Search results for "Regioselectivity"
showing 10 items of 298 documents
Toward an understanding of the unexpected regioselective hetero-Diels-Alder reactions of asymmetric tetrazines with electron-rich ethylenes: a DFT st…
2009
The regioselective hetero-Diels-Alder (HDA) reaction of asymmetric tetrazines (TTZs) with electron-rich (ER) ethylenes has been studied with use of DFT methods at the B3LYP/6-31G* level of theory. The reaction is a domino process that comprises three consecutive reactions: (i) a HDA reaction between the TTZ and the ER ethylene; (ii) a retro-Diels-Alder reaction with loss of nitrogen; and (iii) a beta-hydrogen elimination with formation of the final pyridazines. The first polar HDA reaction, which is associated to the nucleophilic attack of the ER ethylene to the electrophilically activated TTZ, is the rate and regioselectivity determining step of the domino process. The unexpected regiosele…
A DFT study of the role of the Mg complex formation on the mechanism of the 1,3-dipolar cycloadditions of benzonitrile oxides with acryloylpyrazolidi…
2010
Abstract The role of the Mg complex formation on the mechanism, the reaction rate and regioselectivity of the 1,3-dipolar cycloaddition (13DC) reactions of benzonitrile oxides (BNOs) towards electron-deficient acryloylpyrazolidinones has been theoretically studied using DFT methods at the 6-31G* level. These 13DC reactions have asynchronous concerted mechanism with a non-polar character. These cycloadditions present a slight 5-regioselectivity induced by a steric repulsion between the phenyl substituent of the BNOs and the pyrazolidinone appendage present on the acryloyl derivative, which increases with the formation of the bulky Mg–acryloylpyrazolidinone complex. The present DFT study poin…
Reactivity of Nickel(II) Porphyrins in oCVD Processes—Polymerisation, Intramolecular Cyclisation and Chlorination
2019
Abstract Oxidative chemical vapour deposition of (5,15‐diphenylporphyrinato)nickel(II) (NiDPP) with iron(III) chloride as oxidant yielded a conjugated poly(metalloporphyrin) as a highly coloured thin film, which is potentially useful for optoelectronic applications. This study clarified the reactive sites of the porphyrin monomer NiDPP by HRMS, UV/Vis/NIR spectroscopy, cyclic voltammetry and EPR spectroscopy in combination with quantum chemical calculations. Unsubstituted meso positions are essential for successful polymerisation, as demonstrated by varying the porphyrin meso substituent pattern from di‐ to tri‐ and tetraphenyl substitution. DFT calculations support the proposed radical oxi…
Janus Micelles Induced by Olefin Metathesis
2008
A facile one-step procedure for hydrophobic modification and simultaneous TEM contrast enhancement via a regioselective olefin metathesis reaction using Grubbs' catalyst is presented. Polyether diblock copolymers were investigated, and both the chain ends of the hydrophilic and the hydrophobic block were hydrophobically modified. Modification of the hydrophilic block results in nonsymmetric supramolecular structures (Janus micelles) which self-assemble into larger hierarchically organized super-micelles.
Addition of dianions of carboxylic acids to imines. Influence of the acid in the outcome of the reaction
2009
The addition of different carboxylic acids dianions to N-benzylidenebenzeneamine have been studied. The outcome of the reaction depends on the acid. Saturated ones lead to β-aminoacids with good yields and syn-selectivity whereas α,β-unsaturated ones lead to α− and γ− regioisomers with a regioselectivity that depends on the steric hindrance around the reactive centre. From some of these unsaturated acids, polyunsaturated carboxylic acids can be obtained as a change in the reaction conditions lead to the in situ elimination of aniline. From o-methyl aromatic acids δ-aminoacids are isolated with uneven results. Those derived from six member arene carboxylic acids give, on standing, dihydro-2-…
Electronic fluxes during diels-alder reactions involving 1,2-benzoquinones: mechanistic insights from the analysis of electron localization function …
2012
By means of the joint use of electron localization function (ELF) and Thom's catastrophe theory, a theoretical analysis of the energy profile for the hetero-Diels-Alder reaction of 4-methoxy-1,2-benzoquinone 1 and methoxyethylene 2 has been carried out. The 12 different structural stability domains obtained by the bonding evolution theory have been identified as well as the bifurcation catastrophes (fold and cusp) responsible for the changes in the topology of the system. This analysis permits finding a relationship between the ELF topology and the evolution of the bond breaking/forming processes and electron pair rearrangements through the reaction progress in terms of the different ways o…
The catalytic effects of a thiazolium salt in the oxa-Diels–Alder reaction between benzaldehyde and Danishefsky's diene: a molecular electron density…
2021
The oxa-Diels–Alder (ODA) reaction of benzaldehyde with Danishefsky's diene in the presence of a [thiazolium][Cl] salt, as a model of an ionic liquid, has been studied within Molecular Electron Density Theory (MEDT) at the M06-2X/6-311G(d,p) computational level. The formation of two hydrogen bonds (HBs) between the thiazolium cation and the carbonyl oxygen of benzaldehyde modifies neither the electrophilic character of benzaldehyde nor its electronic structure substantially but accelerates the reaction considerably. This ODA reaction presents an activation energy of 4.5 kcal mol−1; the formation of the only observed dihydropyranone is strongly exothermic by −28.8 kcal mol−1. The presence of…
Understanding the participation of 3-nitropyridine in polar Diels–Alder reactions. A DFT study
2015
The reactivity of 3-nitropyridine acting as an electrophilic dienophile in polar Diels-Alder (P-DA) reactions toward three different dienes of increased nucleophilicity has been theoretically studied using DFT methods at the MPWB1K/6-31G(d) level. It has been observed that this aromatic heterocyclic system suffers cycloaddition reactions yielding isoquinoline derivatives. The present DFT study establishes that while the P-DA reactions with isoprene and 1-methoxy-1,3-butadiene take place through a two-stage one-step mechanism, the use of the strong nucleophilic Danishefsky's diene changes the mechanism to a two-step one with formation of a zwitterionic intermediate. These P-DA reactions are …
Stereoselective 1,3-Dipolar Cycloadditions of a Chiral Nitrone Derived from Erythrulose. An Experimental and DFT Theoretical Study
2000
We have investigated several 1,3-dipolar cycloadditions of a chiral nitrone prepared from L-erythrulose. While cycloadditions to carbon-carbon multiple bonds of dipolarophiles such as ethyl acrylate, ethyl propiolate, or dimethyl acetylenedicarboxylate were poorly stereoselective, reaction with acrylonitrile provided predominantly one diastereomeric adduct. Furthermore, the regioselectivity exhibited by the two structurally similar dipolarophiles ethyl acrylate and ethyl propiolate was found to be opposite. The molecular mechanisms of these cycloadditions have thus been investigated by means of density functional theory (DFT) methods with the B3LYP functional and the 6-31G and 6-31+G basis …
Cyclocarbonylation reactions of allylphenols and allylnaphthols catalyzed by Pd/C- 1,4-bis(diphenylphosphine)butane
2002
Palladium/activated carbon (Pd/C) in the presence of 1,4-bis(diphenylphosphino)butane is an efficient heterogeneous catalyst for cyclocarbonylation reactions of 2-allyl phenols affording five-, six-, and seven-membered ring lactones. Various examples of cyclocarbonylation are reported and the regioselectivity is compared with the homogeneous system. Both commercial Pd/C and Pd/C prepared from activated carbons obtained from vegetable and polymeric wastes are investigated.