Substitution pattern elucidation of hydroxypropyl Pinus pinaster (Ait.) bark polyflavonoid derivatives by ESI(−)‐MS/MS

The structure of condensed tannins (CTs) from Pinus pinaster bark extract and their hydroxypropylated derivatives with four degrees of substitution (DS 1, 2, 3 and 4) has been characterized for the first time using negative‐ion mode electrospray ionization tandem mass spectrometry (ESI(?)‐MS/MS). The results showed that P. pinaster bark CTs possess structural homogeneity in terms of monomeric units (C₁₅, catechin). The oligomer sizes were detected to be dimers to heptamers. The derivatives showed typical phenyl‐propyl ether mass fragmentation by substituent elimination (58 amu) and inherent C₁₅ flavonoid fissions. The relative abundance of the product ions revealed a preferential triple, tetra‐/penta‐ and octa‐ hydroxypropylation substitution pattern in the monomer, dimer and trimer derivatives, respectively. A defined order of –OH reactivity towards propylene oxide was established by means of multistage experiments (A‐ring ≥ B‐ring > C‐ring). A high structural heterogeneity of the modified oligomers was detected. Copyright © 2014 John Wiley & Sons, Ltd.     

Photostability of 4,4′‐Dihydroxythioindigo,a Mimetic of Indigo

The photochemical properties of indigo, a widely used industrial dye, has attracted both experimentalists and theoreticians from the beginning. Especially the high photostability of indigo has been the subject of intensive research. Recently, it was proposed that after photoexcitation an intramolecular proton transfer followed by a nonradiative relaxation to the ground state promote photostability. In indigo the hydrogen bond and the proton transfer occur between the opposing hemiindigo parts. Here, we provide experimental and theoretical evidence that a hydrogen transfer within one hemiindigo or hemithioindigo part is sufficient to attain photostability. This concept can serve as an interesting strategy towards new photostable dyes for the visible part of the spectrum.     

Direct Spectroscopic Detection of the Orientation of Free OH Groups in Methyl Lactate–(Water)1,2 Clusters: Hydration of a Chiral Hydroxy Ester

Hydration of chiral molecules is a subject of significant current interest in light of recent experimental observations of chirality transfer from chiral solutes to water in solution and the important roles which water plays in biological events. Using a broadband chirped pulse and a cavity based microwave spectrometer, we detected spectroscopic signatures of the mono‐ and dihydrates of methyl lactate, a chiral hydroxy ester. Surprisingly, these small hydration clusters show highly specific binding preferences. Not only do they strongly prefer the insertion H‐bonding topology, but they also favor specific pointing direction(s) for their non‐H‐bonded hydroxy group(s). We observed that the particular dihydrate conformer identified is not the most stable one predicted. This work highlights the superior capability of high‐resolution spectroscopy to identify specific water binding topologies, and provides quantitative data to test state‐of‐the‐art theory.     

Probing the Zintl–Klemm Concept: A Combined Experimental and Theoretical Charge Density Study of the Zintl Phase CaSi

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl–Klemm concept. The presence of covalent Si? Si interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation–anion interaction cannot be described as purely ionic, but also has partially covalent character. Integrated QTAIM atomic charges of the atoms contradict the original Zintl–Klemm concept and deliver a possible explanation for the unexpected metallic behavior of CaSi.     

A High‐Pressure Polymorph of Phosphorus Nitride Imide

Phosphorus nitride imide, PN(NH), is of great scientific importance because it is isosteric with silica (SiO₂). Accordingly, a varied structural diversity could be expected. However, only one polymorph of PN(NH) has been reported thus far. Herein, we report on the synthesis and structural investigation of the first high‐pressure polymorph of phosphorus nitride imide, β‐PN(NH); the compound has been synthesized using the multianvil technique. By adding catalytic amounts of NH₄Cl as a mineralizer, it became possible to grow single crystals of β‐PN(NH), which allowed the first complete structural elucidation of a highly condensed phosphorus nitride from single‐crystal X‐ray diffraction data. The structure was confirmed by FTIR and ³¹P and ¹H solid‐state NMR spectroscopy. We are confident that high‐pressure/high‐temperature reactions could lead to new polymorphs of PN(NH) containing five‐fold‐ or even six‐fold‐coordinated phosphorus atoms and thus rivalling or even surpassing the structural variety of SiO₂.     

Asymmetric Cascade Reaction to Allylic Sulfonamides from Allylic Alcohols by Palladium(II)/Base‐Catalyzed Rearrangement of Allylic Carbamates

A regio‐ and enantioselective tandem reaction is reported capable of directly transforming readily accessible achiral allylic alcohols into chiral sulfonyl‐protected allylic amines. The reaction is catalyzed by the cooperative action of a chiral ferrocene palladacycle and a tertiary amine base and combines high step‐economy with operational simplicity (e.g. no need for inert‐gas atmosphere or catalyst activation). Mechanistic studies support a Pd^II‐catalyzed [3,3] rearrangement of allylic carbamates—generated in situ from the allylic alcohol and an isocyanate—as the key step, which is followed by a decarboxylation.     

N‐Heterocyclic Carbene,High Oxidation State Molybdenum Alkylidene Complexes: Functional‐Group‐Tolerant Cationic Metathesis Catalysts

We synthesized the first N‐heterocyclic carbene (NHC) complexes of Schrock’s molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16‐electron complexes represent metathesis active, functional‐group‐tolerant catalysts. Single‐crystal X‐ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring‐closing metathesis (RCM), ring‐opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω‐diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock’s catalyst, e.g., sec‐amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers.     

Pentacoordinate Phosphorus in a High‐Pressure Polymorph of Phosphorus Nitride Imide P4N6(NH)

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high‐pressure polymorph of highly condensed phosphorus nitride imide P₄N₆(NH) representing a new framework topology. This is the first example of phosphorus in trigonal‐bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi‐anvil technique. γ‐P₄N₆(NH) has been thoroughly characterized using X‐ray diffraction, solid‐state NMR and FTIR spectroscopy. The synthesis of γ‐P₄N₆(NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high‐pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.