Studies on the Reactivity of Tetrafluoro- and Pentafluorophenyl Trimethylsilyl Ether with Pentafluorobenzenes. Chemistry and X-ray Structural Investigations of Polyfluorodiphenyl Ethers

The introduction of tetrafluoro- and pentafluorophenoxy moieties into a variety of pentafluorobenzenes C6F5R (R = CF3, CN, NO2) is accomplished by employing the trimethylsilyl ethers (siloxanes) 4-HC6F4OSiMe3 (1) and C6F5OSiMe3 (2) as transfer agents. Depending on the nature of the electrophile, the stoichiometry of the reaction, and the reaction conditions, polysubstituted polyfluorodiphenyl ethers are obtained. Excess C6F5R results in the formation of the 1,4-monosubstituted benzenes (diphenyl ethers) 4-(4‘-XC6F4O)C6F4R (R = CF3, X = H (3), F (4); R = CN, X = H (5), F (6); R = NO2, X = H (9), F (10)). When R = NO2, the 1,2-substituted isomers are also detected. Additional byproducts that are isolable are the disubstituted benzenes 2,4-(4‘-XC6F4O)2C6F3R (R = CN, X = H (7), F (8); R = NO2, X = H (11), F (12)). Excess 1 or 2, when reacted with C6F5R, results in the formation of the trisubstituted benzenes 2,4,6-(4‘-XC6F4O)3C6F2R (R = CN, X = H (13), F (14); R = NO2, = H (15), F (16)). The hydrolysis of nitrile-containing diphenyl ethers (5, 6, 13, and 14) under acidic conditions results in the substituted benzoic acids 4-(4‘-XC6F4O)C6F4COOH (X = H (17), F (18)) and 2,4,6-(4‘-XC6F4O)3C6F2COOH (X = H (19), F (21)). These acids are decarboxylated to form the respective hydropolyfluoro aromatics (4-HC6F4)2O (23), 4-(C6F5O)C6F4H (24), and 2,4,6-(4‘-XC6F4O)3C6F2H (X = H (20), F (22)). In addition to acid 17, alkaline hydrolysis of 5 gives the α-hydroxy-substituted acid 4-(4‘-HC6F4O)C6F3(2-OH)COOH (25). Alkaline hydrolysis under milder conditions enables the isolation of the amide 4-(4‘-HC6F4O)C6F4CONH2 (26). The compounds 3, 4, 14−18, 23, and 26 have been characterized by single-crystal X-ray diffraction analysis. The presence of a hydrogen atom in 3, as well as protection of the reactive 4‘-position with a trifluoromethyl group, gives 4-(4‘-CF3C6F4O)C6F4Li (3a) on reaction with n-butyllithium. In situ reactions between 3a and ketones or acid chlorides result in novel mono- or bis(perfluorodiphenyl ether)-substituted tertiary alcohols 4-(4‘-CF3C6F4O)C6F4C(R)(R‘)OH (R/R‘ = CF3 (27), C6F5 (28), C6H5 (29), C3F7/C8F17 (30), C6F5/CH3 (31)), [4-(4‘-CF3C6F4O)C6F4]2C(R)OH (R = CF3 (32), C3F7 (33), C7F15 (34), i-C3H7 (35)). When R = i-C3H7, the major product is the ester [4-(4‘-CF3C6F4O)C6F4]2C(i-C3H7)OC(O)(i-C3H7) (36). The ketone C3F7(C8F17)CO (37) is synthesized and characterized. Reaction of 3a with hexafluoroglutaryl chloride gives [4-(4‘-CF3C6F4O)C6F4]2C(OH)(CF2)3C(O)C6F4O(4‘‘-C6F4CF3) (38), whereas with dimethyl carbonate or carbonyl fluoride, [4-(4‘-CF3C6F4O)C6F4]2CO (39) as well as small amounts of [4-(4‘-CF3C6F4O)C6F4]3COH (40) and [4-(4‘-CF3C6F4O)C6F4]3COC(O)C4H9 (41) are formed. Residual n-butyllithium cleaves the intermediate 4-(4‘-CF3C6F4O)C6F4COOCH3 to form 4-CF3C6F4C4H9 (42) and 4-HOC6F4COOCH3 (43).