ION-PAIR INTERMEDIATES AND EXTREME DEUTERIUM-ISOTOPE EFFECTS IN PARTIALLY DIASTEREOSPECIFIC BASE-PROMOTED ELIMINATION COMPETING WITH BASE-CATALYZED 1,3-PROTON TRANSFER

Reaction of threo-or erythro-l-(l-acetoxyethyl)indene (1-h and 2-h) with quinuclidine (Q) or sodium methoxide (NaOMe) in methanol results in base-promoted 1,2 elimination to give a mixture of {E)-and (Z)-1-ethylidenindene (4a-h and 4b-h). In the reactions with Q, base-catalyzed 1,3-proton transfer producing 3-(l-acetoxyethyl)indene (3-h) competes with the eliminations. The latter compound undergoes base-promoted 1,4 elimination also yielding a mixture of 4a-h and 4b-h. The corresponding 1,3-dideuterated compounds 1-d and 2-d and the 1,1-dideuterated compound 3-d yield, within experimental error, the same ratio of the elimination products (E)-and (Z)-[3-2H]-l-ethylidenindene {4a-d and 4b-d) as the protium analogues. With NaOMe, the kinetic isotope effects are KH14/kD14 = 6.6 + 1.8 or -1.4, KH24/kD24 = 6.9 ± 0.3, and kH34/kD34 = 7.5 ± 1.1 respectively. The isotope effects measured with Q are (kH13 + kH14)/(kD13 + kD14) = 7.1 ± 0.3 and (kH23 + kH24)/(kD23 + kD24) = 7.3 + 1.6 or -1.2, respectively. These deuterium isotope effects are composed of unusually large base-catalyzed 1,3-proton transfer isotope effects kH13/kD13 = 39 ± 7 and kH23/kD23 = 30 + 22 or -11. They are large owing to competition between the 1,2 elimination and 1,3-proton transfer from a common intermediate. The 1,2-elimination isotope effects, on the other hand, have been attenuated by the competition: kH14/kD14 = 3.9 ± 0.3 and kH24/kD24 = 4.4 + 1.2 or -1.0. Since significant incorporation of protium in 3-d was not observed when starting from 1-d, i.e., the 1,3-proton transfer is highly intramolecular, it is inferred that the 1,2 eliminations and the rearrangements are coupled and that they have at least one irreversibly formed ion pair in common, i.e., the 1,2 eliminations take place with (E1cB)ip,I mechanisms. The isotope effect on the 1,4-elimination reaction of 3 with Q is kH34/kD34 = 3.5 ± 0.1 and no incorporation of protium in 4a-d and 4b-d was observed. This indicates that the reaction proceeds via partially reversibly formed ion pairs. The ion pair and tightly solvated carbanion elimination reactions are partially diastereospecific. 1-h gives 61 ± 2%syn 1,2 elimination with NaOMe and 94 ± 2% syn with Q while 2-h yields 8 ± 2% syn with NaOMe and 14 ± 2% syn with Q. The 1,4 elimination gives a product ratio 4b-h/4a-h of 23/77 with NaOMe and 14/86 with Q. The elimination mechanisms of the intermediates and the relationship between deuterium isotope effects and elimination reactions in general are discussed. © 1979, American Chemical Society. All rights reserved.