Ylide Structure Research Articles

Substituent effect on the transition from ionic to covalent bonding in triphenylphosphonium ylide derivatives: reactivity of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2λ5]oxaphosphole with heterocumulenes

X-Ray crystal analysis of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2λ5]oxaphosphole [and the triphenylphosphonium ylide] 1a and its parent compound 1b has been carried out. The 31P and 13C NMR spectral studies of 1a,b and their derivatives 1c,d, and the correlation of their chemical shifts with P1–O1 bond lengths obtained by X-ray analyses for 1a–d were investigated to clarify that compounds 1a–d exist as resonance hybrids of a P–O bonding oxaphosphole structure (structure A) and a phosphonium ylide structure (structures B and C). The contribution of the structure A decreases gradually in the order of 1a > 1c > 1b > 1d in the solid state. On the basis of a linear correlation between P1–O1 bond lengths and θsum (the sum of bond angles between the equatorial bonds), it is clarified that an increase of the P1–O1 bonding character causes change in the configuration of the phosphorus atom from a tetrahedral to a trigonal bipyramidal arrangement. In connection with these studies, inspection of the structure of a related compound, 2,2,2-triphenyl-6,11-methano-2H-cycloundeca[d][1,2λ5]oxaphosphole structure 2, was also carried out. With a view to clarifying the reactivity of 1a in connection with a contribution of canonical structures A, B, and C, compound 1a was allowed to react with phenyl isocyanate, diphenylcarbodiimide, and phenyl isothiocyanate to give heteroazulenes. The reaction pathways depending on the contribution of canonical structures A, B, and C are discussed.

Chemical Bonding in Hypervalent Molecules Revised. 3. Application of the Atoms in Molecules Theory to Y3X−CH2(X = N, P, or As; Y = H or F) and H2X−CH2(X = O, S, or Se) Ylides

The electron pair density in conjunction with the AIM theory and calculated NMR chemical shifts were used to characterize the bonding properties for nine pnicogen and chalcogen ylide structures. The hybrid B3LYP and MP2 methods were employed with the 6-311+G* basis set. No evidence was found to support a banana (Ω) bonding scheme. Instead, different bonding schemes were found to be dependent on the electronegativity of the X atom in the C−X bond. When X is a highly electronegative atom (N,O), the C−X bond is weaker than a single bond, due to electrostatic repulsion. When the X atom has electronegativity similar to carbon, a covalent, yet significantly polar interaction results, and its strength is determined mainly by electrostatic interactions, with a small contribution of negative hyperconjugation.

Are RR′C–PR′′(BH3)2‘electron poor’ phosphorus ylides?— an ab initio–NMR study

The parent RR′C–PR′′(BH3)2, 3 (R, R′, R′′ = H), has no minimum geometry with ylide structure. In contrast to ‘normal’ ylides, RR′C–PR′′3, which are destabilised when the substituents (R, R′) have π donor character, the investigated RR′C–PR′′(BH3)2 require at least one amino group to form a stable ylidic structure (e.g.4NH with R = NH2, R′ = H, and R′′ = H). Without π donor substituents the molecules lack a tautomerisation barrier for the hydroborylation (RR′CPH(BH3)–BH2H′→(RR′H′C–PH(BH3)BH2). Compound 6, cyclo-1-(C–PH(BH3)2)-2,5-(NH)2-3,4-(CH)2, is a model for the ylide recently obtained by Arduengo. Analysis of the electronic structure confirms his suggestion that a considerable delocalisation from the carbene moiety into the ‘electron poor’ PR′′(BH3)2 group occurs. This electron delocalisation is reflected by the partial charge of the ylide group (−0.50 in 4HN, −0.51 in 4NN, and −0.85 in 6) which is negative (positive in ‘normal’ ylides). The term ‘inverse ylides’ could express this special bonding situation.

The first hafnium methandiide complexes: the assembly of an entire triad of group 4 metal ‘pincer’ bis(phosphinimine) complexes possessing the MC carbene–ylide structure

Dichlorobis[bis(trimethylsilylamido)]hafnium(IV) reacts with (R2PNSiMe3)2CH2 (R = Cy, Ph) cleaving the C–H bonds of the P–C–P backbone methylene group and eliminating 2 mol of hexamethyldisilazane to yield the hafnium methandiide (carbene) complexes 3 and 4, thus completing the triad of group 4 metal complexes containing the MC moiety.

Formation of Nitrile Ylide by Addition of Carbene with Acetonitrile in a Low-Temperature Argon Matrix

After the photoirradiation of (biphenyl-4-yl)chlorodiazirine (BCD) in ca. 10 K argon (Ar) matrices, the IR spectrum of (biphenyl-4-yl)chlorocarbene (BCC) was measured because the measured bands were in fair agreement with the bands obtained by vibration analysis. The photolyses of BCD−acetonitrile (MeCN)−Ar tricomponent matrices were also carried out to study the reaction of BCC with MeCN. For [MeCN]/ [BCD] > 10, a spectrum that differed from BCC was obtained after the irradiation. The new bands are stable during the irradiation, during increasing temperature (to 45 K), and during storage. An 2-azonia-1-allenide type nitrile ylide (NY) structure was obtained in an optimization. Because the measured bands are in good agreement with those calculated using the vibration analysis for this NY structure, BCC adducts with MeCN immediately after the degradation of BCD.

Application of Phosphonium Ylides to Latent Catalysts for Polyaddition of Bisphenol A Diglycidyl Ether with Bisphenol A: Model System of Epoxy-Novolac Resin

Phosphonium ylides Ph 3 PC(H)COR (1 H : R = H; 1 Me : Me; 1 Ph : Ph; 1 p-C6H4OMe : p-C 6 H 4 -OMe; 1 p-C6H4Me : p-C 6 H 4 Me; 1 p-C6H4NO2 : p-C 6 H 4 NO 2 ; 1 OMe : OMe) were prepared by the reaction of triphenylphosphine with the corresponding alkyl halides followed by treatment with a base and applied as novel thermally latent catalysts in the polyaddition of bisphenol A diglycidyl ether with bisphenol A-a model system of epoxy-novolac resins. The phosphonium ylides (1) served as non-salt-type thermally latent catalysts. The relationship between the structure and activity was discussed on the basis of spectroscopic analysis and molecular orbital (MO) calculations. The NMR and IR studies on 1 confirmed that the catalytic activity increased with the electron-accepting character of the acyl group. The MO calculations suggested that a catalyst closer to an ylide structure would be more active, compared with an ylene structure. The catalytic mechanism was also discussed.

Reactions of Alkylidene-Bridged Cobaltadithiolene Complexes with Lewis Bases. Formation of M−S Sulfur Ylide Structure

The reactions of a malonate (C(COOMe)2)-bridged complex with Lewis bases give novel sulfur ylide (M−S sulfur ylide) complex due to the cleavage of Co−C bond as determined by X-ray sturucture analysis. Acetate (CHCOOEt)- and methylene-bridged complexes, on the other hand, reacted with Lewis bases via an SN2 type reaction to give unstable M−S sulfur ylide complexes and stable six-membered-ring complexes.

A theoretical ab initio and Monte Carlo simulation study of the pyridine+CCl2 reaction kinetics in the gas phase and in carbon tetrachloride solution using canonical flexible transition state theory

The potential energy surface for the pyridine+CCl2 reaction was studied at the abinitio MP4/6-311G(2df,p)//MP2/6-31G(*) level of theory. The MP4/6-311G(2df,p) energies were evaluated by the additivity approximation E[MP4/6-311G(2df,p)]≈E[MP4/6-31G(*)]+E[MP2/6-311G(2df,p)]-E[MP2/6-31G(*)]. The first step proceeds by the addition of CCl2 to pyridine forming a dipolar ylide structure without an activation barrier. Then this species rearranges to a more stable biradical like ylide on a picosecond time scale. The generalized transition state for dipolar ylide formation occurs at a large center of mass distance between the species, and to calculate the reaction rate constant we have used canonical flexible transition state theory. The configurational integral was solved by Monte Carlo simulation and statistical perturbation theory, and the potential of mean force in the gas phase was obtained. This procedure was extended to the liquid phase by including the solvent coordinates in the configurational integral. The activation free energy in the gas phase and in carbon tetrachloride solution was calculated as 1.44 and 2.62 kcal mol-1, respectively. The corresponding rate constants are 5.5×1011 and 7.5×1010 l mol-1 s-1. The last value is in reasonable agreement with the experimental result of 7×109 l mol-1 s-1 determined in isooctane solution.

Deprotonation of 1-(carbethoxyalkyl)pyridinium halides with strong N-bases

The rate constants were measured for deprotonation of 1-(carbethoxymethyl)pyridinium chloride and 1-(2-carbethoxyethyl)pyridinium bromide with strong bases (DBU, MTBD, TBD and P2-Et) in acetonitrile. The UV spectra and semiempirical calculations are consistent with an ylide structure of the deprotonated species. The ylides obtained slowly decompose, and the reaction products were identified by 1H NMR spectroscopy; 1-(carbethoxymethyl)pyridinium chloride gives N-methylpyridinium cation and ethanol and 1-(2-carbethoxyethyl)pyridinium bromide converts to pyridine and ethyl acrylate. Copyright © 1999 John Wiley & Sons, Ltd.

Unusually direct synthesis of mono- and di-substituted tetrathiafulvalenes

Direct reaction of norbornadiene, tributylphosphine, carbon disulfide and acetylenic diesters affords the norbornene-fused dihydrotetrathiafulvalenes 7 in addition to the bis-adducts 4 and for less reactive acetylenic dipolarophiles only 7 is isolated. Upon flash vacuum pyrolysis the compounds 7 undergo a retro Diels-Alder reaction to give substituted tetrathiafulvalenes 8. By using the norbornadiene diester in place of norbornadiene with DMAD, 8a can be obtained in higher overall yield. Attempted isolation of the zwitterionic structure 9 instead gave a rearrangement product with the novel 1,2,5-trithiepane stabilised ylide structure 11.

Ether Oxides: A New Class of Stable Ylides? A Theoretical Study of Methanol Oxide and Dimethyl Ether Oxide

The potential energy surfaces of neutral methanol oxide and dimethyl ether oxide and their anion and cation radicals have been calculated at the Becke3LYP/6-311++G(d,p) level of theory. Both neutral singlet methanol and dimethyl ether oxides are predicted to correspond to local minima on their potential energy surfaces. Natural bonding orbital (NBO) population analysis reveals a distinct ylidic character for these species. Upon increasing methyl substitution, the R2O+O- ylide structure is stabilized energetically because of the better charge distribution of the formally positive central oxygen atom; thus, the energy differences relative to the R2O + 3O exit channel decrease significantly. The barrier for the 1,2-hydrogen migration in methanol oxide to yield methyl hydroperoxide amounts to only 5 kcal/mol, whereas the methyl shift in dimethyl ether oxide to afford dimethyl peroxide demands >40 kcal/mol and can proceed by retention or inversion of the configuration at the migrating carbon. The kinetic stabilization of the latter is instead determined either by the loss of a methyl radical or by spin crossing to the triplet surface followed by O atom loss. For this process, the minimal-energy crossing point of the two neutral surfaces was located. The corresponding cation radicals of methanol and dimethyl ether oxide rest in rather deep wells, and their geometries are not too different from those of the neutrals. Therefore, neutralization-reionization mass spectrometry may allow generation and identification of the neutral species, provided that the cation-radical precursors can be made. Furthermore, the kinetic stability of neutral dimethyl ether oxide may be sufficient for its detection in matrix isolation experiments.

Donor Complexes of Silylene, Germylene, and Stannylene

AbstractThe complexes of various donor molecules, AH3 (A = N, P, As, Sb, Bi) and AH2 (A = 0, S, Se, Te) with silylene, germylene, and stannylene (1Al ground state) were studied by means of quantum chemical investigations at ab initio level utilizing all valence electron basis sets and relativistic corrected effective core potential methods. Accordingly, the donor molecules are weakly bound, the association energies are in the range of 15–30 kcal per mole. The exothermic energies for adduct formation, i.e., the resulting binding energies for the donor complexes, decrease in the order silylene > germylene > stannylene. The population analysis indicates for NH3 and BiH3 only a weak bonding towards the XH2 fragment (X =Si, Ge, Sn) while the higher homologues (A = P, As, Sb) form ylide structures, 1.2‐dipolaric in nature. The addition of two donor molecules has been studied for silylene and is much less favourable than donor mono‐addition. For the donor mono‐addition a dual parameter relationship between (a) the HOMO energies of the donor (n‐orbital of the AH3 unit, n, p‐orbitals for AH2) and (b) the covalent bond energies (from the literature) versus the binding energies of the donor‐acceptor compounds was examined. This dual parameter equation describes satisfactorily the essential features of the stabilities of the donor‐acceptor structures.

Reaction of CCl2 with CH2NH and the formation of dipolar and biradical ylide structures

The potential energy surface for the reaction between CH2NH and CCl2 has been investigated using ab initio methods. We have performed geometry optimizations at the MP2/6-31G* level of theory and single point calculations at the MP4(SDQ)/6-311++G** level. The reaction step for ylide formation has a free energy of activation predicted to be 5.0 kcal mol–1. The parallel 1,2-cycloaddition reaction has a calculated free energy barrier of 16.5 kcal mol–1, indicating that this second pathway is not competitive with ylide formation. The structure of the azomethine ylide formed in the first reaction step is similar to that found for the ylide resulting from the reaction of methylene with ammonia and corresponds to a dipolar species. This is highly unstable and rearranges to its more stable isomer, the biradical azomethine ylide, which has a structure similar to the corresponding carbonyl ylide. This species has a free energy barrier to ring closure calculated to be 21.2 kcal mol–1, so it has reasonable kinetic stability. The resulting aziridine has a free energy of 24.1 kcal mol–1 lower than the biradical azomethine ylide, and the activation free energy of ring opening is calculated to be 45.3 kcal mol–1.

Structure and properties of phosphonium ylides-betaines, derivatives of 2-phenyl-2-oxazolin-5-one and its thio- and seleno-analogues

The combined chemical, spectral, and X-ray structural investigation has shown that the structure and properties of mesomeric triphenylphosphonium ylides containing the 5-oxo-2-phenyl-4-oxazolidene moiety undergo considerable changes when the carbonyl oxygen atom is replaced by sulfur or selenium. This substitution leads to a clear decrease of the contribution of the ylide structure and an increased importance of the betaine structure, in contradiction with the electronegativity decrease in the O>S>Se series, but relates to the different ability of double bond formation of these elements. The deactivation of the ylide site by the thio- or selenocarbonyl group has been applied to regioselective syntheses.

ChemInform Abstract: The “Phosphonioyl(phosphorandiyl)carbene” ((iPr2N)P(H)CP(NiPr2)2)+ as a Source of Novel Diphosphaallenes with Ylide Structure: The First Carbodiphosphoranes with H‐Substituted Phosphorus.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

C-spirophosphorane-substituted phosphorus ylides

The methylenephosphinophosphorane 2 was methylated by MeI forming the phosphonium salt 4, which is stable in crystalline form but rearranges into isomer 10 when heated in chloroform solution at 80°C in a sealed tube for 16 h. The isomerization occurs by elimination of N,N′-dimethylcarbodiimide and formation of the methylenephosphonium phosphonate 5. Compound 5 reacts with the carbodiimide via its ylide from 5a to give the ylide 9, which is transformed reversibly into 10. Compound 2 shows unusual properties, such as its reaction with hexafluoroacetone via its ylide form 2a forming the addition product 12. The ylide structure of 12 was confirmed by X-ray analysis. The coordination geometry at P(2) is distorted trigonal bipyramidal, with axial substituents O (of the chelate ring) and N (of the four-membered ring). The bond lengths and angles at the ylidic carbon confirm the partial multiple bond character of the PC bonds. There is evidence for a CH···O hydrogen bond from the carbon atom bearing the CF 3 groups. The ylide 13 was obtained by the elimination of HI from 4 with triethylamine. Compound 2 reacted with CCl 4 to give the P-chloroylide 15, as well as a small amount of 16, which contains two phosphorus atoms of opposite formal charge and different coordination number (λ 4P +, λ 6P −). Compound 16 is isomeric to 3, which we obtained earlier and investigated by X-ray analysis.

Synthesis of N-fluorodinitromethylpyridinium salts as first examples of hitherto unknown class of compounds

Pyridinium salts bearing strong electron-withdrawing groups at positively charged cyclic nitrogen are of great interest because of their incredible synthetic possibilities. Thus the chemistry of N-fluoropyridinium salts is being intensively developed now. Even more interesting in a theoretical aspect are N-fluorodinitromethylpyridinium salts, for fluorodinitromethyl group is close to fluorine atom in its −I effect but in contrast to the latter does not possess +M effect. The synthesis of first representatives of previously unknown class of compounds, N-fluorodinitromethylpyridinium salts (I), was carried out by fluorination of pyridine dinitromethylide with cesium fluoroxysulfate (CEFOX). This reaction is the first example of successful fluorination of ylide structures with CEFOX ▪ As expected, salts of this type are of exceptional activity towards nucleophiles. Thus, reactions of I even with low reactive carbanions proceed practically instantly with introduction of C-nucleophiles at position 4. At present we study the chemical transformations of I as the most interesting examples of extremely activated pyridinium salts.

Synthesis and Structure of a 1λ5,4λ5‐Diphosphapentalene

The diphosphapentalene 3 melts at 194°C without decomposition. Compound 3 is formed in 71% yield by reaction of two equivalents of dimethyl acetylenedicarboxylate (1) with 2. The surprising stability of 3 is due to ylide structures.

First isolation of monocyclic thiabenzenes

Monocyclic thiabenzenes, 1-alkyl-2-aroyl- (or 1-alkyl-2-cyano-) 4,5-dimethylthiabenzenes ( 6 ) are successfully synthesized by deprotonation of the corresponding thiopyranium salts ( 5 ) with triethylamine in ethanol. The ylide structures of 6 are elucidated by spectral and chemical evidence. Thermal reaction of 6 in several solvents provides S-alkyl rearranged products and ring-contracted ones, depending upon solvents used.

Epoxide synthesis under interfacial solid-liquid conditions

In this paper an analysis of the factors responsible for discrimination among differing structures of organic reagents by the microcrystalline structure of solids is presented. As a vehicle for this study, the epoxide synthesis using three sulfur ylide precursors (sulfonium and sulfoxonium salts) and four microcrystalline solids was investigated. Neither the surface area nor the number of active sites at the solid surface control the epoxide yield. It appears that the cell lattice of the solid determines the ylide structure, its nucleophilicity, and thus the reaction yield