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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'.
With unstabilised ylides (R 3 = alkyl) this results in -alkene product with moderate to high selectivity. If the reaction is performed in dimethylformamide in the presence of lithium iodide or sodium iodide, the product is almost exclusively the Z-isomer. [16] With stabilized ylides (R 3 = ester or ketone), the (E)-alkene is formed with high ...
Because phosphonium ylides are seldom isolated, the byproduct(s) generated upon deprotonation essentially plays the role of an additive in a Wittig reaction. As a result, the choice of base has a strong influence on the efficiency and, when applicable, the stereochemical outcome of the Wittig reaction.
Georg Wittig (German: [ˈɡeː.ɔʁk ˈvɪ.tɪç] ⓘ; 16 June 1897 – 26 August 1987) was a German chemist who reported a method for synthesis of alkenes from aldehydes and ketones using compounds called phosphonium ylides in the Wittig reaction. He shared the Nobel Prize in Chemistry with Herbert C. Brown in 1979.
In contrast to phosphonium ylides used in the Wittig reaction, phosphonate-stabilized carbanions are more nucleophilic but less basic. Likewise, phosphonate-stabilized carbanions can be alkylated. Unlike phosphonium ylides, the dialkylphosphate salt byproduct is easily removed by aqueous extraction. Several reviews have been published.
Phosphorus ylides are unsaturated phosphoranes, known as Wittig reagents, e.g. CH 2 P(C 6 H 5) 3. These compounds feature tetrahedral phosphorus(V) and are considered relatives of phosphine oxides. They also are derived from phosphonium salts, but by deprotonation not alkylation.
Crystallographic characterization of the colourless ylide reveals that the phosphorus atom is approximately tetrahedral. The PCH 2 centre is planar and the P=CH 2 distance is 1.661 Å, which is much shorter than the P-Ph distances (1.823 Å). [5]
They are derived from phosphonium salts. A strong base such as butyllithium or sodium amide is required for the deprotonation: [Ph 3 P + CH 2 R]X − + C 4 H 9 Li → Ph 3 P=CHR + LiX + C 4 H 10. One of the simplest ylides is methylenetriphenylphosphorane (Ph 3 P=CH 2). [6] The compounds Ph 3 PX 2 (X = Cl, Br) are used in the Kirsanov reaction ...