A Calorimetric Investigation To Safely Scale-Up a Curtius Rearrangement of Acryloyl Azide

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Highly Selective Hydrogenative Conversion of Nitriles into Tertiary, Secondary, and Primary Amines under Flow Reaction Conditions

Flow reaction methods have been developed to selectively synthesize tertiary, secondary, and primary amines depending on heterogeneous platinum-group metal species under catalytic hydrogenation conditions using nitriles as starting materials. A 10 % Pd/C-packed catalyst cartridge affords symmetrically substituted tertiary amines in good to excellent yields. A 10 % Rh/C-packed catalyst cartridge enables the divergent synthesis of secondary and primary amines, with either cyclohexane or acetic acid as a solvent, respectively. Reaction parameters, such as the metal catalyst, solvent, and reaction temperature, and continuous-flow conditions, such as flow direction and second support of the catalyst in a catalyst cartridge, are quite important for controlling the reaction between the hydrogenation of nitriles and nucleophilic attack of in situ-generated amines to imine intermediates. A wide variety of aliphatic and aromatic nitriles could be highly selectively transformed into the corresponding tertiary, secondary, and primary amines by simply changing the metal species of the catalyst or flow parameters. Furthermore, the selective continuous-flow methodologies are applied over at least 72 h to afford three different types of amines in 80-99 % yield without decrease in catalytic activities.

Acryloylnitrenes: Spectroscopic Characterization, Spin Multiplicities, and Rearrangement to Vinyl Isocyanates

The simplest acryloylnitrene, CH₂═CHC(O)N (1b), and two halogenated derivatives, CH₂═CFC(O)N (2b) and CH₂═CBrC(O)N (3b), were generated through the 266 nm laser photolysis of the corresponding azide precursors in solid N₂-matrices at 15 K. The IR spectroscopic characterization of these new acylnitrenes is supported by ¹⁵N-labeling and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. For the three nitrenes 1b, 2b, and 3b, two conformers exhibiting syn and anti configurations between the C═C and C═O bonds with respect to the C-C bonds have been identified. Consistent with the CBS-QB3 calculated singlet-triplet energy gaps (ΔE_ST < 0 kcal mol^-1), the IR spectral analysis suggests that all these acryloylnitrenes adopt oxazirine-like structures with closed-shell singlet spin multiplicity. Upon subsequent green light (532 nm) irradiation, these acryloylnitrenes rearrange to form vinyl isocyanates CH₂═CXNCO (X = H, F, Br), for which the IR spectra have also been obtained. According to the calculations on the α,β-fluorinated acryloylnitrenes at the CBS-QB3 level, their spin multiplicities can be switched from singlet CH₂═CFC(O)N (ΔE_ST = -0.86 kcal mol^-1) to triplet CF₂═CFC(O)N (ΔE_ST = +0.61 kcal mol^-1), whereas CFH═CFC(O)N is magnetically bistable due to a rather small ΔE_ST (+0.09 kcal mol^-1).

Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Organic azides are useful synthetic intermediates, which demonstrate broad reactivity. Unlike most organic azides, allylic azides can spontaneously rearrange to form a mixture of isomers. This rearrangement has been named the Winstein rearrangement. Using allylic azides can result in low yields and azide racemization in some synthetic contexts due to the Winstein rearrangement. Effort has been made to understand the mechanism of the Winstein rearrangement and to take advantage of this process. Several guiding principles can be used to identify which azides will produce a mixture of isomers and which will resist rearrangement. Selective reaction conditions can be used to differentiate the azide isomers in a dynamic manner. This review covers all aspects of allylic azides including their synthesis, their reactivity, the mechanism of the Winstein rearrangement, and reactions that can selectively elaborate an azide isomer. This review covers the literature from Winstein's initial report to early 2019.

Highly Stereoselective Asymmetric Aldol Routes to tert-Butyl-2-(3,5-difluorophenyl)-1-oxiran-2-yl)ethyl)carbamates: Building Blocks for Novel Protease Inhibitors

Enantioselective syntheses of tert-butyl ((S)-2-(3,5-difluorophenyl)-1-((S)-oxiran-2-yl)ethyl)carbamate and ((S)-2-(3,5-difluorophenyl)-1-((R)-oxiran-2-yl)ethyl)carbamate are described. We utilized asymmetric syn- and anti-aldol reactions to set both stereogenic centers. We investigated ester-derived Ti-enolate aldol reactions as well as Evans' diastereoselective syn-aldol reaction for these syntheses. We have converted optically active ((S)-2-(3,5-difluorophenyl)-1-((S)-oxiran-2-yl)ethyl)carbamate to a potent β-secretase inhibitor.

Comparative Catalytic Activity of Group 9 [Cp*M(III)] Complexes: Cobalt-Catalyzed C-H Amidation of Arenes with Dioxazolones as Amidating Reagents

A procedure for the [Cp*Co(III)]-catalyzed direct C-H amidation of arenes with dioxazolone has been developed. This reaction proceeds under straightforward and mild conditions with a broad range of substrates, including anilides. A comparative study on the catalytic activity of Group 9 [{Cp*MCl2}2] complexes revealed the unique efficiency of the cobalt catalyst.

Polyurethanes from isosorbide-based diisocyanates

Benign building blocks: Stereochemically pure diisocyanates were prepared on a multigram scale from succinic anhydride and isosorbide or isomannide. Characterization of polyurethanes that were produced from these diisocyanates revealed low polydispersity, high thermal stability, and stereochemistry-dependent morphology. If biobased succinic anhydride is used, then no stoichiometric petroleum-derived reagents are required in the synthesis of these materials.

Carbonyldiimidazole-mediated Lossen rearrangement

Carbonyldiimidazole (CDI) was found to mediate the Lossen rearrangement of various hydroxamic acids to isocyanates. This process is experimentally simple and mild, with imidazole and CO(2) being the sole stoichiometric byproduct. Significant for large-scale application, the method avoids the use of hazardous reagents and thus represents a green alternative to standard processing conditions for the Curtius and Hofmann rearrangements.

A modular flow reactor for performing Curtius rearrangements as a continuous flow process

The use of a mesofluidic flow reactor is described for performing Curtius rearrangement reactions of carboxylic acids in the presence of diphenylphosphoryl azide and trapping of the intermediate isocyanates with various nucleophiles.