Palladium-Catalyzed Direct 2-Alkylation of Indoles by Norbornene-Mediated Regioselective Cascade C–H Activation

A palladium-catalyzed direct 2-alkylation reaction of free N-H indoles has been developed. This reaction relies on a norbornene-mediated cascade C–H activation process at the indole ring, which features high regioselectivity and excellent functional group tolerance. The reaction represents the first example for a generally applicable, direct C–H alkylation of indole at the 2-position

Evapotranspiration components, soil water content and net primary productivity in a black locust plantation

ET components, i.e. transpiration (T), soil evaporation(E) and canopy interception (I), soil water and net primary productivity (NPP), water use efficiency (WUE) were estimated in a black locust plantation during a drier (2015) and a wetter (2016) growing season (i.e., June to September).</p

Pyridine-Catalyzed Radical Borylation of Aryl Halides

A pyridine-catalyzed transition-metal-free borylation reaction of haloarenes has been developed based on the selective cross-coupling of an aryl radical and a pyridine-stabilized boryl radical. Arylboronates were produced from haloarenes under mild conditions. This borylation reaction features a broad substrate scope, operational simplicity, and gram-scale synthetic ability

Ligand-Enabled Palladium(II)-Catalyzed γ‑C(sp³)–H Arylation of Primary Aliphatic Amines

The Pd(II)/sulfoxide-2-hydroxypyridine catalytic system shows promising activity in C–H activation chemistry. In this study, we showcase how this catalytic system solves the problem of native primary amine-directed γ-C(sp3)–H arylation. Primary amines with different complexities are compatible with the established methodology, and the range of applicable substrates can be expanded to include pyridine, oxime ether, and pyridine N-oxide

Origin of the Relative Stereoselectivity of the β-Lactam Formation in the Staudinger Reaction

The relative (cis, trans) stereoselectivity of the β-lactam formation is one of the critical issues in the Staudinger reaction. Although many attempts have been made to explain and to predict the stereochemical outcomes, the origin of the stereoselectivity remains obscure. We are proposing a model that explains the relative stereoselectivity based on a kinetic analysis of the cis/trans ratios of reaction products. The results were derived from detailed Hammett analyses. Cyclic imines were employed to investigate the electronic effect of the ketene substituents, and it was found that the stereoselectivity could not be simply attributed to the torquoelectronic model. Based on our results, the origin of the relative stereoselectivity can be described as follows:  (1) the stereoselectivity is generated as a result of the competition between the direct ring closure and the isomerization of the imine moiety in the zwitterionic intermediate; (2) the ring closure step is most likely an intramolecular nucleophilic addition of the enolate to the imine moiety, which is obviously affected by the electronic effect of the ketene and imine substituents; (3) electron-donating ketene substituents and electron-withdrawing imine substituents accelerate the direct ring closure, leading to a preference for cis-β-lactam formation, while electron-withdrawing ketene substituents and electron-donating imine substituents slow the direct ring closure, leading to a preference for trans-β-lactam formation; and (4) the electronic effect of the substituents on the isomerization is a minor factor in influencing the stereoselectivity

Pd(II)-Catalyzed Regioselective 2‑Alkylation of Indoles via a Norbornene-Mediated C–H Activation: Mechanism and Applications

A palladium-catalyzed direct 2-alkylation reaction of free <i>N</i>-H indoles was developed based on a norbornene-mediated regioselective cascade C–H activation. The detailed reaction mechanism was investigated by NMR spectroscopic analyses, characterization of the key intermediate, deuterium labeling experiments, and kinetic studies. The results indicate that a catalytic cycle operates, in which an <i>N</i>-norbornene type palladacycle is formed as the key intermediate. Oxidative addition of alkyl bromide to the Pd­(II) center in this intermediate is the rate-determining step of the reaction. The synthetic utility of this indole 2-alkylation method was demonstrated by its application in natural product total synthesis. A new and general strategy to synthesize <i>Aspidosperma</i> alkaloids was established employing the indole 2-alkylation reaction as the key step, and two structurally different <i>Aspidosperma</i> alkaloids, aspidospermidine and goniomitine, were synthesized in concise routes

Rh(I)-Catalyzed Intramolecular [3 + 2] Cycloaddition of <i>trans</i>-Vinylcyclopropane-enes

Rh(I)-Catalyzed Intramolecular [3 + 2] Cycloaddition of trans-Vinylcyclopropane-ene

Pd(II)-Catalyzed Regioselective 2‑Alkylation of Indoles via a Norbornene-Mediated C–H Activation: Mechanism and Applications

A palladium-catalyzed direct 2-alkylation reaction of free <i>N</i>-H indoles was developed based on a norbornene-mediated regioselective cascade C–H activation. The detailed reaction mechanism was investigated by NMR spectroscopic analyses, characterization of the key intermediate, deuterium labeling experiments, and kinetic studies. The results indicate that a catalytic cycle operates, in which an <i>N</i>-norbornene type palladacycle is formed as the key intermediate. Oxidative addition of alkyl bromide to the Pd­(II) center in this intermediate is the rate-determining step of the reaction. The synthetic utility of this indole 2-alkylation method was demonstrated by its application in natural product total synthesis. A new and general strategy to synthesize <i>Aspidosperma</i> alkaloids was established employing the indole 2-alkylation reaction as the key step, and two structurally different <i>Aspidosperma</i> alkaloids, aspidospermidine and goniomitine, were synthesized in concise routes

Tandem Rh(I)-Catalyzed [(5+2)+1] Cycloaddition/Aldol Reaction for the Construction of Linear Triquinane Skeleton: Total Syntheses of (±)-Hirsutene and (±)-1-Desoxyhypnophilin

A tandem reaction involving a Rh(I)-catalyzed two-component [(5+2)+1] cycloaddition and an aldol condensation has been developed to construct the tricyclo[6.3.0.02,6]undecane skeleton and its heteroatom-imbedded analogues. Meanwhile, this method has been successfully applied to natural product synthesis for the first time. The present strategy enables a straightforward approach to the natural linear triquinane skeleton, as demonstrated by concise and step economical syntheses of hirsutene and 1-desoxy-hypnophilin, whereby the linear triquinane core is diastereoselectively established in one manipulation with correct placement of all stereocenters, including two quarternary centers. This first application of the Rh(I)-catalyzed [(5+2)+1] cycloaddition in natural product synthesis highlights the efficiency of this methodology for constructing complex fused ring systems