An Environmentally Friendly Preparation of Iodohydrins from Alkenes Mediated by Clay-Minerals

Iodoidrinas são intermediários úteis em síntese orgânica, por exemplo, na obtenção de epóxidos. Aqui é descrita a preparação de iodoidrinas com bons rendimentos, a partir da reação de alquenos (ciclo-hexeno, estireno, α-metil-estireno e 1-octeno) com iodo e água, à temperatura ambiente, em presença de argilas ácidas comerciais (K-10 e KSF) e argilas naturais brasileiras (F-101 e F-117), sendo os melhores resultados obtidos com as argilas naturais brasileiras (61-90%). As análises de DRX das argilas confirmaram tratarem-se de argilominerais do grupo das esmectitas, havendo outros minerais de argila presentes na KSF, K-10 e F-117.


Introduction
Chloro-and bromohydrins can be prepared in good yields when the halogenation of the alkene is carried out in aqueous media. 1 On the other hand, the preparation of iodohydrins (an alternate and efficient epoxide precursor 2 ) from the reaction of alkenes with I 2 /H 2 O is difficult due to the reversibility of the addition of iodine to the double bond, and an iodide ion scavenger (a metal salt or an oxidizing reagent) is needed to obtain satisfactory yields. 3ther methodologies include reaction of alkenes with HIO 4 .2H 2 O/NaHSO 3 , 3 N-iodoimides, 3 N-iodosaccharin, 3 bis(piridine)iodine(I) salts, 3 dimethydioxirane/MeI, 4 and triiodide ion 5 in aqueous media.Recently, Wakharkar and co-workers published the preparation of iodohydrins by the reaction of styrene and indene with I 2 /H 2 O mediated by EPZ-10 ® (a commercial clayzinc prepared from montmorillonite and ZnCl 2 ). 6ay-minerals are silicates of aluminium with layered structure that present a wide use in organic synthesis 7 and exhibit specific features such as high versatility, gain in yield and/or selectivity, low cost etc. 8Among the several groups of clay-minerals, the smectites are the most useful in organic reactions.Their structure is characterized by presence of an octahedral alumina layer between two tetrahedrical silicate layers.
Herein we communicate our initial results on the reaction of representative alkenes (ciclohexene, 1-octene, styrene, and α-methylstyrene) with I 2 /H 2 O in the presence of commonly used commercial acid-treated clays (K-10 or KSF) and natural Brazilian clays (F-101 or F-117).

General
All chemicals were used without further purification.K-10 and KSF were purchased from Aldrich.F-101 and F-117 were donated by "Bentonit do Brasil" (Brazilian Co.).Analysis by HRGC were performed on a HP-5890-II gas chromatograph with FID by using a 30 m (length), 0.25 mm (ID) and 25 µm (phase thickness) RTX-5 silica capillary column and He (flow rate 50 cm s -1 ) as carrier gas (split 1:20).Mass spectra were obtained on a Hewlett-Packard HP5896-A HRGC-MS using electron impact (70 eV). 1 H NMR and 13 C NMR were acquired on a Bruker DRX-300 (300 MHz and 75 MHz, respectively) or on a Bruker AC-200 (200 MHz and 50 MHz, respectively) spectrometers for CDCl 3 solutions with tetramethylsilane as internal standard.IR spectra were recorded on a Perkin Elmer 1600 FT-IR or on a Nicolet 740 FT-IR spectrometers (NaCl film).XRD analysis of oriented fractions were made using a Rigaku-Geigerflex 2013 diffractometer equipped with a proportional counter and pulse height analyser using Ni filtered CuKa radiation, produced under conditions of 40 kV and 30 mA.Diffraction patterns were collected at a scanning rate of 4 o (2θ)/min in the interval of 2° and 30°.

Typical procedure for iodohydrin formation
To a stirred suspension of the appropriate alkene (5 mmol) and the clay (1 g) in 1,4-dioxane (15 cm 3 ) and water (2 cm 3 ), it was added iodine (10 mmol) in small portions at room temperature.After 24 h, the clay was filtered off, CHCl 3 (20 cm 3 ) was added and the organic layer was washed with a saturated solution of Na 2 S 2 O 3 (3 x 5 cm 3 ) and brine (5 cm 3 ).The organic extract was dried (anhydrous Na 2 SO 4 ) and filteres trough a small column of SiO 2 .The solvent was evaporated on a rotatory evaporator and then concentrated under reduced pressure (267 Pa) and 50 o C (bath) to give the pure iodohydrin as a colorless or light orange oil.The analytical data for the iodohydrins are in accordance with those previously reported. 2ans-2-Iodocyclohexanol

Results
The results of the reaction of the alkenes with I 2 in aqueous 1,4-dioxane in the presence of diverse clays are summarized in Scheme 1 and Table 1.