Immobilization of BINOL by Cross-Linking Copolymerization of Styryl Derivatives with Styrene, and Applications in Enantioselective Ti and Al Lewis Acid Mediated Additions of Et2Zn and Me3SiCN to Aldehydes and of Diphenyl Nitrone to Enol Ethers

Abstract

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.

Self-Assembled Complexes of Non-cross-linked Amphiphilic Polymeric Ligands with Inorganic Species: Highly Active and Reusable Solid-Phase Polymeric Catalysts

“…[134][135][136][137] Hereby, I present development of new chiral catalysts by our methodology and their application to an asymmetric carbonyl-ene reaction. [138][139][140] It is noteworthy that one of the catalysts can be reused five times with retention of its activity and enantioselectivity.…”

Spatially Resolved Derivatization of Solid-Phase-Synthesis Beads with Fluorescent Dendrimers: Creation of Localized Microdomains

“…After introduction of a titanium, these heterogeneous catalysts perform extremely well when compared with their homogeneous counterparts. The chiral ligands BINOL and Salen may be embedded in a similar fashion, thus establishing the generality of this strategy for heterogeneous catalysis [8].…”

“…) Some of the conclusions of this report have been challenged[14] 8. ) On the surface of the bead, active sites are distributed in two dimensions.…”

Synthesis, Characterization and Catalytic Properties of Chiral BINOL Functionalized Mesoporous Silicas for Enantioselective Morita-Baylis-Hillman Reaction

“…Specifically, chiral BINOLs were accepted as optimal candidates for the asymmetric Morita-Baylis-Hillman reaction [18] and asymmetric allylboration of ketones [19,20] as chiral Brønsted acids even though they have been widely used in metal-centered Lewis acid catalysts [21]. Various supports such as organic polymers [22][23][24][25][26][27][28][29][30], inorganic materials [31][32][33], dendrimers [34], have been investigated to immobilize BINOLs as metal-centered Lewis acid catalysts in a lot of asymmetric catalytic reactions. Meanwhile, one organosilane derivative of chiral BINOL was post-grafted onto ordered mesoporous silicas MCM-41 [35] and SBA-15 [36] by Abdi and coworkers to catalyze asymmetric nitroaldol reaction [37] and enantioselective addition of diethylzinc to aldehydes [38] centered with different metal ions.…”