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Polar Felkin-Anh model

A systematic study of methyl ketone aldol additions with a-alkoxy and o ,/5-bisalkoxy aldehydes has been undertaken, under non-chelating conditions.130 With a single a-alkoxy stereocentre, diastereoselectivity generally follows Cornforth/polar Felkin-Anh models. With an additional /5-alkoxy stereocentre, 7r-facial selectivity is dramatically dependent on the relative configuration at a- and /3-centres if they are anti, high de results, but not if they are syn. A model for such acyclic stereocontrol is proposed in which the /5-alkoxy substituent determines the position in space of the a-alkoxy relative to the carbonyl, thus determining the n-facial selectivity. [Pg.18]

Polar Felkin-Anh model for nucleophilic addition to a carbonyl group. [Pg.627]

The concepts of Felkin, Anh, and Eisenstein are now recognized collectively with the term polar Felkin-Anh model, which is illustrated in Figure The favored approach of the nucleophile allows maximum... [Pg.627]

A DFT study of enolborane addition of o -heteroatom-substituted aldehydes has focused on the relevance of the Cornforth and polar Felkin-Anh (PKA) models for asymmetric induction.154 Using chiral substrates, MeCH(X)CHO, polar (X = F, Cl, (g) OMe) and less polar (X = SMe, NMe2, PMe2) substituents have been examined. The former favour Cornforth TS structures, the latter PKA. TS preferences have been correlated with the relative energy of the corresponding rotamer of the uncomplexed aldehyde. An in-depth study of addition of ( > and (Z)-enolborane nucleophiles to 2-methoxypropanal successfully predicts experimentally determined diastereofacial selectivities. [Pg.20]

SCHEME 117. Comparative chair-like transition states using the polar Felkin-Anh and Comforth models for Z(O)568... [Pg.617]

The Felkin-Anh model has also been used to explain the preference for axial attack by nucleophiles on cyclohexanones and the effect of proximate substituents on facial selection. The anti periplanar geometry that Anh regarded as important in nucleophilic attack of carbonyl compounds is compromised by torsional strain in the reactions of cyclohexanones from the equatorial face. Felkin slated Whereas both torsional strain and steric strain can be simultaneously minimised in a reactant-like transition state when the substrate is acyclic... this is not possible in the cyclohexanone case.. ..These reactions all proceed via reactant-like transition states. In the absence of polar effects, their steric outcome is determined by the relative magnitude of torsional strain and steric strain [in the axial and equatorial transition states] [16]. [Pg.160]

The Felkin-Anh Model [CP2, G5, M5, N2, N5, NNl, WHl, WH2, WP2, WTl] The attack of a hydride H on a prochiral carbonyl group can be accomplished either on the Re or Si face of the carbonyl, leading to a pair of diastereomers, as shown in the models in Figure 3.10. In these models L represents the most bulky group, P the most polar, and S the smallest group. Initially Cram [CA2] proposed model 3.18 to interpret the formation of the major isomer. This is called Cram in Figure 3.10, and the other isomer is labeled anti-Cram. ... [Pg.45]


See other pages where Polar Felkin-Anh model is mentioned: [Pg.617]    [Pg.393]    [Pg.617]    [Pg.393]    [Pg.27]    [Pg.19]    [Pg.20]    [Pg.440]    [Pg.451]    [Pg.188]    [Pg.66]    [Pg.78]    [Pg.88]    [Pg.101]    [Pg.67]   
See also in sourсe #XX -- [ Pg.627 ]




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