A method for synthesis of fluorine compounds using abnormal Grignard reaction of halothane. III. Simple synthesis of trifluoromethylchloroolefins

We have reported an abnormal Grignard reaction of halothane (1) with carbonyl compounds (2). The reaction afforded 1-bromo-1-chloro-2, 2, 2-trifluoroethylcarbinols (3) as the major products at -53°C, while at 0°C, the main products were the dehalogenation products, 1-chloro-2, 2-difluoroethenyl carbinols (4). Treatment of 4 with hydrogen fluoride (HF) afforded 1-chloro-1-(trifluoromethyl)ethene derivatives (5) in good yields. However, this reaction required HF, a dangerous reagent. Here, we describe a much easier and safer synthesis of 5. Thus, treatment of 3 with zinc chloride and acetic anhydride gave acetoxy compounds (6). The acetates were converted to the olefins (5) by reductive debromoacetoxylation with zinc in the presence of a catalytic amount of copper(I) iodide. Application of this reaction to an α, β-unsaturated ketone, benzalacetone, gave the diene bearing a trifluoromethyl group.