Search for: All records

The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp³)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)–Pd and K⁺–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)–Pd and η²-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)–H bond lactonization in the presence of C(sp²)–H.

A simple and efficient nitrile‐directedmeta‐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 remotemeta‐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 remotemeta‐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 remotemeta‐selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non‐directedmeta‐C−H activation. Substituted 2‐pyridone ligands were found to be key in assisting the cleavage of themeta‐C−H bond in the concerted metalation–deprotonation (CMD) process.

A simple and efficient nitrile‐directedmeta‐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 remotemeta‐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 remotemeta‐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 remotemeta‐selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non‐directedmeta‐C−H activation. Substituted 2‐pyridone ligands were found to be key in assisting the cleavage of themeta‐C−H bond in the concerted metalation–deprotonation (CMD) process.