6H5	–	Me	120	N.R	–
19	4-MeO-C6H4	–	Me	120	N.R	–
20	4-NO2-C6H4	–	Me	120	N.R	–

^a Isolated yield.

^b The hydroxyl group was also acylated.

^c This reaction progressed in 110°C.

Figure 1. (a) TEM image of Fe₃O₄@SiO₂–SO₃H after 5 runs. (b) FTIR spectra of Fe₃O₄@SiO₂–SO₃H (A) fresh and (B) after 5 runs recycle.

Figure 2. The XRD pattern of reused Fe₃O₄@SiO₂–SO₃H.

Scheme 1. The proposed mechanism.

Table 3. Comparison of result using Fe₃O₄@SiO₂–SO₃H with results obtained by other published works for the synthesis of phenylmethylene diacetate.

Entry	Catalyst	Conditions	Time (min)	Yield (%)	Ref.
1	PVPHS^a	Ultrasonic/rt	8	98	(7)
2	HClO4–SiO2	Solvent-free/rt	2	95	(8)
3	NSPVPC^b	Solvent-free/rt	6	92	(10)
4	MCM-41-SO3H	Solvent-free/rt	3	95	(12)
5	SBA-15-Ph-PrSO3H	Solvent-free/rt	5	100	(13)
6	Fe3O4@SiO2/Schiff base complex of Cr(III)	Solvent-free/rt	15	96	(26)
7	Fe3O4@SiO2–SO3H	Solvent-free/rt	20	95	This work

Note: rt, room temperature.

^a N-sulfonic acid poly(4-vinylpyridinum) chloride.

^b Poly (4-vinylpyridinium)hydrogen sulfate.

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