The following summary may interest people who want to experiment with the conversion of styrenes to nitrostyrenes[13], the oxidation of alcohols to carbonyl compounds [14, 16] or other reactions supported on K10 clay [15]. Check out ref. 16 for the conversion of phenyl acetic acids to benzaldehydes or conversion of phenethylamine to phenylacetaldehyde. -------------------------------------------------------------------------- Reactions assisted by clays and other lamellar solids - a survey. [1] ACIDITY OF CLAYS AND TYPES OF CLAY CATALYSTS The vast majority of clay catalysts used by organic chemists are based on the naturally occurring smectite clay, montmorillonite (also known as bentonite). This clay has an aluminosilicate structure which can be compared to a multi-decker ham sandwich. The bread layers represent the extended aluminosilicate sheets where two external tetrahedral silica groups surround internal octahedral alumina groups in a tetrahedral octahedral tetrahedral ('TOT') structure. The ham represents an interlamellar water layer containing dissolved cations. The montmorillonites are a group of TOT-type clays in which isomorphous substitution of some of the octahedral aluminium(III) atoms by magnesium(II) or iron(II) atoms has taken place, with the result that the sheet retains a residual negative charge. In the naturally occurring form, this charge is balanced by the introduction into the water layer of interlamellar cations such as Na+ or Ca2+, some cations also occupying broken edge sites. Different deposits of montmorillonites can be found in which there are between 25 and 125 mequiv. of exchangeable cation per 100 g of clay. Such smectite clays have the added property of swelling in the presence of either water or a host of organic molecules, when the interlamellar distance between the sheets increases to accommodate the guest molecules. Natural montmorillonite clays have almost no catalytic activity, but it is relatively easy to convert them into useful catalysts by either (a) acid treatment or (b) cation exchange with polyvalent ions such as AI3+ or Cr3+. These cations can polarise their coordinated water molecules to yield protons in the interlamellar zone (equation 1) [Al(OH2)6]3+ --> [Al(OH2)5(OH)]2+ + H+ (equation 1) However, even mild acid ion-exchange of the natural clays can cause partial leaching of aluminium from the octahedral layer, resulting in de-lamination of the aluminosilicate sheets, providing a less crystalline aluminium-exchanged clay. Treatment of natural montmorillonites with strong mineral acids causes considerable de-lamination of the structure, producing an increase in surface area, particularly at the sheet edges, as well as adsorption of quantities of acid onto both external and internal surfaces. The tetrahedrally substituted smectite clay, beidellite, can also be used to prepare acid catalysts with high acidity by cation exchange. Clay catalysts have been shown to contain both Bronsted and Lewis acid sites, with the Bronsted sites mainly associated with the interlamellar region and the Lewis sites mainly associated with edge sites. The acidity of the ion-exchanged clays is very much influenced by the quantity of water between the sheets. If the clay is heated (to around 100 C) so as to remove most of the interlamellar water until only 'one layer' of water remains, at about 5% total water level, the Bronsted acidity increases markedly to that of a very strong acid indeed. Heating to a higher temperature (at around 200-300°C) results in collapse of the clay interlayer structure as the water is driven out, resulting in a decrease in Bronsted acidity but an increase in Lewis acidity. Further heating (to around 450°C and above) results eventually in complete dehydroxylation of the aluminosilicate lattice, producing a completely amorphous solid that retains Lewis acidity. The swelling properties of montmorillonite and beidellites can be employed to manufacture 'pillared clays'. Large polyatomic inorganic ions can be intercalated into the swelled clay and then modified by heat treatment, so as to produce inorganic pillars that hold the roof up at a larger than normal interlamellar distance. Pillared clays can be prepared that have pore sizes larger than most zeolites, as well as having increased thermostability over normal layered clays, and much activity has been concentrated in incorporating transition metal elements into pillared clays to provide useful inorganic catalysts. However, organic chemists with synthesis in mind have so far confined their interest to the swellable montmorillonite clays, and almost all of their clay catalysts have been either (a) acid-treated clays such as K-l0, or ion-exchanged clays such as Al3+, Cr3+ or H+ exchanged Wyoming or Texas bentonites. Further details of clay structures are given in Chapter 1. The acid-treated and cation exchanged clays can be simply regarded as solid acids and act as heterogeneous catalysts, with all of the advantages of easy removal of the catalyst from the products. Acid-treated clays, because of their increased surface area and swelling properties, have also been widely used as solid supports for inorganic reagents such as potassium permanganate, thallium(III) nitrate and both copper(II) and iron(III) nitrates. The ion-exchanged clays have mostly Bronsted acidity in the interlamellar zone and so are characterized by promoting acid-catalysed reactions often of a bimolecular type between protonated and neighbouring unprotonated reactants. PREPARATION OF CLAY CATALYSTS Organic chemists can purchase ready-made acid-treated montmorillonite catalysts from a variety of commercial sources. Sud-Chemie AG, of Munich, West Germany, produce K-10, KSF and Tonsil acid-treated clays, of which K-10 can be obtained in Europe from Fluka AG and in America from Aldrich Chemical Co., and this easy availability has meant that organic chemists have made most use of this material. Additionally, Filtrol clays are manufactured by the Filtrol Corporation in Los Angeles, USA, and a series of clays (Fulmont, Fulbent and Fulcat) are produced by Laporte Industries, Widnes, England. Ion-exchanged clays, e.g. Al3+ exchanged montmorillonite, are simply prepared by suspending the clay in water containing low concentrations (ca. 0.5 M) of salts of the cation to be introduced. The clay is then washed by decantation or centrifugation to get rid of most of the exchanged sodium cations and then dialysed until the supernatant is free of sodium ion. Pillared clays have an important role to play as industrial catalysts, but to date have not been used much by practising organic chemists for synthesis purposes and hence their preparation will not be detailed here. Clay-supported reagents have mainly been prepared by depositing inorganic materials on clay supports. Potassium permanganate/montmorillonite is prepared by grinding potassium permanganate and montmorillonite together as dry powders in approximately equal quantities. Reactions are carried out by refluxing the resultant solid in dichloromethane with the appropriate substrate. Thallium(III) nitrate/K-10 (TTN/K-10) is prepared by stirring K-10 montmorillonite with a solution of thallium(III) nitrate in a mixture of methanol and trimethyl orthoformate, then evaporating to dryness. The resultant free-flowing solid catalyst is stable in a well-stoppered bottle for months. Claycop [copper(II) nitrate/K-10] and Clayfen [iron(III) nitrate/K-10] are prepared by adding K-10 montmorillonite to the acetone solvate of the inorganic nitrates and removing the solvent under vacuum. -------------------------------------------------------------------------- Clay-Supported Copper(II) and Iron(III) Nitrates [ref. 2] Choice of an Acidic Clay as Support Supported reagents have the following assets of which points 4-6 were the most immediately important for our purposes: 1. restricted diffusion of the reaction partners 2. microenvironments of differing polarity, and with acidic or basic sites; 3. enzyme-like pockets to bind the substrate; 4. activation or stabilisation of the reagents; 5. promotion of selective modes of reaction; 6. ease of work-up by immobilisation of by-products, or of toxic chemicals. K10 clay, an inexpensive acidic industrial catalyst [K10 clay is made by Sud-Chemie (Munchen); montmorillonite K10 is also available from Fluka], was chosen as the support after a comparison with other clays (natural or industrial), sand, silica gel, titanium dioxide, acidic alumina, and zeolites. These other possible inorganic supports either deactivate the reagent or present theological properties after impregnation incompatible with an easy experimental set-up and work-up. [3, 4] A considerable amount of data dealing with the interaction of layer silicates with organic molecules, and with clay-activated organic reactions, has accumulated recently. A general and quite exhaustive introduction to the subject was published in 1974.[5] Updated data are available from recent reviews[6, 7, 8], or from individual papers. Unfortunately, this new chapter of organic chemistry has yet to be indexed in a standardised form: it is thus advisable to scan geology and soil science literature, in addition to chemical publications, when seeking bibliographic information. The Reagents Preparation of Clay-Supported Iron(III) Nitrate (Clayfen) Whereas these procedures worked out very safely in our hands, nitrates are potentially dangerous compounds, and appropriate caution is to be applied in each step. In particular, we urge to avoid confinement conditions, and we recommend to proceed to scaling-up only after appropriate safety tests. Clay-Supported Iron(III) Nitrate (Clayfen) [4, 9] Iron(III) nitrate nonahydrate (22.5 g) is added to acetone (375 mL) in a 1 litre, pear-shaped evaporating flask. The mixture is stirred vigorously for ~ 5 min until complete dissolution of the crystals of hydrated iron(III) nitrate. The first-formed homogenous rusty brown solution turns after a short time into a muddy, light brown suspension. K 10 clay (30 g) is added in small amounts and stirring is continued for another 5 min. The solvent is then removed from the resulting suspension under reduced pressure (rotary evaporator) on a water bath at 50 C. After 30 min. the dry solid crust adhering to the walls of the flask is flaked off and crushed with a spatula, and rotary evaporator drying is resumed for another 30 min. This procedure yields K10 clay-supported iron(III) nitrate (Clayfen) as a yellow, floury powder; yield: ~ 50 g. Warnings: 1. Prolonged heating, or use of a bath temperature > 50 C may yield an unstable reagent, which decomposes in a vigorous exothermic reaction, with emission of a large amount of nitrogen dioxide fumes. This decomposition generally takes place within 15 min after the end of an incorrect preparation of Clayfen. 2. Preparation of Clayfen includes flaking off of the dry solid crust formed in the evaporator. The physical state of the reagent at this step is crucial: if this removal is performed while the solid is still a little muddy, it will aggregate in spheres that remain wet inside. and the resulting reagent will display little or no activity, or will decompose as described above. This difficulty will be best avoided by strict observance of the experimental conditions described (quantities, flask volume and shape). 3. Clayfen, spontaneously, slowly loses nitrous fumes, and should never he stored in a closed flask. Stability of Clay-Supported Iron(III) Nitrate (Clayfen) Left in air contact at room temperature, Clayfen retains its activity for only a few hours. It may be stored for a few days and retains its activity when it is covered with n-pentane immediately after preparation; the hydrocarbon is evaporated prior to use. It is a good practice to use only freshly prepared reagent: it loses about 40 % of its reactivity upon standing exposed to air for 4 h, or under n-pentane for 24 h. The thermal stability of Clayfen was investigated by differential calorimetric analysis. Above 59°C, it decomposes with an enthalpy release of ~ 20 cal/g. This is a first order process, with calculated half-lives of 53 min at 69 C (in boiling n-hexane, for instance) or of 14 min at 80 C (the temperature of boiling benzene). In our experience the latter is the upper temperature limit for the practical use of Clayfen. Clay-Supported Copper(II)Nitrate (Claycop) This reagent, also based on a salt forming covalent bidentate complexes upon dehydrations [10,11], shows no loss of reactivity in the applications described hereafter, even after standing in an open powder box for one month. In our hands, it never underwent any spontaneous decomposition. Clay-supported Copper(II) Nitrate (Claycop) [12] Clay-supported copper(II) nitrate (Claycop) is prepared in a process similar to the preparation of Clayfen, by adding K10 clay (30 g) to a solution of copper(II) nitrate trihydrate (20 g) in acetone (375 ml). The resulting suspension is placed in a rotary vacuum evaporator and the solvent is eliminated under reduced pressure (water jet aspirator) on a water bath at 50 C. After 30 min. the dry solid crust adhering to the walls of the flask is flaked off with a spatula, and vaporation is resumed for another 30 min in the same conditions, giving Claycop as a light blue, free-flowing powder; yield: ~ 50 g. For examples of applications of Clayfen, Claycop and K10 see references 13,14,15. References: 1. J A Ballantine in Chp 4. p100-103. In "Bologh, Laszlo, Organic Chemistry Using Clays", 1993, Springer-Verlag. 2. From "Clay-Supported Copper(II) and Iron(III) Nitrates: Novel Multi-Purpose Reagents for Organic Synthesis. A Cornelis, P Laszlo. Synthesis 1985, 909-918. 3. Cornelis, Laszlo & Pennetreau, Clay Miner 1983, 18, 437; CA 1984, 100, 102362. 4. Cornelis, Laszlo & Pennetreau, Bull. Soc. Chim. Belg. 1984, 93, 961 5. B. Theng, The Chemistry of Clay-Organic Reactions, Wiley NY, 1974. 6. McKillop & Young, Synthesis 1979, 401, 481. 7. Theng, Dev. Sedimentol. 1982, 35, 197; CA 1982, 97, 197524; 8. Lagaly, Phil. Trans. R. Soc. Lond. A 1984, 311, 315 9. Cornelis & Laszlo, Synthesis 1980, 849. 10. Addison. Prog. Inorg. Chem. 1967, 8, 195. 11. Addison, Logan, Wallwork, Garner. Q. Rev. Chem. Soc. 1971, 25, 289. 12. Bologh, Hermecz, Meszaros, Laszlo. Helv. Chim. Acta 1984, 67, 2270. 13. Selective Nitration of Styrenes with Clayfen and Clayan: A Solvent-free Synthesis of beta-Nitrostyrenes. Rajender S. Varma, Kannan P. Naicker and Per J. Liesen. Tetrahedron Letters 39 (1998) 3977-3980. 14. Varma, R.S.; Dahiya, R. Tetrahedron Lett. 1997,38. 2043; 15. Sodium Borohydride on Wet Clay: Solvent-free Reductive Amination of Carbonyl Compounds Using Microwaves. Rajender S. Varma and Rajender Dahiya. Tetrahedron 54, 6293-8 (1998) 16. Varma & Dahiya. Tetr. Lett. 39 (1998), 1307-8. --------------------------------------------------------------------------