Non-stabilized ylides

With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins. Seebach et al... 

Bis-ylides (Scheme 31) may function as four-electron donors and become geminally diaurated. The donor capacity of ylides is generally higher than that of most other ligands, and even powerful donors like tertiary phosphines can thus be readily replaced by non-stabilized ylides. Ylides may therefore be used as auxiliary ligands which are retained, while other components of the coordination sphere are subject to substitution. 

Pheromones.—A careful examination of the factors which affect the ratio of the cis-cis- and mww-c/.y-alkenes formed from the reaction of thealdehyde (69) with a non-stabilized ylide has been described.70 Methods were specifically developed to give the... 

In sharp contrast to the olefmation reaction employing stabilized telluronium ylides, semi-and non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides). 

This was the first-described, non-stabilized ylide, obtained by treatment of the corresponding telluronium tetrafluoroborate with hthium 2,2,6,6-tetramethylpiperidide (LiTMP). Epoxides are obtained by reaction with both aldehydes and ketones. ... 

The bis-ylide ligands [R2P(CH2)2] can easily be prepared by treatment of the phosphonium salts [R2PMe2] (R = Me, Ph) with strong deprotonating reagents such as lithium derivatives, NaNH2, or non stabilized ylides R3P=CH2. However, Au Ag, and Cu complexes (99) were prepared by reaction of ClMPMe3 (M = Ag, Au) or CuCl with excess of R3P=CH2 [202, 203] (Scheme 30). 

It has been the practice3 to divide ylides into stabilized, semi-stabilized and non-stabilized. It can be seen that this division breaks down somewhat for structural studies, as shown by the variation in P=C bond lengths and the more varied nature of the substituents in more recent studies. However, an effort has been made in Table 1 to quote non-stabilized ylides first, followed roughly by semi-stabilized, then stabilized and finally sp ylides. 

The results indicate that the kinetic acidity is markedly lower for phosphonium salts than for the corresponding nitroalkanes, but cannot be correlated with pKa values. In contrast, the measurement of kinetic acidity using double potential step chronoamperometry180 allowed the determination of pka values for a series of phosphonium salts corresponding to semi-stabilized or non-stabilized ylides ... 

Non-stabilized ylides lack any substituent on carbon that can conjugate with the P=C double bond. In other terms, the substituents attached to the carbanion cannot assist in delocalizing the anion. Although these types of ylides are reactive, a large number of examples have been prepared and their single-crystal X-ray structures determined. We... 

TABLE 1. X-ray structural data for non-stabilized ylides (Class l)fl... 

The geometry about the P=C double bond for the Class 2 non-stabilized ylides is similar to the structure of the Class 1 ylides. The P=C distances range from 1.618 to 1.727 A. The phosphorus atom is a slightly distorted tetrahedron and the carbon atom is slightly pyramidal. One of the phosphonium substituents bisects the plane formed by the ylidic carbon and its two substituents, and this plane is bent toward the perpendicular P substituent, as shown in Figure 3. 

Table 8 presents the Amax values for a series of stabilized79 and non-stabilized80 ylides. For the non-stabilized ylides, the reference compound is Ph3PCH2 (4), which has its imax at... 

In conclusion, the dominant resonance structures for the ylides are as follows for non-stabilized ylides, A for stabilized ylides, 16b and 16c for P=C=P ylides, 22 (although further study on this class is advised to determine if the gas-phase structure may in fact be linear) and for P=C=C ylides, 28b and 28c. 

In this example, a successful tandem Mn02 oxidation-Wittig reaction is achieved, using a non-stabilized ylide, thanks to the employment of pre-dried Mn02, the guanidine (78) as... 

The Wittig Reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt. The geometry of the resulting alkene depends on the reactivity of the ylide. If R is Ph, then ihe ylide is stabilized and is not as reactive as when R=alkyl. Stabilized ylides give ( )-alkenes whereas non-stabilized ylides lead to (Z)-alkenes. 

With non-stabilized ylides ( R = H or Me) and numerous saturated or unsaturated aldehydes and ketones the Wittig reaction ( i ix) gives the corresponding dithianes 7 in good yields (70-93 %) The autoxidation of these ylides gives the expected products 8 and 9. The semi-stabilized ylides (R1= 0) react only with a reactive aldehyde. 

Ylide type non-stabilized ylide semi-stabilized ylide stabilized ylide... 

Although the Schlosser modification of the Wittig reaction provides access to trans-olefins from non-stabilized ylides, the Julia-Lythgoe olefination has proven to be the method of choice for solving this synthetic problem today, a) M. Schlosser, K.-F. Christmann, Justus Liebigs Ann. Chem. 1967, 708, 1-35 b) M. Schlosser, K.-F. Christmann, A, Piskala, Chem. Ber. 1970, 103, 2814-2820. 

Scheme 4.22 Mechanism of the Wittig reaction of non-stabilized ylides under salt-free conditions...

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