Aromatic sulfonation, 131. Formation of sulfonic acids and sulfonic anhydrides in the sulfur trioxide sulfonation of some dialkylbenzenes and 1,ω-diarylalkanes

Article abstract of DOI:10.1002/jlac.199719971109

An exploratory study has been made on the SO₃ sulfonation of a number of alkylarenes in [D₂]dichloromethane as an aprotic solvent, mostly in the presence of [D₈]1,4-dioxane as a reactivity moderator, in order to obtain information on sulfonic acid and sulfonic anhydride formation. 1,2-Dimethylbenzene (1) with 1.0 mol-equiv. of SO₃ yields predominantly the 4-sulfonic acid (1-4-S), whereas with 4.0 mol-equiv. of SO₃, 18 mol-% of the corresponding (intermolecular) sulfonic anhydride (1-4-SO₂)₂O is additionally formed. 1,4-Dimethylbenzene (2) with 0.8 mol-equiv. of SO₃ yields predominantly 2-2-S, whereas with 4.0 mol-equiv. of SO₃ the sulfonic anhydride (2-2-SO₂)₂O and its 6-S and 6,6′-S₂ derivatives are formed, the eventual main product being (6-S-2-2-SO₂)₂O. Reaction of 1-ethyl-4-methylbenzene (3) with 2.0 mol-equiv. of SO₃-[D₈]dioxane at -78 → -40°C leads to the predominant formation of the 2- and 3-S derivatives in a molar ratio of 26:74, with small amounts of the three corresponding intermolecular anhydrides. Upon performing the sulfonation of 3 at 0 → 22°C, the relative amounts of the anhydrides are larger, and in addition the 3,5-S₂ derivatives of the intermolecular anhydrides are formed. The absence of the isomeric 2,6-S₂ derivative is ascribed to the buttressing effect of the 2-sulfo on the 1-ethyl group, hampering the sulfonation at C(6). Reaction of diphenylmethane (4) with 4.2 mol-equiv. of SO₃ at -20 → 25°C yields initially 4-4′-S and subsequently 4-4′,4″-S₂. Sulfonation of 1,2-diphenylethane (5) with 4.0 mol-equiv. of SO₃ at -78 → 22°C for 4.5 h leads mainly to the cyclic dimeric disulfonic anhydride [5-4′,4″(-SO₂)₂O]₂, which upon hydrolysis affords 9% of 5-2′,4″- and 91% of 5-4′,4″-S₂. However, sulfonation of 5 at 0 → 22°C for ca. 0.8 h was found to lead to the formation of the intermolecular sulfonic anhydride (4′-S-5-4″-SO₂)₂O. On sulfonation of 1,2-di-p-tolylethane (6) with 1.0 mol-equiv. of SO₃, only ca. 20% of the substrate was sulfonated to give, after hydrolysis, 3% of the 2′- and 12% of the 3′-sulfonate, whereas with 4.0 mol-equiv. of SO₃ the initial monosulfonic acids are almost completely converted to yield the 2′,3″,5″-S₃, 3′,3″,5″-S₃, and 3′,3″,5′,5″-S₄. Wiley-VCH Verlag GmbH, 1997.