Understanding Pd-Pd bond length variation in (PNP)Pd-Pd(PNP) dimers.

Analysis of the structures of three (PNP)Pd-Pd(PNP) dimers [where PNP stands for anionic diarylamido/bis(phosphine) pincer ligands] has been carried out with the help of single-crystal X-ray diffractometry and density functional theory (DFT) calculations on isolated molecules. The three dimers under study possess analogous ancillary ligands; two of them differ only by an F versus Me substituent in a remote (five bonds away from Pd) position of the pincer ligand. Despite these close similarities, X-ray structural determinations revealed two distinct structural motifs: a highly symmetric molecule with a long Pd-Pd bond or a highly distorted molecule with Pd-Pd bonds ca. 0.14 Å shorter. DFT calculations on a series of (PNP)Pd-Pd(PNP) dimers (as molecules in the gas phase) confirmed the existence of these distinct minima for dimers carrying large isopropyl substituents on the P-donor atoms (as in the experimental structure). These minima are nearly isoergic conformers. Evidently, the electronically preferred symmetric structure for the dimer (with a square-planar environment about Pd and a linear N-Pd-Pd-N vector) is not sterically possible with the preferred Pd-Pd distance. Thus, the minima correspond to either a symmetric structure with a long Pd-Pd bond distance or a structure with a short Pd-Pd distance but with substantial distortions in the Pd coordination environment to alleviate steric conflict. This notion is supported by finding only a single minimum (symmetric and with short Pd-Pd bonds) for each of the dimers carrying smaller substituents (H or Me) on the P atoms, regardless of the remote substitution.

Heterolytic splitting of H-X bonds at a cationic (PNP)Pd center.

The (PNP)PdOTf complex is a suitable synthetic equivalent of the [(PNP)Pd](+) fragment in reactions with various HX substrates. The [(PNP)Pd](+) fragment either simply binds HX molecules as L-type ligands (X = NH(2), PCy(2), imidazolyl) or heterolytically splits the H-X bond to produce [(PN(H)P)Pd-X](+) (X = H, CCR, SR). DFT calculations analyze the relative energetics of the two outcomes and agree with the experimental data. Calculations also allow to assess the unobserved Pd(IV) isomer [(PNP)Pd(H)(2)](+) and validate its unfavourability with respect to the Pd(II) isomer [(PN(H)P)PdH](+).

Syntheses of novel high affinity ligands for opioid receptors.

A series of novel high affinity opioid receptor ligands have been made whereby the phenolic-OH group of nalbuphine, naltrexone methiodide, 6-desoxonaltrexone, hydromorphone and naltrindole was replaced by a carboxamido group and the furan ring was opened to the corresponding 4-OH derivatives. These furan ring 'open' derivatives display very high affinity for mu and kappa receptors and much less affinity for delta. The observation that these target compounds have much higher receptor affinity than the corresponding ring 'closed' carboxamides significantly strengthens our underlying pharmacophore hypothesis concerning the bioactive conformation of the carboxamide group.

Carbon-halide oxidative addition and carbon-carbon reductive elimination at a (PNP)Rh center.

An unsaturated (PNP)Rh fragment can be generated by means of C-C reductive elimination from (PNP)Rh(Me)(Ar) or (PNP)Rh(Ar)(Ar). This fragment undergoes carbon-halogen oxidative addition with aryl chlorides, bromides, and iodides at room temperature. The C-H oxidative addition products in reactions with haloarenes are not observed, and evidence is presented that carbon-halogen oxidative addition is thermodynamically preferred. C-C reductive elimination from (PNP)Rh(Me)(Ar) and (PNP)Rh(Ar)(Ar) proceeds near quantitatively as a clean, first-order reaction.

Skeletal change in the PNP pincer ligand leads to a highly regioselective alkyne dimerization catalyst.

A Rh complex of a bulky diarylamino-based PNP pincer ligand is a robust catalyst for the dimerization of terminal alkynes and highly selective for the trans-enyne product.

Conformational polymorphism of methacrylamide.

The industrially important compound methacrylamide crystallizes as concomitant conformational polymorphs; the monoclinic form contains only the s-cis conformer, while the orthorhombic form contains only the s-trans conformer.

A nobel cyanogenic glycoside from Semiaquilegia adoxoides.

Phytochemical investigation of Semiaquilegia adoxoides was initiated in view of its common usage in traditional Chinese medicine and the scarcity of previous phytochemical studies. Fractionation of an ethanol extract from the roots of this plant led to the isolation and identification of a novel cyanogenic glycoside, 2-(beta-D-glucopyranosyloxy)-4-hydroxybenzeneacetonitrile (1). The structure of 1 was elucidated on the basis of IR, FAB-MS, 1D and 2D NMR spectral analysis.

Two new quinoline glycoalkaloids from Echinops gmelinii.

Two new glycosidic quinoline alkaloids, 1-methyl-4-methoxy-8-(beta-D-glucopyranosyloxy)-2(1H)-quinolinone (1) and 4-methoxy-8-(beta-D-glucopyranosyloxy)-2(1H)-quinolinone (2), have been isolated from the 1-butanol extract of the aerial parts of Echinops gmelinii (Compositae). Structural elucidation of the two new glycoalkaloids was based on their 1H, 13C, DEPT, HSQC, COSY, HMBC NMR spectra and high-resolution FAB-MS data. These two compounds are rare examples of quinoline alkaloidal glycosides from natural sources.

A new nitroethylphenolic glycoside from Semiaquilegia adoxoides.

The isolation of a new nitroethylphenolic glycoside, 4-hydroxy-1-(2-nitroethyl) benzene 4-O-(6'-O-beta-D-xylopyranosyl)-beta-D-glucopyranoside (1), from the roots of Semiaquilegia adoxoides is reported.

Facile oxidative addition of N-C and N-H bonds to monovalent rhodium and iridium.

An investigation of room-temperature oxidative addition of N-C and N-H bonds to RhI and IrI in solution and in the solid state is presented. The rigid, product-adapted framework of the pincer bis(ortho-phosphinoaryl)amine (PNP) ligand may contribute to the ease of the N-C and N-H cleavage. The migration of Me from N of the coordinated amine moiety to Rh proceeds with near-zero entropy of activation in solution. In the solid state, this transformation is a crystal-to-crystal reaction, transforming only one of the two independent molecules of (PN(Me)P)RhCl into (PNP)Rh(Me)Cl.

Diiminoisoindoline: tautomerism, conformations, and polymorphism.

Two polymorphs of the industrially important compound diiminoisoindoline occur in the amino tautomeric form as a conformational isomorph with a 1 : 1 mixture of the syn- and anti-isomers, and a conformational polymorph containing only the syn-isomer.