MONTMORILLONITE K10

1

Montmorillonite K10

[1318-93-0]

(catalyzes protection reactions of carbonyl and hydroxy groups;
promotes ene,

22

condensation,

25

and alkene addition

27

reactions)

Physical Data:

the surface acidity of dry K10 corresponds to a

Hammett acidity function H

0

= −6 to −8.

13

Form Supplied in:

yellowish-grey dusty powder. Forms with wa-

ter a mud that is difficult to filter, more easily separated by cen-
trifugation; with most organic solvents, forms a well-settling,
easy-to-filter suspension.

6

Handling, Storage, and Precaution:

avoid breathing dust; keep

in closed containers sheltered from exposure to volatile com-
pounds and moisture.

General. Montmorillonite clays are layered silicates and are

among the numerous inorganic supports for reagents used in or-
ganic 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 cata-
lyst, manufactured by alteration of montmorillonite (by calcina-
tion and washing with mineral acid; this is probably a proprietary
process).

The first part of this article specifically deals with representa-

tive 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 Fe

III

) montmorillonite. Clayfen and claycop,

versatile stoichiometric reagents obtained by metal nitrate deposi-
tion 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but have now been superseded by zeolites.

Activation

6

. K10 clay may be used crude, or after simple ther-

mal 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. ‘clayzic’ and ‘clayfec’).

In addition, K10 is a support of choice for reacting salts, for exam-
ple 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 ap-
plications; 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 ex-
haustively in the literature.

Preparation of Acetals. Trimethyl orthoformate (see Triethyl

Orthoformate) impregnated on K10 affords easy preparation of
dimethyl acetals,

14

complete within a few minutes at room tem-

perature in inert solvents such as carbon tetrachloride or hexane
(eq 1). The recovered clay can be reused.

(1)

O

HC(OMe)

3

, K10

OMe

MeO

5 min, 100%

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

OH

OH

O

OEt

O

O

O

O

(CH

2

)

3

+

(2)

K10

benzene, Dean–Stark

2–3 h

80%

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%).

O

O

Cl

O

O

Cl

K10

+

(3)

CCl

4

reflux, 8 h

70%

Preparation of Enamines. Ketones and amines form enam-

ines 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

Acetophe-

none requires longer heating.

19

K10

(4)

O

N

O

N

O

+

toluene

reflux, 70 h

54%

The K10-catalyzed reaction of aniline with β-keto esters gives

enamines chemoselectively, avoiding the competing formation of
anilide observed with other acidic catalysts.

20,21

Synthesis of γ

γ

γ-Lactones via the Ene Reaction. 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

Avoid Skin Contact with All Reagents

2

MONTMORILLONITE K10

precedes the ene step, it results in a mixture of lactones. Using
kaolinite instead of K10 stops the reaction at the ene intermedi-
ate, before lactonization.

O

CO

2

Et

CO

2

Et

O

CO

2

Et

OH

O

O

CO

2

Et

OH

O

(5)

+

+

44:49

78%

50:50 mixture

of diastereoisomers

K10

80 °C
72 h

Synthesis of Enol Thioethers. Using a Dean–Stark water sep-

arator, 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.

(6)

O

S

SH

+

K10

toluene

reflux, 6 h

81%

Preparation of Monoethers of 3-Chloro-1,2-propanediol.

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.

(7)

OH

O

Cl

O

Cl

OH

+

K10

CCl

4

reflux, 2.5 h

60%

α

α

α,β

β

β-Unsaturated Aldehydes via Condensation of Acetals with

Vinyl Ethers.

K10-catalyzed reaction of diethyl acetals with

Ethyl Vinyl Ether leads to 1,1,3-trialkoxyalkanes. Hydrolysis
turns these into trans-α,β-unsaturated aldehydes.

25

The reaction

is performed close to ambient temperatures (eq 8). K10 is supe-
rior 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 analo-
gous reaction.

OEt

OEt

OEt

OEt

OEt OEt

+

(8)

O

H

+

K10

rt, 80%

83%

Protective Tetrahydropyranylation of Alcohols and Phenols.

With an excess of 3,4-Dihydro-2 H-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

OH

O

O

O

K10

(9)

+

CH

2

Cl

2

rt, 30 min

83%

Markovnikov Addition of Hydrochloric Acid to Alkenes. 1-

Chloro-1-methylcyclohexane, the formal Markovnikov adduct of
hydrochloric acid and 1-methylcyclohexene, becomes largely pre-
dominant 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).

Cl

K10

SO

2

Cl

2

Cl

(10)

+

1,1:1,2 = 91:9

CH

2

Cl

2

0 °C, 2 h

98%

Porphyrin Synthesis. Meso-tetraalkylporphyrins are formed

in good yields from condensation of aliphatic aldehy-
des with pyrrole; thermally activated K10 catalyzes the
polymerization–cyclization to porphyrinogen, followed by p-
Chloranil oxidation (eq 11).

28

O

N
H

N

N

N

N

C

5

H

11

C

5

H

11

C

5

H

11

C

5

H

11

1. K10
CH

2

Cl

2

(11)

+

2. p-chloranil
46%

Meso

-tetraarylporphyrins, with four identical or with tuneable

ratios of different aryl substituents, are made by taking advantage
of modified K10 (‘clayfen’ or Fe

III

-exchanged) properties.

29,30

Iron(III)-Doped Montmorillonite.

General Considerations. 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 Fe

III

compounds in water.

8,32

The resulting ma-

terial is filtered and dehydrated to afford the active solid-acid cat-
alyst. 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

Diels–Alder Reactions.

33

Stereoselective Diels–Alder reac-

tions involving an oxygen-containing dienophile are acceler-

A list of General Abbreviations appears on the front Endpapers

MONTMORILLONITE K10

3

ated in the presence of Fe

III

-doped montmorillonite in organic

solvents (eq 12).

34

Furans also undergo Diels–Alder reactions

with Acrolein and Methyl Vinyl Ketone in CH

2

Cl

2

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

CHO

K10–Fe

3+

(12)

CHO

+

CH

2

Cl

2

, –24 °C

60%

O

O

O

O

K10–Fe

3+

(13)

+

endo:exo =

13.5:1

CH

2

Cl

2

, –24 °C

98%

K10–Fe

3+

4-t-butylphenol (10%)

(14)

endo:exo = 4:1

CH

2

Cl

2

, 0 °C

The role of Fe

III

-impregnated montmorillonite, and other

cation-exchanged montmorillonites, in asymmetric Diels–Alder
reactions was found to be limited to the use of small chiral aux-
iliaries; the results obtained from these reactions are similar to
those of homogeneous aluminum catalysts (eq 15).

33

O

O

O

O

CO

2

R*

CO

2

R*

CO

2

R*

CO

2

R*

(15)

+

endo:exo = 98:2

+

+

+

(S)

(S)

(S)

(S)

(S)

(R)

(R)

(R)

(R)

(R)

(R)

(S)

39% de

Friedel–Crafts

Acylation

and

Alkylation.

37

,38

The

Friedel–Crafts acylation of aromatic substrates with vari-
ous acyclic carboxylic acids in the presence of cation-exchanged
(H

+

, Al

3+

, Ni

2+

, Zr

2+

, Ce

3+

, Cu

2+

, La

3+

) montmorillonites

has been reported.

39

Curiously, the use of iron-doped montmoril-

lonite 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 FeCl

3

-impregnated K10 was re-

cently reported.

11

Additionally, Friedel–Crafts chlorination of

adamantane in CCl

4

using the same catalyst was also reported. The

alkylation of aromatic substrates with halides under clay catal-
ysis gave much higher yields than conventional Friedel–Crafts
reactions employing Titanium(IV) Chloride or Aluminum Chlo-
ride 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.

Aldol Condensations.

Cation-exchanged montmorillonites

accelerate the aldol condensation of silyl enol ethers with acetals
and aldehydes.

40

Similarly, the aldol reaction of silyl ketene ac-

etals with electrophiles is catalyzed by solid-acid catalysts. Nei-
ther report discussed the use of iron montmorillonite for these
reactions; however, some reactivity is anticipated.

Miscellaneous Reactions. The coupling of silyl ketene ac-

etals (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

N

NC

R

1

O

R

2

OTMS

R

3

N

R

3

R

2

OR

1

NC

(16)

TMS

+

K10–Fe

3+

O

R

1

= Me, Et; R

2

= H, Me; R

3

= H, Me

The resulting N-silyldihydropyridines easily undergo desily-

lation by treatment with Cerium(IV) Ammonium Nitrate to af-
ford the desired dihydropyridine derivative. The reactivity was
found to be dependent on the montmorillonite counterion and to
follow the order: Fe

3+

>

Co

2+

>

Cu

2+

≈

Zn

2+

>

Al

3+

≈

Ni

2+

≈

Sn

4+

.

1.

Cornélis, A.; Laszlo, P., Synlett 1994, 155.

2.

McKillop, A.; Young, D. W., Synthesis 1979, 401.

3.

Theng, B. K. G. The Chemistry of Clay–Organic Reactions; Hilger:
London, 1974.

4.

Thomas, J. M. In Intercalation Chemistry; Whittingham, M. S.;
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5.

Cornélis, A.; Laszlo, P., Synthesis 1985, 909.

6.

Cornélis, A. In Preparative Chemistry Using Supported Reagents;
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7.

Cornélis, A.; Gerstmans, A.; Laszlo, P.; Mathy, A.; Zieba, I., Catal. Lett.
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8.

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10.

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11.

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, 98, 6750.

13.

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14.

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15.

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16.

Conan, J. Y.; Natat, A.; Guinot, F.; Lamaty, G., Bull. Soc. Fr. Part (2)
1974, 1400.

Avoid Skin Contact with All Reagents

4

MONTMORILLONITE K10

17.

Vu Moc, Thuy; Petit, H.; Maitte, P., Bull. Soc. Chim. Belg. 1980, 89, 759.

18.

Hünig, S.; Benzing, E.; Lücke, E., Chem. Ber. 1957, 90, 2833.

19.

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20.

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21.

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22.

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23.

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24.

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25.

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26.

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27.

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28.

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29.

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30.

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31.

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32.

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Chem. Soc., Dalton Trans. 1974

, 2207.

33.

Cativiela, C.; Figueras, F.; Fraile, J. M.; Garcia, J. I.; Mayoral, J. A.,
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, 4, 223 and references therein.

34.

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35.

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36.

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37.

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39.

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, 2689.

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André Cornélis & Pierre Laszlo

Université de Liège, Liège, Belgium

Mark W.Zettler

The Dow Chemical Company, Midland, MI, USA

A list of General Abbreviations appears on the front Endpapers