Cubane, Cuneane, and Their Carboxylates:  A Calorimetric, Crystallographic, Calculational, and Conceptual Coinvestigation

This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo[4.2.0.0.^2,50.^3,80^4,7]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo[3.3.0.0.^2,40.^3,70^6,8]octane-2,6-dicarboxylate) have been determined by combustion calorimetry, Δ_fH°_m(cr)/kJ·mol^-1 = −232.62 ± 5.84 and −413.02 ± 5.16, respectively. The enthalpies of sublimation have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T = 298.15 K, Δ_cr^gH_m(298.15 K)/kJ·mol^-1 = 117.2 ± 3.9 and 106.8 ± 3.0, respectively. Combination of these two enthalpies resulted in Δ_fH°_m(g., 298.15 K)/kJ·mol^-1 of −115.4 ± 7.0 for dimethyl 1,4-cubanedicarboxylate and −306.2 ± 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements, accompanied by quantum chemical calculations, resulted in values of Δ_fH_m(g, 298.15 K) = 613.0 ± 9.5 kJ·mol^-1 for cubane and 436.4 ± 8.8 kJ·mol^-1 for cuneane. From these enthalpies of formation, strain enthalpies of 681.0 ± 9.8 and 504.4 ± 9.1 kJ·mol^-1 were calculated for cubane and cuneane by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are disordered; the substitution pattern and structure have been confirmed by determination of the X-ray crystal structure of the corresponding diacid.