POLYMER-BOUND AND DENDRIMER-BOUND TI-TADDOLATES IN CATALYTIC (AND STOICHIOMETRIC) ENANTIOSELECTIVE REACTIONS - ARE PENTACOORDINATE CATIONICTI COMPLEXES THE CATALYTICALLY ACTIVE SPECIES

alpha, alpha, alpha', alpha'-Tetraaryl-1,3-dioxolane-4,5-dimethanols ( TADDOLs), containing styryl groups either at C(2) of heterocyclic ring or in the alpha-position, were prepared in the usual way (18-22, 24, 25). These compounds were copolymerized with styrene and divinylbenzen e in a suspension, yielding polymers (33-40, Scheme 3) as beads with a rather uniform particle-size distribution (150-450 mu m), swellable i n common organic solvents. HOCH2- and BrCH2-substituted TADDOLs were a lso prepared and used for attachement to Merrifield resin or to dendri tic molecules (23, 26-32). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer- or dendrimer-bound reagents (stoichiometric) or Lewis acid s (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R(2)Zn and RTi(OCHMe(2))(3); Scheme 5, Table 1) and to ketones (of LiAIH(4), Table 2); enantioselective ring opening of meso-anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3-crotonyl-1,3-oxazolidin-2-one to cyclopentadiene and to (Z)-N-benzylidenephenylamine N-oxide(-->48, 49, Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer-bound or dendritic TADDOL ligands were compar able or identical to those observed with the soluble analogs. The acti vity of the polymer-bound Lewis acids was only slightly reduced as com pared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has bee n demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels-Al der reaction (Scheme 7a), induced by the polymer-bound Cl2Ti-TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis-acid catalysis, and to propose a cationic , trigonal-bipyramidal complex as the catalytically active species (Fi g. 6). It is suggested that similar cations may be involved in other T i-TADDOLate-mediated reactions as well.