Ylide

An ylide is a neutral zwitterionic compound featuring a carbanion directly adjacent to a positively charged heteroatom, most commonly phosphorus, sulfur, or nitrogen, with the overall molecule bearing no net charge.^[1] These 1,2-dipolar species are typically represented with a polarized bond between the carbanionic carbon and the onium center, such as in phosphonium ylides of the form R₃P⁺–CR₂⁻.^[2] Ylides are generated by deprotonation of the corresponding onium salts using strong bases, which activates the alpha-carbon for nucleophilic reactivity.^[1] In organic synthesis, they play a pivotal role as versatile reagents for constructing carbon-carbon and carbon-heteroatom bonds; for instance, phosphorus ylides are central to the Wittig reaction, which converts carbonyl compounds into alkenes with controllable stereochemistry.^[2] Sulfur ylides, such as sulfonium and sulfoxonium variants, facilitate the Corey–Chaykovsky reaction to form epoxides, aziridines, and cyclopropanes from aldehydes, ketones, or electron-deficient alkenes.^[3] Azomethine ylides act as 1,3-dipoles in cycloaddition reactions, enabling the synthesis of pyrrolidines and other heterocycles.^[4] Beyond classical applications, ylides have evolved into ligands in transition metal catalysis due to their strong σ-donor and π-acceptor properties, influencing reactivity in cross-coupling and C–H activation processes.^[5] Their tunable electronic and steric features, modulated by substituents on the heteroatom or carbanion, allow for asymmetric synthesis and stabilization of reactive intermediates like carbenes or low-valent main group species.^[2] The term "ylide" was introduced by Georg Wittig in the mid-20th century to describe these unique dipolar entities, reflecting their hybrid ionic-covalent bonding character.^[6]

Fundamentals

Definition and Nomenclature

Ylides are neutral dipolar molecules characterized by a formally positively charged heteroatom, typically from the p-block elements such as nitrogen, phosphorus, or sulfur, directly adjacent to an anionic site that is usually a carbanion but can involve other atoms.^[7] This 1,2-dipolar arrangement results in a zwitterionic structure, often denoted in charge-separated form as X⁺–Y⁻, where X is the heteroatom and Y is the anionic center, though they are frequently represented with a double bond (ylide or ylene form, X=Y) to reflect partial double-bond character due to resonance.^[7] The generic formula for common ylides features the onium center with substituents, as in R₃P⁺–CH₂⁻ for phosphonium ylides or R₂S⁺–CH₂⁻ for sulfonium ylides, where R represents alkyl or aryl groups.^[7] These structures maintain formal charges while exhibiting significant covalent bonding between the adjacent atoms, distinguishing them from simple ionic compounds. In nomenclature, the term "ylide" functions as a complete descriptor rather than a suffix, with subclasses identified by the central heteroatom, such as phosphorus ylides or sulfur ylides.^[7] According to IUPAC recommendations, phosphorus ylides like the Wittig reagent are named substitutively, for example, methylidenetriphenyl-λ⁵-phosphane for Ph₃P=CH₂, though the retained name methylenetriphenylphosphorane is widely accepted. Ylides are differentiated from betaines, which have separated charges, and from general zwitterions by their specific 1,2-adjacency and reactivity; for onium ylides, the positive charge is emphasized in names like ammonium ylides.^[7] Ylides are subclassified based on the anionic site: those with a carbanion (carbyllides, e.g., R₃P⁺–CR₂⁻) predominate in synthetic applications, while heteroatom-centered variants include oxonium ylides (R₂O⁺–CR₂⁻).^[7] This distinction arises from the heteroatom's ability to stabilize the adjacent negative charge, influencing their chemical behavior.

Historical Development

The earliest phosphonium ylides were prepared in 1894 by August Michaelis and Gimborn via dehydrohalogenation of corresponding phosphonium salts, though their zwitterionic nature was not fully recognized at the time.^[8] A notable early example, triphenylphosphinemethylene (Ph₃P=CH₂), was reported by Hermann Staudinger and J. Meyer in 1910 through deprotonation of the corresponding phosphonium salt.^[9] This compound, though unstable and initially viewed as a curiosity, represented a pioneering example of a carbon-phosphorus ylide, setting the stage for later developments in organophosphorus chemistry. Staudinger's work highlighted the zwitterionic nature of such species, where the negative charge on carbon is adjacent to the positively charged phosphorus. In the late 1940s, Georg Wittig systematically explored phosphonium salts as an extension of his research on nitrogen ylides, leading to the discovery of stable phosphonium ylides.^[6] This work culminated in the 1953 publication of the seminal olefination reaction, demonstrating how phosphonium ylides react with carbonyls to form alkenes and triphenylphosphine oxide, a transformation that revolutionized alkene synthesis. For this contribution, Wittig shared the 1979 Nobel Prize in Chemistry with Herbert C. Brown.^[6] Sulfur ylides followed soon after, with Christopher Ingold and J. A. Jessop identifying the first sulfonium ylide in 1930 through the isolation of a stable dimethylsulfonium fluorenylide. Advancements in the 1960s, particularly by A. W. Johnson, expanded their utility, including the development of nonstabilized sulfonium ylides for cyclopropanation and epoxidation reactions. Concurrently, nitrogen ylides, such as azomethine ylides, were reported in the early 1960s by Rolf Huisgen, enabling 1,3-dipolar cycloadditions for pyrrolidine synthesis. The Corey-Chaykovsky reaction, introduced in 1962 by E. J. Corey and Michael Chaykovsky using sulfonium ylides for epoxide formation from carbonyls, with sulfoxonium variants detailed in 1965, further highlighted sulfur ylides' synthetic importance. Oxonium ylides, featuring oxygen as the onium center, emerged later, with significant developments in metal-catalyzed generation and reactions from the 1970s onward. By the mid-20th century, ylides had transitioned from laboratory novelties to essential synthetic reagents, driven by the Wittig and Corey-Chaykovsky methodologies that enabled precise carbon-carbon bond formation. This shift was marked by increased stability and versatility, facilitating applications in natural product synthesis. Post-2000 developments included iodonium ylides, first synthesized by O. Ya. Neiland in 1957 but revitalized for carbene transfer and fluorination reactions. Boronium ylides emerged around 2006, exemplified by carba-closo-dodecaborane derivatives serving as naked boron nucleophiles.

Structural Aspects

General Structure and Resonance

Ylides are neutral molecules characterized by a formal negative charge on an adjacent atom to a positively charged heteroatom, typically represented as a resonance hybrid between zwitterionic (ylide) and neutral (ylene) forms. This electronic delocalization imparts 1,2-dipolar character, with the actual structure lying between the two canonical contributors, exhibiting partial double-bond order between the heteroatom and the carbanion center.^[10] A prototypical example is the phosphorus ylide, depicted by the resonance structures $\ce{R3P^{+}-CH2^{-} \leftrightarrow R3P=CH2}$\ceR3P+−CH2−↔R3P=CH2, where the zwitterionic form shows charge separation with a single P–C bond and the ylene form features a double bond with octet fulfillment at phosphorus. The dipolar form predominates due to limited d-orbital participation in π-bonding, resulting in a bent geometry, with the P–C bond length intermediate between single and double bonds (approximately 1.66–1.70 Å). Similar resonance applies to other ylides, such as sulfur ylides ($\ce{R2S^{+}-CH2^{-} \leftrightarrow R2S=CH2}$\ceR2S+−CH2−↔R2S=CH2), where the zwitterionic (ylide) form predominates more due to poorer orbital overlap and longer S–C bond lengths (typically 1.78–1.82 Å).^[11][12][10] The stability and bonding of the resonance hybrid are influenced by substituents on the carbanionic carbon; electron-withdrawing groups, such as carbonyl (e.g., $\ce{-COOR}$\ce−COOR) or ester functionalities, enhance the carbanion resonance contributor by delocalizing the negative charge, thereby stabilizing the ylide form and reducing its nucleophilicity. In contrast, alkyl substituents favor the ylene form, promoting greater double-bond character. The central heteroatom, often hypervalent in the zwitterionic representation (e.g., 10-electron phosphorus), achieves octet compliance in the ylene structure through expanded valence, underscoring the hybrid's role in fulfilling electronic requirements without violating the octet rule in either extreme.^[11][12]

Bonding Characteristics

Ylides exhibit distinctive bonding characteristics arising from the interplay between their zwitterionic and ylene resonance forms, which influence their reactivity. The highest occupied molecular orbital (HOMO) of an ylide is predominantly the lone pair on the carbanionic carbon, residing in a p-orbital, while the lowest unoccupied molecular orbital (LUMO) involves contributions from the heteroatom. In phosphorus and sulfur ylides, this carbanionic lone pair contributes to the stabilization of the ylene form (R₃Hetero=CR₂) through resonance hybridization and modulates HOMO-LUMO interactions critical for nucleophilic behavior.^[5][13] Geometrically, the carbanionic carbon in ylides adopts a bent configuration, approximating sp² hybridization in the ylene resonance contributor, with bond angles around the carbon typically near 120°. X-ray crystallographic analyses of phosphorus ylides reveal P-C bond lengths of approximately 1.66 Å in non-stabilized examples, such as derivatives of triphenylphosphonium methylide, reflecting partial double-bond character intermediate between single (∼1.80 Å) and double (∼1.60 Å) bonds. These bond lengths underscore the partial delocalization across the heteroatom-carbon linkage, with variations depending on substituents that modulate electron density.^[14] Steric and electronic effects further define ylide bonding. Bulky substituents, like the triphenyl groups commonly employed on phosphorus, impose steric hindrance that prevents dimerization or self-condensation by shielding the reactive carbanion, thereby enhancing thermal stability without altering the core electronic structure. Electronically, the polarity of the ylide bond increases with the electronegativity of the heteroatom; for instance, oxygen and nitrogen ylides display heightened zwitterionic character compared to phosphorus or sulfur analogs.^[15] Density functional theory (DFT) computations provide deeper insights into these bonding nuances, revealing that the degree of zwitterionic versus ylene character correlates with the heteroatom. For phosphorus ylides like H₃P-CH₂, the smaller HOMO-LUMO gap (compared to amine analogs) supports greater delocalization, whereas nitrogen ylides such as H₃N-CH₂ exhibit more pronounced charge separation and ionic bonding. These studies highlight how heteroatom identity tunes the overall bonding hybrid, influencing reactivity profiles across ylide classes.^[13]

Synthesis

Deprotonation of Onium Salts

The deprotonation of onium salts represents the primary method for synthesizing ylides, particularly those featuring carbanionic centers adjacent to heteroatoms such as phosphorus or sulfur. In this approach, a quaternary onium salt of the general form R₃X⁺–CHR₂ X⁻ (where X is a heteroatom like P or S, and X⁻ is a counterion such as iodide) is treated with a strong base to abstract the acidic proton from the α-carbon, yielding the neutral ylide R₃X=CR₂.^[16][6] This acid-base reaction exploits the enhanced acidity of the α-hydrogen (pK_a ≈ 22 for phosphonium salts), which arises from the stabilization of the resulting carbanion by the adjacent positively charged onium center.^[16] A classic example is the preparation of methylenetriphenylphosphorane (Ph₃P=CH₂), a key reagent in the Wittig reaction, generated from methyltriphenylphosphonium iodide. The reaction proceeds as follows: $$\text{Ph}_3\text{P}^+-\text{CH}_3 \text{ I}^- + \text{base} \rightarrow \text{Ph}_3\text{P}=\text{CH}_2 + \text{HI} + \text{baseH}^+$$Ph3​P+−CH3​ I−+base→Ph3​P=CH2​+HI+baseH+ This transformation was first demonstrated by Georg Wittig in the early 1950s using organolithium bases such as phenyllithium.^[6] Commonly employed bases include n-butyllithium (n-BuLi), sodium hydride (NaH), and sodium amide (NaNH₂), selected for their strength in deprotonating the onium salt while minimizing side reactions.^[17][16] For labile or unstabilized ylides prone to decomposition or nucleophilic attack by the base, non-nucleophilic alternatives like sodium hexamethyldisilazide (NaHMDS) are preferred to enhance selectivity.^[18] Solvent choice plays a critical role in controlling reaction conditions, particularly for thermally sensitive ylides. Tetrahydrofuran (THF) is widely used due to its ability to dissolve the salts and bases while allowing low-temperature operations (e.g., -78 °C) to prevent ylide decomposition or protonation by the conjugate acid.^[17][16] Dimethyl sulfoxide (DMSO) is an alternative for sulfur-based onium salts, often in conjunction with NaH to generate dimsyl sodium in situ.^[19] This method is broadly applicable to the synthesis of most carbanion-stabilized ylides across heteroatom classes, including phosphorus, sulfur, and others, with typical isolated yields ranging from 70% to 90% when performed under anhydrous conditions.^[12][18] The onium salts are typically preformed via alkylation of the neutral heteroatom compound, though the deprotonation step is often conducted in situ for efficiency.

Nucleophilic Displacement and Other Routes

One common route to ylides involves the prior formation of onium salts through nucleophilic displacement reactions, where a neutral nucleophile such as a tertiary phosphine or sulfide attacks an alkyl halide via an S_N2 mechanism. For instance, triphenylphosphine reacts with methyl iodide to yield methyltriphenylphosphonium iodide, which serves as a precursor for subsequent ylide generation via deprotonation.^[20] This approach is versatile for preparing a wide range of phosphonium and sulfonium salts, applicable to both stabilized and non-stabilized ylides, though it requires careful selection of the alkylating agent to avoid side reactions with sensitive substrates.^[16] An alternative synthesis employs diazo compounds, where thermal or transition metal-catalyzed decomposition generates a carbene intermediate that adds to a phosphine or sulfide to directly form the ylide, bypassing the need for strong bases. A representative example is the copper(I)-catalyzed reaction of triphenylphosphine with diazomethane, producing methylenetriphenylphosphorane and nitrogen gas.^[21] This method, developed in the 1960s, is particularly advantageous for non-stabilized ylides prone to decomposition during traditional deprotonation, offering yields up to 80% in some cases, though it requires handling potentially explosive diazo reagents.^[22] Direct carbene addition to onium precursors represents another pathway, often involving electrophilic carbenes from diazo decomposition or alpha-elimination. For example, dichlorocarbene, generated from chloroform and base, adds to triphenylphosphine to form dichloromethylenetriphenylphosphorane. Such additions are mild and regioselective, enabling access to halogenated ylides useful in further synthetic transformations, but they are limited by the availability of suitable carbene sources.^[12] Electrochemical methods provide a base-free alternative for ylide generation, typically through cathodic reduction of onium salts to deprotonate the alpha-carbon. Sulfonium salts, for instance, undergo one-electron reduction at a mercury or glassy carbon electrode to produce sulfur ylides in situ, as demonstrated in the synthesis of dimethyloxosulfonium methylide for epoxidation reactions with yields exceeding 70%.^[23] This technique excels in controlling reaction conditions for air- or moisture-sensitive species but may suffer from electrode fouling or low scalability.^[24] For stabilized ylides bearing electron-withdrawing groups, such as carboxylate esters, a specialized route involves Michael addition of triphenylphosphine to activated acetylenic compounds. The nucleophilic attack on dimethyl acetylenedicarboxylate forms a zwitterionic betaine intermediate, which undergoes proton transfer to yield the ylide (e.g., (Ph₃P⁺–CHC(CO₂Me)₂)^–).^[25] This one-pot process, often conducted at room temperature, provides stable ylides in high purity (up to 95% yield) without halide byproducts, though it is less suitable for non-carbon-based nucleophiles due to competing protonation.^[25] These routes complement deprotonation by enabling access to ylides unstable to bases, with diazo and electrochemical methods particularly noted for improved yields in delicate systems, albeit with constraints on substrate scope for heteroatom variants.^[12]

Classification

Phosphorus Ylides

Phosphorus ylides, also known as phosphonium ylides, represent the most extensively studied class of ylides due to their pivotal role in organic synthesis, particularly in carbon-carbon bond formation. These compounds feature a phosphorus atom bonded to a carbanion, typically represented in two resonance forms: the phosphonium ylide form $\ce{R3P^{+}-CR2^{-}}$\ceR3P+−CR2− and the phosphorane form $\ce{R3P=CR2}$\ceR3P=CR2. The latter depiction emphasizes the double-bond character between phosphorus and carbon, facilitated by the d-orbitals of phosphorus, which allow for partial $\pi$π-bonding and stabilization of the carbanion.^[26] Phosphorus ylides are classified based on the substituents attached to the ylidic carbon, which influence their stability and reactivity. Non-stabilized ylides, such as methylenetriphenylphosphorane ($\ce{Ph3P=CH2}$\cePh3P=CH2), bear alkyl groups on the carbanion and exhibit high nucleophilicity due to minimal electronic delocalization. Semi-stabilized ylides, exemplified by benzylidenetriphenylphosphorane ($\ce{Ph3P=CHPh}$\cePh3P=CHPh), incorporate conjugating groups like phenyl that provide moderate stabilization through resonance. Stabilized ylides, such as (ethoxycarbonylmethylidene)triphenylphosphorane ($\ce{Ph3P=CHCO2Et}$\cePh3P=CHCO2Et), feature electron-withdrawing groups (e.g., ester or carbonyl) adjacent to the carbanion, enabling significant delocalization and greater thermal stability. This classification dictates their behavior in reactions, with non-stabilized variants favoring Z-selective olefinations and stabilized ones promoting E-selectivity.^[27][17] A common preparation route for stabilized phosphorus ylides involves the reaction of triphenylphosphine with α-halo carbonyl compounds, such as ethyl bromoacetate, to form the corresponding phosphonium salt, followed by deprotonation with a base like sodium ethoxide. This method leverages the nucleophilic attack of phosphorus on the alkyl halide, yielding salts that are readily converted to ylides under mild conditions. Non-stabilized ylides are similarly prepared but often generated in situ due to their reactivity. Methylenetriphenylphosphorane, a prototypical non-stabilized ylide, is commercially available as a solution in tetrahydrofuran, facilitating its use in laboratory syntheses without on-site preparation.^[26][28]

Sulfur Ylides

Sulfur ylides constitute a significant subclass of ylides characterized by a sulfur atom bearing a positive charge adjacent to a carbanion, generally formulated as $R_2S^+ - \overline{C}R_2$R2​S+−CR2​. These compounds exhibit zwitterionic nature and are pivotal in synthetic organic chemistry due to their reactivity akin to carbenes. The two primary variants are sulfonium ylides, where sulfur is in the +2 oxidation state ($R_2S^+ - \overline{C}R_2$R2​S+−CR2​), and sulfoxonium ylides, featuring sulfur in the +4 oxidation state ($R_2S(O)^+ - \overline{C}R_2$R2​S(O)+−CR2​). A representative sulfonium ylide is dimethylsulfonium methylide, $(CH_3)_2S^+ - \overline{C}H_2$(CH3​)2​S+−CH2​, while dimethyloxosulfonium methylide, $(CH_3)_2S(O)^+ - \overline{C}H_2$(CH3​)2​S(O)+−CH2​, exemplifies the sulfoxonium type.^[29][30] Preparation of sulfur ylides typically involves the alkylation of dialkyl sulfides or sulfoxides with diazomethane to form the corresponding onium salts, followed by deprotonation with a strong base such as sodium hydride. Alternatively, sulfonium or sulfoxonium salts can be generated from sulfides or sulfoxides reacting with haloforms (e.g., chloroform) in the presence of a base like potassium tert-butoxide, with subsequent deprotonation yielding the ylide; this route draws from the broader deprotonation of onium salts. These methods allow for the in situ generation of ylides under mild conditions, facilitating their immediate use in reactions.^[31][30] In comparison to phosphorus ylides, sulfur ylides display reduced thermal stability, attributed to poorer pπ-dπ bonding between sulfur and the carbanionic carbon compared to phosphorus, due to the smaller size of sulfur leading to suboptimal orbital overlap.^[32] This lower stability enhances their carbene-like reactivity, positioning them as non-metallic carbenoids capable of 1,3-dipolar additions and insertions. Notably, they are employed in the Corey-Chaykovsky reaction for the stereoselective formation of epoxides from aldehydes and ketones, as well as cyclopropanes from α,β-unsaturated carbonyls, providing versatile tools for constructing strained rings in natural product synthesis.^[33][30][29] As hypervalent variants analogous to sulfur ylides, iodonium ylides ($R_2I^+ - \overline{C}R_2$R2​I+−CR2​) share similar structural motifs and reactivity profiles, often serving as carbene precursors in metal-catalyzed processes. These iodine-based compounds extend the utility of sulfur ylide chemistry into realms requiring higher oxidation states or distinct electronic properties.^[34]

Oxygen Ylides

Oxygen ylides encompass two primary classes: oxonium ylides and carbonyl ylides, both characterized by a positively charged oxygen atom adjacent to a carbanionic center, rendering them highly reactive zwitterionic species. Oxonium ylides possess the general structure R₂O⁺–CR₂⁻, where the oxygen is part of an alkoxy or similar group, as exemplified by dimethyloxonium methylide ((CH₃)₂O⁺–CH₂⁻). In contrast, carbonyl ylides feature the form R₂C=O⁺–CR₂⁻, representing a 1,3-dipole with the oxygen integrated into a carbonyl moiety. These structures exhibit significant charge separation, with the carbanion acting as a nucleophilic site, and their bonding reflects heteroatom ylide trends of partial double-bond character between the onium center and carbon.^[35][36] Preparation of oxonium ylides typically involves the metal-catalyzed decomposition of diazo compounds in the presence of ethers or alcohols, generating the ylide in situ at moderate temperatures (above 25°C) using catalysts such as rhodium(II) acetate (Rh₂(OAc)₄) or copper complexes. For instance, α-diazo esters react with dialkyl ethers to form alkoxy-substituted oxonium ylides, which are too unstable for isolation and are handled under catalytic conditions to control reactivity. Carbonyl ylides are similarly produced via rhodium-catalyzed addition of diazo ketones to carbonyl compounds, such as aldehydes or ketones, often in tandem with intramolecular processes; photochemical or thermal cleavage of oxiranes also serves as a route, though less common. These methods ensure transient generation, as persistent oxygen ylides are rare due to their inherent instability.^[37][38][39] Oxygen ylides display extreme reactivity, primarily undergoing rapid rearrangements or cycloadditions before isolation, with stability enhanced only by specific substituents or low temperatures where spectroscopic detection (e.g., via NMR) is possible. Oxonium ylides are particularly labile, prone to proton transfer or elimination, limiting their standalone utility. Carbonyl ylides, while also fleeting, excel in [3+2] cycloadditions as 1,3-dipoles, reacting with alkenes or alkynes to form dihydrofurans; for example, rhodium-catalyzed generation from diazoacetates and aldehydes followed by addition to electron-deficient olefins yields substituted tetrahydrofurans in up to 87% yield with high enantioselectivity using chiral catalysts. These applications highlight their value in synthesizing oxygen heterocycles, such as in alkaloid frameworks, though their handling requires precise in situ protocols.^[35][40][41]

Nitrogen Ylides

Nitrogen ylides encompass a class of zwitterionic species where the positively charged heteroatom is nitrogen, distinguishing them as 1,3-dipoles with significant utility in synthetic organic chemistry. The primary subtypes are ammonium ylides, characterized by the structure $R_3N^+-\overline{C}R_2$R3​N+−CR2​, where the nitrogen bears three substituents and is adjacent to a carbanionic carbon, and azomethine ylides, represented as $R_2\overline{C}-N^+R-CR_2$R2​C−N+R−CR2​ in their zwitterionic form.^[42][43] A representative example of an azomethine ylide is the N-methyl-C-phenyl variant, often depicted with resonance contributing to its reactivity profile. These structures enable nitrogen ylides to participate effectively as dipoles, though their behavior is modulated by electronic delocalization. Preparation of ammonium ylides typically involves the formation of quaternary ammonium salts from tertiary amines and α-halo compounds, followed by deprotonation at the α-carbon using a strong base such as sodium hydride or butyllithium.^[42] For azomethine ylides, generation proceeds from imines via deprotonation at an α-position or through initial condensation of amines with α-halo carbonyl compounds to form iminium intermediates, which are then deprotonated to yield the ylide.^[44] These methods ensure in situ formation, as nitrogen ylides are often transient and require controlled conditions to engage in subsequent reactions. Nitrogen ylides function as effective 1,3-dipoles, particularly in cycloaddition processes that construct nitrogen-containing heterocycles. However, they are less commonly employed than phosphorus or sulfur analogs due to the nitrogen lone pair in the resonance form—such as $R_2C=NR-\overline{C}R_2$R2​C=NR−CR2​ for azomethine ylides—which diminishes the charge separation and ylidic polarity, resulting in reduced stability and synthetic versatility. A prominent application involves azomethine ylides in the Prato reaction, where they undergo [3+2] cycloaddition with fullerenes like C$_{60}$60​ to afford pyrrolidinofullerene derivatives, enabling precise surface functionalization of carbon nanomaterials.^[45]

Other Ylides

Ylides incorporating less common main-group elements, such as boron, silicon, halogens, and arsenic, represent specialized classes with unique structural and reactivity profiles that extend beyond the more prevalent phosphorus, sulfur, oxygen, and nitrogen variants. These "other" ylides typically feature a positively charged heteroatom adjacent to a carbanion, akin to the general zwitterionic motif $\ce{X^{+}-CR2^{-}}$\ceX+−CR2− where X is the heteroatom, enabling ambiphilic behavior in which the heteroatomic center acts as both a leaving group and a carbanion stabilizer.^[46] Boron ylides possess the core structure $\ce{R2B^{+}-CR2^{-}}$\ceR2B+−CR2−, often stabilized by bulky substituents like mesityl groups to mitigate the inherent instability of the boron-carbon interaction. Preparation typically involves deprotonation of alkylboranes, such as alkyldimesitylboranes (Mes₂B-CHR), using strong bases to generate the ylide anion, which can then participate in subsequent reactions.^[47] These ylides exhibit niche applications in catalysis, including stereoselective cyclopropanation of olefins to form gem-diborylcyclopropanes, leveraging the boron center's Lewis acidity for activation.^[46] Additionally, boron ylides serve as analogs in hydroboration-related processes, facilitating carbon-carbon bond formation under mild conditions.^[48] Halonium ylides, exemplified by iodonium species with the structure $\ce{R2I^{+}-CR2^{-}}$\ceR2I+−CR2−, are synthesized through specialized routes such as the reaction of iodosyl compounds (e.g., iodosylbenzene, PhI=O) with diazoalkanes or their derivatives, generating the ylide via nitrogen extrusion and iodine-carbon bond formation.^[49] These ylides are characterized by internal halogen bonding that enhances stability, particularly in heterocyclic variants, and find use in niche catalytic transformations like C-H activation for arene functionalization.^[50][51] Silicon ylides adopt the form $\ce{R3Si^{+}-CR2^{-}}$\ceR3Si+−CR2−, often prepared from α-halo silanes via halogen-metal exchange or carbene insertion, though their isolation remains challenging due to rapid rearrangement.^[52] They exhibit limited but targeted reactivity in epoxidation and cycloaddition contexts, benefiting from silicon's ability to modulate carbanion basicity through hyperconjugation.^[52] Arsonium ylides, structured as $\ce{R3As^{+}-CR2^{-}}$\ceR3As+−CR2−, function as phosphorus mimics and are generated by deprotonation of arsonium salts analogous to the Wittig precursor preparation, often using bases like sodium hydride on triphenylarsonium alkylides.^[53] These ylides display similar olefination capabilities but with altered stereoselectivity, serving in synthetic applications such as epoxide formation and as alternatives in carbonyl condensations where arsenic's larger size influences product distribution.^[54][55] Recent advancements include their incorporation into boron-catalyzed C1 copolymerizations with other ylides, highlighting emerging roles in polymer synthesis.^[56]

Properties

Stability Factors

Ylides exhibit varying degrees of stability depending on their structural features and reaction conditions, with phosphorus ylides serving as the prototypical example. Non-stabilized ylides, characterized by simple alkyl substituents on the carbanionic carbon, possess high nucleophilicity and basicity, leading to rapid decomposition at or near room temperature; these species are typically generated in situ for synthetic applications to avoid self-reaction. In contrast, stabilized ylides bearing electron-withdrawing groups (EWGs) such as ester (CO₂R) or cyano (CN) functionalities achieve greater longevity through resonance delocalization of the negative charge, often existing as isolable, crystalline solids that can be stored under inert conditions.^[57][12] Substituent effects play a central role in modulating ylide stability. EWGs on the α-carbon effectively conjugate with the carbanion, lowering the pKa of the conjugate phosphonium acid (typically 6–10 for stabilized variants compared to 20–25 for non-stabilized alkylphosphonium salts) and reducing the electron density available for unwanted interactions. Additionally, bulky groups on the onium center, such as triphenylphosphine-derived moieties, provide steric shielding that hinders intermolecular proton transfers or nucleophilic attacks, further enhancing kinetic stability.^[58][59] Environmental conditions significantly impact ylide persistence. Aprotic solvents like tetrahydrofuran or benzene are preferred, as they minimize protonation of the basic carbanion (pKa of conjugate acids generally 20–30), whereas protic media promote rapid reversion to the phosphonium salt. Temperature control is crucial: non-stabilized ylides require low temperatures (e.g., below 0 °C) for handling, while stabilized analogs tolerate ambient conditions and even mild heating without decomposition. Inert atmospheres prevent oxidative degradation, a common issue for reactive non-stabilized species.^[16][57] Decomposition pathways for unstable ylides primarily involve proton transfer processes. In non-stabilized cases, one ylide molecule can abstract an α-proton from another, leading to dimerization products such as phosphonium alkylides or elimination to form alkenes and phosphine oxides. Moisture-induced hydrolysis exemplifies this, yielding hydrocarbons (e.g., methane from methylides) via nucleophilic attack on water followed by P–C bond cleavage. Stabilized ylides resist these routes due to reduced basicity, though extreme conditions like high temperatures can trigger thermal elimination. For comparison, sulfur ylides often display even lower intrinsic stability owing to poorer orbital overlap for charge delocalization.^[58][16][57]

Spectroscopic Identification

Ylides are characterized spectroscopically using a variety of techniques that reveal their unique bonding and electronic features, particularly the ylidic carbon's partial double-bond character. Nuclear magnetic resonance (NMR) spectroscopy is a primary method for identifying ylides in solution, providing insights into the chemical environments of key atoms. In ¹H NMR spectra, the proton attached to the ylidic carbon (the carbanion proton) typically appears at 0-5 ppm, which is downfield relative to alkane protons due to the deshielding effect of the adjacent positively charged heteroatom.^[60] For phosphorus ylides, ³¹P NMR is particularly diagnostic, with chemical shifts generally ranging from -10 to +30 ppm, reflecting the phosphorus-carbon interaction and distinguishing ylides from their phosphonium salt precursors (which resonate at more positive shifts around +30 to +50 ppm). These shifts can vary with stabilization; non-stabilized ylides tend toward higher values (20-30 ppm), while conjugated or electron-withdrawing groups shift them upfield. Infrared (IR) spectroscopy complements NMR by probing vibrational modes associated with ylide bonds. The ylidic C-H stretch appears as a weak absorption around 3000 cm⁻¹, indicative of the sp²-hybridized carbon similar to alkenes, though often broadened due to the zwitterionic nature. For phosphorus ylides, the P=C stretch manifests as a medium-intensity band near 1100 cm⁻¹ in phosphorane-like structures, though it may appear higher (1200-1300 cm⁻¹) in stabilized variants owing to resonance contributions that alter bond order. These IR features are useful for confirming ylide formation in situ, as the disappearance of precursor C-H stretches above 2900 cm⁻¹ correlates with ylide generation.^[61] Additional techniques provide supporting evidence for ylide identification. Ultraviolet-visible (UV-Vis) spectroscopy is effective for conjugated ylides, where extended π-systems lead to intense absorptions in the 250-400 nm range, often with bathochromic shifts compared to non-conjugated analogs due to lowered HOMO-LUMO gaps.^[62] Mass spectrometry frequently shows stable molecular ions, enabling confirmation of the ylide formula through high-resolution peaks and fragmentation patterns that retain the ylidic moiety, such as loss of phosphine groups in phosphorus ylides.^[63] X-ray crystallography offers definitive solid-state geometry, revealing P-C bond lengths of 1.70-1.80 Å—intermediate between single (1.85 Å) and double (1.66 Å) bonds—and nearly planar ylidic centers, which validate the ylide resonance hybrid.^[64] The transient nature of many ylides, especially non-stabilized ones, poses challenges for spectroscopic characterization, often necessitating low-temperature or in situ measurements to prevent decomposition or reversion to precursors. Low-temperature NMR (e.g., below -40°C) is commonly employed to capture fleeting intermediates and resolve dynamic equilibria, while cryogenic IR or matrix isolation techniques help observe short-lived vibrations.^[65] These methods ensure accurate identification despite ylides' reactivity toward moisture, oxygen, or nucleophiles.

Reactions and Applications

Olefination Reactions

The Wittig reaction represents a cornerstone of phosphorus ylide chemistry, enabling the conversion of aldehydes and ketones into alkenes through nucleophilic addition to the carbonyl group. In this process, a phosphonium ylide, typically of the form $\ce{Ph3P=CHR}$\cePh3P=CHR, reacts with a carbonyl compound $\ce{R'2C=O}$\ceR′2C=O to yield the corresponding alkene $\ce{RCH=CR'2}$\ceRCH=CR′2 and triphenylphosphine oxide $\ce{Ph3P=O}$\cePh3P=O as a byproduct.^[66] This reaction, first reported by Georg Wittig in 1954, has become indispensable for constructing carbon-carbon double bonds in organic synthesis due to its broad substrate scope and functional group tolerance.^[66] The mechanism of the Wittig reaction proceeds through the initial formation of a betaine intermediate upon nucleophilic attack of the ylide carbon on the carbonyl carbon, followed by intramolecular proton transfer and cyclization to an oxaphosphetane. The oxaphosphetane then undergoes stereospecific decomposition via a four-center elimination to afford the alkene and phosphine oxide.^[67] Under lithium salt-free conditions, the reaction for non-stabilized ylides (where R is an alkyl group lacking conjugating substituents) favors Z-alkene formation through a cis-oxaphosphetane pathway, while stabilized ylides (with electron-withdrawing groups like ester or carbonyl on R) preferentially yield E-alkenes due to the thermodynamic stability of trans-configured intermediates.^[66] Semi-stabilized ylides, bearing aryl substituents, exhibit more balanced E/Z ratios influenced by reaction conditions.^[68] Variants of the Wittig reaction enhance stereocontrol and applicability. The Horner-Wadsworth-Emmons (HWE) modification employs phosphonate-stabilized carbanions, generated from deprotonation of alkylphosphonates, which react with carbonyls to form alkenes and phosphate byproducts; this analog provides superior E-selectivity, particularly for conjugated systems, owing to the more ionic character of the carbanion and the stability of the erythro-betaine intermediate.^[69] For achieving high E-selectivity in Wittig reactions with non-stabilized ylides, the Schlosser modification involves treating the initial betaine with phenyllithium to form a lithiated oxyanion intermediate, which equilibrates to the trans-configured species before protonation and elimination.^[70] Conversely, salt-free conditions using strong, non-coordinating bases like sodium hexamethyldisilazide promote Z-selectivity by minimizing ion-pairing effects that favor isomerization.^[71] The scope of olefination reactions encompasses a wide range of aldehydes and ketones, facilitating the synthesis of both simple and complex alkenes, including those in natural products. A notable application is the industrial production of vitamin A, where sequential Wittig reactions couple C15-phosphonium ylides with C5-aldehydes to construct the polyene chain with defined stereochemistry, enabling efficient large-scale synthesis since the 1950s.^[72] These methods have been pivotal in assembling unsaturated frameworks in pharmaceuticals and agrochemicals, underscoring the versatility of ylide-based olefinations in modern synthetic chemistry.^[66]

Cycloaddition Reactions

Ylides, particularly those of nitrogen and sulfur, participate in 1,3-dipolar cycloaddition reactions, which are powerful methods for constructing five-membered heterocycles through the combination of a 1,3-dipole and a dipolarophile.^[73] Azomethine ylides, a class of nitrogen ylides with the general structure R₂C⁺–N⁻R, serve as 1,3-dipoles and react with electron-deficient alkenes to form pyrrolidines, enabling the stereocontrolled synthesis of substituted pyrrolidine frameworks prevalent in natural products and pharmaceuticals.^[44] These reactions proceed under mild conditions, often without catalysts, and allow for the creation of up to four contiguous stereocenters in a single step.^[74] The regiochemistry of azomethine ylide cycloadditions with unsymmetrical alkenes follows the Huisgen rules, which predict the orientation based on frontier molecular orbital interactions between the dipole's HOMO and the dipolarophile's LUMO; for instance, N-metalated azomethine ylides typically add to acrylates with the ylide carbon attaching to the β-position of the alkene.^[73] This regioselectivity is rationalized by the coefficient matching in the transition state, ensuring the major product aligns with the electron-withdrawing group on the dipolarophile. Experimental studies confirm high regioselectivity (>95:5) in reactions with methyl acrylate, yielding 2,5-disubstituted pyrrolidines as predominant isomers.^[75] A notable application is the Prato reaction, where azomethine ylides generated in situ from glycine esters and aldehydes undergo cycloaddition with fullerenes, such as C₆₀, to afford fulleropyrrolidines; this method, developed in 1993, facilitates the attachment of functional groups to the fullerene surface.^[45] Sequential additions can yield bisadducts with controlled regiochemistry, as the initial monoadduct directs the second ylide via steric and electronic effects, achieving up to 30% isolated yields for specific bisadducts with glycine and paraformaldehyde.^[76] Sulfur and oxygen ylides, including sulfoxonium ylides like dimethyloxosulfonium methylide, engage in Corey-Chaykovsky reactions, where they act as 1,3-dipoles with carbonyl compounds to form epoxides via methylene transfer. These ylides, less basic than sulfonium counterparts, provide cleaner reactions with aldehydes and ketones, yielding trans-epoxides in 70-90% yields for simple substrates like cyclohexanone. With Michael acceptors such as α,β-unsaturated esters, sulfoxonium ylides undergo cyclopropanation, installing a cyclopropane ring adjacent to the acceptor in a stereospecific manner, as demonstrated in the synthesis of donor-acceptor cyclopropanes with >20:1 diastereoselectivity. The mechanism of these 1,3-dipolar cycloadditions is concerted and pericyclic, involving a suprafacial [3+2] addition without intermediates, as supported by stereospecificity studies where cis-alkenes yield cis-pyrrolidines.^[77] Endo/exo selectivity arises from secondary orbital interactions in the transition state; for azomethine ylides with cyclic dipolarophiles like maleimides, the endo approach—where the ylide nitrogen points toward the carbonyls—predominates due to favorable π-π stacking, often achieving endo:exo ratios of 9:1 or higher under thermal conditions.^[78]

Rearrangement Reactions

Rearrangement reactions of ylides, particularly those involving sulfur and phosphorus, play a crucial role in organic synthesis by enabling intramolecular migrations through sigmatropic shifts, often resulting in skeletal reorganization without the formation of new rings. These processes typically proceed via deprotonation of onium salts to generate the ylide intermediate, which then undergoes a concerted pericyclic rearrangement under mild conditions.^[79] The Stevens rearrangement exemplifies a [2,3]-sigmatropic shift in sulfonium ylides bearing a quaternary center adjacent to the sulfur, where a migrating group (such as alkyl or aryl) transfers from sulfur to the alpha-carbon, yielding alpha-substituted amines after sulfur elimination. This anionic mechanism is accelerated by the zwitterionic nature of the ylide, proceeding through a five-membered transition state that preserves stereochemistry in suitable cases. Seminal work by Stevens in the 1950s established this transformation using strong bases like sodium amide for ylide generation, with modern variants employing phase-transfer catalysis—such as benzyltriethylammonium chloride in dichloromethane—aqueous sodium hydroxide systems—to enhance efficiency and regioselectivity, often achieving migrations in yields exceeding 70% for benzylic systems. A representative example involves the rearrangement of (1-phenylethyl)dimethylsulfonium ylide to 1-phenylpropylamine derivatives, demonstrating the utility in constructing branched carbon chains.^[79][80][81] The Sommelet-Hauser rearrangement is a [2,3]-sigmatropic process for benzyl sulfonium or ammonium ylides, in which the carbanionic center migrates to the ortho position of the aromatic ring, generating an o-quinodimethane intermediate that hydrolyzes to ortho-substituted benzaldehydes, such as o-methylbenzaldehyde from dibenzylmethylsulfonium salts. This reaction, developed in the 1950s, uses strong bases like sodium amide for deprotonation at low temperatures in solvents like liquid ammonia or THF, affording yields of 50-80% for electron-rich aromatics. The mechanism includes deprotonation, sigmatropic rearrangement, and aromatization without requiring ortho-lithiation.^[82][83] Allylic rearrangements in phosphonium ylides involve the migration of allyl groups across the ylide, often via [3,3]-sigmatropic shifts, providing access to homoallylic phosphonates or related motifs. These transformations, explored by Vedejs and coworkers, generate allylic phosphonium ylides from allylic alcohols, carbenes, and chlorophosphites, followed by thermal rearrangement at elevated temperatures (80-120°C) in toluene, delivering single diastereomers in up to 95% yield. The mechanism proceeds concertedly through a chair-like transition state, with the allyl moiety inverting configuration during migration, as seen in the conversion of crotyl phosphonium ylides to trans-1,4-dienes bearing phosphonate functionality. Such rearrangements distinguish themselves by enabling stereocontrolled allyl transposition without external nucleophiles.

Emerging Synthetic Applications

Recent advances in ylide chemistry have leveraged photocatalytic methods to generate sulfur ylides for light-mediated transformations, particularly in epoxidation and cyclopropanation reactions. In 2024, visible-light-driven photocatalysis enabled the formation of sulfur ylides as sources of free carbenes or radicals, facilitating metal-free carbene transfer processes that expand their utility beyond traditional ionic pathways.^[84] These approaches promote sustainable synthesis by utilizing mild conditions and avoiding harsh reagents, with applications in constructing epoxides and cyclopropanes from aldehydes and alkenes, respectively.^[85] Iodonium ylides have emerged as versatile, sustainable reagents for C-H functionalization and alkene difunctionalization, offering metal-free alternatives in green chemistry. A 2022 review highlighted their role in transition-metal-free or low-metal protocols for direct C-H activation, enabling efficient installation of carbene-derived groups without toxic byproducts.^[51] These ylides facilitate difunctionalization of alkenes through carbene insertion, providing access to complex motifs with high regioselectivity and yields up to 90% in representative examples.^[34] Their stability and ease of handling further support scalable, environmentally benign processes.^[86] Stereoselective reactions involving sulfoxonium ylides have gained traction for constructing chiral heterocycles and epoxides. In 2025, nickel(II)-catalyzed cyclization of sulfoxonium ylides with 2-aminopyridines afforded C2-substituted imidazo[1,2-a]pyridines in acceptable to excellent yields (typically 70-95%), compatible with diverse functionalities like halogens and nitro groups, using THF as solvent.^[87] Organocatalytic enantioselective epoxidations using sulfur ylides, as reviewed in 2022, achieve high enantiomeric excesses (91-97% ee) for 2,2-disubstituted epoxides from ketones, building on chiral catalysts like La-Li binaphthoxide systems enhanced by phosphine oxide additives.^[88] Ylides serve as diazo-free carbene precursors, enhancing safety in synthesis by mitigating explosion risks associated with diazo compounds. Sulfoxonium ylides, in particular, offer greater stability and lower toxicity, releasing benign sulfoxides as byproducts, which has been adopted in industrial-scale reactions like pharmaceutical intermediate production.^[88] In polymer synthesis, Fe(II)-catalyzed step-growth polymerization of acyloxyamides and diphosphinoethanes incorporates ylides into the main chain, yielding pH-responsive poly(N-acyliminophosphoranes) with Mn ≈ 5.4 kg/mol and controlled degradation (half-life 6.8 hours at pH 5.5).^[89] These materials show promise for biomaterials due to their hydrophilicity and tunable stability.^[90]