Hammett correlation in competition experiments in dissociation of ionised substituted benzophenones and dibenzylideneacetones

A convenient method of applying competition experiments to devise a Hammett correlation in the dissociation by α-cleavage of 17 ionised 3- and 4-substituted benzophenones, YC6H4COC6H5 [Y=F, Cl, Br, CH3, CH3O, NH2, CF3, OH, NO2, CN and N(CH3)2] is reported and discussed. The results given by this approach, which rely on the relative abundance of [M-C6H5]+ and [M-C6H4Y]+ ions in the electron ionisation spectra of the substituted benzophenones, are compared with those obtained by previous methods. Various refinements of the method are considered, including reducing the ionising electron energy, making allowance for the relative abundance of ions such as C6H5+ and C6H4Y+, which may be formed to some extent by secondary fragmentation, and using substituent constants other than the standard σ constants. The reaction constant, ρ, of 1.08, which is in good agreement with that deduced previously, is consistent with a considerable reduction in electron density (corresponding to an increase in positive charge) at the carbon of the carbonyl group during fragmentation. This method has been successfully extended to the corresponding cleavage of 12 ionised substituted dibenzylideneacetones, YC6H4CH=CHCOCH=CHC6H5 (Y=F, Cl, CH3, OCH3, CF3, and NO2), which may fragment to form either a substituted cinnamoyl cation, [YC6H4CH=CHCO]+, or the cinnamoyl cation, [C6H5CH=CHCO]+. The derived ρ value of 0.76 indicates that the substituent, Y, influences the stability of the cinnamoyl cation somewhat less strongly than it does the analogous benzoyl cation.

Hammett Correlation in the Accelerated Formation of 2,3-Diphenylquinoxalines in Nebulizer Microdroplets

The accelerated formation of 2,3-diphenylquinoxalines in microdroplets generated in a nebulizer has been investigated by competition experiments in which equimolar quantities of 1,2-phenylenediamine, C₆H₄(NH₂)₂, and a 4-substituted homologue, XC₆H₃(NH₂)₂ [X = F, Cl, Br, CH₃, CH₃O, CO₂CH₃, CF₃, CN or NO₂], or a 4,5-disubstituted homologue, X₂C₆H₂(NH₂)₂ [X = F, Cl, Br, or CH₃], compete to condense with benzil, (C₆H₅CO)₂. Electron-donating substituents (X = CH₃ and CH₃O) accelerate the reaction; in contrast, electron-attracting substituents (X = F, Cl, Br and particularly CO₂CH₃, CN, CF₃ and NO₂) retard it. A structure-reactivity relationship in the form of a Hammett correlation has been found by analyzing the ratio of 2,3-diphenylquinoxaline and the corresponding substituted-2,3-diphenylquinoxaline, giving a ρ value of -0.96, thus confirming that the electron density in the aromatic ring of the phenylenediamine component is reduced in the rate-limiting step in this accelerated condensation. This correlation shows that the phenylenediamine acts as a nucleophile in the reaction.

Towards a Stronger Halogen Bond Involving Astatine: Unexpected Adduct with Bu3 PO Stabilized by Hydrogen Bonding

The halogen bond is a powerful tool for the molecular design and pushing the limits of its strength is of major interest. Bearing the most potent halogen-bond donor atom, astatine monoiodide (AtI) was recently successfully probed [Nat. Chem. 2018, 10, 428-434]. In this work, we continue the exploration of adducts between AtI and Lewis bases with the tributylphosphine oxide (Bu₃ PO) ligand, revealing the unexpected experimental occurrence of two distinct chemical species with 1:1 and 2:1 stoichiometries. The 1:1 Bu₃ PO⋅⋅⋅AtI complex is found to exhibit the strongest astatine-mediated halogen bond so far (with a formation constant of 10^(4.24±0.35) ). Quantum chemical calculations unveil the intriguing nature of the 2:1 2Bu₃ PO⋅⋅⋅AtI adduct, involving a halogen bond between AtI and one Bu₃ PO molecular unit plus CH⋅⋅⋅O hydrogen bonds chelating the second Bu₃ PO unit.

The Codon Usage of Lowly Expressed Genes Is Subject to Natural Selection

Codon usage bias affects the genomes of organisms from all kingdoms of life and results from both background substitution biases and natural selection. Natural selection on codon usage to increase translation accuracy and efficiency has long been known to affect gene sequences. Such selection is stronger on highly, compared with lowly expressed genes, resulting in higher levels of codon bias within genes with higher expression levels. Additionally, selection on translation accuracy affects more strongly codons encoding conserved amino acids, since these will more often affect protein folding and/or function. By applying tests of selection on the gene sequences of the bacterium Escherichia coli, we demonstrate that both highly and lowly expressed genes display signals of selection on codon usage. Such signals are found for both conserved and less conserved amino acid positions, even within the 10% of E. coli genes expressed at the lowest levels. We further demonstrate experimentally that single synonymous codon replacements within a lowly expressed, essential gene can carry substantial effects on bacterial fitness. Combined, our results demonstrate that even within genes expressed at relatively low levels there is substantial selection on codon usage and that single synonymous codon replacements within such genes can have a marked effect on bacterial fitness.

Relative reactivity of alkenyl alcohols in the palladium-catalyzed redox-relay Heck reaction

The relative rates of alkenyl alcohols in the Pd-catalyzed redox-relay Heck reaction were measured in order to examine the effect of their steric and electronic properties on the rate-determining step. Competition experiments between an allylic alkenyl alcohol and two substrates with differing chain lengths revealed that the allylic alcohol reacts 3-4 times faster in either case. Competition between di- and trisubstituted alkenyl alcohols provided an interesting scenario, in which the disubstituted alkene was consumed first followed by reaction of the trisubstituted alkene. Consistent with this observation, the transition structures for the migratory insertion of the aryl group into the di- and trisubstituted alkenes were calculated with a lower barrier for the former. An internal competition between a substrate containing two alcohols with differing chain lengths demonstrated the catalyst's preference for migrating towards the closest alcohol. Additionally, it was observed that increasing the electron density in the arene boronic acid promotes a faster reaction, which correlates with Hammett σ_p values to give a ρ of -0.87.