Ylides semistabilized

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... 

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]. 

The major limitation of asymmetric sulfur ylide epoxidations is that only aromatic vinylepoxides can be formed efficiently and with high selectivity. When an aliphatic aldehyde is allowed to react with a semistabilized or nonstabilized sulfur ylide, poor diastereoselectivities and yields are observed, due to problems in controlling the ylide conformation and competing ylide rearrangement reactions [71]. However, some racemic, aliphatic vinylepoxides have been successfully formed by sulfur ylide epoxidations, although varying diastereoselectivities were observed [78-80],... 

The results of ab initio (HF and MP2) and MNDO-PM3 molecular modelling of the Wittig reaction of unstabilized (Me3P=CHMe), semistabilized (Me3P=CHC=CH) and stabilized (Me3P=CH-CN) ylides with ethanal have been compared with experimental findings. ... 

Nesmeyanov et al. reported that some stabilized and semistabilized arsonium ylides reacted with a few aldehydes, acetone, and cyclohexanone to form corresponding olefins (71). 

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... 

Trippett and Walker (97) found that the reaction of para-substituted benzylidene triphenylarsoranes with carbonyl compounds led to olefins when the benzylidene para substituent is highly electron withdrawing, but otherwise to epoxide. KumaricY al. (64) prepared two new semistabilized arsonium ylides, p-bromo- and p-iodobenzylidene triphenylarsorane, which were treated with a series of carbonyl compounds to yield exclusively trans-olefins. In no case was an epoxide obtained. 

Recently, Indian workers (61) reported that 2-naphthylmethylene triphenylarsorane reacted with substituted benzaldehydes to give the corresponding epoxides exclusively, whereas l-bromo-2-naphthylmethylene triphenylarsorane reacted with substituted benzaldehydes to give only olefins. In no case were both olefin and epoxide isolated and in all cases only fram-epoxides or trans-olefins were detected. It has also been reported that reaction of indole-3-carboxaldehyde with semistabilized ar-sonium ylides followed only the trans -carbonyl olefination (91). 

Allen et at. found that high yields of the dissymmetric trans-2,3-diaryloxirans result from the reaction between semistabilized arsonium ylides derived from the benzyl salts of the enantiomers of racemic o-phen-ylenebismethyl phenylarsine and of methyl a-naphthyl-p-tolylarsine upon reaction with prochiral aromatic aldehydes. Optical yields of between 4.7 and 38% are obtained using optically active arsonium salts (2). 

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. 

All P ylides for Wittig reactions are obtained by deprotonation of phosphonium salts. Depending on whether one wants to prepare a nonstabilized, a semistabilized, or a stabilized ylide, certain bases are especially suitable (see Table 11.1 an unusual, i.e., base-free, generation of ylides is described in Side Note 11.1). In stereogenic Wittig reactions with aldehydes, P ylides exhibit characteristic stereoselectivities. These depend mainly on whether the ylide involved is nonstabilized, semistabilized, or stabilized. This can also be seen in Table 11.1. 

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. 

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. 

Restrepo-Cossio, A. A., Cano, H., Mari, F., Gonzalez, C. A. Molecular modeling of the Wittig reaction. 6. Theoretical study of the mechanism of the Wittig reaction ab initio and MNDO-PM3 treatment of the reaction of unstabilized, semistabilized and stabilized ylides with acetaldehyde. Heteroatom Chem. 1997, 8, 557-569. 

Beyond their ubiquitous role in organic synthesis, stabilized, semistabilized, or nonstabilized phosphonium ylides are fascinating ligands of transition metals. Their coordination chemistiy is dominated by C-coordination to the metal center they are known to act exclusively as carbon-centered ligands rather than as C=P ligands. 

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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