Infoscience

Journal article

Hetero-Diels-Alder and cheletropic additions of sulfur dioxide to 1,2-dimethylidenecycloalkanes. Determination of thermochemical and kinetics parameters for reactions in solution and comparison with estimates from quantum calculations

Below -60 and without catalyst, 1.2-dimethylidenecyclopentane (16), 1,2-dimethylidenecyclohexane (13), 1,2-dimethylidenecycloheptane (17), and 1,2-dimethylidenecyclooctane (18) add to sulfur dioxide in the hetero-Diels-Alder mode, giving the corresponding sultines 4,5,6,7-tetrahydro-1H-cyclopent[d][1,2]oxathiin 3-oxide (19), 1.4.5,6,7,8-hexahydro-2,3-benzoxathiin 3-oxide (14), 4,5,6,7,8,9-hexahydro-1H-cyclohept[d][1,2]oxathiin 3-oxide (20), and 1,4,5,6,7,8,9,10-octahydrocyclooct[d][1,2]oxathin 3-oxide (21), respectively. Above -40degrees, the sultines are isomerized into the corresponding sulfolenes 3,4,5,6-tetrahydro-1H-cyclopenta[c]thiophene 2,2-dioxide (22), 1,3,4,5,6,7-hexahydrobenzo[c]thiophene 2,2-dioxide (15), 3,4,5,6,7,8-hexahydro-1H-cyclohepta[c]thiophene 2,2-dioxide (23), and 1,3,4,5,6,7,8,9-octahydrocycloocta[c]thiophene 2,2-dioxide (24). Kinetics and thermodynamics data were collected for these reactions. The sultines are ca. 10 kcal/mol Diels-Alder additions (DeltaH(+)(16 + SO2 --> 19) = 6.6 +/- 0.2 kcal mol(-1) and DeltaH(+)(13 + SO2 --> 14) = 7.2 +/- 0.4 kcal mol(-1)) are ca. 2 kcal smaller than the activation enthalpies of the corresponding cheletropic additions. The activation entropies of the hetero-Diels-Alder additions (DeltaS(+)(16 + SO2 --> 19) = -50.3 +/- 1.1 cal mol(-1) K-1 and DeltaS(+)(13 + SO2 --> 14)= -48.7 +/- 1.8 cal mol(-1) K-1) are more negative than the corresponding reaction entropies (DeltaS(r)(16 + SO2 reversible arrow 19) = -40.9 +/- 1.5 cal mol(-1) K-1 and DeltaS(r)(13 + SO2 reversible arrow 14) = -36 +/- 3 cal mol(-1) K-1) in agreement with third-order rate laws that imply that two molecules of SO2 intervene in the transition states of these cycloadditions. Similar observations were made for the cheletropic additions of SO2. Attempts to simulate the thermodynamics and kinetics parameters of the reactions of SO2 with dienes 16 and 13 by density-functional theory (DFT) suggest that the calculations require an appropriate number of polarization functions in the basis set employed. A satisfactory recipe to compute the SO2 additions to large dimes can be: B3LYP/6-31G(d) geometry optimizations followed by B3LYP/6-31 + G(2df,p) single-point calculations or G2(MP2,SVP) estimates on the B3LYP/6-31G(d) geometries.

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