Carbanion reactivity

Sulfur in its higher oxidation states (e.g., sulfone, —S02—) is especially effective in stabilizing adjacent carbanion centers. However, from a synthetic standpoint there are disadvantages to the sulfone grouping in that the better stabilized carbanions also are the least reactive, and subsequent removal of the sulfone grouping can be difficult. A good balance between carbanion stability, carbanion reactivity, and ease of C-S bond cleavage is present in the... 

Scheme 1 Variable Adjustment of Carbanion Reactivity and Selectivity by Conversion into Titanium Reagents...

This review describes some novel reaction types (Section F), but the more extensive area of activity has to do with variable adjustment of carbanion reactivity and selectivity in reactions with carbonyl compounds (Sects. C-E). Organotitanium... 

If the reactivity of a methyl group in aldol or Knoevenagel condensations can be assumed to depend on the ease of release of its proton to form the carbanion, reactivity then will be predicted by inspection of acidity, theoretically estimated by the following two approaches. 

As a rule, however, carbanion reactivities in the coordinative saturated y9-aminoethyl transition metal complexes are generally kinetically too strongly suppressed. One possible way of circumventing this barrier is shown in Scheme 3, along route ( ). If the transition metal is d-electron-rich and can receive the proton by oxidative addition, then the alkylamine can be eventually more easily generated by reductive elimination with formation of the starting transition metal complex. With these principles of catalytic activation as a guideline, the different catalyst systems described in the literature will now be discussed in some detail. 

L. Somsak, Carbanionic reactivity of the anomeric center in carbohydrates, Chem. Rev., 101 (2001) 81-135. 

Carbanionic reactivity of the anomeric center in carbohydrates 01CRV81. 

As already shown by many of the above examples, physical methods, especially spectroscopic ones, are having a very strong impact on the development of physical organic chemistry, and it is only for lack of space that such important techniques as laser flash spectroscopy can only be mentioned here in passing. Having established itself as a highly efficient method for the study of carbenes and carbocations [63] laser flash spectroscopy has also been used to generate carbanions like the trityl anion recently, [64] and promises to become a useful method for the determination of carbanion reactivity as well. 

Tri(cyclopentyl)borane Synthesis via lithium borates Moderation of carbanion reactivity... 

Gronert, S., and Streitwieser, A., Jr. "Carbon Acidity. 74. The Effects of Hetero-Substituted Pendant Groups on Carbanion Reactivity. Solvent-Separated-Conduct Ion Pair Equilibria and Relative pK Li / THF s for 9-Substitued Fluorenyllithiums in Tetrahydrofuran. The Importance of Internal Chelation." J. Am. Chem. Soc., 110,2836 (1988). 

Carbanion Reactivity. An increase in reaction rate is observed for the reaction of alkynyllithium reagents with alkyl halides and oxiranes (eq 10). The strongly coordinating HMPA probably complexes the lithium cation, thereby increasing the negative charge density on the carbon and creating a much more nucleophilic alkynyl anion. A similar effect is observed for (Trimethyl-stannylmethyl)Uthium, which does not react with oxiranes in THF but in THF-HMPA the reaction proceeds readily. ... 

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