Stereoselektive Protonierung von Carbanionen, 1. Diastereoselektive Protonierung von C – H‐aktivierten Cyclohexan‐Derivaten

Abstract

The diastereoselectivity of the kinetically controlled protonation of carbanions derived from 4-tert-butyl-1-X-cyclohexanes (1: X = CN, 2 X = C02Me, 2-Si: are discussed with regard to structure and aggregation of lLi to 4Li and compared with relevant data from the literature. It is concluded that so far an empirical approach to high diastereoselectivities cannot be avoided and that the results cannot be predicted from the ratios obtained by the much slower alkylation reactions.

Control of the Stereochemistry of Kinetic Protonation: Intramolecular Proton Delivery1,2

“…Van der Waals forces along with the electronic preference lead to the cyclohexyl cyanomethide anion being slightly deformed from planarity. Nevertheless, the approach to planarity accounts for kinetic protonation occurring from the less hindered side of the π anionic system as experimentally observed . Hence, for all systemsenolate, nitronate, and cyanokinetic protonation is similar.…”

Diastereoselectivity of Enolate Anion Protonation. H/D Exchange of β-Substituted Ethyl Butanoates in Ethanol-d

“…Though the trends are in the same direction, the numbers themselves differ substantially. Perhaps even more relevant is the research of Hünig and his colleagues on the stereochemistry of protonation and alkylation of enolates produced with LDA in the 4- tert -butylcyclohexyl system, including ester, ketone, nitrile, and sulfone enolates …”

“…Surprisingly few systematic investigations of the stereochemistry of enolate protonation have been reported. Although islands of understanding are available for cyclic and acyclic 10,11 systems, the variable effects of aggregation have been a stumbling block with regard to a fundamental and comprehensive grasp of the steric and electronic factors controlling the stereochemistry. − Current efforts to sort out equilibria and kinetics when aggregation is a factor should make this less difficult in the future . With the current state of knowledge, however, it seems wise to avoid the complexities of the aggregates ubiquitous in nonpolar organic solvents when attempting to study the structural factors that control the diastereoselectivity of the reactions of enolate anions with electrophiles.…”

Investigations on the Efficiency of Regioselective C-Deuteration of Endocyclic Enolates

“…The continuing development of novel synthetic methodologies for the direct incorporation of nonradioactive isotopic labels into organic molecules for chemical and biological studies is a major field of research 1. A great deal of attention has been paid to deuterium incorporation involving simple carbon−hydrogen bond exchange reactions,2 many of which have relied on a deprotonation‐deuteration strategy involving relatively acidic centres,3 most notably adjacent to a carbonyl group 4. For synthetic ease, most H−D exchange reactions are generally performed under thermodynamic control5 by using an excess of the deuterium source to drive the reaction to completion 6.…”