Theoretical study on the photochemistry of furoylazides: Curtius rearrangement and subsequent reactions

Ultrafast photogeneration of a metal-organic nitrene from 1,1'-diazidoferrocene

Ferrocene and its derivatives have fascinated chemists for more than 70 years, not least due to the analogies with the properties of benzene. Despite these similarities, the obvious difference between benzene and ferrocene is the presence of an iron ion and hence the availability of d-orbitals for properties and reactivity. Phenylnitrene with its rich photochemistry can be considered an analogue of nitrenoferrocene. As with most organic and inorganic nitrenes, nitrenoferrocene can be obtained by irradiating the azide precursor. We study the photophysical and photochemical processes of dinitrogen release from 1,1'-diazidoferrocene to form 1-azido-1'-nitrenoferrocene with UV-pump-mid-IR-probe transient absorption spectroscopy and time-dependent density functional theory calculations including spin-orbit coupling. An intermediate with a bent azide moiety is identified that is pre-organised for dinitrogen release via a low-lying transition state. The photochemical decay paths on the singlet and triplet surfaces including the importance of spin-orbit coupling are discussed. We compare our findings with the processes discussed for photochemical dinitrogen activation and highlight implications for the photochemistry of azides more generally.

3-Fluoro-2H-azirine: Generation, Characterization, and Photochemistry

The elusive 3-fluoro-2H-azirine, cyclic NCH2CF, has been generated through the stepwise decomposition of the acryloyl azide CH2CFC(O)N3 in an N2-matrix at 10 K. The characterization of cyclic NCH2CF with matrix-isolation IR spectroscopy is supported by 15N isotope labeling and the calculations with density functional theory (DFT) at the B3LYP/6-311++G(3df,3pd) level of theory. Upon irradiation at 193 nm, cyclic NCH2CF undergoes ring opening by forming the more stable nitrile isomer CH2FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH2CFC(O)N3 in the gas phase at 500 °C yields the Curtius rearrangement product CH2CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH2CN). Calculations on the potential energy profile for the decomposition reactions of CH2CFC(O)N3 demonstrate that the excessive energy, arising from the highly exothermic Curtius rearrangement of the azide, plays a key role in driving further dissociation reactions of CH2CFNCO by overcoming the formidable barriers (>50 kcal mol-1) under the pyrolysis conditions.

A theoretical study on the photochemical generation of phenylborylene from phenyldiazidoborane

Organic borylenes are a kind of highly reactive species, which play important roles in a lot of reactions as vigorous intermediates. In this work, we investigated the photochemical generation mechanisms of phenylborylene (PhB) together with the side product N-phenylnitrenoiminoborane (PhNBN) from phenyldiazidoborane (PhBN₆) by extrusion of dinitrogen in the two lowest electronic singlet states (S₀ and S₁) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the reaction PhBN₆ → PhB + 3N₂ involves stepwise N₂ extrusion three times and the azido region rearrangement. Moreover, we found that the studied photo-induced processes are kinetically feasible because the highest energy barrier is only 0.36 eV and excitation with light of wavelength 254 nm can provide enough excess energy to overcome these energy barriers. Importantly, we revealed that several conical intersections between S₁ and S₀ states participate and facilitate the studied photochemical processes. Our results not only clarify the experimental observations, (H. F. Bettinger, J. Am. Chem. Soc. 2006, 128, 2534), but also provide valuable insights into borylene chemistry.

Curtius-Type Rearrangement of Sulfinyl Azides: A Matrix Isolation and Computational Study

The highly unstable methyl sulfinyl azide, CH₃S(O)N₃, has been synthesized and characterized for the first time. In the gas phase, CH₃S(O)N₃ decomposes quickly at room temperature (300 K) with an estimated half-life (t_1/2) of 7 min. Upon irradiation at 266 nm in cryogenic Ar (10 K) and Ne (3 K) matrices, the azide extrudes molecular nitrogen by yielding the novel sulfinyl nitrene intermediate CH₃S(O)N in the closed-shell singlet ground state, which has been characterized with matrix-isolation IR and UV-vis spectroscopy. Prolonged irradiation at 266 nm causes Curtius rearrangement of the nitrene to form N-sulfinylamine CH₃NSO and S-nitrosothiol CH₃SNO. By high-vacuum flash pyrolysis (HVFP) at 800 K, CH₃S(O)N₃ also decomposes and furnishes CH₃S(O)N with minor fragmentation products HNSO and CH₂ in the gas phase. A similar photo-induced Curtius-type rearrangement of trifluoromethyl sulfinyl azide CF₃S(O)N₃ to CF₃NSO and CF₃SNO has also been observed in matrices. According to the theoretical calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G (3df,3pd) level of theory, the rearrangement of CH₃S(O)N₃ prefers a stepwise pathway by initial formation of the nitrene intermediate CH₃S(O)N. In line with the thermal persistence of CH₃S(O)N in the gas phase, the barriers for its subsequent rearrangement are higher than 30 kcal mol^-1.

Theoretical insights into photo-induced isomerization mechanisms of phenylsulfinyl radical PhSO˙

Sulfinyl radicals (R-SO˙) play important roles in lots of reactions, while the isomer oxathiyl radicals (R-OS˙) and the isomerization between them are rarely observed due to the poor stability of R-OS˙. In this work, the complete active space self-consistent field (CASSCF) and its multi-state second order perturbation (MS-CASPT2) methods were employed to study the photo-induced reaction mechanisms of phenylsulfinyl radical PhSO˙ 1 and its isomer phenoxathiyl radical PhOS˙ 2. Our results show that 1 and 2 have similar singly occupied molecular orbitals in the ground state but different properties in the excited state, which determine their diverse behaviors after irradiation. Radical 1 can generate 2 by light irradiation, but 2 produces isomerization product 3 (2-hydroxyphenylthiyl radical) and ring-opening product 4 (acyclic thioketoketene radical) in two paths via S atom migration intermediate Int1 (2-carbonylcyclohexadienthiyl radical). The former path involves consequent hydrogen shift reactions with a strongly exothermic process while the latter path involves both ring-expansion and ring-opening processes with a high barrier, resulting in a structural and energetic preference for the former path. Moreover, we revealed several conical intersections that participate in the reactions and facilitate the photochemical processes. Our calculations not only remain consistent with and clarify the experimental observations (X. Zeng, et al., J. Am. Chem. Soc., 2018, 140(31), 9972-9978) but also enrich the knowledge of sulfinyl radicals and isomer oxathiyl radicals.

Nitrene formation is the first step of the thermal and photochemical decomposition reactions of organic azides

In this work, the decomposition of a prototypical azide, isopropyl azide, both in the ground and excited states, has been investigated through the use of multiconfigurational CASSCF and MS-CASPT2 electronic structure approaches. Particular emphasis has been placed on the thermal reaction starting at the S₀ ground state surface. It has been found that the azide thermally decomposes via a stepwise mechanism, whose rate-determining step is the formation of isopropyl nitrene, which is, in turn, the first step of the global mechanism. After that, the nitrene isomerizes to the corresponding imine derivative. Two routes are possible for such a decomposition: (i) a spin-allowed path involving a transition state; and (ii) a spin-forbidden one via a S₀/T₀ intersystem crossing. Both intermediates have been determined and characterised. Their associated relative energies have been found to be quite similar, 45.75 and 45.52 kcal mol^-1, respectively. To complete this study, the kinetics of the singlet and triplet channels are modeled with the MESMER (Master Equation Solver for Multi-Energy Well Reactions) code by applying the RRKM and Landau-Zener (with WKB tunnelling correction) theories, respectively. It is found that the canonical rate-coefficients of the singlet path are 2-orders of magnitude higher than the rate-coefficients of the forbidden reaction. In addition, the concerted mechanism has been investigated that would lead to the formation of the imine derivative and nitrogen extrusion in the first step of the decomposition. After a careful analysis of CASSCF calculations with different active spaces and their comparison with single electronic configuration methods (MP2 and B3LYP), the concerted mechanism is discarded.

A comprehensive theoretical analysis of Curtius rearrangement of syn-syn and syn-anti conformers of oxalyl diazide

The complete theoretical study of thermal Curtius rearrangement of syn-syn and syn-anti conformers of oxalyl diazide, in the gas phase and in solution has been established for the first time. The inexplicit solvent effect was taken into account via the self-consistent reaction field (SCRF) method. The gas and solution phases of all optimized geometries of the mentioned conformers associated with the Curtius rearrangement along the concerted and stepwise pathways were reported using the polarized continuum model and non-electrostatic terms from the SMD universal solvation model. The Curtius rearrangement of syn-syn and syn-anti conformers was taken place via concerted and stepwise pathways, respectively. The syn-syn conformer of oxalyl diazide is more stable than the syn-anti conformer in the gas phase and solution, and rearranged to syn-carbonyl azide isocyanate via an exergonic concerted mechanism with a single transition state. Nevertheless, the rearrangement of syn-anti conformer occurred through the two transition states and an intermediate, which the first and second steps are endergonic and exergonic, respectively. Theoretical results point out that the concerted pathway is predominant with 10²-10⁶ and 10⁴-10⁵ times faster than the stepwise mechanism in gas phase and solution, respectively. Topological analysis of the electron localization function at the B3LYP/6-311++G (2d,d,p) level of theory indicate that the catastrophe sequence 1-6-C^†TSC^†F C^†C-0 begins with the N₄-N₅ bond breaking, elimination of nitrogen molecule and increasing of non-bonding monosynaptic attractor on N₄ atom, and then changing of topological signature of C₂-N₄ bond, breaking of C₁-C₂ bond, and formation of pseudo-radical centers on C₁ and C₂ atoms. Subsequently, annihilation of pseudo-radical centers on the C₁ atom, change of topological signature of C₂-N₄ and formation of C₁-N₄ bond were executed. The obtained results of ELF calculations show that the reaction takes place via a concerted mechanism but highly asynchronous process.

Tunable photochemical 6π heterocyclization reactions mediated by a boron Lewis acid

The regulation effect of boron Lewis acid catalyst on the photoinduced 6π heterocyclization was investigated by using multi-configurational ab initio calculations.