Reversible Disulfur Monoxide (S₂O)-Forming Retro-Diels−Alder Reaction. Disproportionation of S₂O to Trithio-Ozone (S₃) and Sulfur Dioxide (SO₂) and Reactivities of S₂O and S₃

5,6-Di-tert-butyl-2,3,7-trithiabicyclo[2.2.1]hept-5-ene 7-endo-oxide (4) was prepared by addition of S2Cl2 to 3,4-di-tert-butylthiophene 1-oxide (3) in high yield. The oxidation of 4 with dimethyldioxirane gave a 7:1 isomeric mixture of 5,6-di-tert-butyl-2,3,7-trithiabicyclo[2.2.1]hept-5-ene 2-endo-7-endo-dioxide (5a) and 2-exo-7-endo-dioxide (5b) quantitatively. The thermally labile 5 was shown to undergo a retro-Diels−Alder reaction that produces S2O and 3 in a reversible way. The resulting S2O was trapped by Diels−Alder reactions with dienes to give 3,6-dihydro-1,2-dithiin 1-oxides in good yields. In the absence of the dienes, S2O disproportionates to SO2 and S3, and the resulting S3 underwent a 1,3-dipolar cycloaddition with 3 on its syn-π-face with respect to the SO bond to give a trithiolane derivative, whereas in the presence of excess norbornene, it produced the 1,3-dipolar cycloadduct with norbornene in good yield. Thus, the retro-Diels−Alder reaction of 5 functions as an S2O and S3 source. DFT calculations at the B3LYP/6-311+G(3df,2p) level were carried out in order to explain why S2O disproportionates to SO2 and S3 and why S2O acts as a dienophile and not a 1,3-dipole, whereas O3 and S3 serve as 1,3-dipoles.