Tetraaryl-l,3-dioxolan-4,5-dimethanol

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

The a,a,a,a-tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) chiral ligands have also been the basis of enantioselective catalysis of the D-A reaction. In a study using 2-methoxy-6-methylquinone as the dienophile, evidence was found that the chloride-ligated form of the catalysts was more active than the dimeric oxy-bridged form.117... 

The use of TADDOL-based ligands offers an important alternative for copper-catalyzed asymmetric 1,4-additions. TADDOLs (a, a, a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol compounds), introduced by Seebach, are among the most successful currently known ligands in asymmetric catalysis. Seebach also developed the first copper-catalyzed 1,4-addition of a Grignard reagent using a TADDOL derivative as a chiral ligand (see Scheme 7.2) [17]. We have reported TADDOL-based... 

Another chiral ligand which plays an increasingly important role in asymmetric catalysis is TADDOL (a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) [63]. Various attempts have been made to immobilize this chiral system to various solid sup-... 

In the catalytic system shown in Scheme 9, a hydrogen bond between one hydroxy function of the diol catalyst and the carbonyl group of the substrate is regarded as the driving force of catalysis. Here, the spatial orientation of the bulky a-1-naphthyl substituents of the TADDOL (a,a,a, a -tetraaryl-l,3-dioxolan-4,5-dimethanol) scaffold generates the chiral environment controlling the enantioselectivity of the reaction. 

Chiral titanium complexes with a, a, a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands are versatile auxiliaries in the Lewis acid catalyzed alcoholysis of racemic 4-(arylmethyl)-2-phenyl-5(477)-oxazolones 234, providing the corresponding enantiomerically enriched N-protected amino acid esters 235 (Scheme 7.73). The enantioselectivity of the reaction is dependent on the solvent, temperature, and chiral ligand. Selected examples of the alcoholysis of saturated 5(477)-oxazolones are shown in Table 7.21 (Fig. 7.23). 

Various tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands have been employed in the syntheses of a series of Ti-TADDOL derivatives578 such as TiCl2(TADDOL),579 TiCl2(TAD-DOL)(THF)2 (54),515 and Ti(OC(Ph)CHC(Ph)0)(TADDOL)(THF)2.580 Much of this chemistry has been reviewed.383,386,581 Ti complexes of these ligands have been extensively used in asymmetric synthesis.515,582-591... 

Comma, P. J. Beck, A. K. Seebach, D., A Simple Batch Reactor for the Efficient Multiple Use of Polymer-bound a,a,a ,a -Tetraaryl-l,3-dioxolane-4,5-dimethanol Titanates in the Nucleophilic Addition of Dialkylzinc Reagents to Aldehydes. Org. Process Res. Dev 1998, 2,18. 

We prepared and purified the air- and moisture-sensitive complex 43 to carry out these studies. We have also found that it is possible to generate this complex in situ, with the starting ArOH (two equivalents) and two equivalents of BuLi, followed, at it, by TiCU, and then at -18° by the diene and two more equivalents of BuLi. The cyclization results are the same as with the preprepared complex 43. We will use this latter approach to quickly screen a variety of other alcohols and diols, including enantiomerically pure diols such as BINOL (1,1 -bi-2-naphthol) and Taddol (a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol), in this cyclization. 

Tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands synthesized from tartaric acid have been extensively employed by Narasaka as the chiral control element in selective Diels-Alder reactions. Initial experiments were conducted with simple dienes and a,P-unsaturated imides using complex 44 (Scheme 36) [104,105]. Several rather subtle features have contributed to the success of these endeavors 1) the use of the acetophenone-derived dioxolane rather than the ac-etonide resulted in an increase of 20% ee 2) the use of alkyl-substituted benzenes as solvent augmented enantioselectivities relative to more common organic solvents e.g., CH2CI2, THF) [106] 3) use of 4 A molecular sieves was typically required to achieve maximum enantioselectivity. 

Ligands having C2-symnietry have also been used with metals that are undoubtedly octahedral, however the analysis of facial selectivity in octahedral complexes is complicated by several possible competing coordination modes of the dienophile. One class of ligand that has been well studied are the TADDOLs (TADDOL is an acronym for a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol). Both the Narasaka [236] and the Seebach [228] groups have evaluated a number of TADDOLs as ligands for titanium in the asymmetric Diels-Alder reaction. Table... 

Other enantioselective catalysts for the D-A reactions have been developed. The chiral ligands used include the TADDOLs (a, a, a, a-tetraaryl-l,3-dioxolane-4,5-dimethanols) ° and BINOL derivatives. These are discussed in Section 6.4 of Part B. 

Based on Frechefs work [20], Luis and coworkers [21] studied the influence of the mode of preparation of the polystyrene backbone functionalized with TADDOL (a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) 2. This immobilized ligand was loaded with titanium on the topicity of an asymmetric transformation. Here, the Diels-Alder reaction of cyclopentadiene with 3-crotonyl-l,3-oxazolidin-2-one was chosen as a model reaction. The TADDOL ligands were incorporated into the polymeric backbone either by polymerization using functionalized styrene derivative 1 or by grafting and coupling of phenol 2 to Merrifield-type resins (Scheme 7). 

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