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Titolo:
PREPARATION AND STRUCTURAL-ANALYSIS OF SEVERAL NEW HA',ALPHA'-TETRAARYL-1,3-DIOXOLANE-4,5-DIMETHANOLS (TADDOLS) AND TADDOL ANALOGS, THEIR EVALUATION AS TITANIUM LIGANDS IN THE ENANTIOSELECTIVE ADDITION OF METHYLTITANIUM AND DIETHYLZINC REAGENTS TO BENZALDEHYDE, AND REFINEMENT OFTHE MECHANISTIC HYPOTHESIS
Autore:
ITO YN; ARIZA X; BECK AK; BOHAC A; GANTER C; GAWLEY RE; KUHNLE FNM; TULEJA J; WANG YM; SEEBACH D;
Indirizzi:
ETH ZURICH,ORGAN CHEM LAB,UNIV STR 16 CH-8092 ZURICH SWITZERLAND ETH ZURICH,ORGAN CHEM LAB CH-8092 ZURICH SWITZERLAND KYUSHU UNIV 33,FAC SCI,DEPT CHEM,HIGASHI KU FUKUOKA 812 JAPAN UNIV BARCELONA BARCELONA SPAIN COMENIUS UNIV BRATISLAVA,CHEM TECHNOL RES INST BRATISLAVA SLOVAKIA UNIV MIAMI,DEPT CHEM CORAL GABLES FL 33124
Titolo Testata:
Helvetica Chimica Acta
fascicolo: 8, volume: 77, anno: 1994,
pagine: 2071 - 2110
SICI:
0018-019X(1994)77:8<2071:PASOSN>2.0.ZU;2-A
Fonte:
ISI
Lingua:
ENG
Soggetto:
PIG-LIVER ESTERASE; RESOLUTION GAS-CHROMATOGRAPHY; DIELS-ALDER REACTIONS; ORGANIC-SYNTHESIS; ASYMMETRIC HYDROGENATION; BETA-CYCLODEXTRIN; MESO-DIESTERS; ALDEHYDES; COMPLEXES; ACID;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
81
Recensione:
Indirizzi per estratti:
Citazione:
Y.N. Ito et al., "PREPARATION AND STRUCTURAL-ANALYSIS OF SEVERAL NEW HA',ALPHA'-TETRAARYL-1,3-DIOXOLANE-4,5-DIMETHANOLS (TADDOLS) AND TADDOL ANALOGS, THEIR EVALUATION AS TITANIUM LIGANDS IN THE ENANTIOSELECTIVE ADDITION OF METHYLTITANIUM AND DIETHYLZINC REAGENTS TO BENZALDEHYDE, AND REFINEMENT OFTHE MECHANISTIC HYPOTHESIS", Helvetica Chimica Acta, 77(8), 1994, pp. 2071-2110

Abstract

Preparation and screening of twenty new ligands, all analogs of pha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL), for the Ti-catalyzed asymmetric addition of methyltri(isopropoxy)titanium and diethylzinc to benzaldehyde are described. These ligands have the dioxolane ring of the TADDOL's replaced by cyclobutane, cyclopentane, cyclohexene, cyclohexane, bicyclo[2.2.1]heptene and -heptane and bicyclo[2.2.2]octene and -octane moieties; several have H-atoms or alkyl groups in place of the aryl groups, and nine of them have C-2 symmetry. X-Ray crystallography and molecular mechanics are used to analyze the structure of the ligands, and two structural features appear to correlate with selectivity: i) the torsion angle for the chelating O-atom and the ortho-C-atom of the axial Ph group (a small, ca. 19 degrees, angle is optimum, Fig. 8) and ii) the 'degree of perpendicularity' of the axial Ph group (Fig.9). Competition experiments indicate that TADDOL 1a catalyzes both the methyltitanium and diethylzinc additions greater than or equal to 50 times faster than the related dioxolane analogs 12a, 12c, and 12e (Scheme 7), indicating that both axial and equatorial aryl groups (see Footnote 6) are necessary for ligand-accelerated catalysis of these reactions. A refined mechanistic hypothesis is presented (Fig. 10) to explain the selectivities observed for these new ligands. Our analysis suggests that a combination of structural features appear necessary for good catalytic efficiency and high selectivity. These features, especially the rather subtle conformational effects, appear to be optimized (among the ligands tested) in the TADDOL's.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 28/03/20 alle ore 10:59:13