Self-consistent acidity scales of neutral and cationic brønsted acids in acetonitrile and tetrahydrofuran

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Relative stability and proton transfer reactions of unsaturated isocyanides and cyanides: Relative Stability and Proton Transfer Reactions of Unsaturated Isocyanides and Cyanides

International audienceThe typical Gibbs free energy difference between hydrocarbon substituted isocyanides and the corresponding cyanides is 25 to 28 kcal/mol in favor of the cyanides and is mostly independent of the substituent. Triple bonded species with a ―C ≡ C―RN,C (RN,C = CN, NC) structure can be considered as exceptions. Because isocyanide and cyanide species have very similar structures, the relative energy is independent of the pressure and temperature conditions. Theoretical and experimental gas-phase investigations show that basicity of isocyanides ranges from 182.1 to 198.2 kcal/mol which is 14.0 to 19.7 kcal/mol higher than the basicity of respective cyanides. The most favored protonation centers are located on isocyanide or cyanide group depending on the species. The biggest increase of basicity was caused by bulkier substituents. The substitutions have greater influence on the basicity of cyanides than on the basicity of isocyanides. In regard to deprotonation, the cyanides are more acidic than the corresponding isocyanides. For most of the unsaturated cyanide and isocyanide species the (N,C)-CHR′ hydrogen (the one connected to the carbon next to cyanide/isocyanide group) is the most acidic. Our work suggests that for derivatives bearing unsaturated substituent the favored deprotonation center may be different and some cyanides and isocyanides are unstable towards gas-phase deprotonation equilibrium as the formed anion tends to isomerize

pKaH values and θH angles of phosphanes to predict their electronic and steric parameters

Phosphanes have numerous important uses and at the same time are an important class of organic bases with basicities spanning more than 30 orders of magnitude. In many cases, their behaviour in specific applications depends strongly on their basicity. Basicities (pKaH values) of many phosphanes have been published but are scattered across different reports and there are prominent gaps in the availability of data. In this report, we present an extensive set of pKaH data of a most diverse set of phosphanes, both newly measured/calculated and collected from the literature. We demonstrate that pKaH values can serve as an alternative to Tolman electronic parameters (TEP values) in evaluating the electronic properties of phosphanes. Additionally, we suggest two easily obtainable parameters for assessing the steric properties of phosphaneswithout need for sophisticated calculations or preparation of metal-ligand complexes

A comprehensive self-consistent spectrophotometric acidity scale of neutral Brønsted acids in acetonitrile. J Org Chem. 2006; 71:2829–2838. [PubMed: 16555839

Abstract For the first time the self-consistent spectrophotometric acidity scale of neutral Brønsted acids in acetonitrile (AN) spanning 24 orders of magnitude of acidities is reported. The scale ranges from pK a value 3.7 to 28.1 in AN. The scale includes altogether 93 acids that are interconnected by 203 relative acidity measurements (∆pK a measurements). The scale contains compounds with gradually changing acidities, including representatives from all the conventional families of OH (alcohols, phenols, carboxylic acids, sulfonic acids), NH (anilines, diphenylamines, disulfonimides) and CH acids (fluorenes, diphenylacetonitriles, phenylmalononitriles). The CH acids were particularly useful in constructing the scale because they do not undergo homo-or heteroconjugation processes and their acidities are rather insensitive to traces of water in the medium. The scale has been fully cross-validated: the relative acidity of any two acids on the scale can be found combining at least two independent sets of ∆pK a measurements. The consistency standard deviation of the scale is 0.03 pK a units. Comparison of acidities in many different media 2 has been carried out and the structure-acidity relations are discussed. The large variety of the acids on the scale, its wide span and the quality of the data make the scale a useful tool for further acidity studies in acetonitrile.

Acidity of Strong Acids in Water and Dimethyl Sulfoxide

Careful analysis and comparison of the available acidity data of HCl, HBr, HI, HClO₄, and CF₃SO₃H in water, dimethyl sulfoxide (DMSO), and gas-phase has been carried out. The data include experimental and computational p<i>K</i>_a and gas-phase acidity data from the literature, as well as high-level computations using different approaches (including the W1 theory) carried out in this work. As a result of the analysis, for every acid in every medium, a recommended acidity value is presented. In some cases, the currently accepted p<i>K</i>_a values were revised by more than 10 orders of magnitude

Experimental Basicities of Superbasic Phosphonium Ylides and Phosphazenes

Experimental basicities of some of the strongest superbases ever measured (phosphonium ylides) are reported, and by employing these compounds, the experimental self-consistent basicity scale of superbases in THF, reaching a p<i>K</i>_α (estimate of p<i>K</i>_a) of 35 and spanning more than 30 p<i>K</i>_a units, has been compiled. Basicities of 47 compounds (around half of which are newly synthesized) are included. The solution basicity of the well-known <i>t</i>-Bu-NP₄(dma)₉ phosphazene superbase is now rigorously linked to the scale. The compiled scale is a useful tool for further basicity studies in THF as well as in other solvents, in particular, in acetonitrile. A good correlation between basicities in THF and acetonitrile spanning 25 orders of magnitude gives access to experimentally supported very high (p<i>K</i>_a > 40) basicities in acetonitrile, which cannot be directly measured. Analysis of structure–basicity trends is presented

Superbasicity of a Bis-guanidino Compound with a Flexible Linker: A Theoretical and Experimental Study

The bis-guanidino compound H2C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H center dot center dot center dot N=" hydrogen bonding to form an eight-membered C3N4H heterocycle. Solid-state CPMAS N-15 NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50-273 K. Examination of the carbon-nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H](+) indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese-Martin for kinetics method was 2.8 kcal mol(-1), with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pK(a) in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pK(a) of 28.98 +/- 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy