Rhodium‐Catalyzed Remote C(sp3)−H Borylation of Silyl Enol Ethers

A rhodium‐catalyzed remote C(sp³)?H borylation of silyl enol ethers (SEEs, E/Z mixtures) by alkene isomerization and hydroboration is reported. The reaction exhibits mild reaction conditions and excellent functional‐group tolerance. This method is compatible with an array of SEEs, including linear and branched SEEs derived from aldehydes and ketones, and provides direct access to a broad range of structurally diverse 1,n‐borylethers in excellent regioselectivities and good yields. These compounds are precursors to various valuable chemicals, such as 1,n‐diols and aminoalcohols.     

Azaruthena(II)‐bicyclo[3.2.0]heptadiene: Key Intermediate for Ruthenaelectro(II/III/I)‐catalyzed Alkyne Annulations

A ruthenium‐catalyzed electrochemical dehydrogenative annulation reaction of imidazoles with alkynes has been established, enabling the preparation of various bridgehead N‐fused [5,6]‐bicyclic heteroarenes through regioselective electrochemical C?H/N?H annulation without chemical metal oxidants. Novel azaruthenabicyclo[3.2.0]heptadienes were fully characterized and identified as key intermediates. Mechanistic studies are suggestive of an oxidatively induced reductive elimination pathway within a ruthenium(II/III) regime.     

Photoredox‐Catalyzed Isomerization of Highly Substituted Allylic Alcohols by C−H Bond Activation

Photoredox‐catalyzed isomerization of γ‐carbonyl‐substituted allylic alcohols to their corresponding carbonyl compounds was achieved for the first time by C?H bond activation. This catalytic redox‐neutral process resulted in the synthesis of 1,4‐dicarbonyl compounds. Notably, allylic alcohols bearing tetrasubstituted olefins can also be transformed into their corresponding carbonyl compounds. Density functional theory calculations show that the carbonyl group at the γ‐position of allylic alcohols are beneficial to the formation of their corresponding allylic alcohol radicals with high vertical electron affinity, which contributes to the completion of the photoredox catalytic cycle.     

Synthesis of Biaryl‐Bridged Cyclic Peptides via Catalytic Oxidative Cross‐Coupling Reactions

Biaryl‐bridged cyclic peptides comprise an intriguing class of structurally diverse natural products with significant biological activity. Especially noteworthy are the antibiotics arylomycin and its synthetic analogue G0775, which exhibits potent activity against Gram‐negative bacteria. Herein, we present a simple, flexible, and reliable strategy based on activating‐group‐assisted catalytic oxidative coupling for assembling biaryl‐bridged cyclic peptides from natural amino acids. The synthetic approach was utilized for preparing a number of natural and unnatural biaryl‐bridged cyclic peptides, including arylomycin/G0775 and RP 66453 cyclic cores.     

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid: 1H, 31P,and 13C{1H} NMR and some comments on corresponding [AX]3 and AXX′2 spin systems and spectra

Abstract

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid 1,3,5-C₆H₃[PO₃^2?]₃ gives rise to deceptively simple ¹H and ³¹P NMR spectra due to a corresponding [AX]₃ spin system. The ¹³C{¹H} NMR spectrum reveals two ¹³C isotopomers which are identified via AXX′₂ systems. Results from analysis and iteration are described.     

Rhodium(III)‐Catalyzed Asymmetric Access to Spirocycles through C−H Activation and Axial‐to‐Central Chirality Transfer

Axial‐to‐central chirality transfer is an important strategy to construct chiral centers, where the axially chiral reagents are mostly limited to atropomerically stable ones. Reported herein is a Rh^III‐catalyzed enantioselective spiroannulative synthesis of nitrones. The coupling proceeds via C?H arylation to give an atropomerically metastable biaryl, followed by intramolecular dearomative trapping under oxidative conditions with high degree of chirality transfer.     

Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C−H Aminations

While direct nitrene insertions into C?H bonds have become an important tool for building C?N bonds in modern organic chemistry, the generation of nitrene intermediates always requires transition metals, high temperatures, ultraviolet or laser light. We report a mild synthesis of carbazoles and related building blocks through a visible light‐induced intramolecular C?H amination reaction. A striking advantage of this new method is the use of more reactive aryl sulfilimines instead of the corresponding hazardous azides. Different catalysts and divergent light sources were tested. The reaction scope is broad and the product yield is generally high. An efficient gram‐scale synthesis of Clausine C demonstrates the applicability and scalability of this new method.     

Rational Development of Remote C−H Functionalization of Biphenyl: Experimental and Computational Studies

A simple and efficient nitrile‐directed meta‐C?H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U‐shaped template to achieve a molecular U‐turn and assemble the large‐sized cyclophane transition state for the remote C?H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta‐C?H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta‐selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand‐containing Pd–Ag heterodimeric transition state (TS) favors the desired remote meta‐selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non‐directed meta‐C?H activation. Substituted 2‐pyridone ligands were found to be key in assisting the cleavage of the meta‐C?H bond in the concerted metalation–deprotonation (CMD) process.     

Asymmetric Total Syntheses of Di‐ and Sesquiterpenoids by Catalytic C−C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C?C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (?)‐microthecaline A, (?)‐leubehanol, (+)‐pseudopteroxazole, (+)‐seco‐pseudopteroxazole, pseudopterosin A–F and G—J aglycones, and (+)‐heritonin. The key step in these syntheses involve a Rh‐catalyzed C?C/C?H activation cascade of 3‐arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C?H amination of the tetralone substrate in the synthesis of (?)‐microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C?C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.     

Photochemical Transformations with Iodine Azide after Release from an Ion‐Exchange Resin

This report discloses the photochemical homolytic cleavage of iodine azide after its formation following release from polymer‐bound bisazido iodate(I) anions. A series of radical reactions are reported including the 1,2‐functionlization of alkenes and the unprecedented chemoselective oxidation of secondary alcohols in the presence of primary alcohols.