Stability of Ylids

In non-stabilised ylids the ylid C atom generally has a pyramidal bond configuration whereas in stabilised ylids the arrangement is usually planar or nearly so. According to NMR, a rapid inversion occurs in non-stabilised ylids such as (6.409f) but (6.409e) is almost planar with nearly equal C-C ring distances. 

Phosphonium ylids are formed from phosphonium halides when bases of sufficient strength are added (6.395). The strength of base required depends on the acidity of the a-hydrogen atom which is in turn controlled by the substituents on the a carbon atom (R,R, R =aryl or alkyl). The more R and R can stabilise an adjacent charge, the more easily protons on the carbon atom can be ranoved and the more stable ylid formed. 

Suitable bases include ammonia, NaOH, NaOEt and Et3N. The reversible nature of (6.410) means that phosphonium salts can be considered as Bronsted acids, and phosphonium ylids as the corresponding bases. 

Phosphonium ylids are formed in reactions between phosphines and carbenes (6.97) and between phosphines and carbon tetrahalides (6.85). Phosphines also react with aliphatic diazo compounds to give ylids, provided cuprous chloride is present to prevent the formation of phosphinazines (6.107). 

Some phosphonium ylids may be prepared by simply heating a phosphonium salt (6.391), or by heating a vinyl phosphonium salt with a metal alkyl (6.411). Dihalophosphoranes react with active methylene compounds under conditions in which hydrogen halide may be removed (6.412). Another reaction is that of carbene transfer (6.413) or the action of sodamide on a phosphonium salt at 0°C. 

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Another six-membered cycle with two isolated Si-Si linkages was formed by Schmidbaur554) while investigating the stability of ylids with disilanyl groups. They found a high tendency to the formation of cyclic ylids ... 

The most convincing evidence concerning the relative stabilities of thiophene S,C-ylids, S,N-ylids, and the sulfoxides comes from ab initio restricted Hartree-Foch molecular orbital (MO) calculations, in which the molecular parameters of the ylid 18 and the S-methyl thiophenium salt 19 were compared (84CC859). The ylid (18) has never been isolated, nor would one intuitively expect it to enjoy any real stability. However, it was chosen to simplify the MO calculations. The stability of ylids such as 14 and 15 is almost certainly due to the ability of these systems to delocalize the formal negative charge over several atoms, and clearly this facility is not present in 18. Thus, 18 represents a highly localized ylidic structure. In principle, the salt 19 can be... 

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

On the other hand, its close relationship to the X -phosphorins as well as the extreme stability of a large series of compounds having hetero atoms at the phosphorus were more in line with a 67r-electron aromatic system. Indeed, many of these compounds fail to display typical ylid reactions. Current arguments support the aromatic nature of X-phosphorins 3 as we shall demonstrate. 

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. 

Nevertheless, the stabilization of the ylid by the cyclic delocalized dtr-electron system presents some interesting theoretical problems. In this connection Markl has coined the term non-classical phosphabenzene . We will return to this point (p.115). 

Addition of DMAD to the carbonyl-stabilized nitrogen ylid 329 gave the furan 330 and the isoindolenine 331. This and similar reactions201 could involve either a symmetry allowed concerted cycloaddition or a nonconcerted reaction followed by elimination of the tertiary amine. 

The third area of interest has been the emergence of use of silylmethylamino derivatives as precursors to 1,3-dipolar, non-stabilized azomethine ylids. The ability of the a-aminosilanes to generate the azomethine ylids in the presence of the dipolarophile attracted the attention of organic chemists (See Section VII). 

A large variety of silylmethylamino derivatives have been shown to be excellent starting materials for the in situ generation of non-stabilized azomethine ylids. Combined with a number of electron-deficient olefinic or acetylenic molecules that have been recognized as good dipolarophiles, ready access to diversely substituted five-membered ring nitrogen heterocycles has consequently been opened. 

Nucleophiles can be added to acceptor-substituted olefins. In that case, enolates and other stabilized carbanions occur as intermediates. Reactions of this type are discussed in this book only in connection with 1,4-additions of ylids (Section 9.2.2), organometallics (Section 8.6), or enolates (Section 10.6) to a,/3-unsaturated carbonyl and carboxyl compounds. 

No base was needed in either of the last two examples the stable ylid itself was used as a reagent. The stability of the enolate ylid means that the Wittig reagent must act as the enol partner and the other compound as the electrophile. 

The reactions of the initial ylid vary depending on the stability of the product(s) and the coreactants—compare equation (237) with equation (241) - If a... 

When we are talking about S ylids or P ylids, stabilized refers to stabilization of the carbanion as explained in Chapter 31. 

It might be expected that the stability of the sodium bromide complex of the ylid would be much lower than the lithium bromide complex. Thus, when the ylid is prepared using a mixture of phenylsodium and phenyl-lithium, decomposition also occurs, but in this case the predominant product is dimethylethylamine 179>. 

This mechanism differs from the preceeding mechanism primarily in that the lithium ion is used to stabilize the ylid and a dilithium reagent is thought to be the intermediate leading to 7 and 10. The existence of dilithium reagents has been noted in the reactions of the tetramethyl-ammonium ion with excess phenyllithium in ether 170> and it is quite possible that they can be formed here. 

The stabilization of the benzhydryl ylid is expected to be quite high since the related fluorenylid (2) can be isolated as a salt free solid. The high yield of 32 was attributed to hydrogen atom abstraction from the cyclohexene by the diphenylcarbene and subsequent coupling of di-phenylmethyl radicals 46>. 

The greater the stabilization of the carbanionic portion of the ylid, e.g., benzyl, the more likely the ylid path is followed. 

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