Geminal substituent effects, 11. The anomeric effect in orthoesters — the concept of geminal pairwise interactions for the interpretation of standard enthalpies of formation

Abstract

New enthalpies of formation AHp(g) are provided for the series of orthoesters 2a-2f, 3 and 4 from their enthalpies of combustion AH; and their enthalpies of vaporization AH& or enthalpies of sublimation AH&b. From these data and from literature data the following new thermochemical increments were calculated [kJ . Table 4) are determined by the number of hydrogen atoms in the group and by the number of painvise geminal interactions between all atoms attached to the central carbon atom. -Using this procedure for orthoesters and orthocarbonates, deviations from additivity for the geminal interaction between oxygen atoms, i. e. the anomeric effect, are observed.In recent publications we reported on quantitative enthalpic data of the anomeric stabilization of acetals and ketalsL21 obtained from their heats of formation AHfo(g). For this purpose AHfo(g) was divided into strain-free group increments for the atoms and groups of the compounds. This means that for strained structures the strain enthalpy H,, which was obtained by MM2 calculations for a reference compound was substracted from the experimental heat of formation AHf'(g).The anomeric stabilization is recognized from the increments of the carbon atoms which are flanked by the two oxygen ligands (see Table 3 kJ . mol-I. The introduction of the first alkoxy group into an alkane results in -16.9 and -16.6 kJ . mol-' stabilization, respectively, of the C or CH increment. The introduction of a second alkoxy group into the geminal position has an even higher stabilizing effect of -37.9 and -43.7 kJ .mol-' for the CH and the C increment, respectively. The difference of the stabilizing effects due to the introduction of the first and the second (gem.) alkoxy groups is, by definition, the anomeric stabilization. It is, accordingly -21.3 kJ . mot-' for acetals and -26.8 kJ . mol-' for ketalsc21 CH [02, C[02, C2] -61.9; C[02, C, Cpd provided that the stereoelectronic requirementsL31 are satisfied. There is evidence from force-field calculations that there is generally no steric inhibition for satisfying this requirement in the orthoesters and orthocarbarnates reported here[lb1.In the present paper we investigate in a similar way the anomeric effect of orthoesters, an effect discussed in the literature mainly with regard to the hydrolysis of orthoesters and for the reactivity in Grignard r e a c t i o n~[~~>~l .Only a few heats of formation of orthoesters have been reported so fad4]; they will be included in our analysis of the anomeric effect.Standard enthalpies of formation AHfo(g) obtained from the calorimetrically measured enthalpies of combustion AH,O(cr) and enthalpies of vaporization AH&,, or enthalpies of sublimation AH&b are reported for the eight orthoesters 2a-2f, 3, and 4. We prepared the 2,6,7-trioxabicyclo[2,2,2]octanes 2a-2f by the BF3 catalyzed rearrangement of the corresponding esters of 3-methyl-3-hydroxymethyl oxetanes l a -l f according to a procedure developed by CoreyL61. Purification was achieved by repeated crystallization, and, if possible, by s...

Thermochemical study of aliphatic and phenyl-substituted geminal diacetates

“…The new values of the geminal interactions in diacetates complement our previous results [6,7]. A summary of the geminal interactions of acyloxy, alkoxycarbonyl, and alkoxy groups in diacetates, malonic esters and acetates is given in Table VIII.…”

“…Apparently the anomeric 7r-a* through bond interaction of the two oxygens is strongly reduced by the interaction of their free electron pairs with the attached acyl groups. Earlier we have proposed from MM3 calculations that destabilizing gem interactions of cyano-and alkoxycarbonyl groups [6,7] are caused by dipolar repulsions. We have now calculated in the same way by MM3 the dipolar repulsions for a gem pair of diacetoxy groups located at different types of central carbon atoms.…”

“…We provided a systematic definition of the geminal interactions of different combinations of substituents, donors (OR, NR2, F) or acceptors (CN, CO2R, COR) and we investigated the interaction of two or three different or identical substituents on the central C-atom. As a result it was shown that electron donating substituents like alkoxy groups (in acetals [2,11] and ortoesters [7]) or amino groups (in aminales [8]) experience synergetic stabilization, generally called "anomeric stabilization," which is mainly caused by 7r-a* conjugation (a through bond effect). On the contrary, combinations of electron-withdrawing substituents (CN, COOR, COR) show a destabilizing synergetic effect [1,5,9,13] mainly caused by repulsive interaction through space.…”

Relationships among strain energies of mono- and poly-cyclic cyclohexanoid molecules and strain of their component rings

“…The greatest change in H m (strain) corresponds to the introduction of three nitrogen atoms into the ring. There are few factors contributing to the strain energy in 1,3,5-trioxane and 1,3,5-tri-methyl-hexahydro-s-triazine: the stabilization from two oxygens (or nitrogens) in 1,3 -position (26) and the "conflicts" between angle distortions of the C-C framework and C-C-H and H-C-H angles, torsion barriers, and nonbonded repulsions. Result of these factors indicates the apparently large strain energies of 1,3,5-trioxane and 1,3,5-tri-methyl-hexahydro-striazine.…”

Thermochemistry of phenols: buttress effects in sterically hindered phenols

“…(37) Strain enthalpy reflects a non-additive component of the enthalpy of a molecule. We define the strain of a molecule as the difference between the experimental standard molar enthalpy of formation f H o m (g) and the calculated sum of strain-free increments (38)(39)(40) of the Benson type (41,42) for ] were determined recently. (45) Appropriate strain enthalpies H m (strain) were derived (table 10, column 4) with help of the strain-free increments mentioned above.…”