Photodecomposition of 1H -Pyrrole Carbonyl Azides: Direct Observation of Singlet 1H -Pyrrole Carbonyl Nitrenes and Triplet 1H -3-Pyrrylnitrene

Synthesis, Characterization and Energetic Performance of Oxalyl Diazide, Carbamoyl Azide, and N,N'-Bis(azidocarbonyl)hydrazine

As pure compounds, small carbonyl azides enjoy a bad reputation, due to the high explosive sensitivity and instability they demonstrate. Consequently, most reported examples have only been poorly characterized. The compounds oxalyl diazide (1), carbamoyl azide (2), as well as N,N'-bis(azidocarbonyl)hydrazine (3) were obtained by performing a diazotation reaction on the corresponding hydrazo precursor. Carbamoyl azide (2) could also be obtained from oxalyl diazide via Curtius rearrangement to the reactive isocyanate, followed by reaction with water. Further, different trapping reactions of the isocyanate with hydroxyl (methanol, oxetan-3-ol) and amino (2-amino-5H-tetrazole) functions are described. All products were extensively analyzed using IR, EA, DTA and multinuclear NMR spectroscopy, and the crystal structures elucidated using single crystal X-ray diffraction. In addition, the sensitivities toward friction and impact were determined and the energetic performances of the carbonyl azides were calculated using the EXPLO5 code.

Acryloylnitrenes: Spectroscopic Characterization, Spin Multiplicities, and Rearrangement to Vinyl Isocyanates

The simplest acryloylnitrene, CH₂═CHC(O)N (1b), and two halogenated derivatives, CH₂═CFC(O)N (2b) and CH₂═CBrC(O)N (3b), were generated through the 266 nm laser photolysis of the corresponding azide precursors in solid N₂-matrices at 15 K. The IR spectroscopic characterization of these new acylnitrenes is supported by ¹⁵N-labeling and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. For the three nitrenes 1b, 2b, and 3b, two conformers exhibiting syn and anti configurations between the C═C and C═O bonds with respect to the C-C bonds have been identified. Consistent with the CBS-QB3 calculated singlet-triplet energy gaps (ΔE_ST < 0 kcal mol^-1), the IR spectral analysis suggests that all these acryloylnitrenes adopt oxazirine-like structures with closed-shell singlet spin multiplicity. Upon subsequent green light (532 nm) irradiation, these acryloylnitrenes rearrange to form vinyl isocyanates CH₂═CXNCO (X = H, F, Br), for which the IR spectra have also been obtained. According to the calculations on the α,β-fluorinated acryloylnitrenes at the CBS-QB3 level, their spin multiplicities can be switched from singlet CH₂═CFC(O)N (ΔE_ST = -0.86 kcal mol^-1) to triplet CF₂═CFC(O)N (ΔE_ST = +0.61 kcal mol^-1), whereas CFH═CFC(O)N is magnetically bistable due to a rather small ΔE_ST (+0.09 kcal mol^-1).

3-Nitrene-2-formylthiophene and 3-Nitrene-2-formylfuran: Matrix Isolation, Conformation, and Rearrangement Reactions

Two new heteroarylnitrenes, 3-nitrene-2-formylthiophene (15/15') and 3-nitrene-2-formylfuran (16/16'), in the triplet ground state have been generated in solid Ar (10.0 K) and N₂ (15.0 K) matrices by the 266 nm laser photolysis of 3-azido-2-formylthiophene (13) and 3-azido-2-formylfuran (14), respectively. According to the characterization with matrix-isolation IR spectroscopy and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level, both nitrenes exhibit two conformations depending on the orientation of the formyl groups. Upon subsequent green-light irradiation (532 nm), both the nitrenes 15/15' and 16/16' undergo ring closure to form 3,2-thienoisoxazole (17) and 3,2-furoisoxazole (18), respectively. Traces of 3-imino-4,5-dihydrothiophene-2-ketene (19), formally formed through the intramolecular 1,4-H shift in the corresponding nitrenes 15/15', have been also identified among the laser photolysis products of the azide 13. In sharp contrast to the photochemistry, the high-vacuum flash pyrolysis (HVFP) of the azide 13 at ca. 1000 K mainly yields imino ketene in two conformations 19/19' together with traces of isoxazole 17. In addition to the reversible conformational interconversion in the imino ketene 19 ↔ 19', the photoisomerization from isoxazole 17 to imino ketene 19 has also been observed. The HVFP of the azide 14 at ca. 1000 K results in complete dissociation to HCN, C₂H₂, CO, CO₂, H₂O, and N₂. Unlike the recently disclosed hydrogen-atom tunneling (HAT) in the transformation from the structurally related 2-formyl phenylnitrene (2) to imino ketene 3 in a cryogenic Ar-matrix, the absence of HAT in nitrenes 15 and 16 can be reasonably explained by the higher barrier heights and also larger barrier widths in the isomerization reactions.

Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl Nitrenes and the Photointerconversion of 4-Pyridylnitrene with Diazacycloheptatetraene

Recently, nicotinoyl nitrene (2) has been generated from the photodecomposition of nicotinoyl azide (1) and used as the key intermediate in probing nucleobase solvent accessibility inside cells. Following the 266 nm laser photolysis of nicotinoyl azide (1) and isonicotinoyl azide (5) in solid N₂ matrices at 15 K, nicotinoyl nitrene (2) and isonicotinoyl nitrene (6) have now been identified by matrix-isolation infrared (IR) spectroscopy. Both aroyl nitrenes 2 and 6 adopt closed-shell singlet ground states stabilized by significant N_nitrene···O interactions, which is consistent with the spectroscopic analysis and calculations at the CBS-QB3 level of theory. Upon subsequent visible light irradiations, 2 (400 ± 20 nm) and 6 (532 nm) undergo rearrangement to pyridyl isocyanates 3 and 7. Further dissociation of 3 and 7 under 193 nm laser irradiation results in CO elimination and formation of ketenimines 12 and 13 via the ring opening of elusive pyridyl nitrenes 4 and 8, respectively. In addition to the IR spectroscopic identification of 8 in the triplet ground state, its reversible photointerconversion with ring expansion to diazacycloheptatetraene 9 has been observed directly. The spectroscopic identification of the nitrene intermediates was aided by calculations at the B3LYP/6-311++G(3df,3pd) level, and the mechanism for their generation in stepwise decompositions of the azides is discussed in the light of CBS-QB3 calculations.