Research Article | Open Access
Tungstosilicic Acid: An Efficient and Ecofriendly Catalyst for the Conversion of Alcohols to Alkyl Iodides
Treatment of a range of benzylic, allylic, and secondary aliphatic alcohols with potassium iodides in the presence of H4SiW12O40 affords the corresponding alkyl iodides in good to excellent yield with straightforward purification at room temperature in CH3CN.
Heteropolyacids (HPAs) and their salts are useful acid and oxidation catalysts for various reactions and there are already many practical applications. There are several advantages from using the HPA catalysts. One of the most interesting aspects may be the fact that they can be used in various kinds of reaction media or fields [1–3]. There are already several large-scale industrial processes utilizing HPA catalysts [4, 5]. The direct conversion of alcohols to alkyl iodides is a transformation that is widely utilized in organic synthesis . Some alternative reagents reported for this purpose are PPh3/I2/imidazole , PPh3/NIS , PPh3/DEAD/MeI , and P(OPh)3/MeI .
A common drawback of all these procedures is generating stoichiometric quantities of triphenyl phosphine oxide or diphenyl methylphosphonate, which can cause difficulties in product purification. Other methods avoid the use of phosphines, the use of HI , TMSI , N,N-diethylaniline : BF3/I2 , TMSCl/NaI , P2I4 , I2 , or of alkali metal iodides in conjunction with Lewis or Bronsted acids such as BF3OEt2 , CeCl3 , MsOH , Al(HSO4)3 , sodium iodide over KSF-clay under microwave irradiation , KI/H2SO4 supported on natural kaolinitic clay under microwave irradiation  cesium iodide/methanesulfonic acid , or MeSCH = I−/imidazole . Also, the use of expensive and not easily available reagents such as polymer-supported triphenylphosphine  or tris [4-(1H,1H-perfluorooctyloxyphenyl)] phosphine  is reported.
In this work, we describe a simple procedure for an efficient conversion of a wide range of allylic, secondary aliphatic, and benzylic alcohols to the corresponding iodides by treatment with the potassium iodide in the presence of H4SiW12O40 in CH3CN at room temperature (Scheme 1).
2. Results and Discussion
As shown in Table 1, a variety of benzylic, allylic, and secondary alcohols that were treated with potassium iodides in the presence of H4SiW12O40 affords the corresponding alkyl iodides in good to excellent yield at room temperature in CH3CN. Primary benzylic alcohols (Table 1, entries 1–10) were easily converted to the corresponding benzyl iodides in good to excellent yields. Also, secondary aliphatic and benzylic alcohols (Table 1, entries 11–13) were converted to the corresponding iodides in good yields. No transformation took place when KI without H4SiW12O40 was heated under reflux for 12 h in acetonitrile. Furthermore, under the same reaction conditions, allylic alcohols formed allylic iodides in appropriate yields. It has been observed that the attacking of the iodide ion does not involve allylic rearrangement (Table 1, entries 14,15).
To show the advantage of using heteropolyacid, some of our results compared with those reported in the literature. As shown in Table 2, the iodination of benzyl alcohol with KI in the presence of H4SiW12O40 is simple, efficient, and higher yields of corresponding iodides are important features of this method.
In conclusion, the present research for the conversion of alcohols into iodides shows that our method may represent a valuable alternative to those reported in the literature. The superiority of this method is the ease of operation, the simplicity of workup, and the environmental advantage that makes the process very useful.
All chemicals were purchased from Merck. Melting points were recorded on an electro thermal melting point apparatus. The NMR spectra were recorded in CDCl3 with TMS as an internal standard on a Bruker Avance DRX 250 MHz spectrometer. IR spectra were determined on a SP-1100, P-UV-Com instrument. Purity determination of the products was accomplished by TLC on silica gel poly gram SIL G/UV 254 plates. Products were identified by comparing IR and 1H NMR spectra with those reported for authentic samples.
3.1. General Procedure for the Conversion of Alcohols to Iodides
To a mixture of H4SiW12O40 (0.3 g), potassium iodide (2 mmol) in a flask were added acetonitrile (5 mL) and alcohol (1 mmol) at room temperature for 1–3 h. The reaction mixture was monitored by TLC in hexane/ethylacetate (4 : 1) as eluent. After completion of the reaction (TLC), ether (10 mL) was added and washed with aqueous saturated sodium hydrogen carbonate solution, followed by deiodination with sodium hydrogen sulfite. The resultant organic layer was extracted with ether (3 × 10 mL), and the combined extract dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure to afford corresponding iodide in almost pure form. If necessary, products were purified by column chromatography (eluent, hexane-ethyl acetate, 95 : 5). The products were identified by IR and 1H NMR.
3.2. Spectral Data for Compounds
Benzyl Iodide (Table 1, entry 1)
1H NMR (CDCl3): δ = 4.5 (s, 2H, ArCH2I), 7.4 (m, 5H, Ar–H).
4-Chlorobenzyl Iodide (Table 1, entry 2)
1H NMR (CDCl3): δ = 4.34 (s, 2H, ArCH2I), 7.23 (d, 2H, Ar–H), 7.32 (d, 2H, Ar–H).
4-Flourobenzyl Iodide (Table 1, entry 3)
1H NMR (CDCl3): δ = 4.44 (s, 2H, ArCH2I), 7.12 (d, 2H, Ar–H), 7.21 (d, 2H, Ar–H).
2-Chlorobenzyl Iodide (Table 1, entry 4)
1H NMR (CDCl3): δ = 4.2 (s, 2H, ArCH2I), 7.29 (m, 4H, Ar–H).
2-Hydroxybenzyl Iodide (Table 1, entry 5)
1H NMR (CDCl3): δ = 4.22 (s, 2H, ArCH2I), 6.83–7.09 (m, 4H, Ar–H).
4-Isopropylbenzyl Iodide (Table 1, entry 6)
1H NMR (CDCl3): δ = 1.31–1.33 (s, 6H, iPr), 2.99 (m, 1H, –CH), 4.58 (s, 2H, ArCH2I), 7. 27 (d, 2H, Ar–H), 7.40 (d, 2H, Ar–H).
4-Methoxybenzyl Iodide (Table 1, entry 7)
1H NMR (CDCl3): δ = 3.8 (s, 3H, OCH3), 3.9 (s, 2H, ArCH2I), 6.82 (d, 2H), 7.07 (d, 2H).
4-Nitrobenzyl Iodide (Table 1, entry 8)
1H NMR (CDCl3): δ = 4.48 (s, 2H, ArCH2I), 7.50 (d, 2H, Ar–H), 8.12 (d, 2H, Ar–H).
4-Methylbenzyl Iodide (Table 1, entry 9)
1H NMR (CDCl3): δ = 2.41 (s, 3H, CH3), 4.54 (s, 2H, ArCH2I), 7.21 (d, 2H, Ar–H), 7.36 (d, 2H, Ar–H).
1-Iodo-2-phenyl-ethane (Table 1, entry 10)
1H NMR (CDCl3): δ = 2.95 (t, 2H, CH2), 3.35 (t, 2H, CH2I), 7.00–7.12 (m, 5H, Ar–H).
1,1-diphenyl Methyl Iodide (Table 1, entry 11)
1H NMR (CDCl3): δ = 6.17 (s, 1H, ArCHI), 7.20–7.32 (m, 10H, Ar–H).
Iodocyclohexane (Table 1, entry 12)
1H NMR (CDCl3): δ = 0.85–0.99 (m, 4H), 1.4–1.75 (m, 6H), 3.89 (m, 1H, CHI).
5-Iodononane (Table 1, entry 13)
1H NMR (CDCl3): δ = 1.24–1.36 (m, 14H), 1.52–1.56 (m, 4H), 4.14 (m, 1H, CHI).
3-Iodo-1-propene (Table 1, entry 14)
1H NMR (CDCl3) δ = 6.0–6.1 (1H, m), 5.27–5.33 (1H, m), 5.09–5.12 (1H, m), 3.89 (2H, d, J = 7.5 Hz, CH2I).
3-Iodopropenyl Benzene (Table 1, entry 15)
1H NMR (CDCl3): δ = 3.4 (d, 2H, CH2I), 6.6 (m, 1H, =CH), 6.8 (d, 1H, PhCH), 7.20 (d, 5H).
The authors are grateful to Islamic Azad University of Rasht Branch for financial assistance of this work.
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