Dynamic stereomutation of vinylcyclopropanes with metalloradicals.

The ever increasing demands for greater sustainability and lower energy usage in chemical processes call for fundamentally new approaches and reactivity principles. In this context, the pronounced prevalence of odd-oxidation states in less precious metals bears untapped potential for fundamentally distinct reactivity modes via metalloradical catalysis1-3. Contrary to the well-established reactivity paradigm that organic free radicals, upon addition to a vinylcyclopropane, lead to rapid ring opening under strain release-a transformation that serves widely as a mechanistic probe (radical clock)4 for the intermediacy of radicals5-we herein show that a metal-based radical, that is, a Ni(I) metalloradical, triggers reversible cis/trans isomerization instead of opening. The isomerization proceeds under chiral inversion and, depending on the substitution pattern, occurs at room temperature in less than 5 min, requiring solely the addition of the non-precious catalyst. Our combined computational and experimental mechanistic studies support metalloradical catalysis as origin of this profound reactivity, rationalize the observed stereoinversion and reveal key reactivity features of the process, including its reversibility. These insights enabled the iterative thermodynamic enrichment of enantiopure cis/trans mixtures towards a single diastereomer through multiple Ni(I) catalysis rounds and also extensions to divinylcyclopropanes, which constitute strategic motifs in natural product- and total syntheses6. While the trans-isomer usually requires heating at approximately 200 °C to trigger thermal isomerization under racemization to cis-divinylcyclopropane, which then undergoes facile Cope-type rearrangement, the analogous contra-thermodynamic process is herein shown to proceed under Ni(I) metalloradical catalysis under mild conditions without any loss of stereochemical integrity, enabling a mild and stereochemically pure access to seven-membered rings, fused ring systems and spirocycles.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis.

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni(I)(µ-Br)PAd2(n-Bu)]2, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

Ruthenium-Catalyzed Synthesis of Aryl and Alkenyl Halides from Fluorosulfonates.

Aryl and alkenyl halides are widely used as key intermediates in organic synthesis, particularly for the formation of organometallic reagents or as radical precursors. They are also found in pharmaceutical and agrochemical ingredients. In this work, the synthesis of aryl and alkenyl halides from the corresponding fluorosulfonates using commercially available ruthenium catalysts is reported. Notably, this is the first conversion of phenols to aryl halides that is efficient with chloride, bromide, and iodide. Fluorosulfonates are readily prepared using sulfuryl fluoride (SO2 F2 ) and less expensive substitutes for triflates. Although aryl fluorosulfonates and their reactions are well known, this is the first report of an efficient coupling of alkenyl fluorosulfonates. To finish, it was demonstrated, by means of representative examples, that the reaction is possible in a one-pot process, starting directly from phenol or aldehyde.

Oxidative Rearrangement of MIDA (N-Methyliminodiacetic Acid) Boronates: Mechanistic Insights and Synthetic Applications.

Herein we report that coordinative hemilability allows the MIDA (N-methyliminodiacetic acid) nitrogen to behave as a nucleophile and intramolecularly intercept palladium π-allyl intermediates. A mechanistic investigation indicates that this rearrangement proceeds through an SN2-like displacement at tetrasubstituted boron to furnish novel DABN boronates. Oxidative addition into the N-C bond of the DABN scaffold furnishes borylated π-allyl intermediates that can then be trapped with a variety of nucleophiles, including in a three-component coupling.

Reaction of Vinyl Aziridines with Arynes: Synthesis of Benzazepines and Branched Allyl Fluorides.

We report the cycloaddition between vinyl aziridines and arynes. Depending on the reaction conditions and the choice of the aryne precursor, the aziridinium intermediate can be trapped through two distinct mechanistic pathways. The first one proceeds through a formal [5+2] cycloaddition to furnish valuable multi-substituted benzazepines. In the second pathway, the aziridinium is intercepted by a fluoride ion to afford allylic fluorides in good yields. Both reactions proceed stereospecifically and furnish enantiopure benzazepines and allylic fluorides.

Conformationally stable peptide macrocycles assembled using the Petasis borono-Mannich reaction.

Macrocyclization of linear peptide precursors using the Petasis borono-Mannich reaction affords a diverse range of macrocycles with an endocyclic amine. Analysis of the corresponding macrocyclic structures underscores that the hydrogen bond between an endocyclic amine and the adjacent amide NH is a powerful control element for conformationally homogenous peptide macrocycles.

Author Correction: Amine hemilability enables boron to mechanistically resemble either hydride or proton.

During the revision of this Article prior to publication, a computational study was reported (Vallejos, M. M. & Pellegrinet, S. C. Theoretical study of the BF3-promoted rearrangement of oxiranyl N-methyliminodiacetic acid boronates. J. Org. Chem. 82, 5917-5925; 2017) that evaluates the nucleophilic boryl transfer mechanism predicted in this Article; this reference has now been added as number 19, and the subsequent references renumbered.

Amine hemilability enables boron to mechanistically resemble either hydride or proton.

Tetracoordinate MIDA (N-methyliminodiacetic acid) boronates have found broad utility in chemical synthesis. Here, we describe mechanistic insights into the migratory aptitude of the MIDA boryl group in boron transfer processes, and show that the hemilability of the nitrogen atom on the MIDA ligand enables boron to mechanistically resemble either a hydride or a proton. The first case involves a 1,2-boryl shift, in which boron migrates as a nucleophile in its tetracoordinate form. The second case involves a neighbouring atom-promoted 1,4-boryl shift, in which boron migrates as an electrophile in its pseudo-tricoordinate form. Density functional theory studies and in situ NMR measurements all suggest that MIDA can act as a dynamic switch. These findings encouraged the development of novel migration processes involving boron that exploit the chameleonic behaviour of boron by acting as both a nucleophile and an electrophile, including the first report of a compound with a boronate functionality bound to carbon in the carboxylic acid oxidation state.

Modular Synthesis of ß-Amino Boronate Peptidomimetics.

Herein, we describe the synthesis of novel ß-amino boronate peptidomimetics from amphoteric α-borylaldehydes in the Ugi multicomponent reaction. A mild deprotection method provided the free and stable boronic acid forms of the target molecules, which display notable stability toward protodeborylation. Despite the presence of Lewis acidic boron, there is no evidence for hydrolysis of the adjacent amide via a 5- or 6-membered ring intermediate. This methodology should facilitate the development of libraries of new boron-containing antibiotics and antifungal agents.

Achieving Skeletal Diversity in Peptide Macrocycles through The Use of Heterocyclic Grafts.

Despite their therapeutic potential, peptide macrocycles often suffer from drawbacks such as low membrane permeability, proteolytic instability, and conformational lability. As a result, there have been significant efforts to "depeptidize" amino acid-rich macrocycles through the incorporation of heterocyclic grafts into their backbones. In this concept article, we summarize selected recent methodologies that can be used to introduce heterocycles into cyclic peptides.

3-Cyanoallyl boronates are versatile building blocks in the synthesis of polysubstituted thiophenes.

We report the preparation of hitherto unprecedented 3-cyanoallyl boronates using condensation of the parent α-boryl aldehyde and nitriles. The resulting allyl boronates have been used to generate a wide range of borylated thiophenes, which represent a valuable class of heterocycles in modern drug discovery. Subsequent Suzuki-Miyaura cross-coupling enabled the synthesis of pharmaceutically important 3,5-disubstituted aminothiophenes. Moreover, late stage functionalization gave access to borylated bromothiophene and thieno[2,3-b]pyridines.

Amphoteric Borylketenimines: Versatile Intermediates in the Synthesis of Borylated Heterocycles.

We report the first synthesis of amphoteric borylketenimines from ethynyl N-methyliminodiacetic acid (MIDA) boronate and sulfonyl azides via copper catalysis. In situ trapping of these intermediates with various nucleophiles provided access to novel borylated azetidimines, iminocoumarins, amides, iminooxetanes, and amidines. The described strategy based on borylketenimines offers high levels of chemo- and regioselectivity, enabling the synthesis of unprecedented borylated molecules. This work highlights the unexplored utility of borylketenimines in the synthesis of potentially bioactive molecules.

A Linchpin Synthesis of 6-Hydroxyceramides from Aziridine Aldehydes.

A chemoselective N-oxidation/Meisenheimer rearrangement protocol was developed to generate vinylaziridine scaffolds from aziridine aldehydes. A subsequent Lewis acid-mediated aziridine ring opening with carboxylic acid nucleophiles followed by N-O bond cleavage furnishes a human skin 6-hydroxyceramide natural product in short order. The utility of this methodology is demonstrated by the preparation of a number of unnatural 6-hydroxyceramide analogues. This modular approach enables the expedient synthesis of poorly understood skin lipids, which may find application in therapeutics and cosmetics.

Synthesis of Chiral Piperazinones Using Amphoteric Aziridine Aldehyde Dimers and Functionalized Isocyanides.

We have evaluated a range of functionalized isocyanides in the aziridine aldehyde-driven multicomponent synthesis of piperazinones. High diasteroselectivity for each isocyanide was observed. A theoretical evaluation of the reaction course corroborates the experimental data. Moreover, the reactivity of cis- and trans-configured aziridine aldehyde dimers has been compared. This study further probes the dimer-driven mechanism of cyclization and enables an efficient access to a wide range of chiral piperazinones bearing functionalized side chains.

Comparison of orthogonal chromatographic and lectin-affinity microarray methods for glycan profiling of a therapeutic monoclonal antibody.

The N-linked glycosylation of four lots of a marketed human therapeutic monoclonal antibody (mAb) was assessed by three orthogonal chromatographic methods and a commercial lectin microarray. For chromatography, the N-glycans were removed enzymatically from the mAbs using PNGase F. Native glycans were determined by HPAEC-PAD using a panel of 21 N-glycan standards and a multi-stage linear gradient eluent profile for sequential analyses of typical neutral and sialylated glycans in one chromatographic run. The monosaccharide contents of these glycans following acid hydrolysis were confirmed by HPAEC-PAD with monosaccharide standards. Glycosylation analysis by HILIC-FD after stoichiometric labelling with two different fluorescent tags (2-AA and 2-AB) enabled direct quantitation. The 2-AA- and 2-AB-labelled versions of the same glycan standard panel yielded distinctive separation profiles suitable for orthogonal identification of mAb glycans. Glycan profiling with the lectin microarray required partial denaturation of the intact mAbs to expose the sequestered Fc N-glycans. Glycosylation fingerprints were obtained using a fluorescently labelled antibody directed against human IgG Fc. Fluorescence intensities from the fingerprints were deconvoluted with a proprietary algorithm to obtain semi-quantitative "glycan structural class" information. Glycosylation analyses of the four mAb lots by these four methods, which separate and detect oligosaccharides according to different principles, provided complementary and corroboratory qualitative and quantitative information. The predominant N-linked structures were core-fucosylated asialo diantennary structures with varying galactosylation. There were also trace amounts of afucosyl and bisected glycans, but no detectable sialylation by any of the four methods. The therapeutic mAb demonstrated a high degree of consistency in the types and amounts of N-linked glycans in the four lots (<6% CV), and between all four analysis methods (<6% CV). The described methods are co-supported by the excellent quantitative agreement of their results, which is particularly notable considering the orthogonality of their separation and detection mechanisms.

Indole Functionalization via Photoredox Gold Catalysis.

The use of photoredox catalyst [Au2(dppm)2]Cl2 to initiate free-radical cyclizations onto indoles is reported. Excitation of the dimeric Au(I) photocatalyst for the reduction of unactivated bromoalkanes and bromoarenes is used for the generation of carbon-centered radicals. Previous to this work, reduction processes leading to indole functionalization utilizing photoredox catalysts were limited to activated benzylic or α-carbonyl-positioned bromoalkanes. This method offers a mild and safe alternative to organostannanes and pyrophoric initiators for access to high energy radicals that were previously inaccessible through catalytic or stoichiometric means.