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

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[5,2,1,03,8]-3-decanone (5), tricyclo[5,3,1,03,8]-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]