Vinylene carbonate 1,1-dioxides

Althiophene 1,1-dioxide, which was reported years ago,118 has recently been reinvestigated,125,126 It was found that the anti head-to-head (119) and anti head-to-tail (120) isomers are formed in a ratio of 2.7. This dimerization was proposed to occur via attack of a molecule in the excited triplet state (Er SO kcal/mol) on another molecule in the ground state. The preferential formation of isomer 119 was attributed to a better stabilization of the more polar transition state in the formation of 119 relative to 120 in view of the observed effect of the polarity of the solvent on the ratio of 119 to 120. Similar dimerizations have been reported for 2-nitrobenzo-l,4-dithiin (121)127 and 1,1-dimethyl-2,5-diphenyl-l-silacyclopenta-2,4-diene (111).128 Other heterocycles that dimerize via (2 + 2)-cycloaddition, either upon direct or upon sensitized irradiation, are l/f-dibenzo[6,/l-azepine derivatives (122),129 vinylene carbonates (123),u,no and 1,3-substituted imidazolin-2-ones (124).,3, ,2... 

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