A Method for Synthesis of Fluorine Compounds Using Abnormal Grignard Reaction of Halothane. III. Simple Synthesis of Trifluoromethylchloroolefins.

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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.

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