Phosphonium Ylides Research Articles

Palladium(ii) pincer complexes of a C,C,C-NHC, diphosphonium bis(ylide) ligand.

A new family of pincer palladium(ii) complexes bearing an electron-rich C,C,C-NHC, diphosphonium bis(ylide) ligand of LX2-type was prepared through the dual N-functionalization of 1H-imidazole by (3-bromopropyl)triphenylphosphonium bromide. Selected basic conditions allowed the sequential coordination of the NHC and phosphonium ylide moieties to Pd(ii). This strategy led to an original ortho-metallated complex where the Pd center is bonded to four carbon atoms of three different natures: carbenic (sp2), arylic (sp2), and chiral ylidic (sp3). Protonation of the latter afforded NHC, diphosphonium bis(ylide) pincer Pd(ii) complexes as a mixture of meso- and dl-diastereomers (de = 50%). The selectivity of C-coordination was rationalized on the basis of DFT calculations, evidencing the quasi-degeneracy of the two diastereomeric forms.

Heteroatom effects toward isomerization of intermediates in Wittig reactions of non‐stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton with benzaldehyde

AbstractIsomerization of intermediates, cis‐ and trans‐1,2‐oxaphosphetanes, in Wittig reactions of non‐stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton containing group 14 (PhSi, PhGe, PhSn, n‐BuSn) and 15 (P, As, Sb, and Bi) elements with benzaldehyde (PhCHO) was investigated by variable‐temperature (VT)31P{1H} NMR spectroscopy. The isomerization from the cis‐1,2‐oxaphosphetane to the trans‐form occurred at lower temperatures as the row number of the same group elements increases. Wittig reactions under the same conditions gave the (Z)‐olefin as a major product in the cases of period 3 elements (PhSi and P) and the (E)‐olefin as a major product in the cases of elements from period 4 and below (PhGe, PhSn, n‐BuSn, As, Sb, and Bi). The selectivity of olefin formation is considered to depend on the isomerization temperature of the intermediates, because each olefin must be obtained from the corresponding 1,2‐oxaphosphetane. The VT‐31P{1H} NMR spectra showed that the cis‐1,2‐oxaphosphetanes were the kinetic products in the first step of Wittig reactions and the trans‐forms were the thermodynamically stable products formed by isomerization from the cis‐forms via ring‐opening and ring‐closing reactions of phosphonium ylides with PhCHO. Density functional theory (DFT) calculations indicated that cis‐1,2‐oxaphosphetanes were less stable than the trans‐forms by ~2 kcal/mol, supporting thermodynamically favorable isomerization from cis‐forms to trans‐forms, as observed by VT‐31P{1H} NMR spectroscopy. Heteroatoms at the bridgehead position of the phosphaheteratriptycene skeleton significantly affected the isomerization temperature as well as the phosphorus‐31 signals in the 31P{1H} NMR spectra, which were observed at lower field as row number of the same group element increases.

Alkene Synthesis Using Phosphonium Ylides as Umpolung Reagents

AbstractWe report a transformation between a well‐known nucleophile, phosphonium ylide, and an electrophile. Subsequent ylide‐ylide coupling achieves unusual (Z)‐selectivity as compared to the conventional Wittig olefination. The synthesis of medium‐sized ring olefins has also been demonstrated, together with a one‐pot olefination from activated halides.

Chemistry of Phosphorus Ylides: Part 46—Efficient Synthesis and Biological Evaluation of New Phosphorus, Sulfur, and Selenium Pyrazole Derivatives

1,5‐Dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazole‐4‐carbaldehyde was allowed to react with different phosphorus, sulfur, and selenium reagents. The reaction products depend on the nature of the reagent and the condition of the reaction used. Treatment of the carbaldehyde with the active phosphacumulene ylides afforded the phosphoranylidene pyranopyrazoles. On the other hand, its reaction with the stable phosphonium ylides gave the oxaphosphetanes. Sulfidodithiaphosphetane pyrazole was generated from the reaction of Japanese reagent with the carbaldehyde. Selenido‐oxaselenaphosphetane and dioxaphospholane pyrazoles were obtained from the reaction of the carbaldehyde with Woolin's reagent and phosphorus triamide, respectively. The antimicrobial screening of the synthesized compounds was also evaluated.

Wittig reactions of non-stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton containing group 14 and 15 elements with benzaldehyde

Wittig reactions of non-stabilized phosphonium ylides bearing a phosphaheteratriptycene skeleton containing Group 14 and 15 elements (PhSi, PhGe, PhSn, n-BuSn, P, As, Sb, and Bi) at another bridgehead position with benzaldehyde provided (Z)-olefins as a major product in the cases of period 3 elements (PhSi, P) and (E)-olefins as a major product in the cases of below period 4 elements (PhGe, PhSn, n-BuSn, As, Sb, and Bi). These results are attributed to stereochemical drift of the intermediates come from the heteroatom effect at another bridgehead position of the phosphaheteratriptycene skeleton.

Further Studies on the Pyrolytic Domino Cyclization of Stabilized Phosphonium Ylides Bearing an Ortho-Aminophenyl Group.

Four new, stabilized phosphonium ylides containing a 2-(benzyl(methyl)amino)phenyl group have been prepared and characterized and are found, upon pyrolysis under gas-phase flow conditions, to lose Ph3PO and benzyl radicals to afford new heterocyclic products resulting from domino cyclization of both C- and N-centered radicals. Most products arise from processes of the former type and have quinoline, phenanthridine, or ring-fused phenanthridine structures, while in one case, a process of the latter type leads to a benzocarbazole product. The X-ray structure of a 2-(methyl(tosyl)amino)phenyl ylide is also reported.

Flash vacuum pyrolysis of oxo-stabilised phosphonium ylides containing methoxythiophene and methylthiophene groups

Ten new oxo-stabilised phosphonium ylides containing substituted thiophene groups, as well as two deuterium-labelled analogues, are prepared and fully characterised. Upon flash vacuum pyrolysis at 800 °C, the simpler examples undergo extrusion of Ph3PO coupled with domino cyclisation to give thieno[3,2-b]furan and thieno[3,4-b]furan products but the corresponding approach to a thieno[2,3-b]furan fails. One ylide designed to give a thieno[3,4-b]furan instead gives phenylbutadiyne and 2-phenyl-3-vinylthiophene at 725 °C and the mechanism of this unusual process is elucidated by deuterium labelling. Attempts to access more complex heterocyclic products from extended methoxythienyl ylides failed. Two ylides bearing a 3-methyl-2-thienylacryloyl group gave 5-benzylbenzothiophene, 5-(α-methylbenzyl)benzothiophene and fluoreno[3,4-b]thiophene upon FVP at 800 °C and the mechanism was again elucidated by deuterium labelling.

Binuclear mercury (II) complexes of ambidentate sulfonium ylides in comparison with analogous phosphonium-based complexes; synthesis, characterization and a comprehensive theoretical study

The dehydrobromination reaction of a new sulfonium salt [(Me)2SCH2(O)C10H7]Br (1) with sodium hydroxide solution led to formation of sulfonium ylide (Me)2SCHC(O)C10H7 (2) which was used to synthesize three binuclear complexes [{(Me)2SCHC(O)C10H7}2HgX2]2 (X = Cl (3), Br (4) and I (5)). Characterization of all compounds was thoroughly performed by elemental analysis, IR, 1H and 13C NMR spectroscopic methods. Also the structure of complex (4) was characterized by X-ray crystalography methode. The structure of prepared complexes was also compared with analogous phosphonium complexes [{Ph3PCHC(O)C10H7}2HgX2]2 (X = Cl, Br and I) using DFT calculations with the M06/def2-SVP level of theory. Based on the theoretical studies of geometry-optimized bidentate structures, the interaction energy values, ΔEint, for C→M bonds in [{(Me)2SCHCOC10H7}HgX2]2 complexes are larger than those in [{Ph3PCHCOC10H7}HgX2]2 (X = Cl, Br and I) complexes. Obviously, in comparison to the phosphonium ylide, the sulfonium ylide forms a stronger bond with Hg2X4 (X = Cl, Br and I) fragments in aforementioned complexes.

Photochemical Synthesis and Electronic Properties of Extended Corannulenes with Variable Fluorination Pattern.

The first family of extended and fluorinated corannulenes is prepared through a highly efficient and modular synthetic strategy. In this strategy, corannulene aldehyde could be combined with the fluorine-carrying phosphonium ylides to furnish stilbene-like vinylene precursors. A photochemically induced oxidative cyclization process of these precursors gives rise to the fluorinated and curved polycyclic aromatic hydrocarbons. A UV-vis absorption study shows that aromatic extension results in a bathochromic shift of about 12 nm. Fluorination further shifts the absorption spectrum to the red region, and a maximum shift of about 22 nm is detected for a compound carrying two trifluoromethyl groups. A cyclic and square-wave voltammetry investigation reveals that the extension of the corannulene scaffold increases the reduction potential by 0.11 V. Placement of fluorine or trifluoromethyl groups further enhances the electron affinities. In this regard, the presence of one trifluoromethyl group equals the effect of three aromatic fluorine atoms. Molecules with two trifluoromethyl groups, meanwhile, exhibit the highest reduction potentials of -1.93 and -1.83 V. These values are 0.37 and 0.46 V higher than those of the parental corannulene and demonstrate the utility of the present design concept by efficiently accessing effective electron acceptors based on the buckybowl motif.

Cyclic (Amino)(Phosphonium Bora-Ylide)Silanone: A Remarkable Room-Temperature-Persistent Silanone.

A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium bora-ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t1/2 =4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t1/2 =5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr2 increases further its stability (no degradation after 3 weeks at room temperature).

Cyclic (Amino)(Phosphonium Bora‐Ylide)Silanone: A Remarkable Room‐Temperature‐Persistent Silanone

AbstractA silanone substituted by bulky amino and phosphonium bora‐ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron‐donating phosphonium bora‐ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t1/2=4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t1/2=5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr2 increases further its stability (no degradation after 3 weeks at room temperature).

Synthesis and reactivity of [M(η3-allyl)(η2-amidinato)(CO)2(phosphonium ylide)] (M = Mo, W): Investigation of the ligand properties of phosphonium ylides

Abstract Phosphonium ylide complexes of Mo and W formulated as [M(η3-allyl){η2-(NPh)2CH}(CO)2(CH2PR3)] (M = Mo, R = Me: 2a-Mo; M = Mo, R = Ph: 2b-Mo, and M = W, R = Me: 2a-W) were prepared by the reaction of amidinato(pyridine) complex, [M(η3-allyl){η2-(NPh)2CH}(CO)2(NC5H5)] (M = Mo: 1-Mo and M = W: 1-W), with a phosphonium ylide, CH2PR3 (R = Me, Ph), which was generated in situ by the reaction of the corresponding phosphonium salt with nBuLi. These complexes were characterized spectroscopically, as well as by the X-ray diffraction. The phosphonium ylide ligand shows stronger electron donating ability toward the metal than N-heterocyclic carbene or phosphine ligands. This trend is supported by the comparison of the spectroscopic data and the DFT calculations. We also investigated the reactivity of the phosphonium ylide complexes 2-Mo with two-electron donors such as PEt3 and NHC. In the case of the PPh3 ylide complex (2b-Mo), the substitution reaction of the ylide ligand for the two-electron donors took place cleanly to yield the corresponding complexes. On the other hand, in the PMe3 ylide complex (2a-Mo), the substituted complexes formed but the unreacted ylide complex 2a-Mo was also present in the reaction mixture. These results show that the bond strength of the M-C(phosphonium ylide) bond is affected by the substituents on the phosphorus atom.

Versatile Pyrolytic Synthesis of Fused Polycyclic Heteroaromatic Compounds

Thirty-six new stabilised phosphonium ylides designed to undergo thermal loss of Ph3PO and radical domino cyclisation have been prepared and are generally found to undergo the desired reaction under flash vacuum pyrolysis conditions at 850 °C. A wide range of ­tetra- and pentacyclic fused-ring heterocycles, many previously unknown, are thus formed in moderate to high yield in a single step. By using suitably substituted starting materials, substituents such as CH3 and Cl can be installed at various positions in the products. The method has also been demonstrated in a combinatorial mode to generate a small library of twelve fused-ring heterocycles in a single pyrolysis.

Gas-phase domino cyclization of phosphonium ylides leading to the total synthesis of Eustifoline D

Six stabilised phosphonium ylides bearing ortho-benzylaminophenyl and cinnamoyl (or a heterocyclic analogue) groups have been prepared and upon flash vacuum pyrolysis at 800°C were found to undergo cascade cyclization processes to give mainly 3-styrylquinolines but also in some cases ring-fused carbazoles and other fused-ring heterocyclic products. By starting with an appropriate ring-methylated precursor the natural product Eustifoline D was obtained in 19% yield in the pyrolysis in addition to the 3-(2-furylethenyl)quinoline (46%).

The value of 2JP–CO as a diagnostic parameter for the structure and thermal reactivity of carbonyl-stabilised phosphonium ylides

A survey of 20 carbonyl-stabilised phosphonium ylides with recently reported X-ray structures shows a strong correlation between the CP to CO torsion angle and the value of 2JP–CO, with high values being associated with an anti configuration and low with syn. Seven new X-ray structural determinations are reported, several for types of ylide not crystallographically characterised before, and these also conform to this pattern. The value of 2JP–CO is then correlated with whether or not thermal extrusion of Ph3PO occurs to give alkynes for over 200 ylides and an empirical rule developed that the extrusion never occurs for ylides where this value is > 11 Hz. This is used to rationalise the anomalous behaviour of some trioxo ylides and cyclic ylides, two of which afford cycloalkynes, isolated after rearrangement as the isomeric 1,3-dienes. The rule also holds for a family of novel highly fluorinated ylides which afford fluorinated alkynes in good yield upon flash vacuum pyrolysis.

Chemistry of Phosphorus Ylides: Part 45 Synthesis of Phosphoranylidene Thietane, Azetidine and Thiazinane Derivatives as Potent Chemo Preventative Agents

Reactions of nucleophilic active phosphacumulenes with iso(thio)cyanate compounds were performed. The reaction products depend on the nature of the reagent, substrate and the condition of the reaction used. New heterocyclic 4-membered or 6-membered sulfur and nitrogen compounds such as phosphoranylidene thietane, azetidine and thiazinane were obtained. On the other hand, the stable phosphonium ylides with the iso(thio)cyanate afforded phosphoranylidene thiocarbamoyl derivatives. Possible reaction mechanisms are considered and the structural assignments are based on compatible analytical and spectroscopic data.

New reactions and reactive intermediates in the pyrolysis of cyclic phosphonium ylides

The authors are grateful to EPSRC and Glaxo-Wellcome (now GSK) for their support of this work through a CASE Studentship to TM.

Electronic and spectral properties of phosphonium ylides-betaines, derivatives of 2 oxazoline-5-one with conjugated and non-conjugated substituents

The spectral and quantum-chemical study of phosphonium ylides derivatives of 2‑oxazoline-5-one with both conjugated and non-conjugated substituents were performed. It was shown that considerable positive charge is located at phosphorus atom, whereas the substantial negative charge is fixed at sulfur atom. It has been found from the calculations and 13 C NMR spectral data that introducing of the non-conjugated and conjugated substituents in the position 2 of the oxazole cycle in thiaphosphonium ylides causes only small change in the molecular equilibrium geometry and in charge distribution in oxazole moiety, whereas spectral characteristics of substituted derivatives are very sensitive to the nature of the lowest electron transitions which reflects in changes of their absorption maxima.

A Phosphonium Ylide as an Ionic Nucleophilic Catalyst for Primary Hydroxyl Group Selective Acylation of Diols

Carbonyl-stabilized phosphonium ylides exhibit great utility in modern organic synthesis, and they are known as an ambident nucleophile at the carbonyl oxygen atom and at the α-carbon atom. However, they have found limited use as catalysts. We focused on the inherent nucleophilicity of the oxygen atom to develop an ionic nucleophilic catalysis, and the phosphonium ylide-catalyzed primary alcohol selective acylation of mixed diols with acid anhydrides has been demonstrated. Mechanistic studies revealed the behavior of a catalyst, which would contribute to the field of ylide chemistry.

A Fairly Stable Crystalline Silanone

AbstractSilanones 2 substituted by bulky amino‐ and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron‐donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2=0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2Si=O owing to the strong π‐electron donation from the phosphonium ylide substituent.