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 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.[4]
Carbonyl ylides (RR'C=O+C−RR') can form by ring-opening of epoxides or by reaction of carbonyls with electrophilic carbenes,[5] which are usually prepared from diazo compounds. Oxonium ylides (RR'-O+-C−R'R) are formed by the reaction of ethers with electrophilic carbenes.
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.
In the presence of the group 3homolepticcatalyst Y[N(SiMe3)2]3, triphenylphosphonium methylide can be coupled with phenylsilane.[6] This reaction produces H2 gas as a byproduct, and forms a silyl-stabilised ylide.
^Ferguson, Marcelle L.; Senecal, Todd D.; Groendyke, Todd M.; Mapp, Anna K. (2006). "[3,3]-Rearrangements of Phosphonium Ylides". J. Am. Chem. Soc.128 (14): 4576–4577. doi:10.1021/ja058746q. PMID16594686.
^Patel, Ramesh M.; Argade, Narshinha P. (2007). "Facile SN2' Coupling Reactions of Wittig Reagents with Dimethyl Bromomethylfumarate: Synthesis of Enes, Dienes, and Related Natural Products". J. Org. Chem.72 (13): 4900–4904. doi:10.1021/jo070728z. PMID17539690.
