Triplet States of Tetrazoles, Nitrenes, and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene

Mechanistic Understanding and Reactivity Analyses for the Photochemistry of Disubstituted Tetrazoles

The photolysis of tetrazoles has undergone extensive research. However, there are still some problems to be solved in terms of mechanistic understanding and reactivity analyses, which leaves room for theoretical calculations. Herein, multiconfiguration perturbation theory at the CASPT2//CASSCF level was employed to account for electron correction effects involved in the photolysis of four disubstituted tetrazoles. Based on calculations of vertical excitation properties and evaluations of intersystem crossing (ISC) efficiencies in the Frank-Condon region, the combination of space and electronic effects is found in maximum-absorption excitation. Two types of ISC (¹ππ* → ³nπ*, ¹ππ* → ³ππ*) are determined in disubstituted tetrazoles, and the obtained rates follow the El-Sayed rule. Through mapping three representative types of minimum energy profiles for the photolysis of 1,5-, and 2,5-disubstituted tetrazoles, a conclusion can be drawn that the photolysis of tetrazoles exhibits reactivity characteristic of bond-breaking selectivity. Kinetic evaluations show that the photogeneration of singlet imidoylnitrene operates predominately over that in the triplet state, which can be confirmed by a double-well model in the triplet potential energy surface of 1,5-disubstituted tetrazole. Similar mechanistic explorations and reactivity analyses were also applied to the photolysis of 2,5-disubstituted tetrazole to unveil fragmentation patterns of nitrile imine generation. All computational efforts allow us to better understand the photoreactions of disubstituted tetrazoles and to provide useful strategies for regulating their unique reactivity.

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp³)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp³)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

Lights on 2,5-diaryl tetrazoles: applications and limits of a versatile photoclick reaction

Recently, photoclick chemistry emerged as a powerful tool employed in several research fields, from medicinal chemistry and biology to material sciences. The growing interest in this type of chemical process is justified by the possibility to produce complex molecular systems using mild reaction conditions. However, the elevated spatio-temporal control offered by photoclick chemistry is highly intriguing, as it expands the range of applications. In this context, the light-triggered reaction of 2,5-diaryl tetrazoles with dipolarophiles emerged for its interesting features: excellent stability of the substrates, fast reaction kinetic, and the formation of a highly fluorescent product, fundamental for sensing applications. In the last years, 2,5-diaryl tetrazoles have been extensively employed, especially for bioorthogonal ligations, to label biomolecules and nucleic acids. In this review, we summarized recent applications of this interesting photoclick reaction, with a particular focus on biological fields. Moreover, we described the main limits that affect this system and current strategies proposed to overcome these issues. The general discussion here presented could prompt further optimization of the process and pave the way for the development of new original structures and innovative applications.

Graphical abstract

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.

Heteroarylcarbene-Arylnitrene Radical Cation Isomerizations

5-Phenyltetrazole 1e is an important source of phenylnitrene or the phenylnitrene radical cation ( m/ z 91) under thermal, photochemical, and electron impact conditions. Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c yield pyridylnitrene radical cations 9a^•+ ( m/ z 92) upon electron impact. In contrast, 2-(5-tetrazolyl)pyridine 12a^•+ generates 2-pyridyldiazomethane 24^•+ and 2-pyridylcarbene 26^•+ radical cations ( m/ z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene-nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyl)pyridine, 12a^•+ → 21^•+, taking place in the radical cations. Subsequent losses of N₂ generate the pyridinium diazomethyl radical 22^•+ or pyridinium-2-carbyne 23^•+. These two ions can isomerize to 2-pyridyldiazomethane 24^•+ and 2-pyridylcarbene 26^•+, the latter rearranging to the phenylnitrene radical cations 9a^•+. ¹³C-labeling of the tetrazole rings confirmed that 2-(5-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the ¹³C label, but 4-(5-tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c^•+, which has lost the ¹³C label. 2-Pyridylcarbene/phenylnitrene radical cations ( m/ z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5- a]pyridine 34. Similarly, 4-pyridylnitrene radical cation 18c^•+ or its isomers ( m/ z 92) is obtained from 1,2,3-triazolo[1,5- a]pyrazine 36. Several other α-heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by ¹³C-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-¹³C. In general, 5-aryltetrazoles generate arylnitrene radical cations under electron impact, but α-heteroaryltetrazoles generate α-heteroarylcarbene radical cations.

Phenylnitrene Radical Cation and Its Isomers from Tetrazoles, Nitrile Imines, Indazole, and Benzimidazole

Phenylnitrene radical cations m/ z 91, C₆H₅N, 8a^•+ are observed in the mass spectra of 1-, 2-, and 5-phenyltetrazoles, even though no C-N bond is present in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate that initial formation of the C-phenylimidoylnitrene 13^•+ and/or benzonitrile imine radical cation 19^•+ from 1 H- and 2 H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13^•+ to the phenylcyanamide ion 15^•+ over a low barrier. A cyclization of imidoylnitrene ion 13^•+ onto the benzene ring offers alternate, very facile routes to the phenylnitrene ion 8a^•+ and the phenylcarbodiimide ion 14^•+ via the azabicyclooctadienimine 16^•+. Eliminations of HNC or HCN from 14^•+ and 15^•+ again yield the phenylnitrene radical cation 8a^•+. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14^•+ to the phenylcyanamide ion 15^•+ requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine -3-phenyl-1 H-diazirine-phenylimidoylnitrene-phenylcarbodiimide/phenylcyanamide rearrangement has parallels in thermal and photochemical processes, but the facile cyclization of imidoylnitrene 13^•+ to azabicyclooctadienimine 16^•+ is facilitated by the positive charge making the nitrene more electrophilic. Furthermore, the benzonitrile imine radical cation 19^•+ can cyclize to indazole 24^•+, and a series of intramolecular rearrangements via hydrogen shifts, ring-openings and ring closures allow the interconversion of numerous ions of composition C₇H₆N₂^•+, including 19^•+, 24^•+, the benzimidazole ion 38^•+ and o-aminobenzonitrile ion 40^•+, all of which can eliminate either HCN or HNC to yield the C₆H₅N^•+ ions of phenylnitrene, 8a^•+, and/or iminocyclohexadienylidene, 34^•+. Moreover, benzonitrile imine 19^•+ can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45^•+. The N-phenylnitrile imine ion 2d^•+ derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a^•+ but may also cyclize to the indazole ion 24^•+. The N-phenylimidoylnitrene 59^•+ derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15^•+ and hence phenylnitrene radical cation 8a^•+.

Phenylnitrene Radical Cation Rearrangements

The electronic structure and the rearrangements of the phenylnitrene radical cation C₆H₅N^.+ 2^.+ have been investigated at DFT and CASPT2(7,9) levels of theory. The ²B₂ state has the lowest energy of five identified electronic states, and it can undergo ring expansion to the 1-azacycloheptetetraene radical cation 4^.+ with an activation energy of ca. 28 kcal/mol. Ring opening and recyclization provide a route to 5-cyanocyclopentadiene radical cation 8^.+, which may undergo facile 1,5-hydrogen shifts. The 2-, 3-, and 4-pyridylcarbene radical cations 31^.+, 35^.+ , and 39^.+ interconvert with the phenylnitrene radical cation via azacycloheptatetraenes with activation barriers <35 kcal/mol. The carbene-carbene and carbene-nitrene rearrangements, ring expansions, ring contractions, ring openings (e.g., to cyanopentadienylidene 28^.+), and cyclizations taking place in all these radical cations are completely analogous to the thermal and photochemical rearrangements.