Search results for "Regioselectivity"
showing 10 items of 298 documents
Synthesis, characterization and X-ray structure of heterocyclic systems prepared via 1,3-dipolar cycloaddition of nitrile oxides with benzimidazolone
2018
Abstract We report here a one-step synthesis of the new isoxazolyl-benzimidazolones and isoxazolinyl-benzimidazolones by 1,3-dipolar cycloaddition of the p-substituted benzonitrile oxides and mesitonitrile oxide with 1-cyclohex-1-en-1-yl-3-propargyl-benzimidazolone or 1-cyclohex-1-en-1-yl-3-allyl-benzimidazolone. In all cases these cycloadditions are completely regio- and chemoselective. The structures of new cycloadducts obtained are elucidated by their analytical and spectral data. The relative stereochemistry of the cycloadduct 8 was confirmed by single crystal X-ray analysis.
A DFT study of the role of Lewis acid catalysts in the mechanism of the 1,3-dipolar cycloaddition of nitrile imines towards electron-deficient acrylo…
2012
Abstract The role of the Mg complex formation in the reaction rate and regioselectivity of the 1,3-dipolar cycloaddition (13DC) reactions of nitrile imines (NIs) towards electron-deficient methacryloylpyrazolidinones has been theoretically studied using DFT methods at the B3LYP/6-31G * level. These 13DC reactions have an asynchronous concerted mechanism with a low polar character, displaying a high C regioselectivity induced by a steric repulsion between the phenyl substituent present at the C3 carbon of the NIs and the pyrazolidinone appendage present in the methacryloyl derivative, which increases with the formation of the bulky Mg–methacryloylpyrazolidinone complex. Solvent effect has be…
The Participation of 3,3,3-Trichloro-1-nitroprop-1-ene in the [3 + 2] Cycloaddition Reaction with Selected Nitrile N-Oxides in the Light of the Exper…
2021
The regioselective zw-type [3 + 2] cycloaddition (32CA) reactions of a series of aryl-substituted nitrile N-oxides (NOs) with trichloronitropropene (TNP) have been both experimentally and theoretically studied within the Molecular Electron Density Theory (MEDT). Zwitterionic NOs behave as moderate nucleophiles while TNP acts as a very strong electrophile in these polar 32CA reactions of forward electron density flux, which present moderate activation Gibbs free energies of 22.8–25.6 kcal·mol−1 and an exergonic character of 28.4 kcal·mol−1 that makes them irreversible and kinetically controlled. The most favorable reaction is that involving the most nucleophilic MeO-substituted NO. Despite P…
A DFT study for the regioselective 1,3-dipolar cycloadditions of nitrile N-oxides toward alkynylboronates
2003
Abstract The mechanism for the 1,3-dipolar cycloaddition of benzonitrile oxide toward ethynyl and propynylboronate has been studied by using density functional theory (DFT) at B3LYP/6-31G* level. These cycloadditions are concerted [3+2] processes. The presence of the two oxygens on the boronic ester precludes the participation of the boron atom on [3+3] processes. The two regioisomeric channels associated to the formation of the isoxazoles bearing the boronic ester unit on the 4- or 5-positions have been characterized. The B3LYP/6-31G* activation parameters are in acceptable agreement with the experiments, allowing to explain the factors controlling these regioselective cycloadditions.
Experimental and theoretical push-pull Chemo- and regioselectivity in 1,3-Dipolar cycloaddition reactions: the case of benzotriazepin-5-one with mesi…
2007
A novel heterocyclic compound 3-mesityl-5-methyl-4,5,11,11a-tetrahydro-6H-[1,2,4]oxadiazolo [5,4-b][1,3,4]benzotriazepin-6-one 4 has been synthesised by a 1,3 dipolar cycloaddition (13DC) reaction of 1,3,4-benzotriazepin-5-one 1 with mesitylnitrile oxide 3. The reaction, beside its synthetic interest, has shown to be completely chemo- and regioselective. The structure of the compound was determined by X-ray crystallography and analysed by spectral methods (NMR and mass spectrometry). The molecular mechanism for the reaction has been studied using quantum mechanical calculations at the B3LYP/6-31G* theory level. Two mechanisms are possible for the formation of the cycloadduct 4. The first on…
Extension of the Squaraine Chromophore in Symmetrical Bis(stilbenyl)squaraines
1997
The bis(stilbenyl)squaraines 6d−j,d‘,j‘ represent a novel class of NIR pigments. Their synthesis was performed by the regioselective 2-fold condensation of highly nucleophilic 3,5-dihydroxystilbene...
A DFT study of the domino reactions between imidazole NHC, ketenimines and DMAD or MP acetylene derivatives yielding spiro-pyrroles
2014
Abstract The domino reactions of imidazole carbene 9, ketenimine 10 and DMAD 5 or MP 7 to yield the formal [3+2] cycloadducts 12 or 14 have been investigated using DFT methods at the B3LYP/6-31G* level. These domino reactions are initialized by the nucleophilic attack of imidazole 9 on ketenimine 10 to yield the zwitterionic intermediate 11. The second reaction is a formal [3+2] cycloaddition (32CA) reaction between the zwitterionic intermediate 11 and acetylene carboxylates DMAD 5 or MP 7 yielding spiro-pyrroles 12 and 14. These formal 32CA reactions are initialized by the nucleophilic attack of the N3 nitrogen of intermediate 11 on the most electrophilic centres of these acetylene derivat…
Photocyclization of a bichromophoric phenol/olefi system substituted at the methylene spacer - Zwitterions versus H-bridged intermediates in the exci…
2002
Photolysis of (E)- and (Z)-2-(1-ethyl-3-phenylpropenyl)phenol (1 and 2) under a variety of conditions produces a mixture of dihydrobenzopyrans 3 and 4, dihydrobenzofuran 5, cyclopropanes 6 and 7, and rearranged olefin 8. Acidic treatment of 1 and 2 gives a mixture of the six-membered ring products 3 and 4. Remarkable differences, associated with the nature of the precursor and the reaction conditions, are observed in the regiochemistry of cyclization and in the stereochemistry of the dihydrobenzopyrans (3/4 ratio). This points in favour of an H-bridged intermediate, rather than a fully zwitterionic species, as the direct precursor of the photocyclized products.
Role of the gem-Difluoro Moiety in the Tandem Ring-Closing Metathesis−Olefin Isomerization: Regioselective Preparation of Unsaturated Lactams
2006
Careful selection of the metathesis catalyst, solvent, and reaction conditions allows for the efficient and regioselective synthesis of isomeric fluorinated and nonfluorinated lactam derivatives II and III from precursor amides I through a ring-closing metathesis (RCM) reaction or a tandem RCM-isomerization protocol, respectively. The presence of the gem-difluoro moiety in the starting materials exerts a pivotal effect by directing the isomerization step, making the overall tandem transformation a regioselective process. The scope, limitations, and synthetic usefulness of this protocol are also discussed.
Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art
2022
Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limit…