Nitrile Imines and Nitrile Ylides: Rearrangements of Benzonitrile <i>N</i>‑Methylimine and Benzonitrile Dimethylmethylide to Azabutadienes, Carbodiimides, and Ketenimines. Chemical Activation in Thermolysis of Azirenes, Tetrazoles, Oxazolones, Isoxazolones, and Oxadiazolones

Flash vacuum thermolysis (FVT) of 1-methyl-5-phenyltetrazole (<b>5b</b>), 2-methyl-5-phenyltetrazole (<b>1b</b>), and 3-methyl-5-phenyl-1,3,4-oxadiazol-2­(3<i>H</i>)-one (<b>3b</b>) affords the nitrile imine (<b>2b</b>), which rearranges in part to <i>N-</i>methyl-<i>N</i>′<i>-</i>phenylcarbodiimide (<b>7b</b>). Another part of <b>2b</b> undergoes a 1,4-H shift to the diazabutadiene (<b>13</b>). <b>13</b> undergoes two chemically activated decompositions, to benzonitrile and CH₂NH and to styrene and N₂. FVT of 2,2-dimethyl-4-phenyl-oxazol-5­(2<i>H</i>)-one (<b>16</b>) at 400 °C yields 3-methyl-1-phenyl-2-azabutadiene (<b>18</b>) in high yield. In contrast, FVT of 3,3-dimethyl-2-phenyl-1-azirene (<b>21</b>) at 600 °C or 4,4-dimethyl-3-phenyl-isoxazolone (<b>20</b>) at 600 °C affords only a low yield of azabutadiene (<b>18</b>) due to chemically activated decomposition of <b>18</b> to styrene and acetonitrile. There are two reaction paths from azirene (<b>21</b>): one (path a) leading to nitrile ylide (<b>17</b>) and the major products styrene and acetonitrile and the other (path b) leading to the vinylnitrene (<b>22</b>) and ketenimine (<b>23</b>). The nitrile ylide PhC^–N⁺C­(CH₃)₂ (<b>17</b>) is implicated as the immediate precursor of azabutadiene (<b>18</b>). FVT of either 3-phenylisoxazol-5­(4<i>H</i>)­one (<b>25</b>) or 2-phenylazirene (<b>26</b>) at 600 °C affords <i>N</i>-phenylketenimine (<b>28</b>). The nitrile ylide PhC^–N⁺CH₂ (<b>30</b>) is postulated as a reversibly formed intermediate. <i>N-</i>Phenylketenimine (<b>28</b>) undergoes chemically activated free radical rearrangement to benzyl cyanide. The mechanistic interpretations are supported by calculations of the energies of key intermediates and transition states