Palladium(II)-Catalyzed C(sp2)–H Bond Activation/C–N Bond Cleavage Annulation of N-Methoxy Amides and Arynes

Ring Expansion via Palladium-Catalyzed Aryne Insertion into C-C Bonds of Benzocyclobutenones

Insertion of arynes into cyclic C-C bonds provides a straightforward approach for ring expansion with an ortho-phenylene moiety; however, the current scope of substrates has been restricted to enolizable ketones. Here we report the first example of aryne-insertion into non-enolizable ketones through Pd-catalyzed C-C bond activation of benzocyclobutenones. 2-Haloaryl boronates were employed as an unconventional aryne precursor. The reaction shows a wide substrate scope and high functional group tolerance. The mechanistic studies reveal an interesting dual role of the Pd catalyst for both aryne generation and C-C bond activation.

Directed regioselective arylation of imidazo[1,2-a]pyridine-3-carboxamides using Rh(III) catalysis

In contrast to previously reported free-radical pathways to functionalize imidazo[1,2-a]pyridines at the C-5 centre, directing group approaches are rare. Herein, we demonstrate a rhodium(III) catalyzed efficient and regioselective strategy for directed C-5 functionalization of imidazo[1,2-a]pyridines using N-methoxyamide as a directing group. This methodology facilitates directed arylation without the necessity for pre-functionalization. It also allows for gram-scale synthesis and post-functionalization.

Palladium(II)-catalyzed annulation of N-methoxy amides and arynes: computational mechanistic insights and substituents effects

CONTEXT

The combined use of transition metal-catalyzed C-H activation with aryne annulation reactions has emerged as an important strategy in organic synthesis. In this study, the mechanisms of the palladium(II)-catalyzed annulation reaction of N-methoxy amides and arynes were computationally investigated by density functional theory. The role of methoxy amide as a directing group was elucidated through the calculation of three different pathways for the C-H activation step, showing that the pathway where amide nitrogen acts as a directing group is preferable. At the reductive elimination transition state, an unstable seven-membered ring is formed preventing the lactam formation. A substituent effect study based on an NBO analysis, Hammet, and using a More O'Ferall-Jenks plot indicates that the C-H activation step proceeds via an electrophilic concerted metalation-deprotonation (eCMD) mechanism. The results show that electron-withdrawing groups increase the activation barrier and contribute to an early Pd-C bond formation and a late C-H bond breaking when compared with electron-donating substituents. Our computational results are in agreement with the experimental data provided in the literature.

METHODS

All calculations were performed using Gaussian 16 software. Geometry optimizations, frequency analyses at 393.15 K, and IRC calculations were conducted at the M06L/Def2-SVP level of theory. Corrected electronic energies, NBO charges, and Wiberg bond indexes were computed at the M06L/Def2-TZVP//M06L/Def2-SVP level of theory. Implicit solvent effects were considered in all calculations using the SMD model, with acetonitrile employed as the solvent.

Radical-mediated sulfonylative/thiolative cyclization of biaryl enones to phenanthrone derivatives

An approach for the assembly of phenanthrone derivatives bearing all carbon quaternary centres has been developed through visible light-promoted tandem sulfonylation/intramolecular-arylation of biaryl enones with sulfonyl chlorides. A series of sulfonylated 10,10-dialkylphenanthrones were obtained in good yields. In addition, the approach has been extended to thiotrifluoromethyl (SCF3) and thiocyanato (SCN) radicals to obtain the corresponding phenanthrones under oxidative conditions. The synthetic utility was also illustrated by the scalability and further transformations of the product.

ortho-Nitro-substituted diaryliodonium salts enabled regioselective cyclization of arylcarboxylic acids toward 3,4-naphthocoumarins

We herein report an efficient regioselective cascade of arylation and cyclization of arylcarboxylic acids via Pd(ii)-activation of both C–I and vicinal C–NO2 bonds of ortho-nitro-substituted diaryliodonium salts.