Relation entre structure et réactivité dans les réactions d'addition nucléophile sur les dérivés carbonylés: influence des interactions diaxiales-1,3 sur la réactivité de cyclanones et cyclanols stériquement encombrés

Abstract

The rate constants for the addition of sodium borohydride to the sterically hindered cyclic ketones, 3,3,5-trimethylcyclohexanone (2), 3,3,5,5-tetramethylcyclohexanone (3), bicyclo[3,2,1]-3-octanone (4), tricyclo[5,2,1,0^3,8]-3-decanone (5), tricyclo[5,3,1,0^3,8]-5-undecanone (6) and bicyclo[3,3,1]-3-nonanone (7) have been determined at 25 °C in a 50/50 by volume mixture of water and dioxane. The values demonstrate energies due to 1,3-diaxial interactions that arise on passing from the initial to the transition states. The measured energies of activation represent almost the total conformational energy corresponding to these interactions, suggesting a late transition state.Comparison of the rates of reduction of these ketones where the carbon atom at the site of attack passes from sp² to sp³ hybridisation, with the rates of chromic acid oxidation of the corresponding alcohols showing therefore the opposite change in hybridisation, indicates that the magnitude of the 1,3-diaxial interactions can serve to determine the position of the transition state along the reaction coordinate.The observed differences in reactivity can also contribute to the elucidation of conformational problems. [Journal translation]