3‑Pyridazinylnitrenes and 2‑Pyrimidinylnitrenes

Abstract

Mild flash vacuum thermolysis of tetrazolo­[1,5-<i>b</i>]­pyridazines <b>8T</b> generates small amounts of 3-azidopyridazines <b>8A</b> (<b>8aA</b>, IR 2145, 2118 cm^–1; <b>8bA</b>, 2142 cm^–1). Photolysis of the tetrazoles/azides <b>8T/8A</b> in Ar matrix generates 3-pyridazinylnitrenes <b>9</b>, detected by ESR spectroscopy (<b>9a</b>: <i>D</i>/<i>hc</i> = 1.006; <i>E</i>/<i>hc</i> = 0.003 cm^–1). Cyanovinylcarbenes <b>11</b>, derived from 4-diazobut-2-enenitriles <b>10</b>, are also detected by ESR spectroscopy (<b>11a</b>: <i>D</i>/<i>hc</i> = 0.362; <i>E</i>/<i>hc</i> = 0.021 cm^–1). Carbenes <b>11</b> rearrange to cyanoallenes <b>12</b> and 3-cyanocyclopropenes <b>13</b>. Triazacycloheptatetraenes <b>20</b> were not observed in the photolyses of <b>8</b>. Photolysis of tetrazolo­[1,5-<i>a</i>]­pyrimidines/2-azidopyridmidines <b>18T/18A</b> in Ar matrices at 254 nm yields 2-pyrimidinylnitrenes <b>19</b>, observable by ESR, UV, and IR spectroscopy (<b>19a</b>: ESR: <i>D</i>/<i>hc</i> = 1.217; <i>E</i>/<i>hc</i> = 0.0052 cm^–1). Excellent agreement with the calculated IR spectrum identifies the 1,2,4-triazacyclohepta-1,2,4,6-tetraenes <b>20</b> (<b>20a</b>, 1969 cm^–1; <b>20b</b>, 1979 cm^–1). Compounds <b>20</b> undergo photochemical ring-opening to 1-isocyano-3-diazopropenes <b>23</b>. Further irradiation also causes Type II ring-opening of pyrimidinylnitrenes <b>19</b> to 2-(cyanimino)­vinylnitrenes <b>21</b> (<b>21a</b>: <i>D</i>/<i>hc</i> = 0.875; <i>E</i>/<i>hc</i> = 0.00 cm^–1), isomerization to cyaniminoketenimine <b>25</b> (2044 cm^–1), and cyclization to 1-cyanopyrazoles <b>22</b>. The reaction mechanisms are discussed and supported by DFT calculations on key intermediates and pathways. There is no evidence for the interconversion of 3-pyridazinylnitrenes <b>9</b> and 2-pyrimidinylnitrenes <b>19</b>