Synthesis of 2,5-disubstituted thienosultines and their thermal reactions with dienophiles and nucleophiles

The 2,5-disubstituted thienosultines (5,7-disubstituted 1,4-dihydro-1H-3λ⁴-thieno[3,4-d][2,3]oxathiin-3-oxides) 5a-d were prepared from the corresponding dichlorides 4a-d with the commercially available Rongalite (sodium formaldehyde sulfoxylate) in 17-60% yields. When heated in the presence of electron-poor dienophiles, sultines 5a-d underwent elimination of SO₂, and the resulting non-Kekulé biradicals 7a-d were intercepted as the 1:1 adducts 8-12 in good to excellent yields. The pyrolysis of sultines and sulfolenes with different concentrations of dienophiles revealed that either a preequilibrium between starting reagents and biradical species or Diels-Alder and retro-Diels-Alder reaction mechanisms may be involved; however, more work is necessary to establish the proposed mechanisms. Reaction of sultine 5b with nBuLi was found to undergo a nucleophilic ring-opening reaction to give sulfinyl alcohol 17 after H₂O workup. When sultine 5a was heated in benzene in a sealed tube in the presence of methanol, methanol-d₄, or 2-mercaptoethanol, the respective 1:1 trapping adducts 19-21 as well as the rearranged sulfolene 6a were isolated in similar amounts. The isolation of adducts 19-21 may be explained by the involvement of either biradical or ionic intermediates during the pyrolysis.