1,3‐Hydron transfer in some 5‐ and 7‐substituted 1‐methylindenes. Enantioselectivities and enantiomer‐dependent kinetic isotope effects

Base‐catalysed substrate‐enantioselective 1,3‐hydron transfer (kinetic resolution) was studied in the indene system. A series of 1‐methylindenes substituted in the aromatic ring [5‐methoxy‐ (2), 5‐fluoro‐ (3), 5‐nitro‐ (4) and 7‐nitro (5)] and 1‐methylindene (1) were employed as substrates. The rate constants, the enantioselectivities and the kinetic isotope effects (KIEs) for the enantioselective reactions [( k H / k D ) + and ( k H / k D ) − ] were determined at 20°C using (+)‐(8 R , 9 S )‐dihydroquinidine as chiral catalyst in the solvent o ‐dichlorobenzene. The rate constants vary according to the elecronic effects of the substituents. The primary deuterium KIE, ranging from 4·73 [for (+)‐( S )‐2] to 11·3 [for (−)‐( R )‐5], is correlated with the rate constants as expected on the basis of the Melander–Westheimer postulate. The introduction of a substituent in the aromatic ring decreases the enantioselectivity. All compounds except 5 show the same sense of the enantioselectivity [ k + / k − > 1; all substrates used are (+)‐( S )/(−)‐( R )]. The enantiomer dependence of the KIE is most pronounced for 1 [( k H / k D ) + = 5·71 and ( k H / k D ) − = 6·46] and vanishes for the most acidic substrates (4 and 5).