Electrochemical fluorination of methyl cinnamates in Et(3)N center dot 3HF-MeCN

Methyl cinnamates, R-C6H4-CH=CH-CO2CH3 (R=CH3O; CH3, H, F and CF3), were electrochemically fluorinated in a 1 M solution of (C2H5)(3)N . 3HF in acetonitrile under potentiostatic conditions. The electrolyses gave, in general, diasteroisomeric mixtures of 2,3-difluoro-3-phenylpropionates (2), 3-acetamido-2-fluoro-3-phenylpropionates (3) and 2-acetamido-3-fluoro-phenylpropionates (4) as the main products and small amounts of higher fluorinated compounds. The overall chemical yields and current efficiencies were 78-94% and 42-70%, respectively. The substituent R greatly influenced both the chemo- and regioselectivity of fluorination. Ratios of di- and polyfluorinated products to fluoroacetamides and of beta-fluoroacetamides (3) to alpha-fluoroacetamides (4) have shown reasonable correlation with the resonance substituent constants sigma(p)(+). The effect of substituent R is discussed in terms of relative stability of intermediate radical-cations. Cis-addition was the predominant steric outcome of both difluorination and fluoroacetamidation reactions. Copyright (C) 1996 Elsevier Science Ltd