An efficient and convenient synthesis of 1,1-diacetates catalyzed by sulfamic acid in the recyclable ionic liquid [bmim]PF6

Li Yiqun
(Department of Chemistry, Jinan University, Guangzhou 510632, China)

The project was supported by the National Natural Science Foundation of China (20272018), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China and the   Guangdong Natural Science Foundation  (974021, 021166)

Abstract A variety of aldehydes can be converted into 1,1-diacetates efficiently and conveniently in the presence of catalytic amounts of sulfamic acid at ambient temperature in the room temperature ionic liquid [bmim]PF6 with excellent yields. The ionic liquid is immiscible with water or diethyl ether and can be reused without noticeable dropdown in activity after separation of the products.

    1,1-diacetates are efficient protecting groups for aldehydes as they are stable in neutral and basic media[1]. Usually, the 1,1-diacetates are prepared from aldehydes and acetic anhydride in the presence of strong protonic acids such as sulfuric acid[2], phosphoric acid[2], or methanesulfonic acid[2], and Lewis acids such as anhydrous zinc chloride[3], phosphorus(III) chloride[4], and anhydrous ferric(III) chloride[5,6]. Recently the use of montmorillonite clay[7], TMCS-NaI[8], Sc(OTf)3[9], Cu(OTf)2[10], PVC-FeCl3[11] SnCl4/SiO2[12], NH2SO3H[13] etc. as catalysts have also been reported. But these methods have the drawback as using of organic solvents such as MeCN, MeNO2, Ac2O or halogenated solvents. It is necessary to develop an alterative solvent for the synthesis of 1,1-diacetates under mild and environmentally benign conditions.
    The use of ionic liquids as a novel class of environmentally friendly solvents is now of topical interest in the organic synthesis. The ionic liquids have several unique properties; they can solvate a wide range of organic and inorganic chemicals, they are highly polar yet non-coordinating and immiscible with a wide range of organic solvents, and they have a negligible vapor pressure. With the current desire to avoid the use of volatile organic solvents in organic synthesis, herein, we decided to investigate the use of potentially cleaner solvents, namely 1-butyl-3-methyl imidazonium hexafluorophosphate, [bmim]PF6, for the synthesis of 1,1-diacetates promoted by sulfamic acid. (Scheme 1)Scheme 1

    The results of the synthesis of 1,1-diacetates in the presence of catalytic amount of sulfamic acid in ionic liquid [bmim]PF6 are summarized in Table 1. The isolated yields of the products are excellent in all cases.

Table 1 Conversion of aldehydes to the corresponding 1,1-diacetates catalyzed by NH2SO3H in [bmim]PF6

entry Aldehydes (1) Productsa (2) Yieldb mp (ºC)
R R (%) Found Reported
a 4-MeO-C6H4 4-MeO-C6H4 86 64-66 64-652
b 4-HO-C6H4 4-AcO-C6H4 92 91-92 89-902
c 4-HO-3-MeO-C6H3 4-AcO-3-MeO-C6H3 92 75-76 79-802
d (E)-PhCH=CH (E)-PhCH=CH 85 82-83 84-8512
e C6H5 C6H5 91 43-44 44-4612
f 4-Cl-C6H4 4-Cl-C6H4 90 80-82 79-802
g 2-NO2-C6H4 2-NO2-C6H4 88 82-84 85.5-86.52
h 3-NO2-C6H4 3-NO2-C6H4 95(93)c 64-65 63-642
i 4-NO2-C6H4 4-NO2-C6H4 93 125-126 124-1252
j 4-Me2N-C6H4 4-Me2N-C6H4 No reaction
aAll the products gave satisfactory spectral analysis for 1H NMR and compared with authentic samples.
bIsolated yield.
cYields in the parenthesis was repeated in the same batch of the ionic liquids over five runs.

As shown the Table 1, the aldehydes containing the electron withdrawing group have greater activiy than that with electron donor group and esterification of phenol group was found to produce the corresponding triacetates under these conditions (entry b and c). Additionally, it is worth noting that 4-N, N-dimethyl- aminobenzaldehyde failed to produce the corresponding 1,1-diacetate and the starting material recovered quantitatively after 48h under the same conditions (entry j). This result is the same with that reported by T. S. Li etal. in the literature[13]. The reason may be due to that the N, N-dimethylamino group is the strong electron donor group in the compund 1j and hence reduce the activity. The equilibrium between quininoid structure with aldehyde structure may exist and decreases the activity of the aldehyde group. (Scheme 2)

Scheme 2

    The ionic liquid [bmim]PF6 acts as an excellent alternative solvent for the synthesis of 1,1-diacetates. This is the first report of using sulfamic acid as catalyst for synthetic chemistry in ionic liquids. The results showed that relatively high activity of this system reported here.
    In conclusion, the present protocol in ionic liquid for the formation of 1,1-diacetates offers significant improvements over the existing procedures and is an attractive method of synthesis since the reaction is rapid, the yields are excellent and the procedure is simple and benign to the environment.
    Melting points were uncorrected. 1H NMR spectra were recorded on a Bruker DRX 400 (400MHz) instrument in CDCl3 with residue CHCl3 at d 7.26 ppm as an internal standard. Benzaldehyde was purified by distillation. All other chemicals used were of commercial grade without further purification. [bmim]PF6 was prepared according to the literature[14].
    4.1 General procedure for the preparation of 1,1-diacetates
    A mixture of the aldehyde (5.0 mmol), acetic anhydride (15.0 mmol) and sulfamic acid (0.5 mmol) in ionic liquid [bmim]PF6 (2 ml) was stirred at room temperature. The progress of the reaction was monitored by TLC. After the completion of the reaction, the saturated sodium hydrogen carbonate solution (10ml) was added. After 10 min the lower layer [BMIM]PF6 was separated from the aqueous layer which was extracted with diethyl ether (10 ml×3). The ionic liquid can be reused for the next run by previously drying for 1h under vacuum at 80ºC. The combined organic layer was washed with brine and then dried over anhydrous sodium sulfate. Evaporation of the solvent under reduced pressure afforded the corresponding product that was purified by crystallization from cyclohexane to give pure 1,1-diacetate.
    2a: 1H NMR(CDCl3) dH: 2.11 (s, 6H, 2CH3CO), 3.81 (s, 3H, CH3O), 6.96 (d, 2H, J=6.4Hz, Ar-H), 7.45 (d, 2H, J=6.4Hz, Ar-H), 7.62 (s, 1H, CH).
    2b: 1H NMR(CDCl3) dH: 2.13 (s, 6H, 2CH3CO), 2.31 (s, 3H, CH3CO), 7.14 (d, 2H, J=6.4Hz, Ar-H), 7.45 (d, 2H, J=6.4Hz, Ar-H), 7.62 (s, 1H, CH).
    2c: 1H NMR(CDCl3) dH: 2.13 (s, 6H, 2CH3CO), 2.32 (s, 3H, CH3CO), 3.81 (s, 3H, CH3O), 7.05 (d, 1H, J=1.6Hz, Ar-H), 7.12 (s, 1H, Ar-H), 7.17 (d, 1H, J=1.6Hz, Ar-H), 7.65 (s, 1H, CH).
    2d: 1H NMR(CDCl3) dH: 2.13 (s, 6H, 2CH3CO), 6.20 (dd, 1H, J=16.0 Hz, J=6.4Hz, CH=), 6.87 (d, 1H, J=16.0Hz, CH=), 7.31-7.43 (m, 5H-Ar + CH).
    2e: 1H NMR(CDCl3) dH: 2.14 (s, 6H, 2CH3CO), 7.41-7.42 (m, 3H, Ar-H), 7.51-7.54 (m, 2H, Ar-H), 7.69 (s, 1H, CH).
    2f: 1H NMR(CDCl3) dH: 2.18 (s, 6H, 2CH3CO), 7.38 (d, 2H, J=6.4Hz, Ar-H), 7.46 (d, 2H, J=6.4Hz, Ar-H), 7.63(s, 1H, CH).
    2g: 1H NMR(CDCl3) dH: 2.15 (s, 6H, 2CH3CO), 7.59-7.73 (m, 1H, Ar-H), 7.70-7.73 (m, 2H, Ar-H), 8.05-8.07 (m, 1H, Ar-H), 8.21 (s, 1H, CH).
    2h: 1H NMR(CDCl3) dH: 2.17 (s, 6H, 2CH3CO), 7.63 (t, J=8.0Hz, 1H, Ar-H), 7.74 (s, 1H, CH), 7.83 (d, J=8.0Hz, 1H, Ar-H), 8.27 (dd, J=8.4Hz, J=1.2Hz, 1H, Ar-H), 8.41 (s, 1H, Ar-H)
    2i: 1H NMR(CDCl3) dH: 2.17 (s, 6H, 2CH3CO), 7.71 (d, 2H, J=8.73Hz, Ar-H), 7.74 (s, 1H, CH), 8.28 (d, 2H, J=8.76Hz, Ar-H).