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Organic reactions catalysed by sheet silicates: intermolecular elimination of ammonia from primary amines
Journal of Molecular
Catalysis,
30 (1985) 373 - 388
373
ORGANIC REACTIONS CATALYSED BY SHEET SILICATES: INTERMOLECULAR ELIMINATION OF AMMONIA FROM PRIMARY AMINES JAMES A. BALLANTINE*, KEVIN J. WILLIAMS Department 8PP (U.K.)
of Chemistry,
J. HOWARD PURNELL, MONGKON RAYANAKORN**, University
College of Swansea,
Singleton
Park, Swansea,
SA2
and JOHN M. THOMAS Department
of Physical
Chemistry,
University
of Cambridge,
Cambridge,
CB2 1EP (U.K.)
(Received March 23,1984)
Summary
(1) Certain ion-exchanged montmorillonites catalyse the intermolecular elimination of ammonia from primary amines to produce dialkylamines. The yields are significant with cycloalkylamines and benzylamine but poor in the cases of alkan-lamines and alkan-2-amines. (2) Cyclic amines, such as pyrrolidine, also evolved ammonia to give coupled, ring-opened products such as 1,4di-(l-pyrrolidyl)butane. (3) However, those ion-exchanged sheet silicate catalysts which catalyse the addition of either water, or alcohols, or thiols or carboxylic acids to alkenes failed to effect the addition of amines to the corresponding alkenes. (4) The products from these amine reactions are consistent with the involvement of interlamellar protons, but the reactions have no counterparts in the proton-catalysed solution chemistry of amines. Similar products have been observed previously, however, over the surfaces of finely divided nickel, palladium and ruthenium.
Introduction
As part of an extensive study of the catalytic activity of ion-exchanged sheet silicates [ 11, we have shown that these materials can act as efficient acidic heterogeneous catalysts for a wide variety of organic reactions, including (a) ether formation by addition of water to alkenes [2], (b) ether *Author to whom correspondence should be addressed. **Present address: Department of Chemistry, Chiang Mai University, Chiang Mai, Thailand. 0304-5102/85/$3.30
@ Elsevier Sequoia/Printed in The Netherlands
374
formation by the addition of alcohols to alkenes [3,4], (c) ester formation by the addition of carboxylic acids to alkenes [ 5,6], (d) ether formation by the intermolecular dehydration of alcohols [3,4] and (e) thioether formation by the intermolecular elimination of hydrogen sulphide from thiols f71. The present publication describes the extension of this study into an investigation of the behaviour of amine compounds under the influence of ion-exchanged sheet silicate catalysts. Although the intercalation of amines into montmorillonites has been studied extensively for many years using a variety of physical techniques [ 8 - 121, there have been no previous reports that amines undergo chemical changes in these systems. The wealth of physical evidence suggested that most intercalated amines were simply protonated, at least in part, by acidic centres in the sheet silicates [ 13 1. The research described in this publication has established that primary amines can be converted to secondary amines, via an intermolecular elimination of ~monia, in the presence of these ion-exchanged sheet silicates. Reactions of this type are unknown in the solution chemistry of amines with homogeneous acidic catalysts, but similar products have been obtained at high temperatures in the presence of f?nely divided nickel [14,15], or palladium [16,17] as heterogeneous catalysts, and also with ruthenium complexes [18 - 201 as homogeneous catalysts. In these cases, which are all catalysed by transition metals, o~da~on/~duction me~h~isms involving intermediacy of the imine have been proposed [17,20,21]. This publication reports on the reactions of a variety of amines in the presence of ion-exchanged sheet silicate (montmorillonite) catalysts, and also on attempts to add amines to alkenes under similar conditions.
Methods Preparation of the ion-exchanged catalysts Cation exchange was effected by gradually adding the finely divided montmo~loni~ (100 g), (usually ‘Wyo~g bentonite’ supplied by B.D.H. Ltd., Poole, U.K.) to solutions of the appropriate cation salts (800 cm3, 0.5 M) with vigorous stirring for 3 h. Typical ionexchanged catalysts have been made using aluminium, copper or ammonium sulphates, dilute sulphuric acid or chromic chloride solutions. The resultant suspension was centrifuged for 30 min at 1000 G, and most of the water removed by deception. The solid material was carefully removed from the centrifuge tube in such a way that the very dense material {e.g. quartz, feldspar, haematite and magnetite) at the bottom of the tube was left behind. The purified slurry was washed with de-ionised water (800 cm3) with vigorous stirring for 1 h before centrifugation at 1000 G for 30 min, to remove water and dense material again. This washing and separation stage was repeated five or six times until chemical tests showed the washings to be free of the appropriate anion.
376
connected to a 4-station Perkin-Elmer Sigma 1OB chromatographic data system, were used for quantitative analysis of the reaction products, whereas Pye Model 104 instruments were used for both manual preparative collection of products prior to ‘H NMR spectroscopic analysis, and for combined gas chromato~phy-rn~ spectrome~ (GC-MS) analysis. For these analyses three interchangeable stainless steel columns were employed; (a) 8 ft X l/8 in o.d. packed with 15% Carbowax 20M/2% KOH on 120 - 140 mesh Chromosorb G AW DMCS, (b) 3 ft X l/8 in o.d. with identical packing or (c) 3 Et X l/8 in o.d. packed with 8% Apiezon M/2% KOH on 120 - 140 mesh Chromosorb P AW DMCS. Nitrogen was used as carrier gas at a flow rate of 20 cm3 min-i at 100 psi. The oven temperature was programmed, typically from 60 to 220 “C at 8 “C mm-‘, with injector and detector held at 250 “C. Injection volumes for quantitative analyses were 0.5 ~1. Glass columns of wider bore (8 ft X l/4 in o.d.) and injection volumes of 2.0 ~1 were employed for the manual collection of individual products in m.p. tubes for ‘II NMR analysis on a Vat&n XL-100 ~st~ment. GC-MS analyses were carried out using an A.E.I. Model MS9 mass spectrometer, modemised with a replacement electronic console and solid state magnet by Mass Spectrometry Services Ltd. The spectrometer was coupled to the gas chromatograph via an A.E.I. silicone rubber membrane single-stage separator and was fitted with a V.G. Model 2015 data system with cartridge disc storage. Helium was used at 20 cm3 mm-’ (70 psi) as carrier gas. Calibrations of the flame ionisation detectors were carried out by repetitive injection of the individual pure components at different concentrations in a suitable solvent, so as to produce a detector response plot for each product at optimised flow rates. Pure samples of the components were either purchased, synthesised or collected by preparative gas chrom~~aphy. In this way the raw area units, obtained from the Sigma 10 data station, could be related directly to the mass of each product. Tabulated yields are expressed in weight % of the total material present in the analysed product mixture after removal of the catalyst. Results Reactions of alkan-1 amines with ion-exchanged montmorillonite A number of alkan-l-amines were treated in the sealed reactor with A13’ ionexchanged montmorillonite at temperatures in excess of 200 “C for varying periods of time. The yields of products for two sets of conditions (215 “C for 14 h and 210 “C for 50 h) are summarized in Table 1. In general, a rather slow reaction took place in which a small amount of the primary amine was converted to a secondary amine with loss of ammonia. A small quantity of the Schiff base was usually observed in addition. It is interesting to note that no alkenes were observed as products in these reactions, even after 50 h reaction, when in most cases more than 85% of the alkan-l-amine was returned unchanged.
93.5 86.3
93.1
94.1
96.5
97.1
ethanamine propan-l amine
butan-l amine
2methylpropan-l-amine
pentan-l amine
hexan-l -amine
Nethylethsnaminea Npropylpropan-1 -aminea N-(1-propylidine)propan-1-aminec N-butylbutan-l-aminea N-(1-butylidine)butan-lamineb N-( 2methylpropyl)2-methylpropan-l-amine’ N-(2-methyl-1propylidine)-2methylpropan-1 -amineC Npentylpentan-l-amineC N-(1pentylidine)pentan-l-amineC Nhexylhexan-1 -amineC N-( 1 -hexylidine)hexan-1 amineC
Product(s)
aIdentified by comparison with commercial sample. bIdentified by comparison with synthetic sample. CIdentified by analysis of mass spectral data. dData for reaction at 210 “C for 50 h in parentheses.
Unreacted amine (wt.%)
Reactant
(4.7) (14.6) (0.8) (11.2) (1.4) (8.3)
1.5 2.0 1.2 1.7
(6.9) (2.1) (3.9) (1.6)
1.0 (1.1)
6.5 13.0 0.7 5.3 1.6 4.9
Wt.%d
GC-MS product analysis from reaction of alkan-l-amines with A13+exchanged montmorillonite
TABLE 1
157(6) 185(7) -
127(5)
78( 18) lOl( 7) 129(7) 129(4)
Molec ion (abd%)
GC-MS data
57(100),
30(81
j 100(100), 44(35), 30(34) 98(100), 84(24j, 43(48) 114( loo), 44( 59). 30(44) 140(23), 112( loo), 70( 23)
84(96),
58( loo), 44( 27), 30( 93) 72( loo), 43( 28): 30( 98) 44(l), 30(3), 28( 100) SS(lOO), 44(87), 30(82) 84(3), 56( 2), 28( 100) 86(100), 57(35), 30(69)
Fragment ions (abd%)
for 14 h at 215 “C!
98.8
94.6
butan-2amine
2-methylpropan-2-amine
69.7
93.2
cyclohexylamine
cycloheptyl~ine
as for Table 1.
81.8
cyclopentylamine
Footnotes:
Unreacted amine Wt.%
Reactant
0.8 0.4 0.8 0.4 4.0 1.4
(2.6) (1.9) (1.6) (0.7) (1.1) (3.4)
Wt.%c
montmorillonite
12.8 5.3 29.8 0.6 4.9 2.0
N-(cyclopentyl)cyclopentylamineC N-(cyclopentylidine)cyclopentyhuninec N-(cyelohexyl)cyclohexylaminea N(cyclohexylidine)cyclohexylamineb N-(~cloheptyl~y~ohep~l~~e~ N(cycloheptylidine)cycloheptylamineC
(28.0) (4.4) (56.6) (0.8) (25.2) (1.7)
Wt.%d
Product
86( 29), 58( 7), 44( 100) 84( 5), 42( 6), ZS(lO0) 100(65), 58(28), 44(100) 98(72), 57(33), 42(100) 55( ZO;, 41(100), 39( 36)
Fragment ions (abd%)
163(18) 151(35) 181(g) 179( 29) 209(11) 207( 14)
Molec ion (abd%)
GC-MS data
124(100), 122(36), 138(100), 136(57), 166(17), 150(32),
84(12), 56(37) 84(64), 54(100) 56(71), 44(53) 98(100), 54(55) 152(100), 56(48) 112(86), 55(100)
Fragment ions (abd%)
for 14 h at 215 “C
101(l) 99(l) 56(8Oj
Molec ion (abd%)
GC-MS data
for 14 h at 215 “C
with A13+excbanged montmorillonite
N-(prop-2-yljpropan-2-aminec N-( 2-propylid~e)pro~n-2-~ine~ N-(but-2-yl)butan-2-amineC N-( 2-butylidine)hutan-2-aminec 2-methylpropenea alkene oligome&
Product
GC-MS product analysis from reaction of cycloalkylamines
TABLE 3
as for Table 1.
98.8
propan-a-amine
Footnotes:
Unreacted
Reactant
GC-MS product analysis from reaction of alkan-2-amines with Al’*-exchanged
TABLE 2
379
The structures of the secondary amines were established to be, in every case, the di-(alk-l-yl)amines, by comparison of their GC-MS characteristics with either authentic or synthetic materials. Reactions of alkan-2-amines with ion-exchanged montmorillonite The reactions of alkan-!&mines with Al’+ ionexchanged montmorillonite were even slower than with the corresponding alkan-l-an-&es and very low yields of the di-(alk-2-yl)amines were obtained (Table 2). A small amount of alkene was obtained only in the case of 2-methylpropan-2-amine by unimolecular elimination of ammonia. Reaction of cycloalkylamines with ion-exchanged montmorillonite The reactions of cycloalkylamines with A13+ ionexchanged montmorillonite were considerably faster than with the corresponding alkan-lamines. High yields of the di-(cycloalkyl)amines were obtained along with small yields of the corresponding Schiff bases under identical conditions (Table 3). It is interesting that the yield of secondary amine obtained from cyclohexylamine was considerably higher than from either cyclopentylamine or cycloheptylamine. The effect of temperature on the rate of the reaction of cyclohexylamine with A13+exchanged montmorillonite (Fig. 1) showed that
Fig. 1. Effect of reaction time and temperature on the yield of dicyclohexylamine the reaction of cyclohexylamine with A13+-exchanged montmorillonite.
from
380
although the reaction was negligible below 160 “C it proceeded rapidly at 220 “c. Reaction of benzylamine with ion-exchanged montmorillonite Benzylamine gave an excellent yield of dibenzylamine in a very clean reaction when treated with A13+exchanged montmorihonite (Table 4). Reaction of cyclic secondary amines with ionexchanged montmorillonite Pyrrolidine reacted with A13+exchanged montmorillonite in a very clean reaction to give two products; 4-(1-pyrrolidyl)butan-l-amine and 1,4di-( 1-pyrrolidyl)butane, in addition to unreacted pyrrolidine only (Table 4). When the reaction was allowed to proceed for longer times, the relative yield of the latter product increased at the expense of the former, suggesting that the former was an intermediate in the formation of the latter. The structures of the two products were confirmed by facile syntheses from lithium aluminium hydride reduction of the corresponding pyrrolidyl succinamines, which were prepared by aminolysis of the appropriate succinate esters with pyrrolidine. Piperidine also reacted with A13+exchanged montmorillonite in the same way to give the corresponding ring-opened products, but in much reduced yield (Table 4). Reaction of aniline with ionexchanged montmorillonite Aniline was reacted with A13+exchanged montmorillonite at 210 “C for periods up to 50 h, and GLC analysis of the reaction mixture established that no reaction had taken place, only unchanged aniline being detected. Reactions using cyclohexylamine hydrochloride A series of comparative reactions was carried out in which cyclohexylamine was mixed with anhydrous cyclohexylamine hydrochloride in a molar ratio of 2:1, and reacted together above 200 “C in the presence and absence of A13+exchanged montmorillonite. The hydrochloride was found to dissolve completely at this concentration at about 130 “C. At the end of the reaction the products were extracted into water and ether, basified with solid potassium hydroxide and the ether layer analysed. It was found that in the absence of the sheet silicate catalyst the mixture reacted to produce a reasonable quantity of dicyclohexylamine after 12 h reaction at 200 “C, but less than that obtained from the reaction between cyclohexylamine and A13+exchanged montmorillonite (Table 5). A blank reaction proved that cyclohexylamine showed little tendency to give the diamine in the absence of catalyst. Influence of the exchanged cation on the catalytic activity of ion-exchanged montmorillonites in amine reactions A wide variety of cationexchanged sheet silicates and related heterogeneous catalysts have been investigated for their efficiency in the cyclo-
68.0
97.5
80.2
pyrrolidine
piperidine
benzylamine
4-( 1-pyrrolidyl)butan-1-amineb 1,4-di-(l-pyrrolidyl)butaneb 5-( 1-piperidyl)pentan-1-amineb 1,5-di-(1-piperidyl)pentaneb N-( benzyl)benzylaminea
Product
27.1 4.2 1.2 0.7 19.8
(41.3) (14.2) (1.3) (0.6) (32.4)
Wt.%”
142( 1) 196( 1) 170(2) 238( 1) 197(4)
Molec ion (ahd%)
GC-MS data
2.7 (10.0)
97.3 (90.0)
86.6
none
Al%-exch. mont.
cyclohexylamine + hydrochloride salt of cyclohexylsmine
cyclohexylamine + hydrochloride salt of cyclohexylamine
aparentheses refer to reactions for 12 h at 200 “C.
13.4
0.1 11.1 (32.1)
99.9 88.9 (67.9)
none Al*exch.
cyclohexylamine cyclohexylamine mont.
N-(cyclohexyl)cyclohexylamine (product wt.%)a
Cyclohexylamine (reactant wt.%)a
Catalyst
84(100), 71(10), 70(17), 42(16) 110(7), 97(17), 84(100), 71(23) 98(iOO), 84(11), 55(8), 41(11) 154(7), 124(11), 98(100), 42(11) 106(45,91, loo), 28(49)
Fragment ions (ahd%)
Reactants
Analysis of products from reactions with cyclohexylsmine hydrochloride for 4 h at 210 “C
TABLE 5
Footnotes: as for Table 1.
Unreacted amine (wt.%)
Reactant
GC-MS product analysis from reaction of various cyclic amines with A13*exchanged montmorillonite for 14 h at 215 “C!
TABLE 4
382 TABLE 6 Relative reactivity of various heterogeneous dicyclohexylamine after 4 h at 220 “C
catalysts as measured by production
Catalyst
Relative reactivity
Al%exchanged mineral colloid (RHSR) A13+exchanged Wyoming bentonite (BDH) NHJexchanged Wyoming bentonite (BDH) H+exchanged Wyoming bentonite (BDH) A13+-exchanged Ca-D montmorillonite (ACC) H+exchanged mineral colloid (RHSR) Co2+-exchanged Wyoming bentonite (BDH) Ni2+exchanged Wyoming bentonite (BDH) Mg2+exchanged Wyoming bentonite (BDH) Cr*exchanged Wyoming bentonite (BDH) A13+exchsnged Gelwhite L (GKC) vanadylexchanged Wyoming bentonite (BDH) non-exchanged (Ca) Ca-D montmorillonite (ACC) Al’+exchanged fuller’s earth (H & W) Al*exchanged ferruginous smectite (SCMR) nonexchanged Fulcat 22A (LAB) Al*exchanged Ca montmorillonite SAx-1 (SCMR) nonexchanged Surrey powder (LAB) Ca2+-exchanged Wyoming bentonite (BDH) non-exchanged Fulcat 14 (LAB) nonexchanged Gelwhite L (GKC) non-exchanged ferruginous smectite (SCMR) H”exchanged Gelwhite L (GKC) nonexchanged (Na) Wyoming bentonite (BDH) non-exchanged mineral colloid (RHSR) H+ Nafion (D) Al*exchanged Barasym Smm-100 (SCMR) hydroxylated aluminium oxychloride (RC) RE zeolite (WRG) no catalyst
100 91 90 87 87 82 79 78 73 72 69 59 57 55 54 52 49 47 35 32 30 26 24 22 19 14 13 4 1 0
of
Footnotes : Sources of original unexchanged material : RHSR - Dr. R. H. S. Robertson BDH - British Drug Houses, U.K. ACC - America Colloid Co., U.S.A. LAB - Laporte Industries Ltd., U.K. GKC - Georgia Kaolin Co., U.S.A. H&W - Hopkin & Williams, U.K. SCMR - Source Clay Mineral Repository, U. of Missouri, U.S.A. D - E. I. DuPont de Nemours, U.S.A. RC - Reheis Co., U.S.A. WRG - W. R. Grace Co., Davison Division, U.S.A.
hexylamine reaction; Table 6 gives an approximate order of catalytic activity as measured by the yield of dicyclohexylamine produced after 4 h reaction at 220 “C in the sealed reactor. Under these conditions, the best catalyst produced a 24.5% yield of dicyclohexylamine.
383
6
12
18
24
Fig. 2. Comparison of the efficiency of NH4+- and A13+-zxchanged montmorillonites as catalysts for the conversion of cyclohexylamine into dicyclohexylamine at 200 “C.
In addition, it was observed that the act&y of certain catalysts varied with reaction time. For although A13+exchanged montmorillonite was a much better catalyst at 200 “C!than was NHzexchanged montmorillonite at an early stage in the reaction, after 10 h the situation was reversed and NH:exchanged material was the better catalyst (Fig. 2). This may have some relation to the fact that in zeolite chemistry, ammonium ions are known to produce protons on heating. Reaction of alkenes with alkanamines in the presence of ion-exchanged montmorillonite The reaction of a wide variety of alkenes with alkanamines in the presence of ionexchanged montmorillonites failed to give any addition products, and the starting materials were recovered with little change. Discussion Reaction with primary amines Sheet silicates, when exchanged with certain cations, are able to act as heterogeneous catalysts for the conversion of primary amines into secondary amines at temperatures above 200 “C. The reaction is slow with alkan-l-
384
amines and alkan-2-amines, but is much easier with cycloalkylamines and with benzylamine. There is a considerable volume of spectroscopic evidence [8 - 13,24 263 for the occurrence of protonation of intercalated amines in sheet silicates and it seems likely that the conversion to secondary amines involves a protonation mechanism, as shown in Scheme 1. This is directly analogous to the mechanism already proposed for the formation of di-(alk-l-yl) ethers from alkan-l-ols [4] and for the formation of di-(alk-l-yl) sulphides from alkan-1-thiols [ 71.
R-CH2-NH2
interlamellar HS
’
R-CHz->H, I H&CH2-R 1
R-CH,-NH-CH,-R
d
-NH3
R-YH2
+NH*---CH2--R
Scheme 1
It is to be noted that the product is a di-(alk-1-yl)amine, and hence any alternative mechanism which involves the intermediacy of either carbocations or alkenes must be invalidated [ 4,6 J. The mechanism in Scheme 1 involves an SN2 intermolecular elimination reaction where an unprotonated amine acts as a nucleophile to displace ammonia from a protonated amine molecule. Such reactions are quite unknown in the solution chemistry of amines, but it has been possible to simulate this type of reaction with a measure of success by heating cyclohexylamine with its hydro~lo~de salt in the absence of catalyst in the sealed reactor. It is possible that sheet silicates are effective catalysts because the micro-environment of the interlamellar region provides a limited source of highly acidic protons, so that both protonated and unprotonated species can co-exist in proximity close enough for substitution reactions to take place within these he~rogeneo~ catalysts. The conversion of primary to secondary amines is already known as a facile reaction over finely divided nickel [ 14,151 or palladium [16,17] at high temperatures, or over homogeneous ruthenium complexes [ 19,201. In all of these cases mechanisms involving oxidation-reduction processes have been proposed, as in Scheme 2. The key steps in the oxidation-~duction process are dehydrogenation to an imine, which then couples with an amine to given an unstable gem-
385
R-CH2-NH2
finely divided Ni
--Hz
+ R-CH=NH I
H2N-CH2-R
1 R-7H
-NH3
<
R-YH-NHz
NH-CH2-R
N-CH*--R Ni
+Hz
\
Scheme 2. Oxidation-reduction
R-CH2-NH-CH2-R mechanism of Winaus and Adkins [14].
diamino derivative. This eliminates ammonia easily to give a Schiff base, which is hydrogenated over the catalyst to the product. Very small amounts of Schiff base are often observed in the sheet silicate reactions, so that it would appear that there is a small tendency for the dehydrogenation and coupling steps to occur in these catalysts also. However in the case of our catalysed reactions, both A13+- and NHz-exchanged montmorillonites are efficient catalysts for the reaction, and it seems very unlikely that an oxidation-reduction mechanism involving a hydrogenation step would be effective in the absence of traditional transition metal hydrogenation catalysts such as Ni, Pd or Ru. In addition, the remarkable results from the competitive reactions between mixtures of cyclohexylamine and benzylamine in the presence of sheet silicate catalysts [27] can be rationalised completely on the basis of a protonation model, but would be uninterpretable using the oxidationreduction model. The protonation mechanism is therefore preferred for the sheet silicatecatalysed reactions. The survey of the reaction of cyclohexylamine with a variety of different catalysts (Table 6) has established that the nature of the cation, not unnaturally, dictates the catalytic activity of these sheet silicate materials. This is to be expected, just as in analogous instances of catalysis by cationexchanged zeolites [28], owing to the occurrence of cation hydrolysis [M”+ + Hz0 + M--OH’“- l)+ + H+]. The most effective catalysts were found to be montmorillonites which had been exchanged with Al 3+, NH: or H+ ions. It is interesting that this is not the same order of catalytic activity which was observed in the production of esters [6] by the addition of carboxylic acids to alkenes using ionexchanged sheet silicates. In the ester case, Cti+-exchanged montmorillonite was a very effective, catalyst and NHzexchanged material was a rather poor catalyst. In addition, although A13+exchanged fuller’s earth was the most effective catalyst for ester formation [6], this catalyst was only just over half as
386
effective as Al’+exchanged montmorillonite for the amine reaction. It therefore seems that it is necessary to ‘tune’ the catalyst for each individual reaction, and simple considerations of interlamellar cation hydrolysis alone are not enough to interpret the observed trends. It is possible that the layer charge and hence the electric field gradient experienced by the ~terc~a~d reaction may be important.
Reaction with cyclic amines A proton mechanism, Scheme 3, can also explain the products obtained from the reaction of pyrrolidine with sheet silicate catalysts, and is preferred to the oxidation-~duction mechanism [17,21] which has been proposed for the formation of analogous products over finely divided nickel and palladium catalysts.
Scheme 3.
The protonation mechanism in Scheme 3 is remarkable in that three molecules of pyrrolidine are involved in the elimination of one molecule of ammonia at the reaction site.
Reaction with ~en~y~rnine The reaction of benzylamine with A13+exchanged montmo~llonite
gave rise to dibenzylamine, which is in accord with the normal reaction of primary amines, although giving a much higher yield than with alkan-lamines. However, this is an unexpected result when compared with the reaction of both benzyl alcohol [4 ] and benzyl thiol [7] with A13+exchanged montmo~~onit~, when the polymeric material poly(phenylene-me~ylene) is obtained. The difference is presumably due to the differences in strength of the various nucleophiles in the displacement reactions [4]. In the case of benzylamine, the stronger amine nucleophile acts to displace ammonia to give dibenzylamine, whereas in the case of benzyl alcohol the stronger nucleophile is the active benzene ring, which displaces water to give a diphenylmethane derivative instead.
381
Reaction with aniline The fact that aniline failed to give products with A13+-exchanged montmorillonite is not surprising, as it would be extremely difficult for aniline to act as a nucleophile to displace ammonia from a protonated aniline molecule. Aromatic nucleophilic substitutions, although not unknown, normally require very forcing conditions.
Reaction with amines and alkenes Amines failed to react with alkenes in the presence of the montmorillonite catalysts, although nucleophiles such as water [ 21, alcohols [ 31, thiols [7] and even carboxylic acids [6] had been shown to add on to protonated alkene molecules in the interlamellar region, This failure can be attributed to the higher basicity of amines, which would result in the amines reacting preferentially with the available interlamellar protons, so that the normal carbocation addition mechanism, which is observed with water, alcohols, thiols and acids, is blocked.
Acknowledgements We acknowledge, with thanks, the interest and financial support of the B.P. Research Centre, Sunbury-on-Thames, for studentships to M.R. and K.J.W. We also record our appreciation to Dr. W. Jones and Miss L. J. Williamson of Cambridge University for assistance in preparing some of the catalysts used in the survey and for some of the X-ray diffraction measurements.
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13 (a) W. Jones,
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
D. T. B. Tennakoon, J. M. Thomas, L. J. Williamson, J. A. Ballantine and J. H. Purnell, Proc. Znd. Acad. Sci. (Chem. Sci.), 92 (1983) 27; (b) J. M. Thomas, in M. S. Whittingham and A. J. Jacobson (eds.), Intercalation Chemistry, Academic Press, New York, 1982, Chapt. 3. C. F. Winaus and H. Adkins, J. Am. Chem. Sot., 54 (1932) 307. K. Kinder, G. Melamed and D. Matthies, Ann,, 644 (1961) 23. K. Kinder, Ann., 485 (1931) 113. N. Yoshimina, I. Moritani, T. Shimumura and S. I. Murahashi, J. Am. Chem. SOC., 95 (1973) 3038. Bui-The-Kai, C. Concilio and G. Porzi, J. Organometall. Chem., 208 (1981) 249. Bui-The-Kai, C. Concilio and G. Porzi, J. Org. Chem., 46 (1981 j 1759. C. W. Jung, J. D. Fellman and P. E. Garron, Organometallics, 2 (1983) 1042. K. Kinder and D. Matthies, Ber., 96 (1963) 924. J. P. Eeckman and H. Laudelout, Kolloid Z., 178 (1961) 99. P. A. Diddams, .W. Jones, D. T. B. Tennacoon, J. M. Thomas, J. A. Ballantine and J. H. Purnell, unpublished work. A. Servais, J. J. Fripiat and A. Leonard, Bull. Sot. Chim. Fr., (1982) 617. A. Leonard, A. Servais and J. J. Fripiat, Bull. Sot. Chim. Fr., (1981) 625. J. J. Fripiat, A. Servais and A. Leonard, Bull. Sot. Chim. Fr., (1981) 635. J. A. Ballantine, J. H. Purnell and J. M. Thomas, Clay Minerals, 18 (1983) 347. J. A. Rabo, Catal. Dev. Sci. Eng., 23, (1981) 293.
Catalysis,
30 (1985) 373 - 388
373
ORGANIC REACTIONS CATALYSED BY SHEET SILICATES: INTERMOLECULAR ELIMINATION OF AMMONIA FROM PRIMARY AMINES JAMES A. BALLANTINE*, KEVIN J. WILLIAMS Department 8PP (U.K.)
of Chemistry,
J. HOWARD PURNELL, MONGKON RAYANAKORN**, University
College of Swansea,
Singleton
Park, Swansea,
SA2
and JOHN M. THOMAS Department
of Physical
Chemistry,
University
of Cambridge,
Cambridge,
CB2 1EP (U.K.)
(Received March 23,1984)
Summary
(1) Certain ion-exchanged montmorillonites catalyse the intermolecular elimination of ammonia from primary amines to produce dialkylamines. The yields are significant with cycloalkylamines and benzylamine but poor in the cases of alkan-lamines and alkan-2-amines. (2) Cyclic amines, such as pyrrolidine, also evolved ammonia to give coupled, ring-opened products such as 1,4di-(l-pyrrolidyl)butane. (3) However, those ion-exchanged sheet silicate catalysts which catalyse the addition of either water, or alcohols, or thiols or carboxylic acids to alkenes failed to effect the addition of amines to the corresponding alkenes. (4) The products from these amine reactions are consistent with the involvement of interlamellar protons, but the reactions have no counterparts in the proton-catalysed solution chemistry of amines. Similar products have been observed previously, however, over the surfaces of finely divided nickel, palladium and ruthenium.
Introduction
As part of an extensive study of the catalytic activity of ion-exchanged sheet silicates [ 11, we have shown that these materials can act as efficient acidic heterogeneous catalysts for a wide variety of organic reactions, including (a) ether formation by addition of water to alkenes [2], (b) ether *Author to whom correspondence should be addressed. **Present address: Department of Chemistry, Chiang Mai University, Chiang Mai, Thailand. 0304-5102/85/$3.30
@ Elsevier Sequoia/Printed in The Netherlands
374
formation by the addition of alcohols to alkenes [3,4], (c) ester formation by the addition of carboxylic acids to alkenes [ 5,6], (d) ether formation by the intermolecular dehydration of alcohols [3,4] and (e) thioether formation by the intermolecular elimination of hydrogen sulphide from thiols f71. The present publication describes the extension of this study into an investigation of the behaviour of amine compounds under the influence of ion-exchanged sheet silicate catalysts. Although the intercalation of amines into montmorillonites has been studied extensively for many years using a variety of physical techniques [ 8 - 121, there have been no previous reports that amines undergo chemical changes in these systems. The wealth of physical evidence suggested that most intercalated amines were simply protonated, at least in part, by acidic centres in the sheet silicates [ 13 1. The research described in this publication has established that primary amines can be converted to secondary amines, via an intermolecular elimination of ~monia, in the presence of these ion-exchanged sheet silicates. Reactions of this type are unknown in the solution chemistry of amines with homogeneous acidic catalysts, but similar products have been obtained at high temperatures in the presence of f?nely divided nickel [14,15], or palladium [16,17] as heterogeneous catalysts, and also with ruthenium complexes [18 - 201 as homogeneous catalysts. In these cases, which are all catalysed by transition metals, o~da~on/~duction me~h~isms involving intermediacy of the imine have been proposed [17,20,21]. This publication reports on the reactions of a variety of amines in the presence of ion-exchanged sheet silicate (montmorillonite) catalysts, and also on attempts to add amines to alkenes under similar conditions.
Methods Preparation of the ion-exchanged catalysts Cation exchange was effected by gradually adding the finely divided montmo~loni~ (100 g), (usually ‘Wyo~g bentonite’ supplied by B.D.H. Ltd., Poole, U.K.) to solutions of the appropriate cation salts (800 cm3, 0.5 M) with vigorous stirring for 3 h. Typical ionexchanged catalysts have been made using aluminium, copper or ammonium sulphates, dilute sulphuric acid or chromic chloride solutions. The resultant suspension was centrifuged for 30 min at 1000 G, and most of the water removed by deception. The solid material was carefully removed from the centrifuge tube in such a way that the very dense material {e.g. quartz, feldspar, haematite and magnetite) at the bottom of the tube was left behind. The purified slurry was washed with de-ionised water (800 cm3) with vigorous stirring for 1 h before centrifugation at 1000 G for 30 min, to remove water and dense material again. This washing and separation stage was repeated five or six times until chemical tests showed the washings to be free of the appropriate anion.
376
connected to a 4-station Perkin-Elmer Sigma 1OB chromatographic data system, were used for quantitative analysis of the reaction products, whereas Pye Model 104 instruments were used for both manual preparative collection of products prior to ‘H NMR spectroscopic analysis, and for combined gas chromato~phy-rn~ spectrome~ (GC-MS) analysis. For these analyses three interchangeable stainless steel columns were employed; (a) 8 ft X l/8 in o.d. packed with 15% Carbowax 20M/2% KOH on 120 - 140 mesh Chromosorb G AW DMCS, (b) 3 ft X l/8 in o.d. with identical packing or (c) 3 Et X l/8 in o.d. packed with 8% Apiezon M/2% KOH on 120 - 140 mesh Chromosorb P AW DMCS. Nitrogen was used as carrier gas at a flow rate of 20 cm3 min-i at 100 psi. The oven temperature was programmed, typically from 60 to 220 “C at 8 “C mm-‘, with injector and detector held at 250 “C. Injection volumes for quantitative analyses were 0.5 ~1. Glass columns of wider bore (8 ft X l/4 in o.d.) and injection volumes of 2.0 ~1 were employed for the manual collection of individual products in m.p. tubes for ‘II NMR analysis on a Vat&n XL-100 ~st~ment. GC-MS analyses were carried out using an A.E.I. Model MS9 mass spectrometer, modemised with a replacement electronic console and solid state magnet by Mass Spectrometry Services Ltd. The spectrometer was coupled to the gas chromatograph via an A.E.I. silicone rubber membrane single-stage separator and was fitted with a V.G. Model 2015 data system with cartridge disc storage. Helium was used at 20 cm3 mm-’ (70 psi) as carrier gas. Calibrations of the flame ionisation detectors were carried out by repetitive injection of the individual pure components at different concentrations in a suitable solvent, so as to produce a detector response plot for each product at optimised flow rates. Pure samples of the components were either purchased, synthesised or collected by preparative gas chrom~~aphy. In this way the raw area units, obtained from the Sigma 10 data station, could be related directly to the mass of each product. Tabulated yields are expressed in weight % of the total material present in the analysed product mixture after removal of the catalyst. Results Reactions of alkan-1 amines with ion-exchanged montmorillonite A number of alkan-l-amines were treated in the sealed reactor with A13’ ionexchanged montmorillonite at temperatures in excess of 200 “C for varying periods of time. The yields of products for two sets of conditions (215 “C for 14 h and 210 “C for 50 h) are summarized in Table 1. In general, a rather slow reaction took place in which a small amount of the primary amine was converted to a secondary amine with loss of ammonia. A small quantity of the Schiff base was usually observed in addition. It is interesting to note that no alkenes were observed as products in these reactions, even after 50 h reaction, when in most cases more than 85% of the alkan-l-amine was returned unchanged.
93.5 86.3
93.1
94.1
96.5
97.1
ethanamine propan-l amine
butan-l amine
2methylpropan-l-amine
pentan-l amine
hexan-l -amine
Nethylethsnaminea Npropylpropan-1 -aminea N-(1-propylidine)propan-1-aminec N-butylbutan-l-aminea N-(1-butylidine)butan-lamineb N-( 2methylpropyl)2-methylpropan-l-amine’ N-(2-methyl-1propylidine)-2methylpropan-1 -amineC Npentylpentan-l-amineC N-(1pentylidine)pentan-l-amineC Nhexylhexan-1 -amineC N-( 1 -hexylidine)hexan-1 amineC
Product(s)
aIdentified by comparison with commercial sample. bIdentified by comparison with synthetic sample. CIdentified by analysis of mass spectral data. dData for reaction at 210 “C for 50 h in parentheses.
Unreacted amine (wt.%)
Reactant
(4.7) (14.6) (0.8) (11.2) (1.4) (8.3)
1.5 2.0 1.2 1.7
(6.9) (2.1) (3.9) (1.6)
1.0 (1.1)
6.5 13.0 0.7 5.3 1.6 4.9
Wt.%d
GC-MS product analysis from reaction of alkan-l-amines with A13+exchanged montmorillonite
TABLE 1
157(6) 185(7) -
127(5)
78( 18) lOl( 7) 129(7) 129(4)
Molec ion (abd%)
GC-MS data
57(100),
30(81
j 100(100), 44(35), 30(34) 98(100), 84(24j, 43(48) 114( loo), 44( 59). 30(44) 140(23), 112( loo), 70( 23)
84(96),
58( loo), 44( 27), 30( 93) 72( loo), 43( 28): 30( 98) 44(l), 30(3), 28( 100) SS(lOO), 44(87), 30(82) 84(3), 56( 2), 28( 100) 86(100), 57(35), 30(69)
Fragment ions (abd%)
for 14 h at 215 “C!
98.8
94.6
butan-2amine
2-methylpropan-2-amine
69.7
93.2
cyclohexylamine
cycloheptyl~ine
as for Table 1.
81.8
cyclopentylamine
Footnotes:
Unreacted amine Wt.%
Reactant
0.8 0.4 0.8 0.4 4.0 1.4
(2.6) (1.9) (1.6) (0.7) (1.1) (3.4)
Wt.%c
montmorillonite
12.8 5.3 29.8 0.6 4.9 2.0
N-(cyclopentyl)cyclopentylamineC N-(cyclopentylidine)cyclopentyhuninec N-(cyelohexyl)cyclohexylaminea N(cyclohexylidine)cyclohexylamineb N-(~cloheptyl~y~ohep~l~~e~ N(cycloheptylidine)cycloheptylamineC
(28.0) (4.4) (56.6) (0.8) (25.2) (1.7)
Wt.%d
Product
86( 29), 58( 7), 44( 100) 84( 5), 42( 6), ZS(lO0) 100(65), 58(28), 44(100) 98(72), 57(33), 42(100) 55( ZO;, 41(100), 39( 36)
Fragment ions (abd%)
163(18) 151(35) 181(g) 179( 29) 209(11) 207( 14)
Molec ion (abd%)
GC-MS data
124(100), 122(36), 138(100), 136(57), 166(17), 150(32),
84(12), 56(37) 84(64), 54(100) 56(71), 44(53) 98(100), 54(55) 152(100), 56(48) 112(86), 55(100)
Fragment ions (abd%)
for 14 h at 215 “C
101(l) 99(l) 56(8Oj
Molec ion (abd%)
GC-MS data
for 14 h at 215 “C
with A13+excbanged montmorillonite
N-(prop-2-yljpropan-2-aminec N-( 2-propylid~e)pro~n-2-~ine~ N-(but-2-yl)butan-2-amineC N-( 2-butylidine)hutan-2-aminec 2-methylpropenea alkene oligome&
Product
GC-MS product analysis from reaction of cycloalkylamines
TABLE 3
as for Table 1.
98.8
propan-a-amine
Footnotes:
Unreacted
Reactant
GC-MS product analysis from reaction of alkan-2-amines with Al’*-exchanged
TABLE 2
379
The structures of the secondary amines were established to be, in every case, the di-(alk-l-yl)amines, by comparison of their GC-MS characteristics with either authentic or synthetic materials. Reactions of alkan-2-amines with ion-exchanged montmorillonite The reactions of alkan-!&mines with Al’+ ionexchanged montmorillonite were even slower than with the corresponding alkan-l-an-&es and very low yields of the di-(alk-2-yl)amines were obtained (Table 2). A small amount of alkene was obtained only in the case of 2-methylpropan-2-amine by unimolecular elimination of ammonia. Reaction of cycloalkylamines with ion-exchanged montmorillonite The reactions of cycloalkylamines with A13+ ionexchanged montmorillonite were considerably faster than with the corresponding alkan-lamines. High yields of the di-(cycloalkyl)amines were obtained along with small yields of the corresponding Schiff bases under identical conditions (Table 3). It is interesting that the yield of secondary amine obtained from cyclohexylamine was considerably higher than from either cyclopentylamine or cycloheptylamine. The effect of temperature on the rate of the reaction of cyclohexylamine with A13+exchanged montmorillonite (Fig. 1) showed that
Fig. 1. Effect of reaction time and temperature on the yield of dicyclohexylamine the reaction of cyclohexylamine with A13+-exchanged montmorillonite.
from
380
although the reaction was negligible below 160 “C it proceeded rapidly at 220 “c. Reaction of benzylamine with ion-exchanged montmorillonite Benzylamine gave an excellent yield of dibenzylamine in a very clean reaction when treated with A13+exchanged montmorihonite (Table 4). Reaction of cyclic secondary amines with ionexchanged montmorillonite Pyrrolidine reacted with A13+exchanged montmorillonite in a very clean reaction to give two products; 4-(1-pyrrolidyl)butan-l-amine and 1,4di-( 1-pyrrolidyl)butane, in addition to unreacted pyrrolidine only (Table 4). When the reaction was allowed to proceed for longer times, the relative yield of the latter product increased at the expense of the former, suggesting that the former was an intermediate in the formation of the latter. The structures of the two products were confirmed by facile syntheses from lithium aluminium hydride reduction of the corresponding pyrrolidyl succinamines, which were prepared by aminolysis of the appropriate succinate esters with pyrrolidine. Piperidine also reacted with A13+exchanged montmorillonite in the same way to give the corresponding ring-opened products, but in much reduced yield (Table 4). Reaction of aniline with ionexchanged montmorillonite Aniline was reacted with A13+exchanged montmorillonite at 210 “C for periods up to 50 h, and GLC analysis of the reaction mixture established that no reaction had taken place, only unchanged aniline being detected. Reactions using cyclohexylamine hydrochloride A series of comparative reactions was carried out in which cyclohexylamine was mixed with anhydrous cyclohexylamine hydrochloride in a molar ratio of 2:1, and reacted together above 200 “C in the presence and absence of A13+exchanged montmorillonite. The hydrochloride was found to dissolve completely at this concentration at about 130 “C. At the end of the reaction the products were extracted into water and ether, basified with solid potassium hydroxide and the ether layer analysed. It was found that in the absence of the sheet silicate catalyst the mixture reacted to produce a reasonable quantity of dicyclohexylamine after 12 h reaction at 200 “C, but less than that obtained from the reaction between cyclohexylamine and A13+exchanged montmorillonite (Table 5). A blank reaction proved that cyclohexylamine showed little tendency to give the diamine in the absence of catalyst. Influence of the exchanged cation on the catalytic activity of ion-exchanged montmorillonites in amine reactions A wide variety of cationexchanged sheet silicates and related heterogeneous catalysts have been investigated for their efficiency in the cyclo-
68.0
97.5
80.2
pyrrolidine
piperidine
benzylamine
4-( 1-pyrrolidyl)butan-1-amineb 1,4-di-(l-pyrrolidyl)butaneb 5-( 1-piperidyl)pentan-1-amineb 1,5-di-(1-piperidyl)pentaneb N-( benzyl)benzylaminea
Product
27.1 4.2 1.2 0.7 19.8
(41.3) (14.2) (1.3) (0.6) (32.4)
Wt.%”
142( 1) 196( 1) 170(2) 238( 1) 197(4)
Molec ion (ahd%)
GC-MS data
2.7 (10.0)
97.3 (90.0)
86.6
none
Al%-exch. mont.
cyclohexylamine + hydrochloride salt of cyclohexylsmine
cyclohexylamine + hydrochloride salt of cyclohexylamine
aparentheses refer to reactions for 12 h at 200 “C.
13.4
0.1 11.1 (32.1)
99.9 88.9 (67.9)
none Al*exch.
cyclohexylamine cyclohexylamine mont.
N-(cyclohexyl)cyclohexylamine (product wt.%)a
Cyclohexylamine (reactant wt.%)a
Catalyst
84(100), 71(10), 70(17), 42(16) 110(7), 97(17), 84(100), 71(23) 98(iOO), 84(11), 55(8), 41(11) 154(7), 124(11), 98(100), 42(11) 106(45,91, loo), 28(49)
Fragment ions (ahd%)
Reactants
Analysis of products from reactions with cyclohexylsmine hydrochloride for 4 h at 210 “C
TABLE 5
Footnotes: as for Table 1.
Unreacted amine (wt.%)
Reactant
GC-MS product analysis from reaction of various cyclic amines with A13*exchanged montmorillonite for 14 h at 215 “C!
TABLE 4
382 TABLE 6 Relative reactivity of various heterogeneous dicyclohexylamine after 4 h at 220 “C
catalysts as measured by production
Catalyst
Relative reactivity
Al%exchanged mineral colloid (RHSR) A13+exchanged Wyoming bentonite (BDH) NHJexchanged Wyoming bentonite (BDH) H+exchanged Wyoming bentonite (BDH) A13+-exchanged Ca-D montmorillonite (ACC) H+exchanged mineral colloid (RHSR) Co2+-exchanged Wyoming bentonite (BDH) Ni2+exchanged Wyoming bentonite (BDH) Mg2+exchanged Wyoming bentonite (BDH) Cr*exchanged Wyoming bentonite (BDH) A13+exchsnged Gelwhite L (GKC) vanadylexchanged Wyoming bentonite (BDH) non-exchanged (Ca) Ca-D montmorillonite (ACC) Al’+exchanged fuller’s earth (H & W) Al*exchanged ferruginous smectite (SCMR) nonexchanged Fulcat 22A (LAB) Al*exchanged Ca montmorillonite SAx-1 (SCMR) nonexchanged Surrey powder (LAB) Ca2+-exchanged Wyoming bentonite (BDH) non-exchanged Fulcat 14 (LAB) nonexchanged Gelwhite L (GKC) non-exchanged ferruginous smectite (SCMR) H”exchanged Gelwhite L (GKC) nonexchanged (Na) Wyoming bentonite (BDH) non-exchanged mineral colloid (RHSR) H+ Nafion (D) Al*exchanged Barasym Smm-100 (SCMR) hydroxylated aluminium oxychloride (RC) RE zeolite (WRG) no catalyst
100 91 90 87 87 82 79 78 73 72 69 59 57 55 54 52 49 47 35 32 30 26 24 22 19 14 13 4 1 0
of
Footnotes : Sources of original unexchanged material : RHSR - Dr. R. H. S. Robertson BDH - British Drug Houses, U.K. ACC - America Colloid Co., U.S.A. LAB - Laporte Industries Ltd., U.K. GKC - Georgia Kaolin Co., U.S.A. H&W - Hopkin & Williams, U.K. SCMR - Source Clay Mineral Repository, U. of Missouri, U.S.A. D - E. I. DuPont de Nemours, U.S.A. RC - Reheis Co., U.S.A. WRG - W. R. Grace Co., Davison Division, U.S.A.
hexylamine reaction; Table 6 gives an approximate order of catalytic activity as measured by the yield of dicyclohexylamine produced after 4 h reaction at 220 “C in the sealed reactor. Under these conditions, the best catalyst produced a 24.5% yield of dicyclohexylamine.
383
6
12
18
24
Fig. 2. Comparison of the efficiency of NH4+- and A13+-zxchanged montmorillonites as catalysts for the conversion of cyclohexylamine into dicyclohexylamine at 200 “C.
In addition, it was observed that the act&y of certain catalysts varied with reaction time. For although A13+exchanged montmorillonite was a much better catalyst at 200 “C!than was NHzexchanged montmorillonite at an early stage in the reaction, after 10 h the situation was reversed and NH:exchanged material was the better catalyst (Fig. 2). This may have some relation to the fact that in zeolite chemistry, ammonium ions are known to produce protons on heating. Reaction of alkenes with alkanamines in the presence of ion-exchanged montmorillonite The reaction of a wide variety of alkenes with alkanamines in the presence of ionexchanged montmorillonites failed to give any addition products, and the starting materials were recovered with little change. Discussion Reaction with primary amines Sheet silicates, when exchanged with certain cations, are able to act as heterogeneous catalysts for the conversion of primary amines into secondary amines at temperatures above 200 “C. The reaction is slow with alkan-l-
384
amines and alkan-2-amines, but is much easier with cycloalkylamines and with benzylamine. There is a considerable volume of spectroscopic evidence [8 - 13,24 263 for the occurrence of protonation of intercalated amines in sheet silicates and it seems likely that the conversion to secondary amines involves a protonation mechanism, as shown in Scheme 1. This is directly analogous to the mechanism already proposed for the formation of di-(alk-l-yl) ethers from alkan-l-ols [4] and for the formation of di-(alk-l-yl) sulphides from alkan-1-thiols [ 71.
R-CH2-NH2
interlamellar HS
’
R-CHz->H, I H&CH2-R 1
R-CH,-NH-CH,-R
d
-NH3
R-YH2
+NH*---CH2--R
Scheme 1
It is to be noted that the product is a di-(alk-1-yl)amine, and hence any alternative mechanism which involves the intermediacy of either carbocations or alkenes must be invalidated [ 4,6 J. The mechanism in Scheme 1 involves an SN2 intermolecular elimination reaction where an unprotonated amine acts as a nucleophile to displace ammonia from a protonated amine molecule. Such reactions are quite unknown in the solution chemistry of amines, but it has been possible to simulate this type of reaction with a measure of success by heating cyclohexylamine with its hydro~lo~de salt in the absence of catalyst in the sealed reactor. It is possible that sheet silicates are effective catalysts because the micro-environment of the interlamellar region provides a limited source of highly acidic protons, so that both protonated and unprotonated species can co-exist in proximity close enough for substitution reactions to take place within these he~rogeneo~ catalysts. The conversion of primary to secondary amines is already known as a facile reaction over finely divided nickel [ 14,151 or palladium [16,17] at high temperatures, or over homogeneous ruthenium complexes [ 19,201. In all of these cases mechanisms involving oxidation-reduction processes have been proposed, as in Scheme 2. The key steps in the oxidation-~duction process are dehydrogenation to an imine, which then couples with an amine to given an unstable gem-
385
R-CH2-NH2
finely divided Ni
--Hz
+ R-CH=NH I
H2N-CH2-R
1 R-7H
-NH3
<
R-YH-NHz
NH-CH2-R
N-CH*--R Ni
+Hz
\
Scheme 2. Oxidation-reduction
R-CH2-NH-CH2-R mechanism of Winaus and Adkins [14].
diamino derivative. This eliminates ammonia easily to give a Schiff base, which is hydrogenated over the catalyst to the product. Very small amounts of Schiff base are often observed in the sheet silicate reactions, so that it would appear that there is a small tendency for the dehydrogenation and coupling steps to occur in these catalysts also. However in the case of our catalysed reactions, both A13+- and NHz-exchanged montmorillonites are efficient catalysts for the reaction, and it seems very unlikely that an oxidation-reduction mechanism involving a hydrogenation step would be effective in the absence of traditional transition metal hydrogenation catalysts such as Ni, Pd or Ru. In addition, the remarkable results from the competitive reactions between mixtures of cyclohexylamine and benzylamine in the presence of sheet silicate catalysts [27] can be rationalised completely on the basis of a protonation model, but would be uninterpretable using the oxidationreduction model. The protonation mechanism is therefore preferred for the sheet silicatecatalysed reactions. The survey of the reaction of cyclohexylamine with a variety of different catalysts (Table 6) has established that the nature of the cation, not unnaturally, dictates the catalytic activity of these sheet silicate materials. This is to be expected, just as in analogous instances of catalysis by cationexchanged zeolites [28], owing to the occurrence of cation hydrolysis [M”+ + Hz0 + M--OH’“- l)+ + H+]. The most effective catalysts were found to be montmorillonites which had been exchanged with Al 3+, NH: or H+ ions. It is interesting that this is not the same order of catalytic activity which was observed in the production of esters [6] by the addition of carboxylic acids to alkenes using ionexchanged sheet silicates. In the ester case, Cti+-exchanged montmorillonite was a very effective, catalyst and NHzexchanged material was a rather poor catalyst. In addition, although A13+exchanged fuller’s earth was the most effective catalyst for ester formation [6], this catalyst was only just over half as
386
effective as Al’+exchanged montmorillonite for the amine reaction. It therefore seems that it is necessary to ‘tune’ the catalyst for each individual reaction, and simple considerations of interlamellar cation hydrolysis alone are not enough to interpret the observed trends. It is possible that the layer charge and hence the electric field gradient experienced by the ~terc~a~d reaction may be important.
Reaction with cyclic amines A proton mechanism, Scheme 3, can also explain the products obtained from the reaction of pyrrolidine with sheet silicate catalysts, and is preferred to the oxidation-~duction mechanism [17,21] which has been proposed for the formation of analogous products over finely divided nickel and palladium catalysts.
Scheme 3.
The protonation mechanism in Scheme 3 is remarkable in that three molecules of pyrrolidine are involved in the elimination of one molecule of ammonia at the reaction site.
Reaction with ~en~y~rnine The reaction of benzylamine with A13+exchanged montmo~llonite
gave rise to dibenzylamine, which is in accord with the normal reaction of primary amines, although giving a much higher yield than with alkan-lamines. However, this is an unexpected result when compared with the reaction of both benzyl alcohol [4 ] and benzyl thiol [7] with A13+exchanged montmo~~onit~, when the polymeric material poly(phenylene-me~ylene) is obtained. The difference is presumably due to the differences in strength of the various nucleophiles in the displacement reactions [4]. In the case of benzylamine, the stronger amine nucleophile acts to displace ammonia to give dibenzylamine, whereas in the case of benzyl alcohol the stronger nucleophile is the active benzene ring, which displaces water to give a diphenylmethane derivative instead.
381
Reaction with aniline The fact that aniline failed to give products with A13+-exchanged montmorillonite is not surprising, as it would be extremely difficult for aniline to act as a nucleophile to displace ammonia from a protonated aniline molecule. Aromatic nucleophilic substitutions, although not unknown, normally require very forcing conditions.
Reaction with amines and alkenes Amines failed to react with alkenes in the presence of the montmorillonite catalysts, although nucleophiles such as water [ 21, alcohols [ 31, thiols [7] and even carboxylic acids [6] had been shown to add on to protonated alkene molecules in the interlamellar region, This failure can be attributed to the higher basicity of amines, which would result in the amines reacting preferentially with the available interlamellar protons, so that the normal carbocation addition mechanism, which is observed with water, alcohols, thiols and acids, is blocked.
Acknowledgements We acknowledge, with thanks, the interest and financial support of the B.P. Research Centre, Sunbury-on-Thames, for studentships to M.R. and K.J.W. We also record our appreciation to Dr. W. Jones and Miss L. J. Williamson of Cambridge University for assistance in preparing some of the catalysts used in the survey and for some of the X-ray diffraction measurements.
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