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

Abstract

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