Carbanions stabilisation

In the above discussion the problem of stereoisomeric control has been set aside. Total stereoisomeric purity is in fact difficult to achieve, and extensive mechanistic studies have been carried out to elucidate the factors which influence the (E/Z) ratio. It thus appears that, in the formation of the alkene R1,CH=CH R2, thermodynamic control leading to predominance of the ( )-isomer is promoted by elevated temperatures, apolar solvents, stabilising salts (lithium or sodium halides), carbanion stabilisation, an electron-rich phosphorus atom [—PO(OR)2], and excess base. On the other hand, kinetic control leading to a predominance of the (Z)-isomer, is promoted by low temperatures,... 

The workers at Merck were able to go from start to finish in a single vessel. Base treatment of 22 + 23 led to attack at the epoxide end of 22 to give the anion 24. This is in equilibrium with the carbanion stabilised by Ar and CN and cyclises to an epoxide that gives a 15 85 mixture of the diastereoisomers 25 and 20. Reduction with borane gave a mixture of the amino alcohols... 

Trithiocarbonate S-oxides are reactive towards alkyllithiums [111] thio-philic additions were carried out at -78 °C. The resulting carbanions, stabilised by three sulfur groups, were quenched by water or by other electrophiles to afford trithioorthoester oxides. With enones, 1,4-addition was observed elimination of an alkanesulfenic acid led to /1-oxoketenedithioace-tals which could be transformed into 4-oxoalkanethioates. This Umpolung route allows the formal use of an (alkylthio)carbonyl anion. 

The stereochemical course of hydrogen-deuterium exchange in homochiral PhEtHC CF3 has been studied [15] and, like systems containing other carbanion-stabilising groups, the extent of racemisation of the product varies with solvent. 

Fluorine attached to carbon, which is itself adjacent to the carbanionic site (7.16A), is carbanion-stabilising and therefore strongly activating, for example when X or Y in 7.16A is CF3. 

Simple sulfonyl carbanions which do not contain additional carbanion-stabilising groups, e.g. carbonyl groups or heteroatoms, can be readily alkylated in high yield by modern techniques with the use of alkyllithiums and lithium amide bases. A number of allylic halides have been successfully used. In allylic halides, the halogen directly attached to the double-bonded carbon is relatively inert towards nucleophilic attack (Scheme 41). In this way, sulfones (96) can be transformed via desulfonation into vinyl halides (97) or into ketones (98) by hydrolysis (Scheme 41). In contrast to ordinary alkyl sulfones, triflones (99) can be alkylated under mildly basic conditions (potassium carbonate in boiling acetonitrile) (Scheme 42). The ease of carbanion formation from triflones (99) arises from the additional electron-withdrawing (-1) effect of the trifluoromethyl moiety. 

A carbanion stabilised by both carbonyl groups in the p-keto ester is formed. This carbanion reacts intramolecularly with the ester, resulting in the expulsion of methoxide and the formation of a ketone. 

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