Big Chemical Encyclopedia

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

Articles Figures Tables About

Lanthanide complexes hetero-Diels-Alder reactions

Lactam, ring formation, 288 Lactones, optically active, 31 Lanthanide complexes epoxy ring opening, 234 hetero-Diels-Alder reactions, 217 nitno-aldol reaction, 228 Laudanosine, 36 Leucine hydrocarboxylation, 168 Lewis acid complexes, 212 Ligands ... [Pg.195]

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. [Pg.918]

Danishefsky and co-workers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is used as an NMR shift reagent, promoted hetero Diels-Alder reactions [30] of aldehydes with siloxydienes and induced enantiomeric enrichment (Sch. 1) [31]. Suitable substituents on the dienes were introduced to improve the extent of asymmetric induction. The best result was obtained in the reaction of benzaldehyde with l-methoxy-2-methyl-3-(trimethyl-siloxy)- , 3-butadiene using 1 mol % Eu(hfc)3 the enantiomerie excess was, however, moderate (58%). The authors maintained that the major advantage of lanthanide catalysis lay in the survival of otherwise labile systems used as adducts. [Pg.923]

L-Iduronyl synthons catalyzed by a vinyl cerium reagent have been shown to open a way to an efficient preparation of l,2,4-tri-0-acetyl-3-0-benzyl-Z,-iduronyl derivatives.1097 The synthetic utility of the cationic lanthanide complex [Cp 2Ce][BPh4] as an effective Lewis acid catalyst for the hetero-Diels-Alder reaction between Danishefsky s diene and substituted benzaldehydes has been demonstrated (Scheme 307). [Pg.168]

Asymmetric catalysis of hetero Diels Alder reactions using chiral lanthanide complexes 02CRV2211. [Pg.161]

Mikami and coworkers also reported the development of lanthanide bis(trif-luoromethanesulfonyl)amides (bistrifylamides) as a new type of asymmetric catalysts for the hetero-Diels-Alder reaction of Danishefsky s diene, wherein the significant effect of water as an additive is observed in increasing not only the enan-tioselectivity but also the chemical yield. Bistrifylamides can be used as effective bidentate Hgands to increase the Lewis acidity of their chiral metal complexes on account of the higher acidity of the conjugated acids than those of afiphatic and aromatic diols, which are commonly used as chiral bidentate hgands [33]. [Pg.1181]

Chiral lanthanide bistrifylamides were prepared through the reaction of lan-thanide(III) triflates and chiral bistrifylamides, which are deprotonated with 2 equivalents of sodium hydride in THF for 1 h. Dichloromethane or toluene was introduced, after evaporation of THF, to the residual complex. The resultant suspension of the lanthanide complex was then used for the hetero-Diels-Alder reaction (Scheme 16). [Pg.1181]

The Lewis acidity of lanthanide complexes has been known for a long time. It was exploited extensively in their use as NMR shift reagents, mainly Eu(fod)3. They show strong affinity toward carbonyl oxygens and, therefore, have been widely used as catalysts for cycloaddition of dienes with aldehydes [25]. Moreover, the ability of catalytic amounts of lanthanide compoimds to activate coordinating nitriles as well as imines has also been recognized [26]. In recent years lanthanide (III) complexes have demonstrated clear effectiveness in catalyzing not only hetero-Diels-Alder reactions, but also Michael, aldol, Strecker and Friedel-Crafts acylation reactions [27]. [Pg.312]

Asymmetric hetero-Diels-Alder reactions of glyoxylate esters and Danishefsky s diene catalyzed by various chiral bis(oxazoline)-lanthanide complexes afforded the corresponding aldol adducts, which upon treatment with... [Pg.692]

In the asymmetric hetero Diels-Alder reactions of Danishefsky s diene by chiral lanthanide bis(trifluoromethane-sulfonyl)amide (bis-triflamide) complexes, particularly 5rttrimn bis-triflamide, a significant effect of water as an additive was observed in increasing not only the enantioselectivity but also the chemical yields (Scheme 28). ... [Pg.453]

Danishefsky and coworkers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is known as a NMR shift reagent, promoted hetero Diels-Alder reactions of aldehydes with siloxydienes and induced enantiomeric enrichment (Scheme 13.19). The best result was obtained... [Pg.128]

Inanaga and coworkers developed another type of lanthanide catalysts for asymmetric hetero Diels-Alder reaction (Scheme 13.23) [59, 60]. Benzaldehyde reacted with l-methoxy-3-(trimethylsiloxy)-l,3-butadiene in the presence of chiral Yb(III) phosphate complex, Yb[(R)-BNP]3, to afford the corresponding adduct with 77% yield and 70% ee. 2,6-Lutidine worked best as achiral additive in the system to improve reactivity as well as enantioselectivity (89% ee) [59b]. In modification studies of the chiral ligand to improve solubility and chiral environment of catalyst, partially reduced (R)-Hg-BNP was found more suitable than original (R)-BNP ligand in this reaction. Yb[(R)-H8-BNP]3 complex promoted the reaction in 98% ee (R=Ph) without any additives (Scheme 13.23a) [59c]. Yb[(P)-BNP]3 complex was... [Pg.133]

The use of lanthanide complexes in asymmetric catalysis was pioneered by Danishefsky s group with the hetero-Diels-Alder reaction,and their utility as chiral Lewis acid catalysts was shown by Kobayashi. The Brpnsted base character of lanthanide-alkoxides has been used by Shibasaki for aldol reactions, cyanosilylation of aldehydes and nitroaldol reactions.The combination of Lewis acid and Brpnsted base properties of lanthanide complexes has been exploited in particular by Shibasaki for bifunctional asymmetric catalysis. These bimetallic lanthanide-main-group BINOL complexes are synthesized according to the following routes ... [Pg.528]


See other pages where Lanthanide complexes hetero-Diels-Alder reactions is mentioned: [Pg.408]    [Pg.453]    [Pg.453]    [Pg.453]    [Pg.1144]    [Pg.1169]    [Pg.389]    [Pg.134]   
See also in sourсe #XX -- [ Pg.217 ]




SEARCH



Diels hetero

Diels-Alder reaction complex

Hetero-Diels-Alder

Hetero-Diels-Alder reaction

Hetero-Diels-Alder reactions complexes

Lanthanide complex

Lanthanide complexation

Lanthanides Diels-Alder reaction

Lanthanides reactions

© 2024 chempedia.info