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Oxocarbenium Ions (oxocarbenium + ion)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Stereoselective Synthesis of Tertiary Ethers through Geometric Control of Highly Substituted Oxocarbenium Ions,

ANGEWANDTE CHEMIE, Issue 34 2010
Lei Liu
Vollständig substituiert heißt vollständig kontrolliert: Die Geometrie in 1,1-disubstituierten Oxocarbeniumionen und die Konformation von Oxocarbeniumionen, die ein tertiäres Stereozentrum enthalten, lassen sich mithilfe einfacher Modelle vorhersagen. Auf diesen Modellen beruhen hoch stereoselektive Synthesen von Tetrahydropyran-Derivaten mit tertiären Ethergruppen (siehe Schema; RL, RS und RN sind große, kleine bzw. nucleophile Gruppen). [source]

Nucleophilic Additions to Fused Bicyclic Five-Membered Ring Oxocarbenium Ions: Evidence for Preferential Attack on the Inside Face.

CHEMINFORM, Issue 11 2004
Deborah M. Smith
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Asymmetric Allylation of Methyl Ketones by Using Chiral Phenyl Carbinols

CHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2009

Abstract To methyl or to alkyl? High induced facial selectivities and excellent yields are obtained in the allylation of aliphatic methyl ketones by using a structurally simple phenylbenzyl auxiliary to give the corresponding homoallylic ethers (see scheme). The transferred auxiliary has a very good protecting quality and can easily be removed. Novel chiral auxiliaries for the stereoselective allylation of aliphatic methyl ketones with allyltrimethylsilane and their use in the synthesis of homoallylic ethers are described. In a multicomponent domino process catalyzed by trifluoromethanesulfonic acid, the allyl moiety and the auxiliary are transferred onto the substrate to yield tertiary homoallylic ethers. The most useful auxiliary for a general application turned out to be the trimethylsilyl ether of phenyl benzyl carbinol with an induced diastereoselectivity of 90:10 using ethyl methyl ketone and 94:6 using isopropyl methyl ketone as substrates. The transferred substituted benzyl moiety has good protecting properties in subsequent transformations and can easily be removed under reductive conditions to provide the corresponding homoallylic alcohol. The origin of the high selectivity could be elucidated by identifying the relevant transition states using quantum-chemical calculations. An excellent agreement between calculated and experimentally observed selectivities was obtained assuming an oxocarbenium ion as intermediate. [source]

Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary-Mediated Crotylation and Pentenylation of Butanone by DFT Calculations

CHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2009

Abstract The solvent makes the difference: While attack at the Re face of the intermediate oxocarbenium ion becomes less energy demanding with increasing size of R,, it is the solvent effect that makes this attack the main trajectory, leading to inversed stereochemistry (see picture). Auxiliary-mediated domino crotylations and pentenylations of butanone yield homoallylic ethers with two newly formed stereogenic centers. With our norpseudoephedrine-derived auxiliary, we observed the formation of anti isomers exclusively, and the nature of the major isomer was independent of the substrate double bond geometry. Interestingly, there is a switch in induced selectivity when going from crotylation to pentenylation. Here, we present the computational rationalization for this behavior by identification of the relevant transition states (TSs), the energies of which were determined by using the B3LYP/6-31+G(d) level of theory in combination with the PCM/UAKS method to include the effects exerted by the solvent dichloromethane. To quickly narrow down the number of potentially relevant TSs from the whole set of 288 and 864 TSs for the crotylation and pentenylation, respectively, we employed a screening process based on B3LYP//AM1 energies. The predicted selectivities are in good agreement with experimentally determined ones. Furthermore, the obtained results also facilitate an explanation of the selectivities obtained in hexenylations and heptenylations. Finally, activation energies were determined that account for the significantly longer reaction times than those for the domino allylation with unsubstituted trimethylallylsilane. [source]