Double Diels−Alder Strategies to Soluble 2,9- and 2,9,6,13-Tetraethynylpentacenes, Photolytic [4 + 4] Cycloadditions, and Pentacene Crystal Packing

Four new classes of organic solvent soluble ethynylpentacene derivatives (2,9-, 2,10-, 2,6,9,13-, 2,6,10,13-) have been prepared by complementary, versatile, double Diels−Alder strategies. Functional groups on the A, C, and E rings can be manipulated to increase the solubility, modulate the electronics, and alter the solid-state packing. Cycloaddition reactions with diene <b>2</b> and 1,4,5,8-anthradiquinone (<b>3</b>) or <i>ortho</i>-quinodimethane <b>19</b> with 1-butyl-3-methylimidazolim iodide (<b>18</b>) as the iodide source (a significant improvement over NaI) and benzoquinone (<b>20</b>) followed by in situ aromatization afforded the quinones <b>4</b>, <b>5</b>, <b>21</b>, and <b>22</b>, respectively. For the 2,9- and 2,10- families, a one-pot desilylation/triflation was developed. Palladium(0) coupling and reductive aromatization afforded 2,9-di(triisopropylsilylethynyl)pentacene (<b>10</b>) and 2,10-di(triisopropylsilylethynyl)pentacene (<b>11</b>), respectively. Photodimerization of these pentacenes afforded the air-stable [4 + 4] cycloaddition pentacene precursors (tetrakisnaphthotricyclo[4.2.2.2<sup>2,5</sup>]dodecanes, <b>12</b><b>−</b><b>15</b>). Thermal cycloreversion of the dimers (∼13 J/g, ∼4 kcal/mol) produces the parent pentacenes (<b>10</b> or <b>11</b>). The tetrasubstituted family utilized a parallel route with extra versatility as the timing of the Grignard and palladium(0) coupling step may be varied depending upon the functional group combinations desired. The subsequent reactions provided the tetraethynylpentacenes <b>28</b><b>−</b><b>30</b>, <b>33</b><b>−</b><b>35</b> (para-isomers), and <b>38</b> (meta-isomer). X-ray crystallography analysis of <b>28</b>,<b> 29</b>, and <b>33</b> revealed various π−π stacked packing motifs that differ from the unfavorable herringbone pattern of pentacene.