Synthesis and Physical Properties of Sterically Congested Cycloalkenes, 1,2-Di-<i>tert</i>-butyl-3,3,5,5-tetramethylcyclopentene and 1,2-Di-<i>tert</i>-butyl-3,3,6,6-tetramethylcyclohexene

Two sterically congested cycloalkenes (<b>9</b> and <b>10</b>), congeners of tetra-<i>tert</i>-butylethylene, were synthesized and characterized. Oxidation of the bicyclic 1,3-dithietane <b>8</b> with dimethyldioxirane (DMD) gave the <i>endo</i>,<i>endo-</i>disulfoxide <b>13</b>, thermal isomerization of which to the <i>endo</i>,<i>exo-</i>disulfoxide <b>15</b> followed by oxidation with DMD gave the trioxide <b>18</b>. Heating <b>18</b> in refluxing 1,3-dimethyl-2-imidazolidinone furnished 1,2-di-<i>tert</i>-butyl-3,3,5,5-tetramethylcyclopentene (<b>9</b>) in 69% yield by a 2-fold extrusion process. The reaction of the 1,6-diketone dihydrazone <b>23</b> with Se<sub>2</sub>Cl<sub>2</sub> gave the selenadiazoline <b>34</b> and the 1,3-diselenetane <b>35</b>. Heating <b>34</b> at 115−130 °C gave 1,2-di-<i>tert</i>-butyl-3,3,6,6-tetramethylcyclohexene (<b>10</b>), a “didehydro” derivative of tetra-<i>tert</i>-butylethylene, in 43% yield. The CC bond in <b>10</b> is strained in degree comparable to those of most strained alkenes reported so far.