Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Metal enolates chirality transfer

If the metal enolate contains a center of chirality, diastereoselection may be exhibited in the C—C bond formation process. Evans has identified three classes of metal enolates in which chirality transfer may occur (i) endo- and exo-cyclic enolates such as (27) or (28), which contain a chiral center ( ) in a ring bonded to the enolate at two points (ii) acyclic enolates such as (29) or (30), in which the moiety containing the chiral center ( ) is bonded to the enolate at only one point and (iii) chelated enolates such as (31) or (32), in which the chiral center is a part of the chelate ring. (Z)-Endocyclic enolates are also possible for large ring cyclic ketones. [Pg.13]

In recent years, investigations of the diastereoselectivity and enantioselectivity of alkylations of metal enolates of carboxylic acid derivatives have become one of the most active areas of research in synthetic organic chemistry. Intraannular, extraannular and chelate-enforced intraannular chirality transfer may be involved in determining the stereochemistry of these alkylations. [Pg.39]

Alkylations of acyclic enolates containing a collection of chiral auxiliary groups have been used successfully for the asymmetric synthesis of carboxylic acids. The chiral, nonracemic substrates that have been used include amides, imides, esters, imine derivatives of glycinates and acyl derivatives of chiral transition metals. In these systems either extraannular or chelate-enforced intraannular chirality transfer may control the sense of the alkylation step. [Pg.44]

Asymmetric protonation of a metal enolate basically proceeds catalytically if a coexisting achiral acid A-H reacts with the deprotonated chiral acid A -M faster than with the metal enolate, a concept first described by Fehr et al. [44]. A hypothesis for the catalytic cycle is illustrated in Scheme 2. Reaction of the metal enolate with the chiral acid A -H produces (R)- or (S)-ketone and the deprotonated chiral acid A -M. The chiral acid A -H is then reproduced by proton transfer from the achiral acid A-H to A -M. Higher reactivity of A -M toward A-H than that of the metal enolate makes the catalytic cycle possible. When the achiral acid A-H protonates the enolate rapidly at low temperature, selective deprotonation of one enantiomer of the resulting ketone by the metallated chiral acid A -M is seen as an alternative possible mechanism. [Pg.1225]

If the Claisen rearrangement is carried out with peptide allylic esters, the transfer of an allylic side chain to the a-position of the C terminal amino acid results in a modification of the peptide chain. This concept is comparable to the alkylations of peptide enolates described by Seebach et al. [93]. If it is possible to carry out the rearrangement not only with amino acids but also with peptide esters, the question arises if it is possible to transfer the chiral information from the peptide chain to the new chiral center formed during the rearrangement process, prohahly via some peptide metal enolate complexes. [Pg.277]

Metal-based asymmetric phase-transfer catalysts have mainly been used to catalyze two carbon-carbon bond-forming reactions (1) the asymmetric alkylation of amino acid-derived enolates and (2) Darzens condensations [5]. The alkylation ofprochiral glycine or alanine derivatives [3] is a popular and successful strategy for the preparation of acyclic a-amino acids and a-methyl-a-amino acids respectively (Scheme 8.1). In order to facilitate the generation of these enolates and to protect the amine substituent, an imine moiety is used to increase the acidity of the a-hydrogens, and therefore allow the use of relatively mild bases (such as metal hydroxides) to achieve the alkylation. In the case of a prochiral glycine-derived imine (Scheme 8.1 R3 = H), if monoalkylation is desired, the new chiral methine group... [Pg.161]

The synthesis of the chiral copper catalyst is very easy to reproduce. The complex catalyses the asymmetric alkylation of enolates of a range of amino acids, thus allowing the synthesis of enantiomeric ally enriched a,a disubstituted amino acids with up to 92% ee. The procedure combines the synthetic simplicity of the Phase Transfer Catalyst (PTC) approach, with the advantages of catalysis by metal complexes. The chemistry is compatible with the use of methyl ester substrates, thus avoiding the use of iso-propyl or ferf-butyl esters which are needed for cinchona-alkaloid catalyzed reactions[4], where the steric bulk of the ester is important for efficient asymmetric induction. Another advantage compared with cinchona-alkaloid systems is that copper(II)(chsalen) catalyses the alkylation of substrates derived from a range of amino acids, not just glycine and alanine (Table 2.4). [Pg.26]

Since samarium diiodide is only a one electron donor, two equivalents of the metal are required in order for the reaction to proceed. The first electron donated from the samarium produces a chiral ketyl radical 30 which undergoes enantioselective addition to the acrylate according to the chelated transition state shown in 32. The second electron donation then provides a chiral samarium enolate intermediate 33 that can potentially undergo stereoselective proton transfer in the formation of a second chiral center. [Pg.467]

It is well established that the reaction of carbenoids with At-alkylindoles delivers zwitterionic intermediates. The reason why this scenario is favored can be ascribed to the fact that the positive charge of the intermediate is stabilized by the electron-rich indole while the negative charge is stabilized by the carbenoid component. In other words, the site of C3 is highly reactive in metal carbenoid insertion reactions. In 2010, Lian and Davies described such a process in their seminal work on Rh-catalyzed [3 + 2] annulation of indoles. In the presence of 1,2-dimethylindole 53, Rh2(S-DOSP)4 induced the decomposition of methyl a-phenyl-a-diazoacetate la and C—H bond insertion of indole, providing the C3 functionalization product 54 in 95% yield but negligible asymmetric induction (<5% ee). It is proposed that the poor chiral induction in the formation of C—H bond insertion product 54 can be attributed to the rapid proton transfer from the zwitterionic intermediate A to the achiral enol B, which can further tautomerize into the observed C—H bond insertion product 54 (Scheme 1.18). [Pg.21]

Using the manganese enolates, the chiraUty is transfered from the chiral ester moiety towards the peptide backbone, but not inside the peptide backbone. Therefore, the influence of the chelating metal salt in the rearrangement of several dipeptide crotyl esters was investigated. [Pg.278]

As an essential component to asymmetric organocatalysis, chiral, metal-free Bron-sted bases have mediated several types of C-C and C-X bond-forming reactions mediated by enamine and enolates. Brmsted bases (Figure 13.1) have the functional capacity to accept a hydrogen (or proton) from an acidic source or equivalent activated species. This proton transfer forms the basis of the key activation component to new-bond formation reactions. [Pg.343]


See other pages where Metal enolates chirality transfer is mentioned: [Pg.101]    [Pg.78]    [Pg.572]    [Pg.94]    [Pg.133]    [Pg.597]    [Pg.267]    [Pg.721]    [Pg.721]    [Pg.68]    [Pg.104]    [Pg.572]    [Pg.164]    [Pg.92]    [Pg.520]    [Pg.520]    [Pg.158]    [Pg.75]    [Pg.153]    [Pg.240]    [Pg.1107]    [Pg.188]    [Pg.172]    [Pg.1]    [Pg.233]    [Pg.712]    [Pg.201]    [Pg.712]    [Pg.996]    [Pg.41]    [Pg.219]    [Pg.16]   
See also in sourсe #XX -- [ Pg.3 , Pg.13 ]

See also in sourсe #XX -- [ Pg.13 ]

See also in sourсe #XX -- [ Pg.3 , Pg.13 ]




SEARCH



Chiral enolate

Chiral metal

Chirality, transfer

Enolates chiral

Metal enolate

Metal enolates

Metal transfer

© 2024 chempedia.info