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 |
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Authors: | Fré dé ric Monnat,Pierre Vogel,José Á ngel Sordo |
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Abstract: | 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]oxathiin 3‐oxide ( 21 ), respectively. Above −40°, 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 (ΔH≠( 16 −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 dienes 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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