Photoracemization-Based Viedma Ripening of a BINOL Derivative

Viedma ripening is a deracemization process that has been used to deracemize a range of chiral molecules. The method has two major requirements: the compound needs to crystallize as a conglomerate and it needs to be racemizable under the crystallization conditions. Although conglomerate formation can be induced in different ways, the number of racemization methods is still rather limited. To extend the scope of Viedma ripening, in the present research we applied UV-light-induced racemization in a Viedma ripening process, and report the successful deracemization of a BINOL derivative crystallizing as a conglomerate. Irradiation by UV light activates the target compound in combination with an organic base, required to promote the excited-state proton transfer (ESPT), leading thereafter to racemization. This offers a new tool towards the development of Viedma ripening processes, by using a cheap and “green” catalytic source like UV light to racemize suitable chiral compounds.     

Chiral Symmetry Breaking of Spiropyrans and Spirooxazines by Dynamic Enantioselective Crystallization

Dynamic enantioselective crystallization enabled the chiral symmetry breaking of two spiropyrans and one spirooxazine. The three spiro compounds afforded racemic conglomerate crystals, and easily racemized in alcoholic solution without irradiation. Optically pure enantiomorphic crystals were obtained by vapor-diffusion crystallization or attrition-enhanced deracemization (Viedma ripening). Their absolute configurations were determined by single-crystal X-ray analysis and each enantiomorphic crystal was correlated with its solid-state circular dichroism (CD) spectrum.     

A Chiral Stationary Phase Based on Guanine Conglomerates Obtained under Viedma Ripening Conditions

Journal of Analytical Chemistry - A new chiral stationary phase based on guanine is proposed. An Inerton NAW-HDMS inert substrate modified with 10% guanine under Viedma ripening conditions is...     

Absolute Asymmetric Synthesis: Viedma Ripening of [Co(bpy)3]2+ and Solvent‐Free Oxidation to [Co(bpy)3]3+

Syntheses of [Co(bpy)₃]²⁺ yield racemic solutions because the Δ‐ and Λ‐enantiomers are stereochemically labile. However, crystallization and attrition‐enhanced deracemization can give homochiral crystal batches of either handedness in quantitative yield. Subsequently, solvent‐free oxidation with bromine vapour fixes the chirality because [Co(bipy)₃]³⁺ does not enantiomerize in solution at ambient temperature. This combination of Viedma ripening and the labile/inert Co^II/Co^III couple constitutes a convenient method of absolute asymmetric synthesis.     

Enantiopure Isoindolinones through Viedma Ripening

Here we demonstrate that deracemization of isoindolinones using Viedma ripening is possible starting from a racemic mixture of conglomerate crystals. Crystals of the enantiopure isoindolinones lose their chiral identity upon dissolution even without the need for a catalyst. This enabled complete deracemization of the reported isoindolinones without a catalyst.     

Racemization and Deracemization through Intermolecular Redox Behaviour

Chiral molecules exhibiting a quinone and/or hydroquinone moiety are ubiquitous in natural products and small molecule drugs. Herein, we describe a chiral quinone-hydroquinone molecule that racemizes through a reversible redox reaction. Using a combined computational and experimental approach, we show that this racemization proceeds via an intermolecular reaction mechanism. Starting from two achiral reactants, this molecule could be obtained in enantiopure form using Viedma ripening.     

A Complex Reaction Network Model for Spontaneous Mirror Symmetry Breaking in Viedma Deracemizations

Attrition-enhanced chiral symmetry breaking in crystals, known as Viedma deracemization, is a promising method for converting racemic solid phases into enantiomerically pure ones under non-equilibrium conditions. However, many aspects of this process remain unclear. In this study, we present a new investigation into Viedma deracemization using a comprehensive kinetic rate equation continuous model based on classical primary nucleation theory, crystal growth, and Ostwald ripening. Our approach employs a fully microreversible kinetic scheme with a size-dependent solubility following the Gibbs–Thomson rule. To validate our model, we use data from a real NaClO₃ deracemization experiment. After parametrization, the model shows spontaneous mirror symmetry breaking (SMSB) under grinding. Additionally, we identify a bifurcation scenario with a lower and upper limit of the grinding intensity that leads to deracemization, including a minimum deracemization time within this window. Furthermore, this model uncovers that SMSB is caused by multiple instances of concealed high-order autocatalysis. Our findings provide new insights into attrition-enhanced deracemization and its potential applications in chiral molecule synthesis and understanding biological homochirality.     

Asymmetric Synthesis of Indoline from Achiral Phthalimide Involving Crystallization-Induced Deracemization

Asymmetric synthesis was performed by combining the photochemical reaction of an achiral substrate followed by crystallization-induced deracemization. The results indicated that a fused indoline produced by photochemical intramolecular δ-hydrogen abstraction and cyclization of N-(5-chloro-2-methylphenyl)phthalimide crystallized as a racemic conglomerate. Since this substrate has an aminal skeleton, racemization involving a ring-opening and ring-closing equilibrium process occurred under suitable conditions. Efficient racemization was observed in acetone containing a catalytic base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Crystallization-induced dynamic deracemization by Viedma ripening from racemic indoline was performed with an excellent enantioselectivity of 99 % ee. Furthermore, one-pot asymmetric synthesis of the indoline was achieved by the photochemical reaction of achiral phthalimide followed by continuous attrition-enhanced deracemization converging to 99 % ee of enantiomeric crystals. This is the first example of asymmetric expression and amplification by photochemical hydrogen abstraction and crystallization-induced dynamic deracemization.     

Spontaneous and induced chiral symmetry breaking of stereolabile pillar[5]arene derivatives upon crystallisation

Stereolabile pillar[5]arene (P[5]) derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation. In the solid state, we showed that crystal enantiomeric excess of a conglomerate-forming P[5] derivative can be obtained by handpicking and Viedma ripening without the intervention of external chiral entities. On the other hand, in the presence of ethyl d/l-lactate as both optically-active solvents and chiral guests, the handedness of P[5] derivative crystals, either forming racemic compounds or conglomerates upon condensation, can be directed and subsequently inverted in a highly controllable manner.

Stereolabile pillar[5]arene derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation.     

Crystallization of tetrahedral patchy particles in silico

We investigate the competition between glass formation and crystallization of open tetrahedral structures for particles with tetrahedral patchy interactions. We analyze the outcome of such competition as a function of the potential parameters. Specifically, we focus on the separate roles played by the interaction range and the angular width of the patches, and show that open crystal structures (cubic and hexagonal diamond and their stacking hybrids) spontaneously form when the angular width is smaller than about 30°. Evaluating the temperature and density dependence of the chemical potential of the fluid and of the crystal phases, we find that adjusting the patch width affects the fluid and crystal in different ways. As a result of the different scaling, the driving force for spontaneous self-assembly rapidly grows as the fluid is undercooled for small-width patches, while it only grows slowly for large-width patches, in which case crystallization is pre-empted by dynamic arrest into a network glass.     

Use of tetrameric cubane aggregates of lithium aryloxides as secondary building units in controlling network assembly

Pre-aggregation of lithium aryloxides into tetrahedral arrangements followed by crystallization with the divergent Lewis base dioxane results in the preparation of three types of coordination polymers: zig-zag chains, (6,3) sheets, and diamondoid lattices.     

Kinetics of nonisothermal polymer crystallization

We adopt a cluster size distribution model to investigate the kinetics of nonisothermal polymer crystallization. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different cooling rates. The incubation period is also investigated at different cooling rates and initial temperatures. The relationship between cooling rates and incubation time is presented graphically and compared with experimental measurements. The initial temperature (relative to melting point) has a significant effect on nonisothermal crystallization. A comparison of moment and numerical solutions of the population balance equations shows the influence of Ostwald ripening. Agreement between modeling results and experimental measurements at different cooling rates supports the application of the distribution kinetics model for nonisothermal crystallization.     

Synthesis of aluminosilicate and gallosilicate zeolites via a charge density mismatch approach and their characterization

Aluminosilicate and gallosilicate zeolite syntheses via a charge density mismatch (CDM) approach are compared at intermediate-silica compositions (Si/Me = 5-16, where Me is Al or Ga). With a variation of the crystallization temperature and of the type and/or concentration of alkali metal ions added as a crystallization structure-directing agent (SDA) to tetraethylammonium-tetramethylammonium, tetraethylammonium-hexamethonium, and strontium-choline mixed-SDA systems, we were able to obtain 11 different zeolite structures. However, only 5 out of a total 40 pairs of aluminosilicate and gallosilicate synthesis runs at otherwise identical chemical compositions were found to give the same zeolite product with no detectable impurities, suggesting that the structure-directing ability of Ga is quite different from that of Al even in intermediate-silica synthesis conditions. The CDM approach to offretite synthesis led to hexagonal plate-like crystals with aspect ratios lower than 0.3, and UZM-22 exhibited no significant preference of Al substitution for particular tetrahedral sites, especially for site T1, unlike its framework type material ZSM-18. More interestingly, the EU-1 zeolite obtained from an aluminosilicate synthesis mixture containing Li(+) as an inorganic crystallization SDA in the tetraethylammonium-hexamethonium double-organic additive system has been characterized to locate about half of its Li(+) ions in the framework, while the Li distribution over the 10 topologically different tetrahedral sites is nonrandom in nature.     

Self-organization of rotaxane thin films into spatially correlated nanostructures: morphological and structural aspects

The self-organization of rotaxane thin films into spatially correlated nanostructures is shown to occur upon a thermal stimulus. The mechanism of formation of nanostructures and their organization has been investigated using atomic force microscopy, bright field transmission electron microscopy, selected area electron diffraction, and molecular mechanics simulations. The evolution of the nanostructures follows a complex pathway, where a rotaxane thin film first dewets from the substrate to form nanosized droplets. Droplets coalesce by ripening, generating spatially correlated motifs. In a later stage, the larger droplets change shape, nucleate, and coalesce to yield crystallites that grow into larger crystals by incorporating the surrounding droplets. The results show the following: (i) the nanostructures represent a metastable state of a crystallization process; (ii) spatial correlations emerge during ripening, but they are destroyed as stable nuclei are formed and crystallization proceeds to completion; iii) crystallization, either on graphite or amorphous carbon films, leads to a precise basal plane, viz. (010), which has minimum surface energy. The inherent degrees of freedom permitted in the rotaxane architecture favors the re-organization and nucleation of the film in the solid state. Low-energy trajectories leading to crystallites with stable surfaces and minimum energy contact plane are found to occur via concerted, small amplitude, internal motions without disruption of packing and intermolecular contacts.     

Tetrahedral onsager crosses for solubility improvement and crystallization bypass

Pure organic molecules exhibiting a suitable concave rigid shape are expected to give porous glasses in the solid state. Such a feature opens new opportunities to avoid crystallization and to improve molecular solubility in relation to the high internal energy of these solid phases. To quantitatively explore the latter strategy, a series of rigid tetrahedral conjugated molecules nC and the corresponding models nR have been synthesized. Related to the present purpose, several properties have been investigated using UV absorption, steady-state fluorescence emission, differential scanning calorimetry, (1)H NMR translational self-diffusion, magic angle spinning (13)C NMR, and multiple-beam interferometry experiments. The present tetrahedral crosses are up to 8 orders of magnitude more soluble than the corresponding model compounds after normalization to the same molecular length. In addition, they give concentrated monomeric solutions that can be used to cover surfaces with homogeneous films whose thickness goes down to the nanometer range. Such attractive features make cross-like molecular architectures promising for many applications.     

The effect of B2O3 addition on the crystallization of amorphous TiO2-ZrO2 mixed oxide

The effect of B₂O₃ addition on the crystallization of amorphous TiO₂-ZrO₂ mixed oxide was investigated by X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG/DTA). TiO₂-ZrO₂ mixed oxide was prepared by co-precipitation method with aqueous ammonia as the precipitation reagent. Boric acid was used as a source of boria, and boria contents varied from 2 to 20 wt%. The results indicate that the addition of small amount of boria (<8 wt%) hinders the crystallization of amorphous TiO₂-ZrO₂ into a crystalline ZrTiO₄ compound, while a larger amount of boria (?8 wt%) promotes the crystallization process. FT-IR spectroscopy and ¹¹B MAS NMR results show that tetrahedral borate species predominate at low boria loading, and trigonal borate species increase with increasing boria loading. Thus it is concluded that highly dispersed tetrahedral BO₄ units delay, while a build-up of trigonal BO₃ promote, the crystallization of amorphous TiO₂-ZrO₂ to form ZrTiO₄ crystals.     

Preferential binding of fluorine to aluminum in high peralkaline aluminosilicate glasses

For two series of fluoride-containing aluminosilicate glasses of high peralkaline type, we apply 27Al, 19F, 29Si, and 23Na NMR spectroscopy to understand the structural changes introduced by the addition of alkali fluorides. Adding fluoride in concentrations above the solubility limit causes crystallization of different phases in sodium and potassium glasses despite identical composition. However, the NMR spectra reveal that the structural evolution of the precrystallized states is similar in both series. In particular, fluorine coordinates exclusively to alkaline cations and aluminum. No indication of direct bonding with silicon was found from 19F --> 29Si cross-polarization experiments. In contrast to other glass systems, double resonance experiments in these peralkaline systems show that halide addition produces at most a minor fraction of tetrahedral aluminum containing fluorine in its coordination sphere. Instead, the fluorine addition prior to crystallization converts up to about 20% of the initial tetrahedral aluminum (1 mol % in absolute units) to 5- and 6-fold coordinated aluminum. A minor portion of five-coordinated aluminum groups is considered as the intermediate to the growing fraction of octahedral aluminum in the silicate matrix. The initialization of the crystallization process is correlated with the saturation of the silicate matrix by octahedral aluminum clusters segregating out under further doping by fluoride. It is suggested that the formation of the nonframework Al-F bonds is responsible for structural relaxation, reflected by the reduction of the glass transition temperature.     

Ti-HMS和TS-1分子筛结构和催化性能的对比研究

 采用长链十二烷基胺为模板剂,成功合成出含钛中孔分子筛Ti-HMS, 并针对其晶化过程、结构特征以及催化性能与修正经典法制备的微孔分子筛TS-1进行了对比. 结果表明,中孔分子筛Ti-HMS的晶化机理不同于微孔分子筛TS-1的晶化机理. 随着晶化温度的提高, Ti-HMS分子筛的相对结晶度降低,且晶化过程中母液的pH值变化不大. TEM照片显示, Ti-HMS分子筛的孔壁整体上呈无定形,在局部区域可能存在晶态或者类晶态物种. 与TS-1分子筛相比, Ti-HMS分子筛的活性中心(骨架钛)处在畸变的四面体环境中. 中孔分子筛Ti-HMS可以满足大分子硫化物(如苯并噻吩)氧化反应的要求; 对于小分子硫化物(如噻吩)的氧化反应,微孔分子筛TS-1催化剂效果更好.     

Formation of uniform CuO nanorods by spontaneous aggregation: Selective synthesis of CuO, Cu2O, and Cu nanoparticles by a solid-liquid phase arc discharge process

Uniform and monodisperse CuO nanorods have been synthesized by directional aggregation and crystallization of tiny CuO nanoparticles generated from a solid-liquid arc discharge process under ambient conditions in the absence of any surfactants. Uniform CuO nanorods with sharp ends are formed from tiny nanoparticles via a process that involves the rapid oxidation of Cu nanoclusters, the spontaneous aggregation of CuO nanoparticles, and the Ostawald ripening process. The spontaneous aggregation and oriented attachment of tiny CuO nanoparticles contributed obviously to the formation of these kinds of nanostructures. By choice of suitable reducing agent to prevent the oxidation of Cu nanoclusters, Cu and Cu2O nanoparticles can be selectively synthesized.     

Unraveling the crystallization mechanism of CoAPO-5 molecular sieves under hydrothermal conditions

The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.