Semistabilized ylide

As the formation of betaines from amide-stabilized ylides is known to be reversible (in contrast with aryl- or semistabilized ylides, which can exhibit irreversible anti betaine formation see Section 1.2.1.3), the enantiodifferentiating step cannot be the C-C bond-forming step. B3LYP calculations of the individual steps along the reaction pathway have shown that in this instance ring-closure has the highest barrier and is most likely to be the enantiodifferentiating step of the reaction (Scheme 1.16) [25]. 

Semistabilized ylides such as benzylidene- (55) or methylene- (59) triphenylarsorane reacted with carbonyl compounds in ether to give olefins as well as oxiranes. For example, reaction of methylene triphenylarsorane with benzophenone gave, besides triphenylarsine oxide... 

This explains the moderate selectivity of only 3 1 obtained here. Because of the triple bond adjacent to the negative charge 15 rather belongs to the semistabilized ylides C than to category A, resulting in the production of T /Z-mixtures. Here neither the kinetically nor the thermodynamically controlled pathway dominates. 

Semistabilized ylides generally react with aldehydes to form mixtures of cis- and trans-oxaphosphetanes before the decomposition to the alkene starts. Therefore, stereogenic reactions of ylides of this type usually give alkene mixtures regardless of whether the work is carried out salt-free or not. 

Arsonium ylides. Semistabilized allylic arsonium ylides are generally not useful in synthesis because they react with carbonyl compounds to form mixtures of epoxides and alkenes. Unexpectedly, the reaction of the semistabilized ylide triphenylarsonium... 

The reaction of p-chlorobenzaldehyde with phenyldiazomethane in the presence of (MeOlsP and catalytic amounts of meso-tetraphenylporphyrin iron chloride (ClFeTPP) resulted in the formation of the corresponding alkenes with an /Z-selectivity of 86 14, but the yield was low (30%). When phenyldiazomethane is generated in situ from the corresponding potassium tosylhydrazone salt, the olefin yield increases to 92% with E/Z-selectivity of 97 3. Thus, high levels of -selectivity are obtained with semistabilized ylides by this method . This process is applied to a wide range of aldehydes and is practical as compared to standard Wittig reaction, and therefore finds applications in industry, o... 

For a recent comparative study of a nonstabilized ylide reacted with an unsaturated aldehyde and a semistabilized ylide with a saturated aldehyde in the synthesis of dienes, see R. Ideses and A. Shani, Tetrahedron, 1989, 45, 3523. 

Figure 1. Proposed mechanistic model for the Wittig reaction of nonstabilized and semistabilized ylides with aldehydes in the presence of soluble metal ions. Formation of the metallated betaines represents the step that determines the stereocontrol of the reaction.

Two mechanistic variations can be envisioned to be operative in some Wittig reactions. In some cases (Z-stereoselective reactions) a very asynchronous reaction with a 2-center "anti" pseudo betaine transition state is involved. The other mechanism involves a more synchronous concerted reaction with a 4-center "syn" transition state. These two mechanisms may compete, e.g., in the reactions of semistabilized ylides with aldehydes where mixtures of Z- and E-alkenes are obtained. Further details of these studies will be provided in future publications, as well as additional data about solvent effects. 

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