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The tetramer of bis(4‐di‐n‐butylaminophenyl)(pyridin‐3‐yl)borane [systematic name: 2λ4,4λ4,6λ4,8λ4‐tetrabora‐1,3,5,7(1,3)‐tetrapyridinacyclooctaphane‐11,31,51,71‐tetrakis(ylium)], C132H192B4N12, was synthesized unexpectedly and crystallized. Its structure contains an unusual 16‐membered ring core made up of four (pyridin‐3‐yl)borane groups. The ring adopts a conformation with pseudo‐S4 symmetry that is very different from the two other reported examples of this ring system. Density functional theory (DFT) computations indicate that the stability of the three reported ring conformations is dependent on the substituents on the B atoms, and that the pseudo‐S4 geometry observed in the bis(4‐dibutylaminophenyl)(pyridin‐3‐yl)borane tetramer becomes significantly more stable when phenyl or 2,6‐dimethylphenyl groups are attached to the boron centers. [ABSTRACT FROM AUTHOR]

Charge‐enhanced chiral thioureas were used in the organocatalysis of the Friedel–Crafts alkylation of indole with trans‐β‐nitrostyrene and the oxa‐Pictet–Spengler reaction of tryptophol and benzaldehyde. The effects of substoichiometric Brønsted acidic additives on the reaction conversions and enantiomeric ratios of these transformations were examined, and the role of these species in the mechanism of the latter reaction was explored using variable time normalization kinetics to elucidate the reaction order of the catalyst, cocatalyst, and reagents. A proposed pathway for the oxa‐Pictet–Spengler cyclization that involves matched and mismatched catalyst and cocatalyst pairs and an off‐cycle racemization of the latter species is provided. [ABSTRACT FROM AUTHOR]

Several charge‐containing TADDOL salts were synthesized and used as organocatalysts in asymmetric Diels–Alder and hetero‐Diels–Alder reactions. Their catalytic activity was found to exceed that of a noncharged analog while maintaining or improving upon the enantioselectivity. The enhanced activities of the TADDOL salts enabled them to act as presumed hydrogen bond donor catalysts in the Diels–Alder and hetero‐Diels–Alder reactions of 1,3‐cyclohexadiene with methyl vinyl ketone at 40°C and 2‐phenoxy‐1,3‐butadiene with ethyl glyoxylate at room temperature, respectively. Given the ionic nature of these charge‐activated catalysts, it also proved possible to recycle and reuse the TADDOL without chromatography or the need for a recrystallization. [ABSTRACT FROM AUTHOR]

All three N‐methylated and N‐protonated hydroxypyridinium BArF4– salt isomers were synthesized and their hydrogen bond donating abilities were investigated. DFT and G4 theory computations along with IR spectroscopic measurements were found to be effective methods for predicting the catalytic activities of these O–H and N–H Brønsted acids. A UV‐vis titration approach for rapidly quantifying hydrogen bond donating ability revealed that carbon‐hydrogen bonds also can participate in electrostatic interactions, but the presence of multiple equilibrium complexes results in a limitation of this method. In the methylated series of hydroxypyridines, the ortho and para isomers displayed modest rate enhancements relative to the meta derivative. Protonation introduces a new acidic site and the ortho hydroxypyridinium ion salt is a significantly more active catalyst than all of the other species examined. This is indicative of bidentate activation by the N–H and O–H acidic sites, and suggests a new design strategy for improving charge‐enhanced catalysts. [ABSTRACT FROM AUTHOR]

Uncertainties and errors in enantiomeric excess determinations obtained by high performance liquid chromatography (HPLC) with a chiral column are evaluated with different peak resolutions and elution orders using (R)‐ and (S)‐ibuprofen standards. Five injections of each sample were made to reduce the variability in the data and each chromatogram was monitored at multiple wavelengths to facilitate the detection of chemical impurities. Using appropriate sample concentrations, the most favorable detection wavelengths, and default instrument parameters led to large errors (>10 %) whereas properly set integration values gave impressive reproducibilities (±0.5 % or better) and accuracies (<1 %) in the most favorable situations. Uncertainties and errors of up to±2.0 % and 5.2 % ee, respectively were also observed under separation conditions routinely reported in chemistry journals. This study aims to provide a practical guide to the accuracy and precision that can be achieved with HPLC, and suggestions for obtaining reliable results. [ABSTRACT FROM AUTHOR]

Review: 193 refs. [ABSTRACT FROM AUTHOR]