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The asymmetric Meerwein-Pondorf-Verley (MPV) reduction of prochiral ketones with 2-propanol catalyzed by chiral aluminum alkoxides

Posted on:2002-10-09Degree:MasterType:Thesis
Country:ChinaCandidate:Campbell, Elbert JosephFull Text:PDF
GTID:2461390011998292Subject:Chemistry
Abstract/Summary:PDF Full Text Request
Tetra- through hexacoordinate alkylaluminum(III) Schiff base complexes were synthesized in high yield (70–90%) and investigated as catalysts for the Meerwein-Pondorf-Verley (MPV) reduction. In the reduction of cyclohexanone (1 equiv cyclohexanone and 4 equiv 2-propanol) the pentacoordinate aluminum complexes (10 mol%) were found to be inactive for the reaction. Although the tetra- and hexacoordinate complexes (10 mol%) exhibited little reactivity within the initial 24 h, over an extended period of time (72 h) high yield (75–80%) of cyclohexanol was obtained.; In the presence of excess 2-propanol (20 equiv), the pentacoordinate complexes were shown to be stable for over two days. However, when the tetracoordinate aluminum complex is treated with 2-propanol (10 equiv), the metal center was alcoholized from the ligand after 24 h to yield a mixture of free ligand and aluminum tris(isopropoxide) ((AlOiPr) 3). The Al(OiPr)3 generated from this alcoholysis reaction was soluble in organic solvents in contrast to commercial, oligomeric Al(OiPr)3. Thus it is believed that the in situ-generated Al(OiPr) 3 was soluble due to a lower aggregation state (low amount of bridging isopropoxy ligands) than commercial Al(OiPr) 3. Rathke et al. have previously postulated that the bridging alkoxy ligands of the aluminum alkoxide reagents used in the MPV reduction were catalytically ineffective. Hence, the efficiency of in situ-generated Al(O iPr)3 in the MPV reduction was tested.; It was found that simple alkylalumiunm reagents (AlMe3 or AlMe2Cl) dealkylated in the presence of excess 2-propanol (40 equiv) and formed low-aggregated aluminum alkoxides that were soluble in organic solvents. Treatment of this reaction mixture with carbonyl substrates (10 equiv) produced the corresponding primary and secondary alcohols in high yield (50–99%) under mild conditions.; This aluminum-catalyzed MPV reaction was also modified for the asymmetric synthesis of chiral alcohols. Treatment of prochiral, aromatic ketones (10 equiv) with a solution of enatiopure BINOL (1 equiv), AlMe3 (1 equiv), and 2-propanol (40 equiv) yields chiral alcohols in high yield (50–99%) and good ee (50–80%) after 16 h. Although the asymmetric aluminum-catalyzed MPV reduction has been sought after for over four decades to date, this thesis reports the first example that is initiated by a chiral aluminum catalyst and an achiral hydride source (2-propanol).
Keywords/Search Tags:Aluminum, 2-propanol, MPV, Chiral, High yield, Reduction, Equiv
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