GEMINAL SUBSTITUENT EFFECTS - 16 - DOES THE ANOMERIC EFFECT IN KETALSDEPEND ON THE HYBRIDIZATION OF THE CENTRAL CARBON-ATOM

Enthalpies of formation Delta H(f)degrees(g) of ketals prepared from c yclopropanone, cyclobutanone, cyclopentanone, cyclohexanone, and 7-nor bornanone with methanol, ethylene glycol, and 2,2-dimethyl-1,3-propane diol, as well as of acetals/ketals of acyclic aldehydes and ketones, h ave been determined by measuring their heats of combustion and their h eats of sublimation/vaporization. Group increments defining the strain -free energy level have been derived from the collected experimentally determined Delta H(f)degrees(g) values of unstrained acetals and keta ls, and the anomeric stabilizations of these groups (given in brackets ) have been calculated from a comparison of their group increments wit h those of ethers: CH2[2O, C] -64.9 (-20.2), CH[2O, C] -63.5 (-21.3), C[2O, 2C]-61.9 (-26.8), CH[2O, Ph] -58.4 (-16.2), C[2O, Ph, C] -56.3 ( -21.2), C[2O, 2Ph] -67.1 (-32.0) kJ mol(-1). Enthalpies of formation D elta H(f)degrees(g) of cyclic and spirobicyclic ketals have also been determined experimentally and compared with values obtained from molec ular mechanics calculations (MM3). The close agreement of the results shows that the anomeric interactions, which are already integrated in the MM3 force field, are not dissimilar in the small-ring cyclic and s pirobicyclic ketals investigated in this study. The hybridization of t he anomeric carbon atom apparently has no influence on the size of the anomeric effect detectable. The strain enthalpies of the cyclic and s pirobicyclic ketals have therefore been calculated from their Delta H( f)degrees(g) values using the group increment scheme. Analysis and int erpretation of all geminal interactions known in acetals/ketals is pos sible by means of a recently developed additivity scheme, and a single value of -38.6 kJ mol(-1) has been obtained for the structural increm ent representing the inherent geminal O-C-O interactions.