Chemistry

Academician A. V. TOPCHIEV, V. G. KRYUCHKOVA, and S. V. ZAVGORODNII

ALKYLATION OF 4-FLUOROPHENOL WITH PROPYLENE AND CYCLOHEXENE IN THE PRESENCE OF THE CATALYSTS BF₃·H₃PO₄ AND BF₃·O(C₂H₅)₂

The reaction of fluorophenols with olefins has not been covered at all in the literature. In continuation of investigations in the field of the alkylation of halophenols with olefins in the presence of catalysts based on boron fluoride (¹–⁶), the present communication studies the reaction of 4-fluorophenol with propylene and cyclohexene in the presence of BF₃·H₃PO₄ and BF₃·O(C₂H₅)₂.

4-Fluorophenol, in contrast to the chloro- and bromophenols studied earlier, possesses higher reactivity and, on interaction with olefins, in particular with propylene, forms a rather complex mixture of products. In the present case, along with the isopropyl ether of 4-fluorophenol, the isopropyl ether of isopropylfluorophenol is constantly formed, and under certain conditions isopropylfluorophenol is also obtained. The yields of the products depend on the nature and amount of catalyst, the temperature, and the molar ratios of the reactants. Thus, in the presence of BF₃·O(C₂H₅)₂, as the mildest catalyst, at a temperature of 60° only ether products are obtained, and phenolic products are absent, at least in appreciable quantities. In the presence of BF₃·H₃PO₄ the yields of phenolic compounds are the higher, the higher the temperature within the range from 40 to 70°.

The best conditions under which the isopropyl ether of 4-fluorophenol is obtained in 54% yield are molar ratios of fluorophenol, propylene, and BF₃·H₃PO₄ equal to 3 : 1 : 0.4 and a temperature of 40°; for obtaining the isopropyl ether of isopropyl-4-fluorophenol in 36% yield, the ratios of reactants and catalyst are 5 : 1 : 0.3 and the temperature is 60°; and for obtaining isopropyl-4-fluorophenol in a maximum yield of 38%, the ratios of reactants and catalyst are 3 : 1 : 0.2 and the temperature is 70°.

The influence of temperature, catalyst quantities, and molar ratios of the reactants on the total yield of the indicated products is seen from the data of the experiments given in Table 1.

4-Fluorophenol with cyclohexene in the presence of BF₃·H₃PO₄ forms a single product—the cyclohexyl ether of 4-fluorophenol—with a maximum yield of 70.7% of theoretical. After the very first distillation of the alkylate, the product boils within a range of 2–3°, as is seen from the data in Table 2. Compounds of the ether type were identified by cleavage to the corresponding phenols and conversion of the latter into phenoxyacetic acids.

Experimental Part

4-Fluorophenol was obtained by demethylation of 4-fluoroanisole (⁷), m.p. 46. Propylene, cyclohexene, and the catalysts were obtained as before (⁵).

Alkylation of 4-fluorophenol with propylene. The reaction was carried out by the usual procedure (⁶) in a three-necked flask. In experiments with the ethyl etherate of boron fluoride, fluorophenol and catalyst were placed in the flask, the mixture was heated to 60°, and, with vigorous stirring, propylene was introduced at a rate of 1.2–2 l/hr. As the propylene was introduced, the reaction mass became brown in color and by the end acquired a dark-brown coloration. After the calculated amount of propylene had been passed through,

Table 1

Alkylation of 4-fluorophenol with propylene in the presence of \(\mathrm{BF_3\cdot H_3PO_4}\) and \(\mathrm{BF_3\cdot O(C_2H_5)_2}\)

Note. For each experiment, 0.1–0.2 mole of propylene and the calculated amounts of 4-fluorophenol and catalyst were taken. Experiments nos. 1 and 2 were carried out in the presence of boron fluoride ethyl etherate. The temperature fluctuated by \(\pm 2^\circ\) from that indicated in the table.

the mixture was stirred for 2 hr at the experimental temperature and left to stand at room temperature for 12 hr. Then, in the form of a rather viscous oil, it was mixed with benzene and treated repeatedly with small portions of 10% aqueous NaOH solution until complete extraction of the phenolic products (test with a drop of hydrochloric acid). The ether products, in the form of a transparent, almost colorless oil, were washed well with water, dried over calcium chloride, and distilled into fractions boiling within the range of 2 to 10°. The phenolic products, soluble in alkali, were treated with conc. HCl, extracted with ether, dried over sodium sulfate, and likewise distilled within a range of 3–6°. For identification, both were redistilled into narrower fractions. By heating with HBr, the isopropyl ether of 2-isopropyl-4-fluorophenol was converted into 2-isopropyl-4-fluorophenol, and from the latter, with chloroacetic acid, 2-isopropyl-4-fluorophenoxyacetic acid was obtained.

Table 2

Alkylation of 4-fluorophenol with cyclohexene in the presence of BF₃·H₃PO₄

Alkylation of 4-fluorophenol with cyclohexene. The reaction was carried out in a three-necked flask with vigorous stirring. To a mixture of 4-fluorophenol and the catalyst BF₃·H₃PO₄, heated to a specified temperature, 0.1 mole of cyclohexene was slowly added from a dropping funnel. The reaction mass, a brown liquid, was treated in the same way as in the reaction with propylene. In this case the sole reaction product is the cyclohexyl ether of 4-fluorophenol, which already during the first distillation of the alkylate is isolated in fairly pure form. After redistillation it is a colorless oil with a pleasant ether-like odor. The physicochemical constants of the products obtained are given in Table 3.

Table 3

Voronezh State University

Received
19 XI 1959

CITED LITERATURE

1. S. V. Zavgorodnii, V. G. Kryuchkova, Tr. Voronezhsk. gos. univ., 32, 25 (1953).
2. S. V. Zavgorodnii, E. M. Faustova, ZhOKh, 23, 1651 (1953).
3. S. V. Zavgorodnii, ZhOKh, 22, 1995 (1952).
4. S. V. Zavgorodnii, V. G. Kryuchkova, Tr. Voronezhsk. gos. univ., 40, 125 (1956).
5. S. V. Zavgorodnii, V. G. Kryuchkova, ZhOKh, 27, 330 (1957).
6. S. V. Zavgorodnii, V. G. Kryuchkova, ZhOKh, 29, 1340 (1959).
7. L. N. Ferguson, J. C. Reid, M. Galvin, J. Am. Chem. Soc., 68, 2502 (1946).
8. V. Jones, J. Chem. Soc., 1938, 1414.