Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R-NHC, X-NHC and X-X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study

The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M⁰/M^II catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, M^II/M^IV catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R]⁺ cations. In this study, the relative stabilities of (NHC)M^II(R)(X)L and (NHC)M^IV(R)(X)₃L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M^II and M^IV), while it is thermodynamically and kinetically more facile for the M^IV complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the M^IV complexes were significantly stronger than that for the M^II complexes. In particular, for the (NHC)₂M^IV(Ph)(Br)₃ complexes, Ph-NHC coupling was facilitated dramatically from Pt (ΔG = -36.9 kcal mol^-1, ΔG^≠ = 37.5 kcal mol^-1) to Pd (ΔG = -61.5 kcal mol^-1, ΔG^≠ = 18.3 kcal mol^-1) and Ni (ΔG = -80.2 kcal mol^-1, ΔG^≠ = 4.7 kcal mol^-1). For the M^II oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the M^IV oxidation state of the metal as the (NHC)₂M^IV(R)(X)₃ complexes were kinetically (4.3-15.9 kcal mol^-1) and thermodynamically (8.0-23.2 kcal mol^-1) significantly less stable than the (NHC)M^IV(R)(X)₃L (L = pyridine) complexes. For the Ni^IV and Pd^IV complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X]⁺ cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising M^IV intermediates.

Pd-Catalyzed regioselective C–H halogenation of quinazolinones and benzoxazinones

A Pd-catalyzed ortho-selective halogenation of benzoxazinone and quinazolinone scaffolds has been described employing N-halosuccinimide via C–H bond activation.

Synthesis and biological screening of novel 2-morpholinoquinoline nucleus clubbed with 1,2,4-oxadiazole motifs

Novel series of 2-morpholinoquinoline scaffolds (6a-n), containing the 1,2,4-oxadiazole and moiety, was designed and synthesized in good yield (76-86%). The synthesized compounds were screened for their preliminary in vitro antimicrobial activity against a panel of pathogenic strains of bacteria and fungi. Molecular docking and pharmacokinetic study were carried out for the prepared compounds. The cytotoxicity of the synthesized compounds was tested at different concentrations using bioassay of S. pombe cells at the cellular level. The effect of synthesized compounds on the DNA integrity of S. pombe was observed on agarose gel. Compounds 6d, 6e, 6g, 6h, 6j and 6n exhibited excellent antimicrobial potency as compared to the standard drugs (i.e Ampicillin, Norfloxacin, Chloramphenicol, Ciprofloxacin). Compounds 6d, 6e, 6g, 6k and 6n were found to have significant antifungal activity as compared to griseofulvin. Compounds 6f, 6i, 6k, 6l were found very less cytotoxic, while compounds 6d, 6e, 6g, 6h were found to exhibit maximum toxicity. The rest of the synthesized compounds were found to be moderately toxic.

Palladium-Catalyzed Carbamate-Directed Regioselective Halogenation: A Route to Halogenated Anilines

This study describes an efficient method for ortho-selective halogenation of N-arylcarbamates under mild conditions for the first time. Although being weakly coordinating, N-arylcarbamates act very well as a removable directing group for activation of C-H bonds. The developed procedure results in extremely valuable halogenated N-arylcarbmates that can further be hydrolyzed to halogenated anilines. The obtained reaction conditions showed broad scope and wide functional group tolerance. All the products were formed in good yields with extremely high selectivity.

Palladium N-heterocyclic carbene catalyzed expected and unexpected C–C and C–N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones

A palladium N-heterocyclic carbene complex of vitamin B₁ developed earlier in our laboratory was successfully employed as an efficient catalyst for the regioselective C–C and C–N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones.