Urea-Catalyzed N–H Insertion–Arylation Reactions of Nitrodiazoesters

Bismuth-catalyzed arylation of ethyl glyoxylate with aniline via N-H insertion

Glycine derivatives such as ethyl 2-(4-aminophenyl)-2-(phenylamino) acetate is an exciting and essential non-proteinogenic class of amino acids. Herein, we report an efficient and novel route to synthesize glycine derivatives using ethyl glyoxylate, aniline, and its derivatives catalyzed by bismuth salts. In our scheme, mild, non-toxic, and commercially viable reagents were utilized. The synthesized moieties were characterized by ESI-MASS, ¹H-NMR, ¹³C-NMR, and XRD techniques. The target glycine derivatives were successfully obtained with a maximum yield of 87%. Moreover, the reaction is very green as water is the only byproduct.

DFT Mechanistic Study of the Cyclopropanation of Styrene and Aryldiazodiacetate Catalyzed by Tris(pentafluorophenyl)borane

Metal-free boron Lewis acids, tris(pentafluorophenyl)borane B(C₆F₅)₃, have the advantages of low toxicity and low cost and are a promising catalyst. A density functional theory (DFT) calculation was used to clarify the mechanism and the origin of the diastereoselective cyclopropanation of aryldiazodiacetate and styrene derivatives catalyzed by B(C₆F₅)₃. Four pathways were calculated: B(C₆F₅)₃-catalyzed N-, C-, and O-bound boron-activated aryldiazodiacetate and without B(C₆F₅)₃ catalysis. By calculating and comparing the energy barriers, the most possible reaction mechanism was proposed, that is, first, B(C₆F₅)₃ catalyzed O-bound boron to activate aryldiazodiacetate, followed by the removal of a N₂ molecule, and finally, styrene nucleophilic attack occurred to produce [2+1] cyclopropane products. N₂ removal is the rate-limiting step, and this step determines the preference of a given mechanism. The calculated results are in agreement with experimental observations. The origin of diastereoselectivity is further explained on the basis of the favorable mechanism. The steric hindrance interference between the styrene aryl group and the large tri(pentafluorophenyl)borane B(C₆F₅)₃ and the favorable π-π stacking interaction between the benzene rings combined to cause the high diastereoselectivity, which resulted in lower energy of the transition state (TS) corresponding to the reaction mechanism. The calculated results not only provide a more detailed explanation of the mechanism for the experimental study but also have certain reference and guiding significance for other catalytic cyclopropanation reactions.

TfOH-Catalyzed N-H Insertion of α-Substituted-α-Diazoesters with Anilines Provides Access to Unnatural α-Amino Esters

A time-economical TfOH-catalyzed N-H insertion between anilines and α-alkyl and α-aryl-α-diazoacetates provides a straightforward approach to access unnatural α-amino esters, which readily undergo various transformations and can thus be used for the synthesis of pharmaceutically relevant molecules. The α-amino esters were obtained in moderate to excellent yields.

Catalytic Friedel-Crafts Alkylation of Electron Rich Aromatic Derivatives with α-Aryl Diazoacetates Mediated by Brønsted Acids

The catalytic protonation of aryl diazoacetates by strong Brønsted acids, followed by a Friedel-Crafts alkylation reaction with electron rich aromatic compounds, is reported. The reaction provided in a direct fashion 24 geminal diarylacetates in yields of ≤92%.

Synthesis of Unsymmetrical α,α-Diarylacetates

Ethyl bromofluoroacetate has been developed as a precursor for the convenient synthesis of unsymmetrical α,α-diarylacetates featuring indoles, anilines, and other electron-rich aromatics. In conjunction with a mild Lewis acid catalyzed C-N → C-C exchange, intermediate arylglycines can be synthesized and transformed into α,α-diarylacetates in a one-pot protocol, resulting in a net diarylation reaction exhibiting a wide scope. In the context of diarylacetates, the synthetic equivalence of the fluorinated reagent with α-nitro-α-diazo carbonyls was established.

Quantification of electrophilic activation by hydrogen-bonding organocatalysts

A spectrophotometric sensor is described that provides a useful assessment of the LUMO-lowering provided by catalysts in Diels–Alder and Friedel–Crafts reactions. A broad range of 33 hydrogen-bonding catalysts was assessed with the sensor, and the relative rates in the above reactions spanned 5 orders of magnitude as determined via ¹H- and ²H NMR spectroscopic measurements, respectively. The differences between the maximum wavelength shift of the sensor with and without catalyst (Δλ_max^–1) were found to correlate linearly with ln(k_rel) values for both reactions, even though the substrate feature that interacts with the catalyst differs significantly (ketone vs nitro). The sensor provides an assessment of both the inherent reactivity of a catalyst architecture as well as the sensitivity of the reaction to changes within an architecture. In contrast, catalyst pK_a values are a poor measure of reactivity, although correlations have been identified within catalyst classes.