Characterization of Natural Brazilian Clays and their Utilization as Catalysts in the Coiodination of Alkenes with Water and Alcohols

As argilas naturais brasileiras F-101, F-117 e caulim foram caracterizadas através de análises química e textural, difração de raios X e RMN no estado sólido de Al e Si. A argila F-101 é uma esmectita, a F-117 é uma estrutura interestratificada contendo esmectita, caulinita e mica como argilominerais. O caulim é essencialmente uma caulinita. As argilas F-101 e F-117 são catalisadores mais eficientes para a coiodação de estireno e ciclo-hexeno com água e álcoois para produzir iodoidrinas e -iodo-éteres, respectivamente, comparadas ao caulim e a argila montmorilonita K-10.


Introduction
The successful application of heterogeneous catalysis in organic reaction has been one of the most fruitful areas of green chemistry.Clay-minerals are aluminosilicates with layered structure that present a wide use in organic synthesis 1 and exhibit specific features such as high versatility, gain in yield and / or selectivity, low cost etc. 2 The electrophilic halogenation of alkenes to produce vicinal dihaloalkanes is a well-known reaction in organic chemistry. 3However, when the halogenation of the alkene is carried out in a nucleophilic solvent (water, alcohols, carboxylic acids, nitriles, etc), difunctionalized products (halohydrins, -haloethers, -haloesters, etc) are obtained.This process is termed 'cohalogenation' and provides useful products for diverse synthetic applications. 4sides the large applicability in organic synthesis, the recent discovery of the bioactivity of naturally occurring organohalogen compounds has increased the interest in such compounds. 5Furthermore, organic iodides are widely used as auxiliaries in radiodiagnosys. 6n contrast to other halogens, the simple iodination of an alkene is very slow and hence, the coiodination of alkenes is only effective using alternate sources of eletrophilic iodine. 7An Indian group published the coiodination reactions of styrene and indene mediated by EPZ-10 ® (a commercial clay zinc prepared from montmorillonite and ZnCl 2 ) 8 and recently we published an effective coiodination of some alkenes with water mediated by clay-minerals and obtained the corresponding iodohydrins in good yields. 9erein we communicate our results on the characterization and utilization of natural Brazilian clays (F-101, F-117 and kaolin) as catalysts in coiodination reactions of styrene and cyclohexene with water and alcohols.

General
All chemicals are commercially available (Aldrich) and were used without further purification.K-10 was purchased from Aldrich, F-101 and F-117 clays (crushed) were donated by "Bentonit do Brasil" (Brazilian Co.) and kaolin was donated by CETEM (Rio de Janeiro, Brazil).

Characterization of the clays
The chemical compositions of the clays were determined by X-ray fluorescence using a Philips 1480 with Rh tub.The X-ray diffractions (XRD) of oriented samples were conducted using a Bruker AXS D5005 diffractometer equipped with a proportional counter and pulse height analyzers Ni filtered by CuK radiation (40 kV/35 mA), diffraction patterns collected at a scanning rate of 0.05º (2 ) min -1 in the interval of 3º to 50º (for F-101, F-117 and K-10 clays) or by CoK radiation (35 kV/40 mA), diffraction patterns collected at a scanning rate of 0.02º (2 ) min -1 in the interval of 5º to 80º (for kaolin).Textural analysis of clays were performed from the nitrogen adsorption isotherms on an ASAP 2000 at 333 K (to avoid collapse in the framework of the clay-mineral) using the multipoint BET method.Solid state NMR measurements (MAS-NMR) were carried out on a Bruker DRX-300 spectrometer operating at a B o of 7.05 Tesla, corresponding to a Larmor frequency of 78.2 MHz for aluminium and 59.6 MHz for silicon.Samples were spun in zirconia rotors equipped with Kel-F caps, at speed of 12 kHz.A short pulse of 7 ms was used and the recycle time was 0.3 s, for 27 Al, while the 29 Si pulse was of 10 s and its recycle time was 2.0 s.The 27 Al chemical shifts were referenced using a solid sample of AlCl 3 .6H 2 O ( = 0 ppm), while the 29 Si signal positions were referenced using a secondary standard of kaolin ( = -91.5 ppm) in relation to TMS.

Characterization of reaction products
HRGC (high-resolution gas chromatography) analysis were performed on a HP-5890-II gas chromatograph with FID by using a 30 m (length), 0.25 mm (ID) and 25 m (phase thickness) RTX-5 silica capillary column and He (flow rate 50 cm/s) as carrier gas (split 1:20).Mass spectra (MS) were obtained on a Hewlett-Packard HP5896-A HRGC-MS using electron impact (70 eV). 1 H 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 TMS as internal standard.IR spectra were recorded on a Perkin Elmer 1600 FT-IR or on a Nicolet 740 FT-IR spectrometers (NaCl film).

Typical procedure for cohalogenation reactions of styrene and cyclohexene
To a stirred suspension of the alkene (5 mmol) and the clay (0.2 g) in dioxane (15 cm 3 ) / water (2 cm 3 ) or in the appropriated alcohol (15 cm 3 ) was added iodine (10 mmol) in small portions at room temperature (25 -30 o C).After 24 h, the clay was filtered off, Et 2 O (10 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 ).The organic extract was dried (anhydrous Na 2 SO 4 ) and filtered through 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 product as a colorless or light orange oil.The analytical data for the products are in accordance with those previously reported. 10

Characterization of the clays
Figure 1 shows the XRD patterns of F-101, F-117 and K-10 clays.The F-101 clay shows a strong reflection at 15.23 Å (2 = 5.78º), which indicates the presence of smectite. 11In addition, this sample shows a peak at 3.30 Å (2 = 27.05º)corresponding to -quartz. 12On the other hand, the XRD patterns of the natural clay F-117 show several peaks: 8.76 Å (2 = 10.15º) and 6.46 Å (2 = 13.69º)suggesting the presence of mica and kaolinite respectively and 3.18 Å (2 = 28.06º)indicating the presence of -quartz.In addition, the broad weak peak corresponding to smectite in F-117 clay suggests that this clay mineral is not the main component and its structure is disarranged. 13ese results indicate that F-117 clay is an interstratified clay and F-101 clay presents a sole kind of clay mineral (smectite).K-10 showed a very weak and broad reflection d(001) in 2 = 6.0° (d = 14.7 Å), suggesting the presence of clay mineral of smectite group.The reflections in 2 = 9.00 o (d= 10,0 Å) and 2 = 18.0° (d = 4,92 Å) suggest the presence of a clay-mineral of the mica group.Claver et al. identified K-10 as an acid treated montmorillonite with a partially destroyed structure. 14According to Cseri et al., K-10 was activated with mineral acid at high temperature and this treatment results in ion exchange, dealumination, and extraction of iron and manganese. 15As seen in Figure 2, the XRD patterns of kaolin indicates that kaolinite is the only clay-mineral component of the clay. 16able 1 shows the results of the chemical analysis of the clays.The results of chemical analysis showed that the amount of K ion in F-117 clay (2.83) is greater than in F-101 clay, suggesting the existence of illite in the former. 17s observed, both F-101 and F-117 clays show iron contents around 10% and Mg cation presented in both clays in the octahedral layer.This conclusion was based on a previous report of Guarino, in which the Mg cation amount was not altered in the homoionic clays studied. 11n the other hand, the low content of elements different from Si, Al and Fe in kaolin suggests it is a 1:1 clay, as other kaolinitic clays. 18he results of textural properties of the smectites are summarized in Table 2.Both F-101 and F-117 clays exhibit analogous hysteresis, characteristic of mesoporous solids. 11lthough the pore diameter were similar too, the K-10 clay presents greater surface area than the other clays.
Figure 3 and Table 3 show the 27 Al MAS-NMR spectra of clays.The 27 Al NMR spectrum of F-101 clay shows a broad peak at 56.4 ppm due to tetrahedral Al ions and another one at -5.1 ppm which can be attributed to octahedral Al sites.In a previous paper, it was observed that tetrahedral Al sites were not present in the structure of this clay 12 as it was expected to be from the composition of feldspar, which was not detected by XRD as it presents low crystallinity. 11On the other hand, the F-117 clay presents a peak at 61.4 ppm attributed to tetrahedral Al sites.All Al is hexacoordinated (structural) in kaolin and the percentage of Al IV in acid commercial K-10 (18.4%) is greater than in the others Brazilian natural clays, probably due to the acid-high temperature treatment of this clay. 15oreover, the Al VI is smaller in K-10 due to its disarranged framework.
Figure 4 shows the 29 Si MAS-NMR spectra of the clays.The region of Q 3 (nAl) in the spectra of F-101 and F-117 clays, detected between -70 and -93 ppm, presents a peak that indicates the presence of Si of the clay framework.compounds or iodohydrin in the case of the reaction with the bulk t-butanol.
The natural Brazilian clays F-101 and F-117 showed better yields, while K-10 clay was intermediate and kaolin was the worst.As kaolin presents an invariable basal distance and K-10 clay has its framework disarranged by previous acid-treatment, 13 our results support the need of a well-defined variable basal distance, which is present in F-101 and F-117 clays.Besides, the iron contents of F-101 and F-117 clays are greater than the other ones and Fe 3+ is known to catalyze coiodination reactions. 7,19On the other hand, the same reactions in the absence of any catalyst led to the same products in considerably lower yields.

Conclusions
The characterization of natural Brazilian clays showed that F-101 clay is constituted of only one clay-mineral (smectite), while F-117 clay is an interstratified clay.The use of these clays as catalysts in coiodination reactions present great advantages over traditional methods, 7,8 such as inexpensive material, mild reactions conditions and minimal environmental impact.Considering the environmental issues that require the substitution of toxic catalysts by more friendly solid catalysts and the advantages of the heterogeneous catalysis in terms of easy work up procedures, our methodology agrees with the green chemistry approach. 20he signal attributed to -quartz and amorphous silica, Q 4 (0Al), observed in the range from -106 to -130 ppm, is also presented in both natural Brazilian clays.K-10 clay shows a Q 4 (0Al) peak more defined than the matural clays, suggesting it has more amorphous silica than the others.As expected, kaolin exhibits a signal at -91.3 ppm related to structural Si, not been detected quartz nor amorphous silica.

Utilization of clays in coiodination reaction of alkenes
The results of the reactions of coiodination of styrene and cyclohexene with water and alcohols in the presence of clays are summarized in Scheme 1 and Table 4.These reactions were carried out at room temperature by stirring together the alkene, iodine and the clay in the appropriated solvent (aqueous dioxane or alcohols).The reaction products were analyzed by HRGC and characterized by spectroscopic methods.It was observed a sole product in each reaction (iodohydrin or -iodoether) with no detectable amount of any other products, such as diiodo

Table 2 .
Textural parameters of clays

Table 1 .
Chemical analysis of clays

Table 4 .
Yields of products according to Scheme 1