Titanium TADDOL-derived catalyst

The TADDOL-derived titanium catalyst has also been used for inverse electron-demand Diels-Alder reactions [124-127], although Posner favored the use of a BINOL-derived titanium catalyst in his inverse electron-demand Diels-Alder reactions for the synthesis of la,25(OH)2D3 (Sch. 47) [125,126]. 

Catalytic enantioselective nitrone cycloadditions were first published in 1994, by Scheeren [84] and by Jorgensen [85, 86]. Jorgensen reported that the TADDOL-derived titanium catalyst 92 [87] promoted the cycloaddition re-... 

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]. 

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... 

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]. 

Titanium-TADDOL derived catalyst enantioselective cyclopropanation of cinnamyl alcohol37... 

Narasaka has demonstrated that TADDOL-Ti dichloride prepared from TADDOL and Cl2Ti(OPr )2 in the presence of MS 4A acts as an efficient catalyst in asymmetric catalytic Diels-Alder reactions with oxazolidinone derivatives of acrylates, a results in extremely high enantioselectivity (Sch. 45) [112]. Narasaka reported an intramolecular version of the Diels-Alder reaction, the product of which can be transformed into key intermediates for the syntheses of dihydrocompactin and dihydromevinolin (Sch. 46) [113]. Seebach and Chapuis/Jurczak [114] independently reported asymmetric Diels-Alder reactions promoted by chiral TADDOL- and 3,3 -diphenyl BINOL-derived titanium alkoxides. Other types of chiral diol ligands were also explored by Hermann [115] and Oh [116]. 

Narasaka reported that TADDOL-TiCl2 was able to catalyze asymmetric DA reaction of cyclopentadiene with oxazolidinone derivatives of acrylates in the presence of 4A MS [148]. A remarkable solvent effect on the enantioselectivity was observed, and high enantioselectivity was attained using mesitylene as the solvent. Cycloadditions to oxazolidinone derivatives of acrylates were also efficiently catalyzed by dendritic or polymer-supported TADDOL-Ti catalysts [149]. From the structural determination of the 3-(( )-3-cinnamoyl)-l,3-oxazolidin-2-one adduct, it can be deduced that the transition state involves binding of the dienophile to the titanium catalyst via the N-acyl-oxazolidinone [19a] (Scheme 14.59). The diastereo-and enantioselectivity of this type of catalyst are thus probably owing to both electronic and steric effects from TADDOL ligand. 

The first catalytic asymmetric version of [2 + 2] cycloaddition reaction was realized by Narasaka in 1989 using chiral titanium catalyst derived from TADDOL. It was found that the reaction of ketene dithioacetal with acryloyloxazolidinone derivatives proceeded smoothly in the presence of 10 mol% of TADDOL-TiCl2 to give the cyclobutane derivatives in high yields (64-96%) and good to excellent enantioselectivities (80-98%) [183]. The reaction is presumed to proceed via a carbonyl substrate chelated TADDOL-TiCb intermediate although the exact reaction mechanism is unclear. Moreover, alkynyl, alkenyl, and 1,2-propadienyl... 

To facilitate catalyst recovery, polymeric and dendrimeric TADDOL, and BINOL ligands have been used for the titanium-catalyzed diethylzinc addition reaction [49]. Moreover, ionic liquids and fluorous solvents have also been used as the reaction media to facilitate the separation of ligands [50]. The microporous metal-organic frameworks prepared from BINOL derivatives were applicable to heterogeneous diethylzinc addition to aldehydes in the presence of excess amount of Ti(O Pr)4 [51]. 

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