Crystal structures of two activated cyclohexanones with opposite pyramidalizations of the carbonyl groups

Article abstract of DOI:10.1021/ja00049a011

The crystal structure analyses of the two 3,3,5,5-tetraalkylcyclohexanones 1 and 2 activated by Li⁺ or SbCl₅ complexation show that 1 and 2 have rather different chair conformations and opposite pyramidalizations of the carbonyl group. The bicychc ketone 1 has an exocyclic C=C double bond acting as an intramolecular nucleophile attacking the carbonyl group from the axial direction, whereas 2 is preferentially attacked by nucleophiles from the equatorial direction. In 1, the axial a protons (but not the C_α-Cβ_β bonds) are well aligned for hyperconjugative interactions with the carbonyl group, whereas, in 2, both the axial C_α-H and the C_α-C_β bonds may interact with the carbonyl group. In all structures, a C=O elongation and a C_C=O-C_α shortening are observed. In addition, 2 shows a slight C_α-C_β elongation. The conformational differences between 1 and 2 are in agreement with the general conformational flexibility of cyclohexanones, as could be shown by comparison of their Cremer-Pople puckering parameters with those of cyclohexanones from the Cambridge Structural Database. The direction of the carbonyl pyramidalization is compared with that of pyramidalized sp² C atoms in a norbornenyl cation and discussed in terms of preferential attacks of nucleophiles, as predicted by current models of stereoselective nucleophile addition (Cieplak, Houk, Klein, Felkin).