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Phase transfer catalyzed reactions of crosslinked chloromethylated polystyrene with vanillin
Reactive & Functional Polymers 50 (2002) 139–147 www.elsevier.com / locate / react
Phase transfer catalyzed reactions of crosslinked chloromethylated polystyrene with vanillin a, b a A.A. Sarhan *, A.A. El-Shehawy , M.Y. Abdelaal a
b
Chemistry Department, Faculty of Science, Mansoura University, ET-35516 Mansoura, Egypt Chemistry Department, Faculty of Education, Tanta University, Kafr El-Sheikh, Tanta ET-33516, Egypt Received 2 February 2001; received in revised form 20 June 2001; accepted 23 June 2001
Abstract Chloromethylated polystyrene was reacted with vanillin under phase transfer catalyzed conditions. This reaction was performed under different conditions, including variations of the phase transfer catalysts, the reaction solvent, the type of base, the reaction time and temperature. In addition, the effect of the reaction technique (i.e., solid–liquid–liquid and solid–liquid–solid systems) on the reaction conversion was studied. Polymer-supported vanillin was evaluated for its utilization in polymer analogue conversions by performing condensation reactions of the aldehyde functionality with malononitrile, ethyl cyanoacetate, cyanoacetamide, cyanothioacetamide, 2-aminophenol, hydrazine hydrate and hydroxylamine hydrochloride. In all cases, the reactions were followed up by means of FTIR spectroscopic analysis. 2002 Published by Elsevier Science B.V. Keywords: Polymer supported; Vanillin; PTC; Chloromethylated polystyrene
1. Introduction Soluble and insoluble polymers with pendant haloalkyl groups have been employed as starting materials for the synthesis of various functional polymers [1]. More recently, many reports have shown that phase transfer catalysts (PTC) such as quaternary onium salts and crown ethers are common reagents for the chemical modification of polymers [2]. Particularly, the chemical modification of pendant chloromethyl groups to produce various functional polymers has been extensively studied [3–6]. The selection of suitable PTC and the *Corresponding author. E-mail address: [email protected] (A.A. Sarhan).
combination of catalyst, reagent, solvent and polymer are factors of importance. We have reported previously on some factors that may affect PT-catalyzed nucleophilic displacement reactions, including P1 as alkylating agent and 1 as substrate [7]. The employed variation significantly affected the degree of conversion and selectivity of the alkylation reaction, i.e. O- versus C-alkylation [8,9]. Statistics alone would give a reactivity ratio for O- /ortho-C-alkylation of 1:1 for 2 and 1:2 for 1 (Scheme 1). The electronic effects of the o-methoxy group may significantly enhance the nucleophilicity of the phenolic OH while deactivating the nucleophilicity of the ring carbon in the meta position. The other two ring carbons are strongly deactivated by the electron with-
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trometer using CDCl 3 as solvent and SiMe 4 as internal standard. Chemical shifts are expressed in d ppm units.
2.1. General procedure for PTC alkylation of 2 with P1
Scheme 1. The possible active sites for the nucleophilic displacement reaction of vanillin (2) and 4-hydroxybenzaldehyde (1) with P1.
drawal effect of the o-aldehydic group. Also, the aldehyde group in the polymer-supported vanillin (P2) facilitates the follow up of the reaction on the basis of its FTIR carbonyl index. Furthermore, the availability of vanillin residues in subsequent polymer analogue conversions of P2 can be examined. Accordingly, the present work aimed at supporting vanillin (2) by P1 under various phase transfer (PT) catalyzed conditions. Some factors that may affect the O-alkylation reaction of 2 and P1 were studied. We also attempted to confirm the accessibility of polymer-supported 2 and its applicability to other polymer analogue conversions.
2. Experimental All chemicals and pure grade solvents were used as supplied by Aldrich. Commercial chloromethylated polystyrene-co-DVB (P1) (MP500 A; Bayer; 2% DVB; specific surface area (BET) 66 m 2 / g and 4.6 mequiv. Cl / g) was used. All the polymeric products were washed successively with water, methanol, DMF, dioxane and, finally, diethyl ether. They were dried under vacuum at 408C for 48 h and subjected to FTIR spectroscopic analysis with KBr pellets using a Mattson 5000 FTIR spectrometer. Melting points were determined by a Stuart SMP-2 melting point apparatus. 1 H NMR spectra were recorded on a JEOL JNM-GX270 MHz spec-
2.1.1. Under SLL conditions P1 (100 mg, 0.46 mmol) was swollen in 5 ml of organic solvent for 24 h. To the swollen polymer were added 0.19 mmol of the PT catalyst, 3.68 mmol of the inorganic base dissolved in 5 ml of water and 560 mg (3.68 mmol) of 2. The reaction mixture was stirred magnetically for the desired time at 80–858C in a water bath. The reaction mixture was filtered off and the modified polymer P2 was prepared for FTIR spectroscopic analysis. The modified polymer P2 showed IR absorption peaks at 2755, 1695 and 1150 cm 21 , corresponding to the aldehydic CH, C=O and C–O–C groups, respectively. The results are presented in Fig. 1. 2.1.2. Under SLS conditions P1 (100 mg, 0.46 mmol) was swollen in 10 ml of organic solvent for 24 h. To the swollen polymer were added 0.19 mmol of the PT catalyst, 3.68 mmol of the solid inorganic base and 560 mg (3.68 mmol) of 2. The reaction mixture was stirred magnetically for the desired time at 80–858C in a water bath. The reaction mixture was filtered off and the modified polymer P2 was prepared for FTIR spectroscopic analysis. The results are presented in Fig. 1. 2.2. Reaction of P2 with 3 a–d 2.2.1. PT-uncatalyzed reaction To 1.0 g (2.93 mmol) of P2, swollen in 10 ml ethanol for 24 h, was added an excess amount (30 mmol) of 3a–d and three drops of piperidine as catalyst, except for 3d, where triethylamine was used as catalyst instead of pyridine. The reaction mixture was refluxed in an oil bath with stirring for a certain time. The reaction product P3 was filtered off and prepared for
A. A. Sarhan et al. / Reactive & Functional Polymers 50 (2002) 139 – 147
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Table 1 FTIR spectral data for P3a–d R
FTIR spectral data
C≡N COOEt
CHO absent, CN at 2224 cm 21 CHO at 1683 cm 21 (8%), CN at 2220 cm 21 , C=O (ester) at 1748 cm 21 CHO at 1684.2 cm 21 (21%), CN at 2210 cm 21 , NH 2 at 3384.6 cm 21 , C=S at 1205 cm 21 CHO absent, OH absent, CN at 2211 cm 21 , C=O (amide) at 1683.9 cm 21 , NH 2 at 3853.9 and 3406.3 cm 21
CSNH 2 CONH 2
oil bath with stirring for 20 h. The reaction products P3(a–d) were filtered off and prepared for FTIR spectroscopic analysis. All of the products obtained showed spectral data similar to that obtained for the PT-uncatalyzed reaction with no characteristic absorption for the aldehydic C=O group at 1695 cm 21 , indicating 100% reaction conversion (Table 1).
2.3. Preparation of 4 -hydroxy-3 methoxycinnamonitrile derivatives (4 a–d)
Fig. 1. Dependence of (a) the reaction conversion percentage and (b) the catalyst efficiency on the type of catalyst under SLL conditions using P1 and solid K 2 CO 3 as the two solid phases and DCE as the liquid phase, and under SLS conditions using P1 as the solid phase and aqueous solutions of K 2 CO 3 and DCE as the two liquid phases at 80–858C for 9 h.
FTIR spectroscopic analysis. The reaction conversion was 100, 92 and 79% for 3a–c, respectively, and 100% for 3d, but after 72 h reaction time. The most important IR spectroscopic data are listed in Table 1.
2.2.2. PT-catalyzed reaction To 1.0 g (2.93 mmol) of P2, swollen in 10 ml of DMF for 24 h, were added an excess amount (30 mmol) of 3a–d, 3.7 g (30 mmol) of solid K 2 CO 3 and 75 mg of BTPPC as PT catalyst. The reaction mixture was refluxed at 808C in an
A mixture of 1.52 g (10 mmol) of 2, 10 mmol of the appropriate active methylene compound 3a–d and three drops of triethylamine in 20 ml of absolute ethanol was refluxed on a steam bath for 2 h. The reaction mixture was left at room temperature for 1 day. The formed precipitate was filtered off and crystallized from ethanol to give the products 4a–d. Melting points of 4a–d were 132, 106, 209 and 2598C, respectively. 1 H NMR spectroscopic data for 4a–d are as follows. 4a (R 5 CN): 3.95 (s, 3H, OCH 3 ), 7.02–7.56 (m, 3H, aromatic), 8.11 (s, 1H, CH=C), 9.52 (s, 1H, OH). 4b (R 5 COOEt): 1.65 (t, 3H, CH 2 CH ] 3 ), 3.83 (s, 3H, OCH 3 ), 4.21 (q, 2H, CH CH ] 2 3 ), 6.91–7.41 (m, 3H, aromatic), 8.89 (s, 1H, CH=C), 10.58 (s, 1H, OH). 4c (R 5 CSNH 2 ): 3.60 (s, 3H, OCH 3 ), 6.49 (s, br., 2H, NH 2 ), 7.18–7.69 (m, 3H, aromatic), 8.64 (s, 1H, CH=C), 9.17 (s, 1H, OH). 4d (R 5 CONH 2 ): 3.82 (s, 3H, OCH 3 ), 5.62 (s, br., 2H, NH 2 ), 6.93–7.42 (m, 3H, aromatic), 8.90 (s, 1H, CH=C), 10.82 (s, 1H, OH).
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2.4. Reaction of P1 with 4 -hydroxy-3 methoxycinnamonitrile derivatives (4 a–d)
aldehyde group, indicating 100% reaction conversion.
To 100 mg (0.46 mmol) of P1, swollen in 10 ml of DMF for 24 h, were added 75 mg (0.19 mmol) of BTPPC, 506 mg (3.68 mmol) of solid K 2 CO 3 and 3.68 mmol of 4a–d. The reaction mixture was stirred magnetically for 20 h in a water bath at 80–858C. After cooling, P3(a–d) was filtered off and prepared for FTIR spectroscopic analysis. P3(a–d) showed absorption spectra identical to those obtained from the reaction of P2 with 3a–d (Table 1).
2.7. Reaction of P2 with hydroxylamine hydrochloride (7)
2.5. Reaction of P2 with 2 -aminophenol (5) Compound 5 (3.2 g, 30 mmol) was added to 1.0 g (2.93 mmol) of P2 swollen in 10 ml pyridine for 24 h. The reaction mixture was refluxed for 15 h in an oil bath with stirring. The obtained modified polymer P4 was filtered off and prepared for FTIR spectroscopic analysis. Subsequent FTIR spectroscopic analysis 21 showed an absorption peak at 1630 cm , corresponding to C=N bonding, and a very broad absorption peak in the region 3600–3100 cm 21 , corresponding to OH, in addition to absorption at 1695 cm 21 , corresponding to the C=O group of the remaining CHO functionality. The concentration of the remaining CHO was found to be 39% and, consequently, the reaction conversion was 61%.
2.7.1. PT-uncatalyzed reaction To 100 mg (0.293 mmol) of P2 suspended in 5 ml of pyridine was added 300 mg (4.5 mmol) of 7 and the reaction mixture was stirred magnetically for 6 h under reflux at 90–958C. After cooling, the resulting polymer P6 was collected by filtration and prepared for FTIR spectroscopic analysis. The final polymer P6 showed absorption at 3430 (broad, OH) and 1602 cm 21 (sharp, C=N), while the absorptions 21 21 at 2755 cm (aldehydic CH) and 1695 cm (C=O) disappeared, indicating 100% reaction conversion. 2.7.2. PT-catalyzed reaction BTPPC (74 mg, 0.19 mmol), 5 ml of a 10 wt% aqueous solution of K 2 CO 3 and 300 mg (4.5 mmol) of 7 were added to 100 mg (0.293 mmol) of P2 swollen in 5 ml 1,2-dichloroethane (DCE) for 24 h. The reaction mixture was then heated on a water bath at 80–858C and stirred magnetically for 6 h. The resulting modified polymer P6 was filtered off and prepared for FTIR spectroscopic analysis. P6 showed absorption peaks similar to those obtained in the case of the uncatalyzed reaction.
2.6. Reaction of P2 with hydrazine hydrate (6)
2.8. Reaction of P6 with some bromo derivatives (8 a–c)
To a suspension of 1.0 g (2.93 mmol) of P2, swollen in 10 ml of absolute ethanol for 24 h, was added 1.0 ml (32 mmol) of 6. The reaction mixture was refluxed in an oil bath with stirring for 15 h. The resulting product P5 was filtered off and prepared for FTIR spectroscopic analysis. This showed absorption peaks at 1627 cm 21 , corresponding to C=N bonding, and at 3340 cm 21 for NH 2 , while there was no absorption at 1695 cm 21 , corresponding to C=O of the
BTPPC (80 mg, 0.2 mmol) and 5 ml of a 10 wt% aqueous solution of K 2 CO 3 were added to 100 mg of P6, swollen in 5 ml DCE for 24 h. The mixture was then treated with 4 mmol of cyanogen bromide (8a), ethyl bromoacetate (8b) and phenacyl bromide (8c) and was stirred magnetically in a water bath under reflux at 80–858C for 25 h. After cooling, the resulting polymer P7(a–c) was collected by filtration and prepared for FTIR spectroscopic analysis. The
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obtained P7(a–c) showed no absorption at 3430 cm 21 corresponding to OH groups, indicating 100% reaction conversion.
3. Results and discussion The reaction of P1 (2% DVB, 4.6 mequiv. Cl / g) with 2 was performed under PT-catalyzed conditions, leading to the formation of a polymer-supported aldehyde functionality (P2) as shown in Scheme 2. Excess amounts of 2 (eightfold equiv. of P1) were used in the presence of catalytic amounts of the desired catalyst (5 mol% of 2). P2 showed IR absorption peaks at 1695 and 1150 cm 21 , corresponding to C=O and C–O– stretching, respectively [8,10–14]. IR spectroscopic analysis showed no evidence for the formation of C-alkylated products, indicating that O-alkylation occurred exclusively under the conditions employed to form P2. The type of catalyst, the base, the reaction time and temperature, in addition to the type of organic solvent, were the factors studied under solid– liquid–liquid (SLL) or solid–liquid–solid (SLS) systems. N-Benzyl-4-(N,N-dimethylamino) pyridinium chloride (BDMAPC), benzyltriethyl ammonium chloride (BTEAC), benzyltriphenylphosphonium chloride (BTPPC), and cetyltrimethylammonium bromide (CTMAB) were used as PT catalysts. The degree of conversion was estimated on
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the basis of the normalized net peak area of carbonyl group absorption at 1695 cm 21 versus the net reference absorption at 1595 cm 21 corresponding to the aromatic nucleus [15]. For this purpose, a FTIR calibration curve was constructed as described earlier [7].
3.1. Effect of the type of catalyst Fig. 1 shows the reaction conversion (Fig. 1a) as well as the conversion relative to the noncatalyzed systems, which reflects the catalyst efficiency (Fig. 1b). It is clear from Fig. 1a that benzyltriphenylphosphonium chloride afforded the highest conversions under both SLL (84%) and SLS (57%) conditions and the catalysts can be arranged in the following order: BTPPC . BDMAPC . CTMAB . BTEAC . blank (for SLL systems) BTPPC . CTMAB . BDMAPC . BTEAC . blank (for SLS systems) The greater efficiency of the BTPPC catalyst in comparison with the other catalysts can be attributed to the higher lipophilic character of the cation (Q 1 ) supplied by the catalyst, leading to the formation of an ion-pair with the phenolate anion ( 2 Oph) derived from 2. These ionpairs are able to enter the less polar organic media in which the displacement reaction takes
Scheme 2. The reaction of chloromethylated polystyrene (P1) with vanillin (2).
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place [16]. This has been discussed in detail previously [7]. Fig. 1b shows the dependence of the catalyst efficiency on the technique employed. Although the reaction conversion under SLL conditions is higher than that under SLS conditions, the efficiency of the catalysts under the latter conditions is greater, except for BDMAPC. This means that catalysts are more efficient under SLS conditions than under SLL conditions, with the exception of BDMAPC.
3.2. Effect of the type of base NaOH, Na 2 CO 3 , NaHCO 3 , K 2 CO 3 and KHCO 3 were selected as examples of salts possessing varying basic strengths in aqueous medium. The reaction was performed under both SLS and SLL conditions using DCE as a water-immiscible solvent and DMF as a miscible solvent. Fig. 2 shows that K 2 CO 3 is the most effective salt. SLS conditions afforded a higher conversion in polar solvents such as DMF than afforded under SLL conditions, while lower conversions were afforded in less polar solvents such as DCE. The reaction can be considered as a SL two-phase system in the case
Fig. 2. Dependence of the reaction conversion percentage on the type of base under SLL and SLS conditions using BTPPC as PT catalyst.
of DMF due to its miscibility with the aqueous phase.
3.3. Effect of reaction time Fig. 3 demonstrates the strong dependence of conversion on the reaction time. This dependence was affected by the type and amount of solvent. In accordance with our previous work [7], a minor amount of solvent, i.e. DCE, under SLL conditions afforded a high conversion (100%) after about 25 h, while a conversion of only 80% was achieved with larger amounts of DCE. On the other hand, DCE afforded the lowest conversion (76% after 25 h) under SLS conditions, while DMF afforded the highest conversion (100% after 9 h).
3.4. Effect of the type of organic solvent Fig. 4 shows that the reaction conversion increases significantly with increasing dielectric constant [17] of the solvent employed, apart from the nature of the solvent and the reaction technique, whether a SLS or a SLL system. Such a dependence can be attributed to the extraction of Q 1 – 2 Oph ion-pairs into the polymer matrix, which increases with the dielectric
Fig. 3. Dependence of the reaction conversion percentage on the reaction time under SLL and SLS conditions using BTPPC as PT catalyst and K 2 CO 3 as base.
A. A. Sarhan et al. / Reactive & Functional Polymers 50 (2002) 139 – 147
Fig. 4. Dependence of the reaction conversion percentage on the dielectric constant of the organic solvent using BTPPC as PT catalyst and K 2 CO 3 as base.
constant of the solvent. This holds true for the SLS system and the extraction of the desired ion-pairs affected mainly by the dielectric constant, while the solvent type would have a minor effect. In the case of the SLL system in the presence of water-immiscible solvents (CCl 4 , benzene (Bz), chloroform (Chl), chlorobenzene (CBz) and DCE), diffusion will be facilitated of both Q 1 Cl 2 and Na 1 Cl 2 ion-pairs into the aqueous phase or Q 1 – 2 Oph ion-pairs into the polymer matrix. The diffused Q 1 – 2 Oph ion-pairs, in this case, will be less hydrated and hence of high mobility and reactivity. This is reflected by the steeply increasing reaction conversion with the dielectric constant of the solvent, higher than that obtained under SLS conditions. On the other hand, in the presence of solvents completely or partially miscible with water (DMF, n-PrOH and n-BuOH), the concentration of water in the organic phase increases with miscibility and the system can be considered as a SL two-phase system. Both reacting and produced ion-pairs will be highly abundant. Hence, the selectivity of Q 1 for 2 Oph will decrease. Also, when the 2 Oph anion is dissolved in a water-containing organic solvent, the oxygen of the phenoxide ion is strongly hydrated via hydrogen bond formation. This means that the
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availability of the oxygen and, therefore, its nucleophilicity for nucleophilic displacements, is greatly reduced. In addition, strong hydration of 2 Oph anions decreases the mobility of Q 1 – 2 Oph ion-pairs and, hence, their diffusion into the polymer matrix. Consequently, the reaction rate decreases, which is reflected by the observed relative reduction of the conversion to values lower than that expected on the basis of the dielectric constant. Support for this solvation concept is evident from the observation that the SLL systems afforded higher conversions than the SLS systems in all of the experiments conducted in DCE in the presence of either different PT catalysts (Fig. 1) or different bases (Fig. 2). On the other hand, DMF afforded higher conversions under SLS-PTC conditions than in cases where DMF / H 2 O was used as the solvent under SL-PTC conditions (Fig. 2).
3.5. Utilization of the aldehyde group in P2 for polymer analogue conversions P2 was reacted in ethanol with malononitrile (3a), ethyl cyanoacetate (3b), cyanothioacetamide (3c) and cyanoacetamide (3d) as representatives of active methylene compounds. A catalytic amount of piperidine was used, except for 3d, where triethylamine was used as the catalyst (Scheme 3). The formation of P3(a–d) was confirmed by FTIR spectroscopic analysis and the reaction conversion was estimated on the basis of the unreacted carbonyl groups in P2. The reaction gave 100, 92 and 79% conversion after 24 h for P3(a–c), respectively, and 100% conversion for P3d after 72 h. The SLS-PTC reaction of 3a–d with P2 afforded P3(a–d) in a 100% conversion. P3(a–d) were also obtained in 100% conversion by the reaction of P1 with the nucleophiles 4a–d under SLS-PTC conditions. This was confirmed by the disappearance of the absorption at 1695 cm 21 of the aldehyde group (C=O) and the appearance of a very sharp peak in the region 2220–2190 cm 21 , corresponding to the CN group (Table 1).
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Scheme 3. Condensation reactions of P2 with active methylene compounds and amino compounds.
Compounds 4a–d were obtained from the condensation of 3a–d with 2, parallel to a reported method [18,19] and confirmed by 1 H NMR spectroscopic analysis (see Experimental). P2 reacted smoothly with excess amounts of amine compounds such as 2-aminophenol (5), hydrazine hydrate (6) and hydroxylamine hydrochloride (7), affording P4–P6, respectively. Moreover, P6 was obtained from the SLL-PTC reaction of P2 with 7. The reaction conversion was 61% for P4 and 100% for P6, obtained from both PT-catalyzed and uncatalyzed reactions, as well as for P5. This was demonstrated by the absence of the absorption at 1695 cm 21 for the aldehyde group (C=O). The resulting P6 was further reacted separately with bromo derivatives such as cyanogen bromide
(8a), ethyl bromoacetate (8b) and phenacyl bromide (8c) under SLL-PTC conditions. This afforded P7(a–c) in 100% conversion as demonstrated by the absence of the absorption at 3430 cm 21 for the OH group. Important IR spectroscopic data for P4–P7 are summarized in Table 2. The formation of P4–P7 demonstrates the practical effectiveness of the supTable 2 FTIR spectral data for P4–P7 Product
FTIR spectral data
P4
C=N at 1630 cm 21 , CHO at 1685 cm 21 (39%), OH at 3600–3100 cm 21 CHO absent, C=N at 1630 cm 21 , NH 2 at 3340 cm 21 CHO absent, C=N at 1620 cm 21 , OH at 3430 cm 21 OH absent, CN at 2226 cm 21 OH absent, C=O at 1685 cm 21
P5 P6 P7a P7b,c
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ported aldehyde functionality and its ability to take part in chemical reactions. References [1] T. Nishikubo, T. Iizawa, J. Synth. Org. Chem. Jpn. 51 (1993) 157. [2] T. Nishikubo, Phase-transfer catalysis, mechanism and synthesis, in: M.E. Halpern (Ed.), ACS Symposium Series, Vol. 659, 1995. [3] I. Artaud, I. Tomasi, G. Martin, D. Petre, D. Mansuy, Tetrahedron Lett. 36 (1995) 869. [4] C. Andraud, T. Brotin, C. Garcia, F. Pelle, P. Goldner, B. Bigot, A. Collet, J. Am. Chem. Soc. 116 (1995) 2094. [5] J. Zyss, J.F. Nicoud, Curr. Opin. Solid State Mater. Sci. 1 (1996) 533. [6] T. Iizawa, T. Nishikubo, Y. Masuda, M. Okawara, Macromolecules 17 (1984) 992. [7] A.A. Sarhan, M.Y. Abdelaal, M.M. Ali, E.H. Abdel-Latiff, Polymer 40 (1998) 233.
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[8] E.V. Dehmlow, Phase Transfer Catalysis, VCH, Weinheim, 1993. [9] A.A. Sarhan, M.Y. Abdelaal, M.M. Ali, D.M.A. Hanna (manuscript in preparation). [10] J.M.J. Frechet, M.D. Smet, M.J. Farral, J. Org. Chem. 44 (1979) 1774. [11] C.M. Starks, in: C.M. Starks, C.L. Liotta, M. Halpern (Eds.), Phase Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives, Chapman & Hall, New York, 1994. [12] H.W. Gibson, F.C. Bailey, Macromolecules 9 (1976) 10. [13] H.W. Gibson, F.C. Bailey, Macromolecules 9 (1976) 688. [14] H.W. Gibson, F.C. Bailey, J. Polym. Sci., Polym. Chem. Ed. 12 (1974) 1241. [15] K.J. Shea, D.Y. Sasaki, J. Am. Chem. Soc. 113 (1991) 4109. [16] M. Makosa, M. Fedorynski, Adv. Catal. 35 (1987) 375. [17] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 67th Edition, CRC Press, Boca Raton, FL, 1986 / 1987. [18] M.A. Abdelaziz, H.H. Moharram, S.A. Essawy, A.A. Mohamed, Phosphorus Sulfur Silicon 48 (1990) 269. [19] G.H. El-Gemeie, N.M. Fathy, A.M. Attia, J. Chem. Res. (S) (1995) 292.
Phase transfer catalyzed reactions of crosslinked chloromethylated polystyrene with vanillin a, b a A.A. Sarhan *, A.A. El-Shehawy , M.Y. Abdelaal a
b
Chemistry Department, Faculty of Science, Mansoura University, ET-35516 Mansoura, Egypt Chemistry Department, Faculty of Education, Tanta University, Kafr El-Sheikh, Tanta ET-33516, Egypt Received 2 February 2001; received in revised form 20 June 2001; accepted 23 June 2001
Abstract Chloromethylated polystyrene was reacted with vanillin under phase transfer catalyzed conditions. This reaction was performed under different conditions, including variations of the phase transfer catalysts, the reaction solvent, the type of base, the reaction time and temperature. In addition, the effect of the reaction technique (i.e., solid–liquid–liquid and solid–liquid–solid systems) on the reaction conversion was studied. Polymer-supported vanillin was evaluated for its utilization in polymer analogue conversions by performing condensation reactions of the aldehyde functionality with malononitrile, ethyl cyanoacetate, cyanoacetamide, cyanothioacetamide, 2-aminophenol, hydrazine hydrate and hydroxylamine hydrochloride. In all cases, the reactions were followed up by means of FTIR spectroscopic analysis. 2002 Published by Elsevier Science B.V. Keywords: Polymer supported; Vanillin; PTC; Chloromethylated polystyrene
1. Introduction Soluble and insoluble polymers with pendant haloalkyl groups have been employed as starting materials for the synthesis of various functional polymers [1]. More recently, many reports have shown that phase transfer catalysts (PTC) such as quaternary onium salts and crown ethers are common reagents for the chemical modification of polymers [2]. Particularly, the chemical modification of pendant chloromethyl groups to produce various functional polymers has been extensively studied [3–6]. The selection of suitable PTC and the *Corresponding author. E-mail address: [email protected] (A.A. Sarhan).
combination of catalyst, reagent, solvent and polymer are factors of importance. We have reported previously on some factors that may affect PT-catalyzed nucleophilic displacement reactions, including P1 as alkylating agent and 1 as substrate [7]. The employed variation significantly affected the degree of conversion and selectivity of the alkylation reaction, i.e. O- versus C-alkylation [8,9]. Statistics alone would give a reactivity ratio for O- /ortho-C-alkylation of 1:1 for 2 and 1:2 for 1 (Scheme 1). The electronic effects of the o-methoxy group may significantly enhance the nucleophilicity of the phenolic OH while deactivating the nucleophilicity of the ring carbon in the meta position. The other two ring carbons are strongly deactivated by the electron with-
1381-5148 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. PII: S1381-5148( 01 )00108-0
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trometer using CDCl 3 as solvent and SiMe 4 as internal standard. Chemical shifts are expressed in d ppm units.
2.1. General procedure for PTC alkylation of 2 with P1
Scheme 1. The possible active sites for the nucleophilic displacement reaction of vanillin (2) and 4-hydroxybenzaldehyde (1) with P1.
drawal effect of the o-aldehydic group. Also, the aldehyde group in the polymer-supported vanillin (P2) facilitates the follow up of the reaction on the basis of its FTIR carbonyl index. Furthermore, the availability of vanillin residues in subsequent polymer analogue conversions of P2 can be examined. Accordingly, the present work aimed at supporting vanillin (2) by P1 under various phase transfer (PT) catalyzed conditions. Some factors that may affect the O-alkylation reaction of 2 and P1 were studied. We also attempted to confirm the accessibility of polymer-supported 2 and its applicability to other polymer analogue conversions.
2. Experimental All chemicals and pure grade solvents were used as supplied by Aldrich. Commercial chloromethylated polystyrene-co-DVB (P1) (MP500 A; Bayer; 2% DVB; specific surface area (BET) 66 m 2 / g and 4.6 mequiv. Cl / g) was used. All the polymeric products were washed successively with water, methanol, DMF, dioxane and, finally, diethyl ether. They were dried under vacuum at 408C for 48 h and subjected to FTIR spectroscopic analysis with KBr pellets using a Mattson 5000 FTIR spectrometer. Melting points were determined by a Stuart SMP-2 melting point apparatus. 1 H NMR spectra were recorded on a JEOL JNM-GX270 MHz spec-
2.1.1. Under SLL conditions P1 (100 mg, 0.46 mmol) was swollen in 5 ml of organic solvent for 24 h. To the swollen polymer were added 0.19 mmol of the PT catalyst, 3.68 mmol of the inorganic base dissolved in 5 ml of water and 560 mg (3.68 mmol) of 2. The reaction mixture was stirred magnetically for the desired time at 80–858C in a water bath. The reaction mixture was filtered off and the modified polymer P2 was prepared for FTIR spectroscopic analysis. The modified polymer P2 showed IR absorption peaks at 2755, 1695 and 1150 cm 21 , corresponding to the aldehydic CH, C=O and C–O–C groups, respectively. The results are presented in Fig. 1. 2.1.2. Under SLS conditions P1 (100 mg, 0.46 mmol) was swollen in 10 ml of organic solvent for 24 h. To the swollen polymer were added 0.19 mmol of the PT catalyst, 3.68 mmol of the solid inorganic base and 560 mg (3.68 mmol) of 2. The reaction mixture was stirred magnetically for the desired time at 80–858C in a water bath. The reaction mixture was filtered off and the modified polymer P2 was prepared for FTIR spectroscopic analysis. The results are presented in Fig. 1. 2.2. Reaction of P2 with 3 a–d 2.2.1. PT-uncatalyzed reaction To 1.0 g (2.93 mmol) of P2, swollen in 10 ml ethanol for 24 h, was added an excess amount (30 mmol) of 3a–d and three drops of piperidine as catalyst, except for 3d, where triethylamine was used as catalyst instead of pyridine. The reaction mixture was refluxed in an oil bath with stirring for a certain time. The reaction product P3 was filtered off and prepared for
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Table 1 FTIR spectral data for P3a–d R
FTIR spectral data
C≡N COOEt
CHO absent, CN at 2224 cm 21 CHO at 1683 cm 21 (8%), CN at 2220 cm 21 , C=O (ester) at 1748 cm 21 CHO at 1684.2 cm 21 (21%), CN at 2210 cm 21 , NH 2 at 3384.6 cm 21 , C=S at 1205 cm 21 CHO absent, OH absent, CN at 2211 cm 21 , C=O (amide) at 1683.9 cm 21 , NH 2 at 3853.9 and 3406.3 cm 21
CSNH 2 CONH 2
oil bath with stirring for 20 h. The reaction products P3(a–d) were filtered off and prepared for FTIR spectroscopic analysis. All of the products obtained showed spectral data similar to that obtained for the PT-uncatalyzed reaction with no characteristic absorption for the aldehydic C=O group at 1695 cm 21 , indicating 100% reaction conversion (Table 1).
2.3. Preparation of 4 -hydroxy-3 methoxycinnamonitrile derivatives (4 a–d)
Fig. 1. Dependence of (a) the reaction conversion percentage and (b) the catalyst efficiency on the type of catalyst under SLL conditions using P1 and solid K 2 CO 3 as the two solid phases and DCE as the liquid phase, and under SLS conditions using P1 as the solid phase and aqueous solutions of K 2 CO 3 and DCE as the two liquid phases at 80–858C for 9 h.
FTIR spectroscopic analysis. The reaction conversion was 100, 92 and 79% for 3a–c, respectively, and 100% for 3d, but after 72 h reaction time. The most important IR spectroscopic data are listed in Table 1.
2.2.2. PT-catalyzed reaction To 1.0 g (2.93 mmol) of P2, swollen in 10 ml of DMF for 24 h, were added an excess amount (30 mmol) of 3a–d, 3.7 g (30 mmol) of solid K 2 CO 3 and 75 mg of BTPPC as PT catalyst. The reaction mixture was refluxed at 808C in an
A mixture of 1.52 g (10 mmol) of 2, 10 mmol of the appropriate active methylene compound 3a–d and three drops of triethylamine in 20 ml of absolute ethanol was refluxed on a steam bath for 2 h. The reaction mixture was left at room temperature for 1 day. The formed precipitate was filtered off and crystallized from ethanol to give the products 4a–d. Melting points of 4a–d were 132, 106, 209 and 2598C, respectively. 1 H NMR spectroscopic data for 4a–d are as follows. 4a (R 5 CN): 3.95 (s, 3H, OCH 3 ), 7.02–7.56 (m, 3H, aromatic), 8.11 (s, 1H, CH=C), 9.52 (s, 1H, OH). 4b (R 5 COOEt): 1.65 (t, 3H, CH 2 CH ] 3 ), 3.83 (s, 3H, OCH 3 ), 4.21 (q, 2H, CH CH ] 2 3 ), 6.91–7.41 (m, 3H, aromatic), 8.89 (s, 1H, CH=C), 10.58 (s, 1H, OH). 4c (R 5 CSNH 2 ): 3.60 (s, 3H, OCH 3 ), 6.49 (s, br., 2H, NH 2 ), 7.18–7.69 (m, 3H, aromatic), 8.64 (s, 1H, CH=C), 9.17 (s, 1H, OH). 4d (R 5 CONH 2 ): 3.82 (s, 3H, OCH 3 ), 5.62 (s, br., 2H, NH 2 ), 6.93–7.42 (m, 3H, aromatic), 8.90 (s, 1H, CH=C), 10.82 (s, 1H, OH).
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2.4. Reaction of P1 with 4 -hydroxy-3 methoxycinnamonitrile derivatives (4 a–d)
aldehyde group, indicating 100% reaction conversion.
To 100 mg (0.46 mmol) of P1, swollen in 10 ml of DMF for 24 h, were added 75 mg (0.19 mmol) of BTPPC, 506 mg (3.68 mmol) of solid K 2 CO 3 and 3.68 mmol of 4a–d. The reaction mixture was stirred magnetically for 20 h in a water bath at 80–858C. After cooling, P3(a–d) was filtered off and prepared for FTIR spectroscopic analysis. P3(a–d) showed absorption spectra identical to those obtained from the reaction of P2 with 3a–d (Table 1).
2.7. Reaction of P2 with hydroxylamine hydrochloride (7)
2.5. Reaction of P2 with 2 -aminophenol (5) Compound 5 (3.2 g, 30 mmol) was added to 1.0 g (2.93 mmol) of P2 swollen in 10 ml pyridine for 24 h. The reaction mixture was refluxed for 15 h in an oil bath with stirring. The obtained modified polymer P4 was filtered off and prepared for FTIR spectroscopic analysis. Subsequent FTIR spectroscopic analysis 21 showed an absorption peak at 1630 cm , corresponding to C=N bonding, and a very broad absorption peak in the region 3600–3100 cm 21 , corresponding to OH, in addition to absorption at 1695 cm 21 , corresponding to the C=O group of the remaining CHO functionality. The concentration of the remaining CHO was found to be 39% and, consequently, the reaction conversion was 61%.
2.7.1. PT-uncatalyzed reaction To 100 mg (0.293 mmol) of P2 suspended in 5 ml of pyridine was added 300 mg (4.5 mmol) of 7 and the reaction mixture was stirred magnetically for 6 h under reflux at 90–958C. After cooling, the resulting polymer P6 was collected by filtration and prepared for FTIR spectroscopic analysis. The final polymer P6 showed absorption at 3430 (broad, OH) and 1602 cm 21 (sharp, C=N), while the absorptions 21 21 at 2755 cm (aldehydic CH) and 1695 cm (C=O) disappeared, indicating 100% reaction conversion. 2.7.2. PT-catalyzed reaction BTPPC (74 mg, 0.19 mmol), 5 ml of a 10 wt% aqueous solution of K 2 CO 3 and 300 mg (4.5 mmol) of 7 were added to 100 mg (0.293 mmol) of P2 swollen in 5 ml 1,2-dichloroethane (DCE) for 24 h. The reaction mixture was then heated on a water bath at 80–858C and stirred magnetically for 6 h. The resulting modified polymer P6 was filtered off and prepared for FTIR spectroscopic analysis. P6 showed absorption peaks similar to those obtained in the case of the uncatalyzed reaction.
2.6. Reaction of P2 with hydrazine hydrate (6)
2.8. Reaction of P6 with some bromo derivatives (8 a–c)
To a suspension of 1.0 g (2.93 mmol) of P2, swollen in 10 ml of absolute ethanol for 24 h, was added 1.0 ml (32 mmol) of 6. The reaction mixture was refluxed in an oil bath with stirring for 15 h. The resulting product P5 was filtered off and prepared for FTIR spectroscopic analysis. This showed absorption peaks at 1627 cm 21 , corresponding to C=N bonding, and at 3340 cm 21 for NH 2 , while there was no absorption at 1695 cm 21 , corresponding to C=O of the
BTPPC (80 mg, 0.2 mmol) and 5 ml of a 10 wt% aqueous solution of K 2 CO 3 were added to 100 mg of P6, swollen in 5 ml DCE for 24 h. The mixture was then treated with 4 mmol of cyanogen bromide (8a), ethyl bromoacetate (8b) and phenacyl bromide (8c) and was stirred magnetically in a water bath under reflux at 80–858C for 25 h. After cooling, the resulting polymer P7(a–c) was collected by filtration and prepared for FTIR spectroscopic analysis. The
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obtained P7(a–c) showed no absorption at 3430 cm 21 corresponding to OH groups, indicating 100% reaction conversion.
3. Results and discussion The reaction of P1 (2% DVB, 4.6 mequiv. Cl / g) with 2 was performed under PT-catalyzed conditions, leading to the formation of a polymer-supported aldehyde functionality (P2) as shown in Scheme 2. Excess amounts of 2 (eightfold equiv. of P1) were used in the presence of catalytic amounts of the desired catalyst (5 mol% of 2). P2 showed IR absorption peaks at 1695 and 1150 cm 21 , corresponding to C=O and C–O– stretching, respectively [8,10–14]. IR spectroscopic analysis showed no evidence for the formation of C-alkylated products, indicating that O-alkylation occurred exclusively under the conditions employed to form P2. The type of catalyst, the base, the reaction time and temperature, in addition to the type of organic solvent, were the factors studied under solid– liquid–liquid (SLL) or solid–liquid–solid (SLS) systems. N-Benzyl-4-(N,N-dimethylamino) pyridinium chloride (BDMAPC), benzyltriethyl ammonium chloride (BTEAC), benzyltriphenylphosphonium chloride (BTPPC), and cetyltrimethylammonium bromide (CTMAB) were used as PT catalysts. The degree of conversion was estimated on
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the basis of the normalized net peak area of carbonyl group absorption at 1695 cm 21 versus the net reference absorption at 1595 cm 21 corresponding to the aromatic nucleus [15]. For this purpose, a FTIR calibration curve was constructed as described earlier [7].
3.1. Effect of the type of catalyst Fig. 1 shows the reaction conversion (Fig. 1a) as well as the conversion relative to the noncatalyzed systems, which reflects the catalyst efficiency (Fig. 1b). It is clear from Fig. 1a that benzyltriphenylphosphonium chloride afforded the highest conversions under both SLL (84%) and SLS (57%) conditions and the catalysts can be arranged in the following order: BTPPC . BDMAPC . CTMAB . BTEAC . blank (for SLL systems) BTPPC . CTMAB . BDMAPC . BTEAC . blank (for SLS systems) The greater efficiency of the BTPPC catalyst in comparison with the other catalysts can be attributed to the higher lipophilic character of the cation (Q 1 ) supplied by the catalyst, leading to the formation of an ion-pair with the phenolate anion ( 2 Oph) derived from 2. These ionpairs are able to enter the less polar organic media in which the displacement reaction takes
Scheme 2. The reaction of chloromethylated polystyrene (P1) with vanillin (2).
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place [16]. This has been discussed in detail previously [7]. Fig. 1b shows the dependence of the catalyst efficiency on the technique employed. Although the reaction conversion under SLL conditions is higher than that under SLS conditions, the efficiency of the catalysts under the latter conditions is greater, except for BDMAPC. This means that catalysts are more efficient under SLS conditions than under SLL conditions, with the exception of BDMAPC.
3.2. Effect of the type of base NaOH, Na 2 CO 3 , NaHCO 3 , K 2 CO 3 and KHCO 3 were selected as examples of salts possessing varying basic strengths in aqueous medium. The reaction was performed under both SLS and SLL conditions using DCE as a water-immiscible solvent and DMF as a miscible solvent. Fig. 2 shows that K 2 CO 3 is the most effective salt. SLS conditions afforded a higher conversion in polar solvents such as DMF than afforded under SLL conditions, while lower conversions were afforded in less polar solvents such as DCE. The reaction can be considered as a SL two-phase system in the case
Fig. 2. Dependence of the reaction conversion percentage on the type of base under SLL and SLS conditions using BTPPC as PT catalyst.
of DMF due to its miscibility with the aqueous phase.
3.3. Effect of reaction time Fig. 3 demonstrates the strong dependence of conversion on the reaction time. This dependence was affected by the type and amount of solvent. In accordance with our previous work [7], a minor amount of solvent, i.e. DCE, under SLL conditions afforded a high conversion (100%) after about 25 h, while a conversion of only 80% was achieved with larger amounts of DCE. On the other hand, DCE afforded the lowest conversion (76% after 25 h) under SLS conditions, while DMF afforded the highest conversion (100% after 9 h).
3.4. Effect of the type of organic solvent Fig. 4 shows that the reaction conversion increases significantly with increasing dielectric constant [17] of the solvent employed, apart from the nature of the solvent and the reaction technique, whether a SLS or a SLL system. Such a dependence can be attributed to the extraction of Q 1 – 2 Oph ion-pairs into the polymer matrix, which increases with the dielectric
Fig. 3. Dependence of the reaction conversion percentage on the reaction time under SLL and SLS conditions using BTPPC as PT catalyst and K 2 CO 3 as base.
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Fig. 4. Dependence of the reaction conversion percentage on the dielectric constant of the organic solvent using BTPPC as PT catalyst and K 2 CO 3 as base.
constant of the solvent. This holds true for the SLS system and the extraction of the desired ion-pairs affected mainly by the dielectric constant, while the solvent type would have a minor effect. In the case of the SLL system in the presence of water-immiscible solvents (CCl 4 , benzene (Bz), chloroform (Chl), chlorobenzene (CBz) and DCE), diffusion will be facilitated of both Q 1 Cl 2 and Na 1 Cl 2 ion-pairs into the aqueous phase or Q 1 – 2 Oph ion-pairs into the polymer matrix. The diffused Q 1 – 2 Oph ion-pairs, in this case, will be less hydrated and hence of high mobility and reactivity. This is reflected by the steeply increasing reaction conversion with the dielectric constant of the solvent, higher than that obtained under SLS conditions. On the other hand, in the presence of solvents completely or partially miscible with water (DMF, n-PrOH and n-BuOH), the concentration of water in the organic phase increases with miscibility and the system can be considered as a SL two-phase system. Both reacting and produced ion-pairs will be highly abundant. Hence, the selectivity of Q 1 for 2 Oph will decrease. Also, when the 2 Oph anion is dissolved in a water-containing organic solvent, the oxygen of the phenoxide ion is strongly hydrated via hydrogen bond formation. This means that the
145
availability of the oxygen and, therefore, its nucleophilicity for nucleophilic displacements, is greatly reduced. In addition, strong hydration of 2 Oph anions decreases the mobility of Q 1 – 2 Oph ion-pairs and, hence, their diffusion into the polymer matrix. Consequently, the reaction rate decreases, which is reflected by the observed relative reduction of the conversion to values lower than that expected on the basis of the dielectric constant. Support for this solvation concept is evident from the observation that the SLL systems afforded higher conversions than the SLS systems in all of the experiments conducted in DCE in the presence of either different PT catalysts (Fig. 1) or different bases (Fig. 2). On the other hand, DMF afforded higher conversions under SLS-PTC conditions than in cases where DMF / H 2 O was used as the solvent under SL-PTC conditions (Fig. 2).
3.5. Utilization of the aldehyde group in P2 for polymer analogue conversions P2 was reacted in ethanol with malononitrile (3a), ethyl cyanoacetate (3b), cyanothioacetamide (3c) and cyanoacetamide (3d) as representatives of active methylene compounds. A catalytic amount of piperidine was used, except for 3d, where triethylamine was used as the catalyst (Scheme 3). The formation of P3(a–d) was confirmed by FTIR spectroscopic analysis and the reaction conversion was estimated on the basis of the unreacted carbonyl groups in P2. The reaction gave 100, 92 and 79% conversion after 24 h for P3(a–c), respectively, and 100% conversion for P3d after 72 h. The SLS-PTC reaction of 3a–d with P2 afforded P3(a–d) in a 100% conversion. P3(a–d) were also obtained in 100% conversion by the reaction of P1 with the nucleophiles 4a–d under SLS-PTC conditions. This was confirmed by the disappearance of the absorption at 1695 cm 21 of the aldehyde group (C=O) and the appearance of a very sharp peak in the region 2220–2190 cm 21 , corresponding to the CN group (Table 1).
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Scheme 3. Condensation reactions of P2 with active methylene compounds and amino compounds.
Compounds 4a–d were obtained from the condensation of 3a–d with 2, parallel to a reported method [18,19] and confirmed by 1 H NMR spectroscopic analysis (see Experimental). P2 reacted smoothly with excess amounts of amine compounds such as 2-aminophenol (5), hydrazine hydrate (6) and hydroxylamine hydrochloride (7), affording P4–P6, respectively. Moreover, P6 was obtained from the SLL-PTC reaction of P2 with 7. The reaction conversion was 61% for P4 and 100% for P6, obtained from both PT-catalyzed and uncatalyzed reactions, as well as for P5. This was demonstrated by the absence of the absorption at 1695 cm 21 for the aldehyde group (C=O). The resulting P6 was further reacted separately with bromo derivatives such as cyanogen bromide
(8a), ethyl bromoacetate (8b) and phenacyl bromide (8c) under SLL-PTC conditions. This afforded P7(a–c) in 100% conversion as demonstrated by the absence of the absorption at 3430 cm 21 for the OH group. Important IR spectroscopic data for P4–P7 are summarized in Table 2. The formation of P4–P7 demonstrates the practical effectiveness of the supTable 2 FTIR spectral data for P4–P7 Product
FTIR spectral data
P4
C=N at 1630 cm 21 , CHO at 1685 cm 21 (39%), OH at 3600–3100 cm 21 CHO absent, C=N at 1630 cm 21 , NH 2 at 3340 cm 21 CHO absent, C=N at 1620 cm 21 , OH at 3430 cm 21 OH absent, CN at 2226 cm 21 OH absent, C=O at 1685 cm 21
P5 P6 P7a P7b,c
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[8] E.V. Dehmlow, Phase Transfer Catalysis, VCH, Weinheim, 1993. [9] A.A. Sarhan, M.Y. Abdelaal, M.M. Ali, D.M.A. Hanna (manuscript in preparation). [10] J.M.J. Frechet, M.D. Smet, M.J. Farral, J. Org. Chem. 44 (1979) 1774. [11] C.M. Starks, in: C.M. Starks, C.L. Liotta, M. Halpern (Eds.), Phase Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives, Chapman & Hall, New York, 1994. [12] H.W. Gibson, F.C. Bailey, Macromolecules 9 (1976) 10. [13] H.W. Gibson, F.C. Bailey, Macromolecules 9 (1976) 688. [14] H.W. Gibson, F.C. Bailey, J. Polym. Sci., Polym. Chem. Ed. 12 (1974) 1241. [15] K.J. Shea, D.Y. Sasaki, J. Am. Chem. Soc. 113 (1991) 4109. [16] M. Makosa, M. Fedorynski, Adv. Catal. 35 (1987) 375. [17] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 67th Edition, CRC Press, Boca Raton, FL, 1986 / 1987. [18] M.A. Abdelaziz, H.H. Moharram, S.A. Essawy, A.A. Mohamed, Phosphorus Sulfur Silicon 48 (1990) 269. [19] G.H. El-Gemeie, N.M. Fathy, A.M. Attia, J. Chem. Res. (S) (1995) 292.