Manoj Kumar Gangwar

Palladium complexes of a new type of N-heterocyclic carbene ligand derived from a tricyclic triazolooxazine framework

AbstractA new type of tricyclic triazolooxazine derived N-heterocyclic carbene precursors were developed by the alkylation reaction of a tricyclic triazolooxazine framework. In particular, the reaction of 5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazine with methyl iodide and ethyl iodide yielded the tricyclic triazolooxazine derived N-heterocyclic carbene precursors, (1−2)a, in 67 −84% yield. The tricyclic triazolooxazinium iodide salts, (1−2)a, underwent metallation in a straight forward manner upon treatment with PdCl2 in the presence of K2CO3 in pyridine to give the trans-{3-(R)-5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazin-4-ylidene} PdI2(pyridine) [R = Me (1b), Et (2b)] complexes in 23 −25% yield. The new tricyclic triazolooxazine derived N-heterocyclic carbene moiety, as stabilized upon binding to palladium in the (1−2)b complexes, was structurally characterized by the X-ray single crystal diffraction studies. Graphical AbstractA new class of a tricyclic triazolooxazine derived N-heterocyclic carbene ligand has been developed by simple alkylation of a tricyclic triazolooxazine framework and the ligand has been stabilized by complexation with palladium in its (NHC)PdI2(pyridine) type complexes.

One-Pot Tandem Hiyama Alkynylation/Cyclizations by Palladium(II) Acyclic Diaminocarbene (ADC) Complexes Yielding Biologically Relevant Benzofuran Scaffolds

A series of palladium acyclic diaminocarbene (ADC) complexes of the type cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10 (2); NC4H8 (3); NC4H8O (4)] were used not only to perform the Csp2–Csp Hiyama coupling between aryl iodide and triethoxysilylalkynes but also to subsequently carry out the one-pot tandem Hiyama alkynylation/cyclization reaction between 2-iodophenol and triethoxysilylalkynes, giving a convenient time-efficient access to the biologically relevant benzofuran compounds. The palladium ADC complexes (2–4) were conveniently synthesized by the nucleophilic addition of secondary amines, namely, piperidine, pyrrolidine, and morpholine on the cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2 in moderate yields (ca. 61–66%).

Potent Anticancer Activity with High Selectivity of a Chiral Palladium N-Heterocyclic Carbene Complex

Five enantiomeric pairs of palladium complexes of 1,2,4-triazole-derived chiral N-heterocyclic carbene ligands were investigated to probe the influence of chirality on the compound’s anticancer activity. Although no chirality-related influence was observed for any of the enantiomeric pair, strong anticancer activity was seen for a particular pair, (1S,2S,5R)-1c and (1R,2R,5S)-1c, which was significantly more active than the benchmark drug cisplatin for human breast cancer cells, MCF-7 (ca. 24–27-fold), and human cervical cancer cells, HeLa (ca. three- to fourfold). Broadening its scope of application, (1R,2R,5S)-1c also exhibited antiproliferative activity against lung cancer (A549), skin cancer (B16F10), and multidrug-resistant mammary tumor (EMT6/AR1) cell lines. Interestingly, (1R,2R,5S)-1c displayed 8- and 16-fold stronger antiproliferative activity toward B16F10 and MCF-7 relative to their respective noncancerous counterparts, L929 (fibroblast skin cells) and MCF10A (epithelial breast cells), thereby upholding the potential of these complexes for further development as anticancer agents. (1R,2R,5S)-1c inhibited tumor-cell proliferation by blocking the cells at the G2 phase. (1R,2R,5S)-1c caused DNA damage in MCF-7 cells, leading to mitochondrial reactive oxygen species production and subsequently cell death. We also present evidence indicating that (1R,2R,5S)-1c induced p53-dependent programmed cell death in MCF-7 cells.

Heterodinuclear Zn(II)−Fe(III) and Homodinuclear M(II)−M(II) [M = Zn and Ni] complexes of a Bicompartmental [N 6 O] ligand as synthetic mimics of the hydrolase family of enzymes

Heterodinuclear mixed valence [Zn(II)-Fe(III)] and the homodinuclear [Zn(II)-Zn(II)] and [Ni(II)-Ni(II)] complexes of a bicompartmental ligand containing a bridging phenoxy as a O-donor and four pyridyl moieties and two amine moieties as the N-donors exhibit phosphoester hydrolysis activity similar to the hydrolase family of enzymes. While the heterodinuclear [Zn(II)-Fe(III)] (2) complex was obtained by the sequential addition of Fe(NO3)3∙9H2O and Zn(OAc)2∙2H2O to the ligand 2,6‑bis{[bis(2‑pyridylmethyl)amino]methyl}‑4‑t‑butylphenol (HL) (1) in moderate yield of 37%, the homodinuclear [Zn(II)-Zn(II)] (3) and [Ni(II)-Ni(II)] (4) complexes were obtained by the direct reaction of the ligand (1) with Zn(OAc)2∙2H2O and Ni(OAc)2∙2H2O respectively, in good to moderate yields (43-63%). Based on the spectrophotometric titration and the mass spectrometry studies, a monoaquated and dihydroxo species 2C, 3C and 4C has been identified as the catalytically active species responsible for the phosphodiester hydrolysis of the bis(2,4 - dinitrophenyl)phosphate (2,4 - BDNPP) substrate in the pH range 5.5-10.5. The kinetic studies further revealed that the homodinuclear [Ni(II)-Ni(II)] complexes (4) (kcat = 1.26 × 10-2 s-1) is more active by 39 times than the homodinuclear [Zn(II)-Zn(II)] complexes (3) (kcat = 3.20 × 10-4 s-1) and 27 times more active than the heterodinuclear [Zn(II)-Fe(III)] complex (2) (kcat = 4.62 × 10-4 s-1) in the phosphodiester hydrolysis activity. Significantly enough, the catalyst-substrate adduct species (2E, 2F and 3F) containing a metal bound bis(2,4‑dinitrophenyl)phosphate has been detected by mass spectrometry for the first time.