Eliminations from dichloroacetyl chloride

Fig. 15.35. Prototypical in situ generation of dichloroketene through NEt3-mediated /(-elimination of hydrogen chloride from dichloroacetyl chloride. Regioselective [2+21-cycloaddition II of this dichloroketene with an alkene whose C=C double bond contains a conjugated substituent at C-a, but not at C-/J.

Fig. 12.31. Orientation-selective [2+2]-cycloaddition with in situ generated dichloroketene I the dichloroketene is generated by way of an NEt3-mediated fi-elimination of HC1 from dichloroacetyl chloride.

The resulting enolate is not stable as it can eliminate chloride ion, a good leaving group, to form a ketene. This works particularly well in making dichloroketene from dichloroacetyl chloride as the proton to be removed is very acidic. 

The carbonyl group is accepting electrons in both the enolization step and the nucleophilic attack. The same compounds that are the most electrophilic are also the most easily enolizable. This makes acyl chlorides very enoUzable. To avoid nucleophilic attack, we cannot use chloride ion as base since chloride is not basic, so we must use a non-nucleophilic base such as a tertiary amine. The resulting enolate is not stable as it can eliminate chloride ion, a good leaving group, to form a ketene. This works particularly well in making dichloro-ketene from dichloroacetyl chloride as the proton to be removed is very acidic. 

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