Carbanions reactions with aldehydes

All that has been said in this section applies with equal force to the use of organo-lithium reagents in the synthesis of alcohols. Grignard reagents are one source of nucleophilic carbon organolithium reagents are another. Both have substantial carbanionic char acter in their- car bon-metal bonds and undergo the same kind of reaction with aldehydes and ketones. 

Lithiation of to form 184 was reported by the Harmata group to be the first example of a sulfoximine-stabilized vinyl carbanion. The resulting organolithium species 184 reacted with various electrophiles to supply structurally diverse benzothiazines <88TL5229>. However, the diastereoselectivity of the reactions with aldehydes was low (Scheme 48). 

Anodic oxidation of the carbamates 17 and 23 in methanol, followed by reaction with chlorodiphenylphosphine affords the a-diphenylphosphinylcarbamates 20 and 25, from which the readily generated carbanions react with aldehydes to give the 4-phosphinyl-2-oxazolidinones 21 and 26. The removal of the diphenylphosphinyl group by a mild thermal treatment provides a route to the 2(3//)-oxazolones 22 and 27 (Fig. 5.6). ... 

The ylid character of X5-phosphorins and their Cr(C0)3 complexes again is evident when one or both groups on phosphorus are CHR2 as one can abstract a+ protpn giving a carbanion. Reaction with electrophiles (e.g. D, CH3, and RCHO) causes side chain addition. No Wittig olefination is found with aldehydes. Instead a 1(2 -hydroxy) product 9 is formed which can be dehydrated to the X5-phosphorin derivative 10. 

The nature of the substituents on a stabilized phosphonate carbanion is known to influence the stereochemical outcome of their reactions with aldehydes. For example, a bis(2,2,2-trifluoroethyl) substituent reverses the normal preference for (E) alkenes in a sequence referred to as the Still modification of the HWE reaction (see Protocol ll).24 This substituent is thought to favour formation of the (Z)-isomeric alkene by greatly enhancing the rate of the elimination of the phosphine oxide to give the alkene, which then suppresses equilibration to the thermodynamic product. 

It may rather be that in the case of third row substituents like R4P the relative acidity of cyclopropyl and isopropyl hydrogens is not as indicative of the carbanion configuration as in the case with the second row substituents C(0)R, CN and possibly NO2. As far as cyclopropyl sulfones are concerned one would therefore predict that the corresponding a-sulfonyl cyclopropyl anions may have a pyramidal configuration. The deprotonation of cyclopropyl phosphonates, followed by reaction with aldehydes, was recently published by Hirao and coworkers ... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

Sulfonyl carbanions undergo aldol-type reactions with aldehydes and ketones to give p-hydroxy sulfones which can be converted into alkenes (the Julia reaction) (see Chapter 10, p. 197). With allyl methyl sulfones (75) and a,p-unsaturated carbonyl compounds (63),... 

Reduction of chlorodifluoromethylketones (32) with tetrakis(N,N-dimethyl-amino)ethene (35) produced carbanion (33) via two-electron reduction, which underwent aldol reaction with aldehydes to form 34 [28]. On the other hand, one-electron reduction of 32 with thiolates produced 36 via radical 35 [29] (Scheme 2.41). 

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. 

These tetrahedral structures are capable of interconversion by pyramidal inversion and C—S bond rotation. Williams suggests that the relative reactivity of the above a-lithio sulfinyl carbanion diastereoisomers with aldehydes (and therefore the stereoselectivity of the reaction) is governed by the steric environment of the process. 

Reactions with Aldehydes and Ketones. The carbanion derived from the treatment of (Me0)2P(0)CH(SMe)2 with a base reacts with cyclic and acyclic, aliphatic and aromatic aldehydes and ketones in a Horner—Wadsworth—Emmons reaction to give ketene 5,5-thioacetals in high yields " (eq 2 ). The carbanion is generated using either butyllithium in THE at —78 °C > or, less commonly, sodium hydride in DME. Alternatively, with aromatic aldehydes the reaction may be performed under two-phase conditions using benzyltriethylammonium chloride (TEBA) as a phase-transfer catalyst. ... 

Anionic Additions to Aldehydes. The /dCa of trans-, >-dithiolane 1,3-dioxide has been determined by Bordwell and disclosed by Aggarwal to be 19.1, a surprisingly low value compared to Trans-1,3-dithiane 1,3-dioxide (24.9). While the deprotonation of 1,3-dithiolane and 1,3-dithiolane 1-oxide leads to unstable carbanions that cleave, the anion of Trans-1,3-dithiolane 1,3-dioxide has shown sufBcient stability to undergo addition reactions with aldehydes. Moreover, because of the Cj-symmetry incorporated into a five-membered ring, its potential to serve as a chiral acyl anion equivalent has been tested. 

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