Organic Chemistry InternationalVolume 2011 (2011), Article ID 835183, 4 pagesdoi:10.1155/2011/835183

Research Article

Tungstosilicic Acid: An Efficient and Ecofriendly Catalyst for the Conversion of Alcohols to Alkyl Iodides

Masoud Mokhtary¹ and Faranak Najafizadeh²

¹Department of Chemistry, Rasht Branch, Islamic Azad University, P.O. Box 41325-3516, Rasht, Iran²Department of Chemistry, Science and Research Amol Branch, Islamic Azad University, P.O. Box 678, Amol, Iran

Received 12 October 2011; Accepted 15 December 2011

Academic Editor: William Ogilvie

Copyright © 2011 Masoud Mokhtary and Faranak Najafizadeh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [6]. Some alternative reagents reported for this purpose are PPh₃/I₂/imidazole [7], PPh₃/NIS [8], PPh₃/DEAD/MeI [9], and P(OPh)₃/MeI [10].

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 [15], TMSI [16], N,N-diethylaniline : BF₃/I₂ [17], TMSCl/NaI [18], P₂I₄ [19], I₂ [20], or of alkali metal iodides in conjunction with Lewis or Bronsted acids such as BF₃OEt₂ [21], CeCl3 [22], MsOH [23], Al(HSO₄)₃ [24], sodium iodide over KSF-clay under microwave irradiation [25], KI/H₂SO₄ supported on natural kaolinitic clay under microwave irradiation [26] cesium iodide/methanesulfonic acid [27], or MeSCH = I⁻/imidazole [28]. Also, the use of expensive and not easily available reagents such as polymer-supported triphenylphosphine [29] or tris [4-(1H,1H-perfluorooctyloxyphenyl)] phosphine [30] 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).

Scheme 1: Synthetic pathway for the conversion of alcohols to alkyl iodides.

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).

Table 1: Conversion of alcohols into alkyl iodides using H₄SiW₁₂O₄₀ at room temperature.

Entry
Substrate
Producta
Time/h
Yield/%b

1

0.5
96

2

0.5
92

3

0.5
95

4

1
86

5

1
94

6

0.5
96

7

0.5
95

8

1
65

9

0.5
95

10

1
97

11

3
80

12

1
60

13

1
60

14

1
92

15

1
94

aAll of the products were identified by comparing melting point and ¹H NMR with those of authentic simples reported in the literature [11–14].

bYields refer to isolated products.

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.

Table 2: Comparison of H₄SiW₁₂O₄₀/KI for the conversion of benzyl alcohol into iodide with other reagents.

Entry
Reagent
Reaction conditions
Time
Yield
Ref.

1
N,N-diethylaniline : BF₃/I₂
RT
12 h
86
[17]

2
Me₃SiCl–NaI
RT
20 min
98
[18]

3
P₂I₄, CS₂
RT
30 min
89
[19]

4
I₂
Refluxing petroleum ether
1.5 h
56
[20]

5
NaI/BF₃Et₂O
RT
25 min
94
[21]

6
CeCl₃7H₂O/NaI
Refluxing acetonitrile
20 h
90
[22]

1
NaI, mont. KSF
MW
5 min
55
[25]

2
KI/H₂SO₄, kaolin clay
MW
5 min
92
[26]

9
CsI/MeSO₃H
RT
15 min
95
[27]

10
H₄SiW₁₂O₄₀/KI
RT
30 min
96
This work

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.

3. Experimental

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).

Acknowledgment

The authors are grateful to Islamic Azad University of Rasht Branch for financial assistance of this work.

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