Parabolic Relationship between the Basicity of the Nucleophile and pi-Face Selection in Addition of the Substituted Acetylide Ions to Cyclohexanone and Cyclohexanethione.
Energy changes, structural variation, and electron density shifts during nucleophilic addition to cyclohexanone and cyclohexanethione were examined by means of the ab initio calculations at the HF/6-31G level. The atomic charge on the carbonyl C was found to become more positive upon approach of the nucleophile; the density deformation maps suggest that the charge polarization occurs to a large extent in the pi bond. Since this effect is not compensated for by charge transfer until in the late stage of addition, the reaction site is considerably electron deficient (more so than the carbonyl C in the substrate) for most of the reaction path, and its interactions with the ligands are dominated by hyperconjugation with the vicinal C-H and C-C bonds. Relative stabilities of the axial and equatorial transition-state structures were examined in the series HN=C, ON&tbd1;C(-), N&tbd1;C(-), FC&tbd1;C(-), HC&tbd1;C(-), SBeC&tbd1;C(2-), O(3)SC&tbd1;C(2-), H(3)AlC&tbd1;C(2-), SC&tbd1;C(2-), H(3)BC&tbd1;C(2-), and C&tbd1;C(2-) and found to depend on polarization of the electrophiles, C=O vs C=S, and basicity of the nucleophiles. The latter dependence is parabolic: the axial preference reaches a maximum for the moderately basic anions, and it is diminished or even reversed for the most and least basic nucleophiles, i.e., in the case of reactions proceeding through the very early or very late transition states. Thus, the stereoelectronic effect is largest in the region of the reaction coordinate where the electron deficiency at the reaction site reaches a maximum. These findings corroborate the premises of the hypothesis of hyperconjugative assistance to bond formation and are consistent with major trends in the experimental data if the stereochemistry of alkylation of 4-tert-butylcyclohexanone is correlated with Pauling electronegativity of metals in the case of methylmetals, methylmetal ate complexes, allylmetals and arylmetals, and with the pK(a) of conjugated acids in the case of sulfur-, carbonyl-, and nitrile-stabilized carbanions.
Cieplak
Andrzej Stanislaw
AS
Department of Chemistry, Yale University, New Haven, Connecticut 06520, and Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794.