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Polymer-bound crown ethers. III. effect of polymeric matrix properties on the rate of crotonization
Reactive Polymers, 12 (1990) 193-200 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
193
P O L Y M E R - B O U N D C R O W N E T H E R S . III. E F F E C T OF P O L Y M E R I C M A T R I X P R O P E R T I E S O N T H E R A T E OF C R O T O N I Z A T I O N * A. ZITSMANIS * *, M. KLYAVINSH, A. ROSKA and I. BASHKIROVA
Laboratory of Sorbents, Institute of Applied Biochemistry, U.S.S.R. Academy of Sciences, Olaine 229014 ( U.S. S. R.) (Received October 13, 1988; accepted in revised form October 14, 1989)
Condensation of aromatic aldehydes with nitromethane or fluorene in the presence of polymer-bound crown ethers complexed with alkali metal or ammonium salts and hydroxides was used to study the kinetics of these catalytic reactions. The course of the reactions was found to depend on the properties of the polymeric matrix used, the rate of condensation being affected by the sorption of substrates and reaction products and by the swelling capacity of the matrix.
INTRODUCTION
The knowledge of how a polymeric matrix affects the rate of reactions being catalysed is important for the development of new types of polymer-bound catalysts. A view has been voiced that the polymeric matrix acts solely as a carrier of catalytically active groups [2,3]; the catalytic activity of such groups can be potentiated b y the removal of steric hindrance b y way of introducing a spacer group between the active site and the polymer chain itself [4]. However, according to several reports, a polymer matrix may be capable of
extracting substrates and reagents from the reaction mixture, thus increasing their concentration in the vicinity of catalytically active groups [5] and contributing to the total effect of the p o l y m e r - b o u n d catalyst. We examined the effects of the polymeric matrix on the reaction rate using the condensation of 4-dimethylaminobenzaldehyde with fluorene and nitromethane in the presence of polymerb o u n d crown ethers complexed with potassium and a m m o n i u m salts (hydroxides) as catalysts.
EXPERIMENTAL
* Paper presented at the 4th International Symposium on Polymer Supported Reactions in Organic Chemistry, Barcelona, Spain, June 26-July 1, 1988. For communication II, see Ref. [1]. * * To whom correspondence should be addressed, 0923-1137/90/$03.50
All solvents were dried and purified by distillation prior to use. Polymer-bound crown ethers (PCE) were prepared b y condensation of dibenzo-18-crown-6 with formaldehyde and toluene, anisole, resorcinol or xylene [6] or
© 1990 - Elsevier Science Publishers B.V.
194
with poly(styrene-co-piperazino(1)-methylstyrene-co-divinylbenzene) according to published procedures [6]. PCE complexed with metal and ammonium salts (hydroxides) were obtained as described in the literature [6]. PMR spectra in DMSO-d 6 solution were recorded on a Bruker WH-90 spectrometer, Chemical shifts were measured relative to tetramethylsilane (TMS). UV spectra were obtained with a Carl Zeiss Specord M-40 spectrometer, and IR spectra with a Unicam SP 200 G apparatus. The melting points of cornpounds were determined on a Mettler FP52 apparatus. Elemental analysis was conducted on a Perkin-Elmer 240 B instrument, Polymer-bound crown ethers ( i v a - f and Va and b) were prepared and their properties studied as described elsewhere [6]. The study of the kinetics of 4-dimethylaminobenzaldehyde condensation with nitromethane was carried out according to the procedure in Part II [1]. The kinetic parameters of the condensation reaction were evaluated by measuring the absorbance of the reaction product (9-benzylidenefluorene) in solution at ?~= 401 nm. To this end, a mixture of fluorene (25 ml, 0.05 mol/1) and 4-dimethylaminobenzaldehyde (25 ml, 0.05 mol/1) solutions and a PCE complex with potassium hydroxide (0.7 g, 6.3 × 10 - 4 mol) were placed in a thermostatically controlled 100-cm3 glass cell and supplemented with solvent (50 ml). The stirred reaction
CHiNOs,• c4POH
A r --
I
~
=
Ar-CHmCH--NO~II
~ Ar--CH ~
mixture was sampled every 20 min. The sampie material was diluted 1000-fold and the absorbance of the solution was measured (pathlength in the cuvette 10 mm). A similar measuring procedure was applied with other concentrations of reagents and catalyst.
RESULTS AND D I S C U S S I O N The use of complexes of inorganic compounds with polycondensation polymerbound crown ethers as catalysts for crotonization reactions is highly attractive, e.g. the reaction of 4-dimethylaminobenzaldehyde with nitromethane and fluorene (Scheme 1). The catalysts can be prepared by condensation of dibenzo-18-crown-6 with formaldehyde and aromatic compounds (toluene, xylene, phenol, resorcinol or anisole). The concentration of aromatic compounds in the course of preparation of PCE IV can be easily controlled (Scheme 2). Such an approach allows one to obtain cross-linked polymers with desirable matrix hydrophobicity. The properties of polycondensation PCE IV were compared with those (catalytic activity included) of PCE V, whose crown ether groups were fixed on a styrene-divinylbenzene copolymer matrix. These complexes, when used as catalysts in the condensation reactions under study ( P C E - N H a O O C C H 3 for the synthesis
(~O
(t..~
in e] ~'-(~CH30~C61"IJ.~J'~2",~'-(HO)I(~H3j~.)4-(.CHJ)2-NCoH#" Scheme I.
195 tAO'h
0
IV
ArH2 = a) CH~C6HI~ bJ ~-(CN~)2C6H4~ ¢) HOC~H~ a) 2~4-(0H)zC~H,~ e~ HaCOCeI44 ; f ) nil
Va -- 15 % dlvlnylbenzene, V b --
Z
400% poroge~ -- i6ooctane
% divlnyibe~z~ne
Scheme 2.
of II and P C E - K O H for the synthesis of Ill), considerably accelerate the reactions when compared with the same processes carried out without any catalyst or in the presence of metal salt alone (Figs. I and 2). It should be pointed out that the efficiency of crown ether-catalysed reactions increased with diminishing polarity of the medium (Figs. 1 and 2). The smallest change between the rates of reactions catalysed by the salt compared with the crown ether complex has been
observed in polar solvents (ethanol or acetonitrile), and the largest in a nonpolar medium (benzene). This is apparently due to solvation changes accompanied by changes in the reactivity of inorganic salts in various solvents. A comparison of the rate of condensation of nitromethane and 4-dimethylaminobenzaldehyde in the presence of complex IVf with that of monomeric dibenzo-18-crown-6
¢'rv-ck9 29 3.0 t.6 2,0
1.2 Q
0.5 t.0 0.4 ~0
60
Fig. 1. Plot of ln(c°/c °
90 't, rnin
vs. time in the reaction of 4-dimethylaminobenzaldehyde with nitromethane at 25 ° C in the presence of PCE IVf-NH4OAc complex - C)
(10 mol%) in acetonitrile (A), ethanol (O), benzene (11), in acetonitrile with monomeric dibenzo-18-crown-6 (zx), in acetonitrile with inorganic salt (o), and in acetonitrile without catalyst (n).
~a.--gO
~0
t00
Fig. 2. Plot of ln(c°/c °
140
1~0
f, rain
vs. time in the reaction of 4-dimethylaminobenzaldehyde with fluorene at 75 o C in the presence of PCE I V f - K O H complex (10 tool%) in acetonitrile (A), ethanol (e), benzene (11), in acetonitrile with monomeric dibenzo-18-crown-6 (zx), in acetonitrile with inorganic salt (o), and in acetonitrile without catalyst (Q). -
c ~)
196 4
a
*z
t0
zO Yr, mm "~
,,.
0.4 0o~
O.2
2,O
0o~ 1.o
. . . ~ _ ~ .--9....~
.
~
.
t0 gO ~O40~0 60 70 ~O Ar~mo! %
t.zS
2.s
s.a t ~ c moVt Fig. 3. Starting reaction rate constant (k) plotted vs. the concentration of 4-dimethylaminobenzaldehyde (zx), nitromethane (o), PCE IVf-NH4OAc catalytic complex (n), and vs. surface area of PCE IVf catalyst particles (e) at 25 °C in acetonitrile.
and ammonium acetate revealed that the polymeric catalyst was more efficient than the monomeric crown ether. In an attempt to characterize the capacity of the polymer matrix to accelerate the reaction, we investigated in greater detail the rate of condensation in the presence of catalysts of various compositions, By varying the condensation of each of the reagents in turn, we have established that the process can be described b y a first-order reaction for each particular reagent, while the reaction overall follows a second-order pattern (Fig. 3). To determine the effect of reagent extraction in the polymer phase, we examined in detail the sorption of reaction components into polycondensation polymers IV as a function of the content and type of aromatic condensation comonomer. Analysis of substrate sorption onto polymers in the condensation reactions under study demonstrates that the concentration of reactants can differ dramatically from that in the surrounding liquid phase, depending on the reaction medium, the properties of the reacting agents
"Fig. 4. Concentration of nitromethane ( - - - - - ) and fluorene ( ) in the solid phase as a function of polymer type and composition: PCE IVa (-), PCE IVh (A), PCE IVd (e), and PCE IVe (o) (at 25°C in acetonitrile; starting concentration of all substrates 0.025 tool/l).
and the polymer matrix composition. It should be pointed out that the observed p h e n o m e n a are further complicated b y changes in the swelling ability of polymers IV depending on the nature of the solvent. Comparison of substrate sorption onto polymers (Figs. 4 and 5) shows that an increase in the content of hydrophobic aromatic segments in the matrix promotes sorption of aromatic reagents, the maximum substrate
~ oE o,4 6 o.a
'~ a,o~
.~___ ~ ~..~'~ ~
0.o
o.z
. ~ . , . . ~ _
4.o.
o.t
---~"'-7,. ~ ~ - . o . . ~o 2o
~o 4o so oo 7o 8 o
A~ mo~ ~,
Fig. 5. Sorption of 4-dimethylaminobenzaldehyde onto polymers as a function of their type, composition (IVa: ,,, IVd: o, IVe: o) and swelling (IVa: A, IVd: II, IVe: e) at 25 ° C, 4-dimethylaminobenzaldehyde concentration 0.025 tool/1 in solvent ( - - , ethanol; - - - - - , acetonitrile).
197 sorption corresponding to a certain o p t i m u m concentration. Nitromethane, a hydrophilic reagent, is absorbed onto PCE IV to a much lesser extent than aromatic reagents (approximately half the concentration in the liquid phase). In some polymers IV, the concentrations of fluorene and 4-dimethylaminobenzaldehyde in the polymer phase exceed more than 10-fold their concentration in solution, Thus, as soon as equilibrium is achieved between the fluorene solution (0.025 mol/1) in acetonitrile and crosslinked polymer IVf (in the solid state), the latter binds to its surface and upper layer the fluorene solution, (0.430 m o l / l as calculated from the volume of the polymer in the solid state). The fluorene concentration in the solid phase has thus increased 17.2-fold in comparison with the surrounding solution (the liquid phase). A 10.0fold molar excess of 4-dimethylaminobenzaldehyde from the ethanol solution (0.025 mol/1) moves into polymer I v f (in the solid phase), the 4-dimethylaminobenzaldehyde concentration reaching a value of 0.25 mol/1, Growing solvent polarity does not enhance sorption in the case of hydrophobic reagents, despite concurrent increase in polymer swelling. Thorough measurements of sorption from the reaction medium onto the polymer performed for the reagent (R) 4-dimethylaminobenzaldehyde and the substrate (S) nitromethane or fluorene reveal that the polymer is capable of redistributing R and S concentration, either increasing or decreasing it. In our view, differences in the reaction rate in the case of PCE and monomers can be due to (1)concentration (dilution)of reagents and substrates in the polymer phase; or (2) the cooperative effect of adjacent groups on the catalytic group or the intermediate state, We believe that changes in the reagent and substrate concentrations in the polymer phase, as compared to those of the surrounding solu-
tion, are caused by the interactions between the former and the polymer backbone or the other groups contained in the polymer. This interaction can have a binding or repelling effect, thus increasing or decreasing the R and S concentrations in the polymer phase. The steric hindrances caused by an excessive degree of polymer crosslinking, as well as by insufficient swelling of the polymer in the chosen solvent, generally lead to decreased R and S concentrations in the polymer phase in comparison with the solution surrounding the polymer. The cooperative effects of polymer structure manifest themselves in different ways. Polymer-bound functional groups locate (or dislocate) the R and S molecules in the polymer network so as to make it more or less easy for these molecules to enter a particular reaction. Secondly, the cooperative interaction can cause the polarization of R and S molecules, thus promoting the course of reaction with them. The aforesaid fully applies also to polymer-bound catalyst groups. The cooperative interactions are responsible for the more or less favourable position of crown ether groups in the polymer network, as well as for their degree of polarization. The polarization is closely connected with the complexforming ability of crown ethers and hence with their catalytic activity. The cooperative effect is the sum of several interactions between the polymer and the monomeric reagents. It is quite possible that the effects of some interactions are still unknown to us. First of all, in the course of the reaction hydrophobic (or hydrophilic) interactions between sufficiently closely located units of the polymer backbone affect the catalytic group in the polymer or the intermediate state (or states). This interaction can be modified by introducing various numbers of more hydrophilic (resorcinol, phenol) or more hydrophobic (xylene, toluene) aromatic segments into polymer IV during its formation. Since at least some of the crown ether groups are
198 located in close proximity to one another, the complexation of cations in the polymer network may be either promoted or hindered. The above process is accompanied by modified anion reactivities: they increase or decrease, respectively. In view of the aforesaid, the kinetic equation for the reaction occurring in the homogeneous ( 3 ) a n d heterogeneous (4)phase would assume different forms:
(ic) action of the PCE backbone on the reaction rate under study: ~ i e = rcaa-----zc (8) rh Fobs -- ~alc ic = rh (9)
r h = kobsJR] [S]
(3)
The total effect (i~; the sum of the two effects) corresponds to the ratio of the reaction values of the observed and homogeneous reactions:
Fob~= ~':ob~[R] IS ] Fcalc= kobs[R] [S ]
(4) (5)
Fobs i~ = ie + i c = r----h
(6)
It is apparent that i~ can assume values above or below 1, depending on whether the given polymer absorbs or repels the reagent and substrate. The parameter i c can be either positive or negative, the " + " sign being indicative of a positive, promoting cooperative effect. The data summarized in Table 1 support the possibility of potentiation of both the extracting and the cooperative effect of the polymer. In the first case this is attained by selecting polymers compatible with R and S, and in the second by introduction of optimum amounts of effective groups into the polymer capable of stabilizing the catalytic group or the intermediate products or states. For instance, a PCE based on styrene-divinylbenzene copolymers appears much more suitable for the reaction of 4-dimethylaminobenzaldehyde (Ig) with fluorene (Reaction 2) than with nitromethane (Reaction 1), the latter being diluted in the polymer phase to a greater extent than in the organic solvent, i.e. being incompatible with the PCE matrix (rows 1-5 in Table 1). At the same time, the total matrix effect on the condensation rate of 4-dimethylaminobenzaldehyde and fluorene was much less pronounced than expected on the basis of excellent sorption of both the reagent and substrate in the polymer phase (i~ was equal
kob s =
kh[cat ]
kobs = It[cat] (7) where r h is the rate of the homogeneous reaction; Fobs and ~ ¢ are the observed and calculated rates of reaction with PCE; k h is the observed reaction rate constant in homogeneous conditions; ICobs is the observed reaction rate constant in the presence of PCE; [R], [S] are the reagent and substrate concentrations in the liquid phase; and [P,.], [5] are the reagent and substrate concentrations found experimentally in the polymer phase, Introduction of experimental [R] and [S] values into the kinetic equation (4) for the reaction catalysed by PCE, and the use of reaction rate constants obtained under homogeneous conditions with the same concentrations, the other conditions being equal [eqn. (5)], leads to the following possibilities for the calculated reaction rate ~alc (5): (1) Fobs = ~alc, if differences in the rate values are due to redistribution of R and S concentrations; (2) robs ~ ~1¢, if other influences are exerted on the reaction, e.g. the influence of adjacent groups on the catalyst group or on the intermediate state, If ~1~ or (Fobs -- ~1~) is divided by rh, one can obtain parameters characterizing the effects of the extracting (i~) and cooperative
(10)
199 TABLE 1 Effect of PCE structure on the rate of 4-dimethylaminobenzaldehyde condensation with nitromethane and fluorene a No.
Reagents
Catalyst
R
S
1 2 3 4 5 6 7
lg
CH3NO 2 lVf IVa IVb IVe IVd Va Vb
8 9 10 11 12 13 14
Ig
fluorene
Parameters characterizing reaction rate b Rate constant, 1/mol s Rate, tool/1 s 10 7 X kob s
IVf IVa IVb IVe IVd Va Vb
107 X
Effect of polymer on reaction rate b i ic iy ~:obs 101°X r b 101°)< robs 101°)< rcalc e ~
0.36
1.20 2.60 2.24 1.88 1.56 8.52 15.17
2.25
7.50 16.25 14.00 11.76 9.75 53.25 86.14
97.60
74.00 34.80 57.20 79.50 64.85 180.00 217.30
61.00
46.20 21.75 35.72 18.80 42.30 112.50 145.25
~
1.50 3.60 3.89 18.40 1.16 0 48.15 10492 390 156 595 460 0 476
0.7 1.6 1.7 8.2 0.5 0 21.4 172.0 6.4 2.6 9.8 7.5 0 7.8
2.6 3.3 5.6 7.2 4.5 6.2 - 2.9 5.3 3.8 4.3 23.7 23.7 16.8 38.2 -171.2 -- 6.0 -- 2.0 -- 9.4 -- 6.8 1.8 -- 5.4
0.8 0.4 0.6 0.4 0.7 1.8 2.4
a Liquid-solid-solid system with PCE and anhydrous ammonium acetate in ethanol at 25 o C (Nos. 1-7) or potassium hydroxide in acetonitrile at 75 °C (Nos. 8-14). b Errors: 107 x kobs = -+_0.0001; 107 X l¢obs= _+0.03; 101° X robs= +0.03; 101° × rcalc = +0.03; i e = -+0.02; i c = -+0.07; i~ = _+0.09.
to 172). T h e c o o p e r a t i v e effect of the m a t r i x is clearly u n f a v o u r a b l e in this case. App a r e n t l y , in the p o l y m e r gel p h a s e the b u l k y f l u o r e n e m o l e c u l e c a n o t easily a s s u m e the spatial p o s i t i o n n e c e s s a r y for the r e a c t i o n to occur. In the m a c r o p o r o u s p o l y m e r phase, w h e r e the p o r e size c o n s i d e r a b l y exceeds the d i m e n s i o n s of the f l u o r e n e molecule, the coo p e r a t i v e effect is positive ( i c = 1 . 8 ) d e s p i t e the lack o f a n e x t r a c t i n g effect o n the p a r t of the p o l y m e r m a t r i x (row 13 in T a b l e 1). R e a c t i o n s using p o l y m e r - b o u n d catalysts generally p r o c e e d m o r e slowly in c o m p a r i s o n with those involving the c o r r e s p o n d i n g m o n o mers. T h e s u m of e x t r a c t i n g a n d c o o p e r a t i v e effects leads to r e d u c e d r e a c t i o n rates w h e n p o l y m e r i c catalysts are e m p l o y e d . R e a c t i o n s p r o c e e d m o r e slowly if the c o n c e n t r a t i o n s of R, S, or b o t h in the p o l y m e r p h a s e are l o w e r t h a n those in the c o r r e s p o n d i n g solution a n d w h e n the c o o p e r a t i v e effect is negative. T h e
p h e n o m e n o n o b s e r v e d b y us c o u l d b e interp r e t e d as h a v i n g a s o m e w h a t u n i q u e character b e c a u s e at p r e s e n t such cases (showing o p p o s i t e effects) are rare; t h e r e f o r e it is clear t h a t p o l y m e r s t r u c t u r e design is n e c e s s a r y for each particular reaction. T h e effect o f solvent o n r e a c t i o n s with P C E is m o r e c o m p l i c a t e d , b e c a u s e p o l y m e r swelling a n d the d i s t r i b u t i o n of R a n d S b e t w e e n the liquid a n d solid p h a s e also p l a y a role. F o r q u a n t i t a t i v e assessment o n e c a n use a rule k n o w n f r o m c h r o m a t o g r a p h y stating that polar, and particularly hydrophilic, solvents c o n t r i b u t e to the d i s t r i b u t i o n o f R a n d S i n t o the h y d r o p h o b i c p o l y m e r i c p h a s e a n d vice versa. C o n s e q u e n t l y , a p a r t f r o m the k n o w n adv a n t a g e s of P C E (easy s e p a r a t i o n of r e a c t i o n mixture, possibility of r e p e a t e d use, etc.), b y using r a t i o n a l l y d e s i g n e d p o l y m e r s the reaction rate c a n b e a c c e l e r a t e d or slowed down.
200
REFERENCES 1 A. Zitsmanis, A. Roska and M. Klyavinsh, Polymerbound crown ethers. II. Catalysis of the several condensation reactions by polymer-bound dibenzo-18crown-6, Reactive Polym., 9 (1988)67. 2 M. Tomoi, Y. Hosokawa and H. Kakiuchi, Phase transfer reactions catalyzed by phosphonium salts bound to macroporous polystyrene supports, J. Polym. Sci., Polym. Chem. Ed., 22 (1984) 1243. 3 S.L. Regen, D. Bolikal and C. Barcelon, Triphase catalysis. Backbone structure-activity relationships, J. Org. Chem., 461 (1981) 2511.
4 P.L. Anelli, B. Czech, F. Montanari and S. Quici, Reaction mechanism and factors influencing phasetransfer catalytic activity of crown ethers bonded to a polystyrene matrix, J. Amer. Chem. Soc., 106 (1984) 861. 5 S.L. Regen and A. Nigam, Selectivity features of polystyrene-based triphase catalysts, J. Amer. Chem. Soc., 100 (1978)7773. 6 A. Zitsmanis, A. Roska and M. Klyavinsh, Polymerbound crown ethers. I. Synthesis and complexation of polymer-bound benzocrown ethers, Reactive Polym., 9 (1988) 59.
193
P O L Y M E R - B O U N D C R O W N E T H E R S . III. E F F E C T OF P O L Y M E R I C M A T R I X P R O P E R T I E S O N T H E R A T E OF C R O T O N I Z A T I O N * A. ZITSMANIS * *, M. KLYAVINSH, A. ROSKA and I. BASHKIROVA
Laboratory of Sorbents, Institute of Applied Biochemistry, U.S.S.R. Academy of Sciences, Olaine 229014 ( U.S. S. R.) (Received October 13, 1988; accepted in revised form October 14, 1989)
Condensation of aromatic aldehydes with nitromethane or fluorene in the presence of polymer-bound crown ethers complexed with alkali metal or ammonium salts and hydroxides was used to study the kinetics of these catalytic reactions. The course of the reactions was found to depend on the properties of the polymeric matrix used, the rate of condensation being affected by the sorption of substrates and reaction products and by the swelling capacity of the matrix.
INTRODUCTION
The knowledge of how a polymeric matrix affects the rate of reactions being catalysed is important for the development of new types of polymer-bound catalysts. A view has been voiced that the polymeric matrix acts solely as a carrier of catalytically active groups [2,3]; the catalytic activity of such groups can be potentiated b y the removal of steric hindrance b y way of introducing a spacer group between the active site and the polymer chain itself [4]. However, according to several reports, a polymer matrix may be capable of
extracting substrates and reagents from the reaction mixture, thus increasing their concentration in the vicinity of catalytically active groups [5] and contributing to the total effect of the p o l y m e r - b o u n d catalyst. We examined the effects of the polymeric matrix on the reaction rate using the condensation of 4-dimethylaminobenzaldehyde with fluorene and nitromethane in the presence of polymerb o u n d crown ethers complexed with potassium and a m m o n i u m salts (hydroxides) as catalysts.
EXPERIMENTAL
* Paper presented at the 4th International Symposium on Polymer Supported Reactions in Organic Chemistry, Barcelona, Spain, June 26-July 1, 1988. For communication II, see Ref. [1]. * * To whom correspondence should be addressed, 0923-1137/90/$03.50
All solvents were dried and purified by distillation prior to use. Polymer-bound crown ethers (PCE) were prepared b y condensation of dibenzo-18-crown-6 with formaldehyde and toluene, anisole, resorcinol or xylene [6] or
© 1990 - Elsevier Science Publishers B.V.
194
with poly(styrene-co-piperazino(1)-methylstyrene-co-divinylbenzene) according to published procedures [6]. PCE complexed with metal and ammonium salts (hydroxides) were obtained as described in the literature [6]. PMR spectra in DMSO-d 6 solution were recorded on a Bruker WH-90 spectrometer, Chemical shifts were measured relative to tetramethylsilane (TMS). UV spectra were obtained with a Carl Zeiss Specord M-40 spectrometer, and IR spectra with a Unicam SP 200 G apparatus. The melting points of cornpounds were determined on a Mettler FP52 apparatus. Elemental analysis was conducted on a Perkin-Elmer 240 B instrument, Polymer-bound crown ethers ( i v a - f and Va and b) were prepared and their properties studied as described elsewhere [6]. The study of the kinetics of 4-dimethylaminobenzaldehyde condensation with nitromethane was carried out according to the procedure in Part II [1]. The kinetic parameters of the condensation reaction were evaluated by measuring the absorbance of the reaction product (9-benzylidenefluorene) in solution at ?~= 401 nm. To this end, a mixture of fluorene (25 ml, 0.05 mol/1) and 4-dimethylaminobenzaldehyde (25 ml, 0.05 mol/1) solutions and a PCE complex with potassium hydroxide (0.7 g, 6.3 × 10 - 4 mol) were placed in a thermostatically controlled 100-cm3 glass cell and supplemented with solvent (50 ml). The stirred reaction
CHiNOs,• c4POH
A r --
I
~
=
Ar-CHmCH--NO~II
~ Ar--CH ~
mixture was sampled every 20 min. The sampie material was diluted 1000-fold and the absorbance of the solution was measured (pathlength in the cuvette 10 mm). A similar measuring procedure was applied with other concentrations of reagents and catalyst.
RESULTS AND D I S C U S S I O N The use of complexes of inorganic compounds with polycondensation polymerbound crown ethers as catalysts for crotonization reactions is highly attractive, e.g. the reaction of 4-dimethylaminobenzaldehyde with nitromethane and fluorene (Scheme 1). The catalysts can be prepared by condensation of dibenzo-18-crown-6 with formaldehyde and aromatic compounds (toluene, xylene, phenol, resorcinol or anisole). The concentration of aromatic compounds in the course of preparation of PCE IV can be easily controlled (Scheme 2). Such an approach allows one to obtain cross-linked polymers with desirable matrix hydrophobicity. The properties of polycondensation PCE IV were compared with those (catalytic activity included) of PCE V, whose crown ether groups were fixed on a styrene-divinylbenzene copolymer matrix. These complexes, when used as catalysts in the condensation reactions under study ( P C E - N H a O O C C H 3 for the synthesis
(~O
(t..~
in e] ~'-(~CH30~C61"IJ.~J'~2",~'-(HO)I(~H3j~.)4-(.CHJ)2-NCoH#" Scheme I.
195 tAO'h
0
IV
ArH2 = a) CH~C6HI~ bJ ~-(CN~)2C6H4~ ¢) HOC~H~ a) 2~4-(0H)zC~H,~ e~ HaCOCeI44 ; f ) nil
Va -- 15 % dlvlnylbenzene, V b --
Z
400% poroge~ -- i6ooctane
% divlnyibe~z~ne
Scheme 2.
of II and P C E - K O H for the synthesis of Ill), considerably accelerate the reactions when compared with the same processes carried out without any catalyst or in the presence of metal salt alone (Figs. I and 2). It should be pointed out that the efficiency of crown ether-catalysed reactions increased with diminishing polarity of the medium (Figs. 1 and 2). The smallest change between the rates of reactions catalysed by the salt compared with the crown ether complex has been
observed in polar solvents (ethanol or acetonitrile), and the largest in a nonpolar medium (benzene). This is apparently due to solvation changes accompanied by changes in the reactivity of inorganic salts in various solvents. A comparison of the rate of condensation of nitromethane and 4-dimethylaminobenzaldehyde in the presence of complex IVf with that of monomeric dibenzo-18-crown-6
¢'rv-ck9 29 3.0 t.6 2,0
1.2 Q
0.5 t.0 0.4 ~0
60
Fig. 1. Plot of ln(c°/c °
90 't, rnin
vs. time in the reaction of 4-dimethylaminobenzaldehyde with nitromethane at 25 ° C in the presence of PCE IVf-NH4OAc complex - C)
(10 mol%) in acetonitrile (A), ethanol (O), benzene (11), in acetonitrile with monomeric dibenzo-18-crown-6 (zx), in acetonitrile with inorganic salt (o), and in acetonitrile without catalyst (n).
~a.--gO
~0
t00
Fig. 2. Plot of ln(c°/c °
140
1~0
f, rain
vs. time in the reaction of 4-dimethylaminobenzaldehyde with fluorene at 75 o C in the presence of PCE I V f - K O H complex (10 tool%) in acetonitrile (A), ethanol (e), benzene (11), in acetonitrile with monomeric dibenzo-18-crown-6 (zx), in acetonitrile with inorganic salt (o), and in acetonitrile without catalyst (Q). -
c ~)
196 4
a
*z
t0
zO Yr, mm "~
,,.
0.4 0o~
O.2
2,O
0o~ 1.o
. . . ~ _ ~ .--9....~
.
~
.
t0 gO ~O40~0 60 70 ~O Ar~mo! %
t.zS
2.s
s.a t ~ c moVt Fig. 3. Starting reaction rate constant (k) plotted vs. the concentration of 4-dimethylaminobenzaldehyde (zx), nitromethane (o), PCE IVf-NH4OAc catalytic complex (n), and vs. surface area of PCE IVf catalyst particles (e) at 25 °C in acetonitrile.
and ammonium acetate revealed that the polymeric catalyst was more efficient than the monomeric crown ether. In an attempt to characterize the capacity of the polymer matrix to accelerate the reaction, we investigated in greater detail the rate of condensation in the presence of catalysts of various compositions, By varying the condensation of each of the reagents in turn, we have established that the process can be described b y a first-order reaction for each particular reagent, while the reaction overall follows a second-order pattern (Fig. 3). To determine the effect of reagent extraction in the polymer phase, we examined in detail the sorption of reaction components into polycondensation polymers IV as a function of the content and type of aromatic condensation comonomer. Analysis of substrate sorption onto polymers in the condensation reactions under study demonstrates that the concentration of reactants can differ dramatically from that in the surrounding liquid phase, depending on the reaction medium, the properties of the reacting agents
"Fig. 4. Concentration of nitromethane ( - - - - - ) and fluorene ( ) in the solid phase as a function of polymer type and composition: PCE IVa (-), PCE IVh (A), PCE IVd (e), and PCE IVe (o) (at 25°C in acetonitrile; starting concentration of all substrates 0.025 tool/l).
and the polymer matrix composition. It should be pointed out that the observed p h e n o m e n a are further complicated b y changes in the swelling ability of polymers IV depending on the nature of the solvent. Comparison of substrate sorption onto polymers (Figs. 4 and 5) shows that an increase in the content of hydrophobic aromatic segments in the matrix promotes sorption of aromatic reagents, the maximum substrate
~ oE o,4 6 o.a
'~ a,o~
.~___ ~ ~..~'~ ~
0.o
o.z
. ~ . , . . ~ _
4.o.
o.t
---~"'-7,. ~ ~ - . o . . ~o 2o
~o 4o so oo 7o 8 o
A~ mo~ ~,
Fig. 5. Sorption of 4-dimethylaminobenzaldehyde onto polymers as a function of their type, composition (IVa: ,,, IVd: o, IVe: o) and swelling (IVa: A, IVd: II, IVe: e) at 25 ° C, 4-dimethylaminobenzaldehyde concentration 0.025 tool/1 in solvent ( - - , ethanol; - - - - - , acetonitrile).
197 sorption corresponding to a certain o p t i m u m concentration. Nitromethane, a hydrophilic reagent, is absorbed onto PCE IV to a much lesser extent than aromatic reagents (approximately half the concentration in the liquid phase). In some polymers IV, the concentrations of fluorene and 4-dimethylaminobenzaldehyde in the polymer phase exceed more than 10-fold their concentration in solution, Thus, as soon as equilibrium is achieved between the fluorene solution (0.025 mol/1) in acetonitrile and crosslinked polymer IVf (in the solid state), the latter binds to its surface and upper layer the fluorene solution, (0.430 m o l / l as calculated from the volume of the polymer in the solid state). The fluorene concentration in the solid phase has thus increased 17.2-fold in comparison with the surrounding solution (the liquid phase). A 10.0fold molar excess of 4-dimethylaminobenzaldehyde from the ethanol solution (0.025 mol/1) moves into polymer I v f (in the solid phase), the 4-dimethylaminobenzaldehyde concentration reaching a value of 0.25 mol/1, Growing solvent polarity does not enhance sorption in the case of hydrophobic reagents, despite concurrent increase in polymer swelling. Thorough measurements of sorption from the reaction medium onto the polymer performed for the reagent (R) 4-dimethylaminobenzaldehyde and the substrate (S) nitromethane or fluorene reveal that the polymer is capable of redistributing R and S concentration, either increasing or decreasing it. In our view, differences in the reaction rate in the case of PCE and monomers can be due to (1)concentration (dilution)of reagents and substrates in the polymer phase; or (2) the cooperative effect of adjacent groups on the catalytic group or the intermediate state, We believe that changes in the reagent and substrate concentrations in the polymer phase, as compared to those of the surrounding solu-
tion, are caused by the interactions between the former and the polymer backbone or the other groups contained in the polymer. This interaction can have a binding or repelling effect, thus increasing or decreasing the R and S concentrations in the polymer phase. The steric hindrances caused by an excessive degree of polymer crosslinking, as well as by insufficient swelling of the polymer in the chosen solvent, generally lead to decreased R and S concentrations in the polymer phase in comparison with the solution surrounding the polymer. The cooperative effects of polymer structure manifest themselves in different ways. Polymer-bound functional groups locate (or dislocate) the R and S molecules in the polymer network so as to make it more or less easy for these molecules to enter a particular reaction. Secondly, the cooperative interaction can cause the polarization of R and S molecules, thus promoting the course of reaction with them. The aforesaid fully applies also to polymer-bound catalyst groups. The cooperative interactions are responsible for the more or less favourable position of crown ether groups in the polymer network, as well as for their degree of polarization. The polarization is closely connected with the complexforming ability of crown ethers and hence with their catalytic activity. The cooperative effect is the sum of several interactions between the polymer and the monomeric reagents. It is quite possible that the effects of some interactions are still unknown to us. First of all, in the course of the reaction hydrophobic (or hydrophilic) interactions between sufficiently closely located units of the polymer backbone affect the catalytic group in the polymer or the intermediate state (or states). This interaction can be modified by introducing various numbers of more hydrophilic (resorcinol, phenol) or more hydrophobic (xylene, toluene) aromatic segments into polymer IV during its formation. Since at least some of the crown ether groups are
198 located in close proximity to one another, the complexation of cations in the polymer network may be either promoted or hindered. The above process is accompanied by modified anion reactivities: they increase or decrease, respectively. In view of the aforesaid, the kinetic equation for the reaction occurring in the homogeneous ( 3 ) a n d heterogeneous (4)phase would assume different forms:
(ic) action of the PCE backbone on the reaction rate under study: ~ i e = rcaa-----zc (8) rh Fobs -- ~alc ic = rh (9)
r h = kobsJR] [S]
(3)
The total effect (i~; the sum of the two effects) corresponds to the ratio of the reaction values of the observed and homogeneous reactions:
Fob~= ~':ob~[R] IS ] Fcalc= kobs[R] [S ]
(4) (5)
Fobs i~ = ie + i c = r----h
(6)
It is apparent that i~ can assume values above or below 1, depending on whether the given polymer absorbs or repels the reagent and substrate. The parameter i c can be either positive or negative, the " + " sign being indicative of a positive, promoting cooperative effect. The data summarized in Table 1 support the possibility of potentiation of both the extracting and the cooperative effect of the polymer. In the first case this is attained by selecting polymers compatible with R and S, and in the second by introduction of optimum amounts of effective groups into the polymer capable of stabilizing the catalytic group or the intermediate products or states. For instance, a PCE based on styrene-divinylbenzene copolymers appears much more suitable for the reaction of 4-dimethylaminobenzaldehyde (Ig) with fluorene (Reaction 2) than with nitromethane (Reaction 1), the latter being diluted in the polymer phase to a greater extent than in the organic solvent, i.e. being incompatible with the PCE matrix (rows 1-5 in Table 1). At the same time, the total matrix effect on the condensation rate of 4-dimethylaminobenzaldehyde and fluorene was much less pronounced than expected on the basis of excellent sorption of both the reagent and substrate in the polymer phase (i~ was equal
kob s =
kh[cat ]
kobs = It[cat] (7) where r h is the rate of the homogeneous reaction; Fobs and ~ ¢ are the observed and calculated rates of reaction with PCE; k h is the observed reaction rate constant in homogeneous conditions; ICobs is the observed reaction rate constant in the presence of PCE; [R], [S] are the reagent and substrate concentrations in the liquid phase; and [P,.], [5] are the reagent and substrate concentrations found experimentally in the polymer phase, Introduction of experimental [R] and [S] values into the kinetic equation (4) for the reaction catalysed by PCE, and the use of reaction rate constants obtained under homogeneous conditions with the same concentrations, the other conditions being equal [eqn. (5)], leads to the following possibilities for the calculated reaction rate ~alc (5): (1) Fobs = ~alc, if differences in the rate values are due to redistribution of R and S concentrations; (2) robs ~ ~1¢, if other influences are exerted on the reaction, e.g. the influence of adjacent groups on the catalyst group or on the intermediate state, If ~1~ or (Fobs -- ~1~) is divided by rh, one can obtain parameters characterizing the effects of the extracting (i~) and cooperative
(10)
199 TABLE 1 Effect of PCE structure on the rate of 4-dimethylaminobenzaldehyde condensation with nitromethane and fluorene a No.
Reagents
Catalyst
R
S
1 2 3 4 5 6 7
lg
CH3NO 2 lVf IVa IVb IVe IVd Va Vb
8 9 10 11 12 13 14
Ig
fluorene
Parameters characterizing reaction rate b Rate constant, 1/mol s Rate, tool/1 s 10 7 X kob s
IVf IVa IVb IVe IVd Va Vb
107 X
Effect of polymer on reaction rate b i ic iy ~:obs 101°X r b 101°)< robs 101°)< rcalc e ~
0.36
1.20 2.60 2.24 1.88 1.56 8.52 15.17
2.25
7.50 16.25 14.00 11.76 9.75 53.25 86.14
97.60
74.00 34.80 57.20 79.50 64.85 180.00 217.30
61.00
46.20 21.75 35.72 18.80 42.30 112.50 145.25
~
1.50 3.60 3.89 18.40 1.16 0 48.15 10492 390 156 595 460 0 476
0.7 1.6 1.7 8.2 0.5 0 21.4 172.0 6.4 2.6 9.8 7.5 0 7.8
2.6 3.3 5.6 7.2 4.5 6.2 - 2.9 5.3 3.8 4.3 23.7 23.7 16.8 38.2 -171.2 -- 6.0 -- 2.0 -- 9.4 -- 6.8 1.8 -- 5.4
0.8 0.4 0.6 0.4 0.7 1.8 2.4
a Liquid-solid-solid system with PCE and anhydrous ammonium acetate in ethanol at 25 o C (Nos. 1-7) or potassium hydroxide in acetonitrile at 75 °C (Nos. 8-14). b Errors: 107 x kobs = -+_0.0001; 107 X l¢obs= _+0.03; 101° X robs= +0.03; 101° × rcalc = +0.03; i e = -+0.02; i c = -+0.07; i~ = _+0.09.
to 172). T h e c o o p e r a t i v e effect of the m a t r i x is clearly u n f a v o u r a b l e in this case. App a r e n t l y , in the p o l y m e r gel p h a s e the b u l k y f l u o r e n e m o l e c u l e c a n o t easily a s s u m e the spatial p o s i t i o n n e c e s s a r y for the r e a c t i o n to occur. In the m a c r o p o r o u s p o l y m e r phase, w h e r e the p o r e size c o n s i d e r a b l y exceeds the d i m e n s i o n s of the f l u o r e n e molecule, the coo p e r a t i v e effect is positive ( i c = 1 . 8 ) d e s p i t e the lack o f a n e x t r a c t i n g effect o n the p a r t of the p o l y m e r m a t r i x (row 13 in T a b l e 1). R e a c t i o n s using p o l y m e r - b o u n d catalysts generally p r o c e e d m o r e slowly in c o m p a r i s o n with those involving the c o r r e s p o n d i n g m o n o mers. T h e s u m of e x t r a c t i n g a n d c o o p e r a t i v e effects leads to r e d u c e d r e a c t i o n rates w h e n p o l y m e r i c catalysts are e m p l o y e d . R e a c t i o n s p r o c e e d m o r e slowly if the c o n c e n t r a t i o n s of R, S, or b o t h in the p o l y m e r p h a s e are l o w e r t h a n those in the c o r r e s p o n d i n g solution a n d w h e n the c o o p e r a t i v e effect is negative. T h e
p h e n o m e n o n o b s e r v e d b y us c o u l d b e interp r e t e d as h a v i n g a s o m e w h a t u n i q u e character b e c a u s e at p r e s e n t such cases (showing o p p o s i t e effects) are rare; t h e r e f o r e it is clear t h a t p o l y m e r s t r u c t u r e design is n e c e s s a r y for each particular reaction. T h e effect o f solvent o n r e a c t i o n s with P C E is m o r e c o m p l i c a t e d , b e c a u s e p o l y m e r swelling a n d the d i s t r i b u t i o n of R a n d S b e t w e e n the liquid a n d solid p h a s e also p l a y a role. F o r q u a n t i t a t i v e assessment o n e c a n use a rule k n o w n f r o m c h r o m a t o g r a p h y stating that polar, and particularly hydrophilic, solvents c o n t r i b u t e to the d i s t r i b u t i o n o f R a n d S i n t o the h y d r o p h o b i c p o l y m e r i c p h a s e a n d vice versa. C o n s e q u e n t l y , a p a r t f r o m the k n o w n adv a n t a g e s of P C E (easy s e p a r a t i o n of r e a c t i o n mixture, possibility of r e p e a t e d use, etc.), b y using r a t i o n a l l y d e s i g n e d p o l y m e r s the reaction rate c a n b e a c c e l e r a t e d or slowed down.
200
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4 P.L. Anelli, B. Czech, F. Montanari and S. Quici, Reaction mechanism and factors influencing phasetransfer catalytic activity of crown ethers bonded to a polystyrene matrix, J. Amer. Chem. Soc., 106 (1984) 861. 5 S.L. Regen and A. Nigam, Selectivity features of polystyrene-based triphase catalysts, J. Amer. Chem. Soc., 100 (1978)7773. 6 A. Zitsmanis, A. Roska and M. Klyavinsh, Polymerbound crown ethers. I. Synthesis and complexation of polymer-bound benzocrown ethers, Reactive Polym., 9 (1988) 59.