Monosubstituted 3,3‐Difluorocyclopropenes as Bench‐Stable Reagents: Scope and Limitations

A highly diastereoselective strain-release Doyle-Kirmse reaction: access to functionalized difluoro(methylene)cyclopropanes

Difluoro(methylene)cyclopropanes (F2MCPs) show better anti-cancer properties and chemical reactivities compared to their nonfluorinated analogues. However, catalytic stereoselective methods to access these privileged motifs still remain a challenging goal. The Doyle-Kirmse reaction is a powerful strategy for the concomitant formation of carbon-carbon and carbon-sulfur bonds. Although the enantioselective variants of this reaction have been achieved with high levels of selectivity, the methods that control the diastereoselectivity have been only moderately successful. Herein, we report a catalytic, highly diastereoselective strain-release Doyle-Kirmse reaction for synthesizing functionalized F2MCPs using an inexpensive copper catalyst. The transformation proceeds under mild conditions and displays excellent functional group compatibility on both diazo compounds and difluorocyclopropenyl methyl sulfane/selane derivatives. Furthermore, the obtained products were efficiently transformed into valuable building blocks, such as functionalized spiroheterocycles, difluorocyclopropanes, and skipped dienes.

[3+2]-Cycloaddition Reactions of gem-Difluorocyclopropenes with Azomethine Ylides - Access to Novel Fluorinated Scaffolds

The introduction of fluorinated moieties into drugs as well as the increase of their overall three-dimensionality have become key strategies amongst medicinal chemists to generate sets of compounds with favorable drug-like properties. However, the introduction of fluorinated cyclopropane ring systems which combines both strategies is not widely exploited to date. This paper reports synthetic strategies exploiting the reactivity of gem-difluorocyclopropenes in dipolar cycloaddition reactions with azomethine ylides to afford sets of new fluorine-containing 3-azabicyclo[3.1.0]hexanes. In addition, the unexpected formation of complex trifluorinated scaffolds arising from proline esters and gem-difluorocyclopropenes is highlighted along with computational studies to elucidate the underlying mechanism. This study presents new avenues towards pharmaceutically relevant fluorinated 3-azabicyclo[3.1.0]hexanes that are accessible via robust and short synthetic sequences.

Borane-Catalyzed C-F Bond Functionalization of gem-Difluorocyclopropenes Enables the Synthesis of Orphaned Cyclopropanes

Herein, we disclose an approach to synthesize tert-alkyl cyclopropanes by leveraging C-F bond functionalization of gem-difluorocyclopropenes using tris(pentafluorophenyl)borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph2(C6D5)C3]+[(C6F5)3BF]-. We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α-tert-alkyl centers (63-93% yield). The regioselectivity of the addition to cyclopropenium ions is controlled by their steric and electronic properties and enables access to 3,3-bis(difluoromethyl)cyclopropenes in short order. The resulting cyclopropene products are readily reduced to the corresponding orphaned cyclopropanes under hydrogenation conditions. Quantum chemical calculations reveal the nature of the C-F bond cleavage steps and provide evidence for catalysis by boron and not silylated oxonium ions, though Si-F bond formation is the enthalpic driving force for the reaction.

Indolizin-1-ols with Charged Electron Acceptors: A Direct Way to 3H-Indolizinium-1-olates with Donor Functions

The reaction of cyclopropenones with pyridines, having an attached integer-charged electron-withdrawing group (pyridinium, imidazolium, and phosphonium) was discovered to afford novel indolizin-1-ol derivatives in high yields with no chromatographic purification required. While being stable as solids, these indolizin-1-ols have a limited lifetime in solution. The study of reasons for such instability uncovered an aerobic oxidative pathway, eventually resulting in indolizine-1,7-dione dimers. The exploration of N-(1-hydroxyindolizin-7-yl)pyridinium salts' chemistry led to a reaction discovery, affording a new type of rare pseudo-cross-conjugated mesomeric betaines (3H-indolizin-4-ium-1-olates with an electron-donating function at C7 position) inaccessible by other means. In this reaction, a sequential introduction of nucleophiles takes place: the first one (Nu₁) is represented by simple anilines, whereas Nu₂ extends to primary, secondary, aliphatic, aromatic amines, and phenols. For the obtained betaines having unsymmetrical aliphatic amino groups at C7 position an increased order of the C7-Nu₂ bond resulting in existence of amide type E/Z-forms (∼1:1 at room temperature) was demonstrated. For aryl amino groups, with typically reduced nitrogen's lone-pair donation, the barrier of rotation around the C7-Nu₂ bond was lower, and for the C7-oxy betaines, no such E/Z-isomerism was revealed. Although primary amines (as Nu₂) introduce a hydrogen atom in the conjugated betaine system, allowing prototropic tautomerism in this way, non-zwitterionic tautomers (3-amino-7-iminoindolizin-1-ones) were rejected by nuclear Overhauser effect spectroscopy experiments.

Catalytic regio- and stereoselective silicon–carbon bond formations on unsymmetric gem-difluorocyclopropenes by capture of silyl metal species

A highly regioselective silylation of unsymmetric gem-difluorocyclopropenes was achieved by the capture of in-situ formed silyl metal intermediates, which gave structurally diverse silyldifluorocyclopropanes with good yields and stereoselectivity.