YLIDES Dr. A. G. Nikalje

Definition of ylide: An ylide or ylid  is  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.

Resonance Structure: 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).

Structure Of Ylide: Ylene form

Types Of Ylides: Phosphonium Ylides. Sulfonium Ylides. Oxonium Ylides. Azomethine Ylides. Halonium Ylides.

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, butyl lithium) to form the ylide.

Structure of the ylide

reaction 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.

Sulfonium Ylides It is based on sulfur.Sulfonium ylides are used in the preparation of epoxides or in the stevens re arrangment. In this quaternary ammonium salts & sulfonium salts with corresponding amines or sulfides in presence of a strong base is used in a 1,2- rearrangment.

reaction

Oxonium Ylides They can be form by ring opening of epoxides. They are prepared by rection of ethers with diazo compounds.

Azomethine ylide They are nitrogen based 1,3-dipoles consisting of an iminum ion next to a carbanion. They are used in 1,3-dipolar cycloaddtion reaction to form 5 membered heterocycles,including pyrrolidines &pyrrolines. These reaction stereo & regioselective. They have potential to 4 new contiguous steroceneters. They have high utility in total synthesis & formation of chiral ligand &pharmaceuticals. They can be generated from many sources including aziridines,imines & iminiums.

Structure of azomethine ylide

Halonium ylide Prepared from allyl halides & metal carbenoids. After a [2,3]-rearrangment a homo-allylhalide is obtained. Like the wittig reagent,it is able to replace the oxygen atom o carbonyl groups with a methylene group. Compared with the wittig reagent it has more functional group tolerance.

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