Synthetic Applications of 3,4-Bis(trimethylsilyl)thiophene:  Unsymmetrically 3,4-Disubstituted Thiophenes and 3,4-Didehydrothiophene<sup>†</sup><sup>,</sup><sup>‡</sup>

1997-04-04T00:00:00Z (GMT) by Xin-Shan Ye Henry N. C. Wong
3,4-Bis(trimethylsilyl)thiophene (<b>1a</b>) was synthesized by three routes:  (a) 1,3-dipolar cycloaddition; (b) modification of 3,4-dibromothiophene; and (c) intermolecular thiazole−alkyne Diels−Alder reaction. 3,4-Bis(trimethylsilyl)thiophene (<b>1a</b>) can function as a versatile building block for the construction of unsymmetrically 3,4-disubstituted thiophenes utilizing its stepwise regiospecific mono-<i>ipso</i>-substitution followed by palladium-catalyzed cross-coupling reactions. In this manner, thiophenes <b>15</b>, <b>16</b>, <b>17a</b><b>−</b><b>j</b>, <b>19a,b</b>, <b>20</b>, <b>22a</b><b>−</b><b>c</b>, <b>23a,b</b>, <b>24a</b><b>−</b><b>d</b>, <b>25a</b><b>−</b><b>c</b>, and <b>27a</b><b>−</b><b>j</b> were prepared. The thiophene-3,4-diyl dimer <b>28</b> and thiophene-3,4-diyl tetramer <b>29</b> were also realized by palladium-catalyzed self-coupling reaction of organoboroxines. The stannylthiophene <b>31</b>, formed by conversion of the C−Si bond to a C−Sn bond <i>via</i> boroxine <b>26c</b> underwent both carbonylative coupling and lithiation followed by quenching with electrophiles to afford unsymmetrically 3,4-disubstituted thiophenes <b>33</b> and <b>36a</b><b>−</b><b>c</b> as well. Moreover, 3,4-bis(trimethylsilyl)thiophene (<b>1a</b>) can be used as the starting material for the generation of the highly strained cyclic cumulene 3,4-didehydrothiophene (<b>2</b>), whose existence was substantiated by its trapping reaction with several alkenes.