Palladium catalyzed annulation of benzylamines and arynes via C–H activation to construct 5,6-dihydrophenanthridine derivatives

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.

Synthesis of 5,6-Dihydrophenanthridines via Palladium-Catalyzed Intramolecular Dehydrogenative Coupling of Two Aryl C−H Bonds

5,6-Dihydrophenanthridines are common aza heterocycle frameworks of natural products and pharmaceuticals. Herein, we reported the first palladium-catalyzed intramolecular C−H/C−H dehydrogenative coupling reaction of two simple arenes to generate 5,6-dihydrophenanthridines. The approach features a broad substrate scope and good tolerance of functional groups, offering an efficient alternative synthesis route for important 5,6-dihydrophenanthridine compounds.

Aryne Multicomponent Reactions by Directed C-H Activation

Arylation via ortho C-H activation by the aid of directing groups has been explored recently by many researchers. Herein, a palladium-catalyzed C-H arylation using 8-aminoquinoline as a bidentate directing group has been developed. The reaction furnishes only C-H arylation, unlike previous methods where cyclization to corresponding isoquinolones is observed. More interestingly, sequential C-H functionalization was observed when methylacrylate and acrylonitrile was added; this led to C-H olefination with the aryl group, which was installed from the aryne precursor.

Selective Synthesis of Dihydrophenanthridine and Phenanthridine Derivatives from the Cascade Reactions of o-Arylanilines with Alkynoates through C-H/N-H/C-C Bond Cleavage

In this paper, an unprecedented selective synthesis of dihydrophenanthridine and phenanthridine derivatives through the cascade reactions of 2-arylanilines with alkynoates is presented. Mechanistic studies showed that the formation of the dihydrophenanthridine scaffold involves an initial C(sp²)-H alkenylation of 2-arylaniline with alkynoate followed by an intramolecular aza-Michael addition. When this reaction is carried out at elevated temperature, the in situ formed substituted dihydrophenanthridine readily undergoes a retro-Mannich-type reaction to give the corresponding phenanthridine through C-C bond cleavage. Compared with literature methods, this novel protocol has advantages such as easily obtainable substrates with a free amino group, pharmaceutically privileged products, cheap catalysts, and conveniently controllable selectivity.

The crystal structure of 5-(benzofuran-2-carbonyl)-N-cyclohexyl-5,6-dihydrophenanthridine-6-carboxamide, C29H26N2O3

C29H26N2O3, triclinic, P1̄ (no. 2), a = 9.6533(8) Å, b = 11.3185(9) Å, c = 11.4469(10) Å, α = 83.747(3)°, β = 85.810(2)°, γ = 71.445(2)°, V = 1177.65(17) Å3, Z = 2, R gt(F) = 0.0464, wR ref(F 2) = 0.1621, T = 296(2) K.