Capture of an Elusive Nitrile Ylide as an Intermediate in Isoxazole–Oxazole Photoisomerization

Photochemical three-component assembly of tri-substituted oxazoles through a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade

Construction of complex molecular skeletons with ubiquitous chemical feedstocks in a single transformation is highly appealing in organic synthesis. We report a novel visible-light-induced three-component reaction for the construction of complex 2,4,5-trisubstituted oxazoles, which are valuable in medicinal chemistry, from simple and readily available iodonium-phosphonium hybrid ylides, carboxylic acids, and nitriles. This reaction features a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade, in which a photo-generated α-phosphonium carbene acts as a sequence trigger. This catalyst- and additive-free transformation exhibits high efficiency and broad substrate scope for synthesizing diverse oxazoles.

Synthesis of Highly Reactive Ketenimines via Photochemical Rearrangement of Isoxazoles

Ketenimines are highly electrophilic species with multiple applications as building blocks in organic synthesis; however, the effective preparation of these versatile entities remains a synthetic challenge. Here we report a continuous photochemical process that generates ketenimines via skeletal rearrangement of trisubstituted isoxazoles. The resulting flow process is noteworthy, as it provides for the first time a straightforward entry into these ketenimines that were previously only observed spectroscopically. The value of this methodology toward heterocyclic transposition reactions is demonstrated by converting isoxazoles via isolable ketenimines into pharmaceutically relevant pyrazoles.

Collision Induced Dissociation of Deprotonated Isoxazole and 3-Methyl Isoxazole via Direct Chemical Dynamics Simulations

Isoxazoles are an important class of organic compounds widely employed in synthesis and drug design. Fragmentation chemistry of the parent isoxazole molecule and its substituents has been the subject of several experimental and theoretical investigations. Collision induced dissociation (CID) of isoxazole and its substituents has been studied experimentally under negative ion conditions. Based on the observed reaction products, dissociation patterns were proposed. In the present work, we studied the dissociation chemistry of deprotonated isoxazole and 3-methyl isoxazole using electronic structure theory calculations and direct chemical dynamics simulations. Various deprotonated isomers of these molecules were activated by collision with an Ar atom, and the ensuing fractionation patterns were studied using on-the-fly classical trajectory simulations at the density functional B3LYP/6-31+G* level of electronic structure theory. A variety of reaction products and pathways were observed, and it was found that a nonstatistical shattering mechanism dominates the CID dynamics of these molecules. Simulation results are compared with experiments, and detailed atomic level dissociation mechanisms are presented.

UV-Induced Photochemistry of 1,3-Benzoxazole, 2-Isocyanophenol, and 2-Cyanophenol Isolated in Low-Temperature Ar Matrixes

The monomers of 1,3-benzoxazole isolated in a cryogenic argon matrix were characterized by infrared spectroscopy. The photochemistry of matrix-isolated 1,3-benzoxazole, induced by excitation with a frequency-tunable narrowband UV light, was investigated. Irradiation at 233 nm resulted in a nearly quantitative conversion of 1,3-benzoxazole into 2-isocyanophenol. The individual photochemical behavior of the in situ produced 2-isocyanophenol was studied upon excitations at 290 nm, where 1,3-benzoxazole does not react. The photochemistry of isomeric matrix-isolated 2-cyanophenol was also studied. The photoreactions of 2-substituted (cyano- or isocyano-) phenols were found to have many similarities: (i) OH bond cleavage, yielding a 2-substituted (cyano- or isocyano-) phenoxyl radical and an H-atom, (ii) recombination of the detached H-atom, resulting in an oxo tautomer, and (iii) decomposition leading to fulvenone, together with HCN and HNC. In another photoprocess, 2-cyanophenol undergoes a [1,5] H-shift from the hydroxyl group to the cyano group yielding isomeric ketenimine. The analogous [1,5] H-shift from the hydroxyl group to the isocyano group must have also occurred in 2-isocyanophenol; however, the resulting nitrile ylide isomer is kinetically unstable and collapses to benzoxazole. All photoproducts were characterized by comparing their observed infrared spectra with those computed at the B3LYP/6-311++G(d,p) level. The mechanistic analysis of the photochemistry occurring in the family of the title compounds is presented.

Time-Resolved Photoelectron Spectroscopy Studies of Isoxazole and Oxazole

The excited state relaxation pathways of isoxazole and oxazole upon excitation with UV-light were investigated by nonadiabatic ab initio dynamics simulations and time-resolved photoelectron spectroscopy. Excitation of the bright ππ*-state of isoxazole predominantly leads to ring-opening dynamics. Both the initially excited ππ*-state and the dissociative πσ*-state offer a combined barrier-free reaction pathway, such that ring-opening, defined as a distance of more than 2 Å between two neighboring atoms, occurs within 45 fs. For oxazole, in contrast, the excited state dynamics is about twice as slow (85 fs) and the quantum yield for ring-opening is lower. This is caused by a small barrier between the ππ*-state and the πσ*-state along the reaction path, which suppresses direct ring-opening. Theoretical findings are consistent with the measured time-resolved photoelectron spectra, confirming the timescales and the quantum yields for the ring-opening channel. The results indicate that a combination of time-resolved photoelectron spectroscopy and excited state dynamics simulations can explain the dominant reaction pathways for this class of molecules. As a general rule, we suggest that the antibonding σ*-orbital located between the oxygen atom and a neighboring atom of a five-membered heterocyclic system provides a driving force for ring-opening reactions, which is modified by the presence and position of additional nitrogen atoms.

Photochemical Conversion of Isoxazoles to 5-Hydroxyimidazolines

Photochemical conversion of isoxazole I to 5-hydroxyimidazoline VII proceeded via the trapping of the photogenerated acylazirine III with amines under UV light irradiation. This is the first report of the efficient synthesis of 5-hydroxyimidazolines that are not readily accessible by other means. 5-Hydroxyimidazolines were also converted into multisubstituted imidazoles in one step by treatment with trifluoroacetic anhydride (TFAA) and 2,6-lutidine in dichloromethane.

Development of a Continuous Flow Photoisomerization Reaction Converting Isoxazoles into Diverse Oxazole Products

A continuous flow process is presented, which directly converts isoxazoles into their oxazole counterparts via a photochemical transposition reaction. This results in the first reported exploitation of this transformation to establish its scope and synthetic utility. A series of various di- and trisubstituted oxazole products bearing different appendages including different heterocyclic moieties were realized through this rapid and mild flow process. Furthermore, the robustness of this approach was demonstrated by generating gram quantities of selected products while also providing insights into likely intermediates.

Synthesis of Isoxazole- and Oxazole-4-carboxylic Acids Derivatives by Controlled Isoxazole-Azirine-Isoxazole/Oxazole Isomerization

The first synthesis of isoxazole-4-carboxylic acid derivatives by domino isoxazole-isoxazole isomerization is reported. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxy-/5-aminoisoxazoles (dioxane, 105 °C) leads to the formation of isoxazole-4-carboxylic esters and amides in good yields. 4-Formyl-5-methoxyisoxazoles give methyl oxazole-4-carboxylates under the same reaction conditions. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxyisoxazoles under milder conditions (MeCN, 50 °C) allows the preparation of transient 2-acyl-2-(methoxycarbonyl)-2H-azirines. The azirines isomerize quantitatively either into isoxazoles under catalytic conditions (dioxane, 105 °C) or into oxazoles under noncatalytic thermolysis (o-dichlorobenzene, 170 °C). The mechanism of the isomerization and dependence of the reaction routes on substituents at starting isoxazole core and reaction conditions are discussed on the basis of DFT calculations.

Development of 4-oxime-1,8-naphthalimide as a bioorthogonal turn-on probe for fluorogenic protein labeling

We report 4-oxime-1,8-naphthalimide as a novel bioorthogonal turn-on probe based on cascade 1,3-dipolar cycloaddition and photoisomerization for fluorogenic protein labeling.

Direct versus Indirect Photodissociation of Isoxazole from Product Branching: A Chirped-Pulse Fourier Transform mm-Wave Spectroscopy/Pulsed Uniform Flow Investigation

The UV photodissociation of isoxazole (c-C₃H₃NO) is studied in this work by chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform Laval flow. This approach offers a number of advantages over traditional spectroscopic detection methods due to its broadband, sub-MHz resolution, and fast-acquisition capabilities. In coupling this technique with a quasi-uniform Laval flow, we are able to obtain product branching fractions in the 193 nm photodissociation of isoxazole. Five dissociation channels are explored through direct detection of seven different photoproducts. These species and their respective branching fractions (%) include the following: HCN (53.8 ± 1.7), CH₃CN (23.4 ± 6.8), HCO (9.5 ± 2.3), CH₂CN (7.8 ± 2.9), CH₂CO (3.8 ± 0.9), HCCCN (0.9 ± 0.2), and HNC (0.8 ± 0.2). Guided by previous electronic structure and dynamics simulations, we are able to elucidate the dissociation dynamics that govern the final product branching fractions observed in this work, which differ significantly from previous reports on the thermal decomposition of isoxazole. Interestingly, both direct and indirect dynamics contribute to its dissociation, and clear signatures of both are manifested in the relative branching ratios obtained. Consistent with previous studies on the unimolecular dissociation of isoxazole, our findings also suggest the importance of the open-shell singlet diradicaloid species vinylnitrene in the dissociation dynamics, regardless of the initially populated excited state. This work, taken together with previous investigations, provides a global picture of the complex dissociation pathways involved in the photodissociation of isoxazole.

α-Diazo oxime ethers for N-heterocycle synthesis

This Feature Article introduces the preparation and synthetic utility of α-diazo oxime ethers. α-Oximino carbenes are useful synthons for N-heterocycles, and can be easily prepared from α-diazo oxime ethers as precursors. We begin with the preparation of α-diazo oxime ethers and their application in [3+2] cycloaddition. It turns out that the nature of metals bound to carbenes plays a crucial role in modulating the reactivity of α-oximino carbenes, in which copper carbenes smoothly react with enamines, whereas the less reactive enol ethers and nitriles require gold carbenes. In Section 3.2, a discussion on N-O and C-H bond activation is presented. Carbenes derived from diazo oxime ethers show unique reactivity towards N-O and C-H bond activation, in which the proximity of the two functionalities, carbene and oxime ether, dictates the preferred reaction pathways toward pyridines, pyrroles, and 2H-azirines. In Section 3.3, the development of tandem reactions based on α-diazo oxime ethers is discussed. The nature of carbenes in which whether free carbenes or metal complexes are involved dissects the pathway and forms different types of 2H-azirines. The 2H-azirine formation turned out to be an excellent platform for the tandem synthesis of N-heterocycles including pyrroles and pyridines. In the last section, we describe the electrophilic activation of 2H-azirines with vinyl carbenes and oximino carbenes. The resulting azirinium species undergo rapid ring expansion rearrangements to form pyridines and pyrazines.

Nunes C, Fausto R, Reva I. Conformational control over an aldehyde fragment by selective vibrational excitation of interchangeable remote antennas

We apply vibrational antennas (OH or NH₂ group) to achieve unprecedented conformational control over the heavy aldehyde fragment in 2-formyl-2H-azirine, using selective vibrational excitations of the OH or NH₂ stretching overtones and combination modes.

Coordinate-polymerized organic ligand towards an efficient catalyst for selective hydrocarbon oxidation

Stable and efficient catalyst design via coordinate-polymerized strategy for the selective oxidation of cyclooctene.

Domino reactions of 2H-azirines with acylketenes from furan-2,3-diones: Competition between the formation of ortho-fused and bridged heterocyclic systems

3-Aryl-2H-azirines react with acylketenes, generated by thermolysis of 5-arylfuran-2,3-diones, to give bridged 5,7-dioxa-1-azabicyclo[4.4.1]undeca-3,8-diene-2,10-diones and/or ortho-fused 6,6a,12,12a-tetrahydrobis[1,3]oxazino[3,2-a:3',2'-d]pyrazine-4,10-diones. The latter compounds, with a new heterocyclic skeleton, are the result of the coupling of two molecules of azirine and two molecules of acylketene and can be prepared only from 3-aryl-2H-azirines having no electron-withdrawing groups in the aryl substituent. Calculations at the DFT B3LYP/6-31G(d) level for the various routes of bis[1,3]oxazino[3,2-a:3',2'-d]pyrazine skeleton formation revealed a new domino reaction of 3-aryl-2H-azirines occurring in the presence of furandiones: acid-catalyzed dimerization to dihydropyrazine followed by consecutive cycloaddition of the latter to two molecules of acylketenes.