Mechanism and structural aspects of thermal Curtius rearrangement. Quantum chemical study

CHNO - Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory

Fulminic and cyanic acids played a decisive role in the conception of isomerism 200 years ago. Cyanic (HOCN), isocyanic (HNCO), and fulminic (HCNO) acids have been detected in several interstellar sources, but isofulminic acid (HONC) is little known. Here we examine the interrelationships between the four acids and formylnitrene, HC(O)N, at the CASPT2 and three DFT levels. Formylnitrene has a triplet ground state, T0, a closed shell singlet (CSS), S0, and an open-shell singlet (OSS), S1, lying ∼7 and 27 kcal/mol above T0, respectively. The CSS is weakly stabilized by a 12 kcal/mol bond between the N and the O atoms. A conical intersection 12 kcal/mol above T0 permits easy T0-S0 interchange. Formyl azide and formylnitrene (T0 and S0) are isomerized thermally to HNCO. HOCN is best obtained via dissociation of the nitrene (or of HNCO) to H• + NCO• radicals ∼46 kcal/mol above the T0 nitrene. Isofulminic acid, HONC, isomerizes readily to cyanic acid, HOCN, in thermal and photochemical reactions. Fulminic acid, HCNO, can isomerize to HNCO via CSS formylnitrene. Easy tautomerization prevents the preparation of HOCN in quantity. The barrier to isomerization is strongly reduced in small hydrogen-bonded aggregates so that trace amounts of HOCN can exist in equilibrium with HNCO.

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.

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

Organic azides are an efficient source of nitrenes, which serve as vigorous intermediates in many useful organic reactions. In this work, the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods were employed to study the photochemistry of 2-furoylazide 1 and 3-furoylazide 5, including the Curtius rearrangement to two furylisocyanates (3 and 7) and subsequent reactions to the final product cyanoacrolein 9. Our calculations show that the photoinduced Curtius rearrangement of the two furoylazides takes place through similar stepwise mechanisms via two bistable furoylnitrenes 2 and 6. However, the decarbonylation and ring-opening process of 7 to 9 prefers a stepwise mechanism involving the 3-furoylnitrene intermediate 8, while 3 to 9 goes in a concerted asynchronous way without the corresponding 2-furoylnitrene intermediate 4. Importantly, we revealed that several conical intersections play key roles in the photochemistry of furoylazides. Our results are not only consistent and also make clear the experimental observations (X. Zeng, et al., J. Am. Chem. Soc., 2018, 140, 10-13), but additionally provide important information on the chemistry of furoylazides and nitrenes.

Chemical Dynamics Simulations of Curtius Reaction of Acetyl- and Fluorocarbonyl Azides

Curtius rearrangement is the elimination of N₂ from carbonyl azides RC(O)N₃ to form isocyanates RNCO. Two mechanisms, viz., stepwise and concerted have been proposed in the literature for this reaction. The stepwise mechanism involves the formation of a nitrene RC(O)N by elimination of N₂ followed by an intramolecular rearrangement of the nitrene to form the isocyanate. The concerted mechanism is a single-step pathway forming the N₂ + RNCO products directly. Previous experimental and theoretical studies have indicated that the mechanism is usually concerted for thermal reactions and both stepwise and concerted are preferred under photochemical conditions. In the present work, we investigated the mechanism of Curtius rearrangement of two carbonyl azides with different substituents (R = CH₃ and F). Atomic level reaction mechanisms were studied using chemical dynamics simulations under thermal reaction conditions. Classical trajectories were generated on-the-fly at the density functional B3LYP/6-31+G* level of electronic structure theory with similar initial conditions for both the molecules. Simulation results showed a dominant concerted mechanism for CH₃C(O)N₃ and the operation of both the mechanisms for FC(O)N₃. The fluorocarbonyl nitrene FC(O)N had an appreciable lifetime before undergoing intramolecular rearrangement to form the isocyanate. In a small number of trajectories, the product isocyanate produced via the concerted dissociation of FC(O)N₃, isomerized back to the nitrene form.

Photoinduced Curtius rearrangements of fluorocarbonyl azide, FC(O)N3: a QM/MM nonadiabatic dynamics simulation

Upon either photolysis or pyrolysis, carbonyl azide can eliminate molecular nitrogen along with the formation of isocyanate.

Theoretical insights into photo-induced Curtius rearrangement of chlorodifluoroacetyl azide

The mechanism of photo-induced Curtius rearrangement is a two-step reaction with a nitrene intermediate.

Synthesis of reduction-sensitive 1,1-diarylhydrazines from 1,1-diarylamines

1-(2-Nitrophenyl)-1-phenylamine and methyl 4-((2-nitrophenyl)amino)benzoate have been transformed into their corresponding urea derivatives through the action of chlorosulfonyl isocyanate. The initial sulfimidate product from the former reaction has sufficient stability so that it can be isolated and characterized as its disodium salt, and this, as well as three other subsequent products, have been characterized by X-ray diffraction. The corresponding intermediary urea was converted into its hydrazine derivative via a Hofmann rearrangement under oxidative conditions. Density functional theory has been used to examine the nature of the intermediates and transition states for the Hofmann rearrangement. There is little theoretical indication for a cyclic aziridinonium intermediate and the transition state between the urea and the isocyanate corresponds to a reactant-like rotation of the planar singlet nitrene before migration and formation of the new N−N bond.

The Curtius rearrangement of cyclopropyl and cyclopropenoyl azides. A combined theoretical and experimental mechanistic study

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.