A ylide or ylid (// or //) 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. Ylides are thus 1,2-dipolar compounds. 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".
Many ylides may be depicted by a multiple bond form in a resonance structure, known as the ylene form:
The actual electron distribution in the molecules and hence the relative importance of the ylide and ylene forms is dependent on the "onium" center and substituent pattern (the identity of the various R groups)
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, butyl lithium) 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.
Other ylide types
Based on sulfur
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
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.
Stable carbenes also have a ylidic resonance contributor e.g.:
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.
An important ylide reaction is of course the Wittig reaction (for phosphorus) but there are more.
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 a by product, and forms a silyl-stabilised ylide.
The initial addition reaction is followed by an elimination reaction.
- Zwitterion: a neutral molecule with one or more pairs of positive and negative charges
- Betaine: a neutral molecule with an onium cation and a negative charge
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- (i) Reaction of allyl alcohol with 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane forms a phosphite ester. (ii) Metal carbene addition (from ethyl diazoacetate and ClFeTPP) forms an ylide. (iii) A rearrangement reaction (in blue) yields a phosphonate.
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