Premium
Preparation and Characterization of TADDOLs Immobilized on Hydrophobic Controlled‐Pore‐Glass Silica Gel and Their Use in Enantioselective Heterogeneous Catalysis
Author(s) -
Heckel Alexander,
Seebach Dieter
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020201)8:3<559::aid-chem559>3.0.co;2-1
Subject(s) - chemistry , silylation , catalysis , benzaldehyde , silica gel , bromide , acetal , heterogeneous catalysis , nucleophile , organic chemistry , imidazole , polymer chemistry , nuclear chemistry
Highly porous silica gel (controlled‐pore glass, CPG, ca. 300 m 2 g −1 ) with covalently attached TADDOLs (loading 0.3–0.4 mmol g −1 ) and Me 3 Si‐hydrophobized surface has been prepared: First, mercaptopropyl groups were attached to the silica gel by treatment with (mercaptopropyl)trimethoxysilane; then the SH groups were trityl‐protected, and the remaining accessible SiOH groups hydrophobized by silylation (heating with Me 3 Si‐imidazole); after deprotection, the SH groups were used as nucleophiles for benzylation with TADDOLs carrying a 4‐bromomethyl‐phenyl group in the 2‐position of their dioxolane rings; alternatively, the SH groups have been benzylated with the 4‐bromomethyl‐benzaldehyde acetal of diethyltartrate, and the diarylmethanol moieties of the TADDOLs created on the solid support by addition of excess phenyl, or 1‐ or 2‐naphthyl magnesium bromide. Each step of the immobilizing procedure was carefully monitored and analyzed (Ellman's test, methyl‐red test), and resulting materials characterized by electron microscopy, DRIFT spectroscopy (IR), 13 C‐ and 29 Si NMR solid‐state NMR spectroscopy, and elemental analysis. The immobilized TADDOLs were titanated to give ( i PrO) 2 Ti‐, Cl 2 Ti‐, or (TosO) 2 Ti‐TADDOLates which were used for catalyzing the additions of Et 2 Zn or Bu 2 Zn to PhCHO and of diphenyl nitrone to 3‐crotonoyl‐oxazolidinone. The following findings are remarkable: i) The enantioselectivities and conversions of the reactions mediated by the CPG‐immobilized Ti‐TADDOLates match those observed under standard homogeneous conditions. ii) If and when the rates and/or the enantioselectivities of reactions have dropped after several applications of the same catalyst batch, washing with aqueous HCl/acetone and reloading with titanate leads to full restoration of its performance. iii) There is no detectable loss of the hydrophobizing Me 3 Si groups after nine acidic washes! iv) There is a seasoning of the catalyst material in the Cl 2 Ti‐TADDOLate‐mediated [3+2] cycloaddition of diphenylnitrone: Initially it is necessary to use 0.5 equivalents of the immobilized catalyst to match the performance of the homogeneous catalyst; after three runs the reaction rate, enantio‐ and diastereoselectivity have dropped considerably; acidic washing after each subsequent run completely restores the performance; after a total of seven runs the amount of catalyst can be reduced to 0.4, 0.3, 0.2, and 0.1 equivalents in the following runs, with identical good results!