Physical Data: the surface acidity of dry K10 corresponds to a Hammett acidity function H0 = -6 to -8.13
Form Supplied in: yellowish-grey dusty powder. Forms with water a mud that is difficult to filter, more easily separated by centrifugation; with most organic solvents, forms a well-settling, easy-to-filter suspension.6
Handling, Storage, and Precautions: avoid breathing dust; keep in closed containers sheltered from exposure to volatile compounds and moisture.
Montmorillonite clays are layered silicates and are among the numerous inorganic supports for reagents used in organic synthesis.1,2 The interlayer cations are exchangeable, thus allowing alteration of the acidic nature of the material by simple ion-exchange procedures.3,4 Presently, in fine organic synthesis, the most frequently used montmorillonite is K10, an acidic catalyst, manufactured by alteration of montmorillonite (by calcination and washing with mineral acid; this is probably a proprietary process).
The first part of this article specifically deals with representative laboratory applications to fine chemistry of clearly identified, unaltered K10, excluding its modified forms (cation-exchanged, doped by salt deposition, pillared, etc.) and industrial uses in bulk. This illustrative medley shows the prowess of K10 as a strong Brønsted acidic catalyst. The second part deals with cation-exchanged (mainly FeIII) montmorillonite. Clayfen and claycop, versatile stoichiometric reagents obtained by metal nitrate deposition on K10,5 are used in oxidation and nitration reactions. They are treated under Iron(III) Nitrate-K10 Montmorillonite Clay and Copper(II) Nitrate-K10 Bentonite Clay.
K10 is often confused, both in name and in use, with other clay-based acidic catalysts (KSF, K10F, Girdler catalyst,
acid treated or
H+-exchanged montmorillonite or clay, etc.) that can be effectively interchanged for K10 in some applications. Between the 1930s and the 1960s, such acid-treated montmorillonites were common industrial catalysts, especially in petroleum processing, but have now been superseded by zeolites.
K10 clay may be used crude, or after simple thermal activation. Its acidic properties are boosted by cation exchange (i.e. by iron(III)7 or zinc(II)8) or by deposition of Lewis acids, such as zinc(II)9,10 or iron(III)11 chloride (i.e.
clayfec). In addition, K10 is a support of choice for reacting salts, for example nitrates of thallium(III),12 iron(III) (
clayfen),5 or copper(II) (
claycop).5 Multifarious modifications (with a commensurate number of brand names) result in a surprisingly wide range of applications; coupled with the frequent imprecise identification of the clay (K10 or one of its possible substitutes mentioned above), they turn K10 into a Proteus impossible to grab and to trace exhaustively in the literature.
Trimethyl orthoformate (see Triethyl Orthoformate) impregnated on K10 affords easy preparation of dimethyl acetals,14 complete within a few minutes at room temperature in inert solvents such as carbon tetrachloride or hexane (eq 1). The recovered clay can be reused.
Cyclic diacetals of glutaraldehyde are prepared in fair yields by K10-promoted reaction of 2-ethoxy-2,3-dihydro-4H-pyran with diols, under benzene azeotropic dehydration (eq 2).15
Diastereoisomeric acetal formation catalyzed by K10 has been applied to the resolution of racemic ketones, with diethyl (+)-(R,R)-tartrate as an optically active vicinal diol.16
1,3-Dioxolanes are also prepared by K10-catalyzed reaction of 1-chloro-2,3-epoxypropane (Epichlorohydrin) with aldehydes or ketones, in carbon tetrachloride at reflux (eq 3).17 In the reaction of acetone with the epichlorohydrin, the efficiency of catalysts varies in the order: K10 (70%) > Tin(IV) Chloride (65%) > Boron Trifluoride (60%) = Hydrochloric Acid (60%) > Phosphorus(V) Oxide (57%).
Ketones and amines form enamines in the presence of K10 at reflux in benzene or toluene, with azeotropic elimination of water (eq 4). Typical reactions are over within 3-4 h. With cyclohexanone, the efficiency depends on the nature of the secondary amine: Pyrrolidine (75%) > Morpholine (71%) > Piperidine (55%) > Dibutylamine (34%).18 Acetophenone requires longer heating.19
The K10-catalyzed reaction of aniline with b-keto esters gives enamines chemoselectively, avoiding the competing formation of anilide observed with other acidic catalysts.20,21
K10 catalyzes the ene reaction of diethyl oxomalonate and methyl-substituted alkenes at a rather low temperature for this reaction (80 °C), followed by lactonization (eq 5).22 When alkene isomerization precedes the ene step, it results in a mixture of lactones. Using kaolinite instead of K10 stops the reaction at the ene intermediate, before lactonization.
Using a Dean-Stark water separator, K10 catalyzes formation of alkyl- and arylthioalkenes from cyclic ketones and thiols or thiophenols, in refluxing toluene (eq 6). A similar catalysis is effected by KSF (in a faster reaction) and K10F.23 The isomer distribution is under thermodynamic control.
Alcohols react regioselectively with 1-chloro-2,3-epoxypropane to form 1-alkoxy-2-hydroxy-3-chloropropanes. The K10-catalyzed process is carried out in refluxing carbon tetrachloride for 2.5 h (eq 7).24 Yields are similar to those obtained by Sulfuric Acid catalysis.
K10-catalyzed reaction of diethyl acetals with Ethyl Vinyl Ether leads to 1,1,3-trialkoxyalkanes. Hydrolysis turns these into trans-a,b-unsaturated aldehydes.25 The reaction is performed close to ambient temperatures (eq 8). K10 is superior to previously reported catalysts, such as Boron Trifluoride or Iron(III) Chloride. The addition is almost instantaneous and needs no solvent. Cyclohexanone diethyl acetal gives an analogous reaction.
With an excess of 3,4-Dihydro-2H-pyran, in the presence of K10 at room temperature, alcohols are transformed quantitatively into their tetrahydropyranyl derivatives. Run in dichloromethane at room temperature, the reaction is complete within 5-30 min (eq 9). The procedure is applicable to primary, secondary, tertiary, and polyfunctional alcohols as well as to phenols.26
1-Chloro-1-methylcyclohexane, the formal Markovnikov adduct of hydrochloric acid and 1-methylcyclohexene, becomes largely predominant when Sulfuryl Chloride is the chlorine source and K10 the solid acid.27 The reaction at 0 °C, in dry methylene chloride, is complete within 2 h (eq 10).
Meso-tetraalkylporphyrins are formed in good yields from condensation of aliphatic aldehydes with pyrrole; thermally activated K10 catalyzes the polymerization-cyclization to porphyrinogen, followed by p-Chloranil oxidation (eq 11).28
Meso-tetraarylporphyrins, with four identical or with tuneable ratios of different aryl substituents, are made by taking advantage of modified K10 (
clayfen or FeIII-exchanged) properties.29,30
The acid strength of some cation-exchanged montmorillonites is between Methanesulfonic Acid (a strong acid) and Trifluoromethanesulfonic Acid (a superacid) and, in some instances, their catalytic activity is greater than that of a superacid.31 Iron montmorillonite is prepared by mixing the clay with various FeIII compounds in water.8,32 The resulting material is filtered and dehydrated to afford the active solid-acid catalyst. These solid-acid catalysts are relatively inexpensive and are generally used in very small quantities to catalyze a wide variety of reactions, including Friedel-Crafts alkylation and acylation, Diels-Alder reactions, and aldol condensations.1,5
Stereoselective Diels-Alder reactions involving an oxygen-containing dienophile are accelerated in the presence of FeIII-doped montmorillonite in organic solvents (eq 12).34 Furans also undergo Diels-Alder reactions with Acrolein and Methyl Vinyl Ketone in CH2Cl2 to give the corresponding cycloadducts in moderate yield (eq 13).35 The iron-doped clay also catalyzes the radical ion-initiated self-Diels-Alder cycloaddition of unactivated dienophiles such as 1,3-cyclohexadiene and 2,4-dimethyl-1,3-pentadiene (eq 14).36
The role of FeIII-impregnated montmorillonite, and other cation-exchanged montmorillonites, in asymmetric Diels-Alder reactions was found to be limited to the use of small chiral auxiliaries; the results obtained from these reactions are similar to those of homogeneous aluminum catalysts (eq 15).33
The Friedel-Crafts acylation of aromatic substrates with various acyclic carboxylic acids in the presence of cation-exchanged (H+, Al3+, Ni2+, Zr2+, Ce3+, Cu2+, La3+) montmorillonites has been reported.39 Curiously, the use of iron-doped montmorillonite was not included in the report; however, some catalysis is expected. Under these conditions, the yield of the desired ketones was found to be dependent on acid chain length and the nature of the interlayer cation.
The direct arylation of a saturated hydrocarbon, namely adamantane, in benzene using FeCl3-impregnated K10 was recently reported.11 Additionally, Friedel-Crafts chlorination of adamantane in CCl4 using the same catalyst was also reported. The alkylation of aromatic substrates with halides under clay catalysis gave much higher yields than conventional Friedel-Crafts reactions employing Titanium(IV) Chloride or Aluminum Chloride as catalyst.8 Higher levels of dialkylation were observed in some cases. The alkylation of aromatic compounds with alcohols and alkenes was also found to be catalyzed with very low levels of cation-exchanged montmorillonites, as compared to standard Lewis acid catalysis; however, iron-doped clays performed poorly compared to other metal-doped clays.
Cation-exchanged montmorillonites accelerate the aldol condensation of silyl enol ethers with acetals and aldehydes.40 Similarly, the aldol reaction of silyl ketene acetals with electrophiles is catalyzed by solid-acid catalysts. Neither report discussed the use of iron montmorillonite for these reactions; however, some reactivity is anticipated.
The coupling of silyl ketene acetals (enolsilanes) with pyridine derivatives bearing an electron-withdrawing substituent, namely cyano, in the meta position is catalyzed by iron montmorillonite and other similar solid-acid catalysts (eq 16).41
The resulting N-silyldihydropyridines easily undergo desilylation by treatment with Cerium(IV) Ammonium Nitrate to afford the desired dihydropyridine derivative. The reactivity was found to be dependent on the montmorillonite counterion and to follow the order: Fe3+ > Co2+ > Cu2+ &AApprox; Zn2+ > Al3+ &AApprox; Ni2+ &AApprox; Sn4+.
André Cornélis & Pierre Laszlo
Université de Liège, Belgium
Mark W. Zettler
The Dow Chemical Company, Midland, MI, USA