3‑Pyridazinylnitrenes and 2‑Pyrimidinylnitrenes

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>

Nitrene-Carbene-Carbene Rearrangement. Photolysis and Thermolysis of Tetrazolo[5,1‑<i>a</i>]phthalazine with Formation of 1‑Phthalazinylnitrene, <i>o-</i>Cyanophenylcarbene, and Phenylcyanocarbene

1-Azidophthalazine <b>9A</b> is generated in trace amount by mild FVT of tetrazolo­[5,1<i>-a</i>]­phthalazine <b>9T</b> and is observable by its absorption at 2121 cm^–1 in the Ar matrix IR spectrum. Ar matrix photolysis of <b>9T/9A</b> at 254 nm causes ring opening to generate two conformers of (<i>o-</i>cyanophenyl)­diazomethane <b>11</b> (2079 and 2075 cm^–1), followed by (<i>o</i>-cyanophenyl)­carbene ³<b>12</b>, cyanocycloheptatetraene <b>13</b>, and finally cyano­(phenyl)­carbene ³<b>14</b> as evaluated by IR spectroscopy. The two carbenes ³<b>12</b> and ³<b>14</b> were observed by ESR spectroscopy (<i>D</i>|<i>hc</i> = 0.5078, <i>E</i>|<i>hc</i> = 0.0236 and <i>D</i>|<i>hc</i> = 0.6488, <i>E</i>|<i>hc</i> = 0.0195 cm^–1, respectively). The rearrangement of <b>12</b> ⇄ <b>13</b> ⇄ <b>14</b> constitutes a carbene–carbene rearrangement. 1-Phthalazinylnitrene ³<b>10</b> is observed by means of its UV–vis spectrum in Ar matrix following FVT of <b>9</b> above 550 °C. Rearrangement to cyanophenylcarbenes also takes place on FVT of <b>9</b> as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes <b>16</b>–<b>18</b>. Thus the overall rearrangement <b>10</b> → <b>11 → 12</b> ⇄ <b>13</b> ⇄ <b>14</b> can be formulated

C₉H₈ Pyrolysis. <i>o</i>‑Tolylacetylene, Indene, 1‑Indenyl, and Biindenyls and the Mechanism of Indene Pyrolysis

<i>o</i>-Tolylacetylene <b>5</b> is obtained by flash vacuum pyrolysis (FVP) of the isoxazolone <b>13a</b> at 800 °C/10^–4 hPa. At 900–1000 °C the acetylene <b>5</b> isomerizes to indene <b>1</b>, which reacts further by elimination of a hydrogen atom and dimerization of the 1-indenyl radical <b>9</b> to 1,1′-biindenyl <b>10</b>. The latter undergoes partial isomerization to 3,3′-biindenyl <b>16</b>, and further pyrolysis of the biindenyls yields higher polycyclic aromatic hydrocarbons (PAHs), particularly chrysene <b>2</b>. C–H bond breakage in indene, which occurs with an activation energy of 80 ± 5 kcal/mol with formation of the 1-indenyl radical <b>9</b>, has been the subject of much investigation in relation to hydrocarbon combustion, in particular the formation of chrysene and other PAHs from indene, which itself is formed in the combustion of toluene and other hydrocarbons. However, C–C bond breakage also needs to be considered. Calculations at the B3LYP/6-311+G­(d,p) level indicate that key C–C bond breakages in indene have free energies of activation of ca. 80 kcal/mol. Positive entropies of activation make all these reactions more facile at high temperatures relevant to hydrocarbon combustion chemistry. C1–C2 bond breakage results in the formation of <i>o</i>-tolylvinylidene <b>6</b> and <i>o</i>-tolylacetylene <b>5</b>. The reversible 1,2-shift interconverting <b>5</b> and <b>6</b> (the Roger Brown rearrangement) can lead to carbon scrambling in C3-labeled indene <b>1a</b>, resulting in indene <b>1d</b> carrying the label in positions 1, 2, and 3 and explaining the ¹⁴C-labeling pattern observed by Badger et al. in the derived chrysene <b>2d</b>. <i>o-</i>Tolylacetylene <b>5</b> and <i>o-</i>tolylvinylidene <b>6</b> should be considered as intermediates in models of the fuel-rich combustion of toluene, indene, and other hydrocarbons

Phenyl­nitrene, Phenyl­carbene, and Pyridyl­carbenes. Rearrangements to Cyanocyclopentadiene and Fulvenallene

Flash vacuum thermolysis (FVT) of phenyl azide <b>29</b> as well as precursors of 2-pyridyl­carbene <b>34</b> and 4-pyridyl­carbene <b>25</b> affords phenyl­nitrene <b>30</b> (labeled or unlabeled), as revealed by matrix isolation electron spin resonance spectroscopy. FVT of 1-¹³C-phenyl azide <b>29</b> affords 1-cyanocyclo­pentadiene (cpCN) <b>32</b>, which is exclusively labeled on the CN carbon, thus demonstrating direct ring contraction in phenyl­nitrene <b>30</b> without the intervention of cyclo­perambulation and 1,3-H shifts. However, the cpCN obtained by rearrangement of pyridyl-2-(¹³C-carbene) <b>34</b> carries ¹³C label on all carbon atoms, including the CN carbon. Calculations at the B3LYP/​6‑31G* level and in part at the CASSCF/​6‑31G* and CASPT2/​cc-pVDZ//​CASSCF­(8,8)/​cc-pVDZ levels support a new mechanism whereby 2-pyridyl­carbene rearranges in part via 1-azacyclo­hepta-1,2,4,6-tetraene <b>36</b> to phenyl­nitrene, which then undergoes direct ring contraction to cpCN. Another portion of 2-pyridyl­carbene undergoes ring expansion to 4-azacyclo­hepta-1,2,4,6-tetraene <b>42</b>, which then by trans-annular cyclization affords 6-azabicyclo[3.2.0]­cyclohepta-1,3,5-triene <b>43</b>. Further rearrangement of <b>43</b> via the spiroazirine <b>44</b> and biradical/​vinyl­nitrene <b>45</b> affords cpCN with the label on the CN group. An analogous mechanisms accounts for the labeling pattern in fulvenallene <b>60</b> formed by ring contraction of 1-¹³C-phenyl­carbene <b>59</b> in the FVT of 1-¹³C-phenyldiazomethane <b>58</b>

Ring Contraction in Arylcarbenes and Arylnitrenes; Rearrangements of 1- and 3‑Isoquinolylcarbenes and 2‑Naphthylnitrene to Cyanoindenes

Flash vacuum pyrolysis (FVP) of 1-(5-¹³C-5-tetrazolyl)­isoquinoline <b>18</b> generates 1-(¹³C-diazomethyl)­isoquinoline <b>19</b> and 1-isoquinolyl-(¹³C-carbene) <b>22</b>, which undergoes carbene–nitrene rearrangement to 2-naphthylnitrene <b>23</b>. The thermally generated nitrene <b>23</b> is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes <b>26</b> and <b>27</b> under the FVP conditions. The ¹³C label distribution in the cyanoindenes was determined by ¹³C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo­[<i>c</i>]­cyclohepta-1,2,4,6-tetraene <b>32</b> bifurcating to 2-naphthylnitrene <b>23</b> and 2-aza-benzobicyclo[3.2.0]­heptatriene <b>39</b> (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo­[<i>d</i>]­bicyclo­[4.1.0]­hepta-2,4,6-triene <b>34</b> and 3-aza-benzo­[<i>d</i>]­cyclohepta-2,3,5,7-tetraene <b>35</b>. Product studies demand that the major path is route a1 via 2-naphthylnitrene <b>23</b>, which then undergoes direct ring contraction to 1-cyanoindene; but the ¹³C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene <b>22</b> is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)­isoquinoline <b>28</b> similarly generates 3-diazomethylisoquinoline <b>29</b> and 3-isoquinolylcarbene <b>30</b>, which rearranges to 3- and 2-cyanoindenes <b>26</b> and <b>27</b>