Extreme deuterium isotope effects as evidence of ion-pair intermediates in base-promoted elimination reactions and base-catalysed 1,3-proton transfer reactions

Abstract

Reactions of 1-(1-acetoxy-l-methylethyl)indene (h-A) or its [1,3-²H₂]-analogue (d-A) with tertiary amines in methanol result in competing base-promoted 1,2-elimination and base-catalysed 1,3-proton transfer to give 1- isopropylideneindene (h-C){or its [3-²H]-analogue (d-C)} and 3-(1-acetoxy-1-methylethyl)indene (h-B){or its [1,1-²H₂]-analogue (d-B)}. Furthermore. (h-B) and d-B) also undergo competing reactions among which a ‘true’ base-promoted 1,4-elimination predominates over 1,3-proton transfer. Measured deuterium isotope effects on the reactions of (A) with the bases N-ethylpiperidine (EP), quinuclidine (Q), and diazabicyclo[2.2.2] octane (DABCO) were 7.0 ± 0.2, 7.1 ± 0.2, and 5.2 _–0.7^+0.9, respectively. These isotope effects were composed of the 1,3-proton transfer isotope effects 14.7_–2.3^+2.0, 18.1 ± 1.1. and 7.9_–0.8^+1.0 and the 1,2-elimination isotope effects 2.6 ± 0.8, 4.0 ± 0.3 and 1.9_–0.5^+0.6; respectively. The results indicate that the 1,2-elimination and the 1,3-proton transfer reactions, which appear to be intramolecular, are coupled with at least one common ion-pair intermediate. These competitive isotope effects provide evidence of a new type, confirming previous indications that ion pairs may be intermediates in elimination and 1.3-proton transfer reactions.

The isotope effects on the reaction of (B), on the other hand were small: 1.33 ± 0.03, 1.63 ± 0.05, and 1.0 ± 0.1 with EP, Q, and DABCO, respectively. As the reactions were not accompanied by significant incorporation of protium into (d-B), these small isotope effects indicate the intermediacy of reversibly formed ion pairs. Thus the observations strongly suggest that all the above reactions involve at least one common ion pair.

Complete internal consistency of the interpretations requires the introduction into the mechanism of another ion pair which is not in equilibrium with the first one.