Does aromaticity in a reaction product increase or decrease the intrinsic barrier? Kinetics of the reversible deprotonation of benzofuran-3(2H)-one and benzothiophene-3(2H)-one

DFT investigation of hydrogen atom-abstraction reactions of NHC-boranes by various carbon-centered radicals: barriers and correlation analyses

In this study, we employed a quantum-mechanical computational method to investigate the hydrogen-atom abstraction reactions of two nitrogen heterocyclic carbene boranes (NHC-boranes), NHC-BH3 and NHC-BH2CN, by a series of carbon-centered radicals bearing various substituents. We explored the degree of correlation of the activation and free energy barriers to their components. Furthermore, we also investigated the effects of the radical and substituent sizes, nucleophilicity/electrophilicity indices, and the spin density distribution of the radical reactants on the three fundamental barriers and the thermal contribution of the reaction energy to the kinetic barrier. Using the generated data, we assessed the abilities of the various radical reactants to abstract the hydrogen atom from NHC-boranes. Further, we performed a similar analysis after dividing those radical reactants into four groups, which were classified based on the dominant factor affecting their electronic density distribution, which involves the inductive effect, conjugation, hyperconjugation, and the feedback of lone-pair electrons. The results and conclusions of this investigation not only provide insight into the relationships between some of the key kinetic and thermodynamic parameters, which is useful for understanding the dynamics of such hydrogen-abstraction reactions, but also provide information for selecting suitable radical reactants for further experimental investigations.

Unusually high α-proton acidity of prolyl residues in cyclic peptides

The acidity of the α-proton in peptides has an essential role in numerous biochemical reactions and underpins their stereochemical integrity, which is critical to their biological function. We report a detailed kinetic and computational study of the acidity of the α-proton in two cyclic peptide systems: diketopiperazine (DKP) and triketopiperazine (TKP). The kinetic acidity (protofugality) of the α-protons were determined though hydrogen deuterium exchange studies in aqueous solutions. The acidities of the α-proton in prolyl residues were increased by 3–89 fold relative to other amino acid residues (prolyl > glycyl ≫ alanyl > tyrosyl). Experimental and computational evidence for the stereoelectronic origins of this enhanced prolyl reactivity is presented. TKPs were 10⁶-fold more reactive than their DKP analogues towards deprotonation, which we attribute to the advanced development of aromaticity in the earlier transition state for proton transfer in these cases. A Brønsted linear free energy analysis of the reaction data was conducted to provide estimates of α-proton pK_as.

Kinetic and computational studies reveal that prolyl residues in cyclic peptides are substantially more acidic than other residues due to a stereoelectronic effect.

DFT studies on quantum mechanical tunneling in tautomerization of three-membered rings

Amino–imino and keto–enol tautomerization processes in three-membered ring systems have been explored to examine the role of quantum mechanical tunneling along with aromaticity. The DFT calculations shed light on the role of aromaticity in tautomerization processes and as perceived this property may not contribute entirely to facilitate the formation of tautomeric forms.

Proton transfers in aromatic systems: How aromatic is the transition state?

The question as to what extent aromaticity in a reactant or product is expressed in the transition state of a reaction has only recently received serious attention. Inasmuch as aromaticity is related to resonance, one might expect that, in a reaction that leads to aromatic products, its development at the transition state should lag behind bond changes as is invariably the case for the development of resonance in reactions that lead to delocalized products. However, recent experimental and computational studies on proton transfers from carbon acids suggest the opposite behavior, i.e., the development of aromaticity at the transition state ismoreadvanced than the proton transfer. The evidence for this claim is based on the determination of intrinsic barriers that show a decrease with increasing aromaticity. According to the Principle of Nonperfect Synchronization (PNS), this decrease in the intrinsic barrier implies a disproportionately large amount of aromatic stabilization of the transition state. Additional evidence for the high degree of transition state aromaticity comes from the calculation of aromaticity indices such as HOMA, NICS, and the Bird Index. Possible reasons why the degree to which aromaticity and resonance are expressed at the transition state is different are discussed.