Search results for "ylid"
showing 10 items of 1155 documents
An MEDT study of the carbenoid-type [3 + 2] cycloaddition reactions of nitrile ylides with electron-deficient chiral oxazolidinones.
2016
The molecular mechanism of the carbenoid-type [3 + 2] cycloaddition (32CA) reactions of a nitrile ylide (NY) with a non-chiral and a chiral oxazolidinone has been studied within Molecular Electron Density Theory (MEDT) at the MPWB1K/6-31G(d) computational level. Topological analysis of the Electron Localisation Function (ELF) of the NY shows that it presents a carbenoid structure. The high nucleophilic character of the NY together with the electrophilic character of the non-chiral oxazolidinone favour a polar 32CA reaction with a very low activation energy, the reaction being completely meta/endo selective. A Bonding Evolution Theory (BET) study of the molecular mechanism makes it possible …
Understanding the regio- and chemoselective polar [3+2] cycloaddition of the Padwa carbonyl ylides with α-methylene ketones. A DFT study
2009
The regio- and chemoselective polar [3+2] cycloaddition (32CA) of the Padwa carbonyl ylide (CY) with α-methylene ketone (αMK) to yield the oxa-bridged spirocycloadduct has been studied using the DFT method at the B3LYP/6-31G(d) computational level. Six reactive channels associated to the stereo-, regio-, and chemoselective approach modes of the CY to the CC and CO reactive sites of the αMK have been analyzed. DFT calculations for this cycloaddition are in complete agreement with the experimental outcome, explaining the reactivity and selectivity of the formation of the [3+2] cycloadduct. Analysis of the global and local electrophilicity and nucleophilicity indices allows an explanation abou…
A five-step synthesis of (±)-tylophorine via a nitrile-stabilized ammonium ylide.
2012
The Stevens rearrangement of a nitrile-stabilized ammonium ylide is the key step of a very short and practical synthesis of the phenanthroindolizine alkaloid (±)-tylophorine. The method requires only five linear steps and is devoid of any protecting group manipulations.
Understanding the origin of the asynchronicity in bond-formation in polar cycloaddition reactions. A DFT study of the 1,3-dipolar cycloaddition react…
2012
The origin of the asynchronicity in bond-formation in polar cycloadditions has been studied by an ELF analysis of the electron reorganisation along the 1,3-dipolar cycloaddition of Padwa's carbonyl ylide 4 with the 1,2-benzoquinone 8. This reaction presents an unexpected asynchronous bond-formation, which is initialised through the nucleophilic attack of Padwa's carbonyl ylide on the carbonyl oxygen atom of the strongly electrophilically activated 1,2-benzoquinone. The present study allows for the establishment that along an asynchronous bond-formation, the more favourable two-center interaction begins at the most electrophilic center, which is the center with the highest spin density achie…
A DFT study of the Huisgen 1,3-dipolar cycloaddition between hindered thiocarbonyl ylides and tetracyanoethylene
2004
Abstract The mechanism for the 1,3-dipolar cycloaddition between the hindered thiocarbonyl ylide 1 and tetracyanoethylene 2 has been studied at the B3LYP/6-31G ∗ level. Formation of the [3+2] cycloadduct 4 takes place through a stepwise mechanism that is initiated by the nucleophilic attack of the thiocarbonyl ylide 1 to the ethylene derivative 2 to give a zwitterionic intermediate IN . The subsequent cyclization of IN yields a seven-membered cyclic ketene imine 6 , which equilibrates with the thermodynamically more stable [3+2] cycloadduct 4 . The computed free energies are in agreement with the experimental outcomes.
Understanding the Electronic Reorganization along the Nonpolar [3+2] Cycloaddition Reactions of Carbonyl Ylides
2011
[EN] The nonpolar [3+2] cycloaddition (32CA) reaction of the carbonyl ylide (CY) 23 with tetramethylethylene (TME) 24 has been studied with DFT methods at the B3LYP/6-31G* level. This cycloaddition reaction, which has a very low activation energy of 4.7 kcal/mol, takes place through a synchronous transition structure. A topological analysis of the ELF along the 32CA reaction provides a new scope of the electronic structure of CY 23 as a pseudodiradical species offering a sound explanation of the high reactivity of this CY in nonpolar reactions. In addition, this analysis points to the nonparticipation of the oxygen lone pairs in the 32CA reaction. This cycloaddition can be seen as a pseudod…
Synthesis of polymeric 1-iminopyridinium ylides as photoreactive polymers
2010
Two synthetic routes to polymeric 1-imino pyridinium ylides as new photoreactive polymeric architectures were investigated. In the first approach, polymerization of newly synthesized 1-imino pyridinium ylide containing monomers yielding their polymeric analogues was achieved by free radical polymerization. Alternatively, reactive precursor polymers were synthesized and converted into the respective 1-imino pyridinium ylide polymers by polymer analogous reactions on reactive precursor polymers. Quantitative conversion of the reactive groups was achieved with pentafluorophenyl ester containing polymers and newly synthesized photoreactive amines as well as by the reaction of poly(4-vinylbenzoy…
Synthesis and Structures of Two Triorganotin(IV) Polymers R3Sn{O2CC6H4[N=C(H)}{C(CH3)CH(CH3)-3-OH]-p} n (R = Me and Ph) Containing a 4-[(2Z)-(3-Hydro…
2009
Two new polymeric triorganotin(IV) complexes R3Sn{O2CC6H4[N=C(H)}{C(CH3)CH(CH3)-3-OH]-p} n ([Me3Sn(LH)] n : 1) and ([Ph3Sn(LH)] n : 2) containing a 4-[(2Z)-(3-hydroxy-1-methyl-2-butenylidene)amino]benzoate (LH) framework were prepared. Both compounds have been characterized by 1H, 13C, 119Sn NMR, IR and 119Sn Mossbauer spectroscopic techniques in combination with elemental analyses. The crystal structures of complexes 1 and 2 reveal that they exist as polymeric zig-zag chains in which the LH-bridged Sn-atoms adopt a trans-R3SnO2 trigonal bipyramidal configuration with R groups in the equatorial positions and the axial sites occupied by an oxygen atom from the carboxylate ligand and the alco…
ChemInform Abstract: Toward an Understanding of the Mechanisms of the Intramolecular [5 + 2] Cycloaddition Reaction of γ-Pyrones Bearing Tethered Alk…
2000
The molecular mechanism for the intramolecular [5 + 2] cycloaddition reaction of β-silyloxy-γ-pyrones bearing tethered alkenes has been characterized using ab initio methods. A comparative study for this sort of cycloaddition carried out at different computational levels points out that the B3LYP/6-31G* calculations give similar barriers to those obtained with the MP3/6-31G* level. Analysis of the energetic results shows that the reaction takes place along a stepwise process: first, the migration of the neighboring silyl group to the carbonyl group of the γ-pyrone takes place to give a weak oxidopyrylium ylide intermediate, which by a subsequent concerted intramolecular [5 + 2] cycloadditi…
The role of the transfer group in the intramolecular [5+2] cycloadditions of substituted β-hydroxy-γ-pyrones: a DFT analysis
2005
The intramolecular [5+2] cycloaddition reactions of a series of 3-OR (R=SiMe3, H, CHO, Me) substituted β-hydroxy-γ-pyrones bearing tethered alkenes were studied using DFT methods at the B3LYP/6–31G* level. The role of the transfer R group was analyzed considering two alternative channels: (i) in channel a the process is initialized by the transfer of the R group with formation of an oxidopyrylium ylide intermediate, which is followed of an intramolecular [5+2] cycloaddition; (ii) in channel b, the process is initialized by the intramolecular [5+2] cycloaddition on the γ-pyrone followed by the transfer of the R group. Copyright © 2005 John Wiley & Sons, Ltd.