Phosphoryl carbanions formation

The successful preparation of (2-hydroxyalkyl)phosphonic acid derivatives by the treatment of an appropriately phosphorylated carbanion with an aldehyde or ketone, has been further extended to the formation of 294 from 293". ... 

The importance of the cation-oxygen interaction has also been pointed out in highly Z-selective olefination reactions of the a-silyl-a-phosphoryl carbanion 19 (Scheme 2.16) [41, 42]. The chelating Li-O interactions are likely to determine the configuration of the carbanion 19 in the approach of the aldehyde. This transition state leads to the formation of the kinetically favored adducts 20, which give the (Z)-alkenes 21 after syn-elimination of the oxophilic silylated moiety and hydrolysis. 

Not only alky] groups, but also aryl [492, 493], vinyl [494], acyl [276, 495—497], alkoxycarbonyl [498], aminocarbonyl [499-501], silyl [502-504], or phosphoryl groups [279, 280] can migrate to a vicinal carbanion (Scheme 5.68). Because some of these groups can be used to stabilize a-heteroatom-substituted carbanions by chelate formation, migration of these groups to the carbanion is a potential side reaction in the generation and alkylation of chelate-stabilized carbanions. 

The mechanism involves initial formation of an acetylated or phosphorylated cation, which reacts with a carbanion to form a salt that is strongly acidic because of its substituent electron-withdrawing groups. This salt is hence readily converted into an ylide by loss of a proton, whose removal is assisted by the acetic anhydride or triethylamine (see equation 27). As with method B the arsine oxide method is limited to the preparation of stable ylides, since its success depends upon the acidity of the methylene compound. Almost all examples of this method have utilized triphenylarsine oxide tri-w-butylarsine oxide has been used in triethylamine but gave only intractable products in acetic anhydride In a modification of this method, an ylide has been prepared by reaction of acetoacetanilide with diacetoxytriphenylarsorane, the latter compound having been prepared from triphenylarsine and lead tetra-acetate. ... 

Michael addition has been applied to the formation of cyclopropanic systems. Thus, the addition of phosphonate carbanions generated by LDA in THF, by NaH in THF, by thallium(I) ethoxide in refluxing THF, or by electrochemical technique to oc,P-unsaturated esters provides a preparation of substituted 2-(alkoxycarbonyl)cyclopropylphosphonates in moderate to good yields via a tandem Michael addition-cyclization sequence (Scheme 8.47). The cyclopropanation has also been achieved via oxidation with iodine in the presence of KF-AI2O3. Nitrile ylides prepared from acyl chlorides and diethyl isocyanomethylphosphonate in the presence of Et3N react with methyl acrylates by a 1,3-dipolar cycloaddition to give phosphoryl pyrrolines or pyrroles. ... 

Scheme 2.60. The Peterson reaction of an a-silyl carbanion bearing a phosphoryl group with ethyl formate.

On the other hand, high Z-selectivity is seen in the olefination reactions of the carbanion 19 derived from 3,3-diethoxybutylphosphonate with aldehydes (Scheme 2.16) [41, 42]. Similarly, Z-selective Peterson reactions of the in situ generated a-phosphoryl-a-(trimethylsilyl)allyl anion 104 with aldehydes or alkyl formates to afford the 2-dienylphosphonates 105 have been reported (Scheme 2.63) [168, 169]. These methods allow access to (Z)-alkenylphosphonates, whereas Wittig-Horner reactions give the thermodynamic ( )-alkenes almost exclusively. These excellent Z-selectivities can be rationalized in terms of the chelation control mechanism (see Section 2.2.2.3). 

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