Neighboring Effect in Fragmentation Pathways of Cage Guanylhydrazones in the Gas Phase

ESI–MS/MS investigation of the mono- and bis­(guanylhydrazone) derivatives <b>1</b>–<b>5</b> based on adamantane and pentacycloundecane (PCU) skeleton was described. Elimination of neutral guanidine is the most abundant reaction channel in the case of 2,4-adamantyl and PCU derivatives <b>4</b> and <b>5</b>, while the elimination of CH₂N₂ fragment is preferred for other compounds. This was attributed to the cage opening of adamantane or PCU skeletons in the former case leading to the formation of the cyclohexyl- or cyclopropylcarbinyl carbocation stabilized by the conjugation with the guanylhydrazone subunit. The main fragmentation pathways observed experimentally were analyzed by using DFT calculations. All investigated bis­(guanylhydrazone)­s formed dications and their abundances were found to be proportional to the interguanidine distance in the considered ions. Calculation of the first and the second proton affinities supported qualitative interpretation of the dication abundance. Close contact of two guanidine subunits is thus confirmed to be crucial in determining preferential fragmentation pathway and to suppress formation of the dication

Correlation Method for Conversion Determination of Biodiesel Obtained from Different Alcohols by ¹H NMR Spectroscopy

This work presents a correlation method that uses ¹H NMR spectra for determining the conversion during transesterification of vegetable oil with 10 alcohols: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, isopentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 1-dodecanol, and benzyl alcohol. Conversions ranging from 70 to 99%, depending on primary alcohol used, are obtained using organic catalyst <i>N</i>,<i>N</i>′,<i>N</i>″-tris­(3-(dimethylamino)­propyl)­guanidine. Catalysts, alcohols, and intermediate products can cause signal overlapping, debilitating the use of the conventional ¹H NMR method. Thus, our method uses the correlation between the triplet signal of α-carbonylmethylene and two signals of unsaturated parts of the fatty acid chain, where signal overlapping does not occur, hence, adding robustness and flexibility to the method. The method is applicable for unsaturated oils and is not affected by the amounts of residual alcohol, catalyst, or intermediate products, making it ideal for conversion determination and <i>in situ</i> monitoring of complex mixtures. In addition, we present a systematic analysis of the ¹H NMR spectra of biodiesels produced with the mentioned alcohols

“Backdoor Induction” of Chirality: Asymmetric Hydrogenation with Rhodium(I) Complexes of Triphenylphosphane-Substituted β‑Turn Mimetics

Bioconjugate bidentate ligands <b>2</b>–<b>10</b> were obtained by tethering triphenylphosphanecarboxylic acid to amino acid substituted spacers with different flexibility, ranging from a rigid enediyne-based β-turn inducer to flexible linear aliphatic chains with up to eight carbon atoms. The 21 synthesized ligands revealed up to 81% ee selectivity in rhodium-catalyzed asymmetric hydrogenation of α,β-unsaturated amino acids. The key feature of the catalysts is the prochiral coordination sphere of the catalytic metal while the chirality is transmitted by “backdoor induction” from distant hydrogen-bonded amino acids. DFT calculations were applied to study the structure and relative stability of the precatalytic organometallic Rh­(I) complexes, with particular emphasis on hydrogen-bonded secondary structures