Primary and secondary β-deuterium kinetic isotope effects in the 1,3-prototropic rearrangement of 1-methylindene using tertiary amines as catalysts in the solvents toluene and dimethyl sulphoxide

Abstract

Rate constants, primary deuterium kinetic isotope effects (k.i.e.s), and secondary β-deuterium k.i.e.s have been determined for the base-catalysed 1,3-prototropic rearrangement reaction of 1-methylindene (1) to 3-methylindene (2) in the solvents toluene and Me₂SO at 20 °C. The structurally similar tertiary amines quinuclidine (3), 1,4-diazabicyclo[2.2.2]octane (4), 1,3,5-triaza-adamantane (5), and 1,3,5,7-tetra-aza-adamantane (6), were used as catalysts. The primary k.i.e.s obtained for amines (3), (5), and (6) are rather strong; 5.75–6.06 in toluene and 7.62–7.83 in Me₂SO, i.e. almost constant over the pK range of ca. 4 (in Me₂SO). Amine (4), despite its intermediary basicity, showed a significantly lower primary k.i.e.: 5.03 in toluene and 7.29 in Me₂SO. The secondary β-deuterium k.i.e.s of ca. 10% were found to correlate inversely with the primary k.i.e.s. The variation of primary and secondary k.i.e.s for the amines (3)–(5) is similar in the two solvents, but the absolute values of the primary k.i.e.s are lower and the secondary k.i.e.s are higher in toluene. The k.i.e. data are related to the degree of proton transfer and anionic hyperconjugation in the TS of the rate-determining proton abstraction step. The possible importance of internal return is discussed.