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TADDOL-derived titanium complex

Enantioselective carbonyl addition of dialkylzinc reagents to aldehydes is one of the most important and fundamental asymmetric reactions [2,15]. Several chiral titanium complexes have been developed to accelerate this type of reaction (Sch. 3) [16,18-26] since Ohno and Kobayashi achieved high enantioselectivity in the presence of Ti(OPr )4 and chiral disulfonylamide [16,17]. Seebach has also demonstrated that TADDOL-derived titanium complexes function as efficient asymmetric catalysts [18]. [Pg.800]

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. [Pg.825]

Asymmetric [2 + 2] cycloaddition reaction affords a practical means of synthesis of optically active cyclobutanes, which can be used as useful intermediates in organic synthesis [138]. Narasaka reported that asymmetric [2 -i- 2] cycloaddition between acryloyl oxazolidinone derivatives and bis(methylthio)ethylene proceeded with high enantios-electivity when catalyzed by TADDOL-derived titanium complex (Sch. 58) [139]. The cyclobutane product was transformed into carbocyclic oxetanocin analogs or (-n)-grand-isol [140]... [Pg.833]

The tartrate (or TADDOL) derived approach to catalyst design has also been applied to the enantioselective a-hydroxylation of p-ketoesters. In this case, an enantiospecihc titanium(IV) complex combines with a sulfonyloxaziridine as the... [Pg.226]

Lewis acid-promoted asymmetric addition of dialkylzincs to aldehydes is also an acceptable procedure for the preparation of chiral secondary alcohol. Various chiral titanium complexes are highly enantioselective catalysts [4]. C2-Symmet-ric disulfonamide, chiral diol (TADDOL) derived from tartaric acid, and chiral thiophosphoramidate are efficient chiral ligands. C2-Symmetric chiral diol 10, readily prepared from 1-indene by Brown s asymmetric hydroboration, is also a good chiral source (Scheme 2) [17], Even a simple a-hydroxycarboxylic acid 11 can achieve a good enantioselectivity [18]. [Pg.97]

Bernadi and Scolastico, and later Evans in a more effective manner, indicated that the enantioselective addition reaction using silyl enol ethers can be catalyzed by Lewis acidic copper(II) cation complexes derived from bisoxazolines [38-40]. In the presence of the copper complex (S,S)-14 (10 mol %), silyl enol ethers derived from thioesters add to alkylidenemalonates or 2-alkenoyloxazo-lidone in high ees (Scheme 12). Bernadi, Scolastico, and Seebach employed a titanium complex derived from TADDOL for the addition of silyl enol ethers to nitroalkenes or 2-cyclopentenone [41-43], although these are stoichiometric reactions. [Pg.157]

Asymmetric catalysis of ene reactions was initially explored in the intramolecular cases, since the intramolecular versions are much more facile than their intermolecular counterparts. The first example of an enantioselective 6-(3,4) car-bonyl-ene cyclization was reported using a BINOL-derived zinc reagent [55]. However, this was successful only when using an excess of the zinc reagent (at least 3 equivalents). Recently, an enantioselective 6-(3,4) olefin-ene cyclization has been developed using a stoichiometric amount of a TADDOL-derived chiral titanium complex (Scheme 17) [56]. In this ene reaction, a hetero Diels-Alder product was also obtained, the ratio depending critically on the solvent system... [Pg.1092]

Recently Mayoral and coworkers have reported the grafting of a TADDOL derivative onto several types of polystyrene resins. The immobilised TADEXDLs were subsequently transformed into their chiral titanium complexes as shown in Scheme 3.6.10. [Pg.242]

Very recently, Seebach and coworkers reported a different approach for the immobilisation of TADEXDL derivatives. Dendritically substituted TADDOLs (90) were co-polymerised with styrene in a suspension polymerisation procedure and subsequently transformed into their titanium complexes. [Pg.243]

Narasaka and coworkers used the titanium complex of the TADDOL (a,a,a, a tetraaryl-l,3-dioxolane-4,5-dimethanol) ligand (8.50) to catalyse Diels-Alder reactions of acyloxazolidinones. Thus, the crotonyl derivative (8.51) was... [Pg.220]

An enantioselective 6-(3,4) olefin-ene cyclization could be achieved using a stoichiometric amount of a TADDOL-derived chiral titanium complex [127]. In this ene reaction, a HDA product was also obtained, where the periselectivity depends critically on the solvent system employed. In both cases, geminal disubstitution is required for good enantiocontrol (Scheme 14.47). [Pg.219]

Some innovative approaches using chiral Lewis acids to promote catalytic enantioselective [2 + 2]-cycloadditions have been documented [36, 148-150]. The earliest example appears to have been reported by Narasaka (Equation 19) [148]. In these cycloaddition reactions, a titanium complex of TADDOL ligand 239 catalyzes the reactions between ketene dithioacetal 237 and acceptors. With fumarate derivative 238, adduct 240 was obtained in 96% yield and 98% ee. [Pg.614]


See other pages where TADDOL-derived titanium complex is mentioned: [Pg.249]    [Pg.249]    [Pg.800]    [Pg.197]    [Pg.311]    [Pg.414]    [Pg.557]    [Pg.217]    [Pg.208]    [Pg.75]    [Pg.196]    [Pg.557]    [Pg.25]    [Pg.227]    [Pg.327]    [Pg.108]    [Pg.34]    [Pg.206]    [Pg.44]    [Pg.265]   
See also in sourсe #XX -- [ Pg.249 , Pg.352 ]




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TADDOL-titanium

TADDOLate

TADDOLates

TADDOLs

Titanium TADDOL complex

Titanium complexe

Titanium complexes

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