An empirical MM2 augmented force field for the cycloimmonium ylides

An original procedure approach taking into account the implementation of parameters determined, using calculations based on density functional theory, for the amidocyano-pyridinium methylide in the MM2 augmented harmonic potential function has been proposed. A good agreement between theoretical force field calculation and X-ray diffraction data has been observed. Thus an empirical force field for cycloimmonium ylides has been established. It provides good quality geometries for cycloimmonium ylide molecules by energy minimization. In this study we proposed a new MM2 augmented atom-type for the ylidic carbon atom. To our knowledge no attempt has been done in this way for such organic systems. Thus, we have shown that parameterization established by the DFT method is able to reproduce or to predict with good accuracy the structures of the cycloimmonium ylide compounds. This study also includes a full conformational analysis.     

Structure of dicyano-pyridinium methylide and amidocyano-pyridinium methylide using ab initio and semi-empirical methods

Taking into account the X-ray diffraction patterns of dicyano-pyridinium methylide and amidocyano-pyridinium methylide, comparative ab initio and semi-empirical studies have been performed. The aim of this study is to elucidate some electronic and structural properties of the cycloimmonium ylides. In fact, we found that the positive charge of the ylide nitrogen atom is delocalized on the pyridine ring. The ylide carbon atom is found to be insignificantly charged. This proves that the formal negative charge often accepted for the ylidic carbanion is delocalized. Thus, we demonstrate the existence of the resonance interaction between the ylidic carbanion and the positive aromatic ring as a factor of stability for planar cycloimmonium ylides. The negative charges found for the cyclic carbon atoms adjacent to the ylide nitrogen atom precludes any nucleophilic addition on the α carbon atoms in the heterocyclic ring. This is in good agreement with the experiments, where no nucleophilic addition on such atoms has ever been observed. Among the six semi-empirical methods used in this paper, the AM1 method is the most appropriate in the study of planar cycloimmonium ylides. This method is found to reproduce the experimental values and ab initio results with good accuracy.     

A comparative study by AM1, PM3 and ab initio HF/3-21G methods on the structure and reactivity of monosubstituted carbanion 1,2,4-triazolium ylides

A full conformational analysis of six 1,2,4-monosubstituted carbanion 1,2,4-triazolium ylides 4 a–f was performed using AM1, PM3 and HF/3-21G methods. The C-type conformers were found as the most stable structures by these different methods. This study also includes a qualitative estimation of the chemical behavior of triazolium ylides 4 a–f as nucleophilic agents on the level of ylide carbon atoms. The ab initio 3-21G method seems to be the most suitable in the characterization of these molecular systems.     

Etude des reactions des diazoderives avec les cycloimmonium ylures-I : Action des sels de diazonium sur quelques pyridinium - ylures

The synthesis of cycloimmonium salts of the type $\text{7}$, $\text{8}$ and the corresponding cycloimmonium ylides $\text{9}$, $\text{10}$ are reported. In the reaction with diazonium salts $\text{11}$, $\text{12}$ the pyridinium monosubstituted carbanion ylides $\text{9}$, $\text{10}$ give the pyridinium disubstituted carbanion ylides $\text{17}$ – $\text{20}$. These are new types of isolable stable ylides. The synthesis involves the formation “in situ” of ylides $\text{9}$, $\text{10}$ which are bonded by their nucleophile ylide carbon atom to the nitrogen atom in diazonium salts. The intermediat formation of cycloimmonium salts $\text{13}$ – $\text{16}$ must be accepted. The structures of all compounds presented in this paper have been established by using elementary analysis, IR and ¹H-RMN spectra.     

1,3-Dipolar cycloadditions of nonstabilized azomethine ylides to planar chalcones via regio- and stereoselective route: A three-component strategy

Simple and efficient strategies toward the synthesis of trisubstituted pyrrolizidines and disubstituted oxazolidine systems by 1,3-dipolar cycloaddition reactions using arylaldehydes and α-amino acids have been developed, followed by a one-pot, three-component strategy. Electron-deficient dipolarophiles, chalcones, were reacted with nonstabilized azomethine ylides derived from arylaldehyde and L-proline in dry dimethyl formamide, leading to substituted pyrrolizidines. The route to substituted oxazolidines involved cycloaddition to the C?O bond of a second molecule of the aldehyde. The structures and stereochemistry of the cycloadducts were established by infrared (IR), NMR spectroscopy, and single-crystal x-ray crystallographic analyses. Condensed Fukui functions and local electrophilicity indices have been computed to characterize the reactive sites and predict the preferred interactions of azomethine ylides to planar chalcones. The softness-matching indices have been evaluated to determine the regioselectivity of the cycloaddition reactions. The theoretical predictions were found to be in complete agreement with the experimental results, implying that the density functional theory (DFT)-based reactivity indices correctly predict the regioselectivities of 1,3-dipolar cycloadditions of azomethine ylides to planar chalcones. The frontier molecular orbital (FMO) energies, electronic chemical potentials, chemical hardness, chemical softness, and global electrophilicity indices of azomethine ylides have been calculated at the DFT/B3LYP/6-31 + G (d, p) level of theory.     

Three‐electron two‐center bonding in cycloimmonium ylides

We explore the possibility that a 3‐electron‐2‐center bonding exists in cycloimmonium ylides. To detect this bonding in a polyatomic system, 3‐electron‐1‐hole density operators, characterizing a Pauling 3‐electron bond, are used in the framework of second quantization formalism. The weights of 3‐electron resonance structures are calculated and compared with the weights of 2‐electron structures for the ylide bond of pyridinium dicyanomethylide; the correlations of (↑↓) and (↑) electronic events, involved in the 3‐electron resonance structures, are also investigated. The calculations are performed in various approximation levels, and both orthogonal and nonorthogonal natural atomic orbitals are adopted. All calculations show that a 3‐electron bond exists between N and C atoms of ylide bond, but this bonding is not extended in C atoms of the pyridinium group. The interactions of α,β electrons (at the configuration interaction [CI] level) increase the localization of electrons, the weights of 3‐electron resonance structures, and thus the probability for 3‐electron bonding. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Studies on ylides: Carbonyl olefination with (p-nitro benzylidene)triphenylarsenane

(p-Nitrobenzylidene)triphenylarsenane, a semistabilized arsonium ylide has been prepared and reacted with carbonyl compounds to yield olefins as opposed to epoxidation products. Treatment of the ylide with a ranged acyl halides gave new disubstituted arsonium ylides. IR and NMR spectral data for the resulting products are reported.     

Synthesis and characterization of bromonium ylides and their unusual ligand transfer reactions with N-heterocycles

Stable aliphatic bromonium ylides (RfSO2)2C--Br+C6H4-p-CF3 (Rf = CF3, CF3(CF2)3) have been synthesized and structurally characterized for the first time. X-ray crystallographic analyses indicated a ylide structure with an sp2 hybridization of the ylide carbanions and with little double-bond character for the ylidic bond. The bromonium ylides selectively undergo transfer of the aryl group to nitrogen heterocycles, such as pyridines, yielding N-arylpyridinium salts. This is in a marked contrast to the reaction of the iodonium ylides, which produces pyridinium ylides through transylidations.     

Palladium(II) complexes with sulphonium ylides

Keto-stabilized sulphonium ylides displace styrene and benzonitrile from their adducts with PdCl₂ to give stable $\text{2}\text{1}$ ylidePdCl₂ complexes. Evidence is given for epimeric equilibrium between trans square-planar structures of these new complexes in solution.A $\text{1}\text{1}$ ylidePdI₂ complex, obtained from dimethylsulphonium methylide, is also described.     

Isomerization and 1,3-dipolar cycloaddition of gem-difluorinated NH-azomethine ylides in the reaction of difluorocarbene with diarylmethanimines

Reaction of difluorocarbene with diarylmethanimines leads to the formation of gem-difluorinated NH-azomethine ylides, two types of competing transformations of which are found to be characteristic: a formal 1,2-H shift into N-(difluoromethyl)imines and 1,3-dipolar cycloaddition to electron-deficient multiple bonds. α,α,α-Trifluoroaceto-phenones are efficient dipolar traps for difluoro NH-ylides, the addition of which to the dipole proceeds regioselectively with the formation of 4-fluoro-2,5-dihydrooxazoles. According to the quantum-chemical calculations by the DFT B3LYP/6-31G* method, 1,3-dipolar cycloaddition of difluorinated NH-azomethine ylides to a C=O bond with the formation of 4-fluoro derivatives of oxazole has lower barrier of activation than the reaction, leading to another regioisomer; the formal 1,2-H shift in the ylide occurs intermolecularly with participation of an imine, a precursor of the ylide.     

Immonium ylides derived from halocarbenes in the synthesis of nitrogen-containing compounds

Reactions of halocarbenes with imines, ketenimines, and aromatic azaheterocycles, which proceed through the corresponding azomethine ylides, keteneimine ylides, and cycloimmonium ylides, are reviewed. New routes for the synthesis of halo- and nitrogencontaining compounds using the transformations of halosubstituted immonium ylides are discussed.The paper is based on an invited lecture at the fifth Conference on Carbene Chemistry (Moscow, September 16–18, 1992).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 646–654, April, 1993.     

The proton and carbon-13 NMR spectra of thioxanthone sulfoxide,thioxanthone sulfone and related ylides - chemical shift assignments

The proton and carbon-13 nmr spectra of thioxanthone sulfoxide, thioxanthone sulfone, thioxanthonium bis(carbomethoxyl)methylide and thioxanthonium bis(carbomethoxyl)methylide S-oxide are assigned using 2-D nmr techniques and compared to those of thioxanthone. The pseudo-equatorial methylide fragment shields C4a/5a by ? 11 ppm relative to the corresponding sulfoxide and by ? 4 ppm relative to thioxanthone. The pseudo-axial methylide fragment in the oxysulfonium ylide has the same effect upon C4a/5a as does the pseudo-axial oxygen of the corresponding sulfone. The sulfoxide and the sulfonium ylide have similar chemical shifts for C2/7 (? 131 ppm) as do the sulfone and oxysulfonium ylide (? 133 ppm).     

Cascade reactions of nitrogen-and phosphorus-containing ylides with methyl diazoacetate and in situ generated diazocyclopropane

Reactions of pyridinium or phosphorus ylides with diazo esters are multistep processes. The first step is covalent bonding between the ylide C atom and the terminal N atom. This is followed by elimination of the ylide-forming molecule and successive addition of one or two ylide fragments. Depending on the nature of the starting reagents, this type of transformations leads to novel polyfunctional phosphorus ylides or, with pyridinium ylides, to polyalkyl 3,4(4,5)-diazaalkadienetri-or tetracarboxylates. A reaction of the in situ generated diazocyclopropane with methyl triphenylphosphoranylideneacetate stops at the first step, giving a cyclopropylazo-containing ylide. Reactions of the latter with acyl chlorides yield six-membered heterocyclic betaines with a triphenylphosphonium substituent. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 108–113, January, 2006.     

The structure of sulfonium ylides stabilized by the malonylide moiety

The crystal structures of four thioxanthenium bis(carbomethoxy)methylides and methylides of di-(p-nitro-phenyl)sulfonium and dimethylsulfonium have been determined. The carbanion moiety in all cases is planar and the pi-system is rotated 90° with respect to the sulfur lone pair. All compounds exhibit a pseudo-equatorial orientation of the carbanion system except for the 2,4-dimethylthioxanthenium ylide. The compounds are described in terms of the geometries around the sulfur atom, the interplanar angles and sulfur-oxygen and oxygen-oxygen interactions. Surprisingly, the sulfur atom is not symmetrically placed with respect to the two phenyl rings in the thioxanthenium compounds. The sulfur is coplanar with one phenyl ring but is significantly out of the plane of the other. The ¹³C and ¹H spectra of the ylides are discussed along with the temperature dependence which indicates restricted rotation about the ⁺S? C^? bond in several cases. The factors contributing to the stability of the ylides are analyzed.     

Regioselective alkylation of amino- and mercapto-1,2,4-triazoles with t-BuOH–HClO4

The behavior of amino- and mercapto-1,2,4-triazoles in a t-BuOH–HClO₄ system has been examined. Under the investigated conditions monoalkylation of 3-amino-1,2,4-triazole proceeds at the endocyclic N1 atoms whereas 1,2,4-triazole-3-thiol undergoes S-tert-butylation. Exhaustive alkylation of the above mentioned triazoles results in di-tert-butyl substituted derivatives, which give 1,3-disubstituted triazoles under the action of base. 4-Amino-1,2,4-triazole undergoes alkylation on the amino group as well as on the endocyclic N1 atom giving a 1,4-disubstituted triazolium salt. An X-ray diffraction investigation of 5-tert-butylsulfanyl-1,2,4-triazole, 1-tert-butyl-3-tert-butylamino-1,2,4-triazol-4-ium, 1-tert-butyl-3-tert-butylsulfanyl-1,2,4-triazol-4-ium, and 1-tert-butyl-4-tert-butylamino-1,2,4-triazolium perchlorates was carried out.     

Ylide及其相应自由基的热力学稳定性研究

本文首次测定了三苯基胂、三丁基磷和二丁基碲系列Ylide的前体化合物中α-C-H键在二甲亚砜(DMSO)溶液中的pKA和气相中的均裂键能(BDE)。通过这些结构相似的Ylide相应性质的类比, 对有关的碳负离子和碳自由基的热力学稳定性以及磷Ylide中P-C键的属性及其对磷Ylide性质的影响从构效关系的更深层次进行了讨论。     

Cycloaddition versus amidation in reactions of 2-amino-2-oxoethyl-phenanthrolinium ylides to activated alkynes and alkenes

A comparative study regarding cycloaddition versus amidation reactions of 2-amino-2-oxoethyl-phenanthrolinium ylides to activated alkynes and alkenes is presented. The reaction pathway is different, depending on dipolarophile (with triple or double bond) and ylide structures. The monoenamidation reaction proved to be stereoselective leading to a Z-stereochemistry on the acrylate double bond. The structures of all newly synthesized compounds have been proved by spectral analysis (NMR and IR) and in some cases by X-ray diffraction.     

Solvatochromic Study of Two Carbanion Monosubstituted 4-Tolyl-1,2,4-triazol-1-ium Phenacylids in Binary Hydroxyl Solvent Mixtures

Two 4-tolyl-1,2,4-triazol-1-ium methylids, namely 4-tolyl-1,2,4-triazol-1-ium-phenacylid and 4-tolyl-1,2,4-triazol-1-ium-4′-nitro-phenacylid, are studied from solvatochromic point of view in binary solvent mixtures of water with ethanol and water with methanol. The contributions (expressed in percent) of the universal and specific interactions are separated from the spectral shifts recorded in the visible range for each composition of the binary solvent mixture. The essential role of the orientation and induction interactions in the studied solutions was demonstrated. Based on the statistic cell model of the binary solvent mixture solutions, the difference between the formation energies of ylid-water and ylid-alcohol complexes is estimated. The composition of the ylid’s first solvation shell was also established using the model of the binary solvent mixture solutions. The results obtained from the statistical cell model were compared with those obtained by using the Suppan’s model, resulting a good agreement.     

Bonding properties and bond activation of ylides: recent findings and outlook

The interaction of phosphorus and nitrogen ylides with metallic precursors has been examined from different points of view. The first one is related to the bonding properties of the ylides. Ylides with a unique stabilizing group bond through different atoms (the Calpha or the heteroatoms); while ylides with two stabilizing groups never coordinate through the Calpha atom. In the second section we examine the cause of the stereoselective coordination of bisylides of phosphorus, nitrogen and arsenic, and of mixed bisylides. We describe here the very interesting conformational preferences found in these systems, which have been determined and characterized. The DFT study of these bisylides has allowed for the characterization of strong intramolecular PO and AsO interactions, as well as moderate CHO[double bond, length as m-dash]C hydrogen bonds as the source of these conformational preferences. The third topic is related to the amazing reactivity of phosphorus ylides in bond activation processes. Depending on the nature of the metallic precursors, ylides can behave as sources of carbenes, of phosphine derivatives, of other ylides or of orthometallated complexes through P[double bond, length as m-dash]C, P-C or C-H bond activation reactions.