Heteroatom-stabilized carbanion

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). 

Additions of a-Heteroatom-stabilized Carbanions (Non-acyl Anion Equivalents) 115... 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabilized carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetralds(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

Organometallics, and Heteroatom-Stabilized Carbanions on the Carboxyl Carbon... 

Many hydride donors, organometallic compounds, and heteroatom-stabilized carbanions react with carboxylic acids and their derivatives. However, the corresponding substitution... 

Fig. 6.40. On the chemo-selectivity of the reactions of hydride donors, organometallic compounds, and heteroatom-stabilized "carbanions with acylating agents (kM t refers to the rate constant of the addition of the nucleophile to the carboxyl carbon, and kadd2 refers to the rate constant of the addition of the nucleophile to the carbonyl carbon).

For the reaction of hydride donors, organometallic compounds and heteroatom-stabilized carbanions with acylating agents or carbonyl compounds one encounters a universal reactivity order RC(=0)C1 > RC(=0)H > R2C=0 > RC(=0)0R > RC C NR It applies to both good and poor nucleophiles, but—in agreement with the reactivity/selectivity principle (Section 1.7.4)—the reactivity differences are far larger for poor nucleophiles. 

Fig. 6.41. Three strategies for the chemoselective acylation of hydride donors, organometallics and heteroatom-stabilized "carbanions" with carboxylic acid derivatives.

Acylation of Organometallic Compounds and Heteroatom-Stabilized Carbanions With Carboxylic Acid (Derivative)s Synthesis of Ketones... 

In Figure 6.41, the reaction of certain heteroatom-stabilized carbanions with carboxylic acid derivatives is presented as strategy 3 of Figure 6.32 for achieving chemoselective acylations. This strategy can be used to convert esters into /i-ketophosphomc acid esters with... 

Most SN reactions of hydride donors, organometallic compounds, and heteroatom-stabilized carbanions at the carboxyl carbon follow the mechanism shown in Figure 6.2. Thus the substitution products, i.e., the aldehydes and ketones C, form in the presence of the nucleophiles. Thus, when the nucleophile and the acylating agent are used in a 2 1 ratio, alcohols F are always produced. 

The SN reaction under consideration is not terminated until water, a dilute acid, or a dilute base is added to the crude reaction mixture. The tetrahedral intermediate B is then protonated to give the compound E. Through an El elimination it liberates the carbonyl compound C (cf. discussion of Figure 6.4). Fortunately, at this point in time no overreaction of this aldehyde with the nucleophile can take place because the nucleophile has been destroyed during the aqueous workup by protonation or hydrolysis. In Figure 6.32 this process for chemoselective acylation of hydride donors, organometallic compounds, and heteroatom-stabilized carbanions has been included as strategy 1. ... 

Related chemistry not covered in this chapter includes the acylation of stabilized organometallics and the broad category of enolate or metalloenamine C-acylation (Volume 2), the Claisen or Dieckman condensations (Chapter 3.6, Volume 2), the acylation of heteroatom-stabilized carbanions (Part 2, Volume... 

---