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The chemoselectivity of geometrically ylide compounds is often hard to assign from experimental techniques, particular system with intramolecular hydrogen bonding (IHB) are even more challenging. Herein, theoretical calculations were performed to investigate whether theoretical results would provide consistent evidence for the existence of IHB to confirm experimental data and to evaluate strength of the N–H···O IHB from geometrical synthesized 2-substituted pyrrole stable phosphorus ylide (dimethyl 2-(1H-pyrrol-2-yl)-3-(triphenylphosphoranylidene) butanedioate in a single chemoselective compound. Topological parameters at the bond critical points (BCP) of intramolecular hydrogen bonds from Bader’s atoms in molecules (AIM) theory and Winhold’s natural bond orbital (NBO) calculations were analyzed at the B3LYP/6-311++g** level in details. A series of gage-including atomic orbital chemical shift (GIAO c.s.) calculations at the HF and DFT levels of theory were carried out to assign the 1H NMR chemical shifts. The best prediction of the experimental 1H NMR values was obtained at the mPW1PW91 levels using the 6-31G** basis set. Theoretical results, in agreement with the experimental data, were confirmed the N–H···O IHB was caused the deshielding of the proton to lower field. The barriers in double bond and single bond rotation were theoretically estimated in detailed and the AIM and NPA approaches were confirmed the loss of charge of the hydrogen atom involving in intramolecular N–H···O hydrogen bonding. The geometrical and topological parameters from AIM and NBO analyses were indicated the medium N–H···O IHB.

The kinetics of the reaction between triphenylphosphine, dialkyl acetylenedicarboxylates and pyrrole in dry organic solvents at different temperatures have been studied spectrophotometrically. The observed overall second rate constant ( k2)for reaction decreases with decreasing solvent dielectric constant and media temperature; k2 follows the Arrhenius equation, and the overall reaction is first order in both the dialkyl acetylenedicarboxylate and triphenylphosphine concentrations. The proposed mechanism has been evaluated, and the activation parameters Δ G≠, Δ S≠ and Δ H≠ for the first, rate-determining step, as an elementary reaction have been determined on the basis of Eyring equation. In addition, the Z- and E-isomers have been optimized at HF and B3LYP levels with the 6-311++G** basis set; the more stable is the E-isomer, in agreement with experimental data. Furthermore, to better understand the intramolecular interactions, atoms in molecules (AIM) calculations and natural population analysis (NPA) methods have been carried out. The 1H, 13C, and 31P NMR data of these ylides are consistent with results obtained from theoretical calculations.

1-Methyl-2-oxopyrrolidinium hydrogen sulfate ([Hpyro][HSO4]) is used as an ionic liquid catalyst for the one-pot three-component synthesis of highly substituted piperidines from aromatic aldehydes, anilines and β–ketoesters in refluxing ethanol. This homogeneous catalyst procedure has the advantages of easy work-up, no need for column chromatography, simple and readily available precursors, and good to high yields.

For the first time, a kinetic investigation of the interchange process in the equilibrium between the Z- and E-isomers of a stable phosphorus ylide involving a 2-indolinone has been undertaken by the 1H NMR technique. Activation parameters such as k-1, k1, ktotal, Ea1, Ea-1, Eatotal, Δ S≠, Δ H≠ and Δ G≠ along with thermodynamic parameters such as Ke, Δ H°, Δ G° and Δ S° were obtained from experimental data for the forward and reverse reactions.

An efficient green protocol is described for the preparation of highly functionalized piperidines via a one-pot five-component reaction between aromatic aldehydes, anilines and β-ketoesters in the presence of oxalic acid dihydrate as catalyst in ethanol at ambient temperature. The structure as well as the relative stereochemistry of these compounds was confirmed by single X-ray crystallographic analysis.

The influence of cation-π and anion-π interactions on the strength and nature of N⋯H hydrogen bond has been investigated by quantum chemical calculations in s-triazine⋯(HF) 3 complex. Ab initio calculations were performed at MP2(FC)/6-311++G(d,p) level of theory. The natural bond orbital (NBO) analysis and the Bader’s quantum theory of atoms in molecules (AIMs) were also used to elucidate the interaction characteristics of these complexes. Some bonding parameters were calculated by natural resonance theory (NRT) to investigate the changes in the geometrical parameters upon complexion. The N⋯H hydrogen bonds can be putted in the categories weak to medium in investigated systems. In addition, both proper and improper hydrogen bonds, which depend on the charge/radius of ion, are seen among interactions.

The addition of triphenylphosphine to acetylenedicarboxylic acid in the presence of N–H heterocyclic compounds such as carbazole, 2-indolinone, 2-benzoxazolinone, 2-mercaptobenzoxazole, 2,4-thiazolidinedione, benzimidazole, isatin, or saccharin leads to stable 1,5-diionic organophosphorus compounds in excellent yields at ambient temperature.

Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of a SH-heterocyclic compound such as 2-thiazoline-2-thiol to generate stable phosphorus ylides. These stable ylides exist in solution as a mixture of the two geometrical isomers as a result of restricted rotation around the carbon–carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. In the recent work, NMR study and the assignment of more stable Z - or E -isomers as the major form were investigated by using natural population analysis (NPA).

The efficient, simple, and mild synthesis of the title compounds (IV) includes a very easy work-up procedure.

p-Toluenesulfonic acid monohydrate (p-TsOH·H2O) is a highly effective and efficient catalyst for the one-pot multicomponent coupling of aldehydes, amines, and β-ketoesters in EtOH at ambient temperature to give highly functionalized piperidines in high yields. This one-pot reaction has some important advantages such as the easy workup procedure, simple and readily available precursors, nontoxic and inexpensive catalyst, and good to high yields.

A simple and convenient one-pot multi-component reaction has been described for the synthesis of functionalized spirolactones. This strategy demonstrated three-component reaction between aromatic ketones (11H-indeno[1,2-b]quinoxalin-11-one) and dimethyl acetylenedicarboxylate (DMAD) in the presence of N-heterocycles, such as pyridine, quinoline, and isoquinoline in CH2Cl2 at ambient temperature without use of any catalyst or activator.

New and stable phosphorous ylides are obtained from a simple and efficient one-pot three-component reaction between hexamethyl phosphorous triamide and dimethyl acetylenedicarboxylate in the presence of NH-heterocyclic compounds in excellent yields at an ambient temperature. These stable ylides exist in solution as a mixture of two geometrical isomers as a result of the restricted rotation around the carbon–carbon partial double bond resulting from the conjugation of the ylide moiety with the adjacent carbonyl group. Dynamic effects are observed in the 1H NMR spectra that are attributed to the restricted rotation around the carbon–carbon double bond. The experimental rotational energy barrier (Δ G#) and other activation parameters on the basis of the 1H NMR study for the rotational interchangeable process of major and minor isomers in ylides 4a–dare reported. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 23:131–137, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20761

An efficient and direct protocol is described for the preparation of α-amino phosphonates derivatives by employing a multi-component, one-pot condensation reaction of aldehyde, amine and trialkyl phosphite in the presence of silica-supported perchloric acid (3 mol%) under solvent-free conditions. The thermal solvent-free green procedure offers advantages such as shorter reaction time, simple work-up, high yield, recovery and reusability of catalyst.

A simple, efficient, and environmentally benign method has been developed for the synthesis of α-amino phosphonates. One-pot, three-component reaction of aldehyds, amines, and trialkylphosphite in the presence of silica-supported sodium hydrogen sulfate (NaHSO4-SiO2) as an inexpensive and reusable catalyst under solvent-free conditions at 80 °C affords the corresponding α-amino phosphonates in good yields and short reaction times.

The reaction between dialkyl acetylenedicarboxylates and NH heterocyclic compounds in the presence of triethyl phosphite leads to stable phosphorus ylide derivatives in good yields. The X-ray crystallographic data and theoretical study show that there is a resonance between two bonds of C19P1 and C191O191 in phosphorus ylide 4d. This compound crystallizes in the orthorhombic system, space group (Pca21), with unit cell parameters a = 17.3699(3) A, b = 13.5500(2) A, c = 18.4627(3) A, α = 90°, β = 90°, γ = 90°, Z = 8, and V = 4345.4(12) A3. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:715–722, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20739

Protonation of the highly reactive 1:1 intermediate produced in the reaction between triphenylphosphite and activated acetylene by sulfonamide leads to a phosphonate ylide, which undergoes methanol elimination in the presence of moisture to produce a highly functionalized sulfonamide phosphonate diester. A dynamic nuclear magnetic resonance (NMR) effect is observed in the 1H NMR spectra of this compound as a result of restricted rotation around the single C‒N bond. The coalescence temperature was observed at TC = 352.5 K, and the free energy of activation (ΔG#) for this process is 75.6 ± 2 kJ.mol−1. GRAPHICAL ABSTRACT

The reaction between triphenyl or trialkyl phosphite and acetylenic esters in the presence of some heterocyclic or aromatic NH compounds such as thiazolidine-2,4-dione, 2-methyl indole, 5-bromoisatine, 3-nitroacetanilide, saccharin, 5,5-dimethylhydantoin, 2-nitroaniline, 4-nitroaniline, benzophenon hydrazine, and anthranilic acid led to the formation of phosphonato esters in high yield. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:630–639, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20725

Herein, a series of separate dynamic 1H NMR effects are reported at different temperatures within a particular enaminoester involving a phenanthridine. These effects are attributed to restricted rotation around the two single bonds such as carbon–carbon (HaCCHb) and nitrogen–carbon (NCCOOCH3). Activation energies (Ea) for these interconversion processes in their rotational isomers are equal to 20 and 20 ± 1 kJ mol−1, respectively. In addition, three dynamic 1H NMR effects are investigated at different temperatures for a particular phosphorus ylide involving a 2-indolinone around the carbon–carbon single bond (HCCPPh3) within the two Z- and E-rotational isomers and partial carbon–carbon double bond (OCH3-CC-PPh3). Activation energies (Ea) for these interconversion processes in rotational isomers are equal to 53, 63 and 73 ± 1 kJ mol−1, respectively. This behavior was also observed for other phosphorus ylide containing 2-mercaptobenzoxazole around the carbon–carbon single bond and partial carbon–carbon double bond with their relevant activation energies containing 13, 10 and 75 ± 1 kJ mol−1, respectively. Copyright © 2011 John Wiley & Sons, Ltd.

The influences of cation–π and anion–π interactions on NMR data have been investigated in complexes of cations and anions with 1,3,5-trifluorobenzene (TFB). Cation–π interaction increases 1 J C–F , 1 J C–H and the chemical shifts of hydrogen and fluorine while it decreases 1 J C–C . The changes are in reverse direction in the presence of anion–π interaction. The role of geometry and electronic effects on the NMR data was considered. The distance dependence of NMR parameters has been studied in these complexes. The NMR data have been investigated in isoelectronic complexes.

A diastereospecific synthesis of N-vinyl heterocyclic compounds is described from a reaction of dialkyl acetylenedicarboxylate and N-H heterocyclic compounds, such as phthalimide, saccharin, 3(2H)-pyridazinone, isatin, maleimide, 2,4-thiazolidinedione, 2-benzoxazolinione and 6-chloro-2-benzoxazolinione, in the presence of triphenylarsine as an efficient catalyst, in excellent yield. The presented one-pot method is simple and effective with mild conditions.

For the first time, α-aminophosphonates were obtained by a simple and efficient one-pot method from the reaction between aldehyde, aniline, and triphenyl phosphite in the presence of acetic acid as a catalyst and solvent at room temperature. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. GRAPHICAL ABSTRACT

A facile and chemoselective one-pot synthesis of novel stable phosphorus ylides has been developed from the reaction between triphenylphosphine and dialkyl acetylenedicarbocxylates in the presence of (heterocyclic) biological bases such as purine, theophylline, 6-azauracil, and 6-aza-2-thiothymine at ambient temperature. GRAPHICAL ABSTRACT

The reaction between dialkyl acetylenedicarboxylates and NH heterocyclic compounds in the presence of trialkyl phosphite leads to stable phosphorus ylide derivatives in good yields. The x-ray crystallographic data and theoretical study show that there is a resonance between two bonds of C9P1 and C91O91 in phosphorous ylide 4c. This compound crystallizes in the triclinic system, space group (), with the following unit cell parameters: a = 8.7522(3)A, b = 8.8513(5)A, c = 18.3469(5)A, α = 99.1220(10)°, β = 90.954(2)°, γ = 118.792(2)°, Z = 2, and V = 1222.72(9)A3. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 22:36–43, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20653

Hydrazine sulfonamide derivatives undergo a smooth reaction with dialkyl acetylenedicarboxylates in the presence of triphenylphosphine in varieties of solvent for generation of phosphorus ylides in a chemoselective manner.

5-Aryl-6-(alkyl- or aryl-amino)-1,3-dimethylfuro [2,3-d]pyrimidine derivatives were obtained by in situ reaction alkyl or aryl isocyanides and pyridinecarbaldehyde derivatives in the presence of 1,3-dimethylbarbituric acid in dichloromethane without any prior activation or modifications.

A new class of enamino esters has been isolated in excellent yields from the 1:1:1 addition reaction between phenanthridine or isoquinoline and activated acetylenic esters in the presence of heterocyclic NH compounds (succinimide, 5-nitroindazole, benzoxazolinone, 6-chlorobenzoxazolinone, carbazole and 3,6-dibromocabazole) or 1,3-dicarbonyl compounds such as 1,3-dimethylbarbituric acid and 1,3-diethyl-2-thiobar- bituric acid.

Protonation of the reactive 1:1 intermediates produced in the reactions between triphenylphosphine and dialkyl acetylenedicarboxylates by 1-amino-anthraquinone or 1,5-diphenylcarbazone as a core dye leads to vinyl phosphonium salts, which undergo Michael addition with conjugate base of NH compounds to produce stable phosphorus ylides as novel dyes in fairly good yields. These ylides can exist in two geometrical isomers (Z) and (E) for 3, because the negative charge of the ylide moiety of these compounds are strongly conjugated with the adjacent carbonyl group. Rotation around the carbon–carbon double bond is slow in the (Z) and (E) geometrical isomers on the NMR time scale at ambient temperature. These compounds are assigned by their IR, 1H, 13C NMR spectral data as well as their mass spectroscopic data.

Kinetic studies were made for the reaction between triphenylphosphine, di-tert-butyl acetylenedicarboxilate in the presence of OH-acid, such as 2-hydroxy-4-methoxybenzaldehyde. To determine the kinetic parameters of the reaction, it was monitored by UV spectrophotometery. The second order fits were automatically drawn by the software associated with a Cary UV spectrophotometer model Bio-300 at appropriate wavelength. The values of the second order rate constant (k2) were calculated using standard equations within the program. At the temperature range studied the dependence of the second order rate constant (ln k2) on reciprocal temperature was in a good agreement with Arrhenius equation. This provided the relevant plots to calculate the activation energy of the reaction. Furthermore useful information was obtained from studies of the effect of solvent and concentration of reactants on the rate of reaction. Proposed mechanism was confirmed according to the obtained results and steady state approximation and first step (k2) of reaction was recognized as a rate-determining step on the basis of experimental data. In addition, assignment of more stable isomers (Z or E) was investigated using the theoretical study.

Stable crystalline phosphorus ylides are obtained in excellent yields from the 1:1:1 addition reactions between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of an SH-heterocyclic compound, such as 2-mercapto-1-methyl imidazole. These stable ylides exist in solution as a mixture of the two geometrical isomers as a result of restricted rotation around the carbon–carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. In the recent work, an NMR study and the stability of the Z- and E-isomers were undertaken for the two rotamers of phosphorus ylides involving 2-mercapto-1-methylimidazole by natural population analysis.

The effect of cation–π interactions on some NMR data of ethylenic complexes has been investigated by B97-1, PBE1KCIS and MPWKCIS1K methods using 6-311++G(3df,3pd) basis set. Unlike 3 J H – H ( cis ) , the chemical shift of ethylenic hydrogen ( δ H ) and 3 J H – H ( trans ) increases by cation–π interaction. The changes of NMR data have been considered with regard to geometry and direct electronic effects. Also, the distance dependence of NMR parameters has been studied in these complexes.

In a new multicomponent reaction phenanthridine reacts with isocyanides and malonitrile in the presence of benzaldehyde derivatives to produce 2-aryl-3-(alkyl- or arylimino)-2,3-dihydropyrrolo[1,2-f]phenanthridine-1,1(12b H)-dicarbonitrile in a simple, mild, and efficient protocol in excellent yields.

The effects of substituent and hybridization of nitrogen atom on hydrogen bonding in the F–H···N CX, F–H···N(H) CX, and F–H···N(H) 2 –CX complexes have theoretically been studied by MP2 and DFT methods with aug-cc-pVDZ and 6-311++G∗∗ basis sets. With respect to the hybridization of nitrogen, sp 3 -hybridized nitrogen forms the strongest bond, followed by sp 2 and then sp. In equilibrium structures, the trend in the two-bond 19 F– 15 N spin–spin coupling constants ( 2h J F–N ) is sp 3 2 . The results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses are in meaningful relationships with other characteristics of hydrogen bonds, especially with the 2h J F–N values.

A new class of enamino esters has been isolated in excellent yields from the 1:1:1 addition reaction between phenanthridine or isoquinoline and acetylenic esters such as ethyl propiolate or dialkyl acetylenedicaboxylates in the presence of heterocyclic NH compounds (succinimide, indole, 2-methylindole, 2-benzoxazolinone, 6-chlorobenzoxazolinone, carbazole and 3,6dibromocarbazole) or 1,3-dicarbonyl compounds like 1,3-dimethylbarbituric acid, acetylacetone and dibenzoylmethane.

A general and practical route has been considered for the synthesis of stable heterocyclic phosphorus ylides 3a–c by a one-pot condensation reaction between dialkyl acetylenedicarboxylate and triphenylphosphine in the presence of -NH heterocyclic compounds such as 2,4-dimethyl-3-acetyl pyrrole. The stable ylides 3a–b exist in solution as a mixture of the two isomers, while 3c indicates only one isomer. For this reason, the assignments of more stable Z- or E-isomers as the major or minor forms were investigated using the theoretical calculations. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

Synthesis of α-amino phosphonates is described under solvent-free conditions at 100°C from reaction between aldehydes and amines in the presence of trialkyl phosphites using Al(H2PO4)3 as an efficient and reusable heterogeneous catalyst. The advantages of this procedure are short reaction time, flexibility and having high to excellent yields.

Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of heterocyclic compounds, such as 2-aminobenzimidazole, 2-hydroxy-3-nitropyridine or 1,2,3,4-tetrahydrocarbazole to generate stable phosphorus ylides. Some ylides exist in solution as a mixture of two geometrical isomers as a result of restricted rotation around the carbon–carbon partical double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group, whereas others occur as a single isomer only. For this reason, the assignments of more stable Z- or E-isomers as the major or minor forms were investigated using theoretical calculations.

In the present work, NMR, theoretical, kinetics, and mechanism investigations were undertaken for a one-pot condensation reaction between 2-mercapto-4,6-dimethyl pyrimidine and dialkyl acetylenedicarboxylates in the presence of triphenyphosphine containing novel stable phosphorus ylides 4a–c. Herein, theoretical calculations have been employed for assignment of the most stable isomers (Z or E) of phosphorus ylides 4a,c by natural population analysis, atoms in molecules methods, and CHelpG keyword, in which E-4(a, c) are more stable forms as the majors. The 1H, 13C, and 31P NMR data of these ylides are consistent with results obtained from theoretical calculations. In addition, kinetic investigation of new ylides was undertaken by ultraviolet spectrophotometry. Useful information was obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates), and also the concentration of reactants on the rate of reactions. The proposed mechanism was confirmed according to the obtained results and a steady-state approximation, and the first step (k2) of the reaction was recognized as a rate-determining step on the basis of the experimental data. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:462–474, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20632

New phosphonate ester derivatives were obtained by in situ stereo-specific reaction between triphenyl phosphite and dialkyl acetylenedicarboxylates in the presence of a series of halogenated anilines. Spectroscopic data and X-ray crystallography analysis are in agreement with the gauche arrangement for the two vicinal protons in the structures. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:222–227, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20600

The reaction between thiobarbituric acid (I) and acetylenic diesters (II) with isocyanides (III) or triphenylphosphine (V) leads to highly functionalized 4H-pyrano[2,3-d]thiopyrimidone (IV) or 1,4-diionic organophosphorous derivatives (VI), respectively.

The effect of carbon hybridization and halogen-acceptor type on some NMR data have been investigated in C−X⋯H−F (X = 19F, 35Cl, 81Br) hydrogen bonds at B3LYP/6-311++G(d,p), PBE1KCIS/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory. In these complexes, halomethanes, haloethylenes and haloacetylenes are considered as proton acceptor and HF as the proton donor. Unlike ΔE, 1hJX–H, and 2hJX–F values. The isotropic value of the proton shielding tensor of hydrogen fluoride increases as follows: C(sp) > C(sp2) > C(sp3). This trend for fluorine of hydrogen fluoride depends on halogen-acceptor type.

The reaction between triphenyl phosphite and acetylenic esters in the presence of some heterocyclic compounds such as oxazolo[4,5-b]pyridine-2(3H)-thione, 2-mercaptobenzothiazole or 2-mercaptopyrimidine led to the formation of phosphonato esters in high yield.

The effect of C–H⋯π and X–H⋯π interactions on some NMR data of haloacetylene⋯benzene, halomethane⋯benzene and hydrogen halide⋯benzene complexes has been studied at PBE0/6-311++G(d, p) and PBE1KCIS/6-311++G(d, p) levels of theory. The complexes were optimized by MP2 method using 6-311++G(d, p) and aug-cc-pVDZ basis sets. In addition to geometrical parameters and binding energies, topological properties of electron charge density calculated by atoms in molecules (AIM) method, and the results of natural bond orbital (NBO) analysis are in a good relationship with calculated NMR data. The consideration of these parameters aids in better understanding of NMR data in these complexes.

A one-pot triphenylarsine-catalyzed synthesis of trans -cyclopropane derivatives is achieved by means of the reaction between acetylenic esters and C–H acids in the presence of triphenylarsine. This procedure is simple and proceeds under mild reaction conditions. Its success depends on the choice of solvent, temperature and C–H acid used.

The synthesis, theoretical study and a kinetics and mechanistic investigation are described as a one-pot condensation reaction between benzamide and acetylenic esters in the presence of triphenyphosphine to generate a novel stable phosphorus ylides. For the first time, theoretical calculations have been employed to assign the most stable isomers ( Z or E) of phosphorus ylides 4a-c by AIM and NBO theory, in which Z-4(a,b) are the more stable forms, whereas Z-4c appears as a single isomer. In these cases, the 1H, 13C and 31P NMR spectra of these ylides are consistent with the results obtained from theoretical calculations. Kinetic investigation of the new ylides was undertaken by UV. Useful information was obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the acetylenic esters), and also the concentration of reactants on the rate of reactions. The proposed mechanism was consistent with the results obtained; from the steady-state approximation, the first step ( k2) of the reaction was recognized as the rate-determining step on the basis of the experimental data.

Stable crystalline phosphorus ylides containing chlorine and sulfur were obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine and dialkyl acetylene-dicarboxylates in the presence of 6-chloro-2-benzoxazolethiol and 2-chloro-phenothiazine. These stable ylides exist in solution as a mixture of two geometrical isomers. This is caused by the conjugation of the ylide moiety with the adjacent carbonyl group, which results in a restricted rotation around the respective carbon-carbon bond.

For the first time, a series of separate dynamic 1 H NMR effects are reported at different temperatures within a particular phosphorous ylide involving a 2-methyl indole namely dimethyl 2( N -2-methyl indole-1-yl)-3-(triphenyl phosphoran ylidene) butandioate. These effects are attributed to restricted rotation around the heteroaryl–carbon and carbon–carbon single bonds and also around carbon–carbon double bond. Rotational energy barrier (Δ G # ) for their interconversion process of rotational isomers equals (55.33, 49.91 and 70.16) ± 2 kJ mol −1 , respectively.

The stacking interactions in the uracil:phenylalanine (U:PHE) and (U:PHE)···Na+ complexes have been studied at different levels of theory, in which the structures were optimized by both standard and gradient counterpoise corrected methods. The Na+ cation can interact with different sites of stacked U:PHE unit. The geometrical parameters of the optimized structures and the calculated binding energies reveal the influence of cation interaction on π–π stacking and vice versa. The interplay between π–π stacking and cation interaction has also been investigated by topological analysis of electron charge density using atoms in molecules (AIM) method. A good agreement between the results of AIM analysis and calculated binding energies has been observed in dimer and complexes.