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Polymer chiral Lewis acid

Keywords acrylates, acrylamides, fumarates, a, -unsaturated ketones, vinyl ethers, vinyl sulphoxides, chiral dienophiles, chiral dienes, chiral catalysts polymer-supported chiral Lewis acids... [Pg.312]

Diels-Alder Polymerization Diels-Alder type polymerization of a bisdienophile monomer (30) and bisdiene monomers (31, 32a,b) by using chiral Lewis acidic catalysts (33-35) affords optically active polymers [68]. For instance, the polymerization of 30 in CH2C12 with 32b by using 33 as a catalyst at -30°C gives a polymer with structure 36 showing molecular rotation of [ ]D +243°. When CHC13 or tetrahydrofuran is used as solvent, the polymers with only low optical activity are produced. [Pg.765]

Chiral (acyloxy)borane (CAB) is known as an effective chiral Lewis acid catalyst for enantioselective allylation of aldehydes. Marshall applied the CAB complex 1 to the addition of crotylstannane to achiral aldehydes and found that the CAB catalyst gives higher syn/anti selectivity than BINOL/Ti catalysts in the reaction [4]. CAB complex 2 was utilized in asymmetric synthesis of chiral polymers using a combination of dialdehyde and bis(allylsilane) [5] or monomers possessing both formyl and allyltrimethylsilyl groups [6]. [Pg.116]

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. [Pg.121]

Itsuno et al. explored the possibility of using polymer-supported chiral Lewis acids in a model Diels-Alder reaction of methacrolein with cyclopentadiene [23a]. By using an insoluble polymer-supported Lewis acid [23], prepared from borane with cross-linked polymers with a chiral moiety such as an A-sulfonylamino acid, the Diels-Alder adduct is obtained in good yield with almost perfect exo selectivity and moderate enantioselectivity (Eq. 23). [Pg.146]

The Diels-Alder reaction of a diene and a dienophile has become one of the most powerful carbon-carbon bond-forming processes [81]. In normal Diels-Alder reactions of an electron-poor dienophile with an electron-rich diene, the main interaction is between the HOMO of the diene and the LUMO of the dienophile. Coordination of a Lewis acid to the dienophile reduces its frontier orbital energies, and this increases the rate of the reaction. Regio- and stereoselectivity are also markedly affected by the Lewis acid. Recent extensive studies on the design of chiral Lewis acids have led to fruitful results in the control of the stereochemistry of a variety of pericyclic reactions. Several chirally modified Lewis acids have been developed for the asymmetric Diels-Alder reaction [82,83] and spectacular advances have recently been achieved in this area. Various kinds of polymer-supported chiral Lewis acid have also been developed. Polymer-supported A1 Lewis acids such as 62 have been used in the Diels-Alder reaction of cyclopentadiene and methacrolein (Eq. 20) [84] as has polymer-supported Ti alkoxide 63 [84]. These Ti catalysts are readily prepared and have high activity in the Diels-Alder reaction. [Pg.965]

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. [Pg.967]

Since the first report on Ti-TADDOLate-mediated Diels-Alder reactions [97,98] several studies of the same reaction have been reported these have shown that Ti-TADDOLate is an efficient chiral Lewis acid in enantioselective Diels-Alder reactions. Polymer- and dendrimer-supported Ti-TADDOLates have been reported and their catalytic activity in several enantioselective reactions has been evaluated [59]. Various kinds of polymeric TADDOLs were prepared both by chemical modification (Eq. 22) and by copolymerization (Eq. 23). [Pg.969]

Catalyzed enantioselective Mukaiyama-aldol reactions have been developed extensively [101] and chiral polymer-supported Lewis acids are the catalysts of choice. Polymer-supported chiral A(-sulfonyloxazaborohdinones 86 and 87, prepared by copolymerization of styrene, divinylbenzene, and chiral monomers derived from L-valine and L-glutamic acid, respectively, have been used for aldol reactions [102]. The rates of reaction using the polymeric catalysts were slow and enantioselectivity was lower than was obtained by use of the low-molecular-weight counterpart (88). The best ee obtained by use of the polymeric catalyst was 90 % ee with 28 % isolated yield in the asymmetric aldol reaction of benzaldehyde with 89 (Eq. 27). [Pg.972]

Optically active l,l -binaphthols are among the most important chiral ligands of a variety of metal species. Binaphthol-aluminum complexes have been used as chiral Lewis acid catalysts. The l,T-binaphthyl-based chiral ligands owe their success in a variety of asymmetric reactions to the chiral cavity they create around the metal center [107,108]. In contrast with the wide use of these binaphthyls, the polymer-supported variety has been less popular. The optically active and sterically regular poly(l,l -bi-naphthyls) 96 have been prepared by nickel-catalyzed dehalogenating polycondensation of dibromide monomer 95 (Sch. 7) [109] and used to prepare the polybinaphthyl aluminum(III) catalyst 97 this had much greater catalytic activity than the corresponding monomeric catalyst when used in the Mukaiyama aldol reaction (Eq. 29). Unfortunately no enantioselectivity was observed in the aldol reaction. [Pg.973]

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. [Pg.84]

A library of chiral dihydropyrans (226) [241] was synthesized using asymmetric hetero-Diels-Alder reactions (HAD) on polymer-bound enol ethers (221) and a, 3-unsaturated oxalyl esters (222). A chiral Lewis acidic Cu -bisoxazoline complex was used because of its high efficiency, the high predictability of the reaction outcome, and its broad substrate tolerance [280]. Enol ethers were used as alkene components bearing a hydroxy function for attachment to the resin via a silyl linkage (Scheme 49). The diene components carried allyl-ester groups, which could be readily displaced by amino functions in subsequent steps of the combinatorial synthesis. [Pg.224]

Most recent research has been focused on the application of polymers as chiral auxiliaries in enantioselective Lewis-acid-catalyzed reactions. Studies of Itsuno and co-workers [44] culminated in the development of a polymer-supported catalyst containing a chiral oxazaborolidinone with oxyethylene crosslinkages which gave the Diels-Alder adduct of cyclopentadiene and methacrolein in 88 % isolated yield with an exotendo ratio of 96 4 and 95 % e. e. for the exo adduct. A variety of polymer-supported chiral Lewis acids was also investigated by Mayoral et al. [45]. Some supported catalysts were more active than their homogeneous analogs, but enantioselectivity was always lower. [Pg.291]

The enantioselective Diels-Alder reaction is another main motif in chiral Lewis acid catalysis. In 1996, Itsuno and coworkers reported an asymmetric Diels-Alder reaction using polymer-supported catalysts under flow conditions. Immobilized chiral oxazoboloridune (34) was prepared from a copolymer of N-sulfonylvabne and borane having styrene moiety, affording the Diels-Alder adduct in an enantioselective manner (up to 71% yield) [126], The authors used a gravity-fed-type column for the flow reaction. Ti-TADDOL-functionalized monolithic resins (35) were developed by Altava and Luis for the asymmetric Diels-Alder reaction (Scheme 7.30). [Pg.177]

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). [Pg.358]

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... [Pg.519]

It has to be taken into account that these catalysts are not yet fully developed. Introduction of electron-withdrawing groups is suspected to increase Lewis acidity, which influences activity, and the optimum spacer length may enable formation of chiral polymers, but such variations are synthetically challenging. [Pg.80]

Enholm [12] has also prepared an enantiomerically pure soluble polymer support 82 by couphng xylose-derived chiral auxiliary 81 with 77 (Scheme 18). The chiral support was then treated with bromopropionic acid 83 to give substrate 84. Eree radical allyl transfer from allyltributyltin imder thermal conditions provided 85 in 93% yield, and basic cleavage from the resin gave (R)-(-)-2-methylpent-4-enoic acid 86 in 80% yield and 97% ee, with a 92% yield of recovered 82. Previous studies of the same process in solution had found the addition of Lewis acids to be crucial for high selectivities to be obtained. Interestingly, the addition of Lewis acids to the reaction on polymer support led to cleavage of the carbohydrate from the polymer backbone. En-... [Pg.105]

Other important titanium alkoxide-based Lewis acids are Ti-TADDOLate (a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol)ates, among the most effective chiral catalysts for several important asymmetric reactions. These will be discussed in the sections on polymer-supported Diels-Alder reactions (Section 21.10) and alkylations (Section 21.9). [Pg.950]

The slow nucleophilic addition of dialkylzinc reagents to aldehydes can be accelerated by chiral amino alcohols, producing secondary alcohols of high enantiomeric purity. The catalysis and stereochemistry can be interpreted satisfactorily in terms of a six-membered cyclic transition state assembly [46,47], In the absence of amino alcohol, dialkylzincs and benzaldehyde have weak donor-acceptor-type interactions. When amino alcohol and dialkylzinc are mixed, the zinc atom acts as a Lewis acid and activates the carbonyl of the aldehyde. Zinc in this amino alcohol-zinc complex is regarded as a kind of chirally modified Lewis acid. Various kinds of polymer-supported chiral amino alcohol have recently been prepared and used as ligands in dialkylzinc alkylation of aldehydes. [Pg.957]


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See also in sourсe #XX -- [ Pg.965 ]




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