Meso Research Articles

Phosphorus-Nitrogen Compounds: Part 76. Design and Syntheses of Dispiro- and Trispiro(N/N)cyclotriphosphazenes: Conformational and Structural Analyses, Chirality, Electrochemical, Dye-Sensitized Solar Cells, and Bioactivity Studies.

The reactions of monospirocyclotriphosphazenes (1 and 2) with N-methyl-1,3-diaminopropane gave unsymmetrical cis-(3 and 5) and trans-(4 and 6) dispirocyclotriphosphazenes. Trans-cis-trans (tct) (7 and 11), cis-cis-cis (ccc) (8 and 12), trans-trans-cis (ttc) (9 and 13), and cis-trans-trans (ctt) (14) trispirocyclotriphosphazenes were obtained from the reactions of 3 and 5 and 4 and 6 with N-methyl-1,3-diaminopropane. cis-Dispirocyclotriphosphazenes (3 and 5) exist as "pseudomesoracemates", while trans-dispirocyclotriphosphazenes (4 and 6) are in "racemates". The existences of cis-3 and trans-4 as "pseudomesoracemate" and "racemate" were confirmed by 31P NMR spectra recorded by the addition of "chiral solvating agent (CSA)". X-ray crystallography proved that the absolute configurations of each enantiomer of cis-5 and trans-6 are SS' and RS'. Trispirocyclotriphosphazenes tct, ttc, ccc, and ctt exist as racemates, pseudomesoracemate, and meso forms. Furthermore, Hirshfeld surface analysis of the crystal structures of cis-5 and trans-6 revealed that the most significant contribution to crystal packing comes from H···H (58.2 and 57.6%, respectively). An oxidation-reduction wave was detected in the reversible cyclic voltammograms of the phosphazenes. The highest power conversion efficiency in dye-sensitized solar cell studies was obtained with cis-5. Additionally, trans-6 exhibited the lowest minimal inhibitory concentration value (78.1 μM) against Candida tropicalis, and it was observed to cleave pBR322 plasmid DNA.

Aluminiumalkylinduzierte Isomerisierung von Gruppe IV meso Metallocen Komplexen

AbstractBei der Synthese von Gruppe IV Metallocen‐Präkatalysatoren für die Polymerisation von Propylen werden im Allgemeinen zwei verschiedene Isomere gebildet: Das racemische Isomer, das isotaktisches Polypropylen (iPP) produziert und das meso‐Isomer, das ataktisches Polypropylen (aPP) produziert. Aufgrund seiner vergleichbar schlechten physikalischen Eigenschaften ist die Anwendung von aPP sehr begrenzt. Um Mischungen beider Polymere zu vermeiden, was die mechanischen und thermischen Eigenschaften von iPP zu verschlechtern würde, müssen die meso Metallocenkomplexe mühsam von den racemischen Komplexen getrennt werden, was die Metallocen‐basierte Polymerisation von Propylen industriell weit weniger attraktiv macht als das Ziegler/Natta‐Verfahren. Um dieses Problem zu umgehen, haben wir ein Isomerisierungsprotokoll zur Umwandlung von meso Metallocenkomplexen in ihre racemischen Analoga entwickelt. Dieses Protokoll steigerte die Ausbeute an iPP um 400 %, wobei die hervorragenden physikalischen Eigenschaften des Polymers beibehalten wurden; es war sowohl auf Hafnocen‐, als auch auf Zirkonocenkomplexe, sowie auf verschiedene Methoden zur Aktivierung des Präkatalysators anwendbar. Durch gezielte Variation der Liganden wurden Methoxygruppen an den Indenylsubstituenten als die für eine Isomerisierung verantwortlichen Strukturmotive gefunden ‐ dieser experimentelle Nachweis wurde durch Dichtefunktionaltheorieberechnungen bestätigt. Flüssig‐Injektion‐Feld‐Desorption‐Ionisierung‐Massenspektrometrie, sowie 1H‐ und 29Si‐Kernresonanzstudien ermöglichten den Vorschlag eines Isomerisierungsmechanismus.

Aluminum Alkyl Induced Isomerization of Group IV meso Metallocene Complexes.

The synthesis of group IV metallocene precatalysts for the polymerization of propylene generally yields two different isomers: The racemic isomer that produces isotactic polypropylene (iPP) and the meso isomer that produces atactic polypropylene (aPP). Due to its poor physical properties, aPP has very limited applications. To avoid obtaining blends of both polymers and thus diminish the mechanical and thermal properties of iPP, the meso metallocene complexes need to be separated from the racemic ones tediously-rendering the metallocene-based polymerization of propylene industrially far less attractive than the Ziegler/Natta process. To overcome this issue, we established an isomerization protocol to convert meso metallocene complexes into their racemic counterparts. This protocol increased the yield of iPP by 400 % while maintaining the polymer's excellent physical properties and was applicable to both hafnocene and zirconocene complexes, as well as different precatalyst activation methods. Through targeted variation of the ligand frameworks, methoxy groups at the indenyl moieties were found to be the structural motifs responsible for an isomerization to take place-this experimental evidence was confirmed by density functional theory calculations. Liquid injection field desorption ionization mass spectrometry, as well as 1H and 29Si nuclear magnetic resonance studies, allowed the proposal of an isomerization mechanism.

Ammonium Zincates as Catalysts for the Microwave-Enhanced Synthesis of Symmetric Piperazines by Regioselective Opening of Aziridines.

2,5-disubstituted N,N'-alkylpiperazines represent an interesting target in organic synthesis both for pharmaceutical or agrochemical applications and as a promising class of ligands in coordination chemistry. We report here a microwave-enhanced synthesis of these compounds starting from non-activated N-alkyl aziridines in the presence of catalytic amounts of simple ammonium metallates. A remarkable TOF of 2787.9 h-1 has been observed in the case of [TBA]2[ZnI4] as the catalyst (catalyst loading 0.1 mol %) and with an almost complete selectivity (up to 97 %) in favor of both diastereoisomers (meso and chiral form) of the target 2,5-disubstituted piperazines, obtained in 1 : 1 ratio. The two isomers are easily separated, because the meso form precipitates in pure from the reaction crude. A stereochemical investigation and the unprecedented isolation of 2,6-disubstituted N,N'-alkylpiperazines allowed us to shed light on the reaction mechanism.

Supramolecular frameworks assembled by [EuL2]3+ units with different coordination configurations

Self-assembly is an important strategy to construct supramolecular frameworks. Small change in the assembling units often leads to significant variations in the self-assembly structures. In the work, we obtain two supramolecular frameworks assembled by different [EuL2]3+ units (L = tris((5,6-dimethyl-1H-benzo[d]imidazol-2-yl)methyl) amine). One meso compound {[EuⅢL2]·(H2O)6·Cl3} (Meso-EuL2) is assembled by an achiral meso-[EuL2]3+ unit, forming a supramolecular three-dimensional framework through six-fold intermolecular π‒π interactions. The other is a racemic compound {[EuⅢL2]·(CH3OH)5·Cl3} (Rac-EuL2), assembled by two chiral rac-[EuL2]3+ units predominantly via hydrogen bonding, leading to a layered supramolecular framework. The luminescent properties are different between Meso-EuL2 and Rac-EuL2, with Meso-EuL2 exhibiting more excimer-like emissions, which is likely caused by its stronger π‒π interactions in the structure.

Meso Compounds Systematization ‒ A Chemical Paradox among Inositols and a Group of Super-symmetric Combinations

Meso Compounds Systematization ‒ A Chemical Paradox among Inositols and a Group of Super-symmetric Combinations

Structural Diversity in Cyclometalated Diiridium(III) Complexes with Bridging syn and anti μ2‐Oxamidato and μ2‐Dithioxamidato Ligands

AbstractSix new diiridium complexes containing 2‐methyl‐6‐phenylpyridyl as the cyclometalating ligand with a μ2‐oxamidato or a μ2‐dithioxamidato ligand as the bridge have been synthesized in 60–73 % yields. These complexes were revealed by multinuclear NMR spectroscopy to contain inseparable mixtures of diastereomers (rac, ΔΔ/ΛΛ and meso, ΔΛ) with bridges in anti and syn configurations. The remarkable variety of isomers present was confirmed by X‐ray crystallography on single crystals grown from mixtures of each complex. In one complex with a N,N’‐bis(4‐trifluoromethylphenyl)‐μ2‐oxamidato bridge, two single crystals of anti and syn isomers were structurally determined. Two single crystals of the μ2‐dithioxamidato bridge complex were found to contain rac and meso forms of the syn isomer. Hybrid DFT computations on the four isomers of each diiridium complex revealed negligible energetic preferences for one isomer despite the methyl groups in the 2‐methyl‐6‐phenylpyridyl cyclometalating ligands being close to the neighboring methyl groups and the bridge, thus supporting the experimental findings of isomer mixtures. Two distinct broad emissions with maxima at 522–529 nm and at 689–701 nm observed in these complexes in dichloromethane are attributed to mixed metal‐ligand to ligand charge transfer (MLLCT) excited states involving the pyridyl and bridge moieties respectively with the aid of electronic structure computations.

Organoplatinum‐Bridged Cyclotribenzylene Dimers

AbstractCyclotribenzylenes (CTBs) combining carbonitrile (−CN) and alkyne (−C2H) substituents were synthesized as racemic mixtures and resolved by HPLC on chiral stationary phases. Two of these compounds were used to prepare platinum‐bridged CTB dimers, in which PtII is bound to the CTBs via Pt−alkynyl bonds in cis configuration. The organometallic complexes were examined by mass spectrometry and NMR spectroscopy, which indicated that they were obtained as mixtures of diastereoisomers (a meso or syn form and a pair of chiral or anti forms) when racemic CTBs were used. Enantiomerically pure complexes were prepared from resolved CTBs, which allowed us to distinguish the NMR signals of the chiral and meso forms in the diastereoisomeric mixtures. In certain conditions, the platinum complexes played the role of a pincer π‐alkynyl ligand for Cu(I) coming from the copper iodide used as a synthetic auxiliary. The Cu+ cations could be easily removed by treatment with NaCN, affording the mononuclear bis‐cyclotribenzylene complexes. These compounds failed to lead to metallo‐cryptophanes by coordination of two [M(dppp)]2+ complex subunits (M=Pd, Pt; dppp=1,3‐bis(diphenylphosphino)propane), each to two carbonitrile substituents belonging to different CTBs, pointing to the superiority of the one pot self‐assembly processes for the preparation of metallo‐cryptophanes.

Revisiting Homochiral versus Heterochiral Interactions through a Long Detective Story of a Useful Azobis-Nitrile and Puzzling Racemate.

This paper documents and reinvestigates the solid-state and crystal structures of 4,4'-azobis-4-cyanopentanoic acid (ACPA), a water-soluble azobis-nitrile of immense utility as a radical initiator in living polymerizations and a labile mechanophore that can be embedded within long polymer chains to undergo selective scission under mechanical activation. Surprisingly, for such applications, both the commercially available reagent and their derivatives are used as "single initiators" when this azonitrile is actually a mixture of stereoisomers. Although the racemate and meso compounds were identified more than half a century ago and their enantiomers were separated by classical resolution, there have been confusing narratives dealing with their characterization, the existence of a conglomeratic phase, and fractional crystallization. Our results report on the X-ray crystal structures of all stereoisomers for the first time, along with further details on enantiodiscrimination and the always intriguing arguments accounting for the stability of homochiral versus heterochiral crystal aggregates. To this end, metadynamic (MTD) simulations on stereoisomer molecular aggregates were performed to capture the incipient nucleation events at the picosecond time scale. This analysis sheds light on the driving homochiral aggregation of ACPA enantiomers.

Effect of Botrytis cinerea Activity on Glycol Composition and Concentration in Wines

The content of 2,3-butanediol ((R,R) and meso isomers) and 1,2-propanediol in grape berries and “liquid samples” (all non-berry extracts) from the Tokaj wine region of Hungary was investigated. Our aim was to find out how the activity of Botrytis cinerea influences the concentrations of these compounds compared with healthy grapes. Based on the measured concentrations, we can make a distinction between healthy berries and noble, rotted, so-called aszú berries. We also investigated if there is a difference between finished aszú wines and liquids intended for aszú production. We wanted to investigate the amount and distribution of the stereoisomers of 2,3-butanediol and their proportions. The results of the HS-SPME-GC-FID analysis of the samples showed significant differences in the 2,3-butanediol content between healthy and botrytised, aszú berries and between liquid samples for aszú production and aszú wines. In the berry samples, meso-2,3-butanediol could not be detected, whereas in the liquid samples, we found good amounts of this isomer. This may be due to the fact that the appearance of the meso form of 2,3-butanediol is a consequence of alcoholic fermentation. Significant differences were found between wines from healthy grapes and wines from botrytised grapes in terms of the levo-2,3-butanediol content, so that from an analytical point of view, a difference can be made between wines from healthy and botrytised grapes. No significant differences were found between berry and liquid samples in terms of 1,2-propanediol concentrations during our tests.

Chiral Self-Sorting in Pd6 L12 Metal-Organic Cages.

Chiral self-sorting during the formation of cage-like molecules continues to fascinate and advance our understanding of the phenomenon in general. Herein, we report the chiral self-sorting in the Pd6 L12 -type metal-organic cages. When a racemic mixture of axially chiral bis-pyridyl ligands undergo coordination-driven self-assembly with Pd(II) ions to form Pd6 L12 -type cages, the system has the option of chiral self-sorting to afford any of at least 70 pairs of (one homochiral and 69 heterochiral) enantiomers and 5 meso isomers or a statistical mixture of everything. However, the system resulted in diastereoselective self-assembly through a high-fidelity chiral social self-sorting to form a racemic mixture of D3 symmetric heterochiral [Pd6 (L6R/6S )12 ]12+ /[Pd6 (L6S/6R )12 ]12+ cages.

Optimizing disordered crystal structures.

A mixed-crystal full-mol­ecule disorder situation is revisited in Parkin et al. [Acta Cryst. (2023), C79, 000–000]. A new inter­pretation of the data leads to the conclusion that the crystal structure is more likely to be a three-component superposition of enantiomers and the meso isomer of an organic compound and the article is thus a good learning example for dealing with a highly disordered structure.

(E,E)-1,1'-[1,2-Bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) revisited: threefold configurational disorder of (S,S), (R,R) and (S,R) isomers, a detailed critique.

Crystal structures described as concomitant triclinic (I) and monoclinic (II) polymorphs of meso-(E,E)-1,1'-[1,2-bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene) [Mohamed et al. (2016). Acta Cryst. C72, 57-62] have been re-investigated. The published model for II was distorted due to forcing the symmetry of space group C2/c on an incomplete structure model. It is shown here to be a likely three-component superposition of S,S and R,R enantiomers with a lesser amount of the meso form. A detailed analysis of how the improbable distortion in the published model aroused suspicion and the subsequent construction of undistorted chemically and crystallographically plausible alternatives having the symmetry of Cc and C2/c is presented. For the sake of completeness, an improved model for the triclinic P-1 structure of the meso isomer I, revised to include a minor disorder component, is also given.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties.

A C4h symmetrically substituted phthalocyanine, 1,8,15,22-tertrakis(2,4-dimethylpent-3-oxy)phthalocyanine (H2 TdMPPc), was used to synthesize Tb3+ -phthalocyanine double-decker complexes ([Tb(TdMPPc)2 ]s). Because H2 TdMPPc has C4h symmetry, S,S, R,R, and meso isomers of [Tb(TdMPPc)2 ] were obtained depending on the difference in the direction of the coordination plane of two C4h -type phthalocyanines with respect to a central Tb3+ ion. We investigated the physical properties of these [Tb(TdMPPc)2 ] isomers, including their single-ion magnetic properties, and found that the spin-reversal energy barrier (Ueff ) of the meso isomer was apparently higher than that of the enantiomers. Detailed crystal structural analyses indicated that the meso isomer has a more symmetrical structure than do the enantiomers, thereby suggesting that the higher Ueff of the meso isomer originated from the more highly symmetrical structure.

Chiral selection of Tröger's base-based macrocycles with different ethylene glycol chains length in crystallization

In the present work, two Trӧger's base-based macrocycles (TBBMs) with different bridging ethylene glycol chains (T1, n = 1; T3, n = 3) were successfully synthesized and studied via the crystal analysis. These two TBBMs possess rare rectangular-like cavities and show chiral selection during the crystallization. T1 with short glycol chain (n = 1) crystallized as racemates, while T3 with long glycol chain (n = 3) was found as meso isomer. In contrast to T1 and T3, for T2 (n = 2) both rac-T2 and meso isomer R2NS2N-T2 has been observed in our previous report. Thus, the synthesis of new TBBMs T1 and T3 with different bridging ethylene glycol chains not only makes the study of TBBMs more systematically, but also helps to understand the relationship between the size of the rectangular cavity and the chiral selection of Tröger's base-based macrocycles during their crystallization.

Discrete chiral organic nanotubes by stacking pillar[5]arenes using covalent linkages

Owing to their unique one-dimensional hollow structures, organic nanotubes have been widely explored in recent years. Covalent organic nanotubes (CONs) can be prepared by stacking building blocks, such as macrocycles, through covalent linkages. However, because of the mismatched covalent connections, controllable synthesis of the discrete CONs with clear structures, such as sidewall and chirality, is a challenging target. In this work, by coupling two pillar[5]arenes through dynamic covalent bonds, thermodynamically stable discrete CONs with 5-fold symmetry are successfully prepared. Three different chiral CONs are separated, including homo-CONs, consisting of two enantiomers (pR, pR and pS, pS), and hetero-CON, consisting of the meso form (pR, pS). These CONs show negative allosteric binding affinities toward guest molecules, which are not observed in individual pillar[5]arenes.

Polygonapholine: A Total Synthesis Questions the Identity for the Purported Structure of the Natural Product.

Polygonapholine was isolated in 1997 from the Polygonatum alte-lobatum rhizome. Based on spectroscopic data, it was assigned a structure comprising an unusual cis-2,6-disubstituted bis-aryl morpholine ring to which is attached a (Z)-4-hydroxycinnamate as an amide and an (E)-4-hydroxycinnamate as an ester. Being a meso compound, polygonapholine should not exhibit an optical rotation as reported. A total synthesis of the purported morpholine alkaloid presented herein reveals discrepancies between the synthetic and the natural product casting doubt on the originally proposed structure.

Synthesis, structural, and stereogenic characterizations of new trispirocyclotriphosphazenes

AbstractIn this study, trispirophosphazenes containing biphenyl‐2,2′‐dioxy and ferrocenyldiamino groups were synthesized to investigate their stereogenic and structural properties. For this purpose, monospiro (1) and trans/cis‐dispiro (2and3) cyclotriphosphazenes were obtained from the reactions of hexachlorocyclotriphosphazatriene (N3P3Cl6; trimer;HCCP) withN‐ethyl‐N′‐ferrocenylmethyl‐ethylenediamine. Subsequently, bis(biphenyl‐2,2′‐dioxy)‐substituted ferrocenylphosphazene (4) was prepared from the reaction of an equimolar amount of monoferroceylphosphazene (1) with two equimolar amounts of dipotassium biphenyl‐2,2′‐dioxide. Reactions of equimolar amounts of2and3with equimolar amounts of dipotassium biphenyl‐2,2′‐dioxide yielded trans‐(5) and cis‐(6) trispirophosphazenes, respectively. Characterizations of all trispiro compounds were made using1H,13C, and31P NMR spectra. In addition, trispirophosphazenes5and6have two equivalent stereogenic P‐centers.Trans‐5andcis‐6are expected to exist in racemate and meso forms. The molecular and crystal structures of the two phosphazenes (4and5) were clarified by single‐crystal X‐ray crystallography. Also, since the absolute configuration of one enantiomer of5is found to be RR, the other must be SS.

Perfectly Isotactic Polypropylene upon In Situ Activation of Ultrarigid meso Hafnocenes.

For more than 40 years, the synthesis of C2 -symmetric indenyl-based racemic metallocenes for the isoselective polymerization of propylene relied on a tedious separation of the produced rac and meso isomers. Status quo, latter are considered wasteful as they produce atactic polypropylene (aPP) rather than isotactic polypropylene (iPP) if activated with methylaluminoxane (MAO). Unexpectedly, the in situ activation of meso hafnocene I yielded perfectly isotactic polypropylene. We verified an isomerization of the meso compound to the corresponding racemic one upon triisobutylaluminum (TIBA) addition via nuclear magnetic resonance (NMR) spectroscopy and established an easy and convenient polymerization protocol, enabling productivities comparable to pure rac-I if applied to pure meso-I or a mixture of both isomers. With this established isomerization protocol, the potential yield of iPP was enhanced by more than 400 %. This protocol was also shown to be applicable to other meso hafnocenes and some initial mechanistic insights were received.

New Chemical Dualities Illustrated by Meso and C2 Symmetrical (CTS) Compounds

The most important relationship between chemistry and genetics is nonetheless the corpuscular nature of their objects, molecules (or atoms) and genes, respectively. On the other hand, one states, without a substantial proof, that philosophy should be one step ahead all sciences. Here is a proof that the reverse can also be true. Two internal enantiomeric halves of meso compounds or the two chiral halves of ­C2 symmetrical isomers constitute pairs of entities suitable to work as duality phenomena in science. Four types of isomers have been identified: (A) meso, (B) C2 symmetrical (CTS), (C) irregular chiral (irrechi) and (D) constitutional (constit.). Meso and CTS are characteristic to plants and microorganisms. Almost all natural micromolecular compounds from vertebrate tissues are asymmetric, i.e. they are constit. isomers. An exception to this rule is meso-inositol, an isomer of hexoses, which are themselves, as their congeners asymmetric. By comparing the real (envisaged) meso isomers of these compounds with the asymmetric ones of vertebrate tissues, the reason for nature selected the latter became quite evident: it is the omission of a suite of structural restrictions. Delivering of meso isomers of natural compounds discloses a huge chemical philosophical potential of this issue. An intrinsic property of meso combinations is their character of dimerism, hence their molecule is formed of two entities that are contrary in a spatial, chemical and optical sense, i.e. good candidates for a duality concept. Moreover, a good deal of material is indicated, i.e. CTS isomers, whose sides are chiral and identical, for a new type of duality in philosophy, strongly expressed in nature by a chemical language.