The versatile chiral ligand for polar metal centers, TADDOL ((R,R)-a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanol), has been incorporated as core building block into dendrimers by way of benzylation of a fourfold phenolic derivative (hexol 2) with Fre  chettype branches of up to fourth generation. These carry either benzyl (3 ± 7) or octyl groups (33 ± 35) at the periphery, or they contain chiral branching units (18 ± 20, 36), derived from (R)-or (S)-3-hydroxybutanoic acid. The dendritic compounds of molecular weight up to 13 626 have been fully characterized, including by MALDI-TOF mass spectrometry, NMR spectroscopy, and optical activity measurements; one of the branch precursors with four octyl groups crystallized in an intriguing packing pattern. From the spectra and from the specific and molar optical rotations, there was no indication for the formation of chiral secondary structures of up to the third generation. The new TADDOLs were converted to Ti TADDOLates, which were employed as catalysts for the addition of Et 2 Zn to PhCHO. The stereoselectivities and the reaction rates observed with the novel catalysts were compared with those of the simple Ti TADDOLate: up to the second generation there was no detectable decrease of selectivity ( 98:2), and the rates hardly decreased up to the third generation; also, enantiomeric branches caused no change of stereoselectivity within experimental error. Thus, there may be applications for the special properties (such as high molecular weight, good solubility, spacing of central site from cross-linked polymer matrix) of dendritically modified chiral catalyst ligands.

The chiral ligand 1,1'-bi-2-naphthol (BINOL) has been succesfully immobilized on polystyrene. Several dendritic and non-dendritic BINOL derivatives (3, and 13-17), bearing at least two polymerizable styryl groups, were prepared and fully characterized. Suspension copolymerization of the MOM- or TIPS-protected cross-linking BINOL ligands (MOM = methyloxymethyl, TIPS = triisopropylsilyl) with styrene, cleavage of the protecting-groups, and loading with a Lewis-acid afforded catalytically active polystyrene-supported BINOLates. The polymer-bound BINOLs p-3, and p-13-p-16 were tested in the Ti-BINOLate-mediated addition of Et2Zn to PhCHO. The enantioselectivities (up to 93%) and conversions obtained with the polymer-bound catalysts were in most cases identical (within experimental error) to those obtained with the unsubstituted 1,1'-bi-2-naphthol and with the non-polymerized BINOL cross-linkers under homogeneous conditions. Special focus was put on the reusability of the supported catalyst: the polymer-beads were used in up to 20 consecutive catalytic runs, with the best polymers showing no or only minor loss of selectivity. BINOL-polymers p-17, obtained by copolymerization of a 3,3'-distyryl-substituted BINOL 17a with styrene, were used in the BINOL. AlMe-mediated cycloaddition of diphenyl nitrone with alkyl vinyl ethers. In all cases the exo/endo selectivity (> or =92:8) and the enantioselectivities with which the exo-cycloadducts were formed (> or =95%) correspond to those observed in the homogeneous reactions. A dendritically cross-linked BINOL-polymer was also employed in the Ti-BINOLate-mediated cyanosilylation of pivalaldehyde. The enantiopurity of the cyanohydrine obtained in the first run was as high as in the homogeneous reaction (72%); surprisingly the catalytic performance of the supported catalyst increased steadily during the first catalytic cycles to reach 83%. Thus, cross-linking BINOLs can be succesfully incorporated into a polystyrene matrix (without racemization!) to give polymer-bound BINOL ligands that give excellent performance over many catalytic cycles with catalytic activities comparable with those of soluble analogues.

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TADDOLs (or,cc,cc',a'-tetraaryl-1,3-dioxolane-4,5-dimethanols) with dendritic branches attached to the aryl groups and with 8 or 16 peripheral styryl double bonds have been synthesized (4 and 5). With these compounds, dendritic molecules were employed for the first time as cross-linkers in a polymerization. The resulting polystyrene was loaded with titanate (Ti(OCHMe,),) to generate polymer heads incorporating Ti-TADDOLate centers (Figs. f and 2) for enantioselective catalysis (Et,Zn addition to PhCHO). Compared with conventionally polymerattached, insoluble Ti-TADDOLates (cf. 3), the new materials have a much higher catalytic activity, rather close to that of soluble analogs (Fig. 3).

Dedicated to Professor Barry M. Trost on the occasion of his 60th birthday A full account is given of the preparation and use of TADDOLates, which are dendritically incorporated in polystyrene beads (Scheme 1). A series of styryl-substituted TADDOLs with flexible, rigid, or dendritically branching spacers between the TADDOL core and the styryl groups (2 ± 16 in number) has been prepared (5 ± 7, 20, 21, 26 in Schemes 2 ± 4 and Fig. 1 ± 3). These were used as cross-linkers in styrene-suspension polymerization, leading to beads of ca. 400-mm diameter (Schemes 5 and 6, b). These, in turn, were loaded with titanate and used for the Lewis acid catalyzed addition of Et 2 Zn to PhCHO as a test reaction (Scheme 6). A comparison of the enantioselectivities and degrees of conversion (both up to 99%), obtained under standard conditions, shows that these polymer-incorporated Ti-TADDOLates are highly efficient catalysts for this process ( Table 1). In view of the effort necessary to prepare the novel, immobilized catalysts, emphasis was laid upon their multiple use. The performance over 20 cycles of the test reaction was best with the polymer obtained from the TADDOL bearing four first-generation Fre¬chet branches with eight peripheral styryl groups (6, p-6, p-6 ¥ Ti(O i Pr) 2 ): the enantioselectivity (Fig. 4), the rate of reaction (Fig. 5), and the swelling factor (Fig. 6) were essentially unchanged after numerous operations carried out with the corresponding beads of 400-mm diameter and a degree of loading of 0.1 mmol TADDOLate/g polymer, with or without stirring (Fig. 7). The rate with the dendritically polymer-embedded Ti-TADDOLate (p-6 ¥ Ti(O i Pr) 2 ) was greater than that measured with the corresponding monomer, i.e., 6 ¥ Ti(O i Pr) 2 (Fig. 8). Possible interpretations of this phenomenon are proposed. A polymer-bound TADDOL, generated on a solid support (by Grignard addition to an immobilized tartrate ester ketal) did not perform well (Scheme 4 and Table 2). Also, when we prepared polystyrene beads by copolymerization of styrene, a zero-, first-, or second-generation dendritic cross-linker, and a mono-styrylsubstituted TADDOL derivative, the performance in the test reaction did not rival that of the dendritically incorporated Ti-TADDOLate ((p-6 ¥ Ti(O i Pr) 2 ) (Scheme 7 and Fig. 10). Finally, we have applied the dendritically immobilized Cl 2 and (TsO) 2 Ti-TADDOLate as chiral Lewis acid to preferentially prepare one enantiomer of the exo and the endo (3 2) cycloadduct, respectively, of diphenyl nitrone to 3-crotonoyl-1,3-oxazolidinone; in one of these reaction modes, we have observed an interesting conditioning of the catalyst: with an increasing number of application cycles, the amount of polymer-incorporated Lewis acid required to induce the same degree of enantioselectivity, decreased; the degrees of diastereo-and enantioselectivity were, again, comparable to those reported for homogeneous conditions (Fig. 9).