Polymer‐ and Dendrimer‐Bound Ti‐TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species?

Abstract

α,α,α′,α′‐T etra a ryl‐1,3‐d ioxolane‐4,5‐d imethan ol s (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α‐position, were prepared in the usual way (18–22, 24, 25). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33–40, Scheme 3) as beads with a rather uniform particle‐size distribution (150–45 μm), swellable in common organic solvents. HOCH 2‐ and BrCH 2‐substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic 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 acids (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 LiAlH 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 comparable or identical to those observed with the soluble analogs. The activity of the polymer‐bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels‐Alder reaction (Scheme 7a), induced by the polymer‐bound Cl 2 Ti‐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 (Fig. 6). It is suggested that similar cations may be involved in other Ti‐TADDOLate‐mediated reactions as well.