Oxides phosphine, ylids form

Isuclectronic with the phosphine oxides are the phosphorus ylids, R,PCH2. As for the oxides, two resonance forms... 

Stereoselective alkenylation reactions have been achieved using chiral addi-tives " or auxiliaries. Ylids formed from phosphine oxides,... 

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

When the ylid derived from the oxide 7 was treated with benzaldehyde, a 2-methoxy-l,3-diene was formed. This led us to attempt a synthesis of (i) ar-turmerone by a sequence (Scheme 3) in which 7 effectively acts as an acyl anion equivalent. The required aldehyde 8 was synthesized in good yield using initially diphenyl(methoxymethyl)phosphine oxide (3), and then chlorotri-methylsilane-sodium iodide (12) to cleave the enol ether. 

Ylids can be isolated, but are usually used in reactions immediately they are formed. They are nucleophilic species that will attack the carbonyl groups of aldehydes or ketones, generating the four-membered ring oxaphosphetane intermediates. Oxaphosphetanes are unstable they undergo elimination to give an alkene (65% yield for this particular example) with a phosphine oxide as a byproduct. The phosphorus-oxygen double bond is extremely strong and it is this that drives the whole reaction forward. 

Another very important reaction initially involving nucleophilic attack on an aldehyde carbonyl is the Wittig reaction. An ylid adds to the carbonyl forming a betaine intermediate which then decomposes to produce an olefin and a tertiary phosphine oxide. 

Primary phosphines react with ketones and aromatic aldehydes in concentrated hydrochloric acid to give phosphine oxides (6.95, 6.96). With carbenes, ylids are formed (6.97), and with organo tin halides, varions ring and cage componnds (Chapter 9). 

Under the action of water, most phosphonium ylids wiU undergo hydrolysis to hydrocarbon and phosphine oxide in the same way as phosphonium salts (6.102, 6.371), which are most certainly formed as intermediates. 

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