Ylids phosphine oxides

There are many examples of Wittig-type reactions in the literature, in which the behavior of phosphonium ylids, phosphine oxide ylids, and phosphonate ylids is significantly different. 

From Azides and x-Acylphosphorus ylids Addition of azides to a-acylphosphorus ylids takes place at room temperature in dichloromethane or at 80°C in benzene, giving triazolines from which a phosphine oxide is spontaneously eliminated. " The ylids exist almost exclusively in the cis-enolate configuration, and a mechanism involving concerted 1,3-dipolar addition has been proposed (Scheme 12) on the basis that there is a low entropy of activation for the reaction, and that the reaction rate is insensitive to changes in solvent polarity. ... 

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. 

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

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. 

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

Phosphine oxide anions are often superior to ylids in olefmation reactions, and the anion of 68, made with lithium di-isopropyl-amide (LDA), has none of the disadvantages of the ylid 67. We have made 41 a range of vinyl ethers 70 this wayc, and as part of a synthesis of stiychnos alkaloids43, we were able41 to convert the acyl indole 71 into the aldehyde 72... 

A.iu. Phosphine Oxides and Phosphonate Esters. Many extensions of the Wittig reaction have been introduced that improve or modify the reactivity and/or stereoselectivity of the ylid. Horner et al. showed that a-lithiophosphine oxides such as that derived from 552 react with aldehydes or ketones to give a p-hydroxy phosphine oxide (553) as an isolable species. Subsequent treatment with base liberates the alkene, (554). Wadsworth and Emmons modified the Horner reaction to use phosphonate ester derivatives such as... 

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). 

Alkyl phosphonium halides are decomposed by strong heating to give tertiary phosphines (6.32), whereas phosphonium hydroxides easily give phosphine oxide and hydrocarbon (6.109). The action of heat on triphenyl ((-alkoxycarbonyl alkyl) phosphonium salts is to produce ylids (6.391) (Section 6.10). 

Phosphonium ylids undergo a most important reaction with aldehydes and ketones, known as the Wittig reaction (3.111). The products of this reaction are olefinic compounds and phosphine oxides. 

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. 

Very reactive ylids must be handled in a nitrogen atmosphere because they react rapidly with oxygen to give phosphine oxide and aldehyde (6.418). In a restricted supply of oxygen, alkenes may be produced (6.419). Very stable yUds react with ozone bnt not with oxygen. 

The monomer Cl3P=NH is a member of the class of compounds with general formulae R3P=NR, known variously as monophosphazenes, phosphine imides, phosphinimines or iminophosphoranes. They are isoelectronic with phosphonium ylids and with phosphine oxides, all of which can be represented by the alternative ionic formulae which can be compared with phosphoranylidene phos-phoranes (6.454) and (7.440). 

Monophosphazenes are more acidic than phosphonium ylids. Amino triphenylphosphonium bromide is converted by triethylamine (7.475), but the corresponding phosphonium ylid requires metal alkyls (7.476). Phosphine oxides can be obtained from monophosphazenes by the action of carbon dioxide, sulphur dioxide, alkyl isocyanates or ketones ((7.477) through (7.480)). 

Over twenty years ago, Wittig found that alkyl substituted phosphonium salts could be deprotonated by strong bases to yield phosphonium ylids which in turn could react with aldehydes and ketones to yield olefins and the corresponding phosphine oxide [1]. The sequence is formulated in equations 1.1-1.3 for the reaction of methyl-triphenylphosphonium halide with cyclohexanone in the presence of base to give me thylenecyclohexane. 

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