Bottlenecks in the prediction of regioselectivity of [4 + 2] cycloaddition reactions: An assessment of reactivity descriptors

Experimental-Density Functional Theory (DFT) Study of the Inhibitory Effect of Furan Residues in the Ziegler-Natta Catalyst during Polypropylene Synthesis

In this experimental-theoretical study, the effect of furan on Ziegler-Natta catalyst productivity, melt flow index (MFI), and mechanical properties of polypropylene were investigated. Through the analysis of the global and local reactivity of the reagents, it was determined that the furan acts as an electron donor. In contrast, the titanium of the ZN catalyst acts as an electron acceptor. It is postulated that this difference in reactivity could lead to forming a furan-titanium complex, which blocks the catalyst's active sites and reduces its efficiency for propylene polymerization. Theoretical results showed a high adsorption affinity of furan to the active site of the Ti catalyst, indicating that furan tends to bind strongly to the catalyst, thus blocking the active sites and decreasing the availability for propylene polymerization. The experimental data revealed that the presence of furan significantly reduced the productivity of the ZN catalyst by 10, 20, and 41% for concentrations of 6, 12.23, and 25.03 ppm furan, respectively. In addition, a proportional relationship was observed between the furan concentration and the MFI melt index of the polymer, where the higher the furan concentration, the higher the MFI. Likewise, the presence of furan negatively affected the mechanical properties of polypropylene, especially the impact Izod value, with percentage decreases of 9, 18, and 22% for concentrations of 6, 12.23, and 25.03 ppm furan, respectively.

Can we predict ambident regioselectivity using the chemical hardness?

The hard/soft acid/base (HSAB) principle is a cornerstone in our understanding of chemical reactivity preferences. Motivated by the success of the original ("global") version of this rule, a "local" counterpart was readily proposed to account for regioselectivity preferences, in particular, in ambident reactions. However, ample experimental evidence indicates that the local HSAB principle often fails to provide meaningful predictions. Here we examine the assumptions behind the standard proof of the local HSAB rule, showing that it is based on a flawed premise. By solving this issue, we show that it is critical to consider not only the charge transferred between the different reacting centers but also the charge reorganization within the non-reacting parts of the molecule. We propose different reorganization models and derive the corresponding regioselectivity rules for each.

SERS of Dopamine: Computational and experimental studies

Computational and experimental studies have been carried out on Dopamine. The calculated Raman spectra of Dopamine with and without Silver clusters (Ag_n (n = 1-4)) are compared with each other and it is shown that the intensity of the Raman activity increases with increasing number of silver atoms. The SERS effect shown by this system is further supported by calculating the Global electrophilicity index ω, the static mean polarizability α₀, and the anisotropy of the polarizabilities Δα. Stabilities of the complexes are analysed using the charge transfer, stabilization energies, and interaction energies. The reactive parameters for these complexes were further supported by looking at the molecular electrostatic potential (MESP) surfaces. SERS substrates were fabricated by sintering silver nanoparticle paste onto a fused silica substrate, using a femtosecond laser. Detection of Dopamine up to 1 μM is reported using the SERS substrates.

DFT study on the regio- and stereo-selectivity of the Diels–Alder reaction between a cycloprop-2-ene carboxylate and some cyclic 1,3-dienes

A study of the reactivity and regio- and stereo-selectivity of Diels-Alder reactions of 2-methyl-3-propionylcycloprop-2-ene carboxylic acid methyl ester with cyclohexa-1,3-diene derivatives and furan, in the gas phase, has been carried out using density functional theory (DFT)-based reactivity indices and activation energies. Four reaction channels associated with ortho (or meta), meta (or para), endo and exo pathways have been explored and characterised. The activation energies indicate that the endo pathways are preferred and DFT-based reactivity indices clearly predict the stereochemistry of the isolated cycloadducts.

Diels–Alder Reactions of α-Cyano α,β-Unsaturated Ketones with 2-Methyl-1,3-Butadiene: DFT Study of Mechanism, Reactivity and Regioselectivity

The mechanism, reactivity and regioselectivity of Diels–Alder cycloaddition reactions of 2-methyl-1,3-butadiene with four dienophiles, ( E)-2-benzylidene-3-oxobutanenitrile, ( E)-2-(furan-2-ylmethylene)-3-oxopentanenitrile, ( E)-2-benzylidene-4,4-dimethyl-3-oxopentanenitrile, and (E)-2-benzoyl-3-(furan-2-yl) acrylonitrile, have been studied by means of several theoretical approaches. The regioselectivity of these reactions has been confirmed by their activation energies, the Gazquez–Mendez rule, Houk's rule and global descriptors of the products. All the studied reactions have normal electron demand character and a polar nature. All calculations were done at the DFT-B3LYP/6-31G(d) level of theory.