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

Amide Synthesis from Decarboxylative Coupling of Isocyanates and Carboxylic Acids

Isocyanates are versatile electrophiles that can react with a wide range of nucleophiles to afford important organic structures. Although the reactions between isocyanates and alcohols, amines and organometallic reagents have been well established, the synthesis of amides through the decarboxylative condensation of carboxylic acids and isocyanates is less appreciated. In this review, the synthesis of isocyanates and its application on amide synthesis through the condensation with carboxylic acids are summarized and discussed. It is our hope that this review will attract more attention to this less mentioned transformation and inspire new developments in the fields of organic synthesis, polymer synthesis and chemical biology.

Diverse functional elements in RNA predicted transcriptome-wide by orthogonal RNA structure probing

RNA molecules can fold into complex structures and interact with trans-acting factors to control their biology. Recent methods have been focused on developing novel tools to measure RNA structure transcriptome-wide, but their utility to study and predict RNA-protein interactions or RNA processing has been limited thus far. Here, we extend these studies with the first transcriptome-wide mapping method for cataloging RNA solvent accessibility, icLASER. By combining solvent accessibility (icLASER) with RNA flexibility (icSHAPE) data, we efficiently predict RNA-protein interactions transcriptome-wide and catalog RNA polyadenylation sites by RNA structure alone. These studies showcase the power of designing novel chemical approaches to studying RNA biology. Further, our study exemplifies merging complementary methods to measure RNA structure inside cells and its utility for predicting transcriptome-wide interactions that are critical for control of and regulation by RNA structure. We envision such approaches can be applied to studying different cell types or cells under varying conditions, using RNA structure and footprinting to characterize cellular interactions and processing involving RNA.

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.

The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

The Curtius rearrangement is a versatile reaction in which a carboxylic acid can be converted to an isocyanate through an acyl azide intermediate under mild conditions. The resulting stable isocyanate can then be readily transformed into a variety of amines and amine derivatives including urethanes and ureas. There have been wide-ranging applications of the Curtius rearrangement in the synthesis of natural products and their derivatives. Also, this reaction has been extensively utilized in the synthesis and application of a variety of biomolecules. In this review, we present mechanistic studies, chemical methodologies and reagents for the synthesis of isocyanates from carboxylic acids, the conversion of isocyanates to amines and amine derivatives, and their applications in the synthesis of bioactive natural products and their congeners.

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.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).