TADDOL moiety

Attached to the periphery of a lst-generation dendrimer, Ti(OCHMe2)2-com-plexes of the six TADDOL moieties in 84 catalyze - in homogeneous solution -the enantioselective addition of diethylzinc to benzaldehyde with about the same selectivity ((S) (R) 97 3) as do six monomeric TADDOL units [105],but, with a molecular weight of only 3833 Da, dendrimer 84 had to be separated by column chromatography rather than by ultrafiltration methods. 

The BIPOLato-Ti-TADDOLato catalysts prepared by addition of BIPOL and TADDOL to Ti(0 Pr)4, catalyze methylation with an achiral methyltitanium reagent to give highly enantiomerically pure methylcarbinol. Since the sterically bulky 3,3 -substituents leads to an increase in enantioselectivity, the chirality of BlPOLato-Ti(0 Pr)2 catalyst 30 can be dynamically controlled by the chiral TADDOL moiety (Scheme 8.25). 3,3 -Dimethoxy derivative affords complete enantioselectivity (100% ee), while the moderate enantioselectivity is obtained with the parent BIPOL (73% ee). 

Highly porous silica gel served as a support for the TADDOL moiety derived from inexpensive and readily available i-tartaric acid, which provided access to htanium-based Lewis acid catalysts (Heckel, 2000). Such entihes are employed successfully for enantioselective reactions. TADDOLs were covalently attached to the trimethyl-silyl-hydrophobized silica gel, controlled-pore glass (CPG) at about 300 m2 g-1, at a loading of 0.3-0.4 mmol gl (Heckel, 2002). In a carefully monitored mulh-step immobilization procedure, the TADDOLs were titanated to yield dichloro-, diisopropyl-, or ditosyl-TADDOLates. These catalysts were employed in dialkylzinc addihon to benzaldehydes and diphenyl nitrone addihon to 3-crotonyloxazolidinone, a [3+2] cycloaddition. 

Alejcakis et al. synthesized a large variety of TADDOL-based phosphites phos-phoramiditeSj and phosphonites 28 and screened these ligands in the Et Zn addition to 2-cydohes enone (Scheme 7.13) [53 54]. While only modest ees were reported for most of these ligands an excellent yield (95%) and enantioselectivity (96%) was observed with ligand 29. The stereocontrol in these ligands is mainly due to the TADDOL moiety. 

Scheme 13. Comparison of Two Polymer-Bound Ti-TADDOLates Generated after Cross-Linking Polymerization of Styrene with Styryl-Substituted TADDOL Monomers (see Scheme 12). Best reproducibility is observed vdth materials of low loading degree (0.10 mmol/g). According to elemental analysis (and rate measurements), ca. 90% of the TADDOL moieties introduced into the polymer (with the monomeric, cross-linking styryl-TADDOLs) carry a Ti-atom [78] [79] and are thus not buried inaccessibly in the cross-linked polymer The dendritic polymer performs better, as far as enantioselectivity of the Et2Zn addition to PhCHO is concerned there is no erratic up and down, and the value obtained in the 20th run is identical to that of the first run (within experimental error) [79].

Scheme 15. Si02 Bound Ti-TADDOLates Tested in Two Reactions [85]. The conlroUed-pore glass (CPG, supplied hy the Grace Company) has an enormously large surface/g. Up to 0.3 mmol/g of TADDOL have been loaded by first attaching Si-(CH2)3S-Trityl and then the TADDOL moiety (through S-alkylation with the corresponding benzyhc bromide) unreacted SiOH groups on the surface are trimethylsilylated. The rates and stereoselectivities of the EtjZn-to-PhCHO addition and of the diphenylnitrone 1,3-dipolar addition shown are comparable to those observed under homogeneous conditions. The material can be washed with aqueous HCl to restore, after reloading with titanate, whatever activity of the solid-state-bound catalyst had been lost after several catalytic cycles [85].

In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

The exo selecdvity of the TiCL-TADDOLate-catalyzed 1,3-thpolar cycloadthdon is improved by the tise of sticcinimide instead of oxazoiidinone as attxiliary for the a,fi-imsatitrated carbonyl moiety (Eq. 8.55). A strong bidentate coo rdinadon of the alkenyl moiety to the metal catalyst is impcrtant in these re... 

Similar to the success achieved with TADDOLs (see Chapter 8) [17], many chiral catalysts and chiral ligands were synthesized from tartaric add, which provides a proper C2-symmetric framework and structural diversity. TaDiAS 1 has remarkable structural diversity because a wide variety of catalysts can be easily synthesized by changing the acetal moieties (R1 and R2), aromatic parts (Ar), and counter anions (X ), making it possible to fine-tune, three-dimensionally, the catalyst (vide infra). Other candidates, such as 2, 3, and 4 (Figure 6.3), produced unsatisfactory results (<10% ee) during preliminary catalyst screening of the N-substituents [4a]. Thus,... 

Convergent dendrimers, with their versatile three-dimensional scaffold, may be tailored to mimic, perhaps crudely, some elements of enzymatic structures. Numerous catalytic moieties, including manganese porphyrins,253,254 bis(oxazoline) copper complexes,304 305 tertiary amines,306 binaphthol titanium complexes,285 307 titanium taddolates,292,308 thiazolio-cyclophanes,309 and fullerene-bound bisoxazoline copper complexes,310 have been incorporated at the core of dendritic molecules to determine the effect of dendritic encapsulation on their catalytic activity. 

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). 

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