Carbanions heteroatom-substituted

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

The Andersen sulphoxide synthesis allows one also to synthesize a variety of a-heteroatom substituted sulphoxides starting from a-heteroatom stabilized carbanions and (—)-(S)-276. The selected examples shown in Scheme 3 are the best illustration of the generality of this approach. The reaction of enolates or enolate like species with (—)-(S)-276 has been used for the synthesis of optically active a-carbalkoxy sulphoxides. For example, treatment of (—)-(S)-276 with the halogenomagnesium enolates of -butyl acetate, t-butyl propionate or t-butyl butyrate resulted in the formation of ( + )-(R)-t-butyl p-toluenesulphinylcarboxylates 298367 (equation 163). 

Since a great number of such transformations were described in the chemical literature, only selected examples of general importance will be presented here. This section will consist of the following parts reactions of the sulphoxide a-carbanions introduction, substitution, transformation and elimination of heteroatomic groups attached to organic substituents in sulphoxides additions to unsaturated sulphoxides other modifications of organic substituents in sulphoxides. 

Me3SiCl reacts with phosphinomethanides I (R=Me) with at least one hydrogen as carbanion substituent (X = Y=H X=H, Y=SiMe3, PMej) via Si-C bond formation to give heteroelement substituted phosphinomethanides [4]. With fully C-heteroatom-substituted I, the reaction depends on the nature of X and Y, as shown by Eqs. (l)-(3) ... 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

Scheme5.20. Possible decomposition reactions of a-heteroatom substituted carbanions. X=0, NR, NR2+.

Such reactive carbanions can also act as reducing agents or as strong bases, and can lead to numerous unexpected reactions [197]. Few solvents are sufficiently inert to withstand them [198], Chelate-stabilized, a-heteroatom-substituted carbanions can, on the other hand, be quite stable, and in recent years numerous useful transformations involving these intermediates have been developed. 

Allylic and propargylic heteroatom-substituted carbanions can yield rearranged or unrearranged products on treatment with an electrophile. The regio- and stereoselectivity of these reactions depends on the precise structure of the carbanion, on the metal and solvent chosen [199], and on the structure of the electrophile [150, 200-203], and can be difficult to predict. 

In addition to deprotonation with strong bases, halogen-metal exchange, or trans-metalation, a-heteroatom-substituted carbanions can also be prepared by 1,5-hydro-gen transfer to vinyl radicals, followed by reduction [204, 205] (Scheme 5.21) or by a related intramolecular 1,4-proton transfer [206]. 

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. 

Some heteroatom-substituted or chelate-stabilized organolithium compounds, on the other hand, can be sufficiently stable toward racemization to enable their use in stereoselective reactions with electrophiles [223, 225, 271, 531, 543, 552-554] (Scheme 5.75). This increased configurational stability of a-heteroatom-substituted carbanions might be due to the stronger pyramidalization of such carbanions [261,555] and fixation of the metal by chelate formation. 

The importance of carbanions a-substituted by heteroatoms in organic synthesis explains the vast amount of literature concerning the use of a-heterosubstituted organotins in transmetallation reactions. As the tin-lithium exchange is assumed to occur with a complete retention of configuration at the carbanion centre639, the enantioselective approach of such stabilized carbanions for synthesis has been the subject of recent developments. 

M. Braun, a-Heteroatom Substituted 1-Alkenyllithium Reagents Carbanions and Carbenoids for C-C Bond Formation , Angew. Chem. Int. Ed. Engl. 1998, 37, 430 451. 

A number of resonance stabilized or heteroatom substituted carbanions have been converted into titanium analogs for the purpose of testing chemoselectivity, e.g., Equation 12 21,77). Generally, an in situ reaction mode was chosen. In all cases a high degree of aldehyde-preference was noted. 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

The use of heteroatom-substituted carbanions has allowed the chemist to access a broad range of relatively stable and selective reagents capable of a wide range of synthetic transformations. ... 

In certain cases, direct elimination of the hydroxysulfone (86) is effective, but generally it is better to go via the acyl derivative (88) which is formed in situ. The Julia reaction essentially resembles the Wittig and Peterson reactions, in that an a-heteroatom-substituted carbanion and a carbonyl compound react together with the subsequent elimination of two vicinal functional groups to form the... 

Thioacetal anions and related anions of type (3 X, Y = SMe or SiMea) can be obtained by addition of organolithiums (RLi) to the corresponding compounds (4)." On the other hand, compounds of type (4 X = Br, SePh, or SnBua) react with Bu"Li to give o-heteroatom-substituted vinyl-lithiums (5), which in cases where Y = SPh or SePh are themselves acyl carbanion equivalents. Interestingly, the vinyl-lithium (5 Y = SPh) can also be used as a source of compounds (4 Y = SPh,... 

A one-pot coupling protocol of arynes with ortho-heteroatom-substituted benzoates to prepare xanthones, thioxanthones, and acridones has been reported by Zhao and Larock (Equation 12.30) [50]. The coupUng presumably proceeds by a tandem intermolecular electrophihc coupling of the aryne with the orthoheteroatom-substituted benzoates and subsequent intramolecular electrophilic cyclization. Proton abstraction by the aromatic carbanion intermediate is a major competing reaction in this cyclization. This side reaction is, however, largely suppressed by employing tetrahydrofuran (THF) as the solvent... 

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