Enthalpies of Combustion and Formation of Severely Crowded Methyl-Substituted 1,3-dioxanes. The Magnitudes of 2,4- and 4,6-diaxial Me,Me-Interactions and the Chair–2,5-twist Energy Difference

Enthalpies of combustion of 2,2-trans-4,6- (1) and 4,4,6,6-tetramethyl- (2) and 2,4,4,6,6- (3) and 2,2,4,4,6-pentamethyl-1,3-dioxanes (4) were determined to estimate their enthalpies of formation in the gas phase. By comparing the latter with the corresponding enthalpies estimated based on the various bond–bond interactions allowed to determine the chair–2,5-twist energy difference (ΔHCT = 29.8 kJ mol–1) for 1 since C-13 shift correlations indicate that it escapes to the 2,5-twist form where the 2-methyl groups are isoclinal and 4- and 6-methyl groups pseudoequatorial to avoid syn-axial interactions. Compounds 2 and 3 in turn give the values 21.0 and 21.6 kJ mol–1 for the 4,6-diaxial Me,Me-interaction. Finally compound 4, which retains the chair conformation to avoid pseudoaxial interactions in the twist forms gives the value 19.5 kJ mol–1 for the 2,4-diaxial Me,Me-interaction indicating that its chair form appears to be somewhat deformed.


Combustion Experiments
The enthalpies of combustion and formation of gaseous trans-2,2,4,6-tetramethyl-1,3-dioxane (1) were published earlier [3,4]. Those of compounds 2-4 were determined as described in Materials and Methods and are listed in Tables 1-3. Table 4 in turn lists the enthalpies of formation of gaseous atoms, the bond energies and the bond-bond interactions [2] for the estimation of the enthalpies of formation of theoretically strain free gaseous compounds 1 to 4 shown in Scheme 1.

Experiments
For explanation of the quantities, see Refs. [3,4,13]. 2 The values refer to 298.15 K.   1 For explanation of the quantities, see Ref. [14]. 2 The values refer to 298.15 K.

Discussion
The enthalpies of vaporization of gaseous 1-4 (Scheme 1) were estimated from Equation (1) which Wadsö derived for weakly associated compounds [12]. E.g., in the case of nine gaseous secondary amines [13] the calculated enthalpies of vaporization deviated on the average from the experimental ones only by ± 0.5 kJ mol −1 .
In order to determine syn-axial 2,4-Me,Me-interaction we must evaluate the theoretical enthalpy of formation for the strain-free gaseous 4: The experimental enthalpy of formation of gaseous 4, −520.9 kJ mol −1 , is given in Table 3. Accordingly, the total interactions in 4 are 552.6 − 520.9 = 31.7 kJ mol −1 . This includes one 2-Me ax ,6-H ax [4], one 4-Me ax ,6-H ax interaction [4], i.e., 8.5 + 3.7 = 12.2 kJ mol −1 . Thus the magnitude of 2,4-diaxial Me,Me-interaction is equal to 31.7 − 12.2 = 19.5 kJ mol −1 . In other words the syn-axial Me,Me-interactions do not differ too much from each other but appear to indicate that the chair form of 4 can be somewhat more deformed than those of 2 and 3. If compound 1 would exist also in the chair form its total strain should be practically equal to that, 31.7 kJ mol −1 , in 4. However, it is only 29.8 kJ mol −1 which supports its existence in the 2,5-twist form.

Conflicts of Interest:
The authors declare no conflicts of interest.