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Does the Anomeric Effect in Ketals Depend on the Hybridization of the Central Carbon Atom?
Author(s) -
Verevkin Sergey P.,
Peng WeiHong,
Beckhaus HansDieter,
Rüchardt Christoph
Publication year - 1998
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199811)1998:11<2323::aid-ejoc2323>3.0.co;2-3
Subject(s) - chemistry , anomeric effect , anomer , geminal , cyclopentanone , standard enthalpy of formation , computational chemistry , cyclohexanone , cyclopropane , carbon atom , medicinal chemistry , stereochemistry , organic chemistry , crystallography , alkyl , ring (chemistry) , catalysis
Enthalpies of formation Δ H f °(g) of ketals prepared from cyclopropanone, cyclobutanone, cyclopentanone, cyclohexanone, and 7‐norbornanone with methanol, ethylene glycol, and 2,2‐dimethyl‐1,3‐propanediol, as well as of acetals/ketals of acyclic aldehydes and ketones, have been determined by measuring their heats of combustion and their heats of sublimation/vaporization. Group increments defining the strain‐free energy level have been derived from the collected experimentally determined Δ H f °(g) values of unstrained acetals and ketals, and the anomeric stabilizations of these groups (given in brackets) have been calculated from a comparison of their group increments with those of ethers: CH 2 [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 Δ H f °(g) of cyclic and spirobicyclic ketals have also been determined experimentally and compared with values obtained from molecular 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 spirobicyclic ketals investigated in this study. The hybridization of the anomeric carbon atom apparently has no influence on the size of the anomeric effect detectable. The strain enthalpies of the cyclic and spirobicyclic ketals have therefore been calculated from their Δ H f °(g) values using the group increment scheme. Analysis and interpretation of all geminal interactions known in acetals/ketals is possible by means of a recently developed additivity scheme, and a single value of –38.6 kJ mol −1 has been obtained for the structural increment representing the inherent geminal O–C–O interactions.