Gas Phase Acidity Measurement of Local Acidic Groups in Multifunctional Species: Controlling the Binding Sites in Hydroxycinnamic Acids

The applicability of the extended kinetic method (EKM) to determine the gas phase acidities (GA) of different deprotonable groups within the same molecule was tested by measuring the acidities of cinnamic, coumaric, and caffeic acids. These molecules differ not only in the number of acidic groups but in their nature, intramolecular distances, and calculated GAs. In order to determine independently the GA of groups within the same molecule using the EKM, it is necessary to selectively prepare pure forms of the hydrogen-bound heterodimer. In this work, the selectivity was achieved by the use of solvents of different vapor pressure (water and acetonitrile), as well as by variation of the drying temperature in the ESI source, which affected the production of heterodimers with different solvation energies and gas-phase dissociation energies. A particularly surprising finding is that the calculated solvation enthalpies of water and the aprotic acetonitrile are essentially identical, and that the different gas-phase products generated are apparently the result of their different vapor pressures, which affects the drying mechanism. This approach for the selective preparation of heterodimers, which is based on the energetics, appears to be quite general and should prove useful for other studies that require the selective production of heterodimers in ESI sources. The experimental results were supported by density functional theory (DFT) calculations of both gas-phase and solvated species. The experimental thermochemical parameters (deprotonation Δ<i>G</i>, Δ<i>H</i>, and Δ<i>S</i>) are in good agreement with the calculated values for the monofunctional cinnamic acid, as well as the multifunctional coumaric and caffeic acids. The measured GA for cinnamic acid is 334.5 ± 2.0 kcal/mol. The measured acidities for the COOH and OH groups of coumaric and caffeic acids are 332.7 ± 2.0, 318.7 ± 2.1, 332.2 ± 2.0, and 317.3 ± 2.2 kcal/mol, respectively

Intramolecular C–C Coupling Reactions of Alkynyl, Vinylidene, and Alkenylphosphane Ligands in Rhodium(III) Complexes

Electrophilic attack with methyl trifluoro­methane­sulfonate or tetrafluoro­boric acid, to new alkynyl rhodium complexes containing alkenylphosphanes, leads to butenynyl coupling products or to the unprecedented rhodaphosphacycle complexes [Rh­(η⁵-C₅Me₅)­{κ⁴-(<i>P</i>,<i>C</i>,<i>C</i>,<i>C</i>)-<i>ⁱ</i>Pr₂PCH₂C­(CH₂)­C­(CH₂R)­CC­(R)}]­[BF₄] (R = Ph (<b>11a</b>), <i>p</i>-tol (<b>11b</b>)). These complexes <b>11a</b>,<b>b</b> can be explained as a result of the coupling of three organic fragments in the molecule, the alkynyl, the vinylidene, generated <i>in situ</i> by reaction with HBF₄ (<b>A</b>), and the C–C double bond from the alkenylphosphane. DFT computational studies on the formation of complex <b>11a</b> suggest the [2 + 2] intramolecular cycloaddition between the double bond of the allylphosphane and the Cα–Cβ of the vinylidene <b>A</b> as the most plausible pathway for this reaction

Acidities of <i>closo</i>-1-COOH-1,7‑C₂B₁₀H₁₁ and Amino Acids Based on Icosahedral Carbaboranes

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol^–1, computed GA = 1321 kJ·mol^–1) and liquid-phase acidity (measured p<i>K</i>_a = 2.00, computed p<i>K</i>_a = 1.88) of the carborane acid <i>closo</i>-1-COOH-1,7-C₂B₁₀H₁₁. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid–base context, to corresponding <i>ortho</i>, <i>meta</i>, and <i>para</i> amino acids derived from icosahedral carborane cages, 1-COOH-<i>n</i>-NH₂-1, <i>n</i>-R with {R = C₂B₁₀H₁₀, <i>n</i> = 2, 7, 12}, and from benzene {R = C₆H₄, <i>n</i> = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids

Phyllanthus emblica

The experimentally observed stereomutation of spiro-1,2-oxaphosphetanes is shown by DFT calculations to proceed through successive M_B2 or M_B4 and M_B3 mechanisms involving two, four, and three Berry pseudorotations at phosphorus, respectively. Oxaphosphetane decomposition takes place in a single step via a polar transition state. The calculated activation parameters for this reaction are in good agreement with those determined experimentally