Amines achiral, tertiary

Influence of Achiral Tertiary Amines on the Enantioselective Hydrogenation of a,p-diketones Over Cinchonidine-Pt/Al203 Catalyst... 

Enantioselective hydrogenation of 2,3-butanedione and 3,4-hexanedione has been studied over cinchonidine - Pt/Al203 catalyst system in the presence or absence of achiral tertiary amines (quinuclidine, DABCO) using solvents such as toluene and ethanol. Kinetic results confirmed that (i) added achiral tertiary amines increase both the reaction rate and the enantioselectivity, (ii) both substrates have a strong poisoning effect, (iii) an accurate purification of the substrates is needed to get adequate kinetic data. The observed poisoning effect is attributed to the oligomers formed from diketones. 

Pt/Al2C>3-cinchona alkaloid catalyst system is widely used for enantioselective hydrogenation of different prochiral substrates, such as a-ketoesters [1-2], a,p-diketones, etc. [3-5], It has been shown that in the enantioselective hydrogenation of ethyl pyruvate (Etpy) under certain reaction conditions (low cinchonidine concentration, using toluene as a solvent) achiral tertiary amines (ATAs triethylamine, quinuclidine (Q) and DABCO) as additives increase not only the reaction rate, but the enantioselectivity [6], This observation has been explained by a virtual increase of chiral modifier concentration as a result of the shift in cinchonidine monomer - dimer equilibrium by ATAs [7],... 

The [3+2]-cycloaddition or [2+2]-cycloaddition of OSO4 and alkene followed by rearrangement forms the cyclic osmate ester. Tertiary amines (achiral ligand) such as 4-dimethyl-aminopyridine (DMAP) or pyridine (Py) accelerate the addition reaction (Scheme 7.24). 

In the presence of a catalytic amount of chiral lanthanide triflate 63, the reaction of 3-acyl-l,3-oxazolidin-2-ones with cyclopentadiene produces Diels-Alder adducts in high yields and high ee. The chiral lanthanide triflate 63 can be prepared from ytterbium triflate, (R)-( I )-binaphthol, and a tertiary amine. Both enantiomers of the cycloaddition product can be prepared via this chiral lanthanide (III) complex-catalyzed reaction using the same chiral source [(R)-(+)-binaphthol] and an appropriately selected achiral ligand. This achiral ligand serves as an additive to stabilize the catalyst in the sense of preventing the catalyst from aging. Asymmetric catalytic aza Diels-Alder reactions can also be carried out successfully under these conditions (Scheme 5-21).19... 

Based on nucleophilic addition, racemic allenyl sulfones were partially resolved by reaction with a deficiency of optically active primary or secondary amines [243]. The reversible nucleophilic addition of tertiary amines or phosphanes to acceptor-substituted allenes can lead to the inversion of the configuration of chiral allenes. For example, an optically active diester 177 with achiral groups R can undergo a racemization (Scheme 7.29). A 4 5 mixture of (M)- and (P)-177 with R = (-)-l-menthyl, obtained through synthesis of the allene from dimenthyl 1,3-acetonedicar-boxylate (cf. Scheme 7.18) [159], furnishes (M)-177 in high diastereomeric purity in 90% yield after repeated crystallization from pentane in the presence of catalytic amounts of triethylamine [158], Another example of a highly elegant epimerization of an optically active allene based on reversible nucleophilic addition was published by Marshall and Liao, who were successful in the transformation 179 — 180 [35], Recently, Lu et al. published a very informative review on the reactions of electron-deficient allenes under phosphane catalysis [244]. 

A new and general approach to chiral aliphatic or aromatic sulfinates has been recently described by Mikofajczyk and Drabowicz (107). It consists of the asymmetric condensation of racemic sulfinyl chlorides at low temperature with achiral alcohols in the presence of chiral tertiary amines as asymmetric reagents. The optical purity (up to 45%) of the sulfinates formed is strongly dependent on the structure of all the reaction components. 

When chiral amines are used in the oxyamination reaction, stereogenicity is induced in the amination step and in this way optically active amino alcohols are obtained after oxidation69. An optically active secondary or tertiary amine can be used as a ligand for palladium in the intermediate 7t-complex, to which an excess of an achiral amine can be added (reagent-induced diastereoselectivity). Here, a pair of diastereomeric tt-complexes are formed which may be in equilibrium with each other, the degree of asymmetric induction is dependent on the ratio between the diastereomeric complexes and/or on their different reactivity. 

Boron Azaenolates from Oxazolines. The reagent is useful for asymmetric aldol condensations of achiral oxazolines. Treatment of 2-ethyl-4-dimethyl-2-oxazoline with IpC2BOTf in the presence of a tertiary amine furnishes a boron azaenolate. Without isolation, treatment with an aldehyde in ether at —78 °C provides an alkylated oxazoline, which is hydrolyzed and converted to p-hydroxy ester via treatment with Diazomethane. Although the yields for the four-step sequence are only moderate, the anti selectivities of the hydroxy acids are excellent with enantioselec-tivities of 77-85% ee (eq 1). ... 

Kobayashi and co-workers exploited the use of lanthanide in a variety of achiral reactions and extended them into several catalytic asymmetric reactions. Their work commenced with catalytic asymmetric Diels-Alder reactions [32], The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, binaphthol and a tertiary amine. The amine significantly influenced reaction selectivity. When triethyl-amine was used in the preparation of the catalyst, the desired product was obtained in moderate ee (33%) (Table 8, entry 1). After screening several reaction conditions, they found that, in general, bulky amines gave better results (entries 2-6). They suggested interesting explanations of this experimental result on the basis of investigations into catalyst structure. Consequently, the use of cw-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (4A MS) was found to produce the best result (yield 77%, endolexo = 89/11, endo = 95% ee) (entry 6). 

CSj, with subsequent alkylation with methyl iodide, led to (i )-l-ferrocenylethyl-5-methyltrithiocarbonate, which involves a rearrangement with retention (Fig. 4-17, top) [100]. Sulfides [101], tertiary phosphines [101], and tertiary amines [102] are also reasonably good nucleophiles and form reactive ionic products, e.g., pyridinium salts [103], but this has only been verified for achiral or racemic substrates. Pyridinium salts may be considered as a storage form of a-ferrocenylalkyl carbocations, and show almost the same behaviour towards nucleophiles [103, 104]. Primary and... 

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