Chengyun Guo

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.

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.

Stereospecific solid-state cyclodimerization of bis(trans-2-butenoato)calcium and triaquabis(trans-2-butenoato)magnesium

60Co γ-irradiation of bis(trans-2-butenoato)calcium (4) and triaquabis(trans-2-butenoato)magnesium (11) affords cyclodimerization products cis,trans- and trans,trans-nepetic acids (5 and 15), respectively. The products are produced in high yield, based upon conversion, by a γ-ray induced radical chain pathway and provide unprecedented, stereospecific, one-pot syntheses of two of the four possible nepetic acid diastereomers, adding significant generality to the synthetic scope of the γ-ray induced reactions of crystalline metal trans-2-butenoates. Stereochemical analysis of the products from the analogous trans-2-butenoato-3-d complexes 19 and 27 establishes unequivocally that hydrogen atom transfer is also stereospecific and not part of a random process. The stereochemistry of the diastereomers is that predicted by the analysis of crystal packing, consistent with the least-motion principles of the topochemical postulate. Analysis of the crystal structures, with respect to the nearest neighbors, is consistent with the hypothesis that the formation of both carbon–carbon bonds and hydrogen atom transfer are topochemical and controlled by the crystal lattice. Analysis of the packing diagrams provides a pathway for chain propagation throughout the crystal that consumes all the molecules in the unit cell.

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.