Light-induced cis/trans isomerization of cis-[Pd(L-S,O)2] and cis-[Pt(L-S,O)2] complexes of chelating N,N-dialkyl-N′-acylthioureas: key to the formation and isolation of trans isomers

Irradiation cis-[M(Lⁿ-S,O)₂] complexes (M = Pt^II, Pd^II) derived from N,N-dialkyl-N′-benzoylthioureas (HLⁿ) with various sources of intense visible polychromatic or monochromatic light with λ < 500 nm leads to light-induced cis?→?trans isomerization in organic solvents. In all cases, white light derived from several sources or monochromatic blue-violet laser 405 nm light, efficiently results in substantial amounts of the trans isomer appearing in solution, as shown by ¹H NMR and/or reversed-phase HPLC separation in dilute solutions at room temperature. The extent and relative rates of cis/trans isomerization induced by in situ laser light (λ = 405 nm) of cis-[Pd(L²-S,O)₂] was directly monitored by ¹H NMR and ¹⁹⁵Pt NMR spectroscopy of selected cis-[Pt(L-S,O)₂] compounds in chloroform-d; both with and without light irradiation allows the δ(¹⁹⁵Pt) chemical shifts cis/trans isomer pairs to be recorded. The cis/trans isomers appear to be in a photo-thermal equilibrium between the thermodynamically favored cis isomer and its trans counterpart. In the dark, the trans isomer reverts back to the cis complex in what is probably a thermal process. The light-induced cis/trans process is the key to preparing and isolating the rare trans complexes which cannot be prepared by conventional synthesis as confirmed by the first example of trans-[Pd(L-S,O)₂] characterized by single-crystal X-ray diffraction, deliberately prepared after photo-induced isomerization in acetonitrile solution.     

Recognition of chiral anions using calix[4]arene-based ureido receptor in the 1,3-alternate conformation

A new type of calixarene-based receptor designed for the recognition of chiral anions was prepared by the introduction of (S)-2-methylbutan-1-ol moieties into the lower rim of calixarene. The immobilization of calixarene skeleton in the 1,3-alternate conformation enabled the construction of a cavity consisting of preorganised ureido functions and chiral substituents in close proximity. This cavity is capable of chiral discrimination of selected anions as demonstrated on d- and l-phenylalaninates.     

Further revisions on the diterpenoid alkaloids reported in a JNP paper (2012, 75, 1145–1159)

We have corrected (Tetrahedron2013, 69, 5859–5866) the structures of diterpenoid alkaloids reported in the Journal of Natural Products2012, 75, 1145–1159. Our follow-up experiments compel us to present further revisions and clarifications on the diterpenoid alkaloids:     

Experimental and computational studies of molecules with close,non-bonded hydrogen–hydrogen contacts: common computational methods grossly overestimate some ‘through-space’ NMR scalar coupling constants

The NMR spin–spin scalar coupling constants (J_HH's) of closely contacting, but non-bonded hydrogen atoms in a series of highly strained molecules (including a new in,in-cyclophane made specifically for this study) have been examined both experimentally and computationally. The experimental J_HH's are invariably quite small (0.1–0.6 Hz), but common DFT methods with modest basis sets nearly always overestimate these values, by factors of 10–30, and even with quite large basis sets (up to cc-pVQZ) the J_HH's of two of the molecules are overestimated by a factor of 10 or more. Possible reasons for these discrepancies are discussed.     

Naryl-substituted anthranilamides with intramolecular hydrogen bonds

Hydrogen bonding interaction as one type of non-covalent force has proven itself to be highly efficient for constructing structurally unique artificial secondary structures. Here, the structure of N_aryl-substituted anthranilamide in solution is demonstrated by various NMR technique, the intramolecular hydrogen bonds between amide attached to arylamine of the same ring is proposed, which is supported by its crystal structure in the solid phase. The substituent on the nitrogen atom of arylamine plays an important role in forming the presence of intramolecular hydrogen bonds. The chemical shift of the N_aryl-H downfield changes obviously, due to the formation of intramolecular hydrogen bonds and the deshielding effect of oxygen, and the neighboring C–H is activated and shows downfield protonic signal too. The presence of intramolecular hydrogen bonds probably provides the explanation for the transformation from N_aryl-substituted anthranilamide to imine, which could be converted into 2-aryl quinazolinone finally.     

Revisit of a series of ICT fluorophores: skeletal characterization,structural modification,and spectroscopic behavior

We have revisited the synthesis of a series of ICT fluorophores, which were reported to have a core structure of 8-oxo-8H-acenaphtho[1,2-b]pyrrol-9-carbonitrile. However, based on the 2D NMR and X-ray diffraction analysis, their core structure was corrected as 1-oxo-1H-phenalene-2,3-dicarbonitrile (1). Compound 1 shows a highly electron-deficient nature and can easily undergo oxidative S_NAr^H reaction on the naphthyl ring to produce a series of novel ICT fluorophores. The regioselectivity of this substitution reaction was studied by introduction of representative nucleophiles. Moreover, due to the strong rigidity and efficient ICT nature, the obtained fluorescent dyes display very good spectroscopic properties even in an aqueous environment.     

Head group specificity of novel functionalized surfactants: synthesis,self-assembly and calcium tolerance

The present work describes the synthesis, characterization and application of functionalized surfactants derived through simple organic reaction steps. These surfactants have been particularly tailor made to resist hardness due to calcium ions in water. It is unique of its kind because here the surfactants have an analogous hydrophobic chain but differ structurally in the composition of the head groups in terms of the position of attachment of the chain. The effect of this small variability in the head group on the surfactant property, adsorption, self assembly and calcium tolerance behaviour has been studied in detail. This kind of phenol–keto surfactants has not been reported before. It was also found that one of the surfactants was more tolerant towards Ca²⁺ ion than the other. The individual packing behaviour of the surfactants at the air–water interface has been projected to cause this difference which is very interesting.     

Formation of Periodic γ‐Turns in α/β‐Hybrid Peptides: DFT and NMR Experimental Evidence

Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains.     

Mixed Aggregates of 1-Methoxyallenyllithium with Lithium Chloride

A combined computational and ¹³C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method.     

X‐Band DNP Hyperpolarization of Viscous Liquids and Polymer Melts

NMR studies of synthetic polymers and biomacromolecules, which provide insight into the conformation and dynamics of these materials, can benefit strongly from the increased sensitivity offered by dynamic nuclear polarization (DNP) and other hyperpolarizing methods. In this study ¹H DNP nuclear spin hyperpolarization of two polybutadiene samples, representing a supercooled liquid and an entangled polymer melt, is demonstrated at 0.35 T magnetic field strength and at temperatures between −80 and +50 °C. Electron spin polarization transfer from the α,γ‐bisdiphenylene‐β‐phenylallyl radical to the sample nuclei is achieved by the Overhauser and solid effect. DNP signal enhancements are studied, varying the electron spin resonance offset, microwave power, and sample temperature. The influence of spin relaxation times, line widths, and molecular dynamics are discussed. The results show promising, up to 15‐fold NMR signal enhancements using noncryogenic temperatures and an inexpensive setup that is less technically demanding than current high‐field DNP setups.