Alkenations phosphorus stabilized

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Gosney, I., Rowley, A. G. Transformations via phosphorus-stabilized anions. 1 Stereoselective syntheses of alkenes via the Wittig reaction. Organophosphorus Reagents Org. Synth. 1979,17-153. 

Alkenation of carbonyl compounds can be used as a general method for the synthesis of enol ethers. The Homer-Wittig reaction of the phosphorus-stabilized carbanion (8) with aldehydes or ketones gives the adducts (9), which on heating eliminate to give the enol ethers (10) as a mixture of ( )- and (Z)-isomers. Since the two diastereomeric adducts (9) can be separated and the elimination reaction is stereospecific, this method can be used to prepare the individual geometrical isomers of (10) according to Scheme 2. 

This chapter will discuss carbanion-like reactions that utilize enolate anions. The acid-base reactions used to form enolate anions will be discussed. Formation of enolate anions from aldehyde, ketones, and esters will lead to substitution reactions, acyl addition reactions, and acyl substitution reactions. Several classical named reactions that arise from these three fundamental reactions of enolate anions are presented. In addition, phosphonium salts wiU be prepared from alkyl halides and converted to ylids, which react with aldehydes or ketones to form alkenes. These ylids are treated as phosphorus-stabilized car-banions in terms of their reactivity. 

Stereodefined alkenes are ubiquitous structural motifs in many natural products and pharmaceutics, and, moreover, they serve as a foundation for a broad range of chemical transformations. Nowadays, carbonyl olefination, elimination, alkyne addition, alkenylation, and alkene metathesis constitute the most widely used methods for the stereoselective synthesis of various alkenes [1-3]. Whereas no single method provides a universal solution to stereoselective alkene synthesis, the olefination reactions of aldehydes and ketones with phosphorus-stabilized carbon nucleophiles have enjoyed widespread prominence and recognition owing to their simplicity, convenience, complete positional selectivity, and generally high levels of geometrical control [4-9]. 

The employment of phosphorus-stabilized carbon nucleophiles for alkene synthesis was initiated by the discovery of the Wittig reaction [10], which provides a convenient method for the preparation of a wide variety of polysubstituted alkenes with complete positional selectivity and generally high levels of geometrical control. Moreover, the phosphonium ylides used in the Wittig reaction are readily formed by the addition of suitable bases to the corresponding phosphonium salts, which are commonly prepared by treating alkyl halides with phosphines. 

In contrast to many other methods commonly employed for stereoselective alkene synthesis such as elimination, alkenylation, alkene metathesis, alkyne addition, the JuUa olefination, and the Peterson olefination [1-3], the olefination reactions of phosphorus-stabilized carbon nucleophiles remain very powerful for modem stereoselective alkene synthesis owing to their convenience, complete positional selectivity, and generally high levels of geometrical control. However, further modifications of these olefination reactions are definitely needed to broaden substrate scope, enhance stereoselectivity, and improve environmental impacts. 

It has been suggested that the preferential formation of ( )-alkene on Wittig reaction of amide-substituted phenyl 3-pyridyl ketones with non-stabilized phosphorus ylides which contain a carboxyl terminus is a consequence of either hydrogen bonding or salt... 

The synthetic utility of a-phosphorus- and a-thio-stabilized carbanions is the subject of numerous reviews.21 Notable are additions of phosphonium ylides (237),183 sulfonium ylides (238),l84 ° oxosulfo-nium ylides (239)184 " and sulfoximine ylides (240)184,1 to electron-deficient alkenes which afford nucleophilic cyclopropanation products. In contrast, with a-(phenylthio)-stabilized carbanions, which are not acyl anion equivalents, either nucleophilic cyclopropanation or retention of the hetero substituent occurs, depending on the acceptor and reaction conditions used. For example, carbanion (241) adds to 1,1-... 

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