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The diesters (E)‐RO₂CCH=CHCO₂R [R = Et, CH₂Ph, iPr, iBu, tBu, 2‐ethylhexyl, (–)‐menthyl, (–)‐bornyl] undergo 1,4‐trichloromethylation with Me₃SiCCl₃ in 84–95 % yield with Bu₄NX (X = OAc, Cl, F; 0.5–10 mol‐%) as an initiator. Optimal catalysis is attained with X = OAc and F; chloride is less effective, bromide is inadequate. Poor regioselectivity but high yields are demonstrated by mixed diesters (2 examples, both > 80 %). Heating of the trichloromethyl adducts, in the presence of Bu₄N(OAc), leads to equilibrating, approximately equimolar, mixtures of RO₂CC(=CCl₂)CHCO₂R and RO₂CCH=CClCHClCO₂R above 100 °C (7 examples). The latter component is produced under thermodynamic control, which can be isolated pure in some cases. The former is the kinetic product of E2 elimination.