Ab initio and experimental studies on the hetero-Diels-Alder and cheletropic additions of sulfur dioxide to (E)-1-methoxybutadiene: a mechanism involving three molecules of SO(2)

Kinetics on the cheletropic addition of sulfur dioxide to (E)-1-methoxybutadiene (1) to give the corresponding sulfolene 2 (2-methoxy-2,5-dihydrothiophene-1,1-dioxide) gave the rate law d[2]/dt = k[1][SO(2)](x)() with x = 2.6 +/- 0.2 at 198 K. Under these conditions, no sultine 3 [(2RS,6RS)-6-methoxy-3,6-dihydro-1,2-oxathiin-2-oxide] resulting from a hetero-Diels-Alder addition was observed, and the cheletropic elimination 2 --> 1 + SO(2) did not occur. Ab initio and DFT quantum calculations confirmed that the cheletropic addition 1 + SO(2) --> 2 follows two parallel mechanisms, one involving two molecules of SO(2) and the transition structure with DeltaG(++) = 18.2 +/- 0.2 kcal/mol at 198 K (exptl); 22.5-22.7 kcal/mol [B3LYP/6-31G(d,p)], the other one involving three molecules of SO(2) with DeltaG(++) = 18.9 +/- 0.1 kcal/mol at 198 K (exptl); 19.7 kcal/mol [B3LYP/6-31G(d,p)]. The mechanism involving only one molecule of SO(2) in the transition structure requires a higher activation energy, DeltaG(++) = 25.2 kcal/mol [B3LYP/6-31G(d,p)]. Comparison of the geometries and energetics of the structures involved into the 1 + SO(2) --> 2, 3 and 1 + 2SO(2) --> 2, 3 + SO(2) reactions obtained by ab initio and DFT methods suggest that the latter calculation techniques can be used to study the cycloadditions of sulfur dioxide. The calculations predict that the hetero-Diels-Alder addition 1 + SO(2) --> 3 also prefers a mechanism in which three molecules of SO(2) are involved in the cycloaddition transition structure. At 198 K and in SO(2) solutions, the entropy cost (TDeltaS(++)) is overcompensated by the specific solvation by SO(2) in the transition structures of both the cheletropic and hetero-Diels-Alder reactions of (E)-1-methoxybutadiene with SO(2).     

Substituent Effect on the Competition between Hetero‐Diels‐Alder and Cheletropic Additions of Sulfur Dioxide to 1‐Substituted Buta‐1,3‐dienes

The reactivity of sulfur dioxide toward variously substituted butadienes was explored in an effort to define the factors affecting the competition between the hetero‐Diels‐Alder and cheletropic additions. At low temperature (<−70°), 1‐alkyl‐substituted 1,3‐dienes 1 that can adopt s‐cis‐conformations add to SO₂ in the hetero‐Diels‐Alder mode in the presence of CF₃COOH as promoter. In the case of (E)‐1‐ethylidene‐2‐methylidenecyclohexane ((E)‐ 4a ), the [4+2] cycloaddition of SO₂ is fast at −90° without acid catalyst. (E)‐1‐(Acyloxy)buta‐1,3‐dienes (E)‐ 1c , (E)‐ 1y , and (E)‐ 1z with AcO, BzO, and naphthalene‐2‐(carbonyloxy) substituents, respectively also undergo the hetero‐Diels‐Alder addition with SO₂+CF₃COOH at low temperatures, giving a 1 : 10 mixture of the corresponding cis‐ and trans‐6‐(acyloxy)sultines c‐ 2c,y,z and t‐ 2c,y,z , respectively). Above −50°, the sultines undergo complete cycloreversion to the corresponding dienes and SO₂, which that add in the cheletropic mode at higher temperature to give the corresponding 2‐substituted sulfolenes (=2,5‐dihydrothiophene 1,1‐dioxides) 3 . The hetero‐Diels‐Alder additions of SO₂ follow the Alder endo rule, giving first the 6‐substituted cis‐sultines that equilibrate then with the more stable trans‐isomers. This statement is based on the assumption that the S=O group in the sultine prefers a pseudo‐axial rather than a pseudo‐equatorial position, as predicted by quantum calculations. The most striking observation is that electron‐rich dienes such as 1‐cyclopropyl‐, 1‐phenyl‐, 1‐(4‐methoxyphenyl)‐, 1‐(trimethylsilyl)‐, 1‐phenoxy‐, 1‐(4‐chlorophenoxy)‐, 1‐(4‐methoxyphenoxy)‐, 1‐(4‐nitrophenoxy)‐, 1‐(naphthalen‐2‐yloxy)‐, 1‐(methylthio)‐, 1‐(phenylthio)‐, 1‐[(4‐chlorophenyl)thio]‐, 1‐[(4‐methoxyphenyl)thio]‐, 1‐[(4‐nitrophenyl)thio]‐, and 1‐(phenylseleno)buta‐1,3‐diene, as well as 1‐(methoxymethylidene)‐2‐methylidenecyclohexane ( 4f ) do not equilibrate with the corresponding sultines between −100 and −10°, in the presence of a large excess of SO₂, with or without acidic promoter. The hetero‐Diels‐Alder additions of SO₂ to 1‐substituted (E)‐buta‐1,3‐dienes are highly regioselective, giving exclusively the corresponding 6‐substituted sultines. The 1‐substituted (Z)‐buta‐1,3‐dienes do not undergo the hetero‐Diels‐Alder additions with sulfur dioxide.     

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]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.     

17O NMR spectroscopy of sulfolenes (2,5-dihydrothiophene-1,1-dioxides) and sultines (3,6-dihydro-1,2-oxathiin-2-oxides)--experiment and quantum calculations: synthesis of 4,9-dioxo-1,2-oxathiacyclodecane-2-oxide, a new heterocycle

The products of hetero-Diels-Alder reactions (sultines) and cheletropic addition reactions (sulfolenes) between 1,3-dienes and sulfur dioxide can be distinguished by their 17O NMR shifts. Experimental data have been collected for derivatives of 3,6-dihydro-1,2-oxathiin-2-oxide and of 2,5-dihydrothiophene-1,1-dioxide. This data was then compared with that calculated by the gauge independent atomic orbital (GIAO) method at the HF/6-31 + G(d,p) and HF/6-311 + G(3df, 2p) levels of theory with geometries optimized by MP2/6-31G(d) calculations. GIAO-MBPT(2) calculations were also performed with the 6-31 + G(d,p) basis set. The adduct between (E)-1-methoxybutadiene and SO2 is sulfolene 3, the ozonolysis of which in SO2 followed by work-up with ethanol provided (2RS,3SR,6SR)-(31), (2RS,3RS,6SR)-(32), and (2RS,3RS, 6RS)-2,6-diethoxy-3-methoxy-1,4-oxathiane-4,4-dioxide (33). Single-crystal X-ray diffraction studies are reported for 32 and 33. Ozonolysis of the hetero-Diels-Alder adduct of SO2 with 1,2-dimethylidenecyclohexane produced 4,9-dioxo-1,2-oxathiacyclodecane-2-oxide (34), the first member of a new class of sulfur heterocycles.