Anylide (/ˈɪlaɪd/)[1] orylid (/ˈɪlɪd/) is aneutraldipolarmolecule containing a formally negatively chargedatom (usually acarbanion) directly attached to aheteroatom 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 acovalent and anionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass ofzwitterions.[2] They appear inorganic chemistry asreagents orreactive intermediates.[3]
The class name "ylide" for the compound should not be confused with thesuffix "-ylide".
Many ylides may be depicted by amultiply bonded form in aresonance structure, known as the ylene form, while the actual structure lies in between both forms:[citation needed]
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.[citation needed]
Phosphonium ylides are used in theWittig reaction, a method used to convertketones and especiallyaldehydes to alkenes. The positive charge in theseWittig reagents is carried by aphosphorus atom with threephenyl substituents and a bond to acarbanion. Ylides can be 'stabilised' or 'non-stabilised'. A phosphonium ylide can be prepared rather straightforwardly. Typically,triphenylphosphine is allowed to react with analkyl halide in a mechanism analogous to that of anSN2 reaction. Thisquaternization 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 astert-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 withbiological and pharmacological activities. Much of the interest in the coordination properties of a-keto stabilized phosphorusylides stems from their coordination versatility due to the presence of different functional groups in their molecular structure.
The a-keto stabilized ylides derived from bisphosphines likedppe,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 bothphosphine 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 likepalladium andplatinum.[4]
Other common ylides includesulfonium ylides andsulfoxonium ylides; for instance, theCorey-Chaykovsky reagent used in the preparation ofepoxides or in theStevens rearrangement.
Carbonyl ylides (RR'C=O+C−RR') can form by ring-opening ofepoxides or by reaction ofcarbonyls with electrophiliccarbenes,[5] which are usually prepared fromdiazo compounds.Oxonium ylides (RR'-O+-C−R'R) are formed by the reaction ofethers with electrophiliccarbenes.
Certainnitrogen-based ylides also exist such asazomethine ylides with the general structure:
These compounds can be envisioned asiminium cations placed next to acarbanion. Thesubstituents R1, R2 areelectron withdrawing groups. These ylides can be generated by condensation of an α-amino acid and analdehyde or by thermal ring opening reaction of certain N-substitutedaziridines.
The further-unsaturatednitrile ylides are known almost exclusively as unstable intermediates.
A rather exotic family of dinitrogen-based ylides are theisodiazenes (R1R2N+=N–), which generally decompose by extrusion of dinitrogen.
Stable carbenes also have a ylidic resonance contributor,e.g.:
Halonium ylides can be prepared from allyl halides and metalcarbenoids. After a [2,3]-rearrangement, a homoallylhalide is obtained.
The active form ofTebbe'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.
An important ylide reaction is of course theWittig reaction (for phosphorus) but there are more.
Some ylides are1,3-dipoles and interact in1,3-dipolar cycloadditions. For instance an azomethine ylide is a dipole in thePrato reaction withfullerenes.
In the presence of thegroup 3homolepticcatalyst Y[N(SiMe3)2]3, triphenylphosphonium methylide can be coupled withphenylsilane.[6] This reaction produces H2 gas as a byproduct, and forms a silyl-stabilised ylide.
Many ylides react insigmatropic reactions.[7] TheSommelet-Hauser rearrangement is an example of a [2,3]-sigmatropic reaction. TheStevens rearrangement is a [1,2]-rearrangement.
A [3,3]-sigmatropic reaction has been observed in certain phosphonium ylides.[8][9]
Wittig reagents are found to react as nucleophiles inSN2' substitution:[10]
The initial addition reaction is followed by anelimination reaction.
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