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Substituted benzyl bromides asymmetric alkylation

The normal U-shaped Hammett plots were found for both the catalysed [by a copper(II)salen complex (31)] and uncatalysed asymmetric alkylation of enolates by substituted benzyl bromides,126 indicating that both reactions occur via an. S N2 mechanism (Scheme 15). Because both reactions were faster when electron-withdrawing substituents were on the benzyl bromide, it was concluded that there was more bond formation than bond rupture in the. S N2 transition states. Because the curvature of the Hammett plot was greater for the catalysed reaction, it was concluded that the catalysed reaction has a later transition state with a greater negative charge on Ca. The role of the catalyst was to increase the nucleophilic character of the enolate anion. [Pg.239]

On the basis of the previous results. Park, Jew, and co-workers [101] developed in 2009 an efficient synthetic methodology for enantiomerically pure a-alkyl-a,3-diaminopropionic acid. They described the asymmetric PTC alkylation of Af(l)-Boc-2-phenyl-2-imidazoline-4-carboxylic acid /cr/-butyl esters (52) with propargyl, allyl, and substituted benzyl bromides under catalysis with the binaphthalene-derived PTC XXV (Scheme 8.19). Alkylated products were obtained in high yields with excellent enantioselectivities and their acidic hydrolysis furnished corresponding optically active a-alkyl-a,(3-diaminopropionic acids. Another example of PTC alkylation of heterocyclic compounds, namely 1-cyanotetrahydro-(3-carbolines using a binaphthyl-modified V-spiro-type catalyst L, was reported by Maruoka and co-workers [103]. [Pg.286]

Asymmetric Alkylation. 7Y-[4-(Trifluoromethyl)benzyl]-cinchoninium bromide (1) has been used as chiral phase-transfer catalyst in the alkylation of indanones (eq 1). For the alkylation of a-aryl-substituted carbonyl compounds the diastere-omeric 7Y-[4-(trifluoromethyl)benzyl]cinchonidinium bromide (2) was used to obtain the opposite stereochemistry (eqs 2 and 3). The asymmetric alkylation of oxindoles was used as the key step in an asymmetric synthesis of (—)-physostigmine (eq 4). ... [Pg.518]

Asymmetric PTC is an important method in the synthesis of a-alkyl and a-amino acids. Belokon et al. [7] reported that the compound (47 ,57 )-2,2-dimethyl-Q ,Q ,Q , Q -tetra-phenyl-l,3-dioxolane-4,5-dimethanol (TADDOL) was used to catalyze the C-alkylation of C-H acids with alkyl halides to the asymmetric synthesis of a-methyl-substituted a-amino acids under PTC conditions. The alkylations of the substrate C-H acids with benzyl bromide or allyl bromide were conducted in dry toluene at ambient temperature with NaH or solid NaOH as base and TADDOL as a chiral promoter. The type of base is important in the asymmetric C-alkylation of C-H acids. [Pg.296]

KHMDS has been used to effect a-deprotonation of O-sUyl protected cyanohydrins derived from 2-/7-tolylsulfinyl henzalde-hyde followed by trapping of the C-nucleophile with diverse C-electrophiles, providing a powerful alternative approach to cyanohydrins of ketones. The remote 1,4-asymmetric induction was equally effective for either epimer (diastereomer) of the 0-TIPS protected cyanohydrin, and an equimolar mixture of the two epimers was employed. Both KHMDS and LHMDS bases provided the substituted cyanohydrins from reactions with highly reactive electrophiles (ClCOOMe and ClCOMe) in excellent yields and diastereoselectivities (dr > 98 2) (eq 65). The deprotonation induced by KHMDS led to more reactive nucleophiles, shortening the reaction times. Notably, in alkylations of Eschen-moser s salt, and benzyl and allyl bromides, the application of LHMDS instead of KHMDS improved the diastereoselectivity. The stereoselectivity of the alkylations mediated by KHMDS could be increased by the inclusion of the 18-crown-6 ether... [Pg.323]


See other pages where Substituted benzyl bromides asymmetric alkylation is mentioned: [Pg.372]    [Pg.372]    [Pg.46]    [Pg.399]    [Pg.150]    [Pg.47]    [Pg.75]    [Pg.127]    [Pg.525]    [Pg.166]    [Pg.256]    [Pg.212]    [Pg.71]    [Pg.443]    [Pg.15]   


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2-Substituted alkyl 3-

Alkyl bromide alkylation

Alkyl bromides

Alkyl substitute

Alkylation bromide

Alkylations, asymmetric

Alkyls benzyls

Asymmetric bromide

Benzyl bromide

Benzyl bromides, substituted

Benzylic bromide

Benzylic substitution

Substitution alkylation

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