Intramolecular reactions. Part VI. Rates of ring formation in reactions of ω-halogenoalkylmalonic esters with bases

Abstract

Reactions of ω-halogenoalkylmalonic esters, X·[CH₂]_n·CH(CO₂Et)₂(X = Cl or Br; n= 2,3,4, or 5), with potassium t-butoxide in t-butyl alcohol and with sodium ethoxide in ethanol have been investigated. In both solvent systems and with each ester studied, the sole product is the cycloalkane-1,1-dicarboxylate which results from intramolecular nucleophilic substitution. In reactions with t-butoxide, the relative rates of closure of 3-, 4-, 5-, and 6-membered rings are 650,000 : 1 : 6500 : 5, and the kinetics show that the halogeno-ester is converted into a carbanion which subsequently cyclises in the rate-determining step. The results are discussed in terms of conjugative control of ring closure against the background of other data on the kinetics of ring formation. Diethyl cycloalkane-1,1-dicarboxylates undergo transesterification on treatment with potassium t-butoxide in t-butyl alcohol under conditions in which acyclic esters are inert. The rate of transesterification is inversely proportional to the size of the ring, and this observation is explained on the basis of increase in steric interference at the reaction site occasioned by expansion of the ring from three to five members.