Enlarged Deuterium Isotope Effects in Oxyanion-Catalyzad 1,3 Proton Transfer Competing with 1,2 Elimination as a Probe of a Common Tightly Hydrogen-Bonded Intermediate

Article abstract of DOI:10.1021/ja00342a035

The reaction of 1-(2-acetoxy-2-propyl)indene (1-h) or 1-(2-acetoxy-2-propyl)<1,3-2H2>indene (1-d) with p-NO2PhO^- in methanol buffered with p-NO2PhOH results in base-catalyzed 1,3 proton transfer, yielding 3-(2-acetoxy-2-propyl)indene (2-h) and 3-(2-acetoxy-2-propyl)<1,1-2H2>indene (2-d), respectively, in competition with base-promoted 1,2-elimination producing 1-isopropylideneindene (3-h) and 1-isopropylidene<3-2H>indene (3-d), respectively.The overall deuterium isotope effect on the reaction of 1 was measured as (k₁₂^H + k₁₃^H)/(k₁₂^D + k₁₃^D) = 5.2, which is composed of the rearrangement isotope effect k₁₂^H/k₁₂^D = 12.2 +/- 1.0 and the elimination isotope effect k₁₃^H/k₁₃^D = 3.6.The enlarged rearrangement isotope effect shows that the intramolecularity of the 1,3 proton-transfer reaction is substantial.The intramolecularity was determined as ca. 87percent for = 0.24 M and ca. 80percent for = 0.71 M by analyzing the 2H content of the product 2-d.The amplified isotope effect on the 1,3-prototropic shift together with the attenuated elimination isotope effect shows that the two reactions are coupled via at least one common intermediate, which is concluded to be a tightly hydrogen-bonded complex between the protonated base and the carbanion.An increase in basicity of the oxyanion favors elimination at the expense of rearrangement.Reaction of 2-h and 2-d predominantly give 1,4 elimination accompanied by a trace of competing 1,3 proton transfer.The isotope effect k₂₃^H/k₂₃^D was measured to 2.5; it is small owing to a large amount of internal return.