ylide

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      An ylide () or ylid () is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass of zwitterions. They appear in organic chemistry as reagents or reactive intermediates.
      The class name "ylide" for the compound should not be confused with the suffix "-ylide".


      Resonance structures


      Many ylides may be depicted by a multiply bonded form in a resonance structure, known as the ylene form, while the actual structure lies in between both forms:

      The actual bonding picture of these types of ylides is strictly zwitterionic (the structure on the right) with the strong Coulombic attraction between the "onium" atom and the adjacent carbon accounting for the reduced bond length. Consequently, the carbon anion is trigonal pyramidal.


      Phosphonium ylides



      Phosphonium ylides are used in the Wittig reaction, a method used to convert ketones and especially aldehydes to alkenes. The positive charge in these Wittig reagents is carried by a phosphorus atom with three phenyl substituents and a bond to a carbanion. Ylides can be 'stabilised' or 'non-stabilised'. A phosphonium ylide can be prepared rather straightforwardly. Typically, triphenylphosphine is allowed to react with an alkyl halide in a mechanism analogous to that of an SN2 reaction. This quaternization forms an alkyltriphenylphosphonium salt, which can be isolated or treated in situ with a strong base (in this case, butyllithium) to form the ylide.

      Due to the SN2 mechanism, a less sterically hindered alkyl halide reacts more favorably with triphenylphosphine than an alkyl halide with significant steric hindrance (such as tert-butyl bromide). Because of this, there will typically be one synthetic route in a synthesis involving such compounds that is more favorable than another.
      Phosphorus ylides are important reagents in organic chemistry, especially in the synthesis of naturally occurring products with
      biological and pharmacological activities. Much of the interest in the coordination properties of a-keto stabilized phosphorus
      ylides stems from their coordination versatility due to the presence of different functional groups in their molecular structure.


      Non-symmetric phosphorus ylides


      The a-keto stabilized ylides derived from bisphosphines like dppe, dppm, etc., viz., [Ph2PCH2PPh2]C(H)C(O)R and [Ph2PCH2CH2PPh2]C(H)C(O)R (R = Me, Ph or OMe) constitute an important class of hybrid ligands containing both phosphine and ylide functionalities, and can exist in ylidic and enolate forms. These ligands can therefore be engaged in different kinds of bonding with metal ions like palladium and platinum.


      Other types




      = Based on sulfur

      =
      Other common ylides include sulfonium ylides and sulfoxonium ylides; for instance, the Corey-Chaykovsky reagent used in the preparation of epoxides or in the Stevens rearrangement.


      = Based on oxygen

      =
      Carbonyl ylides (RR'C=O+C−RR') can form by ring-opening of epoxides or by reaction of carbonyls with electrophilic carbenes, which are usually prepared from diazo compounds. Oxonium ylides (RR'-O+-C−R'R) are formed by the reaction of ethers with electrophilic carbenes.


      = Based on nitrogen

      =
      Certain nitrogen-based ylides also exist such as azomethine ylides with the general structure:

      These compounds can be envisioned as iminium cations placed next to a carbanion. The substituents R1, R2 are electron withdrawing groups. These ylides can be generated by condensation of an α-amino acid and an aldehyde or by thermal ring opening reaction of certain N-substituted aziridines.
      The further-unsaturated nitrile ylides are known almost exclusively as unstable intermediates.
      A rather exotic family of dinitrogen-based ylides are the isodiazenes (R1R2N+=N–), which generally decompose by extrusion of dinitrogen.
      Stable carbenes also have a ylidic resonance contributor, e.g.:


      = Other

      =
      Halonium ylides can be prepared from allyl halides and metal carbenoids. After a [2,3]-rearrangement, a homoallylhalide is obtained.
      The active form of Tebbe's reagent is often considered a titanium ylide. Like the Wittig reagent, it is able to replace the oxygen atom on carbonyl groups with a methylene group. Compared with the Wittig reagent, it has more functional group tolerance.


      Reactions


      An important ylide reaction is of course the Wittig reaction (for phosphorus) but there are more.


      = Dipolar cycloadditions

      =
      Some ylides are 1,3-dipoles and interact in 1,3-dipolar cycloadditions. For instance an azomethine ylide is a dipole in the Prato reaction with fullerenes.


      = Dehydrocoupling with silanes

      =
      In the presence of the group 3 homoleptic catalyst Y[N(SiMe3)2]3, triphenylphosphonium methylide can be coupled with phenylsilane. This reaction produces H2 gas as a byproduct, and forms a silyl-stabilised ylide.


      = Sigmatropic rearrangements

      =
      Many ylides react in sigmatropic reactions. The Sommelet-Hauser rearrangement is an example of a [2,3]-sigmatropic reaction. The Stevens rearrangement is a [1,2]-rearrangement.
      A [3,3]-sigmatropic reaction has been observed in certain phosphonium ylides.


      = Allylic rearrangements

      =
      Wittig reagents are found to react as nucleophiles in SN2' substitution:

      The initial addition reaction is followed by an elimination reaction.


      See also


      1,3-dipole
      Betaine: a neutral molecule with an onium cation and a negative charge
      Zwitterion: a neutral molecule with one or more pairs of positive and negative charges


      References

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    Ylide - Wikipedia

    An ylide (/ ˈ ɪ l aɪ d /) [1] or ylid (/ ˈ ɪ l ɪ d /) is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in …

    Ylide - Chemistry LibreTexts

    Feb 28, 2022 · A ylide is a neutral species in which there is a carbon atom bearing a formal charge of -1 bound to a heteroatom bearing a formal charge of +1. Ylides are classified based on the nature of the positively charged heteroatom.

    Preparation of Ylides - Chemistry LibreTexts

    Jan 23, 2023 · Some characteristic preparations of ylide reagents are shown below. Very strong bases, such as butyl lithium, are required for complete formation of ylides.

    Ylide: Definition, Types, Synthesis, Uses - Chemistry Notes

    Nov 11, 2022 · Types of Ylide. There are various types of ylides but the most common types are given below. Phosphorus ylide; Sulfur ylide; Nitrogen ylide; Of these three classes of ylides, phosphorus ylide is a widely studied compound due to its stability and ability to generate alkene from carbonyl compounds.

    Ylide - an overview | ScienceDirect Topics

    A ylide is defined as a neutral dipolar molecule with a negatively charged atom directly attached to a positively charged heteroatom. It serves as a versatile building block in organic synthesis, enabling various reactions like cyclization and olefination. AI generated definition based on: Tetrahedron Letters, 2014

    The Wittig Reaction - Chemistry LibreTexts

    Jan 23, 2023 · An ylide is defined as a compound with opposite charges on adjacent atoms both of which have complete octets. For the Wittig reaction discussed below an organophosphorus ylide, also called Wittig reagents, will be used.

    The Ylide Has Landed - PMC

    The formation of the ylide completely transforms the reactivity of the methyl group carbon atom from electrophilic (electron-pair acceptor) to nucleophilic (electron-pair donor), which the enzyme CmoA facilitates and exploits, in conjunction with CmoB, in the biosynthesis of 5-oxyacetyluridine.

    Ylide - (Organic Chemistry) - Vocab, Definition, Explanations

    An ylide is a type of zwitterion, a molecule that contains both a positively and a negatively charged atom, typically carbon and a heteroatom like phosphorus or sulfur. Ylides are important intermediates in organic reactions, particularly in the Wittig reaction, which is used to form carbon-carbon double bonds.

    Mononuclear Ylide Complexes, Ylide Formation and Examples of Ylide …

    A ylide is a chemical molecule in which two neighbouring atoms have both a positive and a negative formal charge. One charge of phosphorus ylides is created at a time. The positive charge on phosphorus must first be organised into a phosphonium ion.

    IUPAC - ylides (Y06728)

    Feb 24, 2014 · If X is a saturated atom of an element from the first row of the periodic system, the ylide is commonly represented by a charge-separated form; if X is a second, third, etc. row element uncharged canonical forms are available R m X=YR n.