Carbanionic centres, stabilization

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Benzylic compounds have—compared to the corresponding methyl derivatives—a higher thermodynamic acidity by 10 to 15 pATa units . Mesomeric stabilization requires a considerable flattening of the carbanionic centre towards sp hybridization (the sum of bond angles is 360° for sp and 328° for sp ). However, we should be aware that even if the carbanionic framework would be completely planar, the ion pair 209 is a planar-chiral species. For epimerization, the cation has to migrate from one face to the other one (equation 48). Due to a more facile flipping of the carbanionic centre and an easier formation of solvent-separated ion pairs, most of chiral benzyUithium compounds 208/ewi-208 racemize with great ease. 

Allylic compounds have acidities similar to those of the corresponding benzylic analogues . Due to the mesomeric stabilization, the carbanionic centre is considerably... 

The regioselective functionalization of nitrobenzene and benzonitrile derivatives has been performed via nucleophilic aromatic substitution of hydrogen by phosphorus-stabilized carbanions.41 Lithium phosphazenes have been found to be the most suitable nucleophiles for the substitution of hydrogen in nitrobenzene. This method represents a convenient alternative to the vicarious nucleophilic substitution for the synthesis of benzylic phosphorus derivatives using phosphorus-stabilized anions that do not bear a leaving group at the carbanionic centre. 

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 centre , the enantioselective approach of such stabilized carbanions for synthesis has been the subject of recent developments. 

On the other hand, the pKa values of a series of substituted nitromethanes [83] (Table 4.11) suggest that, whilst chlorine bonded directly to the carbanion centre increases acidity relative to hydrogen, fluorine decreases acidity and, therefore, decreases the stability of the corresponding carbanion. 

However, in some cases, HF is eliminated in preference to dehydrobromination, e.g. in the succinic acid series [5, 6] (Figure 6.4). In these less common processes, the transition state has significant carbanion (ElcB-like) character, and the products are probably governed by the relative stabilities of the possible carbanionic transition states, 6.5A and 6.5B (Figure 6.5), where a fluorine atom situated (3 to a developing carbanion centre, as in 6.5B, is more stabilising than when directly attached, as in 6.5A. This effect is also seen in eliminations from dihaloacenaphthenes [7]. 

Sulfur ylides are useful reagents in organic synthesis. The ylide is formally a zwitterion in which a carbanion is stabilized by interaction with an adjacent sulfonium centre. They are usually prepared by proton abstraction from a sulfonium salt with a suitable base or by reaction of a sulfide with an alkylating agent such as Me30+BF4 or a carbene formed, for example, by metal-catalysed or photolytic decomposition of a diazo compound (1.103). 

Dithioacetals and related compounds have a number of uses in synthesis, both in Friedel-Crafts-type reactions and in reactions which exploit the high reactivity of carbanions stabilized by two or more sulphur atoms bonded at the carbanion centre. Benzyl orthothioformate, CH(SCH2Ph)3, may be used to introduce a — CH(SCH2Ph)2 group into 2- and 3-positions of an indole, with TiCU as catalyst at - 5 °C in CHQ3, and has been used in this way for 2-substitution of (79), giving (80) and (81), with further elaboration of (80) into 2-methyl-lysergic acid. Studies of ethyl dithio-... 

Mono-, bis-, and tris-(phenylseleno)methyl-lithium compounds have been prepared. They are the first representatives of their class, and early results suggest that the PhSe group stabilizes an adjacent carbanion centre nearly as well as PhS. 

The species (10) and (11), may be considered as synthons for HSCH2 and H0(CH2) CH2 respectively, whilst (12) reacts with aldehydes and ketones to give oxiranes. Selenium is capable of stabilizing an adjacent carbanion centre in the same way as sulphur, and reaction of diselenoketals with Bu"Li provides a convenient approach to reagents of type (13). After alkylation, hydroxyalkylation, or acylation of (13) the selenium can be removed reductively, by oxidative elimination, or, in the case of 2-hydroxy compounds, by elimination to form an oxirane, so the synthetic potential is clear. 

In contrast to the early theoretical work of Rank and coworkers , C-NMR investigations had revealed that the metallated carbon atom in the a-sulphinyl carbanion is nearly planar . A four-centre chelate structure 315 has been proposed for a-lithiosulphoxides, and it is believed to be responsible for the planar configuration of the anionic carbon atom and for the greater stability of o(-sulphinyl carbanions in comparison with a-sulphenyl carbanions This chelation favours one of the two diastereoisomeric carbanions and for this reason a-sulphinyl carbanions react with electrophiles in a highly stereoselective manner (see below). 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

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