Hybrid crystal NH₃(CH₂)₄NH₃SiF₆ as an efficient catalyst for the synthesis of benzoxazoles, benzimidazoles and benzothiazoles under solvent-free conditions

Figures & data

Figure 1. The molecular structure of the NH₃(CH₂)₄NH₃SiF₆ compound with the atom-labeling scheme as has been determined. The symmetry codes are: (i) −x−1, −y+1, −z+1; (ii) −x+1, −y, −z.

Figure 2. The three-dimensional plot of the NH₃(CH₂)₄NH₃SiF₆ compound, showing inorganic sheets linked through N–H … .F hydrogen bonds to the organic layers (dashed lines) (34).

Scheme 1. Model reaction for the synthesis of 2-phenyl benzimidazole.

Table 1. Screening of the reaction conditions for the reaction of 1,2-phenylenediamine 1a and benzaldehyde 2a^a.

Entry	Catalyst (mol%)	Solvent (1 mL)	Time^b (min)	Yield (%)^c
1	–	EtOH	240	12
2	5	EtOH	14	93
3	5	MeOH	10	93
4	5	Butanol	23	80
5	5	Isopropanol	30	77
6	5	CH3CN	40	67
7	5	THF	25	69
8	5	ACOEt	15	79
9	5	Solvent free	4	94
10	4	Solvent free	4	94
11	3	Solvent free	4	95
12	2	Solvent free	4	95
13	1	Solvent free	4	98

^aReaction conditions: benzaldehyde (1a, 1 mmol), o-phenylenediamine (2a, 1 mmol), room temperature.

^bTime reported in min monitored by thin layer chromatography (TLC).

^cIsolated yield.

Scheme 2. Synthesis of compounds 3, 4 and 5 derivatives catalyzed by NH₃(CH₂)₄NH₃SiF₆.

Table 2. Substrate scope for synthesis of desired heterocycles derivatives.

Product	R	X	Time min^a	Yield %^b	Melting point (°C)		Ref.
					Found	Reported
3a	H	NH	4	98	>266	292–294	33
3b	Cl	NH	2	97	>266	288–290	33
3c	Me	NH	6	95	>266	275–276	33
3d	NO2	NH	2	99	>266	318–320	33
4a	H	S	4	97	112–113	110–114	33
4b	Cl	S	6	94	114–116	114–116	33
4c	Me	S	3	90	85–87	86–89	43
4d	NO2	S	5	99	226–227	227–229	33
5a	H	O	5	98	100–102	100–102	43
5b	Cl	O	3	97	144–145	144–146	43
5c	Me	O	6	96	88–90	88–89	43
5d	NO2	O	5	99	261–262	262–264	33
5e	OH	O	2	98	220–221	220–223	43

^aIsolated yields.

^bTime reported in min monitored by TLC.

Table 3. Reusability of the catalyst in the synthesis of 2-phenyl benzimidazole 3a.

Run	Time (min)	Yield (%)^a
1	4	98
2	4	97
3	4	97
4	4	96
5	4	95
6	4	95

^aIsolated yields.

Table 4. Comparative synthesis of desired heterocycles using the reported catalysts versus NH₃(CH₂)₄NH₃SiF₆.

Catalyst	Amount of catalyst/reaction conditions	Time	Yield %
NH3(CH2)4NH3SiF6	1 mol% solvent free, RT	2–6 min	90–99
Fe(III)–Schiff base/SBA-15 (33)	0.01 g, 0.0014 mmol (0.01 based on metal ion), water, reflux	3 h	79–92
ZnBr2/ABM (44a)	100 mg, 5 mL, toluene, 111°C	10–25 min	83–96
Indion 190 resin (44b)	10%/weight, 5 mL EtOH, 70°C	4 h	78–92
Nano-ZnO (29)	10 mol%, 10 mL EtOH, reflux	80–100 min	64–88
CuO-np/SiO2 (26)	10 mol %, 10 mL, MeOH, RT	4–14 h	68–93
ZnO PNs (43)	5 mg; solvent free, or in EtOH, RT	2–8 min	90–99
t-Butyl hypochloride (44c)	2 mmol, 10 mL CH3CN, reflux	1–4 h	65–92
ABMs-ZnBr2 (44d)	Toluene; air, 110°C	10–25 min	81–96
PIFA (44e)	1.05 equiv, 3 mL EtOH , 800C, μW	15 min	59–90
Cu NPs/SiO2 (44f)	10 mol%, methanol, RT	4–8 h	72–86
CeO2 NPs (44g)	10 mol%, H2O, RT	20–40 min	58–97
PTSA (44h)	10 mol%, water, 70°C	1–5 h	60–97
FeCl3/montmorilonite K-10 (44i)	10%w/w, CH3OH, condensation, ultrasonic irradiation	0.7–5 h	33–95

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