Intra- and inter-molecular hydron abstraction from allylic carbocation intermediates in aqueous solvent. Observation of a substantial deuterium isotope effect for 1,4-elimination of acetic acid from one of the ion-pair intermediates

Abstract

The allylic carbocation intermediate formed from 3-(2-chloropropan-2-yl)indene (1-Cl) and from (1,1-²H₂)-(1-Cl) in 75 vol % water–acetonitrile at 35 °C reacts rapidly with solvent water to give the allylic alcohols but is also subject to hydron abstraction by general bases to produce 1-isopropylideneindene (3) and 3-isopropenylindene (4). The Brønsted parameters measured with substituted acetate anions were found to be small, β 0.16 for formation of (3) and 0.14 for production of (4). The kinetic deuterium isotope effect on hydron abstraction from the intermediate with acetate anion to form the olefin (3), (k^H/k^D) is 3.0 ± 0.4. The corresponding intramolecular elimination of acetic acid from the contact ion pair formed from 3-(2-acetoxypropan-2-yl)indene (1-OAc) to yield (3) shows a substantial isotope effect, k^H/k^D= 5.2 ± 1.0. The allylic isomer 2-acetoxy-1-isopropylideneindan (2-OAc) yields a contact ion pair that reacts to give (3) with a considerably smaller isotope effect, k^H/k^D= 2.9 ± 0.3. The free carbocation generated from protonated 3-(2-hydroxypropan-2-yl)indene (1-OH) and its (1,1-²H₂)-analogue undergo hydron abstraction to give the olefin (3) with an isotope effect k^H/k^D= 4.1 ± 0.3. These isotope effects do not include the isotope effect on the rate-limiting ionization step, which were found to be k^H/k^D= 1.07 and 1.22 in the reactions of (1-OAc) and (2-OAc), respectively. The elimination isotope effects for the ion pairs are evaluated by assuming that the formation of alcohols from the intermediates has an isotope effect of unity.