Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening in 2-Quinazolylnitrenes
2006-05-26T00:00:00Z (GMT) by
Tetrazolo[1,5-<i>a</i>]quinazoline (<b>9</b>) is converted to 2-azidoquinazoline (<b>10</b>) on sublimation at 200 °C and above, and the azide−tetrazole equilibrium is governed by entropy. 2-Quinazolylnitrenes <b>11</b> and <b>27</b> and/or their ring expansion products <b>14</b> and <b>29</b> can undergo type I (ylidic) and type II (diradicaloid) ring opening. Argon matrix photolysis of <b>9</b>/<b>10</b> affords 2-quinazolylnitrene (<b>11</b>), which has been characterized by ESR, UV, and IR spectroscopy. A minor amount of a second nitrene, formed by rearrangement or ring opening, is also observed. A diradical (<b>19</b>) is formed rapidly by type II ring opening and characterized by ESR spectroscopy; it decays thermally at 15 K with a half-life of ca. 47 min, in agreement with its calculated facile intersystem crossing (<b>19T </b>→<b> 19OSS</b>) followed by facile cyclization/rearrangement to 1-cyanoindazole (<b>21</b>) (calculated activation barrier 1−2 kcal/mol) and <i>N</i>-cyanoanthranilonitrile (<b>22</b>).<b> 21 </b>and <b>22</b> are the isolated end products of photolysis. <b>21</b> is also the end product of flash vacuum thermolysis. An excellent linear correlation between the zero-field splitting parameter <i>D</i> (cm<sup>-1</sup>) and the spin density ρ on the nitrene N calculated at the B3LYP/EPRIII level is reported (<i>R</i><sup>2</sup> = 0.993 for over 100 nitrenes). Matrix photolysis of 3-phenyltetrazolo[1,5-<i>a</i>]quinazoline (<b>25</b>) affords the benzotriazacycloheptatetraene <b>29</b>, which can be photochemically interconverted with the type I ring opening product 2-isocyano-α-diazo-α-phenyltoluene (<b>33</b>) as determined by IR and UV spectroscopy. The corresponding carbene <b>37</b>, obtained by photolysis of <b>33</b>, was detected by matrix ESR spectroscopy.