1314
Ind. Eng. Chem. Res. 1991,30, 1314-1319
Preparation of the Partially Substituted (Phenoxy)chlorocyclotriphosphazenes by Phase-Transfer Catalysis Y. W. Chen-Yang,*S. J. Cheng, and B. D. Tsai Department of Chemistry, Chung Yuan Christian University, Chung-Li, Taiwan 32023, ROC
In this study, phase-transfer catalysis was carried out for the substitution reaction of hexachlorocyclotriphosphazene, N3P3C1, (I), by phenol to synthesize the partially substituted (phenoxy)chlorocyclotriphosphazenes,N3P3CL(OC6H5)x,x = 1-5. The kinetics of the reaction with the molar ratio of phenol and I equal to 2 was studied as a model reaction. It showed that the model reaction was mainly mass-transfer controlled and was second order with respect to the phenoxide. The optimum conditions were found for producing each derivative of the partially substituted (phenoxy)chlorocyclotriphosphazenes, N3P3C~(OC6H5),, x = 1-5, such that the derivative predominates with high yield (over 75% of theory) in the mixture of the products. It was found that phase-transfer catalysis is a much more convenient and economic method than the conventional method for synthesizing partially substituted (phenoxy)chlorocyclotriphosphazenes.
Introduction It is known that the alkoxy- and (ary1oxy)cyclotriphosphazenes can be synthesized by the conventional nucleophilic substitution method. In this method, the sodium salt of the corresponding alcohol is usually used. Due to the low solubility of the salt in the solvent and the consideration of hydrolysis, elevated temperature, long reaction time (about 2 days), and inert atmosphere are usually required (Allcock, 1972a,b). In order to improve the reaction rate, phase-transfer catalysis has been applied in the conventional method by Allcock et al. (Austin et al., 1983). Nevertheless, the yield and the reaction time were not significantly improved, possibly due to lower efficiency of the transferability of the catalyst between solid phase and the organic solvent phase. Hence, a rapid method that can be run in common equipment under atmospheric conditions is valuable. Phase-transfer catalysis (PTC) carried out in a two-immiscible-solvent system has long been known as a simple, effective, and rapid method in preparation of some organic compounds (Dehmlow and Dehmlow, 1983). On the other hand, this method was known as an interfacial condensation method and applied to the preparation of many polymers (Morgan, 1981). In the phosphazene field, the method has also been applied to the polycondensation of cyclotriphosphazenes with hydroquinone by Brandt (Brandt, 1986, 1988, 1989). Recently Carr and Nichols filed a patent of process on the preparation of phosphazene esters (Carr and Nichols, 1986). It demonstrated that PTC was an efficient method for preparing fully substituted (ary1oxy)phosph”s and (fluoroalkanoxy)phosphazenes. Wang et al. also reported the study of the mass-transfer effect of the reaction of hexachlorocyclotriphosphazene and 2,2,2-trifluoroethanol (Wang and Wu, 1990). In this study, PTC was carried out to study the partial replacement of I by phenol because of the potential use of the partially substituted products (Allcock et al., 1980, 1986,1988; Kumar et al., 1983a,b, 1984,1985,1986). In order to understand the effect of the variables in the reaction, the kinetics of the reaction with the molar ratio of phenol and I (MR) equal to 2 was studied first. The study of the optimum reaction conditions for each derivative with different degrees of substitution, the total yields, and the distribution of the products were followed, and finally, the result was compared with that of the conventional method.
* To whom correspondence should be addressed. 0888-5885/91/2630-1314$02.50/0
Experimental Section Apparatus and Materials. The nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AC 200 spectrometer. Chemical shifts upfield of the reference were assigned a negative sign. Elemental analyses were performed on a Heraeus CNH-O-RAPID analyzer by the Instrument Center of NSC at Tainan, Taiwan. The concentration of phenoxide in the aqueous phase was determined with a Shimadzu UV 160 spectrophotometer at 268.8 nm. An Eyela DS-3S mechanical stirrer was used for kinetic study, and an Iwaki Glass PC-351hot plate with magnetic stirrer was used for syntheses of the individual derivatives; Hexachlorocyclotriphoaphazene(I) was kindly provided by the Nippon Fine Chemical Company, Osaka, Japan. Except that sodium hydroxide and 1,2-dichIoroethanewere purchased from Osaka Shimakyu Chemical Co., Osaka, Japan, all other reagents and solvents were purchased from Merck Chemical Co. All reagents and solvents were used as received. Reaction Kinetics. In this study, five series of reaction kinetics were studied. The effect of solvent system was studied first. In this part, a solution of 3.48 g (0.01 mol) of hexachlorotricyclophosphazene (I) in 100 mL of organic solvent was mixed with a 200-mL aqueous solution containing 1.88 g (0.02 mol) of phenol, 1.60 g (0.04 mol) of sodium hydroxide, and 0.24 g (0.75 mmol) of tetrabutylammonium bromide (TBNB), as phase-transfer catalyst, and stirred at a speed of 400 rpm. The condition of the second series was the same as the first series, except that dichloromethane was used as the organic phase and the PTC was varied. On the other hand, only the amount of PTC, the concentration of NaOH, and the stirring speed were varied in the third, fourth, and fifth series, respectively.
A typical procedure was carried out as follows: The organic solution in which I was dissolved was prepared in a 500-mL three-necked flask equipped with a mechanical stirring system. With the mechanical stirrer running at a speed of 400 rpm, the aqueous solution containing phenol, sodium hydroxide, and PTC (except MRNC, which was dissolved in the organic phase due to low solubility in aqueous phase) was discharged into the flask as fast as possible. The reaction mixture was taken out with a 20-mL pipet at time intervals of 2 or 3 min. For good solvent systems, the (phenoxy)cyclox = 1-6, could be triphosphazenes, N3PSClx(OC6H6)ex, dissolved in the organic phase, and a well-defined two@ 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1315
toluene diethyl ether benezene chlcrobenezene
~~- 1 -
\,
,
,
o-dlchlorobenezene
chloroform dichlorcmethane 1 2-dlchlcroethane
00 0
20
40
60
Timdmln)
0
2
4
6
IO
8
4 IBlxlO
Figure 2. Effect of various organic solvents on change of phenoxide concentration with reaction time. Table I. Effect of Various Organic Solventsa
Figure 1. Relationship between UV absorbance at 268.8 nm and concentration of phenoxide.
phase system was observed. The two phases could be separated rather easily, so the reaction mixture collected at each time interval was separated into an organic phase and an aqueous phase. The concentration of phenoxide in the aqueous phase was, then, determined by using the UV spectrophotometer at a wavelength of 268.8 nm. The dependence of the absorbance on the concentration of a standard solution of phenoxide at 268.8 nm is shown in Figure 1. The separated aqueous phase was always diluted to a suitable level, so that the concentration of phenoxide to 1 X mol/L. was ranged from 5 X Synthesis of (Phenoxy)chlorocyclotriphosphazenes, N3P3CL(OCgH6)x,x = 1-6. A typical example was carried out as follows: A 20-mL aqueous solution containing 1.88 g (0.02 mol) of phenol and 1.60 g (0.04mol) of sodium hydroxide was prepared. A 30-mL dichloromethane (CH2C12)solution, containing 3.48 g (0.01 mol) of I and 0.211 g (0.0005 mol) of MRNC (MRNC, CH3bNCI;R = primary C a l 7 and Cl&21), was prepared in a 250-Lthree-necked flask. With the highest stirring speed (>>500 rpm), the aqueous solution was discharged dropwise into the flask and was reacted for 20 min at room temperature. After reaction, the organic layer was separated from the aqueous layer, washed by water, and dried by magnesium sulfate. The organic solvent was then removed under vacuum to give a colorless liquid. This liquid was further separated by conventional column chromatography to individual derivatives. Finally each derivative was identified by 31PNMR and elemental analysis and weighted to calculate the individual yield. Results and Discussion The reaction of I and phenol by PTC can be expressed as follows: N3P3C1, + xNaOH T
1
.
+ xC8H50H org solvent/HgO cah13Rt
N3P3Ck+x(OC6H6)x + xH20 + xNaCl
Kinetic Study. In order to understand the effects of the variables, which included the solvent system, the amount and kind of phase-transfer catalyst, the concentration of base, and the stirring speed in the reaction, the reaction with MR 3 2 was selected as a model reaction to study ita kinetica in detail. (a) Influence of Solvent System. The hydrolysis of the chlorocyelotriphosphenes, N3P3Cls_x(OCsH6),, x = Ck5, is the main side reaction in the phasetransfer catalysis of the substitution of I by phenol in the water-immisci-
solvent 1,2-dichloroethane dichloromethane chloroform o-dichlorobenzeneb chlorobenzeneb benzene diethyl ether
I, % 0 0 23.1 61 78.8 94.3 93.6
hydrolyais, % 6.9 8.4 6.1 5.7 6.4
total
rate, L/(mobs) 109 x
yield, % 93.1 91.6 70.8 38.3 21.2 TBNO >
Figure 6. Effect of various PTC on kinetics. Table 11. Effect of Various PTC on the Rate Constant' PTC ?4 rate, L/(mobs) x 109 MRNC 2.00 295 TBNB 5.00 205 3.15 TBNO 146 3.15 TBNB 108 15 MRNC 0.80 TBNB 2.50 51 TBNB 20 1.25 4.1 TPNB 3.15 TENB 3.15
