Elimination and addition reactions. Part 39. Variation of nucleofugality with transition state structure – 1,3- and 1,2-eliminations from carbanions compared

Abstract

Phenylsulphonyl-activated 1,3-eliminations have been investigated with the objective of determining nucleofugalities of six leaving groups. In cyclohexane-1,3-diyl and propane-1,3-diyl systems unactivated 1,2-elimination of ‘poor’ leaving groups occurs faster than 1,3-elimination. In 2,2-dimethylpropane-1,3-diyl substrates, however, quantitative 1,3-eliminations occur and the nucleofugalities of Br, Cl, OTs, PhSO₂, PhS, PhO, have been determined. In a linear free energy relationship extending over 15 pK_a units and 12 rank units, an excellent correlation of nucleofugality with the pK_a of the conjugate acid of the leaving group is found. It is concluded from the comparison with 1,2-eliminations previously studied that 1,3-elimination involves considerable leaving group separation and ring formation in the transition state. With bromide as leaving group, the 2,2-dimethylpropane-1,3-diyl system is 10³ times more reactive in cyclopropane formation than the propane-1,3-diyl system; this is the first quantitation of the Thorpe–Ingold effect in homocyclic ring formation. With chloride and tosylate leaving groups, smaller accelerations are observed.