Interplay between non-covalent interactions in 1D supramolecular polymers based on 1,4-bis(iodoethynyl)benzene.

A study of the solution-phase, solid-state structures of halogen-bonded co-crystals of 1,4-bis(iodoethynyl)benzene (p-BIB) with three salts, namely, decyltrimethylammonium bromide (DTMABr), tetrapropylammonium bromide (TPABr), and tetrabutylammonium bromide (TBABr), has been carried out, along with theoretical calculations. Isothermal titration calorimetry (ITC) showed that the binding constant of bromide with p-BIB in THF is not strongly dependent on the cation, and that the entropic term clearly dominates the enthalpic one in the free energy of binding. In the three crystal structures, the bromide anion acts as a doubly connected node for halogen bonding interactions, which results in linear or angular open chains. The intrachain angles (IBr-I) of the 1D supramolecular polymers based on p-BIB depend on the geometry and size of the cation and vary from 180° for DMTABr to 75° for TBABr. Non-covalent interaction (NCI) analysis of selected motifs and optimized crystals demonstrates that the balance between halogen bonds, hydrogen bonds, and van der Waals interactions, especially type-I halogenhalogen contacts, determines the crystal structures.

Gold Catalyzed Multicomponent Reactions beyond A³ Coupling.

The preparation of complex architectures has inspired the search for new methods and new processes in organic synthesis. Multicomponent reactions have become an interesting approach to achieve such molecular diversity and complexity. This review intends to illustrate important gold-catalyzed examples for the past ten years leading to interesting skeletons involved in biologically active compounds.

A facile method to determine the absolute structure of achiral molecules: supramolecular-tilt structures.

Achiral compounds 4-methoxy-4-(p-methoxyphenyl)cyclohexanoneethylene ketal (2), 4-hydroxy-4-(p-methoxy phenyl)cyclohexanoneethylene ketal (3), and 3,5-dimethyl-4-nitropyrazole (4) crystallized in chiral structures and the samples showed an enantiomeric excess. We have determined the absolute structures of these compounds by using X-ray diffraction with copper radiation at low temperatures. Moreover, we have also established the prevalent absolute structures in these samples, by comparing their calculated and solid-state vibrational circular dichroism (VCD) spectra. The consistency of this method was confirmed by using (R,R)-2,8-diiodo-4,10-dimethyl-6 H,12H-5,11-methano-dibenzo[b,f][1,5]diazocine, Tröger's base, (R,R)-1, as a chiral compound of known absolute configuration.

Crystallization of an achiral cyclohexanone ethylene ketal in enantiomorphs and determination of the absolute structure.

The achiral 4-methoxy-4-(p-methoxyphenyl)-cyclohexanone ethylene ketal (1) resolves spontaneously. The crystal structure is solved in chiral spatial group P2(1). Because compound 1 is composed of only light atoms (C, H, O) it is not possible to determine its absolute structure configuration. An efficient procedure for the absolute structure configuration determination of flexible molecules containing only light atoms is proposed, based on the combination of X-ray diffraction, solid-state VCD, and DFT calculations.