Transmission of substituent effects through unsaturated systems. Part 6. Kinetics of reduction of β-substituted α,β-unsaturated ketones with sodium borohydride

Abstract

The rate constants for addition of sodium borohydride to 3-substituted 5,5-dimethylcyclohex-2-enones (1), para-substituted 3-phenylcyclohexenones (2), and para-substituted acetophenones (3) have been determined in an alkaline solution (NaOH, 0.025 mol l^–1) of water–dioxane (1:1 v/v) at 298 K. The conjugated cyclohexenone systems (1) and (2) undergo exclusive 1,2-reduction in these conditions to produce the corresponding allylic alcohols. The regioselectivity of reduction is discussed. The linear free energy relationships obtained between the rate constants and σ_p or σ_p⁺ confirm that these 3-substituted cyclohexenone structures are good models for the investigation of the substituent effects on the reactivity of ethylenic systems. Comparison of reaction constants for the three series leads to the conclusion that, as for borohydride reductions of acetophenone, the cyclohexenone reduction must have a late transition state in the final quarter of the reaction co-ordinate.