Carbenic Nitrile Imines: Properties and Reactivity

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD­(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD­(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm^–1 have propargylic structures with a CN triple bond (RCNNSiMe₃ and R₂BCNNBR₂), and those with IR absorptions below ca. 2150 cm^–1 are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900–2200 cm^–1 are carbenic (R–(<i>C</i>:)–NN–R′). Electronegative but lone pair-donating groups NR₂, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize “normal” singlet carbenes, including <i>N</i>-heterocyclic carbenes. NBO analyses at the CCSD­(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes <b>21</b> with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1<i>H-</i>diazirenes of the type <b>24</b> in mildly exothermic reactions with activation energies in the range 29–38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis

Rearrangements of Acyl, Thioacyl, and Imidoyl (Thio)cyanates to Iso(thio)cyanates, Acyl Iso(thio)cyanates to (Thio)acyl Isocyanates, and Imidoyl Iso(thio)cyanates to (Thio)acyl Carbodiimides, RCX-YCN ⇌ RCX-NCY ⇌ RCY-NCX ⇌ RCY-XCN (X and Y = O, S, NR′)

Two types of rearrangements have been investigated computationally at the B3LYP/6-311+G­(d,p) level. The activation barriers for rearrangement of acyl thiocyanates RCO–SCN to the corresponding isothiocyanates RCO–NCS are 30–31 kcal/mol in agreement with the observation that the thiocyanates are in some cases isolable albeit very sensitive compounds. Alkoxycarbonyl-, (alkylthio)­carbonyl- and carbamoyl thiocyanates are isolable and have higher calculated barriers (ca. 40 kcal/mol) toward rearrangement to isothiocyanates, whereas all thioacyl thiocyanate derivatives are rather unstable compounds with barriers in the range 20–30 kcal/mol for rearrangement to the isothiocyanates. Acyl-, alkoxycarbonyl-, and carbamoyl cyanates R–CO–OCN are predicted to be in some cases isolable compounds with barriers up to ca. 40 kcal/mol for rearrangement to the isocyanates RCO–NCO. All of the rearrangements of this type involve the HOMO of a nearly linear (thio)­cyanate anion and the LUMO of the acyl cation, in particular the acyl CX π* orbital. The second type of rearrangement involves 1,3-shifts of the groups R attached to the (thio)­acyl groups, that is, acyl isothiocyanate–thioacyl isocyanate and imidoyl isothiocyanate–thioacyl carbodiimide rearrangements. These reactions involve four-membered cyclic, zwitterionic transition states facilitated by lone pair–LUMO interactions between the migrating R group and the neighboring iso­(thio)­cyanate function. Combination of the two rearrangements leads to the general reaction scheme RCX–YCN ⇌ RCX–NCY ⇌ RCY–NCX ⇌ RCY–XCN (X and Y = O, S, NR′)

Formation of (Aza)fulvenallene, Cyanocyclopentadiene, and (Aza)fluorenes in the Thermal Rearrangements of Indazoles, Azaindazoles, and Homoquinolinic Anhydride

Flash vacuum pyrolysis (FVP) of pyrazoles and indazoles constitutes a valuable route to carbenes and nitrenes. In this study, we employed M062X and CCSD(T) calculations to provide a mechanistic rationale for the formation of fulvenallene and fluorenes from indazoles and the corresponding formation of azafulvenallene 15, cyanocyclopentadiene 19, and azafluorenes, e.g. 45, from azaindazoles, e.g. 12, and from homoquinolinic anhydride. The results reveal the importance of initial tautomerization in the pyrazole moiety of 7-azaindazole 12, which drives the mechanism toward 2-diazo-3-methylene-2,3-dihydropyridine 29 and hence 3-methylene-2,3-dihydropyridin-2-ylidene 26, followed by Wolff-type ring contraction to 1-azafulvenallene 15. This path has a calculated activation energy ∼10 kcal/mol lower than that for an alternate route involving ring opening to 3-diazomethylpyridine, dediazotization, and rearrangement of 3-pyridylcarbene to azacycloheptatetraene and phenylnitrene 24. FVP of 2,5-diphenyltetrazoles and phenyl(pyridyl)tetrazoles leads to nitrile imines, which cyclize to 3-phenylindazoles and -azaindazoles. Nitrogen elimination from these (aza) indazoles results in the formation of (aza) fluorenes, for which two alternate mechanisms are described: route A by rearrangement of (aza) indazoles to diazo(aza)cyclohexadienes and (aza)cyclohexadienylidenes and route B by rearrangement to diaryldiazomethanes and diarylcarbenes. Both paths are energetically feasible, but path A is preferred and corresponds to the azafluorenes obtained experimentally

Mechanistic Diversity in Thermal Fragmentation Reactions: A Computational Exploration of CO and CO₂ Extrusions from Five-Membered Rings

The mechanisms of a variety of thermal pericyclic fragmentation reactions of five-membered heterocyclic rings are subjected to scrutiny at a density functional level by computation of transition state free energy barriers and intrinsic reaction coordinates (IRCs). The preferred computed products generally match those observed in flash vacuum thermolysis experiments. For certain reactions, which also have the highest reaction temperatures and computed barriers, a degree of multireference character to the wave function manifests in an overestimation of the DFT-computed barrier, with a more reasonable barrier obtained by a CASSCF single point energy calculation. Many of the IRCs exhibit “hidden intermediates” along the reaction pathway, but conversely reactions that could be considered to involve the formation of an intermediate nitrene prior to alkyl or aryl migration show no evidence of such an intermediate. Such exploration of the diversity of behavior in a class of compounds using computational methods with interactive presentation of the results within the body of a journal article is suggested as being almost a <i>sine qua non</i> for laboratory-based research on reactive intermediates

Laser-Induced Carbene–Carbene Rearrangement in Solution: The Diphenylcarbene–Fluorene Rearrangement

Diphenylcarbene (DPC) generated by high-intensity laser photolysis of diphenyldiazomethane rearranges to fluorene (FL) by two distinct mechanisms as revealed by methyl-group labeling. Thus, excimer laser irradiation of <i>p</i>,<i>p</i>′-dimethyldiphenyldiazomethane generates 3,6-dimethylfluorene (3,6-DMF) and 2,7-dimethylfluorene (2,7-DMF), which were identified by fluorescence measurements as well as GC-MS and comparison with authentic materials. 3,6-DMF corresponds to direct bond formation between <i>ortho</i> positions in DPC, referred to as <i>ortho</i>,<i>ortho</i>′ coupling. 2,7-DMF corresponds to a carbene–carbene rearrangement, whereby DPC undergoes ring expansion to phenylcycloheptatetraene (PhCHT) followed by ring contraction to <i>o</i>-biphenylylcarbene (<i>o</i>-BPC), which then cyclizes to FL. The carbene–carbene rearrangement dominates over the <i>ortho</i>,<i>ortho</i>′ coupling under all conditions employed. The <i>ortho</i>,<i>ortho</i>′ coupling must take place in a higher excited state (most likely S₂ or T₁) of DPC, because it is not observed at all under thermolysis conditions, where only S₁ and T₀ are populated. The carbene–carbene rearrangement may take place either in a hot S₁ state or more likely in a higher excited state (S₂ or T₁)

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

C₁₅H₁₀ and C₁₅H₁₂ Thermal Chemistry: Phenanthrylcarbene Isomers and Phenylindenes by Falling Solid Flash Vacuum Pyrolysis of Tetrazoles

2-Phenyl-5-(phenylethynyl)­tetrazole <b>44</b> provides a new entry to the C₁₅H₁₀ energy surface. Flash vacuum pyrolysis of <b>44</b> using the falling solid flash vacuum pyrolysis (FS-FVP) method afforded cyclopenta­[<i>def</i>]­phenanthrene <b>31</b> and cyclopenta­[<i>jk</i>]­fluorene <b>52</b> as the principal products. The products are explained in terms of the formation of <i>N-</i>phenyl-<i>C</i>-phenylethynylnitrile imine/(phenylazo)­(phenylethynyl)­carbene <b>45</b> and its cyclization to 3-(phenylethynyl)-3<i>H-</i>indazole <b>46b</b>. Pyrolytic loss of N₂ from <b>46b</b> generates C₁₅H₁₀ intermediate <b>48</b>. Cyclization of <b>48</b> to a dibenzocycloheptatetraene derivative and further rearrangements with analogies in the chemistry of phenylcarbene and the naphthylcarbenes leads to the final products. Similar pyrolysis of 2-phenyl-5-styryltetrazole <b>43</b> afforded 3-styrylindazole <b>58</b>, which on further pyrolysis eliminated N₂ to generate 3- and 2-phenylindenes <b>61</b> and <b>62</b> via C₁₅H₁₂ intermediates

Nitrile Imines: Matrix Isolation, IR Spectra, Structures, and Rearrangement to Carbodiimides

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H <b>6</b>, Ph-CNN-CH₃ <b>17</b>, Ph-CNN-SiMe₃ <b>23</b>, Ph-CNN-Ph <b>29</b>, Ph₃C-CNN-CPh₃ <b>34</b>, and the boryl-CNN-boryl derivative <b>39</b>). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm^–1 have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe₃ <b>23</b>, PhCNNPh <b>29</b>, and boryl-CNN-boryl <b>39</b>). Nitrile imines with IR absorptions below ca. 2200 cm^–1 are more likely to be allenic (e.g., HCNNH <b>1</b>, PhCNNH <b>6</b>, HCNNPh <b>43</b>, PhCNNCH₃ <b>17</b>, and Ph₃C-CNN-CPh₃ <b>34</b>). All nitrile imines isomerize to the corresponding carbodiimides both thermally and photochemically. Monosubstituted carbodiimides isomerize thermally to the corresponding cyanamides (e.g., Ph-NCN-H <b>5</b> → Ph-NH-CN <b>8</b>), which are therefore the thermal end products for nitrile imines of the types RCNNH and HCNNR. This tautomerization is reversible under flash vacuum thermolysis conditions

Ketene–Ketene Interconversion. 6‑Carbonylcyclohexa-2,4-dienone–Hepta-1,2,4,6-tetraene-1,7-dione–6-Oxocyclohexa-2,4-dienylidene and Wolff Rearrangement to Fulven-6-one

6-Carbonylcyclohexa-2,4-dienone (<b>1</b>) has been generated by flash vacuum thermolysis (FVT) with Ar-matrix isolation of methyl salicylate (<b>7</b>), 2-phenylbenzo-1,3-dioxan-4-one (<b>8</b>), phthalic peranhydride (<b>9</b>), and benzofuran-2,3-dione (<b>11</b>) and also by matrix photolysis of <b>9</b>, <b>11</b>, and 2-diazocyclohepta-4,6-dien-1,3-dione (<b>12</b>). In each case, FVT above 600 °C results in decarbonylation of <b>1</b> and Wolff rearrangement to fulven-6-one (<b>13</b>) either concertedly or via open-shell singlet 6-oxocyclohexa-2,4-dienylidene (<b>18</b>). Ketenes <b>1</b> and <b>13</b> were characterized by IR spectroscopy. Photolysis of matrix-isolated <b>1</b> at 254 nm also results in the slow formation of <b>13</b>. The sequential formation of ketenes <b>1</b> and <b>13</b> from <b>7</b> has also been monitored by FVT-mass spectrometry, and <b>13</b> has been trapped with MeOH to afford methyl 1,3-cyclopentadiene-1- and -2-carboxylates <b>15</b> and <b>16</b>. FVT of methyl salicylate-1-¹³C <b>7a</b> revealed a deep-seated rearrangement of the ¹³C-labeled <b>1a</b> to hepta-1,2,4,6-tetraen-1,7-dione (<b>17a</b>) by means of electrocyclic ring opening followed by a facile 1,5-H shift and recyclization prior to CO-elimination and ring contraction to ¹³C-labeled <b>13</b>. The rearrangement mechanism is supported by M06-2X/6-311++G­(d,p) calculations, which predict feasible barriers for the FVT rearrangements and confirm the observed labeling pattern in the isolated methyl salicylate <b>7a/7b</b> and methyl cyclopentadienecarboxylates <b>20</b> and <b>21</b> resulting from trapping of <b>13</b> with MeOH