Download ORGANIC REACTIONS IN A CLAY MICROENVIRONMENT

Clay Minerals (1983) 18, 347-356.
ORGANIC
REACTIONS
IN A CLAY
MICROENVIRONMENT
J. A. B A L L A N T I N E ,
J. H. P U R N E L L
AND J. M. T H O M A S *
Department of Chemistry, University Collegeof Swansea, Singleton Park, Swansea SA2 8PP, and
*Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP
(Received 17 May 1983)
A B S T R A C T : Although natural Na-bentonite has little catalytic activity, various cationexchanged bentonites have proved to be effective catalysts for a wide variety of organic
reactions. In the presence of these catalysts alkenes can be induced to add (a) water, to yield
branched-chain symmetrical ethers; (b) alcohols, to give a variety of ethers; (c) thiols, to yield
thio-ethers; (d) carboxylic acids, to give esters. The number of products obtained in each
reaction depends on the ease of rearrangement of the carbocation intermediates. High yields
are obtained where a single carbocation intermediate is formed, A variety of elimination
reactions is also catalysed by these sheet silicates. Water is eliminated from alcohols to
produce ethers, ammonia is eliminated from amines to make secondary amines, and hydrogen
sulphide is eliminated from thiols to give dialkyl sulphides. In most cases the ion-exchanged
bentonites react as acidic heterogeneous catalysts.
REACTIONS
OF ALKENES
2 R - - C H = C H 2 + H20 Cu2+-bent~
WITH
R--CH--CH 3
,
0I
R - - C] H - C H 3
+
WATER
CH3--CH--R
0I
R_~H_CH 3
Research into catalysis by ion-exchanged bentonites started in Swansea in 1976 following
a request from Professor J. M. Thomas's group (cf. Thomas, 1982) in Aberystwyth. They
had found that when hex-l-ene was refluxed for a short time with a CuE+-exchanged
bentonite, the hex- 1-ene underwent an unexpected chemical reaction to produce a low yield
of an unknown product.
Examination of this product in Swansea using gas/liquid chromatography (GLC)
columns separated it into two roughly equal peaks which were analysed by combined gas
chromatography-mass spectrometry (GC-MS) techniques. It was also found that both
hept-1-ene and oct-1-ene behaved in a similar fashion when treated with CuE+-bentonite,
producing pairs of roughly equal peaks at longer retention times in their gas
chromatograms.
GC-MS analysis established that the two hexene products had identical mass spectra
and, although the retention times of the compounds corresponded roughly to bimolecular
species, no molecular ions could be observed. The mass spectra contained characteristic
fragmentations at m/z 129 (CsH170 +) and 85 (C6H13+). The heptene and octene products
also contained homologous ions at m/z 143 ( C 9 H 1 9 0 +) and m/z 159 (C10H210+),
9 1983 The Mineralogical Society
348
J . A . Ballantine et al.
respectively. It became clear that these pairs of peaks corresponded to the diastereoisomers of the di (alk-2-yl) ethers (Adams et al., 1979) which, although they could be
separated by the G L C column, would have identical mass spectra. The loss of the C4H 9
fragment in the mass spectra with no loss of a CsH n fragment was completely
characteristic of the branched chain ether in which branching was at C2 in both alkyl
chains. The identity was confirmed by 1H nuclear magnetic resonance (NMR)
spectroscopy of the separated components.
The reaction can be represented by Scheme 1 in which the role of the Cu2+-bentonite
is to supply the proton and the water molecule. The Cu2+-bentonite contained about 8%
interlamellar water and once this was used up the reaction ceased. Yields of ethers were
obtained which corresponded to over 90% of the interlamellar water being used in ether
formation. The mechanism of this reaction must clearly involve the Markovnikoff addition
of water to the most stable secondary carbocation intermediate to give a molecule of the
alkan-2-ol, which must then react with another molecule of the secondary carbocation to
give the di-(alk-2-yl) ether. Small quantities of the alkan-2-ols were detached in these
reactions.
SCHEME1
R--CH=CH2
+H+
interlamenar~
+
a
R--CH-CH3 --+ R--CH--CH
I
eo
/\
H H
HH
R--~H--CH 3
OH
R--CH=CH2 ~
+H+
/
R--Cq-I-CH
R-CH-CH 3
0[
R--CH--CH 3
c --H+
R_~H_CH 3
+6--H
]
R--CH--CH
a
It is remarkable that this reaction occurs at all, as five reactants have to come together
to produce the di-(alk-2-yl) ether, and one of these, the water, is only present to a
maximum of 8% of the weight of clay used.
It should be noted that it has not been possible to distinguish between the above
protonation mechanism and the alternative mechanism in which the clay acts by means
of Lewis acid sites. In the Lewis acid mechanism the secondary carbocations are generated
by addition of the alkene double bonds to Lewis acid sites in the sheet silicate prior to the
water addition.
REACTIONS
R - C H 2 - C H = C H 2 + EtOH
OF ALKENES
WITH
Al-bentonit%R _ C H 2 _ C H O E t _ C H
ALCOHOLS
3 + R_CHOEt_CH2_CH 3
Organic reactions in clays
349
As the water reaction was proposed as a nucleophilic addition of water to a carboeation
intermediate, an obvious extension of this principle was to attempt to add alternative
nucleophiles in reactions where the ion-exchanged bentonite would act as a true
heterogeneous catalyst.
Accordingly, various alkenes were reacted in Swansea with a number of alcohols
under the influence of ion-exchanged bentonites. It was found that for alk-l-enes the
reaction proceeded at about 150~ using Al-exchanged bentonite in a small, sealed,
stainless steel reactor.
For example, hex-l-ene was reacted with ethanol to give a 16% yield of a mixture of
ethers (2-ethoxyhexane and 3-ethoxyhexane), with trace quantifies of hexan-2-ol and
hexan-3-ol as the only other products.
When hexan-l-ol was used the products were a mixture of hex-l-yl, hex-2-yl ether and
hex-1-yl,hex-3-yl ether, and when pentan-2-ol was used the products were pent-2-yl,hex-2yl ether and pent-2-yl,hex-3-yl ether. In all cases the hex-2-yl product predominated over
the hex-3-yl product (Davies, 1982).
The mechanism of these reactions must involve protonation of the alkene to give the
alk-2-yl carbocation which can then rearrange to the alk-3-yl carbocation. The alcohol
nucleophile (or intercalated water molecule) can then add to either carbocation to give
the observed products as shown in Scheme 2 below.
interlamellar
it+
+
+ROH
+ROH
_H §
--a +
OR
OR
R = alkyl: hex-2-yl ether
R = H : hexan-2-ol
R = alkyl: hex-3-yl ether
R = H : hexan-3-ol
When 2,2-dialkylalk-l-enes were used, the etherification reaction proceeded well at
lower temperatures to give the corresponding tertiary ethers. For example, with 2-methylpropene a 58% yield of 2-ethoxy-2-methylpropane was obtained at 50~ in the sealed
reactor using Al-bentonite; 2% of t-butanol was also produced (cf. Adams et aL, 1982).
Yields were much reduced when longer chain 2,2-dialkylalkenes were used but the only
products were the t-ether and t-alcohol (Patel, unpublished data). In these cases a
stabilized tertiary carbocation is being produced which can readily be attacked by the
alcohol nucleophile.
With 2,2-dialkylalkenes, at temperatures much above 150~ the etherification reaction
fails and alkene dimerization and trimerization predominate.
REACTIONS
OF ALKENES
C 3 H T - C H 2 - C H = C H 2 + RSH
WITH
THIOLS
Al-bentonit% C3HT_CH2_CHE_CH2SR
+ C 3HT-CH2-CH(SR)--CH 3
+ C3HT--CH(SR)-CH2--CH 3
350
J . A . Ballantine et al.
In attempts ato use alkyl thiols as the nucleophile in the alkene-clay reactions it was found
that alk-l-enes reacted with thiols at high temperatures to produce good yields of
thio-ethers.
For example, hex-l-ene reacted with butane-l-thiol in a sealed reactor at 210~ with
Al-bentonite to give a mixture of hex-l-yl, but-l-yl sulphide (12%), hex-2-yl,but-l-yl
sulphide (30%), and hex-3-yl, but-1-yl sulphide (23%). At lower temperatures the hex-1-yl
isomer was almost the sole product and when the temperature was raised the yields of the
2- and 3-isomers increased and that of the 1-isomer decreased (Galvin, 1983).
It seems likely that, although the 2- and 3-isomers are the result of normal Markovnikoff
addition of butane-1-thiol, the 1-isomer is formed by a completely different mechanism,
probably involving a free radical intermediate as these are common in the reactions of
thiol compounds.
REACTIONS
OF ALKENES
WITH
AMINES
Attempts to use alkan-l-amines as nucleophiles in alkene-clay reactions failed completely
(Williams, 1982). Presumably these, being much stronger bases than the alkenes, compete
much more favourably for the available protons and stop alkene addition reactions from
proceeding.
REACTIONS
OF ALKENES
R - - C H 2 - - C H = C H 2 + AcOH
WITH
CARBOXYLIC
ACIDS
AI bentonite
R - - C H 2 - - C H ( O A c ) - C H 3 + R--CH(OAc)--CH2--CH 3
Carboxylic acids were found to be good nucleophiles in alkene-clay reactions giving rise
to ester products.
For example, hex-1-ene was found to react with acetic acid in a sealed reactor at 200 ~C
in the presence of Al-bentonite to yield hex-2-yl acetate (16%) and hex-3-yl acetate (7%) as
well as some alkene isomers (Ballantine et al., 1981a). The reaction was found to achieve
equilibrium after about 6 h at 200~
and if pure hex-2-yl acetate was treated with
Al-bentonite at 200~ it decomposed to give the same equilibrium mixture. The multiple
equilibria which participate in this reaction sequence are shown in Scheme 3 which
explains the formation of the observed products.
If, however, alkenes are chosen which can only give a single carbocation intermediate
(e.g. ethene, propene and cyclohexene), the yields of ester are improved dramatically. For
example, when acetic acid was reacted with excess ethene in a pressure reactor at 200~
with Al-bentonite, yields of 92% ethyl acetate (based on acetic acid) with a 96% selectivity
could be obtained.
The nucleophilic addition of carboxylic acids also works well with 2,2-dialkylalk- 1-enes
but at much lower temperatures. When 2-methylpropene was reacted with acetic acid at
18~ in the presence of Al-bentonite, yields of 65% 2-methylprop-2-yl acetate were
obtained (Patel, unpublished data).
Esters have also been produced with formic, propanoic and butanoic acids and clearly
the addition of carboxylic acids to alkenes is a realistic alternative to existing methods for
Organic reactions in clays
351
SCHEME 3
H
--I-I+
C3HT-CH2-CH-CH 3~
Hex-2-yl acetate
Ac
C3HT-CHz-CH--CH3
] I+AcOH
+H +
+
C3HT-CH2-CH:CH2 ~
C3HT-CHz-CH-CH3
Hex- 1-ene
-H+//
//
c,H-CH=CI-CH,
JI
C3H7--CH-CH2-CH 3
1[+AcOH
C2Hs--CH--CH-C2H5
__H+
C3H7CH-CH2--CH 3 <
OAc
LI
"J_|
Hex-a-y~ acetate
CaHT--CH--CH2-CH 3
I
/0+\
H Ac
the production of certain esters. The work-up is so much easier than conventional methods
as the catalyst is simply removed by filtration or centrifugation prior to distillation of the
product.
ELIMINATION
OF WATER
FROM
ALCOHOLS
R _ C H 2 _ C H 2 _ O H AIbentonit%R _ C H 2 _ C H 2 _ O _ C H 2 _ C H 2 _ R + H20
Following the successful use of ion-exchanged bentonites for proton addition to alkenes it
seemed obvious to examine their reactivity in other acid-catalysed reactions. Accordingly
their reactivity in the dehydration of alcohols was examined.
When primary alkanols were heated at 200~ in a sealed reactor in the presence of
Al-bentonite, good yields of the di (alk-l-yl) ether were obtained from inter-molecular
dehydration and only very small yields of the corresponding alkene from the competitive
intra-molecular dehydration (B allantine et aL, 1981 c). For example, propan- 1-ol gave 63 %
di-(prop-1-yl) ether and only 3% propene.
Secondary alkanols behaved very differently and the intra-molecular dehydration to the
alkene dominated their reactions. For example cyclohexanol yielded 88% cyclohexene
and only 4% dicyclohexyl ether at 200~ The intra-molecular dehydration of tertiary
alcohols was also facile under these conditions when t-butanol gave only alkene dimers and
trimers at 200~
Clearly the intra-molecular and inter-molecular dehydration reactions are in
competition, as illustrated in Scheme 4. The inter-molecular dehydrations must be of an
J.A. Ballantine et al.
352
SN2 type where the carbocation does not have a discrete lifetime, otherwise rearrangement
to the much more stable secondary carbocation would have taken place and branchedchain ethers would have resulted. Note that in these SN2 reactions a protonated alcohol
molecule must react with a non-protonated alcohol molecule. Hence, if there is an
abundant supply of protons this etherification reaction will fail, as all of the alcohol
molecules will be protonated, as is the case with homogeneous catalysis with concentrated
acids.
If diols are used in the clay reaction at 200 ~C, inter-molecular dehydration can produce
both oligomers and cyclizations. In this way ethylene glycol and diethylene glycol gave
both 1,4-dioxan and oligomers of ethylene glycol (Rayanakorn, 1980; Davies, 1982).
Benzyl alcohol also underwent inter-molecular dehydration but did not produce the
expected diphenyl ether; instead nucleophilic displacement occurred from the reactive
aromatic ring system, as shown in Scheme 5, to give an aromatic substitution reaction
SCHEME 4
R C
-- H2-CH2-OH
H+
~+/H
~ R--CHz--CH2--Q
~.
\ H
In r
--H20 /
9 t a-molecular/
- H + /
R-CH=CH z
alk-1-erie
HO-CH2--CH2-R
] _H20
~int. . . . lecular
R--CH2--CH 2
/H~_~_CH2_CH2_ R
R--CHz--CHz--O-CHzCHz--R
di-(alk- 1-yl) ether
SCHEME 5
~
CHzOH
§ §
~/CH2--OHz
~
"CH2OH
H
,,~
H2OH
-~
CH2OH
1,4- + 1,2-Poly(phenylene-methylene)
-(--CH 2--C6H4-C H2-C 6H4-~n
Organic reactions in clays
353
which gave the polymeric material poly(phenylene-methylene), having a molecular weight
distribution up to 200 000 a.m.u., as the sole product. Essentially this is just an alternative
mode of SN2 nucleophilic displacement to that leading to ethers.
ELIMINATION
R-CH2-CH2--NH 2
OF AMMONIA
FROM
AMINES
Al-bentonite
~ R--CH2-CH2--NH--CH2-CH2--R + NH 3
Primary alkanamines proved to be rather unreactive in ammonia-elimination reactions in
the presence of ion-exchanged bentonites. Only 14% butan-l-amine was found to have
reacted after 15 h at 220~ with Al-bentonite, with di-(but-1-yl) amine as almost the sole
product of the elimination reaction.
In contrast to the behaviour of secondary alcohols, the amine cyclohexylamine gave a
good yield of di-cyclohexylamine (57%) by inter-molecular elimination of ammonia, but
failed to give any cyclohexene via the intra-molecular elimination process (Ballantine
et aL, 198 lb). The reaction was found to be extremely temperature-dependent with only
~5% reaction after 24 h at 160~ rising to >50% reaction after 24 h at 220~
The tertiary amine t-butylamine was little affected by heating at 210~ with
Al-bentonite for 50 h, when only 6% reaction had taken place to give intra-molecular
elimination to 2-methylpropene and its oligomers.
Benzylamine reacted differently to benzyl alcohol in that no polymeric material was
produced; instead inter-molecular elimination of ammonia produced di-benzylamine in low
yield.
The reaction of the cyclic amine pyrrolidine with Al-bentonite gave only two products;
the first resulting from two pyrrolidine molecules reacting together to open one of the rings,
and the second resulting from three molecules reacting together with loss of ammonia.
Scheme 6 illustrates a protonic mechanism for this process, which is consistent with the
normal reactions observed with primary amines.
SCHEME6
H
H2
H3
H2
(I)
--NH
H 9
\H
"~
(II)
J.A. Ballantine et al.
354
Secondary amines
.....
40-
cyclohexyl,benzyl
- -
/
30-
,
\
/'
7:
'\
.....
'\
/
/
\
'\ \,\
~11
'
0
dibenzy[
"\
1'
20-
dicyclohexyl
0125
0"%0
0'J75
1'0
Molar ratio benzylamine
FIG. 1. Yieldsof secondary amines from 18 h reactions of mixtures of benzylamineand cyclohexylaminewith Cr3+-bentoniteat 205~
Some unexpected results were obtained in the bentonite-catalysed reactions of mixtures
of amines (Williams, 1982). When mixtures of benzylamine and cyclohexylamine were
reacted with CP+-bentonite at 205~ the quantities of the secondary amine products
depended on the molar ratios of the two reactants in the manner shown in Fig. 1. Points
of interest are: (a) there is more of the cross-product formed than either of the symmetrical
di-amines; (b) the cross-product is at a maximum when only about 1/4 of the original
mixture is cyclohexylamine; (c) the yield of dibenzylamine is only ~4% with pure
benzylamine, this rising to ~ 10% as more cyclohexylamine is added.
These strange results can be explained if one remembers that protonated species react
with un-protonated species to produce the secondary amines. As cyclohexylamine is a
stronger base (pKb 3.34) than benzylamine (pKb 4.67), it should compete more
favourably for the available protons. If the low yield of di-benzylamine with pure benzylamine reactant is due to there being very little un-protonated species available for reaction,
then the addition of a little cyclohexylamine, which will preferentially react with the
available protons, should increase the quantity of un-protonated benzylamine and thus
increase the yield of di-benzylamine. Also, the very high yield of cross-product is simply
due to the stronger base capturing most of the available protons and then reacting with
the large quantity of un-protonated benzylamine which will become available in this
situation.
The mechanistic model of the bentonite acting as a reactive source of a limited quantity
of protons in the interlamellar space would, therefore, explain these unexpected results.
ELIMINATION
OF HYDROGEN
R--CH2--CH2--SH
Al-bentonite
SULPHIDE
FROM
THIOLS
~ R-CH2--CH2-S--CH2--CH2--R + H2S
Organic reactions in clays
355
Both primary and secondary thiols readily underwent inter-molecular elimination of
hydrogen sulphide under the influence of Al-bentonite in a sealed reactor at 200~ Little
alkene production took place. (Ballantine et al., 1981d). Butan-l-thiol was found to
produce di-(but-l-yl) sulphide (21%) with 75% of the original reactant being returned
unchanged, whereas the cyclohexyl thiol, a secondary thiol, gave di-cyclohexyl sulphide
(76%) in a reaction in which 20% of the original reactant was returned unchanged.
t-Butyl thiol reacted in an expected manner under these conditions to yield
2-methylpropene and its oligomers by an intra-molecular elimination.
Benzyl thiol was found to react with Al-bentonite in a similar manner to benzyl alcohol
when poly(phenylene-methylene) was produced as the sole product.
Although no reaction was observed with either phenol or aniline in the presence of
Al-bentonite, thiophenol was found to react readily at 200 ~C to give a variety of products,
including diphenyl sulphide (15%) and benzene (12%). It is most unlikely that diphenyl
sulphide is produced by nucleophilic displacement of H2S from this aromatic compound,
as such reactions are very unfavourable, and it therefore seems likely that a free-radical
mechanism is taking place with this sulphur compound. The production of benzene by the
elimination of sulphur from thiophenol is a very unexpected reaction, the mechanism of
which is being investigated.
REACTION
WITH
ETHYLENE
O Albentonite~
/\
'
OXIDE
O~O
+
k___J
Ethylene oxide was found to react readily with Al-bentonite at reasonable temperatures to
give high yields of the dimeric ring compounds 1,4-dioxan (65%) and 2-methyl-l,3dioxolane (16%) (Davies, 1982).
KETAL
FORMATION
REACTIONS FROM
KETONES
O
MeOH ~
Al-bentonit7
~
ALDEHYDES
OMe
L-OMe
AND
+ H20
OMe
Al-bentonite
+ HCO2Me
Ion-exchanged bentonites are efficient catalysts for the formation of ketals from aldehydes
or ketones. C~,clohexanone reacted with methanol in the presence of Al-bentonite at room
temperature to give a 33% yield of the dimethyl ketal after 30 min reaction. The reaction
between cyclohexanone and trimethyl orthoformate in the presence of Al-bentonite was
most spectacular. On addition of the catalyst to the mixture of the two liquids at room
356
J.A. Ballantine et al.
temperature, the exothermic reaction caused the liquid to boil and an almost quantitative
yield of the dimethyt ketal was produced in a 5 rain reaction simply by removal of the
catalyst followed by distillation (O'Neil, unpublished data). It is interesting to note that
the raw Na-bentonite completely fails as a catalyst for this facile reaction.
CONCLUSION
The interlayer microenvironment of certain ion-exchanged bentonites has been shown to
be conducive to effective heterogeneous catalysis for a wide variety of organic chemical
reactions. The reactions are consistent, in the main, with a mechanism of action for the
catalysis which suggests the involvement of interlamellar protons.
ACKNOWLEDGMENTS
The authors wish to acknowledge the support of the BP Research Centre, Sanbury-on-Thames, for the
award of studentships, fellowships and financial assistance, and to the SERC for research studentships.
REFERENCES
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ADAMS J.M., CLEMENT D.E. & GRAHAM S.H. (1982) Synthesis of methyl-t-butyl ether from methanol and
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Chemical conversions using sheet silicates: Facile ester synthesis by direct addition of acids or alkenes.
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