The relationship between crystal structure and methyl and t-butyl group dynamics in van der Waals organic solids

We report x-ray diffractometry in a single crystal of 2-t-butyl-4-methylphenol (TMP) and low-frequency solid state nuclear magnetic resonance (NMR) proton relaxometry in a polycrystalline sample of TMP. The x-ray data show TMP to have a monoclinic, P2(1)/c, structure with eight molecules per unit cell and two crystallographically inequivalent t-butyl group (C(CH(3))(3)) sites. The proton spin-lattice relaxation rates were measured between 90 and 310 K at NMR frequencies of 8.50, 22.5, and 53.0 MHz. The relaxometry data is fitted with two models characterizing the dynamics of the t-butyl groups and their constituent methyl groups, both of which are consistent with the determined x-ray structure. In addition to presenting results for TMP, we review previously reported x-ray diffractometry and low-frequency NMR relaxometry in two other van der Waals solids which have a simpler structure. In both cases, a unique model for the reorientational dynamics was found. Finally, we review a similar previously reported analysis in a van der Waals solid with a very complex structure in which case fitting the NMR relaxometry requires very many parameters and serves mainly as a flag for a careful x-ray diffraction study.     

Determinations cinetiques par analyse microcalorimetrique differentielle. XVII. Decomposition spontanee et induites du percarbonate de O,O-t-butyle et O-vinyle en solution

The mechanism of the free-radical decomposition of O,O-t-butyl and O-vinyl peroxycarbonate looks like that of the decomposition of the O,O-t-butyl and O-isopropenyl homolog. In such solvents as triisopropylbenzene, di-n-butyl phthalate or diphenyl oxide oxo-2-ethyl [HC(O)CH₂] and acetyl [CH₃C(O)^·] radicals add themselves to the peroxycarbonate double bond. This addition gives rise to an induced decomposition which is associated with the “spontaneous” thermolysis. In octadecane, the induced process originates in the addition of radicals produced from the solvent. As for its intrinsic thermal behaviour, O,O-t-butyl and O-vinyl peroxycarbonate ranks, before O,O-t-butyl and O-isopropenyl peroxycarbonate or t-butyl peroxybenzoate, among the most stable peroxyesters.     

A method for the one-pot regioselective folding of the heat-stable bacterial enterotoxin ST peptide residues 5–18

A one-pot method for the regioselective formation of the three disulfide bridges of the heat-stable bacterial enterotoxin STa, residues 5–18, employing a temperature-controlled orthogonal protecting group scheme is reported. The protecting groups trityl, t-butyl and 4-methylbenzyl were chosen for the selective formation of the three cystine residues. Cleavage of the peptide from the solid support afforded a partially S-protected crude product with two free thiol groups. The first disulfide linking Cys 6 to Cys 14 was formed by oxidation of the dithiol peptide in a mixture of water/acetonitrile/DMSO. Following solvent removal, the second disulfide between Cys 5 and Cys 10 was formed directly by simultaneous cleavage and oxidation of the t-butyl groups in TFA/DMSO/anisole at room temperature. Subsequent heating of this solution initiated cleavage of the 4-methylbenzyl groups with concomitant oxidation of Cys 9 and Cys 17 yielding the desired three disulfide product.     

α-氨基酸及其酯化物侧链对其β-环糊精复合物稳定常数的影响

为了探索α-氨基酸及其酯化物的侧链R基团对其与环糊精非共价复合物结合强度的影响, 将一定摩尔比的β-环糊精(β-CD)分别与L型正缬氨酸(n-Val)、 亮氨酸(Leu)、 苯丙氨酸(Phe)、 天冬氨酸(Asp)、 天冬氨酸-4-苄酯(Asp-4-benzyl ester)和天冬氨酸-4-叔丁酯(Asp-4-t-butyl ester)在室温下混合, 反应平衡后采用电喷雾电离质谱进行竞争反应检测, 并以改进的质谱滴定结合曲线拟合法计算结合常数. 结果表明, 它们均可形成摩尔比为1∶1的非共价复合物. 在2组竞争反应中, 复合物的结合强度顺序分别为[β-CD∶Asp-4-benzyl ester+H]⁺>[β-CD∶Asp-4-t-butyl ester+H]⁺>[β-CD∶Asp+H]⁺以及[β-CD∶Phe+H]⁺>[β-CD∶Leu+H]⁺>[β-CD∶n-Val+H]⁺. 质谱滴定曲线拟合法测得[β-CD∶n-Val+H]⁺, [β-CD∶Asp+H]⁺, [β-CD∶Asp-4-t-butyl ester+H]⁺, [β-CD∶Asp-4-benzyl ester+H]⁺, [β-CD∶Leu+H]⁺和[β-CD∶Phe+H]⁺的稳定常数(lgK_st)分别为1.81, 2.54, 3.14, 3.26, 3.36和3.67, 结合强度依次增强. 竞争反应的定性分析结果与质谱滴定定量法测得结合强度结果的趋势一致. 由于所选用的α-氨基酸及其酯化物客体的羧基端(—COOH)和氨基端(—NH₂)均相同, 且都为亲水基团, 仅有侧链R基团不同, 因此在溶液中客体分子受疏水驱动与β-CD主体靠近并结合时, 侧链R基团的疏水力和极性2个因素起重要作用. 由于客体分子体积小, 其碳端的羧基还可与β-CD大口或小口边缘的羟基形成氢键, 使复合物更加稳定.     

E-(hydroxyimino)(hydroxymethoxyphosphinyl)acetic acid: Synthesis and pH-dependent fragmentation

In contrast to both its parent “troika” acid (E-1, a phosphorylating agent at pH 7 and 25 °C) and its C-methyl isomer (E-2, which is stable at both acidic and neutral pH), (E)-(hydroxyimino)(hydroxymethoxyphosphinyl)acetic acid E-3 was unreactive at pH 7 and 25 °C but at pH 1.5 fragmented to methyl phosphate 10 (15%) and methyl phosphorocyanidate 11 (85%). The minor product is consistent with solvent phosphorylalion, the reaction exclusively observed with E-1. The non-phosphorylating fragmentation pathway is proposed to involve a preliminary E→ Z isomerizalion of 3 prior to C-C_β cleavage. Dual fragmentation pathways were also detected (³¹P NMR) when the DCHA⁺ salt of E-3 (E-9) was heated in acetonitrile or EtOH; in addition to phosphorylation products (16–19%), 11 was formed (81–84%). Reaction of E-9 in refluxing EtOH:t-BuOH (1:1) showed low stereoselectivity in product formation (~3:1 ethyl methyl phosphate:t-butyl methyl phosphate), supporting a dissociative phosphorylation process.     

Electron spin polarization of free radicals induced by electron-nuclear cross relaxation

Transient radicals ^•C (CH₃)₂X are generated in solution by laser flash photolysis, and chemically induced electron polarization (CIDEP) is investigated by time-resolved ESR spectroscopy. The sign reversal of multiplet polarization at longer times, observed for t-butyl and 2-hydroxy-2-propyl radicals is studied as a function of temperature and solvent and is explainable by efficient electron-nuclear cross relaxation due to modulation of the nonplanarity at the radical centre.     

Microwave activation of catalytic transformation of t-butylphenols

Kinetics of catalytic transformation of 2- and 4-t-butylphenol (2TBP and 4TBP) in the liquid phase on a heterogeneous KSF catalyst has been studied in detail under conventional and microwave conditions. The process includes dealkylation, isomerization and transalkylation reactions. Its kinetics has been described using the method of initial reaction rates. It was found that microwaves affect both the reaction rate and the selectivity. The results were explained in terms of “microwave-induced polarisation” assuming an interaction of microwaves with a highly polarised reagent molecule in adsorbed state on the acidic active site. Temperature and solvent effects were also examined. The reaction mechanism of t-butylphenol transformation is discussed on the basis of electrophilic aromatic substitution via bimolecular reactions.     

New developments in the chemistry of organoaluminum and organogallium hydrides

A survey of our recent work on organoaluminum and organogallium hydrides is presented. Three types of ligand system have been employed for the stabilization of monomeric aluminum and gallium mono- and dihydrides. The “two-armed” 2,6-bis (dimethylaminomethyl) phenyl ligand is effective when intramolecular bis(base) stabilization is necessary; its use has permitted the isolation of the first examples of monomeric aluminum and gallium dihydrates. The use of the corresponding “one-armed” 2-(dimethylaminomethyl) phenyl ligand resulted in the formation of gallium mono- and dihydride monomers and an aluminum dihydride dimer. The first base-free aluminum and gallium monohydrides and gallium dihydride have been stabilized by employing the bulky 2, 4, 6, -tris(t-butyl) phenyl ligand.     

Cyclodimerization of the 1-methyl-1,3-diferrocenylallyl cation into 1,3,5-triferrocenyl-4-(1-ferrocenylethenyl)cyclohexene

An unusual cyclodimerization reaction of the 1-methyl-1,3-diferrocenylallyl cation into 1,3,5-triferrocenyl-4-(1-ferrocenylethenyl)cyclohexene has been studied. Two isomeric compounds were isolated and their structures were established as 1,r3,c5-triferrocenyl-4-t-(1-ferrocenylethenyl)cyclohexene and 1,r3, t5-triferrocenyl-4-c-(1-ferrocenylethenyl)cyclohexene on the basis of ¹H NMR and ¹³C NMR spectral data.     

Carbonylation of tert-butyl alcohol over H-zeolites

The Koch-type carbonylation of tert-butyl alcohol was studied over H-type zeolites. It was found that the catalytic carbonylation of a large amount of tert-butyl alcohol relative to the acidic sites of the H-zeolites in organic solvents requires an elevated temperature and CO pressure, although previous solid state NMR studies have revealed that the transformation of tert-butyl alcohol of an amount comparable to the acidic sites into 2,2-dimethylpropanoic acid proceeds just upon the CO co-adsorption in the H-zeolites at room temperature and atmospheric pressure. The catalytic performance of different H-zeolites and the influence of CO pressure, H₂O addition and solvent effects on the carbonylation of tert-butyl alcohol have been investigated. H-ZSM-5 gives the highest selectivity for 2,2-dimethylpropanoic acid due to its adequate pore dimensions. The present work indicates the possible industrial application of solid acids as carbonylation catalysts instead of liquid acids for the Koch reaction to produce tert-carboxylic acids.     

苯甲酰咪唑过氧化反应

以苯甲酰咪唑和叔丁基过氧化氢为反应物,1,8-二氮杂二环十一碳-7-烯(DBU)为催化剂,乙睛(CH₃CN)作为反应溶剂,合成了过氧化叔丁酯类化合物。 分别考察了不同催化剂及其用量、原料配比、溶剂、温度和时间对反应的影响,在最优条件下,共得到25个目标产物,收率范围为45%~99%。 目标化合物经核磁共振氢谱、核磁共振碳谱以及质谱进行了结构表征。 该方法具有操作简单、反应条件简单温和、产物收率高及底物适用性好等优点,为过氧化叔丁酯类化合物的合成提供了新的思路。     

Synthesis and solid state structure of fluorous probe molecules for fluorous separation applications

A series of colored hydrocarbon and fluorocarbon tagged 1-fluoro-4-alkylamino-anthraquinones and 1,4-bis-alkylamino-anthraquinone probe molecules were synthesized from a (fluorinated) alkyl amine and 1,4-difluoroanthraquinone to aid in the development of fluorous separation applications. The anthraquinones displayed stacking of the anthraquinone tricycle and interdigitation of the (fluorinated) alkyl chains in the solid state. Furthermore, intramolecular N-H?O hydrogen bonds forced the hydrocarbon and fluorocarbon tags into a conformation pointing away from the anthraquinone tricycle, with the angle of the tricycle plane normal and the main (fluorinated) alkyl vector ranging from 1° to 39°. Separation of the probe molecules on fluorous silica gel showed that the degree of fluorination of the probe molecules plays only a minor role with most eluents (e.g., hexane/ethyl acetate and methyl nonafluorobutyl ethers/ethyl acetate). However, toluene as eluent caused a pronounced separation by degree of fluorination for fluorocarbon, but not hydrocarbon tagged probe molecules on both silica gel and fluorous silica gel. These studies suggest that hydrocarbon and fluorocarbon tagged anthraquinones are useful probe molecules for the development of laboratory scale fluorous separation applications.     

Synthesis and Supramolecular Assemblies of Tripodal 1,3,5‐Tris(phenoxymethyl)‐2,4,6‐triethylbenzene Analogues

Tripodal 1,3,5‐tris(phenoxymethyl)‐2,4,6‐triethylbenzene analogues have been synthesized and structurally characterized by IR, ¹H NMR and ¹³C NMR spectroscopy and HRMS, and additionally, the single crystal structures of compounds bearing ortho‐ ( 7 ), meta‐ ( 9 ) and para‐hydroxymethyl ( 11 ) functions have been determined by X‐ray diffraction analysis. The structural study revealed that compounds 7 , 9 , and 11 do not adopt the expected 1,3,5‐alternate conformation in the solid state. The packing diagrams of compounds 7 , 9 , and 11 revealed that six hydrophilic hydroxymethyl groups from six individual molecules ( 7 , 9 and 11 ) were arranged in close contact via intermolecular hydrogen‐bond interactions. For compounds 7 and 9 , the six hydroxyl groups formed a distorted hexagonal ring; however, formation of such a hexagonal ring was not clear in the case of compound 11 . Compounds 9 and 11 were found to form hydrophobic cavities via intermolecular hydrogen‐bond interactions in the solid state, and the cavities were occupied by two ethyl groups from the two cavity‐forming molecules.     

外围取代基团对卟啉铂(Ⅱ)配合物发光性能的影响

设计合成了具有不同外围取代基的卟啉铂(Ⅱ)配合物PtTEMP, PtTBMP, PtOMPP和PtDMPP, 并对其结构和光电性能进行了表征. 晶体结构分析结果表明, 这些卟啉铂(Ⅱ)配合物具有较理想的平面配位构型, β-位叔丁基的引入有效抑制了分子间的π-π相互作用. 外围取代基几乎不影响配合物的吸收和发光性质, 最大发射峰位于646~656 nm之间, 为配体中心的 ³π^*-π磷光发射. 空间位阻效应更强的叔丁基取代配合物(PtTBMP)的溶液态荧光量子效率和外量子效率最高, 分别为0.58和6.3%. 3个甲氧基取代的PtDMPP的发光效率优于2个甲氧基取代的PtOMPP, 二者的溶液态荧光量子效率分别为0.36和0.29, 外量子效率分别为2.4%和1.7%.     

Differences in Dynamics between Crosslinked and Non‐Crosslinked Hyaluronates Measured by using Fast Field‐Cycling Relaxometry

The dynamic properties of water molecules in gels containing linear and crosslinked hyaluronic acid polymers are investigated by using an integrated approach that includes relaxometry, solid‐state NMR spectroscopy, and scanning electron microscopy. A model‐free analysis of field‐dependent nuclear relaxation is applied to obtain information on mobility and the population of different pools of water molecules in the gels. Differences between linear and crosslinked hyaluronic acid polymers are observed, indicating that crosslinking increases both the fraction and the correlation time of water molecules with slow dynamics.     

Crystal structure of inclusion compounds: The complexes of thecis-syn-cis and thecis-anti-cis isomers of dicyclohexano-18-crown-6 with 4-methylbenzenesulfamide

The crystal structure of two complexes of the isomerscis-syn-cis (isomer A) andcis-anti-cis (isomer B) of dicyclohexano-18-crown-6 with 4-methylbenzenesulfamide have been determined by X-ray single crystal diffraction methods. The two structures have been solved by direct methods and refined to agreement values of 0.067 and 0.038 for isomers A and B respectively. The first isomer forms an inclusion compound with a host/guest ratio of 1 : I; the second one of I:2. The amino groups of the guest molecules are connected by N-H...O hydrogen bonds with oxygen atoms of the polyether molecules. The methyl groups of 4-methylbenzenesulfamide do not form hydrogen bonds.[/p]The host-guest interactions in the molecular complexes, the reciprocal influence of the two molecules on their conformation and the intermolecular contacts between the molecules in the crystal are discussed.     

Palladium-catalyzed asymmetric alkylation of 2-azaallyl acetates for the synthesis of β-carboxyaspartic acid derivatives

Palladium-catalyzed asymmetric alkylation of 2-azaallyl acetate, N-(diphenylmethylene)acetoxyglycine ester with a sodium salt of malonate ester was successfully carried out. High enantioselectivities were achieved using sodium dimethyl methylmalonate (98%ee) or sodium dimethyl malonate (93%ee) as a nucleophile with t-butyl ester of 2-azaallyl acetate in the presence of (S)-BINAP in acetonitrile.     

Stereoselective synthesis and optical resolution of chiral calix[4]arenes with mixed ligating functionalities

Exhaustive alkylation of syn-proximal bis[(2-pyridylmethyl)oxy]calix[4]arene 1 with t-butyl bromoacetate or 2-(chloromethyl)quinoline hydrochloride in the presence of Cs₂CO₃ affords selectively a new type of inherently chiral calix[4]arenes 2a,b in the partial cone conformation. (±) 2b has been resolved into its optical antipodes by an HPLC method.     

The first application of titanocenes in the asymmetric oxidation of sulfides

Investigations were carried out on the oxidation of sulfides to sulfoxides catalyzed by the commercial Cp₂Ti(X)₂ (X=Cl, OTf)/t-butyl hydroperoxide (TBHP). The asymmetric version of prochiral sulfides oxidation was achieved using Cp₂TiCl₂, as transition metal catalyst, in the presence of (+)-(R)-BINOL, as chiral ligand and activated 4 Å molecular sieves (m.s.).     

Synthesis, characterisation and liquid crystal properties of 2,5-bis[5-alkyl(alkoxy)phenyl-1,3,4-oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis-1,3,4-oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT-IR and ¹H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature-dependent wide-angle X-ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert-butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally-induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.