Fluorocarbon synthesis via free-radical reactions
The effect of substituents on carbon-hydrogen bond reactivity in free radical addition to fluoroalkenes has been investigated. Ethers, esters, amines, amides, and borates all gave free radical adducts, and an order of reactivity has been compiled. This was explained by the inductive electron withdrawing effects of substituents. The reactivity of cyclic ethers and amides has been discussed in terms of the stereo- electronic effect. The ease of hydrogen abstraction from the substrates was estimated using a method based on the thermal decomposition of di-t-butyl peroxide. A correlation between the ease of hydrogen abstraction and free- radical reactivity was demonstrated. Further reactions of the adducts has given good synthetic routes to other functionally substituted fluorocarbons. Polyfluorinated acid chlorides were obtained in good yields from the oxidative chlorination of borate and alcohol adducts. Remarkably high yields of perfluorinated ethers were produced from the high temperature cobalt trifluoride fluorination of ether adducts. The best yields were obtained with cyclic ether and ethyl and propyl dialkyl ether adducts using a reaction temperature of 440ºC. Methoxy groups and, chlorine atoms in positions susceptible to elimination, were less stable under these conditions. The structure of polyfluorinated ethers isolated when using lower reaction temperatures indicate that the first steps of fluorination occur adjacent to oxygen and via unsaturated intermediates. Chlorination of a methyl ether adduct gave attack at the most nucleophilic site, while bromination gave attack at the position which gave the most stable radical. Attempts at the dehydrodimerization of adducts were unsuccessful.